Category: Army

This is an update to the previous article about the performance of the Type 4 Fighter (Ki-84), addressing some errors and new information that I have found since writing it.

It is not so much an article as a set of separate points.

The original article is here:

Ha-45 Special is Ha-45-11, not Ha-45-12

The engine fitted to the first three prototypes of the Ki-84 was designated the “Ha-45 Special” by the Japanese Army. Despite the name, it is in fact an inferior engine to the “Ha-45” (Ha-45-21). The “Ha-45 Special” is a Ha-45-10 series engine.

In the first article, I wrote that the “Ha-45 Special” was the Ha-45-12. In fact, it seems that the Ha-45 Special was actually the Ha-45-11. This is stated in the document「発動機名称一覧表」[Engine Designation List] created by the Navy Aviation Headquarters on April 11, 1945.

This is further supported by the document “Specifications of Special Army Planes” created by the Army Aviation Examination Dept on August 1, 1943. Here, the projected performance of Ki-84 is written to be 660km/h at 5700 meters, which is the critical altitude of the Ha-45-11. This document was captured and translated by the United States, and the relevant page is pictured below.

This is an important distinction, because the Ha-45-11 and Ha-45-12 have different altitude performance. The supercharger’s first and second speeds were stepped up 5.47⇒5.81 and 7.49⇒7.95, respectively, in the Ha-45-12. This raised the second speed critical altitude from 5700m to 6550m. Below is an excerpt from the Homare manual showing the specifications of the Model 11, 12, 21, and 22, as of December 1943.

It appears to check out that the initial prototypes were equipped with the Ha-45-11. The official performance of the Ki-84, which originates from a test with one of these prototypes, shows that the top speed was achieved at 6550 meters. This indicates that the critical altitude of the engine was increased by perhaps 850 meters due to ram pressure at the Ki-84’s top speed. It would make less sense if this plane was using a Ha-45-12, because that would indicate that there was no increase in the critical altitude from ram pressure.

Furthermore, we can see in the climb test obtained from the same aircraft that the engine began to lose manifold pressure just before 6000 meters when flying at low speed.

Ki-84 Prototype vs. Service Plane

The performance achieved by a Ki-84 prototype is considered to be the “official performance” of the Ki-84, and is written in its piloting manual. However, knowing that the Ki-84 prototype probably used a Ha-45-11 engine, we would expect the performance of a service plane to actually be somewhat different.

Mass production examples of the Ki-84 that were put into service used a Ha-45-21 engine (which the Japanese Army simply called Ha-45, or Type 4 1850 HP Engine). The Ha-45-21 was restricted to prevent engine failures, as is well known. The RPM was limited to 2900, and the manifold pressure to +250mm, just like that of the 10 series engines. With this, the critical altitude rose to about 6500m, which is essentially the same as the Ha-45-12.

Because the Ha-45-21 in its de-rated condition has (on paper) a critical altitude 800 meters higher than the Ha-45-11, as well as 60 more horsepower at critical altitude, we would expect a service Ki-84 to actually have a slightly higher top speed than the 624km/h of the Ki-84 prototype if it was operating in ideal condition.

Every Ki-84 Had Exhaust Thrust

This is not an error with the previous article but an elaboration. It is sometimes said that the early Ki-84s did not generate exhaust thrust. All units of the Ki-84 had exhaust pipes which produced thrust.

The initial prototypes and early pre-production units had a thrust-generating collective exhaust. Later, it was changed to individual thrust-generating exhaust pipes. We can be sure that the thrust force was improved to some extent, but it will not be as marked as the change from standard exhaust pipes to thrust-generating individual exhaust pipes.

For reference, the change from standard (non-thrust generating) exhaust to individual thrust-producing exhaust pipes netted the A6M5 about 10 kts (19km/h) of top speed, with its Sakae 21 engine that provided 980 HP at its top-speed altitude.

The Full-Rated Performance Gap

With a fully-rated Ha-45-21 engine, there exists a record showing a top speed of 634km/h for the Ki-84, which is referenced in the first article.

The idea that the Ki-84 only gained about 10km/h of top speed (624km/h ⇒ 634km/h) when using a “Ha-45” (Ha-45-21) engine seems difficult to believe to some. The fact that the prototype which achieved the lower speed probably had a Ha-45-11, rather than Ha-45-12 engine, makes this performance gap even more egregious.

It is relatively well accepted that the Ki-84’s propeller was inadequately sized for its class of engine, which greatly hindered its ability to make use of greater power. The Ha-45-21 had an increase in output of at least 120 horsepower compared to the Ha-45-12 their critical altitudes. However, the critical altitude of the 21 is about 400 meters lower than that of the 12 (ultimately, it is thought that the critical altitude of the fully-rated 21 was perhaps just 6100 m).

On the other hand, the Ha-45-11 not only has even less power (1440 HP at 2nd speed) than the Ha-45-12, but its critical altitude is lower than even the Ha-45-21, as previously noted. This means that the Ki-84 with a fully-rated Ha-45-21 engine had not only greater power, but at a greater altitude, and still only marked 634km/h.

Additional Evidence for Full-Rated Top Speed

The origin and condition of the fully-rated Ki-84’s 634km/h top speed record has been questioned, and though it is said to be a translated document, there is little contextual information about it.

However, there seems to be additional evidence supporting this fully-rated top speed. This comes from the document「陸軍航空技術沿革史」[History of Army Aviation Technical Development] which was published by the 1st Demobilization Bureau in May, 1947. Though it is post-war, this document was written by former staff of the Japanese Army. Within this document is the table「陸軍制式飛行機諸元表 (A)」[Army Service Airplanes Specification Table (A)].

This table lists performance for both the “Type 4 Fighter (Ki-84I)” and the “Type 4 Fighter Performance Improved“. The engine column shows that the former represents the Ki-84 with a de-rated engine, while the latter represents a plane with a fully-rated engine (referred to here as Type 4 1850HP Kai). These differing names are purely informal.

The top speed listed with the fully-rated Ki-84 is 635km/h at 6400m. The climb time is 5’50” to 5000m.

This compares closely with the values from the other fully-rated test mentioned in the last section, which showed a top speed of 634km/h at 6650m, and a climb time of 5’37” to 5000m.

I believe that 635km/h is a reasonable value for the “true performance” of the fully-rated Ki-84, on rated power. Use of “takeoff power” (essentially war emergency power, WEP) at altitude may have yielded a slightly higher top speed, but not markedly, as the critical altitude would be around 1000 meters lower.

If the Ha-45 engine was able to be produced with a compression ratio of 8.0, as was originally planned, the performance would have been greater, but this was not the case.

(The reduction of the Ha-45-21 CR from 8.0 to 7.2 is most likely the explanation for its fully-rated 2nd-speed horsepower output being reduced from the ballpark of the 1700s to the 1600s).

Expected vs. Achieved Performance

According to the “Famous Airplanes of the World” issue on the Type 4 Fighter, the required top speed issued for the Ki-84 was 680km/h, and the climb time was 4’30” to 5000m.

On the other hand, the “Specifications of Special Army Planes” document shows what seems to be a calculated or required top speed performance of 660km/h, along with the same climb time of 4’30” to 5000m.

Was the requirement reduced to 660km/h? Did Nakajima calculate 660km/h? Or was 660km/h simply the expected performance when equipped with the weaker 10-series engine? The latter would seem to be the obvious guess, but it’s not clear because the climb time remains the same.

If we interpret the expected performance of Ki-84 to be 660km/h, the actual plane seems to have fallen short by about 25km/h. The climb time, more clearly, was more than 1 minute slower than the requirement to 5000m.

Extra Explanation for the Small Performance Gap?

This last section is to be taken less seriously, and is not anything more than an idea. An awfully inadequate propeller could be the sole explanation for the lack of performance in the fully-rated Ki-84.

Anyway, the first prototype of the Ki-84 was initially completed with a smaller 19m² main wing according to numerous secondary sources, such as the “Famous Airplanes of the World” issue. All subsequent planes had a larger 21m² main wing, and it seems that the first unit was later modified to use the final wing.

I have not located a source which specifies which of the initial 3 Ki-84 prototypes achieved the 624km/h at 6550m top speed. If we consider the possibility that it could have been the first prototype with its original, smaller wing, it makes sense that the performance gap between this record and the fully-rated, longer-wing Ki-84 would be more negligible in terms of top speed.

It seems unlikely that the 19m² wing unit recorded this test, however, because this performance was later used as the “official performance” of the Ki-84 in general.

Sources

• Famous Airplanes of the World No. 19: Army Type 4 Fighter ‘Hayate’. Tōkyō: Bunrindo, 2019.
• Rep. CINCPAC-CINCPOA Translations & Interrogations No. 8. 1943.
• Rep. 誉発動機取扱説明書 [Homare Engine Operating Manual]. 1943.
• Rep. 「キ八十四」操縦法 [Ki-84 Piloting Manual]. 1944.
• Rep. 発動機名称一覧表 [Engine Designation List]. 1945.
• Rep. ADVATIS Translation 92: Specifications and Performance Data of Ki-84. 1945.
• Rep. 陸軍航空技術沿革史 [History of Army Aviation Technical Development]. 1947.

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.

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 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.

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.

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.

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.

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)

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-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

• Rep. German Technical Aid to Japan, 1945.
• Rep. キ201「火龍」要目, 1945.
• Rep. 研究試作部内調査表, 1945.
• Rep. Turbojets and Rocket Engines (JAF), 1945.
• Rep. Japanese Army Aircraft Production (Intelligence Memorandum No. 5), 1945.
• Rep. 極秘キ-201機体説明書, 1945.
• Rep. Airborne Radar: Operations and Tactics (Interrogation No. 493), 1945.
• Rep. Corporation Report: Mitaka Plant, 1945.
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