The A6M4 is an “unknown” variant of the Zero Fighter that has been described as a variety of things over the years. The most common theory in English writing is that A6M4 was a designation for a type of Zero fitted with a turbocharger to its Sakae engine. Another common theory is that the number was skipped to avoid using the unlucky number “4”, which can be pronounced the same way as “death” (shi) in Japanese language.
A handful of original Japanese documents exist which can be used to paint a vague outline of what the A6M4 actually was. This article serves as a summary of the various wartime references to an “A6M4”, or otherwise a “Model 40” Zero Fighter, that are known to me.
A6M4 as a Turbocharged Zero Fighter
In the February 1942 Arsenal Gazette of the Navy Aviation Technical Arsenal (Kūgishō), a report dated February 4th outlined a research meeting that was to take place on the 9th of the same month at 15:00. The subject of the meeting was a structural examination of a partially wooden model of a Sakae Model 10 engine fitted with a turbocharger. The purpose was for the eventual installation of a turbocharger in the Zero Fighter.
The next reference in the Arsenal Gazette was dated February 10th, and it outlined the schedule of the first research meeting concerning the fitting of the Zero Fighter with an Ishikawajima turbocharger. This meeting was to take place on the 19th of the same month at 13:00.
It should be noted that these brief meeting schedules did not state which model of Zero Fighter was to be equipped with the turbocharged engine, nor was a designation for this prospective variant given. However, at this time, the engine under examination was a Sakae Model 10-series, which is of the same series as the engine installed in the A6M2.
The next reference to the turbocharged Zero Fighter is more well-known: an English translation of a Japanese document that was captured on Saipan, titled “Quarterly Report on Research Experiments,” and dated October 1, 1942. This is where the first mention of the “A6M4” as a turbocharged Zero variant is known to appear, and the relevant text is in the images below.
Assuming that the translator did not make a typing error (which is more common in translated documents than you may hope), at this time, the “A6M4” was a designation for a development of the A6M3 with an intercooler — almost certainly indicating that it would have a turbocharged engine. The next step in development was for wind tunnel testing to be carried out, according to the record.
We can be fairly sure that this “A6M4” was a development of the A6M3, because it is written under the general section concerning the “Type 0 Mark 2 Carrier Fighter”, which was an earlier designation of the Zerosen Model 32 (A6M3).
On the other hand, Francillon wrote the following entry in his 1967 title “The Mitsubishi A6M3 Zero-Sen (Hamp).”
Francillon stated that Jirō Horikoshi (the head designer of the Zero Fighter) personally informed him that the A6M4 designation referred to two A6M2s that were fitted with a turbocharger in 1943. Unsurprisingly given the situation of turbocharger implementation in Japan at that time, this testing was described as a failure.
Whether it is the case that the turbocharged Zero Fighters were converted from the A6M2 or A6M3, there is reasonable evidence here to state that the “A6M4” designation was most likely applied to the turbocharged Zero Fighter project by late 1942. However, it seems that upon the failure and abandonment of these prototypes, the designation was later re-used for other projects.
Turbocharged Sakae Engine
A Sakae Model 11 engine with a wooden mockup of a turbocharger installation attached to it is pictured below. This is likely to be the same model that was examined at the Kūgishō in early 1942. It can be observed that the compact installation model did not actually include an intercooler at this time.
The turbocharger represented by this model was the Ishikawajima Model IET4. The Model IET4 was designed to maintain the full pressurization of the Sakae engine up to an altitude of 7000 meters.
Fake A6M4
The image on the left below is supposed to show an A6M2 with a turbocharger. In fact, to this day it is a top image result when searching online for “A6M4” or “Zero with turbocharger”. However, it’s just a photoshop of an image of a standard A6M2, compositing a picture of the turbocharger from the Army prototype fighter Ki-87.
A6M4 (?) as an A6M2 with Belt-Fed Guns
Another document possibly related to the “A6M4,” titled “Matters Pending Approval Regarding Airplane Remodeling Experiments,” was created on April 28th, 1943. This document shows various tentative decisions regarding aircraft models in development, including future variants of the Zero fighter.
The following relevant text is quoted:
Type 0 Fighter
(a) Zerosen Model 21
Changed 20mm fixed machine gun ammo capacity from 100 rounds per gun to 150 rounds per gun (belt feed). Provisionally designated as Zerosen Model 41 and ordered to Nakajima.
(b) Zerosen Model 22
(1) Changed 20mm fixed machine gun ammo capacity from 100 rounds per gun to 150 rounds per gun (belt feed). (2) Abolished wingtip folding mechanism and shorten wingspan by about 1 meter. Provisionally designated as Zerosen Model 52 and ordered to Mitsubishi.
Therefore, we can say that as of April 28, 1943, it had been tentatively decided to give the designation “Zerosen Model 41” to a Zerosen Model 21 (A6M2) fitted with belt-fed 20mm machine guns. The designation “Zerosen Model 52” was to be tentatively provided to a Zerosen Model 22 (long-wing A6M3) with belt-fed 20mm machine guns and shortened wings.
In the Japanese Navy’s aircraft naming nomenclature at the time, the first numeral of the model number represented airframe modifications, while the second number represented engine modifications. According to this system, the reasoning behind these provisional designations is briefly as follows:
Model 21 + 1 airframe modification (belt-fed guns) ⇒ Model 41 (3_ is skipped because it is occupied by the shortened square wing modification of Model 32).
Model 22 + 2 airframe modifications (belt-fed guns, shortened round wingtips) ⇒Model 52 (3_ is skipped for the same reason as prior, because Model 32 had shortened wings with square wingtips, while Model 52 had round wingtips).
No code names are listed in this document, and just because it was a “Model 4_” Zero Fighter does not mean that it would necessarily be designated A6M4.
Furthermore, there is no known information which would suggest that the Model 41 was ever built. The Model 52, of course, would go on to be mass produced.
However, when the Model 52 was actually adopted into service on August 23rd, 1943, it was recorded that its prior tentative designation had still been the “Type 0 Ship-Based Fighter Model 22 Kai”, as can be seen in the following document (which also shows the adoption of the Gekkō Model 11).
Type 0 Ship-Based Fighter Model 22 Kai (shortened wingtips of main wing) is adopted as a weapon and designated as Type 0 Ship-Based Fighter Model 52.
A6M4 as an Early Name for A6M5
To quickly recap from the previous section, this is what had been tentatively decided as of April 1943:
Model 2X: 12 meter wingspan with folding wingtips.
Model 3X: 11 meter wingspan with square wingtips, no folding mechanism.
Model 4X: 12 meter wingspan (Model 2X airframe) with belt-fed 20mm MGs.
Model 5X: 11 meter wingspan with round wingtips, no folding mechanism, and belt-fed 20mm MGs.
In reality, the belt-fed 20mm MG (that is, the Type 99 20mm Mark 2 Fixed Machine Gun Model 4) was not actually ready in time for the mass production of the Model 52.
So while the Model 5X had been defined as having 2 modifications over the Model 2X, belt-fed 20mm guns (which is what brought it to 4X) and clipped, rounded wingtips, the actual first production Model 52 or A6M5 only had the latter modification.
This is important to consider when looking at the next and final document regarding the A6M4, which is a translated document about aircraft armament, captured on Peleliu. The data comes from a Japanese notebook and was probably created in mid-1944. Here the “Model 42” is listed, with the code name “A6M4” specified.
Above the Model 42 is two Model 52 (A6M5) with differing armaments. The top Model 52 would later be known as the Model 52 Otsu, or A6M5b, and the lower Model 52 is the standard initial production model without belt-fed MGs – its designation would not change.
Relevant trivia: the designation system that introduced the ability to define minor “subvariants” of Navy aircraft with “Kō, Otsu, Hei…” was only introduced in November 1944. Before this, all Model 52 armament varieties were simply “Model 52”.
As we can see in the document above, the A6M4 has the same armament as the initial A6M5: the Type 99 20mm Mark 2 Fixed MG Model 3, which is not belt-fed. Unfortunately this table is focused purely on armament, so there are no other details to compare.
So, what is the A6M4?
Considering that this Model 42 clearly does not have belt-fed MGs, we can assume that the concept of the “Model 4X” constituting “a Model 2X airframe with belt-fed MGs” had been abandoned at this time. So we may forget that the “Model 41” was ever proposed, which leaves an empty space for an airframe modification in the Zero Fighter’s designation list.
The Model 52 was originally defined as having both belt-fed guns and the clipped, rounded wingtips, but was at first produced with only the latter modification. When changing to belt-fed guns justified an increase in airframe model number in the first place, logically, losing the belt-fed guns would seem to justify regressing the model number by one. As the Model 41 was abandoned without being constructed, there is no conflict.
Therefore my theory is that the Model 42 (A6M4) is the initial Model 52 (A6M5). In my assumption, it was most likely a tentative designation, and was renamed as “A6M5” to avoid unnecessary confusion.
CONCLUSION
In consideration of all of the above, I would say that the “A6M4” is:
A provisional designation that was never officially adopted, and that was used by at least two models of the Zero Fighter at different points in time.
The sparse available evidence suggests that “A6M4” once referred to both the turbocharged Zero, and later the initial model of the A6M5, but neither was ever set in stone.
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 aType 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
Caliber
75 mm
Length
Overall
3,892 mm
Cartridge
75 x 497 mm R
Barrel
3,312 mm
Muzzle Velocity
720 m/s
Weight
Overall
740 kg
Rate of Fire
20 rpm
Barrel
490 kg
Max Effective Range
1,500 ~ 2,000 m
Mount
250 kg
Capacity
15 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
Name
Prototype
Ki-109
Engine
Name
Type 4 1900 HP Engine (Ha-104)
Service
n/a
Output(T.O.)
1,900 hp @ 2,450 RPM
Dimensions
Length
17.950 m
Output(Nom.)
1,810 hp @ 2,300 RPM (2,200 m) 1,610 hp @ 2,300 RPM (6,100 m)
Span
22.500 m
Height
5.800 m
Performance
Top Speed
550 km/h @ 6,090 m
Wing Area
65.85 m2
Climb
n/a
Weights
Empty
7,424 kg
Range
2,200 km
Loaded
10,800 kg
Ceiling
n/a
Wing Loading
164 kg/m2
Armament
Guns
Ki-109 Mtd. Cannon (7.5 cm) x 1 Type 1 12.7mm Flex. M.C. x 1
Crew
4 (pilot, loader, gunner, radio)
Production Ki-109 after the surrender. The propellers were removed. The forced cooling fans in the cowls are clearly visible.
This Ki-109 is wrapped up and painted with a spray gun for transportation.
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.
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
Name
Internal
Y30
Engine (x2)
Name
MK10C Ha-42-21
Code
R1Y1
Prototype
Experimental Gyōun
Output (TO)
2,500 hp @ 2,800 rpm
Dimensions
Length
15.0 m
Performance
Top Speed
648 km/h @ 6,000 m 685 km/h @ 8,000 m
Span
19.0 m
Wing Area
50.0 m2
Range
7,408 km @ 4,000 m
Weights
Empty
10,500 kg
Armament
Type 2 13 mm Flexible MG (400 r/g) x1
Loaded
14,000 kg
Crew
3 (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
Name
Internal
Y40
Engine
Name
Ha-70-01 (AE1T Twin)
Code
R2Y1
Prototype
Experimental Keiun
Output (TO)
3,400 hp @ 3,000 rpm
Dimensions
Length
13.050 m
Output(Nom.)
3,000 hp @ 8,000 m
Span
14.000 m
Performance
Top Speed
741 km/h @ 10,000 m
Height
4.240 m
Climb
21’0″ to 10,000 m
Wing Area
34.0 m2
Range
3,611 km
Weights
Empty
6,015 kg
Ceiling
11,700 m
Loaded
8,100 kg
Armament
none
Overload
9,400 kg
Crew
2 (pilot, recon)
Wing Loading
238.2 kg/m2
‘Keiun’ Reborn as a JetAircraft
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’sNe-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
Name
Internal
Y40
Engine (x2)
Name
Ne-330
Code
R2Y2
Output (Static)
1,320 kgf @ 7,600 rpm
Prototype
Experimental Keiun Kai
Output(Nom.)
990 kgf @ 740 km/h, 7,600 rpm (2,680 hp)
Dimensions
Length
13.050 m
Performance
Top Speed
783 km/h @ 6,000 m 741 km/h @ 10,000 m
Span
14.000 m
Height
4.240 m
Climb
7’0″ to 6,000 m
Wing Area
34.0 m2
Range
1,269 km
Weights
Empty
5,700 kg
Ceiling
10,500 m
Loaded
8,850 kg
Armament
none, later up to 1 ton of weapons
Overload
9,950 kg
Crew
2 (pilot, recon)
Wing Loading
260.3 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 destruction of the first Keiun at Kisarazu AB. The extended prop shaft sits nose-down in front of the airframe.
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.
Two views of an unfinished Keiun airframe.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.
