Author: qaz

  • About the Performance of Type 4 Fighter (Ki-84)

    About the Performance of Type 4 Fighter (Ki-84)

    Update Jun 26 2024: some additional information and corrections to this article in Part 2:

    • In the engine table within this article, the CR of Ha-45-21 was changed to 7.17, because the performance used in the table reflects the actual engine with its production CR (rather than 8.0, which was planned and used in prototypes).
    • The performance of the “Ha-45 Special” was changed from that of Ha-45-12 to that of Ha-45-11 (which it seems to be, see 2nd article).

    The performance of the Type 4 Fighter (project name ‘Ki-84’) is a bit of a can of worms, and a subject of frequent debate. This is due to a significant amount of differing data with varying credibility, and often a lack of information to illustrate the background of each record.

    When discussing Japanese aircraft performance in World War II, there has been a general concept of separating “actual performance” and “potential performance” — A result of negative factors introduced by the declining state of Japanese industry and the lack of resources, especially towards the end of the war.

    “Actual performance” is what was able to be demonstrated by a plane under Japanese fuel, oil, and maintenance conditions. “Potential performance” is said to be what could be demonstrated if the same plane was provided with higher quality conditions, such as those of the United States.

    Type 4 Fighter Mod.1 Kou

    There is a common belief that the Type 4 Fighter exhibited substantially improved performance (speed as high as 687 km/h!) when captured examples were tested by the US, due to the use of high-octane fuel. While it is certainly true that a Japanese fighter could benefit from the US testing medium, the reality is more complex than “higher grade fuel increased the performance”.

    In this article, the true performance of the Type 4 Fighter (Ki-84) will be examined from both angles. This is mainly a technical article, with less focus on history.

    (This article may be rewritten if more detailed sources come to light.)


    About Ki-84’s Engine ー Ha-45

    The most important aspect that dictated the performance of Ki-84 was naturally its engine: the Ha-45. One reason is that depending on the time the airframe was made, a different model (or at least differently performing) engine may have been mounted.

    Japanese engine naming systems are also quite confusing to those not informed, so this will first be clarified. The two relevant engines mounted to Ki-84 were named the ‘Ha-45-11’, and ‘Ha-45-21’ by unified convention. The Army called these engines ‘Ha-45 Special‘ and ‘Ha-45‘ respectively. Ha-45 was serviced in the Army with the name ‘Type 4 1850HP Engine’. The corresponding Navy service names may also be encountered: ‘Homare Model 11’ and ‘Homare Model 21’.

    Because Ki-84 was an Army fighter, I will be using the Army short names ‘Ha-45 Special‘ and ‘Ha-45‘.

    Ha-45

    Ha-45 was a very technically impressive engine, and was developed by Nakajima Airplane Company on a rapid timeframe from 1940-1942. The 18-cylinder air cooled radial engine initially was designed to produce ~1,800 horsepower, but was increased to a maximum output at sea level of 2,000 horsepower. At the same time, it was of a significantly smaller form factor than its global contemporaries. Only slightly larger than the famous Sakae engine it descended from, Ha-45 had a 1,180 mm overall diameter, 830 kg dry weight, and 35.8 l displacement. For comparison, the American 2,000 horsepower-class R18 engine ‘R-2800’ had a 1,342 mm diameter, ~1,050 kg dry weight, and 46 l displacement.

    The Ha-45’s high power for its size of >100 hp per cylinder was achieved with a higher rotational speed of up to 3,000 RPM and higher pressure boost of +500 mmHg compared to prior Japanese engines. Originally, the engine was designed to use 100 octane fuel in order to avoid ‘knock’ at high pressure (premature combustion). However, wartime Japan did not have the means to procure high-grade fuel, and it was ultimately designed to meet its specified power with 92 octane fuel, instead employing a water-methanol injection system within the intake path to reduce the air temperature and avert knock.

    +500 mmHg boost was a high manifold pressure for Japanese aircraft engines, and could only be obtained with water injection on the highest quality of fuel available for service (92 octane in the Navy, 91 octane in the Army). In fact, even the rated power of the engine at +350 mmHg required WM injection. It’s important to know that high-power Japanese engines had to be designed within the constraint of using WM injection to reach boost pressures that were obtained and even surpassed by Allied AC engines without it, as the Allies had a distribution of much higher octane fuel. So resultingly, Allied AC engines that did have WM injection could reach extreme boost pressures Japanese engines could never approach.


    Power Restriction of the Ha-45

    The above specifications describe the extraordinary performance of the Ha-45 engine as it was developed… but a compact, high-power piston engine never had a chance in the actual state of late-war Japan. It’s well established that the mass-produced Ha-45 suffered relentless problems in field service, leading to a terrible operational rate. These problems were caused by a multitude of factors, such as:

    • Lack of skilled production. The Ha-45 required precision manufacturing which was not possible on a mass scale by that time.
    • Declining fuel quality. It is likely that even 91 octane fuel was not actually up to spec near the end of the war.
    • Lack of quality lubricating oils.
    • Delicate electrical system.
    • Insufficient maintenance capacity. The Army simply did not have the depth of maintenance ability to keep the Ha-45 in good shape during widespread service.

    The culmination of these factors caused widespread rises in cylinder temperature, oil temperature, uneven fuel distribution, and various other malfunctions and reliability issues. Resultingly, the Army decided to govern the mass-produced Ha-45 engines down to the about same level as the Ha-45 Special that had been installed in the initial Ki-84 prototypes. The max RPMs were reduced from 3,000 to 2,900, the maximum intake manifold pressure was reduced from +500 mm to +400 mm, and the cylinder compression ratio was reduced from 8.0 to 7.17.

    Army Ha-45 Performance Table
    NameFormatPower
    Takeoff
    Power Rated
    (1st Speed)
    Power Rated
    (2nd Speed)
    CRWeightLen x DiaBoreStroke
    Ha-45 Special2-row R181,820hp @ 2,900RPM (+400mm)1,650hp @ 2,900RPM, 2,000m (+250mm)1,440hp @ 2,900RPM, 5,700m (+250mm)7.0830kg1,690 x 1,180mm130mm150mm
    Ha-452-row R182,000hp @ 3,000RPM (+500mm)1,860 hp @ 3,000RPM, 1,750m (+350mm)1,620 hp @
    3,000 RPM, 6,100m (+350mm)
    7.17830kg1,690 x 1,180mm130mm150mm
    Ha-45 (Governed)2-row R181,850hp @ 2,900RPM
    (+400mm)
    1,680hp @ 2,900RPM, 2,300m
    (+250mm)
    1,500hp @ 2,900RPM, 6,500m (+250mm)7.17830kg1,690 x 1,180mm130mm150mm

    Even with the governed Ha-45 providing as much as 200 less horsepower, the operational rate of the Type 4 Fighter suffered until the end of the war and the power restrictions were rarely relaxed. The operational rate was usually around 40%, and in some units (especially those in the south) it could be as poor as 0-20%. However, there were a few units that managed to tame the suffering Ha-45.

    While the readiness of other Type 4 Fighter units continued to decline, the maintenance team of the Flying 47th Squadron managed to reach up to 100% operational rate at a point in time. This was achieved by implementing a command platoon specifically overseeing aircraft maintenance, which was different from the conventional structure within Army squadrons. Led by Captain Masai Kariya, who was nicknamed the “God of Maintenance”, thorough examinations, maintenance, and overhauls were constantly performed on all aircraft. The Army took note of the 47th Squadron’s ingenuity and ordered the maintenance team to instruct other squadrons, but it was too late to make a major difference.

    Even under the most exhaustive care of the 47th Squadron, which may have been closer to US standards, the Type 4 was unlikely to demonstrate its potential due to factors out of the team’s control (poor oil, fuel, manufacturing precision).

    The purpose of this section was to introduce the multitude of factors that could have caused disparity in the performance of each plane. Now, the performance numbers will be examined.


    About ‘US Testing Data’ of Ki-84

    Before talking about the historical performance records for Type 4, it is first necessary to look at the (supposed) US testing performance numbers which have appeared in numerous publications. It is often said that a captured Type 4 managed to reach an impressive top speed of 687 km/h (427 mph) with American high-octane fuel and test conditions. Similar claims have also been made for other Japanese aircraft, such as Ki-83 reaching 762 km/h (473 mph), Saiun reaching 694 km/h (431 mph), and Raiden reaching 671 km/h (417 mph).

    While the origins of the numbers for some such as Ki-83 and Saiun have not been verified, the Type 4’s numbers are easily located. These numbers are published in a 1946 AAF T-2 report as ‘Factual Data’ and are used to illustrate the claim that the Type 4 compared favorably with the most advanced US service piston fighters ‘P-51H’ and ‘P-47N’.

    As it turns out, this data was not the result of an actual flight test by the US. Rather, these exact numbers were originally created in March 1945 and included in a supplement to the Technical Air Intelligence Center (TAIC) manual on Japanese aircraft. At the time, captured Type 4s had not been extensively tested for performance. As per the TAIC manual:

    Except where otherwise stated, performance figures represent estimates of the Technical Air Intelligence Center and have been calculated after a careful analysis of information derived from intelligence, captured equipment, drawings, and photographs, using power ratings derived from the same sources. When authoritative evidence is not available, it is the policy of TAIC to give the Japanese Aircraft Performance every benefit of the doubt within reasonable limits.

    Japanese Aircraft Performance & Characteristics, TAIC Manual No.1

    As there is no indication in the document that Type 4’s performance was derived from real test data, there is no reason to assume so. These numbers, which were calculated in Japanese operating conditions (92 octane fuel), were probably created by deliberately generous estimates so as to not underestimate an enemy fighter. Not dissimilar to the evaluation of Raiden, which was calculated by TAIC to have speed performance significantly higher than what was actually demonstrated. Furthermore, it is overtly stated in the T-2 report that detailed performance was not measured, and mainly flight characteristics were evaluated.

    This is not to say that it was impossible for Type 4 to demonstrate speed performance of this magnitude. For example, if the Saiun (which used the same engine as Type 4) was truly able to reach 694 km/h in postwar US testing, it would be reasonable for Type 4 to reach 687 km/h and even more in the same conditions. However, such performance would in all likelihood only be met using higher-than-design manifold pressure by modifying the supercharger ratio and throttle valve control, together with high octane fuel and quality American parts.

    As it stands, we seem to have no real detailed data on a US performance test of Type 4 at all.

    In Masai Kariya’s book “Story of Japanese Army Prototype Planes“, it is said that Type 4 achieved 689 km/h using 140 octane fuel during US testing after the war. While this is a slightly different number, it seems quite likely to only be a slight distortion of the TAIC calculations.


    Japanese Performance Data

    As we have no satisfactory US testing data on the Type 4’s performance, we are left with the Japanese performance records.

    There were notable differences in the method of establishing aircraft performance between Japan and the USA at the time. For example, the US recorded top speeds using War Emergency Power (WEP), which is the highest possible output the engine may exhibit for a limited time, while the Japanese standard of recording top speed was at ‘Rated Power’, which could be called ‘Military Power’. This is a lower throttle setting that could be maintained for a longer period of time.

    Furthermore, as previously touched on, the state of late-war Japan left many possible disparities in the performance of each airframe. Whether a prototype with a Ha-45 Special, a production plane with a governed or ungoverned Ha-45, a prototype suffering from malfunctions at a stage of testing, or a production plane with subpar manufacturing and maintenance. At the very least, we can assume the fuel quality for official performance tests to be ‘adequate’, and the airframe to be clean.

    The first record is well known as the “official top speed” of the Type 4. 624 kilometers per hour at an altitude of 6,550 meters was recorded by Major Iwahashi in one of the initial Ki-84 prototypes with a Ha-45 Special engine. This was the highest performance among Army single-engine fighters. In this condition, the plane climbed to 5,000 meters in 6 minutes and 26 seconds. The detailed performance record was published in the ‘Ki-84 Pilot Manual’, and a copy of this test was later captured by the US at Clark Field on Luzon. The speed and climb are as follows.

    Ki-84 Prototype Speed Test (Ha-45 Special)
    Altitude (m)TAS (km/h)RPMBoost (mmHg)Supercharger
    10005442,900+250speed 1
    20005652,900+250speed 1
    30005862,900+250speed 1
    33705942,900+250speed 1
    40005912,900+185speed 2
    49005842,900+95speed 2
    50005802,900+250speed 2
    60006102,900+250speed 2
    65506242,900+250speed 2
    70006152,900+200speed 2
    80005942,900+95speed 2
    90005692,900+40speed 2
    Ki-84 Prototype Climb Test (Ha-45 Special)
    Altitude (m)TimeRate (m/s)IAS (km/h)RPMBoost
    10001’09”14.42602,900+250
    20002’18”14.32602,900+250
    30003’34”12.82602,900+190
    40004’00”11.72602,900+250
    50006’26”11.02602,900+250
    60008’00”10.02602,900+200
    70009’48”8.32402,900+100
    800012’16”6.32302,9000
    900015’34”3.82202,900-180

    This speed was quickly usurped by the 4th Ki-84 prototype (1st pre-production) which seems to have been equipped with a fully rated Ha-45 engine. Lieutenant Funabashi flew this test and achieved 631 kilometers per hour at 6,120 meters with a starting loaded weight of 3,794 kilograms. The climbing time to 5,000 meters was improved to 5 minutes and 54 seconds, a difference of 32 seconds. There is no detailed record of this exact test, but there is a record of a Ki-84 with a fully-rated Ha-45 flying with a much lighter weight of ~3,400 kilograms, which is as follows.

    Ki-84 Unknown Speed Test (Ha-45)
    Altitude (m)TAS (km/h)RPMBoost (mmHg)Supercharger
    10005453,000+350speed 1
    20005703,000+350speed 1
    30005953,000+350speed 1
    37006143,000+350speed 1
    40006103,000+300speed 2
    50006123,000+350speed 2
    60006303,000+350speed 2
    66506343,000+350speed 2
    70006253,000+300speed 2
    80006053,000+200speed 2
    Ki-84 Unknown Climb Test (Ha-45)
    Altitude (m)TimeIAS (km/h)RPM
    10001’10”2653,000
    20002’15”2653,000
    30003’25”2653,000
    40004’30”2553,000
    50005’37”2503,000
    60006’50”2453,000
    70008’15”2403,000
    800010’18”2353,000

    From the available numbers, it seems that the increase in rated engine power only caused a speed increase of about 10 kilometers per hour in the prototype stage. This may be due to the plane’s small 3.0-3.1 meter ‘Pe-32’ propeller, which is considerably smaller than even other Japanese 2,000hp class propellers, such as Shiden’s 3.3 meter propeller, or Saiun’s 3.5 meter propeller. The American Hellcat, Thunderbolt, and Corsair all had propellers around 4 meters in diameter. The choice of Type 4’s propeller is said to have been done to reduce the length of the landing gear and overall weight, but may have negatively affected the transfer of power.

    The highest speed allegedly reached during Japanese testing was 660 kilometers per hour with the Ki-84 Otsu prototype, which is written in numerous secondary sources. The only major difference between the former Model ‘Kou’ was that the ‘Otsu’ was equipped with Type 2 20 mm Autocannons in place of the two nose-mounted Type 1 12.7 mm Autocannons. While on its own this should actually slightly burden the flight performance, there are several factors that could have benefitted this airframe due to its later construction date.

    One possible factor was an improved force of thrust from the engine exhaust. In the initial Ki-84 prototypes and 1st stage of pre-production airframes, the exhaust of each cylinder was routed through a collective exhaust pipe on each side.

    From at least the second stage of pre-production planes, the thrust exhaust method was changed to the evidently superior individual exhaust stacks. The increase in thrust improved the top speed to a certain marginal, but unknown extent. Most photos of Type 4, especially deployed airframes, have this exhaust type. At least some of the 1st stage pre-production planes were also remodeled as such.

    Another factor is that the Type 4 Fighter was seemingly equipped with a slightly larger 3.1-meter Pe-32 propeller after the initial testing phase. This modest increase helped the engine demonstrate its power and evidently did not require a change of reduction ratio to keep the tip velocity in check. Captured production planes were measured to have this propeller diameter.

    Supposing such conditions, it could be possible that the Ki-84 Otsu prototype achieved a top speed 29 km/h higher than the 4th prototype, especially if it was at a lighter load and/or using WEP. But as I do not have the original source for this speed claim, it is only conjecture. Factors such as weight or whether it was achieved during WEP versus military/rated power, or in a dive, are totally unknown.

    It’s also possible that the Ki-84 Otsu did not even possess these features (at least as originally completed). According to Nakajima Airplane Company’s data prepared after the war for the US, two prototypes of Ki-84 with 20 mm machine cannons (known internally as ‘Ki-84-Y’) were completed in 1943. During this time, only the first 3 prototypes and 24 pre-production airframes of the Ki-84 had been built. If correct, this information could suggest that the Ki-84 Otsu prototypes were not equipped with the later performance-enhancing features.


    Conclusion

    From the available data, it seems reasonable to state that Type 4’s maximum speed at rated/military power, gross weight, in Japanese engine settings and fuel grade, was around 631-634 km/h. Perhaps closer or slightly superior to the higher end, assuming both full-rated tests were done with the old exhaust type. But as the engine was largely governed and often suffered from malfunctions for multitudes of reasons, the actual rated speed of a service plane was probably 624 km/h at the most optimistic, even though the ‘Ha-45 Special’-equipped prototype that demonstrated this had the early type of exhaust thrust.

    The top speed when using War Emergency Power (+500 mmHg) would naturally be slightly higher than each of these speeds, but likely only by a matter of about ~10-15 km/h using very tentative estimates. My rough guess would be that ~650 km/h was probably the best case top speed of a Type 4 with emergency power, and a fully rated engine in a ‘clean prototype’ airframe at gross weight with all beneficial improvements.

    As has been discussed, the supposed 687 km/h obtained in US testing with high-octane fuel was only a wartime calculation for Japanese conditions (92 octane fuel) using optimistic data. While it certainly could be possible that Type 4 would have seen a significant performance boost with US high-octane fuel and modifications to produce a higher boost pressure, no such data seems to exist. Furthermore, it’s probable that the US only tested the Type 4s at Japanese power settings, and it’s unknown how much additional pressure a Japanese mass-produced Ha-45 could even handle.

    According to the US report on Type 4 made with a captured plane at Middletown, Ohio in 1946, the aircraft either demonstrated or was estimated to exhibit a climb time of about 6 minutes 40 seconds to 6,100 meters. The starting weight of the plane seems to have been 3,350 kg or 3,500 kg (the two reports list different weights), and this climb time is slightly superior to the 6’50” to 6,000 m time of the Japanese 3,400 kg test, but far from the TAIC calculation of 5’48” to 6,100 m.

    There is little doubt that the Type 4 Fighter was “the most powerful Japanese fighter of World War II”, but its ultimate performance was dictated by the harsh conditions it was developed in, and even then it was rarely able to demonstrate its full potential.


    Sources

    • Ando, Atsuo. Nihon Rikugun-ki no Keikaku Monogatari. 1980.
    • Akimoto, Minoru. Nihon Rikugun Seishiki-ki Taikan. Tōkyō: Kantosha. 2002.
    • Maema, Takanori. Higeki no Hatsudou-ki ‘Homare’. Tōkyō: ‎Soshisha, 2015.
    • Kariya, Masai. Nihon Rikugun Shisaku-ki Monogatari. Tōkyō: Ushio Shobō Mitsuhito Shinsha. 2017.
    • Famous Airplanes of the World No. 19: Army Type 4 Fighter ‘Hayate’. Tōkyō: Bunrindo, 2019.
    • Rep. Ki-84 Pilot Manual. 1944.
    • Rep. Technical Air Intelligence Center Report No. 31, Homare 11 and 21 Engines, Principal Characteristics and Performance. 1945.
    • Rep. ADVATIS Translation 92: Specifications and Performance Data of Ki-84. 1945.
    • Rep. Kenkyū Shisakubu-nai Chōsa-hyō. 1945.
    • Rep. Desc of Experimental Aircraft and Experimental Engines Under Development by the Japanese Army and the Imperial Japanese Navy. 1946.
    • Rep. Manual on Japanese Aircraft, TAIC No. 1. 1945.
    • Rep. Manual on Japanese Aircraft, TAIC No. 2. 1945.
    • Rep. “Ki” 84 Performance Data. 1945.
    • Rep. T-2 Report on Frank-1. 1946.
    • Rep. [Frank-1 Middletown Report]. 1946.
    • If you are interested in Japanese aircraft performance, read the great articles at warbirdperformance.livedoor.blog.
  • Ki-109 & the Ki-109-Mounted Cannon: Type 88, Not ‘Ho-501’

    Ki-109 & the Ki-109-Mounted Cannon: Type 88, Not ‘Ho-501’

    As the threat of strategic bombing loomed over Japan from the middle of the Pacific War, the development of effective extreme-caliber aircraft guns was expedited by the Japanese Army. New weapons with calibers ranging from 47 millimeters to as much as 150 millimeters were developed and were planned to be deployed on various interceptor platforms.

    Type 88 7cm Field-AA

    These weapons – capable of destroying a strategic bomber in just one hit – were naturally only suitable for installation in the heaviest fighters of the time, due to their massive size, weight, and recoil force. For cannons 75 millimeters or more, even most two-engine fighters would be insufficiently sturdy or suffer severe performance detriment.

    For this reason, the Army’s Type 4 Heavy Bomber ‘Hiryū’ (Ki-67) was selected as the basis for such an interceptor. The Type 4 was closer to the international standard of a ‘medium bomber’ but possessed an excellent top speed (537 km/h) and exceptional maneuverability for its class. The weapon of choice was the Type 88 7 cm Field-AA Cannon, a mobile 75mm anti-aircraft gun used by the Japanese Army since 1928.

    The aircraft-adaption of this cannon has often been identified as the ‘Ho-501‘ in both Japanese and English publications. However, through the examination of extant historical materials, it seems evident that this was an error. Ki-109, the features of its gun, and the theory about the Ho-501 will be explained in this article.

    Type 4 Heavy Bomber belonging to the Flying 74th Sentai. The project number was ‘Ki-67’.

    Planning of Special Air Defense Fighter, Ki-109

    The genesis of the Type 4 Heavy Bomber’s interceptor-adaption was with a new prototype order issued by the Army on November 20th, 1943. By this time, Japanese intelligence had already perceived the impending threat of the B-29 Superfortress.

    The request was to Mitsubishi Heavy Industries for the development of a Type 4 Heavy Bomber modification designated ‘Ki-109’ in two models: ‘Ki-109 Kō’: a Patrol & Air Defense Fighter equipped with double dorsal, upward-firing ‘Ho-204’ 37mm machine cannons, and ‘Ki-109 Otsu’: a Foe-Searching & Illuminating Plane equipped with a 40cm search-light. In night operations, these two planes were planned to cooperate in “hunter-killer teams” to bring down strategic bombers.

    Ho-204 37mm Browning-style machine cannon.
    Two of these in an inclined mount were the guns of the original Ki-109 plan.

    However, it was not long before this plan was adjusted by the opinion of Army Major Hideo Sakamoto. Major Sakamoto preferred the concept of mounting a Type 88 7cm Field-AA Cannon, which he theorized could fire from outside the envelope of the B-29’s defensive guns and score a certain kill in one hit. This replacement plan was ordered in January 1944, abolishing the variants with night-fighting equipment and making Ki-109 a single model. Within the adjusted plan, the first prototype of Ki-109 was scheduled to be completed in May 1944, followed by the second in June.

    The design team was led by Mitsubishi Engineer Ozawa, and the process progressed rapidly during the early part of 1944 under the expectation of the B-29’s arrival. The examination of the full-scale wooden mockup was held on February 11th, 1944. To improve visibility while diving, it was requested that the shape of the nose be changed to a shape steeply curved downward in comparison to the original Type 4, and this was reflected in the design.

    Ki-109 Prototype No.1. Note the steeper downward angle of the nose for enhanced visibility.
    The defensive armament remained intact for the prototypes.

    The design was completed in March 1944, and construction of the first two prototypes started. Rather than building from scratch, two Ki-67s owned by the Army Examination Department were remodeled with a new nose, and the defensive armament remained intact. It seems that in the early stage of development, it was hoped that the Ki-109 could retain its defensive weaponry to have a fighting chance against escort fighters.

    The first prototype was completed in August, three months later than planned. Its maiden flight took place on August 30th, where it was demonstrated that the maneuverability of the plane had not significantly deteriorated from the original Type 4. The prototype was subsequently flown to the Fussa Airfield in September for ground firing tests, and the second prototype was completed at the end of October.


    ‘Ki-109 Mounted Cannon’

    Type 88 barrel & cradle,
    length, weight, CG.

    The design of the aerial adaption of the Type 88 7 cm Field-AA Cannon was carried out by the 1st Army Technical Research Institute in cooperation with Mitsubishi, and the Osaka Army Arsenal was responsible for manufacturing. On March 6th, 1944, it was decided that the 1st Institute would complete the design drawings by March 20th. The Type 88 cannons #3582 and #3583 from Osaka Arsenal were chosen as the prototypes, and were to finish being modified to the ‘Ki-109 Mounted Cannon‘ a month after the design was submitted.

    Under the guidance of Major Makiura of the 1st Research Institute, testing was done on standard Type 88 cannons to confirm the recoil characteristics in various conditions. This was to ensure the safe mounting of the gun in an aerial configuration and was carried out from March 22nd to the 28th; the completion report was submitted on the 31st.

    The design modifications to convert the Type 88 to the ‘Ki-109 Mounted Cannon’, in simple summary, were as follows. The land-based mount and pedestal were replaced with an aerial gun cradle. The firing mechanism was changed to be electrically actuated. The length of the gun’s recoil was reduced from 1.40 meters to 1.32 meters. The operation of the weapon consisted of automatic shell ejection and manual shell loading, with the ammunition stored in a 15-round magazine.

    Type 90 AA Sharp Shell,
    length, weight, CG.

    The final specifications of the modified gun were as follows: overall length of 3,892 mm, a barrel length of 3,312 mm, 740 kg overall weight (490 kg barrel weight + 250 kg mount weight), 720 m/s muzzle velocity, and actual caliber of 75 mm. Aiming and firing were done by the pilot, and rather than a co-pilot, there was a dedicated loader’s position. The weapon could, of course, fire any of the shells common to the Type 88. This ammunition pool included the Type 90 & Type 94 HE Shells, Type 90 & Type 3 AA Sharp Shells, and Type 1 & Type 4 AP Shells.

    The prototype cannons completed their modification in April 1944 and were tested at the Otsugawa Range from April 24th to the 28th. 86 shells were fired collectively from both prototypes. The completion report for the tests was submitted on May 2nd, and it was considered that the general functions were in order, including the recoil systems, but the electric firing mechanism needed to be revised. There was no damage to the mount when inspected.

    The adjusted re-test of the Ki-109 Mounted Cannon was carried out from May 26th to 29th at the Irago Range, with the completion report being submitted on June 1st. The main purpose of this test was to confirm the functionality of the electric firing mechanism and the length of recoil at various angles of fire. Both proved adequate, and the test was completed successfully.

    At this point, there is a disparity: primary documentation states that the Ki-109 ground test was scheduled at Fussa and the aerial firing test at Mito. Some secondary sources and recollections state that the ground test was done at Mito and the aerial test at Fussa. I am going with the schedule presented in the documents of the time.

    With the arrival of the first prototype Ki-109 to Fussa in early September, the ground firing test of the airframe-installed cannon was carried out. The schedule was to perform the test from September 8th to the 10th, but the actual date is not known. 24 rounds were to be fired with various amounts of propellant to primarily examine the recoil resistance of the airframe. As a result of the testing, it is known that there was damage to the windshield, entry door, and landing light. However, there were no major structural failures, so the result was considered successful, and the necessary reinforcements were made.

    ‘Ki-109 Mounted Cannon’.

    The prototype was then flown to Mito Airfield, where aerial firing tests were run at the Mito Range. 39 rounds were to be fired with various amounts of propellant. The target aircraft was a Ki-43II, and targets included 10-meter fabric boards and streamers. When firing on ground targets, the Ki-109 was appraised as having “unparalleled accuracy”, requiring almost no compensation of aim to accurately destroy the target. However, aiming at aerial targets was not as simple. Determining the necessary adjustment of aim was tested by shooting with a 16mm-film gun-cam at a target plane flying over a lake. As the Major flying most of the flights said:

    The issue was aiming. It didn’t go well until the end.

    Major Sakamoto, 未知の剣 (translated)
    ‘Ki-109 Mounted Cannon’ Specifications
    Caliber75 mmLengthOverall3,892 mm
    Cartridge75 x 497 mm RBarrel3,312 mm
    Muzzle Velocity720 m/sWeightOverall740 kg
    Rate of Fire20 rpmBarrel490 kg
    Max Effective Range1,500 ~ 2,000 mMount250 kg
    Capacity15 rounds

    Despite the opinions of the pilot, the firing trials were still generally appraised to be very successful due to high accuracy against static targets. Following these trials, the mass production of 44 aircraft was urgently ordered from Mitsubishi on October 13th, 1944. It was expected that this plane would still be effective against formations of massive B-29s.


    Misidentification as “Ho-501”

    Since the end of the war, various researchers have identified the aircraft adaption of the Type 88 with the designation ‘Ho-501‘. This seems to be an error.

    The Japanese Army developed a large range of aircraft cannon projects during World War II, many of which remain with almost no readily available data. In this situation, it’s easy to see how the error has occured. Army aircraft guns with a calibre of over 11mm used the project-name prefix ‘Ho’ (ex: Ho-5, the Army’s mainline 20mm). Fragmentary data about a 7.5 cm cannon named ‘Ho-501’ seemed to be the solution for the missing ‘Ho’ designation of the Ki-109’s cannon.

    An end-war production table.

    As it turns out, the ‘Ho’ prefix only applied to machine cannons. For example, the manually-loaded Type 94 37 mm Tank Gun used on the Ki-45 Kai Otsu did not have a ‘Ho-‘ designation. In the same way, the aero Type 88 was only ever referred to in documentation as the ‘Ki-109 Mounted Cannon‘.

    There are a few documents that definitively separate ‘Ho-501’ from the ‘Ki-109 Mounted Cannon’. First of all, a table of machine cannons made in July 1944 by the Army. At this point, Ki-109’s cannon was already being tested, but the completion of the Ho-501 is still “scheduled”.

    Ref.C14010984100

    In the American document ‘Ordnance Technical Intelligence Report #19‘ made after the war with Japanese data, the Ho-501 is identified as a 7.5 cm recoil-operated machine cannon with a velocity of about 500 m/s and a rate of fire of about 80 rounds per minute. From the specs it can be deduced that this is essentially an automatic adaption of an ‘infantry’ type gun, like the 37mm Ho-203 and 57mm Ho-401, giving a mediocre velocity and fire rate probably intended for use on large bombers or ground targets. The gun was not completed before the end of the war according to the document.

    Finally we are fortunate enough to have a detailed piece of data submitted by the Army to the US occupational authorities regarding this machine cannon: the diagram of the Ho-501’s shell. The diagram shows a high-explosive shell with the same cartridge as the Type 41 Mountain Gun, 75x185R. Therefore, we can say that the Ho-501 was sort of an ‘automatic Type 41’, albeit with a higher velocity.

    Ho-501’s shell, Ref.C13070009100

    It is proven from these materials that the Ho-501 was an entirely different weapon than the ‘Ki-109 Mounted Cannon’. Furthermore, the Army had projects for several more 7.5cm machine cannons, and even 12cm and 15cm machine cannons, which may be part of a future article.


    Ki-109 in Combat

    Starting from November 1944, either of the Ki-109 prototypes were piloted in real interception missions against B-29 formations over the region around Fussa. This was the ‘actual combat testing’ of the Ki-109.

    On the first mission, Major Sakamoto was at the controls. He climbed to an altitude of 10,000 meters over Fussa and waited to intercept a formation of 30-40 B-29s at 9,000 meters. Gently descending towards a group of five planes, he fired around 10 successive shots with a 1,500-meter timed fuse. No effective hits were scored, and the B-29s increased throttle to escape. The non-turbocharged engines of the Ki-109 had obviously inferior retention of power than those of the B-29, and they pulled away.

    B-29s of the 500th BG approaching the Tokyo area.

    Captain Otsuka was the pilot of the second attempt. Rather than approaching from behind as in the first try, he attacked from the front. All the same, no hits were scored, and the Ki-109 sustained .50 caliber hits to the left wing in return. Once again, it returned to the airfield without success.

    For the third and final attempt, Sakamoto was back in the pilot’s seat. But the conditions were hazy with poor visibility, and although he performed an attack, the end results could not be observed.


    Thus the Ki-109 failed to achieve any tangible results from its combat testing, due in major part to its inadequate performance at high altitudes. Multiple methods were employed to try and remedy the lack of performance compared to the B-29 at high altitudes during the beginning of 1945.

    The production model Ki-109s were to be stripped down in weight and streamlined to improve climbing performance and airspeed. Their upper and side defensive guns were removed, leaving only the tail flexible 12.7mm cannon. Any equipment used for the bombing was taken out. All defensive steel plating was removed except for the plates in front of the ammunition storage and instrument panel, and the fire extinguishing systems were also removed, along with the fuel tanks inside the wings. Furthermore, the two existing prototypes were used for more substantial experiments.

    The first prototype of Ki-109 was experimentally equipped with a ‘Toku-Ro Mk.1‘ liquid-fuel rocket engine in the former bomb bay, which was intended to provide 500 kilograms of thrust for 5 minutes. Though the rocket system added over 2 tonnes of weight when mounted, it was expected to increase the top speed by 70 – 150km/h from altitudes of 6,000 to 10,000 meters when active. Regardless, ground testing showed that the performance was inadequate, and the increase in weight in normal conditions without rocket power was too severe.

    In February 1945, the second prototype of Ki-109 was equipped by Hitachi technicians with Ru-3 turbochargers onto its Ha-104 engines to improve the retention of power at high altitude. Much like the Ki-67I Kai (Ki-67I with turbo) before it, testing proved that the turbocharger was far from being suitable for practical use. Due to the incessant reliability issues common to Japanese turbochargers of the period, it was abandoned.


    Diverted to Anti-Shipping, The End of the War

    Ki-109 of the Flying 107th Sentai.

    Failing to considerably enhance the performance of the Ki-109, production was halted in March 1945 during a reorganization of the priority of aviation projects. Only 20 of the planned 44 production planes were completed, due in part to the bombing of Mitsubishi Nagoya. The total count thus became 22 planes, including prototypes.

    Some of the Ki-109s were serviced in the Flying 107th Sentai during the summer of 1945. The unit had been formed on November 10th, 1944 at Hamamatsu, and was trained on Type 4 Heavy Bombers in preparation for using the Ki-109 as an interceptor. However, because of the Ki-109’s inadequate performance in the role of interception, the 2nd Chūtai of the unit relocated to Daegu, in occupied Korea. Here they were used only to patrol the Korea Strait for ships and to escort the Kampu Ferry between Japan and Korea. The 107th Sentai was subsequently disbanded on July 30th to apply the personnel to more useful roles.

    In July 1945, a test was carried out off the coast of Izumisano in Osaka Prefecture to appraise the Ki-109’s ability to destroy American vessels. Such a duty would have been critical in the expected decisive battle of the Japanese mainland. The first target, an Army ‘Daihatsu’ landing craft, was obliterated by a single hit. The second target was an 800-ton ship. The Ki-109 fired four shots with different attack incidences. All four shots hit, “forming a neat row on the waterline”, and the ship sank. Resultingly, the Ki-109s were to be reserved for the defense of the Japanese home islands from anticipated American landings.


    After the end of the war on August 15th, the US occupational authorities planned to requisition Ki-109 production No.’s 10 & No.11 for shipment to the USA. Photographs show that a single Ki-109 painted in overall black was loaded onto the deck of the USS Core, one of the aircraft carriers which shipped Japanese aircraft to the United States for examination. However, it is unknown if the Ki-109 was ever test flown by American pilots. Every existing Ki-109 was scrapped within a couple of years of the war’s closure, and there is no survivor today.

    Special Air Defense Fighter Ki-109 Specifications
    NamePrototypeKi-109EngineNameType 4 1900 HP Engine (Ha-104)
    Servicen/aOutput (T.O.)1,900 hp @ 2,450 RPM
    DimensionsLength17.950 mOutput (Nom.)1,810 hp @ 2,300 RPM (2,200 m)
    1,610 hp @ 2,300 RPM (6,100 m)
    Span22.500 m
    Height5.800 mPerformanceTop Speed550 km/h @ 6,090 m
    Wing Area65.85 m2Climbn/a
    WeightsEmpty7,424 kgRange2,200 km
    Loaded10,800 kgCeilingn/a
    Wing Loading164 kg/m2ArmamentGunsKi-109 Mtd. Cannon (7.5 cm) x 1
    Type 1 12.7mm Flex. M.C. x 1
    Crew4 (pilot, loader, gunner, radio)

    Production Ki-109 after the surrender. The propellers were removed. The forced cooling fans in the cowls are clearly visible.
    Ki-109 and Ki-51 attack planes, Kurume Airfield, Fukuoka Prefecture.
    The destruction of a Ki-109 by the US occupational authorities.
    This plane has the same mottled camo as prototype #1.
    This production Ki-109 was prepared in US insignia, but it is unknown if testing occurred.

    Conclusion

    The Ki-109 was a flawed solution for a desperately necessary requirement: any means to stymie the B-29 Superfortress bombing raids that were correctly expected to severely ramp up from late 1944; an effect of the fall of the Mariana Islands.

    Ultimately, the Ki-109 was useless in its original role because Japanese turbochargers could not be put into practical use in time for the war. The window for intercepting B-29s at high altitudes was brief due to this situation, which similarly choked the ability of single-engine fighters.

    As B-29 raids switched to low-altitude nighttime tactics in early 1945, the purpose of the Ki-109 became redundant. Even if the Ki-109 had been fit for the role, P-51 Mustang fighters began operating as bomber escorts from Iwo in April 1945, and the fate of a bomber airframe as an interceptor was very much sealed.

    Diverting this plane to the anti-shipping role was certainly a more practical mission, but under certain Allied air superiority during an invasion of the home islands, the actual efficacy is very doubtful: a Ki-109 would probably be downed before firing a single shot.


    Sources

    • Aircraft Machine Cannon and Ammunition Code Name Table (Ref.C14010984100)
    • (1st Army Research Institute Document Binding) Primarily Related to the ‘Ki-109’ Mounted Cannon (Ref.A03032209800)
    • Ho-501 Shell Diagram (Ref.C13070009100)
    • Technical Data. Report No. 16A(9).
    • Material on Ki-67. Jap/Ki-67/5-43.
    • Data on Japanese Aircraft Shipped to the United States for Study Purposes. Report No. 15C.
    • ORD TIR No.19: Research, Development and Production of Small Arms and Aircraft Armament of the Japanese Army
    • Mikesh, Robert. (1993). Broken Wings of the Samurai: The Destruction of the Japanese Air Force. Naval Institute Press.
    • Nohara, Shigeru. (1999). The XPlanes of Imperial Japanese Army & Navy. Green Arrow.
    • Watanabe, Yoji. (2002). Unknown Sword: The Battlefield of Army Test Pilots. Bungeishunjū.
    • Akimoto, Minoru. (2002). All the Formal Aircraft in Japanese Army. Kantosha.
    • (2003). Famous Airplanes of the World No.98: Army Type 4 Heavy Bomber ‘Hiryū’. Bunrindo.
    • Ogawa, Toshihiko. (2003). Mysterious New Planes. Kōjinsha.
    • Ikari, Yoshirō. (2003). Mystery Fighters. Kōjinsha.
    • Sahara, Akira. (2006). Prototype & Planned Planes of the Japanese Army 1943~1945. Ikaros Publishing.
    • Sayama, Jirō. (2010). Cannons of the Japanese Army: AA Guns. Kōjinsha.
    • Kariya, Masai. (2017). Japanese Army Prototype Planes Story. Ushioshōbokojinshinsha.

    *August 17th 2023: Corrected information about fate of Ki-109

  • R2Y Keiun: Satisfactory… but the Engine Caught on Fire

    R2Y Keiun: Satisfactory… but the Engine Caught on Fire

    At the outset of the Second Sino-Japanese War in 1937, the Japanese Navy’s Air Service assumed a leading role in the strategic bombardment of China with their modern force of attack planes. As the battle quickly advanced into the interior of the continent, the Navy developed a new requirement to maintain force projection: the need for a land-based, high-speed reconnaissance plane to conduct recon missions far into China while being untouchable by enemy fighters.

    Type 96 Land-based Attackers.

    As it turned out, this was a requirement the Navy could not fulfill by its own developments until the end.

    First, the ’13-Shi High-Speed Land-based Recon Plane’ requirement was issued by the Navy in 1938, calling for a recon plane faster than current fighters. Aichi Watch & Electric Co developed this plane, given the code name ‘C4A1’, and the mockup was completed in March 1939. However, it was decided instead to simply adopt the Army’s existing Type 97 HQ Recon Plane (Ki-15) as the Navy’s Type 98 Land-based Recon Plane (C5M) in November 1939.

    At the outbreak of the Pacific War in December 1941, the Navy was now challenged by Allied fighters with superior speeds to Chinese types. By this time, the Army had developed the Type 100 HQ Recon Plane (Ki-46), a plane with an unprecedented high speed of over 600 km/h. The Navy borrowed some of these planes from the Army in 1942, supplementing the deployment of their own land-based recon conversion, J1N1-C, which had a similar range but a speed of only 507 km/h.

    Type 100 HQ Recon Plane.

    This arrangement was still far from satisfactory, and the Navy had not abandoned the plans for their own land-based recon plane. Starting at the Navy’s own Air Technical Arsenal (Kūgishō) from the beginning of the Pacific War, the resulting developments would seek to challenge the limits of aircraft performance at the time, and when the fortunes of war shifted out of favour, it was revived as one of the earliest Japanese jet aircraft proposals.


    The Predecessor: Y30, R1Y1, ‘Gyōun’

    In the year 1939, the Navy’s Kūgishō was developing three novel aircraft plans to set performance records. These were the ‘Y10’ (for speed record), ‘Y20’ (for range record), and ‘Y30’ (for altitude record). But because of the need to devote the Kūgishō’s capabilities to practical service aircraft, in the following year, the Y10 was canceled, the Y20 was redeveloped into the famous high-speed, long-range bomber P1Y, ‘Ginga’ (Galaxy), and the remaining Y30 is the subject at hand.

    It was decided to redevelop the Y30 plan as a land-based recon plane, and the Navy Aviation HQ ordered the ’17-Shi Land-based Recon Plane’ requirement to the Kūgishō. The requirements were a top speed of 360 knots (667 km/h) at 6,000 meters to outrun any enemy fighter planes, and a range of 4,000 nautical miles (7,410 km) at 4,000 meters to perform long-range recon missions. Additionally, the ability to implement a pressurized cabin in the future was necessary. This plane became known as R1Y1, Experimental Gyōun (Dawn Cloud), and basic planning started in December 1941 under Technical Lieutenant-commander Yukio Ōtsuki.

    (US intel also picked up the name R1Y1, ‘Seiun’ (Blue Cloud), as well as ‘Gyōun’, so the naming situation is uncertain.)

    Draw over of ‘Gyōun’ from #5.

    Initially, the plane was to be powered by a single Mitsubishi ‘Nu-Gō Twin’ in the nose, consisting of two Nu-Gō (ME2A) 24-cylinder liquid-cooled H engines coupled together to produce 5,000 takeoff horsepower. But due to technical issues relating to the huge size of the engine and the fact that it would take a long time to develop, this proposal was abandoned.

    Instead, the plane was designed as a twin-engine type equipped with a Mitsubishi MK10A in each nacelle, an 18-cylinder two-row air-cooled radial with a two-stage supercharger (1st stage: twin continuously variable-speed Vulkan-coupling superchargers, 2nd stage: 1-speed mechanical supercharger) producing 2,400 takeoff horsepower.

    To facilitate production the structure of R1Y1 was made to be very similar to the Y20 (P1Y, Ginga), even using the same jigs, main spar caps, and longerons. The area that was the bomb bay of the P1Y was instead used to contain fuel tanks, and the upper surface of the main wings also featured semi-integral fuel tanks (where part of the wing structure is used as a fuel tank).

    The concrete design of this plane had begun in the summer of 1942, but a snag occurred concerning the engine. It was realized that the MK10A with its advanced Vulkan-coupling superchargers would not be ready in time for the production of the airframe, so it was necessary to abandon the Vulkan-coupling supercharger and use the ‘MK10C’ engine which featured a turbocharger feeding into a two-speed mechanical supercharger.

    An MK10B (Ha-42-41) still exists today in the NASM collection, which is an MK10A with a two-speed prop reduction gear. However, it is incorrectly mislabeled by the NASM as ‘Ha-214 Ru’ which is the Army designation of Ha-42-21.
    Photo: NASM

    With the turbocharged MK10C, the top speed at 6,000 meters was calculated to only be 350 knots (648 km/h), 10 knots below the speed requirement. Even though the turbocharger allowed improved performance at higher altitudes, such as 370 knots (685 km/h) at 8,000 meters, the Navy’s tacticians firmly held the 360 knots (667 km/h) at 6,000 m speed requirement until the end.

    While the plan was stalled due to the performance shortage, the momentum of the war had changed to a situation that no longer demanded such extreme reconnaissance range. Another plan being simultaneously developed in the Kūgishō, ‘Y40‘ was predicted to reach a top speed of 400 knots (741 km/h) (despite only having half the range), and quickly caught the attention of Navy officials who desired extravagant performance. As a result, even though it was far closer to realization, the development of R1Y1 was suspended in the summer of 1943. The design was 85% complete, and the factory work was 20% complete.

    Gyōun Planned Specifications
    NameInternalY30Engine (x2)NameMK10C
    Ha-42-21
    CodeR1Y1
    PrototypeExperimental GyōunOutput (TO)2,500 hp @ 2,800 rpm
    DimensionsLength15.0 mPerformanceTop Speed648 km/h @ 6,000 m
    685 km/h @ 8,000 m
    Span19.0 m
    Wing Area50.0 m2Range7,408 km @ 4,000 m
    WeightsEmpty10,500 kgArmamentType 2 13 mm Flexible MG (400 r/g) x1
    Loaded14,000 kgCrew3 (pilot, recon, radio)

    He 119 and Y40

    The Y40 plan originated with the Japanese testing of the He 119, a novel record-breaking aircraft that was developed by Germany’s Heinkel Aircraft Works in the later 1930s, and purchased by the Japanese Navy.

    The He 119 was developed as a high-speed reconnaissance plane, and in order to achieve exceptional speeds, many of the latest aeronautical innovations were implemented into the design. The low-drag airframe lacked a canopy, rather featuring a glazed nose, and due to the evaporative cooling system, originally exhibited drag-less cooling without an external radiator intake (but due to cooling deficiencies, a radiator scoop was added).

    The engine was located in the middle of the fuselage in order to streamline the shape and consisted of two ‘DB601A’ 12-cylinder inverted-V engines (takeoff power 1,175 hp each) coupled together in parallel, an arrangement designated ‘DB606’ (takeoff power 2,350 hp). This engine drove an extended shaft that skewered through the cockpit to the four-bladed constant-speed propeller on the nose.

    A Heinkel He 119 with the Navy’s representatives.
    You can see the prop shaft inside the cockpit.

    While the He 119 was not a record-setter in terms of maximum airspeed (the top speed was about 590 km/h), one example set the average speed over 1,000 km distance record of 505 km/h in 1937.

    After Heinkel’s development was completed, the Japanese Navy purchased two examples of the He 119 in 1940. The planes arrived by an Italian warship, disassembled, in May 1941. They were assembled at the Kūgishō and immediately prepared for test flights at Kasumigaura Airfield with Heinkel test pilot Captain Gerhard Nitschke.

    The purpose of the Navy’s testing and examination of the He 119s was to obtain reference material for the development of their own high-speed practical aircraft. The flight testing was scheduled to begin on July 7th and to last until the end of that month. To obtain additional development data, various airframe tests such as strength, vibration, and wind tunnel testing were also planned.

    However, the first plane was damaged during a ground run while piloted by Captain Nitschke, and the second plane was later damaged in a landing accident, so the flight testing was not completed. As a result, only the structural tests were conducted to completion.

    DB 606. This consisted of two DB 601A engines coupled as an ‘inverted-W’.

    Based on the data obtained, the high-speed aircraft plan ‘Y40’ was preliminarily started by Technical Lieutenant-commander Masao Yamana at the Kūgishō’s Airframe Department in 1941.

    The main interest of the Navy regarding the He 119 seemed to have been in the mid-fuselage parallel-coupled DB 606 engine and the characteristics of the extended propeller shaft. By reference, the Y40 was to implement a very similar power plant installation in pursuit of extreme speed, based on the Navy’s domestic production model of the DB 601 – the ‘Atsuta’ engine produced by the Aichi Aircraft Company.


    Y40, R2Y1, ‘Keiun’: Aiming for 400 Knots

    With the suspension of the Y30 project, the preliminary Y40 plan was officialized by the Navy Aviation HQ with the issuance of the ’18-Shi Land-based Recon Plane’ specification in the summer of 1943. The parameters were to achieve a speed of 400 knots (741 km/h) at 10,000 meters, faster than any aircraft in service at the time, with a range of 2,000 nautical miles. Technical Lieutenant-commander Ōtsuki, who was the chief designer of the R1Y1, was again placed in charge of this design. The Navy code name ‘R2Y1‘ was assigned, with the prototype designation ‘Experimental Keiun‘ (roughly: Scenic Cloud).

    The initial prototype construction plan was to complete one aircraft each in June, July, and August of 1944, followed by two in September, and another in October.

    Atsuta 31, 1,400hp inverted-V12

    The engine installation involved up-rating an Atsuta Model 30-series (later-series of domestic DB 601) from 1,400 up to 1,700 takeoff horsepower by increasing the manifold pressure, compression ratio, rotational speed, and installing a turbocharger. This variant was named ‘AE1T‘, and shared the same 150 mm bore by 160 mm stroke as the Atsuta, but with a compression ratio of 7.5 along with a maximum manifold boost of +520 mmHg and maximum RPM of 3,000.

    It should be emphasized how incredible this engine configuration was. The Navy’s original ‘Atsuta Model 21’ was roughly equivalent to an early DB601, and produced about 1,200 takeoff horsepower. In essence, this original engine was iteratively modified to finally produce 1,700 horsepower, exceeding the power of the German base model ‘DB 605A’ (1,475 hp) and approaching the power of the water-methanol injected DB605DB (1,800hp) or base DB 603A (1,750 hp), both of which had larger cylinders than the DB 601/Atsuta. The practical application of such a boosted engine is immediately questionable.

    Furthermore, in my research, I could not ascertain whether the AE1T engine implemented water-methanol injection to reduce the tendency of ‘knocking’ (premature combustion). Without this, the Keiun was likely to need at least 100 octane fuel to operate, which was in extremely low supply by the end of the war. Even the comparatively low-boost Atsuta Model 31 (AE1P) required 95 octane fuel to run in take-off condition, so the AE1T would be quite demanding.

    Two AE1T were coupled together into the single unit designated the ‘AE1T Twin‘ or by unified Army-Navy convention, the ‘Ha-70-01‘. Together, this was a 24-cylinder inverted-W arrangement with 3,400 takeoff horsepower. Engine cooling was provided with a retractable, split ventral scoop to the radiators, the oil cooler intakes were positioned in each wing root, and the turbocharger and intercooler air was routed through a large intake at the top of the fuselage forming a distinctive ‘hump’. The Ha-70 engine drove a 4-meter propeller shaft on a 0.4 reduction ratio to turn the Keiun’s large 3.8 meter, 6-bladed constant-speed VDM propeller at the nose.

    As for the airframe of the Keiun, the all-metal structures of the fuselage and wings were designed simply in consideration of production. The lowly-mounted wings consisted of a laminar flow airfoil with a moderate aspect ratio. They had a two-spar structure that was thickly skinned, containing integral fuel tanks. The vertical tail was angled 2 degrees to the right to counteract the torque of the propeller. Keiun sat atop tricycle landing gear, which was rare in Japanese propeller aircraft of the time. This landing gear setup was developed in reference to the analysis of the captured US Douglas A-20 Havoc attack plane.

    The teardrop-shaped canopy was to be pressurized for high-altitude operations and housed the pilot and a radio/reconnaissance member, who operated a camera aiming through the underside of the fuselage. As a pure-reconnaissance plane, the R2Y1 was not equipped with ordnance of any type, which would create additional drag and weight.

    The development of the R2Y1 proceeded under delays relating to the complexity of the power plant setup, which drew criticism from those who doubted its practicality.

    Keiun Planned Specifications
    NameInternalY40EngineNameHa-70-01
    (AE1T Twin)
    CodeR2Y1
    PrototypeExperimental KeiunOutput (TO)3,400 hp @ 3,000 rpm
    DimensionsLength13.050 mOutput (Nom.)3,000 hp @ 8,000 m
    Span14.000 mPerformanceTop Speed741 km/h @ 10,000 m
    Height4.240 mClimb21’0″ to 10,000 m
    Wing Area34.0 m2Range3,611 km
    WeightsEmpty6,015 kgCeiling11,700 m
    Loaded8,100 kgArmamentnone
    Overload9,400 kgCrew2 (pilot, recon)
    Wing Loading238.2 kg/m2

    ‘Keiun’ Reborn as a Jet Aircraft

    In late June 1944, the Japanese Navy suffered a crippling defeat at the Battle of the Philippine Sea, often referred to as the ‘Great Marianas Turkey Shoot’. The main force of carrier aviation in the Navy, the 1st Mobile Fleet, was annihilated by Task Force 58 after failing to cooperate effectively with land-based forces. Hundreds of aircraft and three aircraft carriers were lost.

    Under a situation of such devastating defeats, the Navy Aviation HQ reorganized the development of all aircraft types. Only the designs most essential to the war effort were to be prioritized for the limited allowance of resources remaining. The true value of an advanced, ultra-high-speed recon plane like the Keiun was now heavily in doubt, and the Navy planned to cancel the development of this plane entirely in July 1944.

    The saving grace for this plane was the allure of the ‘turbine rocket’ – what is today known globally as the turbojet.

    Around this time, the Navy had become enthusiastic about the development of the turbojet engine after hearing reports of its practical use and effectiveness in Germany. Compared to the piston engine, the basic turbojet at that time was simpler, cheaper, and faster to produce, while offering far superior efficiency at high speeds and not requiring high-grade fuel. All of these factors were very ideal for the situation of late-war Japan, where resources, skilled labor, and the performance of aircraft were steadily declining.

    TR30 axial-centrifugal turbine rocket.

    Commander Tanegashima’s group at the Kūgishō had been developing a turbojet by their own efforts since 1942, and had finally received major interest from the Navy at this time. The Navy ordered the mass-prototyping of 70 ‘TR10’ model engines. The TR10 was a small centrifugal turbojet with a thrust of only about 300 kgf, but an upscaled model named ‘TR30’ was planned to produce 850 kgf. The TR30 was somewhat similar in scope to the British Rolls-Royce Derwent that originally powered the Gloster Meteor, although the TR30 would never run at maximum power due to being technically underdeveloped.

    Turbojet engines had a higher specific fuel consumption than piston engines, and the nearly three times higher thrust output of the TR30 only exacerbated this. Any aircraft equipped with such an engine would need a large internal fuel volume in order to have acceptable endurance, and the R2Y1’s thick fuselage for housing the coupled Ha-70 engine was clearly suited to such an application.

    In order to salvage their hard work, the design team of the Keiun proposed to the Navy Aviation HQ the aircraft’s conversion to an attack plane powered by two TR30 units. This proposal was immediately accepted by the Navy staff, now eager to apply jet technology, and was tentatively designated R2Y2, Experimental Keiun Kai. To gather aerodynamic data for the development of the R2Y2, the design team suggested the completion of the R2Y1 prototype as a research plane, which too was easily approved.

    Thus, under the aim of ultimately becoming a high-speed jet-propelled attack plane, the Keiun was saved.

    Mitsubishi Ne-330 turbine rocket.

    In the autumn of 1944, more promising axial-flow turbojets based on the format of the German ‘BMW 003A’ were put into development for high-performance aircraft. The largest and most powerful was Mitsubishi’s Ne-330, which was expected to produce up to 1,320 kg of thrust. This would provide significantly higher performance to the Keiun Kai, and although it was also expected to be the most fuel-hungry Japanese turbojet at full throttle, it was adopted as the design engine. Overall, the huge Keiun was probably the most suitable airframe for this engine among the models in development.

    Keiun Kai Planned Specifications
    NameInternalY40Engine (x2)NameNe-330
    CodeR2Y2Output (Static)1,320 kgf @ 7,600 rpm
    PrototypeExperimental Keiun KaiOutput (Nom.)990 kgf @ 740 km/h, 7,600 rpm (2,680 hp)
    DimensionsLength13.050 mPerformanceTop Speed783 km/h @ 6,000 m
    741 km/h @ 10,000 m
    Span14.000 m
    Height4.240 mClimb7’0″ to 6,000 m
    Wing Area34.0 m2Range1,269 km
    WeightsEmpty5,700 kgCeiling10,500 m
    Loaded8,850 kgArmamentnone, later up to 1 ton of weapons
    Overload9,950 kgCrew2 (pilot, recon)
    Wing Loading260.3 kg/m2

    To the Test Flight of ‘Keiun’

    In February 1945, the design responsibility of the Keiun project was taken over from Commander Ōtsuki by Lieutenant-commander Ichi Aburai, who headed it from that point until the end of the war. According to one member of the team, this did not bode well for the continuation of the project.

    The first prototype of the R2Y1 was hurriedly completed as a research plane for the R2Y2 in April 1945. To increase the speed of construction, the turbocharger had not been installed, giving the engine a less impressive power curve at altitude. The aircraft was also not outfitted with the planned pressurized canopy system. These things were not terribly important, given that the use of the Keiun as a high-altitude reconnaissance plane had been abandoned.

    Ground testing started on April 27th when the plane was unloaded at Yokosuka Airfield. Test pilot Susumu Takaoka, who would later perform Japan’s first turbojet aircraft flight in the Kikka Kai, was at the controls of the Keiun for this early period. Starting with a slow taxi, by the afternoon faster ground runs were being conducted, and a violent shimmy in the nose wheel was observed. The dampener for the nose gear was quickly replaced and retested, before the plane was loaded onto a transport ship on April 30th and sent to Kisarazu Airbase.

    Perhaps the most iconic photograph of the Keiun, clearly displaying the huge 6-bladed propeller.

    The test was resumed at Kisarazu the same day, with Lieutenant-commander Kitajima in the cockpit. By May 8th a high-speed ground run was attempted again, and the nose wheel shimmy re-appeared, along with a crack in the fork of the main gear leg. Furthermore, during this test, the temperature of the engine rose abnormally around two of the inner exhaust pipes.

    The crack in the fork was subsequently repaired, and in an attempt to temporarily ease the cooling problem, holes were drilled in the side of the fuselage for additional intake and exhaust pipes to be installed. This reduced the sleek outward appearance of the fuselage.

    Due perhaps in part to the reference data obtained from the vibration testing of the Douglas A-20’s nose and landing gear, the Keiun had little oil leakage in operation, which was remarkable for a Japanese aircraft at the time.

    After all of the preceding tests were finally cleared, Lieutenant-commander Kitajima performed a short 4-second “hop” in the Keiun on May 22nd. This was successful, and no bad behaviors were detected in the landing gear or the temperature of the engine at this time.

    May 23rd was scheduled to be the first flight test of the Keiun. However, as the aircraft was being prepared for takeoff, a Ryūsei attack plane that was taking off prior suffered a landing gear failure, caught fire, and exploded on the runway. Despite this catastrophic event, the test flight of the R2Y1 proceeded on the same day.

    After a 400-meter run, Kitajima brought the Keiun into the air and entered a climb. The aircraft left a trail of black smoke that was characteristically unique to the Ha-70 engine. At about 1,000 meters, Kitajima leveled the plane out with a speed of 180 knots (330 km/h). Suddenly, he noticed an abnormal rise in the engine oil temperature gauge, and immediately powered the engine off, diving back towards the airfield.

    Kitajima successfully landed the aircraft and taxied over to the ground crew with the engine shut off. A fire was spotted at the rear of the engine, which was luckily extinguished quickly before any considerable damage was done to the engine or airframe. Apart from the major cooling issues, the stability and handling of the airframe in flight was said to have no observed problems by Kitajima.


    The End of ‘Keiun’

    The cause of the aerial fire was ascertained to have been overheating at a bent section of the inner exhaust pipe. In an attempt to remedy this, another larger intake scoop was added to the fuselage. Ground testing continued in a trial to solve the cooling issues completely, but a total solution was not reached by the time of mid-June. It was then that an error from the ground crew caused the engine to disastrously overheat, rendering it completely unusable and in need of replacement.

    It would take weeks for a new Ha-70-01 engine to arrive (likely due to bombing damage at the Aichi Atsuta prototype sector) and while waiting for the replacement, a US Mobile Task Force raid in late July destroyed the sole R2Y1 on the ground with a direct bomb hit, scattering the sole flying airframe to pieces.

    The assembly progress of the remaining prototypes at the Kūgishō had been paralyzed by raids, so there was no prospect of completing another example before the end of the war was reached on August 15th.

    At the end of the war, two unfinished Keiun airframes were inventoried at the Kūgishō. The US noted these in their survey of aircraft to ship to the USA, but neither were sent, likely due to being too far from completion. All examples of the Keiun were scrapped following the end of hostilities.

    Both unfinished Keiun at the Kūgishō in the same frame.

    After the war, Lieutenant Takogo Toyoda from the Kūgishō was interrogated for the US Strategic Bombing Survey. Among other subjects, he was questioned about the ‘Keiun’.

    You helped design the KEIUN (“Beautiful Cloud”). Describe it to us.

    “Commander OTSUKI was the chief designer and I helped him. KEIUN was a twin seater, single-engine experimental scouting and reconnaissance plane produced in late 1944 by YOKOSUKA First Air Technical Arsenal. It had an AICHI KEN No 1 engine (AE1T) behind the pilot. The one experimental model was manufactured in late 1944 and test flown in January 1945, but was never flown in combat. The tempo of war was too fast to warrant production of the plane for combat use. […]”

    When was your plane test flown?

    “January 1945” [erroneous]

    How did the plane perform on the test flight?

    “Satisfactorily, but the engine caught on fire. A safe landing was made, however. Only this one Plane (KEIUN) was ever built.”

    Rep. Organization and Operation of First Naval Air Technical Arsenal, 1945.

    What Became of R2Y2, ‘Keiun Kai’?

    The existence of the R2Y2, Experimental Keiun Kai project is something that has always been controversial, and is often even considered dubious as of late. This is especially due to representations showing various different methods of engine placement; most notably its depiction in the video game War Thunder.

    As explained earlier, the R2Y2 was not only a real project, but actually planned as the definitive version of the Keiun since the summer of 1944. At least, this was the case in the papers of the Navy. There is almost no extant documentation concerning this plane, but the clearest possible picture will be painted using what survived.

    According to the Navy’s data on prototype planes compiled at the end of the war, the design of the R2Y2 only started in February 1945, which coincides with the time that the main designer of the Keiun switched from Commander Ōtsuki to Lieutenant-commander Aburai. The specifications provided (already detailed above) give an airframe of the exact same outer dimensions as the original R2Y1, suggesting that the plane was more of a conversion than a major redesign.

    Immediately after the war, Ōtsuki was interviewed by the ATIG. He explained that the R2Y2 plan was equipped with twin Ne-330 turbojets mounted in the wings, and that the design had not advanced much because of the lack of progress on the Ne-330 engine. Later, Ōtsuki wrote a memo on the development of the Keiun including a simple sketch showing the general arrangement of the R2Y2. Emphasis was placed on the location of a large fuel tank in the central fuselage, where the Ha-70 had once been.

    Ōtsuki’s sketch of the basic R2Y2 layout, emphasizing the fuselage fuel tank.

    Another member of the Kūgishō, Ichiro Naito, also recounted about 13 years after the end of the war that the R2Y2 was proposed with under-wing mounted engines and a large fuel tank in the fuselage. He also wrote that 1,000 kg was reserved for ordnance, which is useful to note considering that the actual R2Y2 specifications recorded from the wartime seemingly had not yet considered any armament. Accordingly, the name was still just ‘R2Y2’ at the time, while a concrete change to an attack plane would probably have necessitated the code name ‘R2Y2-G’.

    The slowness of the Ne-330 turbojet’s development is a good explanation for the lack of clarity on the R2Y2. The development of the Ne-330 started in the autumn of 1944 at Mitsubishi Nagoya, but when the first unit was rapidly completed in April 1945, it was almost immediately destroyed by a bombing raid on the 7th of the same month which devastated the plant. After that, work was dispersed to Meidō Industrial School in Matsumoto, before being again shifted to the Niigata Ironworks in Niigata in June, causing further delays. In early August, members of the design group were moved back to Matsumoto to work on the higher-priority KR10 rocket engine for the J8M1 interceptor, and at the end of the war, the second and succeeding units were still under construction at Niigata. The engine was not even remotely near to the stage of service, so the prospects of completing the R2Y2 itself were very slim even by the end of the war.


    An often supposed R2Y2 version
    with a nose air intake.

    Many depictions of the R2Y2 that appeared long after the war in books and magazines show alternate ‘plans’ with the turbojet engines mounted inside the fuselage and fed through intakes in either the nose or the wing roots. The actual veracity of these plans cannot be confirmed by historical materials: they are at best brief concepts, at worst, fictional.

    Both of the turbojets planned for the Keiun Kai at different times, the Ne-30 and Ne-330, had rather high rates of fuel consumption, even compared to other Japanese turbojets. To obtain a satisfactory operational range, the implementation of a large fuel tank in the central fuselage was resultingly necessary. Such a fuel tank would have to be diminished or even removed if shoulder or nose-mounted air intakes were used.

    Mounting the engines inside the fuselage would have increased the complexity of maintenance, the quality of which was falling near the end of the war, and would also likely increase production time. Furthermore, it would have required research into channel losses caused by feeding a jet engine with a long air intake.

    In consideration of the technical problems and the state of the war, along with extant documented materials, it seems certain that the simple “underwing nacelle format” was the final appearance of the Keiun Kai, and probably the sole format ever seriously considered.


    Conclusion

    The Keiun leaves a strong impression as one of the most unique in appearance and technically impressive Japanese aircraft of the Second World War. The monstrous 3,400 horsepower coupled engine and sleek aerodynamics sought to deliver an unprecedented performance of 741 kilometers per hour, exceeding even the highest performance of piston-engined service aircraft around the world at the time.

    However, the real-world practicality of the design is easy to question.

    The preceding variants of the liquid-cooled Atsuta engine already suffered from operational maintenance and production issues in its use with the D4Y ‘Suisei’ bomber. Majorly up-rating and coupling two of these engines together would only have exacerbated these problems. In the complete version of the Keiun, the engine was supposed to be fitted with a turbocharger, which was a technology not mastered by the Japanese until the end of the war.

    In hindsight, it is known that the development period of the R2Y1 eclipsed its potential usefulness on the battlefield due to the deterioration of the war. It may have been best to have continued the development of its predecessor, the R1Y1, instead. Though this plane was expected to miss its performance requirement by a small margin, it had more than a year of concrete development in advance.

    Even when re-proposed as a turbojet attack plane, the completion of the project never came close to realization. From a practical point of view, the decision to continue developing the Keiun until the end of the war was only another resource drain in a situation that was already hopeless by any possible means.


    Sources

    • Sekai no Kōkūki Issue 1953/7.
    • Mitsubishi Jūkō Shashi. 1956.
    • Aireview Issue 1958/5.
    • Nozawa, Tadashi. Nihon Kōkūki Sōshū: Aichi・Kūgishō-hen. Tōkyō: Shuppan Kyōdō-sha, 1981.
    • Ogawa, Toshihiko. Moboroshi no Shinei-ki. Tōkyō: Kōjin-sha, 2003.
    • Kaigun Kōkū Gijutsu-shō. Tōkyō: Gakken Plus, 2008.
    • Nihon Kōkū Gakujutsu-shi: 1910-1945. Tōkyō: Miki Shobō, 2021.
    • Rep. CINCPAC-CINCPOA Translations: No. 8, 1944.
    • Rep. Kaigun Shisaku-ki Seinō Yōmoku Ichiranpyō, 1945.
    • Rep. Japanese Aviation Experimental Budget and Some Design Features of Japanese Aircraft, 1945.
    • Rep. Data on Japanese Aircraft Shipped to United States for Study Purposes, 1945.
    • Rep. Organization and Operation of First Naval Air Technical Arsenal, 1945.
    • Rep. Desc of Experimental Aircraft and Experimental Engines Under Development by the Japanese Army and the Imperial Japanese Navy. 1946.
    • Photos of He 119 and DB606 were taken from https://oldmachinepress.com/.
    • Some photos of Keiun from the SDASM Archive.
  • Kōgiken-IST Ne-130: Driven to Destruction After the War

    Kōgiken-IST Ne-130: Driven to Destruction After the War

    On July 26th, 1944, the Japanese Navy submarine I-29 ‘Matsu’ was sunk by the USS Sawfish near the Philippines while en route to Kure. Onboard ‘Matsu’ were potentially instrumental materials about secret German aircraft technology. Of relevance, there were survey sketches and other data on the Messerschmitt Me 262 and Me 163 and actual examples of their respective jet and rocket engines.

    Eiichi Iwaya
    Wikimedia

    Not all the data was lost, as Navy attaché Technical Commander Eiichi Iwaya had departed the submarine with some of the materials while it was docked at Singapore on July 14th. He boarded a Type 0 Transport Plane on the 17th for a flight to Japan and arrived at Haneda on the 19th. He reported to the Navy Air HQ and then traveled to the Navy 1st Air Technical Arsenal (Kūgishō) where a study meeting was held.

    The materials Iwaya brought were a single 1/15th-reduced side diagram of the BMW 003A turbojet, observation notes on the BMW and the Jumo 004B turbojet, construction drawings of the Walther liquid rocket engine, and operation manuals for the Me 262 and Me 163 fighters. The fine details on the BMW 003A diagram were impossible to see when blown up to a useful size, so questions were telegrammed to another Navy attaché stationed in Germany.

    A joint study meeting was subsequently held at the Kūgishō on July 27th between officials of the Navy, Army, and six private companies that had been ordered to produce jet engines. Before this point, the development of jet and rocket engines had been conducted independently by both military branches, without sharing research, while ordering the same manufacturers. With the declining war situation causing a scarcity of materials and time, they now had no choice but to collaborate.

    The BMW 003A cutaway.

    (Note that the following history is constructed largely from the recollections of those involved, rather than historical documents, of which very few survive.)


    Turboprops-turned-Turbojets

    As a result of the joint meetings, which also took place in August and September, the division of work between both military branches was decided. The Army, probably because of their ‘Toku-Ro Mk.1’ liquid rocket already under development, was to oversee the development of liquid rockets, and the Navy was similarly to oversee the development of jet engines. To this end, the Army’s previous series of jet engines was entirely canceled.

    However, both the Army and Navy each had a turboprop engine also under development at the time. The Army’s was the ‘Ne-201’ (planned output: 1,870 shaft horsepower + 582 kg thrust), and the Navy’s was the ‘GTPR’ (planned output: 3,000 shaft horsepower + ~700 kg thrust). Both of these turboprops had been ordered to be constructed by Ishikawajima Shibaura Turbine. But with the focus now heavily shifting to the turbojet format after the arrival of the BMW diagram, the Navy ordered the conversion of the GTPR to a turbojet named ‘TR140’, and the Army also ordered the design adaption of the Ne-201 to a turbojet called ‘Ne-201II’.

    The 19-stage axial air compressor of the IJA’s ‘Ne-201’ turboprop.

    It was decided at these meetings that three private company groups would develop new turbojet engines of a high output suitable for fighter planes, overseen by the Navy. The TR140 was naturally the responsibility of Ishikawajima Shibaura Turbine, while Nakajima Airplane Company and Hitachi Aircraft Company were assigned the ‘TR230’, and Mitsubishi Heavy Industries along with Niigata Ironworks were assigned the ‘TR330’. The latter two engines were from scratch, based on the BMW 003A’s diagram.

    (The names of all turbojet engines were supposed to be standardized to the Army’s prefix of ‘Ne’ at this time, rather than the Navy’s ‘TR’, but it took a while for the Navy to actually start using this change in practice.)

    It seems that the development of the TR140 proceeded at Ishikawajima Shibaura Turbine for a few months, and was still the definitive engine planned by Ishikawajima until the end of September, but for unknown reasons, it was canceled by December 1944.

    It was decided instead that the Army’s 2nd Air Technical Research Institute (henceforth: Nigiken) would take charge of the development of an engine based on the BMW 003A named ‘Ne-130’ in collaboration with Ishikawajima Shibaura Turbine. Inheriting the design experience of ‘Ne-201II’ and ‘TR140’, the project was set into motion at the end of 1944, trailing behind the other jet engines developed by the Navy and private companies.


    The Development of ‘Ne-130’ by Young Engineers

    The meeting to establish the basic design policy of the Ne-130 engine began at the Matsumoto dispersal site on December 13th, 1944. Present were members of the Nigiken, including the head of the institute Lieutenant General Enozawa, Engineer Kaneko, Lieutenant Colonel Kihara, and five young engineers: Colonel Okazaki, Colonel Harada, Colonel Akiyaka, Colonel Maeda, and First Lieutenant Nakamura.

    “For me, I was just shocked. It was a truly terrifying order that told all of us young people to develop an engine more than two times as powerful as the Kūgishō [TR10] that was shown to us the other day.”

    Yoshio Nakamura, Kuruma yo Konnichiwa Turbo-Jet. Ne-130

    Kūgishō TR10. Nakamura saw this Navy jet engine in mid-1944.

    Under the guidance of professors from the Tokyo Imperial University Aviation Department who had researched the jet engine for years, Professor Nakanishi, Assistant Professor Awano, and Assistant Professor Hatta, the basic design targets of the engine were decided on December 15th after discussing the compressor, combustor, turbine and other aspects. The pressure ratio was to be 3, the number of revolutions at full power 9,000, flow volume 22.8 kg/s, maximum static thrust 900 kg, weight 900 kg, and turbine inlet temperature 750 °C.

    After the basic policy meeting, the engineers from the Nigiken and Ishikawajima Shibaura Turbine grouped up at the Ishikawajima Shibaura Turbine Tsurumi Factory, where the basic and detailed design work on the Ne-130 was conducted. The division of development leads was as follows:

    2nd Army Air Technical Research Institute Team:
    Captain Taku Okazaki – Whole project and the turbine.
    Captain Akiyama – Compressor and combustion chamber.
    Lieutenant Yoshio Nakamura – Auxiliary drive and fuel system.
    Captain Harada – External affairs with the Army Institute and Army Aviation HQ.
    Captain Maeda – Internal affairs and test coordination with labs of the Army Institute.

    Ishikawajima Shibaura Turbine Team:
    Toshio Dokō – Company president, directing overall.
    Mr. Ogura – Design lead.
    Mr. Matsui – Turbine.
    Izumi Iguchi and Mr. Enjōji – Compressor.
    Ōmi Kishi – Auxiliary mechanisms and casing.

    However, this division of work was only official, and at night in the Tsurumi dormitory, ideas were freely exchanged from all sides.

    While the Ishikawajima Shibaura Turbine engineers were experienced with the development of steam turbine engines, this did not translate perfectly to aero engines, which use lighter and more streamlined construction, so the role of the Army in developing the design was crucial. Even the president of the IST company, Toshio Dokō, took part in the design discussions at times and often warned the young engineers “Don’t work yourselves too hard”, worrying about their health. Nonetheless, with a monumental effort, the design drawings of the Ne-130 were completed at the end of January 1945. The process was about a month and a half.

    Ne-130 side cutaway diagram.

    Around this time, the Army’s group from the Nigiken was reorganized into the “Army Special Weapons Department”.

    The Ishikawajima Shibaura Turbine wooden models shop did not have experience in making fine models for aircraft engines, so the making of the Ne-130’s wooden mockup parts was outsourced to about 10 small wooden model shops distributed around the Fukagawa area. In many cases, dimensional errors were found in the drawings due to the very rushed work, but these were quickly corrected, and the wooden models were completed swiftly around the time of February 1945.

    The first prototype of the Ne-130 was scheduled to be completed at the end of March 1945, but the Tsurumi factory experienced a significant degree of absenteeism around this time due to the start of B-29 strategic bombing raids in the area. The first unit was finally assembled in late May and delivered to the Army Special Weapons Department at Tachikawa.

    Ne-130 on the test stand, looking at the exhaust nozzle.

    A static running test of the engine was conducted on June 26th using a modified test stand originally for piston engines. When the test run began, the first failure was in the auxiliary mechanism drive due to a design flaw. After that, the engine was successfully raised to 8,000 RPMs for about one minute until a failure occurred in the compressor. The blades of the first axial stage shattered off and broke the blades of the second stage, continuing on to damage the rest of the stages. The Ne-130 was heavily damaged.

    The cause of the failure was deemed to be a hair fracture that occurred during the creation of the blades. Luckily, there was no damage to the combustion chamber or turbine, but the damage was severe and would take considerable time to repair. It was decided that testing would continue when the second prototype was completed, and the first engine was sent back to the Tsurumi factory for repairs. The design of the compressor blades was strengthened to prevent future accidents.


    Evacuation to Matsumoto

    By mid-1945, the B-29 strategic bombing raids over Japan had intensified to a terrible degree. Much of Tōkyō and the surrounding region had been reduced to a charred wasteland. The engineers knew that Japan’s defeat was only a matter of time. Any day, the lab at Tachikawa could be hit.

    Lt-cmdr. Osamu Nagano.
    The lead designer of the Navy’s Ne-20.

    In this situation, a meeting was held at the Tsurumi factory to decide where the Ne-130 project would be evacuated to continue development. It was supposed to be a joint meeting between the Army and Navy, but because of a bombing raid just prior, the higher ranking Army officials were not able to attend, and all of the ‘brass’ was on the Navy’s side. The young Army engineers could not speak from a place of authority.

    The Navy had already evacuated the development of their jet engine, the ‘Ne-20’ to Hadano, Kanagawa prefecture at this time. In order to consolidate work on jet engines, they wanted to move the Ne-130 work to Hadano as well, and have the Navy’s jet department take over. The Army engineers, having come this far, were not pleased with the proposal, and nervously appealed to be able to evacuate to Matsumoto with the engine, where Ishikawajima Shibaura was also located.

    Only one Navy official would understand the request. It was Technical Lieutenant-commander Osamu Nagano, the lead designer of the Ne-20 engine. He understood the connection between the young engineers and their engine, having also struggled with the hardships of a pioneer in the same technology himself.

    “This should remain entrusted to the young men of the Army.”

    Lt. Cmdr. Osamu Nagano, quoted in Jetto Enjin ni Toritsukareta Otoko

    Encouraged by Nagano’s support, the Ishikawajima Shibaura team spoke up, insisting that their work would suffer if separated from the Army after cooperating for so long. Because of Nagano’s experience, the proposal to evacuate the Army’s team to Matsumoto was accepted.


    The dispersal of the team and engine was completed by early July, and testing started at the schoolyard of Meidō Industrial School with the second prototype of the Ne-130. Testing was smooth as far as 6,000 RPMs, but above that many issues would occur such as the malfunction of the auxiliary drive gear bearing, oil leaks, damage to the fuel pipe or failure of the jet cone.

    Technical Captain Tokiyasu Tanegashima, the pioneer of jet development in the Navy, visited the test site of the Ne-130 one day in mid-July. At this time, the team managed to reach a state of steady operation at 8,000 RPMs with the engine. Tanegashima walked to the rear of the engine and threw small pebbles behind the exhaust nozzle. The pebbles were blown far away by the jet, and he smiled.


    Driven to Destruction After the War

    In early August, the full-power test of the Ne-130 was achieved when the engine reached the design point of 9,000 RPMs. The flow rate, compression ratio, and thrust were observed to almost meet the designed values, but before it was possible to measure adequately, part of the measuring apparatus broke.

    Army Special Weapons Department Jet Team

    The part was repaired by August 14th, but the end of the war occurred the very next day.

    On August 15th at noon, the Army Special Weapons Department at Matsumoto heard the ‘Jewel Voice Broadcast’ announcing the surrender of Japan on the radio. Nobody could quite understand much of what was spoken, but it was clear that Japan’s defeat in the war had come.

    The head of the department, Mr. Ōtsubo, fell to his knees and began crying at the revelation of defeat. Gathering himself together, he announced that he planned to deny official orders and occupy the Army facilities in the districts of Tachikawa and Fussa to fight to the end. He ordered the officers to gather at the school grounds by 5 pm, but it seems that the resistance was not carried out in the end.

    The jet team gathered separately and discussed what to do. The orders from the Ministry of Munitions were to destroy the Ne-130 and all related materials immediately. Everything had come to an end, but the Ne-130 was repaired, so why not have one last run?

    With the intent being to run the engine at maximum power until it was destroyed, the final bench test was started on August 16th. However, before the engine was completely destroyed, a foreign object was sucked into the intake and damaged the blades. After that, in compliance with the orders, the materials related to the Ne-130 were incinerated, and the engine was hidden in a hillside tunnel somewhere in Nagano prefecture and sealed off.

    The first and third prototypes of Ne-130 had been destroyed when the Tsurumi factory was destroyed by bombing on August 1st, leaving no engine intact for the US occupational forces at the end of the war.


    Ne-130 Design and Specification

    As previously mentioned, the Ne-130 turbojet was designed by the collaboration of the Army’s Nigiken and the Ishikawajima Shibaura Turbine Company, based mainly on the layout of the German ‘BMW 003A’ turbojet’s side-view. It was derived from the BMW 003A, but the actual engine had a larger size and higher performance target.

    While the Nigiken group did not have in-depth experience with the design of jet engines and was composed of young engineers only a couple of years graduated, concept guidance was initially given by professors of the Tōkōken. The Ishikawajima Shibaura Turbine company side had experience with the design and manufacture of land-based turbine engines, and several gas turbines ordered by the Army and the Navy previously. The design of ‘Ne-201II’ was inherited, along with the ‘TR140’.

    Compressor
    Ne-130 axial 7-stage compressor.

    The basic compressor format consisted of 7 axial rotor stages surrounded by 10 stators, adapted from the BMW 003A. The inner diameter ranged from 504 to 584 mm, and the outer diameter was 650 mm. The first stage had 36 blades.

    The maximum rotational speed was set at 9,000 RPMs, intended to achieve a flow mass rate of 22.8 kg/s and a pressure ratio of 3.56 at static conditions. This is higher than the BMW 003A’s flow mass rate of 19.3 kg/s and pressure ratio of 3.1.

    The compressor’s degree of reaction was 50%, which is lower than most of the other gas turbines ordered to Ishikawajima Shibaura Turbine around that time, Kō Mk.7, Ne-201, and GTPR, which had about 100% degree of reaction, but higher than the BMW 003A’s 30% reaction. The Navy’s Ne-20 turbojet compressor also had a 100% degree of reaction.

    The compressor absorbed 4,155 HP from the turbine and had a planned efficiency ratio of 83%. From compressor testing after the war, it was found that the Ne-130’s compressor type probably had an actual efficiency of about 80%. The efficiency is therefore about the same as the BMW 003A’s (78 – 80%), and higher than Ne-20’s (73%).

    Combustion Chamber
    Ne-130 annular combustor.

    A cannular and annular-type combustion chamber were both experimented with. Both types had 12 fuel injectors and were made of 18-8 stainless steel.

    The BMW 003A employed an annular combustion chamber, and this seems to be the type that was primarily tested on the Ne-130. However, thorough experiments to determine the efficiency of each combustion chamber type do not seem to have been conducted by the end of the war.

    Turbine

    The turbine format was of the axial single-stage, which was the standard of jet engines at that time. A diaphragm was positioned in front of the turbine to increase efficiency.

    Ne-130 single-stage axial turbine.

    Each of the 80 turbine blades was firmly welded to the disc, using a method developed by Hitachi for turbochargers. This was less robust than the innovative ‘Christmas Tree’ slotted blade roots developed by Frank Whittle’s team in Britain. Unlike the BMW 003A, which used hollow air-cooled turbine blades, the turbine blades of the Ne-130 were solid.

    At 9,000 RPMs, the Ne-130 turbine produced 4,390 HP. Due to the low strength of materials available under wartime conservation, the turbine inlet temperature was 750 °C. This is somewhat higher than the 700 °C inlet temperature of Ne-20, allowed by the lower rotational force exerted on the turbine compared to Ne-20’s 11,000 RPMs. The material used for the turbine was likely I-309, a stainless steel alloy composed without nickel.

    Considering the low durability of materials, this was probably a turbine with a very low degree of reaction, allowing a thicker blade profile for strength.


    Ne-130 Turbojet Overall Specifications (Plan)
    Length3,850 mmFlow Mass Rate22.8 kg/s
    Diameter767 x 850 mmPressure Ratio3.56
    WeightDry: unknownCompressor Efficiency83%
    Wet: 900 kgRevolutions9,000 RPM
    Compressor7 axial rotors, 10 statorsStatic Thrust908 kgf
    Combustion ChamberAnnular, 12x injectorsFuel Consumption Rate1.28 kg/kgf/hr
    Turbine1 axial, 4,390 HPTurbine Inlet Temp.750°C

    Conclusion

    The Ne-130 turbojet has a rather obscure place in the history of Japanese engine technology. Not unreasonably so: while the Ne-130 was one of the most powerful engines actually built by Japan during World War II, it was also only experimental, uncertainly far from the stage of practical use, and destroyed with little trace for the US occupation to recover.

    Nonetheless, the story of the engine was a considerable achievement.

    When it comes to Japanese jet engines from that era, the only model given a wide coverage (in any language) is the ‘Ne-20’ made by the jet engine group at the Navy’s Kūgishō. The Ne-20 was the only practical Japanese turbojet engine and had an astonishingly quick development time — 3 months from the start of design to a prototype, and another 3 months until it was cleared for service. It is regarded as one of the great technological feats of the time and was considered “doubtful” by the analysis of the US occupation side.

    The young engineers of the Army’s Nigiken were given an order to supervise the development of an engine almost twice as powerful as the Ne-20, more ambitious than the original German ‘BMW 003A’, without the prior experience of the Navy. And even so, the initial development pace of the Ne-130 was not dissimilar to that of the Ne-20. The entire design was completed in just 1.5 months, and the first prototype could have been ready as early as March, the same month that the first Ne-20 was completed.

    It is a testament to the persistence of the engineers at the Nigiken and Ishikawajima Shibaura Turbine that the Ne-130 was able to exhibit its full power operation before the end of the war. They knew the hopelessness of the situation, but their efforts would have been instrumental for the next generation of a fighter plane.

    According to the recollection of Yoshio Nakamura, a Ne-130 was sealed in a tunnel in the Nagano prefecture after the end of the war. There remains the uncertain possibility that this piece of history may exist, in some form, today.


    Sources

    • Okazaki, Takurō. 「ネ130 – 軸流圧縮機を中心として」機械の研究 6(8) (1954): 674-678.
    • Tanegashima, Tokiyasu. 「わが国におけるジェットエンジン開発の経過 (1)」機械の研究 21(11) (1969): 46-49.
    • Tanegashima, Tokiyasu. 「わが国におけるジェットエンジン開発の経過 (2)」機械の研究 21(12) (1969): 46-48.
    • Tanegashima, Tokiyasu. “Technical History of the Development of the Jet Engine in Japan.” Memoirs of the Defense Academy, Japan X, No. 1 (1970): 1–32.
    • Nagano, Osamu. 「戦時中のジェットエンジン事始め」鉄と鋼 64(5) (1978): 659-663.
    • Nakamura, Yoshio. 「車よこんいちはTurbo-Jet. ネ130」Motor Fan 50(5) (1986): 128-131.
    • Yoshida, Hideo. “Japanese Pioneers in Research and Development of Gas Turbine (1).” Journal of the Gas Turbine Society of Japan 46(1) (2018): 1-34.
    • Yoshida, Hideo. “Japanese Pioneers in Research and Development of Gas Turbine (2).” Journal of the Gas Turbine Society of Japan 46(3) (2018): 55-71.
    • Torikai, Tsuruo. 知られざる軍用機開発(下). Tōkyō: Kantōsha, 1999.
    • Maema, Takanori. ジェットエンジンに取り憑かれた男(上). Tōkyō: Kodansha, 2003.
    • 大戦末期航空決戦兵器 – 橘花・火龍・秋水・キ74. Tōkyō: Gakushū Kenkyūsha, 2006.
    • Maema, Takanori. 技術者たちの敗戦. Tōkyō: Sōshisha, 2013.
    • Rep. Binding Related to Science and Technology, 1944.
    • ATIG. Rep. Turbojets and Rocket Engines (JAF), 1945.
    • US Naval Technical Mission to Japan. Rep. Miscellaneous Reports of Various Japanese Naval Research Activities, 1946.

  • Tanegashima’s Group: Birth of the Japanese Jet Engine

    Tanegashima’s Group: Birth of the Japanese Jet Engine

    By the 1930s, the practical speed limit of piston-engined aircraft was being approached for the first time. Before a propeller-driven airplane can approach the speed of sound, there is a period where the blade tips must reach supersonic speed, resulting in massive turbulence and loss of efficiency. This effect imposes a rough ‘limit’ on the horizontal top speed of piston-engined airplanes at around 800 kilometers per hour. Globally, the Macchi M.C.72 seaplane set an airspeed record of 709.2 km/h in 1934, followed by the 755.1 km/h record set by a Messerschmitt Me 209 in 1939, which remained the piston airspeed record for the following 3 decades, illustrating the difficulty to advance near the practical limit.

    The World’s First Turbojet W.U. 1.

    The limits of piston-engined aircraft were the main drive behind the development of the gas turbine or jet engine, discarding the propeller, which rapidly loses thrust at high speeds, for propulsion by the exhaust of a high-speed jet of heated gas. This method of propulsion would eventually enable aircraft to reach much higher speeds with greater efficiency than propeller planes.

    Early developments of gas turbine propulsion began to take shape in Europe by the 1930s and culminated with the nearly simultaneous prototyping of the ‘Whittle Unit’ in Britain and the ‘HeS 1’ in Germany; the first two examples of the turbojet engine, in 1937. Further development to make such designs practical resulted in the flight of the first jet plane, Germany’s Heinkel He 178, in 1939, followed by the Britain’s Gloster E.28/39 in 1941.

    As research advanced overseas, the situation within Japan, which up to this point had only recently caught up to the conventional aero-engine technology of the Western world, was isolated. While the experimentation into jet engines in Britain and Germany eventually resulted in widespread development and practical applications in these countries, globally it was largely unknown and had not made an impression. Because the jet engine was not yet attractive to the Japanese military, and companies were tied up in the development of conventional piston engines regardless, the conception of the Japanese jet engine was left to pioneers working independently on their own initiative.


    Early Interest Within Japan

    Kouichi Hanashima

    Interest in Japan regarding technology related to the jet engine dates back as far as the 1920s. Around this time, Navy Lieutenant Kōichi Hanashima (花島 孝一) was interested in turbines and other rotating machines from Europe. In 1920, shortly after his promotion to Lieutenant-commander, he went on a business trip to France. During this trip, Hanashima purchased 10 Rateau turbochargers for the Suiza engine and returned to Japan with these souvenirs the same year.

    Hanashima, who graduated from the Navy Engineering College at the top of the class, was a well-educated man who always had a keen eye on the latest developments of aviation technology. He foresaw possibilities in the application of this technology, however, at that time he could not promote domestic interest in this field. One Rateau turbocharger ended up in the possession of the Tokyo Imperial University for study.

    In December 1930, now a Navy Captain, Hanashima became the General Manager of the Yokosuka Arsenal Engine Testing Department. At that time, the group under him only consisted of Lieutenant Toshio Kondou and a few other members. In 1932, the various Navy aviation research departments such as the Engine Testing Department and Aircraft Testing Department of the Yokosuka Arsenal, and the Giken Aviation Research Department at Kakamigahara, were reorganized into the Yokosuka Air Arsenal, where Hanashima retained his position in the Engine Department. Over time the facilities of the arsenal were expanded, and by 1936 it was almost complete.

    1930 Whittle patent turbojet.

    Early European jet development was now in swing, and at the time, somewhat public, due to lack of government interest. As fragments of this information began to trickle into Japan from overseas, the attention of a few researchers in Japan was caught. Rear-admiral Hanashima, who by 1936 had become aware of jet patent literature from the likes of Robert Goddard, Frank Whittle, and Secondo Campini, was highly captivated with the concept. He quickly formed a research group, and with the cooperation of Tokyo Imperial University and Mitsubishi Heavy Industries, began studies on the jet engine. While European researches at that time were largely focused on the motorjet and turbojet configurations, Hanashima was most interested in the simple ramjet format.

    Tokyo Imperial University started the study of propulsive ducts in 1937. This theoretical research was mainly headed by Professor Keizo Hatta and Professor Fujio Nakanishi. Attention was focused on the ramjet, and experiments were conducted relating to airflow through various ramjet internal ducts. One of their students, Yoshitarō Hibi, was highly interested in the experiments, and after graduating from the Tokyo Imperial University joined Mitsubishi and started working with ramjets. Experiments with scale models in external airflow were conducted in 1939 and later.

    The theoretical and material development that goes into the research of jet engines was not so high in Japan at that time, due to its isolated position from the rest of the technological world. Due to this, the aforementioned research works were slow, troubled, and full of unknowns.

    However, the situation changed when Tokiyasu Tanegashima arrived on the scene around 1938.


    Tokiyasu Tanegashima

    Tokiyasu Tanegashima

    Tokiyasu Tanegashima (種子島 時休) was born in Yokosuka City on July 20, 1902. A descendant of the Tanegashima clan, his ancestry included Tokitaka Tanegashima, who is famous for introducing the firearm to Japan in the 1540s. Due to this, he picked up the nickname ‘Teppou’ (gun) in his childhood. Mr. Tanegashima entered the Naval Engineering College in 1918, graduated in 1921, and served on the largest Japanese battleship at that time ‘Mutsu’, becoming interested in power plants while often visiting the boiler room. In 1927, he discovered the book “Steam and Gas Turbines” by Aurel Stodola, which enclosed an explanation of the gas turbine engine at that time.

    Afterward, Tanegashima entered the Navy Air Service and was appointed to investigate aircraft engines. He passed the technical course of the Navy Staff College in 1929 and entered the Tokyo Imperial University Aeronautics Department the next year. Here he studied the basics of aircraft engines, including early research regarding gas turbines, which was highly rare at the time. He graduated from the Aeronautics Department in 1933 and became the head of the Hiro Navy Arsenal Engine Department Assembly Plant, meeting Osamu Nagano (永野 治), his subordinate and later an invaluable designer of jet engines. Tanegashima was ordered to travel to Paris in 1935 to survey aero engines and fighter-mounted machine guns, touring various related companies throughout Western Europe.

    Brown-Boveri Gas Turbine (1938).

    Tanegashima used this time to also investigate the situation of the gas turbine in Europe, but he was dissatisfied by the lack of information in France, Germany, Italy, and England. However, in 1936 while visiting Switzerland for the Oerlikon company, Tanegashima visited Brown-Boveri, an electrical engineering company that was involved in the manufacture of turbines for general industry. The chief engineer Mr. Klingerfuss displayed drawings of gas turbines, as well as an actual example of an exhaust gas turbine. Tanegashima ordered Brown-Boveri to manufacture two turbochargers for 1,000 hp aircraft engines at 10,000 yen each and was recalled to Japan in 1937 enthusiastic about the possibilities of gas turbines.

    Upon his return, Tanegashima was immediately appointed to Yokosuka Air Arsenal (henceforth ‘Kūgishō’) as the Chief of Service Engineers for the 1st Engine Factory, chief of the factory. The main responsibilities of the Chief of Service Engineers were to examine and test prototype engines, improve existing engines, perform failure tests to apply countermeasures, maintain engines for flight testing planes, and support experiment planning. So the 1st Engine Factory was constantly tied up with work applying countermeasures for various engine malfunctions, which were very serious and labor-intensive due to the pilots trusting their lives in the reliability of those machines.

    However, around the time of 1940, complaints about aircraft engines disappeared due to war enthusiasm, and Tanegashima used this newfound free time to study the gas turbine concept closely in his lab. The second volume of the English translation of “Steam and Gas Turbines”, which could be considered Tanegashima’s bible, was always on his desk. During the early period, working only with his assistant engineer Watanabe Sadaki, he created two crude apparatus for confirming the theoretics of jet ducts and combustion chambers. The results were satisfactory, which reinforced the feeling that it was possible to manufacture a jet engine. Gradually he took in more engineers that gathered around in interest, such as Osamu Nagano, who was assigned to the same department.

    Jumo 205C.

    Commander Tanegashima’s greatest interest was mainly in the format of a propeller-driving gas turbine (turboprop) supplied by a free-piston compressor. At that time, the Navy had been developing the top-secret flying boat ‘H7Y1’ as a reconnaissance aircraft intended to make a round trip from the Marshall Islands to Hawaii. In order to achieve this range requirement of 5,000 nautical miles, four Jumo 205C opposed-piston engines imported from Germany were employed as a result of their low fuel consumption. However, when the prototype was completed in 1939 it suffered from many issues including lack of strength and rigidity, insufficient engine output, poor maneuverability, and more.

    The prototype was canceled in July 1939, and all related materials including drawings and photos were destroyed, but the Jumo 205C engines were left over. The format of this engine was good for Tanegashima’s purposes, and he started a project with Mitsui Co. in 1940 to prototype a free-piston compressor by diverting the cylinders of the Jumo 205C.

    Osamu Nagano

    However, the design of the compression piston diverted from the Jumo 205’s opposed pistons required a large amount of design innovation, and while the project was idling, Commander Osamu Nagano who worked in the Engine Department under Tanegashima designed a tiny model gas turbine with a free-piston. In 1941, it was manufactured by Mr. Masanori Miyata, head of the electronics factory, and after work applying corrections, it was able to reciprocate smoothly and operate continuously.

    This micro gas turbine only had an output of 1/10th horsepower, but it was able to demonstrate itself by driving a small magneto to light an electric lamp, and inspired hope and courage in the young engineers working day and night. Although it was only a ‘toy’, it was technically the first gas turbine manufactured in Japan, and sadly disappeared in time without even a photograph.


    The Tanegashima Group, 2nd Research Branch

    Gloster E.28/39, the first
    British jet plane.

    Then came about the 8th of December 1941, when the Japanese Navy conducted an attack on the United States Navy at Pearl Harbor. Tanegashima recalled an overwhelming sense of dread that a giant was being awoken. Around this time, he had also learned from a Navy intelligence officer that England had tested a jet plane earlier in May. After a few days, Tanegashima decided that the jet engine must be developed at any cost to vastly increase the performance of aircraft, lest Japan fall far behind the western world in aviation technology which would undoubtedly spread from England to the USA. He immediately appealed to his superior, chief of the Engine Department Rear-admiral Kiyoshi Matsukasa, to be able to focus solely on this subject. The request was immediately accepted, and in January 1942 Tanegashima was appointed as the chief of the 2nd Research Branch focused on jet engine development, with several skilled engineering officers and roughly 200 workmen at his disposal. One of these engineers was Shigeo Katō, a 1st Technical Lieutenant who was said to have an even deeper understanding of the gas turbine than Tanegashima.

    Caproni Campini N.1: World's Second Jet Aircraft | Comando Supremo
    Caproni Campini N.1

    After the failure to manufacture a free-piston based on the Junkers engine, Tanegashima had realized that the current Japanese industry was unable to manufacture such a machine. During the middle of the year 1942, it was learned that the Italian Caproni Campini N.1 had flown without a propeller, garnering much public interest, and it was also disclosed by confidential means that the Germans had tested a jet plane long ago. Thus, the focus drifted away from the free-piston, and eventually it was shelved in favor of the conventional turbojet engine, which was speculated to be the power plant of what was the ‘He 178’.

    Axial turbojet explanatory diagram from the first theoretical report on the turbojet in Japan, April 1942.

    During the early phase of official jet engine development under Tanegashima’s Group, several theoretical reports on different types of jet engines and gas turbines were published. The main types studied were the TR (turbine rocket, now turbojet), GTPR (gas turbine propeller rocket, now turboprop), similar compound variations, and the Campini type (motorjet). From these reports, it can be seen that Tanegashima was far in favor of the axial compressor, as opposed to the centrifugal-type common among the early jet engines of the world. However, the quickest way to procure an experimental jet engine was to convert from a turbocharger, which contain a centrifugal compressor.

    In August 1942, one Hitachi turbocharger for a 2,000hp aero-engine was converted to a jet by installing a combustion chamber. During wind tunnel testing, though, it could not self-drive with combustion due to fundamental differences between a small turbocharger and a jet engine. This small unnamed machine, albeit a failure, was the first attempt to prototype a turbojet engine in Japan.


    The First Japanese Turbojet Spins

    TR. The first functional
    Japanese turbojet engine.

    A much larger turbocharger designed by Shigeo Katō was delivered from Ebara during the next month. It was the YT15, a huge device with a turbine diameter of 600 mm and compressor diameter of 500 mm designed for engines of 2,500 hp at altitudes of 15,000 m. However, due to low turbine strength, the YT15 was not adopted as it was. Instead, Katō proposed its conversion into a turbojet due to its large size, and the conversion started at Ebara in February 1943. A complicated folded combustion chamber was installed due to the small distance between the compressor and turbine, and the exhaust pipe was remade as a jet nozzle. The initial engine was completed in June 1943 as Japan’s first functional turbojet, ‘TR’. The design targets were 16,000 RPM, 4.0 pressure ratio, and 300 kg thrust output.

    Thrust300 kgf (Plan)
    Revolutions16,000 RPM
    Pressure Ratio4.0 (Plan)
    Format1C-AN-1AT
    TR Specifications

    Testing with TR started in July when the engine was placed at the mouth of a wind tunnel at 5 m/s wind speed. When it started to turn, vaporized fuel was injected into the combustion chamber and ignited with an electric spark. When the wind was stopped, all that was left was the hum of TR operating on its own power. The fuel quantity was increased, and the engine rapidly reached a speed of 14,000 – 17,000 RPM with a pressure ratio of 3.5, but there was no nozzle, so thrust was not produced. After about 5 minutes of full-power operation, it was stopped. Cracks were observed in the turbine blades.

    Nonetheless, Tanegashima, Katō, and all were elated, as their theories had been proven. Now what was left was to produce a reliable jet engine, which proved to be the longest road of all.

    One day, when running at full operation, there was a tremendous explosion and the TR was left in ruin. The centrifugal compressor, which was only made of cast aluminum due to lack of strong light alloy, burst into three pieces. One pierced the tin roof of the test bench, and two stuck into the surface plate on the floor. Fortunately, nobody was injured.

    Example of turbine failure,
    Whittle Unit 3

    Soon, the second TR engine was delivered from Ebara. Changes to this unit included a forged compressor wheel and slightly reinforced turbine blades. During the first test of unit two, it was carefully brought up to maximum driving speed, then stopped after a few minutes and completely disassembled to inspect its condition. Once again, there were cracks in the base of the turbine blades. However, there was no time to perform repairs, so in order to record the thrust as quickly as possible, Tanegashima ordered Katō to mount the tailpipe and scraped off the cracked parts of the turbine with a file.

    The engine was brought back up to full power operation, and as expected, the weakened turbine blades splintered off. Still not stopping the experiment, Tanegashima cut off the blades diagonal to those lost during operation and continued to drive it at full power several times. As the number of blades decreased, the backpressure increased and the RPM of the turbine decreased, but the corresponding thrust was still generated. Eventually, only a bladeless and tattered turbine wheel remained, but the generation of 250 kg of thrust had been confirmed.

    After that, the third unit was built and testing continued for several months. However, constant issues including cracking of turbine blades, failure of compressor bearings, burn-through of the combustion chamber folds, and distortion of the nozzle plate into the turbine were inescapable, and the environment around Tanegashima began to sour as people became fearful of the dangerous engine. To counteract the complaints, sandbags were piled around the unit in a ‘bunker’, and viewing during operation was done through a mirror placed behind it.


    Improvement of the ‘Turbine Rocket

    Ne-10 design drawing.

    Up until this time, the Navy had generally been indifferent to the jet engine experiments conducted by Tanegashima, and even planned to dissolve his research branch at one point in order to contribute to more practical areas.

    However, in the early summer of 1944 a group of elite air officers stepped into a class where Tanegashima was teaching jet propulsion to officer candidates. Interest among the higher officers of the Navy had been piqued by reports concerning the application of jets in Germany. Immediately work on jet engines was given high priority, and the head of the Kūgishō, Vice-admiral Misao Wada ordered the work to be completed as soon as possible, instructing other departments to assist the development.

    In July 1944 the experiments with the first 3 TR engines concluded, and the results plainly showed that there was still a long road to practicality. In an attempt to remedy the various issues, the design was improved and renamed ‘TR10’, and the Navy General Staff issued a rapid improvement plan. TR10 was ordered to be mass prototyped with iteratively refined designs in 70 units by 6 companies (Kūgishō, Mitsubishi, Nakajima, Hitachi Manufacturing, Ishikawajima Aircraft, Ishikawajima Shibaura Turbine) from the beginning of July to the end of August 1944 for rapid trial and error. The 6 companies formed 3 groups for production: Mitsubishi & Nagoya Kobe, Nakajima & Hitachi, and Ishikawajima Aircraft & IST. Each group was expected to complete 20 engines, with the Kūgishō completing another 10, for the 70 total.

    DimensionsLength: 1,600 mm
    Diameter: 850 mm
    Weight250 kg
    Thrust300 kgf
    Revolutions16,000 RPM
    Pressure Ratio3.5
    Format1C-AN-1AT
    Ne-10 Specifications

    The Navy had suddenly promoted such an unlikely plan due to enthusiasm about combat reports of the German jet fighter ‘Me 262’. Unfortunately, there was a lack of confidence in the design of the obviously unfinished TR10 throughout the manufacturing companies, even if it was improved, and most companies were already overwhelmed with work on piston engines. As a result, the mass-prototype plan progressed painfully slowly. Only a few units seem to have been built.

    Furthermore, at the end of that July, Commander Eiichi Iwaya returned from Germany with a cutaway drawing of the BMW 003A turbojet engine that was in practical use at the time, garnering a massive interest from the Army and Navy. The companies initially assigned to mass-prototype the TR10 were instead assigned three different large-scale turbojets for jet fighters, and the TR10 rapid improvement plan was all but canceled, once again solely in the hands of the Kūgishō. At this time, the jet development of the Army and Navy was merged into a joint effort, and the TR10 was renamed once again to the joint designation ‘Ne-10’ (‘Ne’ for ‘nenshō’, combustion).

    Ne-10 without tailpipe.

    When Tanegashima observed the BMW 003A drawing, he immediately recognized that it was of the same principle as the Ne-10, but employed a 7-stage axial compressor and lower turbine RPM, which greatly reduced the stress on the turbine blades. However, Tanegashima considered himself to be an ‘experimental researcher’ pioneering new technology by his own intuition, and recognized that if the Ne-10 was to be practicalized in a timely manner, a professional designer was necessary.

    So, even before the first ‘Ne-10’ engine was completed in September and proved to be unsatisfactory, Osamu Nagano joined the Tanegashima group in August and started the design of a new engine in the series, Ne-10 Kai (Ne-10 Improved). Ne-10 Kai employed a 4-stage axial compressor ahead of the centrifugal compressor, which lowered the rotations of the turbine to 15,000, a much-needed stress reduction.

    At the same time, a huge version of Ne-10 Kai called ‘Ne-30’ with a powerful projected output of 850 kgf was also planned. Furthermore, during October another version of the Ne-10 Kai with two fuel pumps for mounting to an aircraft (G4M) was designed, called the ‘Ne-12’.

    Large centrifugal turbojet Ne-30, after the war.

    The first unit of the Ne-10 Kai, Ne-12, and Ne-30 were all completed in November 1944. One more unit each of Ne-10 Kai and Ne-12 were completed before the end of that year. By this point, the material strength situation had improved somewhat. During running tests to gather data, these two engine models apparently managed 30 minutes of full-power operation before turbine cracking began, and the planned thrust output of about 320kgf was reached. However, the same issues such as damage to the compressor, unstable combustion, and turbine cracking continued, so this result was still far from satisfactory. The future of the Ne-10 series was in doubt.

    On the other hand, the massively upscaled Ne-30 was deeply troubled for the same fundamental reasons as the rest of the series, only magnified by the increased size of the turbine and other components, and never could operate at full power before its abrupt cancellation.


    The End of the Ne-10 Series

    Ne-12B design drawing.

    The final model of the Ne-10 series, the ‘Ne-12B’, was a finished design at the end of December 1944. This version most notably reduced the weight of Ne-12 from 388 kg to 315 kg and further reinforced the various components.

    At the same time, Tanegashima expressed that the Ne-12 and its series were fundamentally flawed, and if the jet engine was to become practical, it was necessary to pursue a design based more closely on the format of the German BMW 003 type.

    “I would like to express my gratitude for the efforts that brought Ne-12B to this point, but it seems that this engine is underdeveloped. At this time, it would be wise to tear down all of the past and restart with reference to the BMW 003A.”

    Tokiyasu Tanegashima, Wagakuni ni okeru Jetto Enjin Kaihatsu no Keika

    As it was, the strength of materials available at that time in Japan simply could not bear the heat and stress placed upon the turbine of a centrifugal turbojet, even with the additive compressor, and there were many other unsolved issues such as combustion problems which would be negated by adopting the German straight flow style. Furthermore, Tanegashima always believed that the pure axial compressor was the best method from the beginning, and the decision to use a centrifugal compressor was only due to the time and manufacturing constraints.

    Kikka Ne-12 rough sketches.

    Between the arrival of BMW 003A’s drawing in Japan to this point, the Kūgishō had been conceptually drafting a new engine model based on the Ne-12. This engine called ‘Ne-15’ had the same performance requirement, but featured an 8-stage axial compressor derived from the BMW 003A format. By December, this plan had evolved into the ‘Ne-20’ with an improved projected output of 480kgf. Now all efforts were to be focused on the prototyping and testing of Ne-20, and the design improvement of the Ne-10 series was put on hold.

    Even if the development of the ‘Ne-10’ series was discontinued, around this time the prototype of the jet-propelled special attack plane ‘Kikka’ was announced. Although heavily flawed, ‘Ne-12’ was the most advanced turbojet model available in actual testing, so there was no choice but to ignore the problems for the time being and select it as the powerplant. Furthermore, manufacturing and testing models of the Ne-12B as a sort of ‘prototype’ for the Ne-20 could provide valuable developmental data, as it was the most improved version of the Ne-10. For these reasons, the production of Ne-12B engines was ordered from the Yokosuka Yard, Hokushin Electric Works, Ishikawajima Shibaura Turbine, Masada, and Ebara. This order was very reminiscent of the ‘TR10’ mass-prototyping plan, and unsurprisingly, similarly failed to proceed.

    DimensionsLength: 2,102mm (Ne-12) / 1,800 mm (Ne-12B)
    Diameter: 855 mm (Ne-12 & Ne-12B)
    Weight388 kg (Ne-12) / 315 kg (Ne-12B)
    Thrust315 kg (Ne-12) / 320 kg (Ne-12B)
    Revolutions15,000 (Ne-12 & Ne-12B)
    Pressure Ratio1.67 x 2.0 (Ne-12 & Ne-12B)
    Fuel Efficiency1.65 kg/h/kgf (Ne-12B)
    Format4A-1C-AN-1AT (Ne-12 & Ne-12B)
    Ne-12/Ne-12B Specifications

    From January to April 1945, just 12 units of the Ne-12B were constructed, 6 by the Kūgishō and 6 by the Yokosuka Navy Yard, with evidently none being built by private organizations. The first unit was completed in February. After some time, one unit was able to operate at full power for an hour, but the characteristic flaws such as combustion issues and turbine cracking continued. Even after almost two years of development labor, the final model of the ‘Ne-10’ series, with many incremental improvements, was little different from the first in practice.

    The final nail in the coffin for the Ne-10 series came with the testing of the first unit of ‘Ne-20’ in late March 1945. Very quickly this format of an engine, running out of a cave behind the Kūgishō, proved to be more reliable, durable, and powerful than the entire centrifugal series before it. This is not to imply that the Ne-20 did not have its own set of issues, but these were far more manageable. The last tether of the Ne-12, its application to the special attacker ‘Kikka’, was severed on the 19th of April during a meeting at the newfound Jet Department of the Kūgishō, the results of which were phoned from the Navy Air HQ to Nakajima the same day.

    “It was decided that Kikka will be equipped with Ne-20. Carry forward the prototype according to that point.”

    Mr. Nozaki of Navy Air HQ to the 2nd Mfg. Plant

    As a result, the production of the Ne-12B stopped in April. The development of the first Japanese turbojet series was over without practical use. Nonetheless, the Ne-10 series was an impressive technological endeavor from a nation that had only recently become aeronautically independent, with essentially no outside assistance and a lower level of materials science than the western world at the time. These engines provided invaluable data and experience that boosted the indigenous development, which continued at the Kūgishō throughout 1945 until the end of the war. The Ne-20 engine was advanced from the start of design to completing service trials in a staggeringly quick 6-month period – perhaps the fastest turbojet development process of the war globally – and famously succeeded in flight mounted below the wings of the ‘Kikka’ on the 7th of August.


    Fate of the Ne-10 Series

    The whereabouts of these engines after the war are poorly recorded. In the photographs taken by the US Navy at the Kūgishō, among the various jet engines of the department, the following of the Ne-10 series are approximately identified by the author: 1x Ne-10, 1x Ne-30, 1x Ne-30 Mockup, 1x Ne-12(B).

    Ne-12B 1st compressor stage. From the 4th Ne-12B completed by Yokosuka Yard.

    Each of the Ne-10 and Ne-12 turbojets seem to have been scrapped at some point following the conclusion of the war. However, the 1st stage axial compressor fan of a Ne-12 remains preserved at the National Museum of Nature and Science in Tōkyō, Japan. This item bears the inscription “12B NO 4”, seemingly identifying it as part of the 4th Ne-12B made by the Yokosuka Yard.

    Surprisingly, the largest of the Ne-10 series, ‘Ne-30’ still survives to this day. The sole prototype and its mockup have ended up in the storage of the Smithsonian Air and Space Museum, where they remain. At the time this article was written, Ne-30 is not displayed publically.

    Unfortunately, apart from these three artifacts, no other remains are known to exist of Japan’s first turbojet series.

    Sources

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    • Rep. Reaction propulsion by Axial Flow Compressor, 1942.
    • Rep. Free Piston Two-Stage-Combustion Internal Combustion Turbine and Turbine Rocket, 1942.
    • Rep. Binding Related to Science and Technology, 1944.
    • Rep. Development of Gas Turbine Propulsion in Japan, 1945.
    • Rep. Miscellaneous Reports of Various Japanese Naval Research Activities, 1946.
  • Ki-201 ‘Karyū’: The Me 262 Domestic Production Plan

    Ki-201 ‘Karyū’: The Me 262 Domestic Production Plan

    At the dawn of 1944, the German jet fighter Messerschmitt ‘Me 262’ was nearing the beginning of its service life. Due to issues with its power plant and interference from the high command, the aircraft had been in the testing stage since 1941. In the coming months, it would finally enter mass production. This aircraft achieved revolutionary performance; exhibiting a top speed of 870 km/h, a cruising distance of 1,050 km, and a climb rate of 1,200 m/min. The bomber-devastating armament consisted of a quartet of 30 mm machine cannons and 24 rockets. On paper, it was the world’s best interceptor at the time.

    It is comparatively little known that Japan had indigenous jet engine programs prior to being influenced by German technology. The development of original Japanese jet engines began in 1941-1942, but they wouldn’t materialize as prototypes until 1943. In the typical fashion of the Japanese military, the Navy and Army did not collaborate on this ordeal. As such, duplicate research efforts were conducted simultaneously.

    The testing of indigenous jet engines was plagued with troubles ー to be brief; major issues such as total failure of the engine itself during operation, to performance problems like low thrust output and high fuel consumption rate, were unavoidable. By 1944, the most advanced Japanese turbojet developments from both sides only provided about 300 kilograms of thrust. At this point, the Japanese were several years behind their German counterparts. However, with limited assistance, an impressive technological leap was soon to be achieved.

    (This article was revised in early 2022 with new information suggesting that a 1:1 scale mockup of at least the Karyu’s cockpit area was completed.)


    Japanese Interest in the Me 262

    It was in the early months of 1944 that the Luftwaffe High Command revealed the existence of their secret jet and rocket-propelled fighters to Japanese representatives in Berlin for the first time. In previous years the Germans had been reluctant to disclose their experimental weaponry to the Japanese, but as development progressed and the war situation worsened, they opened up more or less entirely. The Japanese did not waste any time to request more information, and in March, Hitler and Göring agreed to release such material to Japan. Three requests were subsequently made on April 1st:

    1. Send Messerschmitt jet technicians to Japan
    2. Permit the training of Japanese technicians in Germany
    3. Allow the purchase of rights for the licensed manufacture of the Me 163 B and Me 262 A.

    In addition, by the beginning of that month, basic survey sketches and illustrations of the Me 163 B, Me 262 A, and various German jet & rocket engines were already turned over to Japanese attaches within Germany. Submarines Ro-501 and I-29 departed weeks later en route to Japan with these limited materials distributed among their cargo. Neither of these submarines would actually arrive in Japan, both being intercepted and sunk en-route. Only a very small amount of technical data survived with Commander Eīchi Iwaya, who would later depart I-29 during its stop at Singapore and arrive in Japan during mid-July of the same year.

    The Germans agreed to release the manufacturing rights for the Me 262 to Japan in May, but the negotiations did not conclude this early, and the plans weren’t to be made available to the Japanese until the autumn of that year. During the interim, Japanese representatives visited production facilities for the Me 262. They were instructed on the manufacturing techniques by August Bringewald, an overseer of Me 262 production in Germany. It was clear that Japan could not mass produce Me 262 without modifying the production techniques accordingly to their own, and would require German specialists to supervise the manufacturing process.

    Finally, in July, orders were issued to Messerschmitt to begin preparing the blueprints and materials for the manufacture of secret aircraft and engines to be delivered to the Japanese. On the 23rd of the same month, Göring approved the delivery of one Me 163 B and one Me 262 A to Japan, but this decision was upended by Hitler in August.


    Around this time in Japan, the limited technical information pages to survive with Commander Iwaya from I-29 were received, as previously mentioned. Among this of relevance was an Me 262 operations manual and a single cutaway of the BMW 003 turbojet. Despite only being a copy of a cutaway reduced to 10x15cm, this drawing was studied extensively and garnered a massive interest, because in Germany the BMW 003 was already in practical use. For small parts that were not clear on the drawing, as the lines were blurred by the microform, educated guesses were made. Using what could be learned from the layout of this drawing, the Japanese paused and re-examined their entire jet program.

    In the necessity of efficient development given the war situation, it was decided to unite jet development cooperatively between the Army and Navy, a practice that scarcely occurred in earlier years. The Army’s turbojet projects were completely canceled, while the Navy’s turbojet developments were to be furthered with modifications for the time being. In addition, a tri-company project was begun to procure high-thrust class axial turbojets, reverse engineered from the diagram of BMW 003. These engines were the following:

    1. Ishikawajima Shibaura Turbine’s Ne-130 (908 kgf)
    2. Nakajima Airplane & Hitachi Manufacturing’s Ne-230 (885 kgf)
    3. Mitsubishi Heavy Industries & Niigata Ironworks’ Ne-330 (1320 kgf)

    (The Navy also privately developed the ‘Ne-20’, though this engine is smaller in scope)

    Concurrently with the planning of these aforementioned engines, an airframe to mount them was devised. The summary of a ‘Rocket Plane’ assigned to Kawasaki Aircraft was included in the Army’s September 1944 aircraft prototype plan. In the general outline, it was labeled as the ‘Me 262’, and the engine model was listed as the TR230 or TR330. Within the engine prototype plan issued in the same month, the engines noted as “for Me 262” were the TR140 and TR330, but curiously not the TR230.

    (TR140 later became the Ne-130, and TR230 and TR330 are early names of Ne-230 and Ne-330 respectively.)

    From these extant materials, it has been deduced that Me 262 was initially assigned to be designed and produced domestically by Kawasaki, and would mount the most successful of the three new turbojet models in development. According to the prototype plan, the order of development should be issued during October 1944, the first prototype should be completed in December 1945, and the practical examination should conclude by June 1946. No prototype ‘Ki’ number was assigned to this plane, so the plan was clearly very preliminary. Unsurprisingly, the development order was not issued at the scheduled time, possibly a result of the ongoing negotiations with Germany. Complete technical and manufacturing plans for the Me 262 were delivered to the head of the Messerschmitt foreign export branch, a certain Dr. Thun, in October. Later that month, Japanese representatives advised the Germans that only the Army was planning to put the Me 262 into mass production. Two mass production plans seem to have been requested, one for 100 aircraft a month, and another for 500. By December, all the necessary contracts regarding the Me 262’s licensing had been signed and concluded.


    Although Kawasaki had been originally selected as the Me 262’s development company, at some point between October and December 1944, it is evident that the plan was transferred to Nakajima. The reason for this is not recorded, though it was possibly due to Nakajima’s position close to the development and construction of jet engines, with the Ne-230 under development. Kawasaki had also experienced an increased assignment of work at this time, which may have rendered the company unable to viably develop such a national first as a high-performance jet plane. A plan name for the aircraft was now established — it was the ‘Ki-201’ with the unconventional designation “Karyū” — the Fire Dragon, and development was ordered by the Japanese Army Air Headquarters. The project would be held cooperatively between the Army and Navy, with the Army in charge of the development of the airframe, and the Navy the engines. The design team was assembled at Nakajima under Iwao Shibuya and began basic research on the Ki-201 design in January 1945.

    The principle outline of the aircraft required was a twin-jet fighter-attacker capable of engaging enemy jet fighters, rocket planes, and high-altitude bombers. The performance requirements were a maximum speed of 800 km/h or more, a practical ceiling of 12,000 m or more, and a cruising range of 800 – 1,000 km or more. According to the ‘Rocket-Weaponry Military Strength Improvement Plan’ drafted in December, where the plane is first known to have been mentioned, prototype #1 was rescheduled to be completed in July 1945, 5 months earlier than the originally planned date of the “Kawasaki Rocket Plane”, with two more aircraft in August, followed by three more in September. It was also desired to increase production beyond this, and service 20 aircraft in August as well as September. Around 100 aircraft were generally expected to be serviced throughout 1945 when production plans were achieved, with two to three squadrons (112–168 planes) active by March 1946.

    Unfortunately for these expectations, Germany’s final attempts at technological assistance did not proceed smoothly. On February 9, 1945, the German submarine U-864 was sunk four kilometers west of Fedje, Norway by the British HMS Venturer. It had experienced numerous setbacks, delaying its intended embark to Japan. Onboard were the parts and plans for manufacturing the Me 262 A, Me 163 B, BMW 003, Jumo 004, and HWK 509. Also lost in the interception were two instrumental Messerschmitt engineers, Riclef Schomerus and Rolf von Chlingensperg, who were intended to assist with the development of jet aircraft and direct the manufacture of Me 163 & Me 262 in Japan, respectively. The Japanese were now left almost entirely in the dark — the only substantial data on German jet technology within their possession was still the very few pages departed from I-29 the previous year.


    The 6 Month Development Life of ‘Karyū’

    Due to the situation the development schedule was delayed, the planned completion of the basic design was now set for June 1945, the first prototype was reverted to the original schedule of December 1945, and the first 18 production aircraft were to be delivered by March 1946. Even in the absence of German manufacturing prints, the team at Nakajima began the basic design process of Ki-201 in April 1945. The last German mission to Japan, submarine U-234, departed on the 15th of the same month. Among its expansive cargo were the actual airframes Me 262 A, Me 163 B, and evidently, Me 309, divided into many crates and complete with manufacturing drawings. But there was no more time to spare, the war was quickly deteriorating and the likelihood that any further German data would make it to Japan was incredibly slim.

    Then, immediately following the capitulation of Germany, U-234 surrendered to the USS Sutton on the 14th of May, dashing any last chance for the arrival of German technology. Even as the captured German technicians expressed the notion that Japan would never be able to develop an Me 262 of their own without the onboard materials, the design of Karyū was nevertheless progressing, unknown to the rest of the world.

    In the initial draft, Karyū had a conventional linear wing, with the airframe dimensions at a span of 12.56 m and a length of 10.55 m, a size nearly identical to the Me 262. Ultimately though, the airframe design settled on a shape that appeared closely to Me 262, with a larger footprint of 13.7 m span and 11.5 m length (a size exceeding Me 262, at 12.6 m span and 10.6 m length). Accordingly, the wings were swept, and the cross-section of the fuselage was distinctly triangular in the mid-section. A tricycle-type landing gear configuration was adopted. The engine selected was the Ne-230 turbine rocket, or alternatively the somewhat more powerful Ne-130, and one was suspended under each wing. Two 1,000 kg powder rockets installed under the fuselage would aid takeoff.

    Around the time of late May or early June, the cockpit mockup examination of the Ki-201 was conducted. Iwao Shibuya took suggestions from pilots and other observation personnel, among them was Yoshio Nakamura, an engineer assigned to the development of the Ne-130 engine.

    “Though I couldn’t even pilot an ordinary airplane properly, I settled into the cockpit of the Karyū, and dreamed of the appearance of the real Karyū, not made of plywood.”

    -Yoshio Nakamura, member of the Army’s Ne-130 design team.

    The basic design drawings of Ki-201 were finalized in June, almost perfectly to schedule. The basic shape of Karyū almost perfectly matches its parent, though it is considerably larger in dimensions. This was a sharp contrast to the Navy’s ‘Kikka’, also developed at Nakajima — due to Kikka’s low thrust engines, it had to be designed as a very small aircraft in order to be practical. On the other end, with the development of high-thrust turbojets as the engine for Karyū, the domestic production of a larger jet like the ‘Me 262’ was possible for the first time. However, it was around this time that troubles with the development of these very engines delayed the projected completion of Karyū No. 1 to March 1946, with the full-scale mock-up to be reviewed in August of the preceding year.

    The detailed design of Karyū was begun in June, immediately after the basic stage was finalized. Though it bore an extremely similar resemblance to Me 262 externally, the detailed structure and materials were quite different due to the circumstances such as the lack of manufacturing plans and the severe material shortages at the time. Me 262’s construction had to be reverse-engineered manually using Japanese methods without any detailed design prints. One could say that the typical Japanese method of aircraft design was incorporated into the shape of the Me 262 to create the Karyū.


    Nakajima’s original designs were applied in areas including the canopy, lateral shape, and vertical tail. The main aircraft material was the lightweight duralumin alloy SDH, and other materials such as silicon-manganese steel, carbon steel, and tin were used in various components on a smaller scale. Just like the Me 262, the airframe structure is semi-monocoque, and the wings were of single-spar (with an ‘auxiliary’ spar) construction, with slotted flaps and leading-edge slats splitting around the engines. Two main fuel tanks of 1,200 liters were located in front and behind the cockpit, with a 600 liter auxiliary tank set behind the rear tank, for a total fuel capacity of 3,000 liters. All fuel tanks were self-sealing, and the main tanks were equipped with automatic fire extinguishers. An 8 mm steel plate is provided in front of the cockpit, with 8 mm at the back and 12 mm at the head of the seat. The front of the windshield is composed of 70 mm of bulletproof glass.

    Compared to Me 262 A, Karyū mounted engines of roughly the same power while increasing the size of the airframe. As such, the maximum top speed estimated by the designers was somewhat lower, though curiously it was projected to exceed Me 262 at extreme altitudes when utilizing Ne-130 engines. Karyū’s increased wing area granted it a lighter wing loading and a higher estimated climb rate.

    Me 262 A was well armed with a quartet of MK 108 autocannons in the nose for bomber interception, and Karyū, aiming to take down the B-29 bomber tormenting Japan, was similarly heavily equipped. The machine cannons on the lower outboard of the nose were 30 mm caliber, and the upper inboard two guns were 20 mm. For the Japanese Army, these guns were the Ho-155 Model II & Type2 respectively, powerful cannons loading fuseless shells able to down a heavy bomber in only a few hits. Both possessed a muzzle velocity roughly 200 m/s over that of the MK 108 and thus were more desirable for firing on air targets. Ki-201 would also be able to load a bomb as large as 800 kg, larger than the fighter-bomber Me 262 A-2a’s maximum bomb load of 500 kg, or a single 600l drop-tank for long-range missions. Radar ordnance consisted of a Ta-Ki Mk. 15 Friend-Foe Identification Radar, and a Ta-Ki Mk. 13 Low-Altitude Altimeter, both stored behind the cockpit along with the radio.

    The detailed design work on the Karyū continued throughout July, and basic aerodynamics examinations were completed together with the wind tunnel testing of scale models at around the same time. Construction preparations of the first prototype also began this month, immediately before the end of the war. With just five months elapsed from the start of the design to this point, the startlingly frantic pace of Karyū’s development can be seen.

    Unfortunately for Japan’s Me 262, it was on August 15th that the end of the war finally arrived. Although design work had progressed at a remarkably fast rate for the situation at the time, development was canceled here and the project ended wholly incomplete. If any actual manufacturing of components apart from the mockup preparation took place, it was not significant enough for the airframe to begin any considerable level of assembly. The IJA’s first and last jet fighter, Karyū, was never to grace the skies over Japan. This anticlimactic ending is a simple reality of most advanced wartime projects. It was a wasteful act for the Navy and Army to order Nakajima to develop a jet aircraft inspired by ‘Me 262’ at the same time, and Karyū’s development suffered as a result. In the end, had efforts been focused on one aircraft, more progress could have been made.

    The status points taken from data submitted by Nakajima Aircraft at the war’s end follow:

    • About 50% completion of the design
    • About 0% completion of the prototype
    • Status
      • Started manufacturing full-scale mockup.
      • Only full-scale construction drawings complete.

    The principle of Karyū was to create a high-performance jet aircraft sporting a devastating offensive armament capable of taking down the American Boeing B-29, as well as having the capability to equip a large bomb to attack the US fleet. Additionally, it was aiming to confront the Allied jet aircraft of a similar role developing at the time, such as the American Lockheed P-80 & British Gloster Meteor, noted by Nakajima. The prototype was to have been assembled near the Mitaka Institute, at a large hangar originally built for the canceled G10N “Fugaku” super-heavy bomber. The production of Karyū was scheduled to commence at the Nakajima Iwate factory, which was the dispersal factory of the Mitaka Institute.

    The Mitaka Institute was remodeled into the International Christian University after the war, and the prototype Karyū’s assembly-site-to-be is now occupied by only a thicket of trees.

    The head of examinations for the Ki-201 prototype was scheduled to have been Major Yoshitsugu Aramaki.

    Ki-201 (estimated) Main Specifications:

    (from June 1945 & August 1945 data sheets)

    DimensionsFull Width: 13.700 m
    Full Length: 11.500 m
    Full Height: 4.05 m
    Wing Area: 25.0 m2
    Mounted EngineNe-230 (x2): 885 kgf each
    or
    Ne-130 (x2): 908 kgf each
    WeightsEmpty Weight: 4,465 kg
    Equip. Weight: 2,497 kg
    Normal Load: 6,962 kg
    Special Load: 8,469 kg
    Top Speed
    Ne-230 (Ne-130)
    726 km/h (740 km/h) @ SL
    792 km/h (811 km/h) @ 6,000 m
    812 km/h (852 km/h) @ 10,000 m
    Wing Loading278.48 kg/m2Climb Rate
    Ne-230 (Ne-130)
    18.9 m/s @ SL
    726 km/h (740 km/h) @ SL
    792 km/h (811 km/h) @ 6,000 m
    812 km/h (852 km/h) @ 10,000 m
    Crew1 (pilot)Cruising Range100% Thrust: 794 km @ 8,000 m
    80% Thrust: 888 km @ 8,000 m
    60% Thrust: 980 km @ 8,000 m
    Fuel CapacityNormal Load: 2,120 l
    Special Load: 2,590 l
    Practical Ceiling13,600 m
    Oil CapacityNormal Load: 80 l
    Special Load: 100 l
    Never Exceed1,000 km/h
    ArmamentHo-155II 30 mm (120 x2)
    Type 2 20 mm (200 x2)
    or
    Type 2 20 mm (200 x4)
    TakeoffNormal Load: 200 km/h / 949 m
    Special Load: 210km/h / 1,580 m
    OrdnanceNo. 50 Bomb (500 kg) x1
    or
    No. 80 Bomb (800 kg) x1
    RadarTa-Ki 15 IFF
    Ta-Ki 13 Low Altimeter

    High-Power Engine Development for ‘Karyū’

    Both of the engines scheduled for Karyū, Ishikawajima Shibaura Turbine’s Ne-130 and 1st Munition Arsenal (formerly Nakajima Airplane)–Hitachi Manufacturing’s Ne-230, were at approximately the same stage of development when the war ended. Neither was ready for use. The larger and heavier Mitsubishi Ne-330, as previously mentioned, wasn’t considered for the final Ki-201. It is quite remarkable that the Japanese were able to engineer these turbojets, most famously the smaller Ne-20 for Kikka, with little more than a cutaway of a BMW 003 and even less material availability than Germany.

    Ne-201-II / Ne-130 (article)

    The first unit of Ne-130 was completed at the end of May 1945, and the team at Tachikawa tested it as far as 8,000 RPM when the engine heavily damaged itself. The cause was hairline fractures in the construction of the compressor blades, which caused the blades to splinter off during high-stress operations. The second engine was completed in early July and eventually successfully tested at full power in August. However, when testing again with accurate measuring equipment on August 16th, one day after the war’s end, the compressor blades were damaged by a foreign object being inhaled. Unit three was completed but had been destroyed on August 2nd when the Tsurumi factory was bombed. As such, there ultimately were no functional Ne-130 engines in the possession of the Japanese.

    Ne-130SpecificationsNe-230Specifications
    DimensionsLength: 3,850 mm
    Diameter: 850 mm
    DimensionsLength: 3,430 mm
    Diameter: 762 mm
    Weight900 kgWeight870 kg
    Thrust908 kgfThrust885 kgf
    Revolutions9,000 rpmRevolutions8,100 rpm
    Configuration7 stage axial compressor
    annular combustion chamber
    1 stage axial turbine
    Configuration7 stage axial compressor
    annular combustion chamber
    1 stage axial turbine
    Pressure Ratio3.56Pressure Ratio3.6
    Fuel Consumption1.39 kg/hr/kgfFuel Consumption1.84 l/hr/kgf
    Ne-230

    *Info about Ne-230 is scarce, and this section is not accurate to date.
    The first Ne-230 was completed at Mitaka in March 1945. Unit two was finished in May, with the final unit in June. During the testing at Takahagi, while applying countermeasures for faults in the engine’s testing, it is said that the engines (a number or all) were damaged by a bombing raid. No engine was transferred to the US for testing after the war, and as such it’s fairly likely that no functional engine survived the war. In late 2017 the parts of two Ne-230 engines were found in the International Christian University, which was formerly known as the Mitaka Institute. The remains included two nozzles and a cover. There is a possibility that these were not ever part of a functional engine, as they show no obvious signs of being bolted to other pieces. Of the late engines, only Ne-230’s drawing is not present.


    In the end, the only successful Japanese turbojet to reach mass production was the Ne-20. This engine was developed for the Navy’s Kikka, and was smaller and less powerful than the engines for the Army’s Karyū, providing only about 490 kgf of thrust. Development progressed quickly as a result, and Kikka flew for the first time in August 1945, the first and last Japanese turbojet aircraft to do so in World War II. From Kikka to Karyū, it could be said that the great driving force of the jet development program in Japan was always the inspiration of the “Me 262”.


    [Translation of Ki-201 Airframe Manual Here]


    Sources

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