It is well established that the performance of Japanese aircraft engines in WWII was limited by the suboptimal fuels available at the time, among other factors. As a result, in order to achieve boost pressures in the ballpark of high-power Allied engines, it was necessary to rely on water injection even at nominal operation.
Nonetheless, aviation enthusiasts often speculate on the performance of Japanese aircraft if they were supplied with high-octane fuels and up-rated appropriately. This is partially due to the fact that there are widespread myths of superior performance numbers being achieved with Japanese aircraft using American high-octane fuel, when in reality, in most if not all cases these numbers are wartime calculations using inadequate data.
Homare
Nonetheless, it is an interesting question. In a past article on the Ki-84, I wrote that “it’s unknown how much additional pressure a Japanese mass-produced Ha-45 could even handle.” Thanks to an article I recently read in the book「日本航空学術史 1910-1945」(Aeronatical Researches in Japan 1910-1945), there is a satisfactory answer to this.
Single-Cylinder Test Units
From April 1939 to March 1944, the Engine Department of the Navy Aviation Technical Arsenal (Kūgishō) manufactured single cylinder testers and conducted tests on the power enhancement of aircraft engines using these machines.
In the past, these experiments relied on foreign imported single-cylinder testers manufactured by the American SPE Company (Self-Priming Pump and Engineering Company) and the German DVL (Deutsche Versuchsanstalt für Luftfahrtforschung). However, this posed a problem when experiments resulted in a broken part of the test units, as they were not domestically produced.
To solve this problem, it was decided to manufacture a domestic single-cylinder tester that could use Japanese mass-produced engine components. The single-cylinder tester was to have easily changed experimental parameters and be able to withstand harsh operation.
For changing the engine compression ratio, the DVL method was adopted in which the engine cylinder mounting base could be vertically adjusted with a handle. An SPE type balancing rod was used to manage the balancing of dynamic forces at high operating RPM. The valvetrain was designed so that it could be adjusted with a dolly lever so the angle of the valves and pushrods would not have to be changed according to whichever cylinder of an actual engine was installed, and the cams and dolly levers could be made to match each engine. Crankshafts were made in several different strokes with a shared diameter. The auxiliary shaft had a large amount of attachment points so that many engine accessories could be installed.
About 20 units were manufactured in total, with examples made for the Homare, Kinsei, Amakaze, and Kamikaze engines. These single-cylinder testers were distributed to fuel depots and the Central Aviation Research Institute. Later models were being developed for water-cooled engines and prototype engines, but were not completed due to the deteriorating war situation.
Power Enhancement Tests
For tests on enhancing the power of engines, the Kinsei and Homare units were chiefly employed. Various factors would be adjusted individually to increase power output such as the compression ratio, RPM, boost pressure, air/fuel mixture, ignition timing, and type of fuel. The results of testing were plotted on a graph with power as the horizontal axis and cylinder temperature as the vertical axis. The objective was to increase power with as little rise in cylinder temperature as possible, so a shallow curve was desired. Generally speaking, the most promising results were achieved by increasing the RPM or boost pressure.
With the Homare test unit, a two-stage roots supercharger with a supercharging capacity of up to +1500 mmHg boost (2.974 atm, 88.98 inHg) was attached, and tests were conducted using highly detonation-resistant fuel consisting of iso-octane mixed with 0.15% tetraethyl lead, benzol, toluol, and other aromatics, in conjunction with water-methanol injection. It was confirmed that the Homare could withstand being supercharged up to about +800 mmHg (2.053 atm, 61.42 inHg). This is the mechanical limitation of the engine.
As designed, the nominal operating boost pressure of the Homare Model 21 / Ha-45 engine was +350 mmHg (1.461 atm, 43.70 inHg), or +500 mmHg (1.658 atm, 49.61 inHg) at takeoff/emergency operation.
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.
Following the surrender of Japan in August 1945, the American occupational authorities sought to gather whatever aircraft developments were of potential interest. Naturally, the most cutting edge planes and engines developed in Japan were high on this list. More than a few of these projects had already been destroyed by Japanese orders immediately issued to prevent that from occurring.
None of the most powerful Japanese turbojets, Ne-130, Ne-230, or Ne-330, were left in Japan to recover. These prototype axial-flow jet engines were to be comparable in performance to the late German models, but each had been destroyed or hidden, one way or another, by September. Even the well known ‘Ne-20’, the turbojet of the Kikka, had risked destruction. The units under the Navy’s direct jurisdiction were sabotaged on such orders; luckily, a few survived at other organizations.
A couple of photos taken on October 16, 1945, show us some of the more obscure Japanese engines that were seized by the US. These photos display a group largely consisting of prototype jet engines awaiting preparation for shipment to the US by the 7th Air Service Area Command of the USAAF. The purpose of this article is to identify each engine shown in these photos and their subsequent fate.
Photos
Left side view of the engine roundup.Right side view of the engine roundup.
Almost every engine visible and identifiable in this roundup was designed, if not manufactured, at the Navy 1st Air Technical Arsenal (Kūgishō). The Kūgishō was a center of Japanese jet engine development until the end of the war. Here, pioneering efforts led by Tokiyasu Tanegashima from the year 1941 resulted in a variety of test engines. Only in mid-1944 did the jet engine receive appropriate attention from the upper brass of the Navy, after which increased funding and restless development by Tanegashima’s group managed to yield the successful flight of Kikka with its twin Ne-20 turbojets just before the end of the war.
Engine Identifications – Left Side
‘Sakae IPR’ Blower
First on this list is a very obscure, almost unknown jet engine project. No doubt inspired by the engine of the Caproni Campini N.1 which flew in 1940, the ‘Sakae IPR’ was a motorjet using a Sakae Mod.11 piston engine to drive a five-stage axial blower, which was followed by a burner. ‘IPR’ stood for “Internal Propeller Rocket”.
It was one of the very first Japanese air-breathing jet engines, built during 1943. This engine produced 604 kgf of thrust, and had large dimensions of 4.17 m length by .91 m diameter. It was designed and probably manufactured at the Navy’s Kūgishō.
Only the blower seems to have been recovered at the time of this photo. Though the Sakae IPR blower was possibly transported to the USA, it was likely of little interest, and no piece of it is known to survive today.
Sakae IPR in its complete form. The blower is the cylindrical centerpiece, facing the opposite direction of the US photo.
Ne-12
Next we have the Ne-12B, the last model in the original series of Japanese turbojets. This was a follow-on design to the original centrifugal ‘Ne-10 series’ turbojet. The four-stage axial compressor, clearly visible in this photo, leads into the main centrifugal compressor.
The Ne-10 Kai was the first model to implement this axial compressor, followed by the Ne-12 which featured reinforcements to improve durability and features for fixing to an aircraft. The final Ne-12B was a model which lightened the weight of the Ne-12 by as much as 70 kg. It is most likely that the engine in this photo is in fact the Ne-12B based on the date, but the visual differences from Ne-12 are unknown. All of these engines were designed at the Kūgishō.
The Ne-12B had a weight of 315 kg and dimensions of 1.80 m length by .86 m diameter. It ran at 15,000 rpm and aimed to produce 320 kgf of thrust. This engine only was able to demonstrate a lifespan of perhaps 1 hour by the end of the war, and had been abandoned in April 1945 after a production run of 12 units, split between the Kūgishō and the Yokosuka Arsenal.
This is, to my knowledge, the only known photo of the Ne-12/B. The engine did not survive to this day, but the 1st stage of a Ne-12B axial compressor is displayed at the National Museum of Nature and Science in Tokyo.
YE3B
Behind the Ne-12 is not a jet, but the YE3B piston engine, a very unusual design. This is a 24-cylinder liquid-cooled engine of the X arrangement, which aimed to produce 2,500 horsepower. It had been abandoned by the end of the war per US intel in favor of the YE3E, a 3,200 horsepower development of the same engine.
Based on very little documented data, the YE3B had cylinders with 145 mm bore and 160 mm stroke. The total displacement was 63.4 liters. It was designed and built by the Kūgishō. In this photo, the engine faces with the back side towards the camera, featuring the supercharger. The upper left row of cylinders and exhaust pipes are visible.
The engine survives today in storage under the ownership of the Smithsonian. Dimensional data of 2.29 m length by 1.75 m width is provided, which contrasts with the (likely erroneous) documented US data of 1.10 m length.
Tsu-11 / Hatsukaze Rocket
The small piston engine here is actually part of a jet. This is the ‘Hatsukaze’ engine portion of the ‘Hatsukaze Rocket’, a Japanese informal name of the Tsu-11 motorjet. Only the Hatsukaze itself is visible in this photo, but based on later photos, it can be confirmed that this is a Tsu-11 setup.
The Tsu-11 consisted of a Hatsukaze piston engine driving at 3,000 rpm, which was stepped up to 9,000 rpm to rotate a single-stage axial fan, followed by a burner. The overall setup weighed 200 kg and produced a very modest 220 kgf of thrust. The dimensions were 2.20 meters length by .64 meters width. Tsu-11 was designed by the Kūgishō but produced at Hitachi Aircraft.
This engine was only ever fitted to the Ōka Model 22 piloted missile, and the ‘Ginga’ bomber as a test auxiliary power unit. It had been contemplated as a temporary engine for the Kikka (in a quad installation of two per wing) if the main turbojets were not ready in time. The performance was very poor, the engine could not be started in the air, and would spontaneously seize at altitudes higher than around 4,000 meters. It did, however, provide the Ōka with a better standoff range than the previously used rocket engines. This would have improved the survive-ability of the mother plane, but perhaps not the missile itself.
A Tsu-11 survives today, installed in the sole Ōka Model 22 preserved at the Smithsonian National Air and Space Museum.
The exhaust nozzle of a Tsu-11 extending from the back of Oka Model 22.
Unidentified
The last two clear objects in this photo I have not identified, although it should be possible. These are a radial engine and an unknown engine to the right of it with an exposed propeller fitting, sitting behind the IPR blower and the Ne-12. If you can help identify these, you could leave a comment on this post.
The rest of the engines to the lower right of the whole photo are not included in this section, as they are more clearly visible in the right-side photograph.
Engine Identifications – Right Side
Ka-10 / Maru-Ka, & Small Model
Here is not only the Ka-10 pulsejet (also named ‘Maru-Ka’ ㋕), but almost hidden behind it, its smaller initial prototype version. These were the only pulsejets built by Japan during World War II. They were directly based on the German As 014 pulsejet used to power the V1 flying bomb.
The full-sized Ka-10 had dimensions of 3.70 meters length by .58 meters width, weighed 150 kilograms and was designed to produce 300 kilogram-force of thrust. The specifications of the smaller test version are unknown.
According to Japanese records, the small test model was completed in early 1945 and tested until June; the full-sized version followed it at the end of July, and remained under testing when the war ended in August. According to a member of the Army special weapons team, five units were built. This engine was to power the ‘Baika’: an ultra low-cost, manned flying bomb designed by Kawanishi Aircraft, which was similar in concept to the Fieseler Fi 103R ‘Reichenberg’, though somewhat more sophisticated in airframe design. However, the Baika had only lapsed one month of design progress when the war ended.
These are the only known photos of the Maru-Ka, which does not survive to this day.
Low-quality side view of both Maru-Ka.
KR10
To the right of the Maru-Ka is the rather well known KR10 liquid-rocket engine, which powered the Shūsui, Japan’s version of the Me 163B Komet rocket interceptor. Technically speaking, this could be the variant ‘KR20’, or ‘KR22’, which differed by thickening the turbopump shaft or increasing its structural support respectively. It is impossible to determine from this photo, but all versions of the engine are typically referred to as ‘KR10’ informally.
KR10 had dimensions of 2.52 meters length by .90 meters width, and weighed 170 kg. It produced 1500 kilogram-force of thrust, identical to the initial model of the German Walther HWK 109-509. The liquid fuel used consisted of the ‘Kō’ and ‘Otsu’ liquids, analogous to the ‘T-Stoff’ and ‘C-Stoff’ used in Germany.
The engine was designed by the Kūgishō with contribution from Mitsubishi, and was manufactured at the Kūgishō and various naval arsenals. The engine ‘KR22’ made by Hiro Naval Arsenal was the unit actually fitted to the Shūsui which flew on July 7th, 1945. This flight met with failure and death of the pilot due to the layout of the fuel system, which failed to feed with a reduced fuel load in a steep climb angle.
Ne-20
The Ne-20 turbojet is the most famous Japanese jet engine from World War II. It is typically referred to as “the first Japanese jet engine”; though it was not the first built by any measure, it is true in the sense that it was the first successful unit.
This engine was designed at the Kūgishō under the leadership of Osamu Nagano. Ne-20, based on the BMW 003A format, had an incredibly rapid developmental pace – advancing from merely a concept to an initial prototype in only three months, and passing trials in another three. Due to this impressive feat, the special attack plane ‘Kikka’ was able to successfully fly on August 7th, 8 days before the end of the war.
Ne-20 had dimensions of 2.70 meters length by .62 meters diameter. It weighed 470 kilograms, rotated at a maximum of 11,000 rpm, and produced thrust from 475 to 490 kilogram-force. The prototypes and first production engines were built at the Kūgishō, with additional production units being made at the Yokosuka Naval Arsenal. Around 20 examples are known to have been completed in total.
The engine in this photo appears to be marked ’19’. Perhaps this was the 19th engine, one of the production examples built at the Yokosuka Naval Arsenal. Some of these engines had been rejected due to poor workmanship related to the lack of experience building jets at the naval yard.
Three Ne-20 turbojets survive to this day: two at the Smithsonian National Air and Space Museum (one on display), and the other example at the Ishikawajima-Harima company museum.
‘Ne-201’ or ‘GTPR’ Turbine-Nozzle Mockup
The Ne-201 and the GTPR are practically unknown engines, especially in English sources. These were both turboprops, designed by the Army and Navy respectively from about the same time (~1942).
Ne-201 was designed by the Kogiken (Army Aero Tech Lab) and Kōken (Tokyo Imperial Uni Aero Dept), manufactured by Ishikawajima Shibaura Turbine. GTPR was designed at the Kūgishō, to be manufactured by Ishikawajima Shibaura Turbine as well. GTPR stood for ‘Gas Turbine Propeller Rocket’.
Both of these engines have been listed here due to the ambiguity of their history. I can state with certainty that the Ne-201 and GTPR were, at the outset, independent projects. However, a few accounts from first-hand suggest that they were the same thing, and a US report identifies this mockup as the ‘GTPR’ component, even though it almost exactly matches a known ‘Ne-201’ design drawing.
Currently, I’ve theorized that at the time of August 1944, when jet development between the Army and Navy was unified, the more developed turboprop project was probably taken (Army Ne-201), but placed under unified leadership. Thus, what was once just Ne-201 likely came to be known by either designation, and developed a bit further until the end of the war. This is only an assumption.
Clearer view of the Ne-201 (GTPR?) turbine/exhaust, with possibly the compressor section mockup in the background, which was also recorded as captured.
The Ne-201 had been built in 1944, and the original GTPR was ordered but never completed. Both designs were to be converted to turbojets in 1944, as priority was placed upon that type of engine, but ultimately Ishikawajima Shibaura Turbine created the turbojet ‘Ne-130’ from scratch. In December 1944 the Ne-201/GTPR damaged itself, by April 1945 it was ready for a re-test, but due to focus on the Ne-130 it received little attention until the end of the war.
Ne-201 had dimensions of 5.75 meters length by 1.10 meters diameter, and weighed 2,500 kilograms. It rotated at 4,200 rpm and produced 862 kilogram-force of thrust (prop 1870 shp/280 kgf + 582 kgf exhaust thrust). An iteration of the GTPR had dimensions of 5.50 meters length by .85 meters diameter and weighed 2,500 kilograms. It rotated at 5,000 rpm and aimed to produce 5,000 equivalent horsepower.
After the war, Tanegashima prepared a report on the GTPR for the US, although the details he provides are unlike either design. It is likely that as a personal passion project, he continued to work on the GTPR design aside more pressing matters until the end of the war, incorporating data learned from BMW 003A and Ne-20.
This turboprop mockup did not survive to this day.
Ne-30 & Ne-30 Mockup
The Ne-30 was one engine in the initial ‘Ne-10 series’ of Navy turbojets. It was a unique departure as an attempt to gain high thrust from the relatively low-performance engine design by upscaling it in size. Effectively, this was a larger Ne-12, with the same features. The left engine installed on a stand is the actual prototype, while the object to the right is the mockup. “Mock of TR30” is possibly written on the side.
The Ne-30 had dimensions of 2.47 meters length by 1.03 meters width, and weighed 900 kilograms. It rotated at 15,000 rpm and aimed to produce 850 kilogram-force of thrust. It was designed by the Kūgishō and built there in November 1944. However, the Ne-30 never demonstrated its intended performance and was abandoned, as with the other Ne-10 series engines. This engine had been contemplated as the original engine of the R2Y2, Keiun Kai, and the Tenga, a jet-version of the Ginga bomber.
Both the engine and its mockup were brought to the USA and still remain in the storage of the Smithsonian today.
Clearer view of the sole Ne-30 prototype.
Ne-10 & Ne-10 Exhaust Nozzle
This engine on the right seems to be the Ne-10, the first functional Japanese turbojet. This can be deduced by the apparent lack of axial compressor stages extending from the front side (which faces away from the camera). The visible side is the turbine at the rear.
The design of the entire Ne-10 series was, put simply, a huge turbocharger converted to a turbojet by installing a folded combustion chamber. First built in mid-1943 as the ‘TR’ (Turbine Rocket), the design was renamed ‘TR10’ in 1944 and prepared for mass production to perform trial-and-error testing. By the end of 1944, it had again been renamed as the ‘Ne-10’ due to unifying development with the Army, which created shared nomenclature.
On the left is presumably the exhaust nozzle to produce thrust from the Ne-10. You can see the attachment points both on the edges of the nozzle and the engine, circling the turbine. An early problem with the Ne-10 had been the nozzle warping into the turbine under heat due to a lack of resistant materials.
Ne-10 had dimensions of 1.60 meters length by .85 meters diameter. It rotated at 16,000 rpm and was designed to produce 300 kilogram-force of thrust. It was designed by Tanegashima’s group at the Kūgishō and only a handful were built, perhaps less than ten. It is somewhat surprising that an original Ne-10 survived to the end of the war, as these engines had a very short lifespan, and a tendency to fail disastrously.
No example of the Ne-10 survived to this day, nor any known components.
YE2H
Lastly, this engine is rather hard to spot. Behind the Ne-10 nozzle is the YE2H prototype – an 18-cylinder, liquid-cooled piston engine of the W-layout. Luckily, a view from the same side of the surviving engine provided by the Smithsonian shows identical features which can be compared to this image.
YE2H has dimensions of 2.46 meters length by 1.12 meters width, and weighs about 1,200 kilograms. The cylinders had the same 145 mm bores and 160 mm stroke as the YE3B shown prior, with a 47.5 liter total displacement. YE2H was to produce 2,500 horsepower. It was designed and built by the Kūgishō, and undergoing a breakdown test there when the war ended.
As mentioned, the YE2H survives today in the storage of the Smithsonian.
YE2H 18-cylinder liquid-cooled W engine. Photo: Smithsonian
Unidentified
The only object I cannot readily identify in the right-side view is this large jet exhaust nozzle. It is clearly considerably wider than the exhaust nozzle of the Ne-20 in the foreground, and even the centrifugal Ne-10. Unlike known exhaust sections from larger Japanese jets, the exit cone protrudes far from the end of the nozzle. It does not appear to be the exhaust nozzle for the Ne-30, nor Ne-130, or Ne-330.
I have speculated that this could be the turbine and exhaust section of the Ne-140, which was the huge turbojet converted from the GTPR turboprop design. However, it is unlikely that any part of the Ne-140 was built before development was apparently terminated in late 1944 or early 1945. The only source (of few overall) that contradicts this is Senshi Sōsho 87, which suggests that Ne-140 was tested by the end of the war, although I suspect that this is a mistake.
Alternatively, it could be possible that this is the turbine and exhaust nozzle of the MTPR, a compound “engine-turbojet” consisting of an Atsuta (DB601) piston engine which drove a prop, and also transferred some power to the compressor of a turbojet linked to it. According to limited information, MTPR was under construction from 1943 before being canceled in mid-1944.
*July 9th 2023: Correction on completion/test dates of Maru-Ka
This article is an attempt to clarify the details of the different training aircraft that were developed for the Japanese Navy’s late-war special attacker ‘Ōka’. The designations and purposes of the Ōka trainers are often confused, not only in English but even in Japanese publications. Using a few historical materials, we can correctly identify them and better understand their true details.
Although the focus of this article is on the Ōka trainers, a brief about the Ōka Model 11, the main mass production model, follows.
The Ōka Model 11 was a manned missile for attacking naval vessels developed by the Japanese Navy in August 1944 under the dire situation of the late war period. The project first received the secret designation ‘Maru-Dai’ (A circle or ‘maru’ around the kanji ‘dai’「大」), and although it was atypical to give Navy code-names to suicide aircraft, it was also designated ‘MXY7’ due to being created by the Navy 1st Air Technical Arsenal (Kūgishō). The Navy formal name ‘Ōka’「櫻花」(Cherry Blossom) was granted for service.
Ōka Model 11 in front of a pile of other dilapidated Ōka, warheads, and bombs.
This plane consisted of a tiny 6.06-meter-long by 5.12-meter-wide airframe with mid wings and a twin tail. It was constructed from duralumin, steel, and wood to conserve resources, and was designed to resist speeds up to 1000 km/h. Its sole armament was a 1.2 ton semi armour piercing warhead in the nose delivered by a ramming suicide attack, and the power plant consisted of three Type 4 Mk.1 Rocket providing 800 kg of thrust each with burn times of about 9 seconds, which were contained in the rear of the fuselage.
Ōka Mod.11 could not take off by its own power and was brought up to the target area by a G4M2e attack plane. The pilot had only the basic instruments and controls necessary to arrive at the target, and the operational range was poor: as little as 20 km when dropped from 3500 m. Due to this latter fact, Ōka could not be deployed effectively, and the mother planes were often intercepted before reaching the drop point. Later developments were centered around extending the range of the Ōka by using alternative jet power plants, but the war ended before any could be utilized.
The first examples of the Ōka were manufactured in September 1944, one month after the start of design. 755 examples of the Mod.11 had been constructed overall by March 1945 when mass production was terminated.
MXY7-K1ー Single Seat Ōka Trainer
Naturally, the training variant of the Ōka was developed at the outset of the project, as unlike conventional aircraft, the manned missile could not take off or land in its operational configuration. This initial training model received the code name ‘MXY7-K1‘, ‘K’ being the Navy code for training aircraft, and ‘1’ denoting that it was the first of this type (a successive two-seat trainer was already planned).
The MXY7-K1 had a few differences from the base aircraft in order to temper its flying characteristics for trainees. The wingspan was extended slightly by 12 centimeters, and the wings were equipped with flaps to decrease the landing speed. Inside the fuselage, which was extended by about 4 centimeters, the warhead & power plant areas were replaced by two water ballast tanks to maintain the proper weight and center of gravity. At the time of landing, these tanks were both dumped by the pilot to reduce weight and further decrease the landing speed. Even with the aforementioned measures, the landing speed was still a rather quick 203 km/h.
MXY7-K1
As for the means of landing, the MXY7-K1 was equipped with a central landing skid below the nose, similar to the method used with the Shūsui rocket fighter. The wings had a guard extending below each wingtip to stop the underside of the wing’s surface from being damaged as the plane leaned to one side and scraped against the ground after touching down.
The first manned test flight of the Ōka was via an MXY7-K1 and took place on October 31st, 1944, with test pilot Kazutoshi Nagano in control. At the drop altitude of 3,500 meters, the G4M released the trainer. The K1 immediately fell sharply from its mother plane but began to glide as the airspeed increase generated more lift from its small wings. Nagano quickly ignited the twin wing-mounted powder rockets, but due to unequal thrust causing the plane to yaw, he released them from their mounts almost instantly. The rockets, still burning powder, flew ahead of his aircraft while spewing smoke, which attracted alarm from the observers on the ground until the plane continued to fly as normal.
Nagano emptied the water ballasts as intended on the approach to the runway, and made a successful landing in front of the crowd of onlookers. Nagano had mainly praises for the aircraft, giving the opinions that the stability and control authority were perfect, that there were no problems with flight while emptying the ballasts, and that it could be used for training without issue. The wing-mounted rockets, however, did not function correctly due to unequal thrust, and were eventually abandoned.
A hangar containing many single-seat MXY7-K1 trainers after the war. Wing flaps, which were a feature unique to the trainers, are clearly visible in the second photo.
K1 trainers arrived at the 721st Naval Air Group (God Thunder Corps) at Kōnoike Air Base in November 1944, which would later become the first unit to operate the Ōka in combat. Here, the first landing-training test flight was conducted by Lieutenant Tsutomu Kariya on November 13th. The drop altitude this time was 3,000 meters, as the previous drop at 3,500 meters had initially frozen the ballast water. However, when Kariya began to dump the ballasts on his approach, the K1 immediately pitched sharply upwards, stalled, and fell from the sky. He could not recover flight, and crashed into the sand, flipping the K1 trainer end over end.
Lieutenant Kariya was still conscious when recovered from the trainer, but he died just hours later.
It was ascertained that Kariya’s crash was caused by pilot error: the front ballast was emptied before the rear one, the incorrect order, and so the accident occurred. But from this point onward, the water ballasts were no longer loaded during training. It was said that, in the official report, the true cause of the accident was likely the nose ballast leaking into the cockpit and blinding the pilot.
Training with the K1 continued immediately and through to the end of the war, initially for the combat operations of the Ōka Model 11, and later for the expected deployment of the Ōka Model 22 (a development to extend the range by using a motorjet engine). By the end of the war, 86 MXY7-K1 trainers had been produced, and out of the few hundred trainees, two deaths and two injuries occurred.
MXY7-K2ー Two Seat Interim Trainer?
The two-seat Ōka trainer is far less understood than its single-seat counterpart, and lots of misinformation floats around this aircraft. The names ‘K1 Kai’ and ‘Wakazakura’ are frequently used to designate this plane in English, but its actual name is ‘MXY7-K2‘. This can be verified by the original nameplate on the rear-left of the surviving example, which is under the ownership of the National Air and Space Museum.
MXY7-K2. A two-seat Ōka trainer produced experimentally.
MXY7-K2 is almost universally described by secondary sources (English and Japanese alike) as the trainer for the ‘Ōka Model 43 Otsu’. The Model 43 Otsu variant of the Ōka was much larger than the preceding models (8.16 meters long by 9.00 meters span), and operated completely independently by launching from land catapults. Utilizing a single Ne-20 turbojet engine for propulsion, it also had a far superior range. The Ōka Model 43 Otsu was expected to correct the problems with the previous models and become the primary special attacker for the final defense of the Japanese home islands. Coastal catapults were constructed around various expected areas of the US invasion fleet. But the war ended before a single Model 43 Otsu finished construction.
The K2 does exhibit some specific features that would seem to imply that it was developed for this task. The wing span of K2 is ~7 meters, significantly larger than the ~5-meter wings of the Model 11 & K1. It could easily be assumed that this wingspan was chosen to emulate flight characteristics closer to the Model 43, which had a ~9-meter wingspan. Also, it is plausible to speculate that the two-seater layout was chosen for safely instructing trainees with the unfamiliar takeoff method of rocket catapulting. Lastly, it could be equipped with a single Type 4 Rocket in the rear for extending the glide range.
From the rear view of each aircraft, you can see the difference in wing length between Ōka Mod.11 and K2. The exhaust venturi at the rear of K2 is partially visible here.
Based on contemporary evidence, however, I would like to present the theory that the MXY7-K2 was only a two-seat trainer for the Ōka Model 11, and was not developed for the Model 43.
To start off, when did the K2 originate? As the development of the Ōka Model 43 Otsu only began in March 1945, this would seem to be an easy point to immediately separate K2 from being a ‘Model 43 Trainer’. While it’s not totally clear from the materials available to me, it is certain that a two-seater Ōka was planned from essentially the very beginning of development:
In September [1944], 9 dummy planes, 1 actual single-seater, 2 two-seaters, and 5 trainers will be manufactured.
Results of a General Staff meeting on August 28th, 1944, quoted in ‘Senshi Sōsho 45’
The precise date of when the K2 was actually completed is yet unknown, but based on this schedule, it appears that the construction of two double-seat Ōka was thought to be imminent just before September. It’s known that initial Ōka prototype constructions proceeded smoothly. This also coincides with the fact that only two examples of K2 are known to have been completed overall.
Both MXY7-K2 trainers seized by US authorities.
Before continuing with the historical analysis, there are also physical features of K2’s airframe relevant to this theory. There is clearly a loop for mounting K2 to a mother plane located between its two canopies. Such a feature would be unnecessary on a dedicated trainer for the Model 43, which only launched from catapults. Furthermore, the scale of the K2, though larger than K1, is not consistent with the giant Model 43 – at roughly 6.4 meters long by 7 meters wide, it’s almost 2 meters shorter in length and span. The fuselage in particular is clearly a direct adaption of the Model 11 design.
The point where K2 seems to become related to the Ōka Model 43 is during June 1945. At this time, the design of Model 43 was already completed, and production plans were progressing. Starting on June 27th and lasting two days, the K2 was launched for a series of very successful flights using the rocket catapult for Mod.43 constructed on the shore of Takeyama. The pilot was Commander Hiromitsu Ito, and the observer seat was occupied by Ōka’s chief designer Tadanao Miki.
“How about starting an aerial sight-seeing company with this plane after the war is over!”
-Commander Ito quoted in ‘Thunder Gods’
One fact that seems to be disregarded, however, is that in the recollections of this event, the K2 is described as “a two-seater Model 11 training plane”. This poses another question, though: If the K2 was truly built prior to the design of Model 43 as a ‘Model 11 trainer’, why would it have been designed with the capability to launch from Model 43 catapults, and utilized in these tests?
The answer to this can be derived from the text of a slightly later document, Av HQ Aero Secret No. 5392 from July 24, 1945, which concerns the development of a two-seat trainer for the Ōka Model 43:
In relation to Chiefs of Staff Aero Secret No. 823, conduct testing research after modifying to allow launching from the experimental rocket catapult, evaluate the practical two-seater, and obtain improvement data.
Extract from ‘Av HQ Aero Secret No. 5392’, quoted in ‘Mysterious Ōka Model 43 Otsu Turbojet Special Attacker (First Part)’.
The wording of this document is a bit vague, but it seems to state that the decision to develop a two-seater Mod.43 trainer only occurred in July 1945, after the catapult test of the MXY7-K2. Furthermore, this document concerns modifying a single-seat Ōka Model 43 trainer design to the newly decided twin-seat type, but also seems to state that an aircraft should be modified to allow catapult launching, to ‘evaluate the practical two-seater’. This likely just means to adjust the future Mod.43 trainer for launching as necessary — Regardless, it definitively separates the two-seater Mod.43 trainer as a later aircraft from K2.
In summary, based on the existing evidence from the period, my theory is that ‘MXY7-K2’ was only a prototype two-seat trainer for the Ōka Model 11. Due to the rapid construction of Ōka Mod.43 rocket catapult sites in 1945 before aircraft could be completed, modifications were done to allow K2 to be catapulted from these sites for early evaluations. For this purpose it was ideal due to having two crew. As K2 was not the true Model 43 trainer, mass production did not proceed afterward. At the end of the war, the US recovered the sole two MXY7-K2 trainers at the Kūgishō, of which the most intact example was sent to the US and remains in the ownership of the Smithsonian NASM.
The intact MXY7-K2 trainer was shipped to the United States after the war. The right image shows K2 next to a Type 4 Heavy Bomber on the carrier USS Core.
Wakazakuraー Ōka Model 43 Otsu Catapult Trainer
*Dec 13, 2023: Information on Wakazakura updated.
Having tentatively concluded that the trainer for the Ōka Model 43 Otsu was not the MXY7-K2, let’s establish what the Mod.43 trainer actually was. In truth, there are almost no materials in my possession to define the Mod.43 trainer with, save for one primary document which coincides with the previous data, and largely is the reason I am confident in this theory. It’s the ‘Navy Prototype Planes Performance Chart’ from August 22, 1945, submitted by the Kūgishō to the US authorities following Japan’s surrender.
At the bottom of this document, a two-seat trainer named ‘Wakazakura’「若櫻」(Young Cherry) is vaguely outlined, which seems likely to be the two-seat trainer of Ōka Model 43 Otsu.
Name
Experimental Wakazakura
Maker
Kūgishō
Format
High[wing] – Mono[plane]
Crew
2
Span (m)
9.000
Length (m)
9.000
Height (m)
3.200
Empty Weight (t)
.600
Gross Weight (t)
.750
Engine
Powder Rocket
Summary
Training glider for catapulting
Progress (Schedule)
Start: 07/1945 | Unit 1: 11/1945 | Finish: 03/1946
Status
Being designed
‘Chikara’ two-seat acrobatic glider.
The Wakazakura is otherwise briefly described by a few Japanese secondary sources. It was a modification of the Navy glider ‘Chikara’ (Power). The Chikara was designed and first built by Japan Small Airplanes in 1941 as a two-seat trainer for the experimental ‘MXY5’ transport glider developed by the Kūgishō. Two pilots are seated in tandem for the purpose of training to be towed, gliding, and landing. The Chikara was rather large for a glider, with a wingspan of 11.25 meters, and a length of 8.8 meters. The empty weight of the airframe was 326 kilograms, while loaded it weighed 516 kilograms. The structure was made of wood but designed to a high strength for unlimited aerobatic potential. The main wheels for landing were semi-recessed into the fuselage, while a skid was positioned below the nose.
Right side view of ‘Chikara’ showing the cockpit.
Therefore, if the specifications given in the previously mentioned Kūgishō table are correct, the design of the ‘Wakazakura’ differed in the following manner from the Chikara. The wingspan was reduced from 11.25m to 9.0m, which coincides with the wingspan of the Ōka Model 43 Otsu. The length slightly increased to 9.0 meters, probably due to the installation of rocket(s) (unclear if a single or multiple). Also due to the powder rockets, the empty weight had increased by about 274kg, and the loaded weight by about 234kg.
It is stated in the 88th volume of Senshi Sōsho that the Wakazakura was to be used as an intermediate trainer for not only the Ōka Mod.43, but also the Kikka. This is not clarified by other sources. Regardless, the Wakazakura was also to utilize the same powder rocket catapult system as the K2 and the Ōka Mod.43. The training grounds was to be at the Mt. Hiei catapult site, where the training of the Ōka Mod.43 was being organized.
Rather important for this theory is the design starting date listed as July 1945 in the Kūgishō table, which coincides with the documented decision to create a two-seater Ōka Model 43 trainer, as explained in the previous section. Furthermore, the initial prototype was only expected to be completed by November 1945, which is quite late. Under this situation, it seems apparent why it was necessary to modify the K2 for testing the rocket catapults beforehand.
A cliff-side catapult for launching Ōka Model 43 Otsu was completed on Mt. Hiei near the end of the war.
For training the pilots of the Ōka Model 43 Otsu, the 725th Naval Air Group was formed at Shiga on July 1, 1945, to operate from the Mount Hiei catapult site. Just before the end of the war, a wooden model of the Mod.43 Otsu was loaded onto this catapult and launch-tested with rocket propulsion. The necessary adjustments were made to the catapult system, the glider landing zone was constructed, and the pilots waited for the arrival of Wakazakura trainers.
Thankfully, the war reached its conclusion before the deployment of Ōka Model 43 Otsu. Not a single prototype of the actual plane nor its Wakazakura trainer was fully completed by the end of hostilities on August 15th, leaving behind little material evidence for researchers. With such little clarifying data and prominent misinformation, it’s easy to see how the unusual ‘MXY7-K2’ and the scarcely documented ‘Wakazakura’ are typically conflated even to this day.
Sources
Nomura, Minoru. Senshi Sousho 45, Imperial General Headquarters Navy Department/Combined Fleet (6), Third Stage Operations Late Period. Tokyo: Asagumo. 1971.
Senshi Sousho 88.
Naito, Hatsuho. Thunder Gods. New York: Kodansha International. 1989.
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.
By the 1930s, the practical speed limit of piston-engined aircraft was being approached for the first time. Before a propeller-driven airplane can approach the speed of sound, there is a period where the blade tips must reach supersonic speed, resulting in massive turbulence and loss of efficiency. This effect imposes a rough ‘limit’ on the horizontal top speed of piston-engined airplanes at around 800 kilometers per hour. Globally, the Macchi M.C.72 seaplane set an airspeed record of 709.2 km/h in 1934, followed by the 755.1 km/h record set by a Messerschmitt Me 209 in 1939, which remained the piston airspeed record for the following 3 decades, illustrating the difficulty to advance near the practical limit.
The World’s First Turbojet W.U. 1.
The limits of piston-engined aircraft were the main drive behind the development of the gas turbine or jet engine, discarding the propeller, which rapidly loses thrust at high speeds, for propulsion by the exhaust of a high-speed jet of heated gas. This method of propulsion would eventually enable aircraft to reach much higher speeds with greater efficiency than propeller planes.
Early developments of gas turbine propulsion began to take shape in Europe by the 1930s and culminated with the nearly simultaneous prototyping of the ‘Whittle Unit’ in Britain and the ‘HeS 1’ in Germany; the first two examples of the turbojet engine, in 1937. Further development to make such designs practical resulted in the flight of the first jet plane, Germany’s Heinkel He 178, in 1939, followed by the Britain’s Gloster E.28/39 in 1941.
As research advanced overseas, the situation within Japan, which up to this point had only recently caught up to the conventional aero-engine technology of the Western world, was isolated. While the experimentation into jet engines in Britain and Germany eventually resulted in widespread development and practical applications in these countries, globally it was largely unknown and had not made an impression. Because the jet engine was not yet attractive to the Japanese military, and companies were tied up in the development of conventional piston engines regardless, the conception of the Japanese jet engine was left to pioneers working independently on their own initiative.
Early Interest Within Japan
Kouichi Hanashima
Interest in Japan regarding technology related to the jet engine dates back as far as the 1920s. Around this time, Navy Lieutenant Kōichi Hanashima (花島 孝一) was interested in turbines and other rotating machines from Europe. In 1920, shortly after his promotion to Lieutenant-commander, he went on a business trip to France. During this trip, Hanashima purchased 10 Rateau turbochargers for the Suiza engine and returned to Japan with these souvenirs the same year.
Hanashima, who graduated from the Navy Engineering College at the top of the class, was a well-educated man who always had a keen eye on the latest developments of aviation technology. He foresaw possibilities in the application of this technology, however, at that time he could not promote domestic interest in this field. One Rateau turbocharger ended up in the possession of the Tokyo Imperial University for study.
In December 1930, now a Navy Captain, Hanashima became the General Manager of the Yokosuka Arsenal Engine Testing Department. At that time, the group under him only consisted of Lieutenant Toshio Kondou and a few other members. In 1932, the various Navy aviation research departments such as the Engine Testing Department and Aircraft Testing Department of the Yokosuka Arsenal, and the Giken Aviation Research Department at Kakamigahara, were reorganized into the Yokosuka Air Arsenal, where Hanashima retained his position in the Engine Department. Over time the facilities of the arsenal were expanded, and by 1936 it was almost complete.
1930 Whittle patent turbojet.
Early European jet development was now in swing, and at the time, somewhat public, due to lack of government interest. As fragments of this information began to trickle into Japan from overseas, the attention of a few researchers in Japan was caught. Rear-admiral Hanashima, who by 1936 had become aware of jet patent literature from the likes of Robert Goddard, Frank Whittle, and Secondo Campini, was highly captivated with the concept. He quickly formed a research group, and with the cooperation of Tokyo Imperial University and Mitsubishi Heavy Industries, began studies on the jet engine. While European researches at that time were largely focused on the motorjet and turbojet configurations, Hanashima was most interested in the simple ramjet format.
Tokyo Imperial University started the study of propulsive ducts in 1937. This theoretical research was mainly headed by Professor Keizo Hatta and Professor Fujio Nakanishi. Attention was focused on the ramjet, and experiments were conducted relating to airflow through various ramjet internal ducts. One of their students, Yoshitarō Hibi, was highly interested in the experiments, and after graduating from the Tokyo Imperial University joined Mitsubishi and started working with ramjets. Experiments with scale models in external airflow were conducted in 1939 and later.
The theoretical and material development that goes into the research of jet engines was not so high in Japan at that time, due to its isolated position from the rest of the technological world. Due to this, the aforementioned research works were slow, troubled, and full of unknowns.
However, the situation changed when Tokiyasu Tanegashima arrived on the scene around 1938.
Tokiyasu Tanegashima
Tokiyasu Tanegashima
Tokiyasu Tanegashima (種子島 時休) was born in Yokosuka City on July 20, 1902. A descendant of the Tanegashima clan, his ancestry included Tokitaka Tanegashima, who is famous for introducing the firearm to Japan in the 1540s. Due to this, he picked up the nickname ‘Teppou’ (gun) in his childhood. Mr. Tanegashima entered the Naval Engineering College in 1918, graduated in 1921, and served on the largest Japanese battleship at that time ‘Mutsu’, becoming interested in power plants while often visiting the boiler room. In 1927, he discovered the book “Steam and Gas Turbines” by Aurel Stodola, which enclosed an explanation of the gas turbine engine at that time.
Afterward, Tanegashima entered the Navy Air Service and was appointed to investigate aircraft engines. He passed the technical course of the Navy Staff College in 1929 and entered the Tokyo Imperial University Aeronautics Department the next year. Here he studied the basics of aircraft engines, including early research regarding gas turbines, which was highly rare at the time. He graduated from the Aeronautics Department in 1933 and became the head of the Hiro Navy Arsenal Engine Department Assembly Plant, meeting Osamu Nagano (永野 治), his subordinate and later an invaluable designer of jet engines. Tanegashima was ordered to travel to Paris in 1935 to survey aero engines and fighter-mounted machine guns, touring various related companies throughout Western Europe.
Brown-Boveri Gas Turbine (1938).
Tanegashima used this time to also investigate the situation of the gas turbine in Europe, but he was dissatisfied by the lack of information in France, Germany, Italy, and England. However, in 1936 while visiting Switzerland for the Oerlikon company, Tanegashima visited Brown-Boveri, an electrical engineering company that was involved in the manufacture of turbines for general industry. The chief engineer Mr. Klingerfuss displayed drawings of gas turbines, as well as an actual example of an exhaust gas turbine. Tanegashima ordered Brown-Boveri to manufacture two turbochargers for 1,000 hp aircraft engines at 10,000 yen each and was recalled to Japan in 1937 enthusiastic about the possibilities of gas turbines.
Upon his return, Tanegashima was immediately appointed to Yokosuka Air Arsenal (henceforth ‘Kūgishō’) as the Chief of Service Engineers for the 1st Engine Factory, chief of the factory. The main responsibilities of the Chief of Service Engineers were to examine and test prototype engines, improve existing engines, perform failure tests to apply countermeasures, maintain engines for flight testing planes, and support experiment planning. So the 1st Engine Factory was constantly tied up with work applying countermeasures for various engine malfunctions, which were very serious and labor-intensive due to the pilots trusting their lives in the reliability of those machines.
However, around the time of 1940, complaints about aircraft engines disappeared due to war enthusiasm, and Tanegashima used this newfound free time to study the gas turbine concept closely in his lab. The second volume of the English translation of “Steam and Gas Turbines”, which could be considered Tanegashima’s bible, was always on his desk. During the early period, working only with his assistant engineer Watanabe Sadaki, he created two crude apparatus for confirming the theoretics of jet ducts and combustion chambers. The results were satisfactory, which reinforced the feeling that it was possible to manufacture a jet engine. Gradually he took in more engineers that gathered around in interest, such as Osamu Nagano, who was assigned to the same department.
Jumo 205C.
Commander Tanegashima’s greatest interest was mainly in the format of a propeller-driving gas turbine (turboprop) supplied by a free-piston compressor. At that time, the Navy had been developing the top-secret flying boat ‘H7Y1’ as a reconnaissance aircraft intended to make a round trip from the Marshall Islands to Hawaii. In order to achieve this range requirement of 5,000 nautical miles, four Jumo 205C opposed-piston engines imported from Germany were employed as a result of their low fuel consumption. However, when the prototype was completed in 1939 it suffered from many issues including lack of strength and rigidity, insufficient engine output, poor maneuverability, and more.
The prototype was canceled in July 1939, and all related materials including drawings and photos were destroyed, but the Jumo 205C engines were left over. The format of this engine was good for Tanegashima’s purposes, and he started a project with Mitsui Co. in 1940 to prototype a free-piston compressor by diverting the cylinders of the Jumo 205C.
Osamu Nagano
However, the design of the compression piston diverted from the Jumo 205’s opposed pistons required a large amount of design innovation, and while the project was idling, Commander Osamu Nagano who worked in the Engine Department under Tanegashima designed a tiny model gas turbine with a free-piston. In 1941, it was manufactured by Mr. Masanori Miyata, head of the electronics factory, and after work applying corrections, it was able to reciprocate smoothly and operate continuously.
This micro gas turbine only had an output of 1/10th horsepower, but it was able to demonstrate itself by driving a small magneto to light an electric lamp, and inspired hope and courage in the young engineers working day and night. Although it was only a ‘toy’, it was technically the first gas turbine manufactured in Japan, and sadly disappeared in time without even a photograph.
The Tanegashima Group, 2nd Research Branch
Gloster E.28/39, the first British jet plane.
Then came about the 8th of December 1941, when the Japanese Navy conducted an attack on the United States Navy at Pearl Harbor. Tanegashima recalled an overwhelming sense of dread that a giant was being awoken. Around this time, he had also learned from a Navy intelligence officer that England had tested a jet plane earlier in May. After a few days, Tanegashima decided that the jet engine must be developed at any cost to vastly increase the performance of aircraft, lest Japan fall far behind the western world in aviation technology which would undoubtedly spread from England to the USA. He immediately appealed to his superior, chief of the Engine Department Rear-admiral Kiyoshi Matsukasa, to be able to focus solely on this subject. The request was immediately accepted, and in January 1942 Tanegashima was appointed as the chief of the 2nd Research Branch focused on jet engine development, with several skilled engineering officers and roughly 200 workmen at his disposal. One of these engineers was Shigeo Katō, a 1st Technical Lieutenant who was said to have an even deeper understanding of the gas turbine than Tanegashima.
Caproni Campini N.1
After the failure to manufacture a free-piston based on the Junkers engine, Tanegashima had realized that the current Japanese industry was unable to manufacture such a machine. During the middle of the year 1942, it was learned that the Italian Caproni Campini N.1 had flown without a propeller, garnering much public interest, and it was also disclosed by confidential means that the Germans had tested a jet plane long ago. Thus, the focus drifted away from the free-piston, and eventually it was shelved in favor of the conventional turbojet engine, which was speculated to be the power plant of what was the ‘He 178’.
Axial turbojet explanatory diagram from the first theoretical report on the turbojet in Japan, April 1942.
During the early phase of official jet engine development under Tanegashima’s Group, several theoretical reports on different types of jet engines and gas turbines were published. The main types studied were the TR (turbine rocket, now turbojet), GTPR (gas turbine propeller rocket, now turboprop), similar compound variations, and the Campini type (motorjet). From these reports, it can be seen that Tanegashima was far in favor of the axial compressor, as opposed to the centrifugal-type common among the early jet engines of the world. However, the quickest way to procure an experimental jet engine was to convert from a turbocharger, which contain a centrifugal compressor.
In August 1942, one Hitachi turbocharger for a 2,000hp aero-engine was converted to a jet by installing a combustion chamber. During wind tunnel testing, though, it could not self-drive with combustion due to fundamental differences between a small turbocharger and a jet engine. This small unnamed machine, albeit a failure, was the first attempt to prototype a turbojet engine in Japan.
The First Japanese Turbojet Spins
TR. The first functional Japanese turbojet engine.
A much larger turbocharger designed by Shigeo Katō was delivered from Ebara during the next month. It was the YT15, a huge device with a turbine diameter of 600 mm and compressor diameter of 500 mm designed for engines of 2,500 hp at altitudes of 15,000 m. However, due to low turbine strength, the YT15 was not adopted as it was. Instead, Katō proposed its conversion into a turbojet due to its large size, and the conversion started at Ebara in February 1943. A complicated folded combustion chamber was installed due to the small distance between the compressor and turbine, and the exhaust pipe was remade as a jet nozzle. The initial engine was completed in June 1943 as Japan’s first functional turbojet, ‘TR’. The design targets were 16,000 RPM, 4.0 pressure ratio, and 300 kg thrust output.
Thrust
300 kgf (Plan)
Revolutions
16,000 RPM
Pressure Ratio
4.0 (Plan)
Format
1C-AN-1AT
TR Specifications
Testing with TR started in July when the engine was placed at the mouth of a wind tunnel at 5 m/s wind speed. When it started to turn, vaporized fuel was injected into the combustion chamber and ignited with an electric spark. When the wind was stopped, all that was left was the hum of TR operating on its own power. The fuel quantity was increased, and the engine rapidly reached a speed of 14,000 – 17,000 RPM with a pressure ratio of 3.5, but there was no nozzle, so thrust was not produced. After about 5 minutes of full-power operation, it was stopped. Cracks were observed in the turbine blades.
Nonetheless, Tanegashima, Katō, and all were elated, as their theories had been proven. Now what was left was to produce a reliable jet engine, which proved to be the longest road of all.
One day, when running at full operation, there was a tremendous explosion and the TR was left in ruin. The centrifugal compressor, which was only made of cast aluminum due to lack of strong light alloy, burst into three pieces. One pierced the tin roof of the test bench, and two stuck into the surface plate on the floor. Fortunately, nobody was injured.
Example of turbine failure, Whittle Unit 3
Soon, the second TR engine was delivered from Ebara. Changes to this unit included a forged compressor wheel and slightly reinforced turbine blades. During the first test of unit two, it was carefully brought up to maximum driving speed, then stopped after a few minutes and completely disassembled to inspect its condition. Once again, there were cracks in the base of the turbine blades. However, there was no time to perform repairs, so in order to record the thrust as quickly as possible, Tanegashima ordered Katō to mount the tailpipe and scraped off the cracked parts of the turbine with a file.
The engine was brought back up to full power operation, and as expected, the weakened turbine blades splintered off. Still not stopping the experiment, Tanegashima cut off the blades diagonal to those lost during operation and continued to drive it at full power several times. As the number of blades decreased, the backpressure increased and the RPM of the turbine decreased, but the corresponding thrust was still generated. Eventually, only a bladeless and tattered turbine wheel remained, but the generation of 250 kg of thrust had been confirmed.
After that, the third unit was built and testing continued for several months. However, constant issues including cracking of turbine blades, failure of compressor bearings, burn-through of the combustion chamber folds, and distortion of the nozzle plate into the turbine were inescapable, and the environment around Tanegashima began to sour as people became fearful of the dangerous engine. To counteract the complaints, sandbags were piled around the unit in a ‘bunker’, and viewing during operation was done through a mirror placed behind it.
Improvement of the ‘Turbine Rocket‘
Ne-10 design drawing.
Up until this time, the Navy had generally been indifferent to the jet engine experiments conducted by Tanegashima, and even planned to dissolve his research branch at one point in order to contribute to more practical areas.
However, in the early summer of 1944 a group of elite air officers stepped into a class where Tanegashima was teaching jet propulsion to officer candidates. Interest among the higher officers of the Navy had been piqued by reports concerning the application of jets in Germany. Immediately work on jet engines was given high priority, and the head of the Kūgishō, Vice-admiral Misao Wada ordered the work to be completed as soon as possible, instructing other departments to assist the development.
In July 1944 the experiments with the first 3 TR engines concluded, and the results plainly showed that there was still a long road to practicality. In an attempt to remedy the various issues, the design was improved and renamed ‘TR10’, and the Navy General Staff issued a rapid improvement plan. TR10 was ordered to be mass prototyped with iteratively refined designs in 70 units by 6 companies (Kūgishō, Mitsubishi, Nakajima, Hitachi Manufacturing, Ishikawajima Aircraft, Ishikawajima Shibaura Turbine) from the beginning of July to the end of August 1944 for rapid trial and error. The 6 companies formed 3 groups for production: Mitsubishi & Nagoya Kobe, Nakajima & Hitachi, and Ishikawajima Aircraft & IST. Each group was expected to complete 20 engines, with the Kūgishō completing another 10, for the 70 total.
Dimensions
Length: 1,600 mm Diameter: 850 mm
Weight
250 kg
Thrust
300 kgf
Revolutions
16,000 RPM
Pressure Ratio
3.5
Format
1C-AN-1AT
Ne-10 Specifications
The Navy had suddenly promoted such an unlikely plan due to enthusiasm about combat reports of the German jet fighter ‘Me 262’. Unfortunately, there was a lack of confidence in the design of the obviously unfinished TR10 throughout the manufacturing companies, even if it was improved, and most companies were already overwhelmed with work on piston engines. As a result, the mass-prototype plan progressed painfully slowly. Only a few units seem to have been built.
Furthermore, at the end of that July, Commander Eiichi Iwaya returned from Germany with a cutaway drawing of the BMW 003A turbojet engine that was in practical use at the time, garnering a massive interest from the Army and Navy. The companies initially assigned to mass-prototype the TR10 were instead assigned three different large-scale turbojets for jet fighters, and the TR10 rapid improvement plan was all but canceled, once again solely in the hands of the Kūgishō. At this time, the jet development of the Army and Navy was merged into a joint effort, and the TR10 was renamed once again to the joint designation ‘Ne-10’ (‘Ne’ for ‘nenshō’, combustion).
Ne-10 without tailpipe.
When Tanegashima observed the BMW 003A drawing, he immediately recognized that it was of the same principle as the Ne-10, but employed a 7-stage axial compressor and lower turbine RPM, which greatly reduced the stress on the turbine blades. However, Tanegashima considered himself to be an ‘experimental researcher’ pioneering new technology by his own intuition, and recognized that if the Ne-10 was to be practicalized in a timely manner, a professional designer was necessary.
So, even before the first ‘Ne-10’ engine was completed in September and proved to be unsatisfactory, Osamu Nagano joined the Tanegashima group in August and started the design of a new engine in the series, Ne-10 Kai (Ne-10 Improved). Ne-10 Kai employed a 4-stage axial compressor ahead of the centrifugal compressor, which lowered the rotations of the turbine to 15,000, a much-needed stress reduction.
At the same time, a huge version of Ne-10 Kai called ‘Ne-30’ with a powerful projected output of 850 kgf was also planned. Furthermore, during October another version of the Ne-10 Kai with two fuel pumps for mounting to an aircraft (G4M) was designed, called the ‘Ne-12’.
Large centrifugal turbojet Ne-30, after the war.
The first unit of the Ne-10 Kai, Ne-12, and Ne-30 were all completed in November 1944. One more unit each of Ne-10 Kai and Ne-12 were completed before the end of that year. By this point, the material strength situation had improved somewhat. During running tests to gather data, these two engine models apparently managed 30 minutes of full-power operation before turbine cracking began, and the planned thrust output of about 320kgf was reached. However, the same issues such as damage to the compressor, unstable combustion, and turbine cracking continued, so this result was still far from satisfactory. The future of the Ne-10 series was in doubt.
On the other hand, the massively upscaled Ne-30 was deeply troubled for the same fundamental reasons as the rest of the series, only magnified by the increased size of the turbine and other components, and never could operate at full power before its abrupt cancellation.
The End of theNe-10 Series
Ne-12B design drawing.
The final model of the Ne-10 series, the ‘Ne-12B’, was a finished design at the end of December 1944. This version most notably reduced the weight of Ne-12 from 388 kg to 315 kg and further reinforced the various components.
At the same time, Tanegashima expressed that the Ne-12 and its series were fundamentally flawed, and if the jet engine was to become practical, it was necessary to pursue a design based more closely on the format of the German BMW 003 type.
“I would like to express my gratitude for the efforts that brought Ne-12B to this point, but it seems that this engine is underdeveloped. At this time, it would be wise to tear down all of the past and restart with reference to the BMW 003A.”
Tokiyasu Tanegashima, Wagakuni ni okeru Jetto Enjin Kaihatsu no Keika
As it was, the strength of materials available at that time in Japan simply could not bear the heat and stress placed upon the turbine of a centrifugal turbojet, even with the additive compressor, and there were many other unsolved issues such as combustion problems which would be negated by adopting the German straight flow style. Furthermore, Tanegashima always believed that the pure axial compressor was the best method from the beginning, and the decision to use a centrifugal compressor was only due to the time and manufacturing constraints.
Kikka Ne-12 rough sketches.
Between the arrival of BMW 003A’s drawing in Japan to this point, the Kūgishō had been conceptually drafting a new engine model based on the Ne-12. This engine called ‘Ne-15’ had the same performance requirement, but featured an 8-stage axial compressor derived from the BMW 003A format. By December, this plan had evolved into the ‘Ne-20’ with an improved projected output of 480kgf. Now all efforts were to be focused on the prototyping and testing of Ne-20, and the design improvement of the Ne-10 series was put on hold.
Even if the development of the ‘Ne-10’ series was discontinued, around this time the prototype of the jet-propelled special attack plane ‘Kikka’ was announced. Although heavily flawed, ‘Ne-12’ was the most advanced turbojet model available in actual testing, so there was no choice but to ignore the problems for the time being and select it as the powerplant. Furthermore, manufacturing and testing models of the Ne-12B as a sort of ‘prototype’ for the Ne-20 could provide valuable developmental data, as it was the most improved version of the Ne-10. For these reasons, the production of Ne-12B engines was ordered from the Yokosuka Yard, Hokushin Electric Works, Ishikawajima Shibaura Turbine, Masada, and Ebara. This order was very reminiscent of the ‘TR10’ mass-prototyping plan, and unsurprisingly, similarly failed to proceed.
Dimensions
Length: 2,102mm (Ne-12) / 1,800 mm (Ne-12B) Diameter: 855 mm (Ne-12 & Ne-12B)
Weight
388 kg (Ne-12) / 315 kg (Ne-12B)
Thrust
315 kg (Ne-12) / 320 kg (Ne-12B)
Revolutions
15,000 (Ne-12 & Ne-12B)
Pressure Ratio
1.67 x 2.0 (Ne-12 & Ne-12B)
Fuel Efficiency
1.65 kg/h/kgf (Ne-12B)
Format
4A-1C-AN-1AT (Ne-12 & Ne-12B)
Ne-12/Ne-12B Specifications
From January to April 1945, just 12 units of the Ne-12B were constructed, 6 by the Kūgishō and 6 by the Yokosuka Navy Yard, with evidently none being built by private organizations. The first unit was completed in February. After some time, one unit was able to operate at full power for an hour, but the characteristic flaws such as combustion issues and turbine cracking continued. Even after almost two years of development labor, the final model of the ‘Ne-10’ series, with many incremental improvements, was little different from the first in practice.
The final nail in the coffin for the Ne-10 series came with the testing of the first unit of ‘Ne-20’ in late March 1945. Very quickly this format of an engine, running out of a cave behind the Kūgishō, proved to be more reliable, durable, and powerful than the entire centrifugal series before it. This is not to imply that the Ne-20 did not have its own set of issues, but these were far more manageable. The last tether of the Ne-12, its application to the special attacker ‘Kikka’, was severed on the 19th of April during a meeting at the newfound Jet Department of the Kūgishō, the results of which were phoned from the Navy Air HQ to Nakajima the same day.
“It was decided that Kikka will be equipped with Ne-20. Carry forward the prototype according to that point.”
Mr. Nozaki of Navy Air HQ to the 2nd Mfg. Plant
As a result, the production of the Ne-12B stopped in April. The development of the first Japanese turbojet series was over without practical use. Nonetheless, the Ne-10 series was an impressive technological endeavor from a nation that had only recently become aeronautically independent, with essentially no outside assistance and a lower level of materials science than the western world at the time. These engines provided invaluable data and experience that boosted the indigenous development, which continued at the Kūgishō throughout 1945 until the end of the war. The Ne-20 engine was advanced from the start of design to completing service trials in a staggeringly quick 6-month period – perhaps the fastest turbojet development process of the war globally – and famously succeeded in flight mounted below the wings of the ‘Kikka’ on the 7th of August.
Fate of the Ne-10 Series
Left image: Ne-12B (upper right). Right Image: Ne-10 (centre-left) Ne-30 and mockup (upper right).
The whereabouts of these engines after the war are poorly recorded. In the photographs taken by the US Navy at the Kūgishō, among the various jet engines of the department, the following of the Ne-10 series are approximately identified by the author: 1x Ne-10, 1x Ne-30, 1x Ne-30 Mockup, 1x Ne-12(B).
Ne-12B 1st compressor stage. From the 4th Ne-12B completed by Yokosuka Yard.
Each of the Ne-10 and Ne-12 turbojets seem to have been scrapped at some point following the conclusion of the war. However, the 1st stage axial compressor fan of a Ne-12 remains preserved at the National Museum of Nature and Science in Tōkyō, Japan. This item bears the inscription “12B NO 4”, seemingly identifying it as part of the 4th Ne-12B made by the Yokosuka Yard.
Surprisingly, the largest of the Ne-10 series, ‘Ne-30’ still survives to this day. The sole prototype and its mockup have ended up in the storage of the Smithsonian Air and Space Museum, where they remain. At the time this article was written, Ne-30 is not displayed publically.
Unfortunately, apart from these three artifacts, no other remains are known to exist of Japan’s first turbojet series.
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