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.
This is an update to the previous article about the performance of the Type 4 Fighter (Ki-84), addressing some errors and new information that I have found since writing it.
It is not so much an article as a set of separate points.
The engine fitted to the first three prototypes of the Ki-84 was designated the “Ha-45 Special” by the Japanese Army. Despite the name, it is in fact an inferior engine to the “Ha-45” (Ha-45-21). The “Ha-45 Special” is a Ha-45-10 series engine.
In the first article, I wrote that the “Ha-45 Special” was the Ha-45-12. In fact, it seems that the Ha-45 Special was actually the Ha-45-11. This is stated in the document「発動機名称一覧表」[Engine Designation List] created by the Navy Aviation Headquarters on April 11, 1945.
This is further supported by the document “Specifications of Special Army Planes” created by the Army Aviation Examination Dept on August 1, 1943. Here, the projected performance of Ki-84 is written to be 660km/h at 5700 meters, which is the critical altitude of the Ha-45-11. This document was captured and translated by the United States, and the relevant page is pictured below.
This is an important distinction, because the Ha-45-11 and Ha-45-12 have different altitude performance. The supercharger’s first and second speeds were stepped up 5.47⇒5.81 and 7.49⇒7.95, respectively, in the Ha-45-12. This raised the second speed critical altitude from 5700m to 6550m. Below is an excerpt from the Homare manual showing the specifications of the Model 11, 12, 21, and 22, as of December 1943.
It appears to check out that the initial prototypes were equipped with the Ha-45-11. The official performance of the Ki-84, which originates from a test with one of these prototypes, shows that the top speed was achieved at 6550 meters. This indicates that the critical altitude of the engine was increased by perhaps 850 meters due to ram pressure at the Ki-84’s top speed. It would make less sense if this plane was using a Ha-45-12, because that would indicate that there was no increase in the critical altitude from ram pressure.
Furthermore, we can see in the climb test obtained from the same aircraft that the engine began to lose manifold pressure just before 6000 meters when flying at low speed.
Ki-84 Prototype vs. Service Plane
The performance achieved by a Ki-84 prototype is considered to be the “official performance” of the Ki-84, and is written in its piloting manual. However, knowing that the Ki-84 prototype probably used a Ha-45-11 engine, we would expect the performance of a service plane to actually be somewhat different.
Mass production examples of the Ki-84 that were put into service used a Ha-45-21 engine (which the Japanese Army simply called Ha-45, or Type 4 1850 HP Engine). The Ha-45-21 was restricted to prevent engine failures, as is well known. The RPM was limited to 2900, and the manifold pressure to +250mm, just like that of the 10 series engines. With this, the critical altitude rose to about 6500m, which is essentially the same as the Ha-45-12.
Because the Ha-45-21 in its de-rated condition has (on paper) a critical altitude 800 meters higher than the Ha-45-11, as well as 60 more horsepower at critical altitude, we would expect a service Ki-84 to actually have a slightly higher top speed than the 624km/h of the Ki-84 prototype if it was operating in ideal condition.
Every Ki-84 Had Exhaust Thrust
This is not an error with the previous article but an elaboration. It is sometimes said that the early Ki-84s did not generate exhaust thrust. All units of the Ki-84 had exhaust pipes which produced thrust.
The initial prototypes and early pre-production units had a thrust-generating collective exhaust. Later, it was changed to individual thrust-generating exhaust pipes. We can be sure that the thrust force was improved to some extent, but it will not be as marked as the change from standard exhaust pipes to thrust-generating individual exhaust pipes.
For reference, the change from standard (non-thrust generating) exhaust to individual thrust-producing exhaust pipes netted the A6M5 about 10 kts (19km/h) of top speed, with its Sakae 21 engine that provided 980 HP at its top-speed altitude.
The Full-Rated Performance Gap
With a fully-rated Ha-45-21 engine, there exists a record showing a top speed of 634km/h for the Ki-84, which is referenced in the first article.
The idea that the Ki-84 only gained about 10km/h of top speed (624km/h ⇒ 634km/h) when using a “Ha-45” (Ha-45-21) engine seems difficult to believe to some. The fact that the prototype which achieved the lower speed probably had a Ha-45-11, rather than Ha-45-12 engine, makes this performance gap even more egregious.
It is relatively well accepted that the Ki-84’s propeller was inadequately sized for its class of engine, which greatly hindered its ability to make use of greater power. The Ha-45-21 had an increase in output of at least 120 horsepower compared to the Ha-45-12 their critical altitudes. However, the critical altitude of the 21 is about 400 meters lower than that of the 12 (ultimately, it is thought that the critical altitude of the fully-rated 21 was perhaps just 6100 m).
On the other hand, the Ha-45-11 not only has even less power (1440 HP at 2nd speed) than the Ha-45-12, but its critical altitude is lower than even the Ha-45-21, as previously noted. This means that the Ki-84 with a fully-rated Ha-45-21 engine had not only greater power, but at a greater altitude, and still only marked 634km/h.
Additional Evidence for Full-Rated Top Speed
The origin and condition of the fully-rated Ki-84’s 634km/h top speed record has been questioned, and though it is said to be a translated document, there is little contextual information about it.
However, there seems to be additional evidence supporting this fully-rated top speed. This comes from the document「陸軍航空技術沿革史」[History of Army Aviation Technical Development] which was published by the 1st Demobilization Bureau in May, 1947. Though it is post-war, this document was written by former staff of the Japanese Army. Within this document is the table「陸軍制式飛行機諸元表 (A)」[Army Service Airplanes Specification Table (A)].
This table lists performance for both the “Type 4 Fighter (Ki-84I)” and the “Type 4 Fighter Performance Improved“. The engine column shows that the former represents the Ki-84 with a de-rated engine, while the latter represents a plane with a fully-rated engine (referred to here as Type 4 1850HP Kai). These differing names are purely informal.
The top speed listed with the fully-rated Ki-84 is 635km/h at 6400m. The climb time is 5’50” to 5000m.
This compares closely with the values from the other fully-rated test mentioned in the last section, which showed a top speed of 634km/h at 6650m, and a climb time of 5’37” to 5000m.
I believe that 635km/h is a reasonable value for the “true performance” of the fully-rated Ki-84, on rated power. Use of “takeoff power” (essentially war emergency power, WEP) at altitude may have yielded a slightly higher top speed, but not markedly, as the critical altitude would be around 1000 meters lower.
If the Ha-45 engine was able to be produced with a compression ratio of 8.0, as was originally planned, the performance would have been greater, but this was not the case.
(The reduction of the Ha-45-21 CR from 8.0 to 7.2 is most likely the explanation for its fully-rated 2nd-speed horsepower output being reduced from the ballpark of the 1700s to the 1600s).
Expected vs. Achieved Performance
According to the “Famous Airplanes of the World” issue on the Type 4 Fighter, the required top speed issued for the Ki-84 was 680km/h, and the climb time was 4’30” to 5000m.
On the other hand, the “Specifications of Special Army Planes” document shows what seems to be a calculated or required top speed performance of 660km/h, along with the same climb time of 4’30” to 5000m.
Was the requirement reduced to 660km/h? Did Nakajima calculate 660km/h? Or was 660km/h simply the expected performance when equipped with the weaker 10-series engine? The latter would seem to be the obvious guess, but it’s not clear because the climb time remains the same.
If we interpret the expected performance of Ki-84 to be 660km/h, the actual plane seems to have fallen short by about 25km/h. The climb time, more clearly, was more than 1 minute slower than the requirement to 5000m.
Extra Explanation for the Small Performance Gap?
This last section is to be taken less seriously, and is not anything more than an idea. An awfully inadequate propeller could be the sole explanation for the lack of performance in the fully-rated Ki-84.
Anyway, the first prototype of the Ki-84 was initially completed with a smaller 19m2 main wing according to numerous secondary sources, such as the “Famous Airplanes of the World” issue. All subsequent planes had a larger 21m2 main wing, and it seems that the first unit was later modified to use the final wing.
I have not located a source which specifies which of the initial 3 Ki-84 prototypes achieved the 624km/h at 6550m top speed. If we consider the possibility that it could have been the first prototype with its original, smaller wing, it makes sense that the performance gap between this record and the fully-rated, longer-wing Ki-84 would be more negligible in terms of top speed.
It seems unlikely that the 19m2 wing unit recorded this test, however, because this performance was later used as the “official performance” of the Ki-84 in general.
Ki-84 Prototype #1
Sources
Famous Airplanes of the World No. 19: Army Type 4 Fighter ‘Hayate’. Tōkyō: Bunrindo, 2019.
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.
In the engine table within this article, the CR of Ha-45-21 was changed to 7.17, because the performance used in the table reflects the actual engine with its production CR (rather than 8.0, which was planned and used in prototypes).
The performance of the “Ha-45 Special” was changed from that of Ha-45-12 to that of Ha-45-11 (which it seems to be, see 2nd article).
The performance of the Type 4 Fighter (project name ‘Ki-84’) is a bit of a can of worms, and a subject of frequent debate. This is due to a significant amount of differing data with varying credibility, and often a lack of information to illustrate the background of each record.
When discussing Japanese aircraft performance in World War II, there has been a general concept of separating “actual performance” and “potential performance” — A result of negative factors introduced by the declining state of Japanese industry and the lack of resources, especially towards the end of the war.
“Actual performance” is what was able to be demonstrated by a plane under Japanese fuel, oil, and maintenance conditions. “Potential performance” is said to be what could be demonstrated if the same plane was provided with higher quality conditions, such as those of the United States.
Type 4 Fighter Mod.1 Kou
There is a common belief that the Type 4 Fighter exhibited substantially improved performance (speed as high as 687 km/h!) when captured examples were tested by the US, due to the use of high-octane fuel. While it is certainly true that a Japanese fighter could benefit from the US testing medium, the reality is more complex than “higher grade fuel increased the performance”.
In this article, the true performance of the Type 4 Fighter (Ki-84) will be examined from both angles. This is mainly a technical article, with less focus on history.
(This article may be rewritten if more detailed sources come to light.)
About Ki-84’s Engine ー Ha-45
The most important aspect that dictated the performance of Ki-84 was naturally its engine: the Ha-45. One reason is that depending on the time the airframe was made, a different model (or at least differently performing) engine may have been mounted.
Japanese engine naming systems are also quite confusing to those not informed, so this will first be clarified. The two relevant engines mounted to Ki-84 were named the ‘Ha-45-11’, and ‘Ha-45-21’ by unified convention. The Army called these engines ‘Ha-45 Special‘ and ‘Ha-45‘ respectively. Ha-45 was serviced in the Army with the name ‘Type 4 1850HP Engine’. The corresponding Navy service names may also be encountered: ‘Homare Model 11’ and ‘Homare Model 21’.
Because Ki-84 was an Army fighter, I will be using the Army short names ‘Ha-45 Special‘ and ‘Ha-45‘.
Ha-45
Ha-45 was a very technically impressive engine, and was developed by Nakajima Airplane Company on a rapid timeframe from 1940-1942. The 18-cylinder air cooled radial engine initially was designed to produce ~1,800 horsepower, but was increased to a maximum output at sea level of 2,000 horsepower. At the same time, it was of a significantly smaller form factor than its global contemporaries. Only slightly larger than the famous Sakae engine it descended from, Ha-45 had a 1,180 mm overall diameter, 830 kg dry weight, and 35.8 l displacement. For comparison, the American 2,000 horsepower-class R18 engine ‘R-2800’ had a 1,342 mm diameter, ~1,050 kg dry weight, and 46 l displacement.
The Ha-45’s high power for its size of >100 hp per cylinder was achieved with a higher rotational speed of up to 3,000 RPM and higher pressure boost of +500 mmHg compared to prior Japanese engines. Originally, the engine was designed to use 100 octane fuel in order to avoid ‘knock’ at high pressure (premature combustion). However, wartime Japan did not have the means to procure high-grade fuel, and it was ultimately designed to meet its specified power with 92 octane fuel, instead employing a water-methanol injection system within the intake path to reduce the air temperature and avert knock.
+500 mmHg boost was a high manifold pressure for Japanese aircraft engines, and could only be obtained with water injection on the highest quality of fuel available for service (92 octane in the Navy, 91 octane in the Army). In fact, even the rated power of the engine at +350 mmHg required WM injection. It’s important to know that high-power Japanese engines had to be designed within the constraint of using WM injection to reach boost pressures that were obtained and even surpassed by Allied AC engines without it, as the Allies had a distribution of much higher octane fuel. So resultingly, Allied AC engines that did have WM injection could reach extreme boost pressures Japanese engines could never approach.
Power Restriction of the Ha-45
The above specifications describe the extraordinary performance of the Ha-45 engine as it was developed… but a compact, high-power piston engine never had a chance in the actual state of late-war Japan. It’s well established that the mass-produced Ha-45 suffered relentless problems in field service, leading to a terrible operational rate. These problems were caused by a multitude of factors, such as:
Lack of skilled production. The Ha-45 required precision manufacturing which was not possible on a mass scale by that time.
Declining fuel quality. It is likely that even 91 octane fuel was not actually up to spec near the end of the war.
Lack of quality lubricating oils.
Delicate electrical system.
Insufficient maintenance capacity. The Army simply did not have the depth of maintenance ability to keep the Ha-45 in good shape during widespread service.
The culmination of these factors caused widespread rises in cylinder temperature, oil temperature, uneven fuel distribution, and various other malfunctions and reliability issues. Resultingly, the Army decided to govern the mass-produced Ha-45 engines down to the about same level as the Ha-45 Special that had been installed in the initial Ki-84 prototypes. The max RPMs were reduced from 3,000 to 2,900, the maximum intake manifold pressure was reduced from +500 mm to +400 mm, and the cylinder compression ratio was reduced from 8.0 to 7.17.
Army Ha-45 Performance Table
Name
Format
Power Takeoff
Power Rated (1st Speed)
Power Rated (2nd Speed)
CR
Weight
Len x Dia
Bore
Stroke
Ha-45 Special
2-row R18
1,820hp @ 2,900RPM (+400mm)
1,650hp @ 2,900RPM, 2,000m (+250mm)
1,440hp @ 2,900RPM, 5,700m (+250mm)
7.0
830kg
1,690 x 1,180mm
130mm
150mm
Ha-45
2-row R18
2,000hp @ 3,000RPM (+500mm)
1,860 hp @ 3,000RPM, 1,750m (+350mm)
1,620 hp @ 3,000 RPM, 6,100m (+350mm)
7.17
830kg
1,690 x 1,180mm
130mm
150mm
Ha-45 (Governed)
2-row R18
1,850hp @ 2,900RPM (+400mm)
1,680hp @ 2,900RPM, 2,300m (+250mm)
1,500hp @ 2,900RPM, 6,500m (+250mm)
7.17
830kg
1,690 x 1,180mm
130mm
150mm
Masai Kariya in front of a Type 2 Fighter
Even with the governed Ha-45 providing as much as 200 less horsepower, the operational rate of the Type 4 Fighter suffered until the end of the war and the power restrictions were rarely relaxed. The operational rate was usually around 40%, and in some units (especially those in the south) it could be as poor as 0-20%. However, there were a few units that managed to tame the suffering Ha-45.
While the readiness of other Type 4 Fighter units continued to decline, the maintenance team of the Flying 47th Squadron managed to reach up to 100% operational rate at a point in time. This was achieved by implementing a command platoon specifically overseeing aircraft maintenance, which was different from the conventional structure within Army squadrons. Led by Captain Masai Kariya, who was nicknamed the “God of Maintenance”, thorough examinations, maintenance, and overhauls were constantly performed on all aircraft. The Army took note of the 47th Squadron’s ingenuity and ordered the maintenance team to instruct other squadrons, but it was too late to make a major difference.
Even under the most exhaustive care of the 47th Squadron, which may have been closer to US standards, the Type 4 was unlikely to demonstrate its potential due to factors out of the team’s control (poor oil, fuel, manufacturing precision).
The purpose of this section was to introduce the multitude of factors that could have caused disparity in the performance of each plane. Now, the performance numbers will be examined.
About ‘US Testing Data’ of Ki-84
Before talking about the historical performance records for Type 4, it is first necessary to look at the (supposed) US testing performance numbers which have appeared in numerous publications. It is often said that a captured Type 4 managed to reach an impressive top speed of 687 km/h (427 mph) with American high-octane fuel and test conditions. Similar claims have also been made for other Japanese aircraft, such as Ki-83 reaching 762 km/h (473 mph), Saiun reaching 694 km/h (431 mph), and Raiden reaching 671 km/h (417 mph).
While the origins of the numbers for some such as Ki-83 and Saiun have not been verified, the Type 4’s numbers are easily located. These numbers are published in a 1946 AAF T-2 report as ‘Factual Data’ and are used to illustrate the claim that the Type 4 compared favorably with the most advanced US service piston fighters ‘P-51H’ and ‘P-47N’.
Data from ‘T-2 Report on Frank-1’
As it turns out, this data was not the result of an actual flight test by the US. Rather, these exact numbers were originally created in March 1945 and included in a supplement to the Technical Air Intelligence Center (TAIC) manual on Japanese aircraft. At the time, captured Type 4s had not been extensively tested for performance. As per the TAIC manual:
Except where otherwise stated, performance figures represent estimates of the Technical Air Intelligence Center and have been calculated after a careful analysis of information derived from intelligence, captured equipment, drawings, and photographs, using power ratings derived from the same sources. When authoritative evidence is not available, it is the policy of TAIC to give the Japanese Aircraft Performance every benefit of the doubt within reasonable limits.
Japanese Aircraft Performance & Characteristics, TAIC Manual No.1
TAIC March 1945 data for Ki-84
As there is no indication in the document that Type 4’s performance was derived from real test data, there is no reason to assume so. These numbers, which were calculated in Japanese operating conditions (92 octane fuel), were probably created by deliberately generous estimates so as to not underestimate an enemy fighter. Not dissimilar to the evaluation of Raiden, which was calculated by TAIC to have speed performance significantly higher than what was actually demonstrated. Furthermore, it is overtly stated in the T-2 report that detailed performance was not measured, and mainly flight characteristics were evaluated.
This is not to say that it was impossible for Type 4 to demonstrate speed performance of this magnitude. For example, if the Saiun (which used the same engine as Type 4) was truly able to reach 694 km/h in postwar US testing, it would be reasonable for Type 4 to reach 687 km/h and even more in the same conditions. However, such performance would in all likelihood only be met using higher-than-design manifold pressure by modifying the supercharger ratio and throttle valve control, together with high octane fuel and quality American parts.
As it stands, we seem to have no real detailed data on a US performance test of Type 4 at all.
In Masai Kariya’s book “StoryofJapanese Army Prototype Planes“, it is said that Type 4 achieved 689 km/h using 140 octane fuel during US testing after the war. While this is a slightly different number, it seems quite likely to only be a slight distortion of the TAIC calculations.
Japanese Performance Data
As we have no satisfactory US testing data on the Type 4’s performance, we are left with the Japanese performance records.
There were notable differences in the method of establishing aircraft performance between Japan and the USA at the time. For example, the US recorded top speeds using War Emergency Power (WEP), which is the highest possible output the engine may exhibit for a limited time, while the Japanese standard of recording top speed was at ‘Rated Power’, which could be called ‘Military Power’. This is a lower throttle setting that could be maintained for a longer period of time.
Furthermore, as previously touched on, the state of late-war Japan left many possible disparities in the performance of each airframe. Whether a prototype with a Ha-45 Special, a production plane with a governed or ungoverned Ha-45, a prototype suffering from malfunctions at a stage of testing, or a production plane with subpar manufacturing and maintenance. At the very least, we can assume the fuel quality for official performance tests to be ‘adequate’, and the airframe to be clean.
The first record is well known as the “official top speed” of the Type 4. 624 kilometers per hour at an altitude of 6,550 meters was recorded by Major Iwahashi in one of the initial Ki-84 prototypes with a Ha-45 Special engine. This was the highest performance among Army single-engine fighters. In this condition, the plane climbed to 5,000 meters in 6 minutes and 26 seconds. The detailed performance record was published in the ‘Ki-84 Pilot Manual’, and a copy of this test was later captured by the US at Clark Field on Luzon. The speed and climb are as follows.
Ki-84 Prototype Speed Test (Ha-45 Special)
Altitude (m)
TAS (km/h)
RPM
Boost (mmHg)
Supercharger
1000
544
2,900
+250
speed 1
2000
565
2,900
+250
speed 1
3000
586
2,900
+250
speed 1
3370
594
2,900
+250
speed 1
4000
591
2,900
+185
speed 2
4900
584
2,900
+95
speed 2
5000
580
2,900
+250
speed 2
6000
610
2,900
+250
speed 2
6550
624
2,900
+250
speed 2
7000
615
2,900
+200
speed 2
8000
594
2,900
+95
speed 2
9000
569
2,900
+40
speed 2
Ki-84 Prototype Climb Test (Ha-45 Special)
Altitude (m)
Time
Rate (m/s)
IAS (km/h)
RPM
Boost
1000
1’09”
14.4
260
2,900
+250
2000
2’18”
14.3
260
2,900
+250
3000
3’34”
12.8
260
2,900
+190
4000
4’00”
11.7
260
2,900
+250
5000
6’26”
11.0
260
2,900
+250
6000
8’00”
10.0
260
2,900
+200
7000
9’48”
8.3
240
2,900
+100
8000
12’16”
6.3
230
2,900
0
9000
15’34”
3.8
220
2,900
-180
This speed was quickly usurped by the 4th Ki-84 prototype (1st pre-production) which seems to have been equipped with a fully rated Ha-45 engine. Lieutenant Funabashi flew this test and achieved 631 kilometers per hour at 6,120 meters with a starting loaded weight of 3,794 kilograms. The climbing time to 5,000 meters was improved to 5 minutes and 54 seconds, a difference of 32 seconds. There is no detailed record of this exact test, but there is a record of a Ki-84 with a fully-rated Ha-45 flying with a much lighter weight of ~3,400 kilograms, which is as follows.
Ki-84 Unknown Speed Test (Ha-45)
Altitude (m)
TAS (km/h)
RPM
Boost (mmHg)
Supercharger
1000
545
3,000
+350
speed 1
2000
570
3,000
+350
speed 1
3000
595
3,000
+350
speed 1
3700
614
3,000
+350
speed 1
4000
610
3,000
+300
speed 2
5000
612
3,000
+350
speed 2
6000
630
3,000
+350
speed 2
6650
634
3,000
+350
speed 2
7000
625
3,000
+300
speed 2
8000
605
3,000
+200
speed 2
Ki-84 Unknown Climb Test (Ha-45)
Altitude (m)
Time
IAS (km/h)
RPM
1000
1’10”
265
3,000
2000
2’15”
265
3,000
3000
3’25”
265
3,000
4000
4’30”
255
3,000
5000
5’37”
250
3,000
6000
6’50”
245
3,000
7000
8’15”
240
3,000
8000
10’18”
235
3,000
From the available numbers, it seems that the increase in rated engine power only caused a speed increase of about 10 kilometers per hour in the prototype stage. This may be due to the plane’s small 3.0-3.1 meter ‘Pe-32’ propeller, which is considerably smaller than even other Japanese 2,000hp class propellers, such as Shiden’s 3.3 meter propeller, or Saiun’s 3.5 meter propeller. The American Hellcat, Thunderbolt, and Corsair all had propellers around 4 meters in diameter. The choice of Type 4’s propeller is said to have been done to reduce the length of the landing gear and overall weight, but may have negatively affected the transfer of power.
The highest speed allegedly reached during Japanese testing was 660 kilometers per hour with the Ki-84 Otsu prototype, which is written in numerous secondary sources. The only major difference between the former Model ‘Kou’ was that the ‘Otsu’ was equipped with Type 2 20 mm Autocannons in place of the two nose-mounted Type 1 12.7 mm Autocannons. While on its own this should actually slightly burden the flight performance, there are several factors that could have benefitted this airframe due to its later construction date.
Ki-84 pre-production w/ collective exhaust.
One possible factor was an improved force of thrust from the engine exhaust. In the initial Ki-84 prototypes and 1st stage of pre-production airframes, the exhaust of each cylinder was routed through a collective exhaust pipe on each side.
From at least the second stage of pre-production planes, the thrust exhaust method was changed to the evidently superior individual exhaust stacks. The increase in thrust improved the top speed to a certain marginal, but unknown extent. Most photos of Type 4, especially deployed airframes, have this exhaust type. At least some of the 1st stage pre-production planes were also remodeled as such.
Another factor is that the Type 4 Fighter was seemingly equipped with a slightly larger 3.1-meter Pe-32 propeller after the initial testing phase. This modest increase helped the engine demonstrate its power and evidently did not require a change of reduction ratio to keep the tip velocity in check. Captured production planes were measured to have this propeller diameter.
Supposing such conditions, it could be possible that the Ki-84 Otsu prototype achieved a top speed 29 km/h higher than the 4th prototype, especially if it was at a lighter load and/or using WEP. But as I do not have the original source for this speed claim, it is only conjecture. Factors such as weight or whether it was achieved during WEP versus military/rated power, or in a dive, are totally unknown.
It’s also possible that the Ki-84 Otsu did not even possess these features (at least as originally completed). According to Nakajima Airplane Company’s data prepared after the war for the US, two prototypes of Ki-84 with 20 mm machine cannons (known internally as ‘Ki-84-Y’) were completed in 1943. During this time, only the first 3 prototypes and 24 pre-production airframes of the Ki-84 had been built. If correct, this information could suggest that the Ki-84 Otsu prototypes were not equipped with the later performance-enhancing features.
Conclusion
From the available data, it seems reasonable to state that Type 4’s maximum speed at rated/military power, gross weight, in Japanese engine settings and fuel grade, was around 631-634 km/h. Perhaps closer or slightly superior to the higher end, assuming both full-rated tests were done with the old exhaust type. But as the engine was largely governed and often suffered from malfunctions for multitudes of reasons, the actual rated speed of a service plane was probably 624 km/h at the most optimistic, even though the ‘Ha-45 Special’-equipped prototype that demonstrated this had the early type of exhaust thrust.
The top speed when using War Emergency Power (+500 mmHg) would naturally be slightly higher than each of these speeds, but likely only by a matter of about ~10-15 km/h using very tentative estimates. My rough guess would be that ~650 km/h was probably the best case top speed of a Type 4 with emergency power, and a fully rated engine in a ‘clean prototype’ airframe at gross weight with all beneficial improvements.
As has been discussed, the supposed 687 km/h obtained in US testing with high-octane fuel was only a wartime calculation for Japanese conditions (92 octane fuel) using optimistic data. While it certainly could be possible that Type 4 would have seen a significant performance boost with US high-octane fuel and modifications to produce a higher boost pressure, no such data seems to exist. Furthermore, it’s probable that the US only tested the Type 4s at Japanese power settings, and it’s unknown how much additional pressure a Japanese mass-produced Ha-45 could even handle.
According to the US report on Type 4 made with a captured plane at Middletown, Ohio in 1946, the aircraft either demonstrated or was estimated to exhibit a climb time of about 6 minutes 40 seconds to 6,100 meters. The starting weight of the plane seems to have been 3,350 kg or 3,500 kg (the two reports list different weights), and this climb time is slightly superior to the 6’50” to 6,000 m time of the Japanese 3,400 kg test, but far from the TAIC calculation of 5’48” to 6,100 m.
There is little doubt that the Type 4 Fighter was “the most powerful Japanese fighter of World War II”, but its ultimate performance was dictated by the harsh conditions it was developed in, and even then it was rarely able to demonstrate its full potential.
Sources
Ando, Atsuo. Nihon Rikugun-ki no Keikaku Monogatari. 1980.