Year: 2023

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

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.

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.

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.

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.

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.

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.

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.

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.

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.

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 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.

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.

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.

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.

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.

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.

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.
• Kaigun Kōkū Gijutsu-shō. Tōkyō: Gakken Plus, 2008.
• Tokko Issue 96: Kawamura, Iwao. Mysterious Ouka Model 43 Otsu Turbojet Special Attacker (First Part). 2013.
• Tokko Issue 97: Kawamura, Iwao. Mysterious Ouka Model 43 Otsu Turbojet Special Attacker (Last Part). 2013.
• Katō, Hiroshi. God Thunder Corps Record. Tokyo: Hobby Japan, 2021.
• Rep. 海軍試作機性能要目一覧表 [List of Navy Prototype Planes Performance Specifications], 1945.
• Rep. Japanese Power Plants For Jet Propulsion. 1946.
• Rep. Desc of Experimental Aircraft and Experimental Engines Under Development by the Japanese Army and the Imperial Japanese Navy. 1946.
• Rep. Full Picture of Navy Military and War Preparations Part 6 (War Preparations and Special Attack Preparations on Defeat). 1952.

*December 13th 2023: Corrected information about Wakazakura