Thursday, March 5, 2020

Japan's First Jet Plane [1/3]: The Unofficial Development of 'Kikka'

  From June 19-20, 1944, a large scale air attack against the US Navy task force covering the invasion of Saipan Island ended in a devastating defeat for the Japanese Navy.

The 1st Mobile Fleet, the main carrier aviation force of the Japanese Navy, failed to effectively cooperate with the exhausted land-based 1st Air Fleet which was supposed to offset the US numerical superiority. As a result, the 1st Mobile Fleet launched the attack alone, and was annihilated by the US Task Force 58 which outnumbered it nearly two to one in an event appropriately dubbed by US aviators as the "Great Marianas Turkey Shoot".

This 'decisive battle' Operation Mk. A (ア号作戦) ended quite decisively in favor of the American forces, with the loss of the Japanese fleet carriers 'Taihou', 'Shoukaku', and 'Hiyou' along with the grand majority of Japan's carrier aircraft groups, crippling the Japanese Navy's carrier aviation force beyond repair. The subsequent fall of the Marianas triggered the collapse of Tojo's Cabinet and placed the mainland of Japan within the range of large-scale B-29 strategic bombings for the first time.

However, despite the heavy blow, land-based units of the Navy's Air Service still remained, and the Army's Air Service had not been subject to the annihilation at the Mariana Islands. Moreover, Japan's 'life-line', the southern resource transfer route, was still intact. Though ultimately set for a total defeat, Japan still possessed the ability to continue the war for the time being, and to this end, the characteristically Japanese plan for a 'decisive battle' against the US Navy was borne once again.

It was plainly clear to the Japanese military that the location of this air battle would be at the Philippines, where the US forces would undoubtedly move in to sever the aforementioned southern resource transfer route, by estimation, about 6 months after the fall of the Marianas Islands. The Army and Navy launched joint research into the preparation of forces for this event in August of 1944. This newly planned 'decisive air battle' was thought to be the last time the Japanese military could be capable of dealing a critical blow to the US Navy.

Special Weapons Development for the Decisive Battle

To increase the Japanese air power in the Philippines for the next decisive battle, the Army adopted extreme prioritization measures. Part of these prioritizations was the first formal application of deliberate suicidal aerial ramming attacks, to which the Navy followed a similar course. The reason for officially adopting the use of suicidal ramming attacks was a desperate move to make victory achievable, under a situation where few skilled pilots remained, and the ability of production was declining. The first purpose-built 'ramming' weapons of each service, the 'To-Gou' plane (modified Ki-67) and the flying bomb 'Maru Dai' (later Ouka) respectively, were ordered into development at the same time in the middle of August 1944.

Due to the deterioration of the war situation
 and the resignation of the strongest
supporter, Hideki Toujou, the design team
 of Nakajima's 'Fugaku' super-heavy
 bomber was dismantled by August 1944.
On August 25th, a team of surplus Nakajima Airplanes engineers from the suspended 'Fugaku' super-heavy bomber project, along with engineers from Kawanishi, were convened at the Kuugishou (1st Aviation Technical Arsenal, at Yokosuka) for a new research task. The Nakajima team consisted of Engineer Kenichi Matsumura and 5 other individuals, and a second Nakajima team of four people was also convened on September 1st.

It was here that a new set of special attack aircraft, tentatively named 'Prosperous Country Weapons' (Koukoku Heiki, 興国兵器) was introduced by the Navy.

At these meetings, multiple types of special weapons were explained, but the main task given to Nakajima and Kawanishi's engineers was the research of a simplified attack plane for the upcoming Philippines decisive battle, called the 'Prosperous Country Mk. 1 Weapon'. However, the basic concept of the 'Prosperous Country Mk. 2 Weapon', a new grade of special attack plane utilizing the novel technology of jet propulsion, was also explained during this time at the Kuugishou.

Jet-propelled Special Weapon 'Prosperous Country Mk. 2'

It may be questioned why an advanced new technology such as the jet engine would be put into the development of special attack weapons. The explanation is in the reality of the situation of Japanese jet engine development at the time of August 1944. The only engines becoming available at this time were of poor thrust output, so creating a fighter-jet like the 'Me 262' was impossible. Thus, the only practical application of these engines, more so when considering the ease of manufacturing and fueling jet engines compared to their piston counterparts, was into small special attackers.

Engineer Shigeru Arai, chief of the Nakajima Koizumi prototype factory, traveled to Kuugishou on September 12 to prepare production plans for the Nakajima model of 'Prosperous Country Mk. 1'. On the 14th of the same month, Kenichi Matsumura returned to the Nakajima Koizumi factory and made an interim report. Matsumura's interim report describes three types of special weapons planned, the simple attack plane 'Prosperous Country Mk. 1', current planes remodeled to carry 800kg bombs, and 'Prosperous Country Mk. 2'.
 Research on TR30, TR10 Model is Mk. 2 Weapon Nakajima Plan. 
TR10 Centrifugal Turbojet
Engineer Matsumura's words may suggest that the Mk. 2 Weapon was planned by other companies than just Nakajima, and moreover, that outside of Nakajima's plan, this plane may have not necessarily been a jet at all. However, no other plans than Nakajima's are known to be recorded. Most importantly, this interim report explains that the first jet engines planned for the Mk. 2 Weapon were the TR10, and more unexpectedly, the TR30.

The TR10 was the improved model of the Japanese Navy's first jet engine, the centrifugal turbojet TR, which had been tested by 3 examples in the spring of 1943. Using what had been learned from the failures of TR, a mass production of 70 TR10 engines for rapid trial and error testing had been planned from July to August 1944, but failed to materialize. The first TR10 engine was finally trial-run in September of that year. A variant known as the TR10 Kai also finished design in this month, by implementing 4 axial stages in front of the centrifugal compressor, and a further strengthened model of this, TR12, was being planned.

The intended thrust output of TR10 was 300kgf, with a maximum RPM of 16,000 (TR10 Kai: 15,000), and the overall weight was 250kg.  TR10 and its derivatives were deeply flawed engines that never escaped issues of performance and breakage. It is consequently easy to understand the fundamental issue with the TR30, which was in an almost exact sense, the TR10 Kai engine upscaled by a factor of 3. The estimated thrust output was a formidable 850kgf, but it can be said that this engine was already destined to fail before it had even reached the prototype stage.

Tsu-11 Axial Motorjet
Engineer Matsumura's interim report also explains that the Navy was aware that the necessary jet engine would not be available in time for the prototyping of Mk. 2 Weapon, and a tentative plan was prepared to provisionally equip prototypes with 'Hatsukaze' engines, and later swap over to the intended turbojet engines when available.

The meaning of this 'Hatsukaze' provisional plan was to initially outfit the aircraft with Tsu-11 engines. Tsu-11 is a low-power motorjet engine often informally referred to as the 'Hatsukaze Rocket'. The simple configuration of this engine is a 160hp Hatsukaze piston motor driving a single-stage compressor, followed by a combustion chamber to the nozzle. The static thrust output is 230kgf. There were plans for both two and four of these engines to be equipped.

On September 16, the head of the Kuugishou, Vice Admiral Misao Wada, requested for the head of the Koizumi prototype factory, Shigeru Arai, to rapidly commence development on the Mk. 2 Weapon. Vice Admiral Wada instructed Arai to research the possible preparation of production, with 3 planes in December, 100 the next January, 200 in February, and 300 in March. The unrealistic demand for production to begin in just three months clearly introduces the ambitious encouragement of Vice Admiral Wada, who became the leading character in the advancement of development for the Mk. 2 Weapon.

A document concerning 'Army and Navy Emergency War Preparation' was dated on the 25th, relating to the 'Prosperous Country' weapons development.
Considering current national power and the tension of the current war situation, the Army and Navy will unite and strive for the urgent development of decisive-battle military forces for certain victory.
Attached was a priority ranking of 'Prosperous Country' weapons. The first rank, "Emergency Procedure 1" was assigned to the 'Mk. 1 Weapon' and 'Current Planes Remodeled' (Shiden, Raiden, Reisen with large bombs and RATO). The second rank, "Emergency Procedure 2" was attributed to the 'Mk. 2 Weapon'. It can be seen that the importance of the Mk. 2 Weapon was lower than the rest at this time. The engine is listed as both Hatsukaze Rocket and TR10.

Air Technical Arsenal Chief
Vice Admiral Misao Wada
On the same day, a construction meeting of the Mk. 2 Weapon held at the Kuugishou between the Navy and Nakajima confirmed that the previous plan instructed by Vice Admiral Wada could not be achieved. The Navy's production plan was greatly scaled back to expect the beginning the production of 'Hatsukaze Rocket'-equipped aircraft in January 1945, and the first 3 'TR12'-equipped aircraft to be delivered in February, 7 in March, 30 in April, and 100 in May. The respective TR12 engines were to be produced with 10 in January, 20 in February, 100 in March, 300 in April, and 750 in May.

Why is it, now, that the engine of the Mk. 2 Weapon was said to be 'TR12' rather than 'TR10'? There is no exact date of this change, because as the TR12 itself is only a slight modification of the 'TR10 Kai', it can be considered a natural gradient. When the improved 'TR12' exists, there would no longer be a reason for the 'TR10' to be equipped. Anyway, the main engine planned for the Mk. 2 Weapon was inaugurated the TR12, as recorded in the October 5th revised version of the previously mentioned document of 'Army and Navy Emergency War Preparation'.

On this day it was also reported that the primary drawings of the TR12-equipped aircraft were to be completed within December, but Vice Admiral Wada was holding onto a highly optimistic plan, and on October 8th gave a notice to complete the airframe outline drawings along with a wooden mockup during October, test fly the TR12 mounted to a Type1 Medium Attacker (G4M) the next month, and complete 30 planes in December. Relentlessly, the Vice Admiral ordered research to be advanced on this possibility, keeping the design department at Nakajima extremely busy.

Frontal projection of the Bell P-59 Airacomet.
Very little is certain about the design situation of the preliminary development stage for this airplane. However, according to the recollection of engineers involved in this project, working under Chief Engineer Kenichi Matsumura, there was initially a concept studied in which two TR10 engines were mounted side-by-side within fuselage, perhaps similar in format to the Bell P-59 Airacomet. Of course, there was also the alternative plan to mount each engine in an under-wing nacelle referential to the 'Me 262', which was ultimately adopted. Although the former plan was recognized as superior in terms of aerodynamics, the latter was more practical from the standpoint of technological simplicity and productivity.

Around this early period of time, the top speed of the Mk. 2 Weapon while tentatively equipped with Hatsukaze rockets was projected to be about 556km/h at sea level with an 800kg bomb, and about 593km/h with a 500kg bomb. The airframe's empty weight was to be 1,480kg, overall weight 3,630kg, strength 7G, and top speed with TR12 engines, about 639km/h at sea level. It was also planned to equip takeoff assistance rockets (RATO).

A similar layout should not give the perception that this plane was a copy of any other aircraft. If it was not expressed clearly enough thus far, this 'Mk. 2 Weapon' was a purely Japanese design, stemming from a unique Japanese requirement. There is a common theory in western sources that upon learning of the German jet fighter 'Me 262', the Navy ordered a copy into development as this aircraft. When observing the actual situation, this theory is baseless, though a similar course was followed by the Army with the Ki-201. The true attribution of the Me 262 to the Japanese Navy was the inspiration to further advance the development of jet engines, due to the success of practical usage in Germany.

Nakajima Airplane Company prepared the arrangement of staff reassignment for the production of Mk. 2 Weapon on October 20, and Vice Admiral Wada visited the Koizumi factory the following 22nd, where he was informed of current circumstances with Mk. 2 Weapon design, including the shape of the wing planform, issues with the undercarriage, and issues with mounting the 'Kuurai Model 7' gliding torpedo.

So, by late October, the shape of the main wing was still undecided. The equipment of a gliding torpedo to this plane is a strange matter that was never mentioned again, so it seems to have been dropped. On the 23rd, Vice Admiral Wada was informed of the state of progress toward the prototype. Returning to the Kuugishou, he dispatched a staff member to inform the factory that a plan research meeting would be held on the November 9.

The Plan of 'Mk. 2 Weapon' Becomes Formal

The agenda of this meeting attended by representatives of the Navy General Staff, Navy Aviation HQ, Kuugishou, and Nakajima was to explain "the value of this weapon, issues with the TR, and issues of the bomb quantity". From this content, it can be seen that even while Vice Admiral Wada was enthusiastically promoting Nakajima's research, the Navy itself still needed to receive explanations on the importance of the 'Prosperous Country Mk. 2 Weapon' in general.

In fact, despite currently being past the date by which, if following the earliest plans, actual examples of this plane would have been completed, the Navy had not even issued a formal instruction of prototyping, nor a plan request, to Nakajima. 'Prosperous Country Mk. 2' was still entirely unofficial. The fact that it had even progressed this far can be attributed almost entirely to the orders of Vice General Wada alone.

On the 15th of November, a meeting was held regarding 5 types of engines that could be used for the Mk. 2 Weapon, and finally on December 9th, a second plan research meeting was held which confirmed the current plan of the aircraft would be proceeded. The production schedule was moved to the end of February 1945 for unit 0 (strength tester), and the end of March 1945 for units 1-3. Nakajima also submitted the required personnel (approximately 950 people) for prototyping and production, floor space (800m² for parts, 1,150m² for assembly), estimated materials for machinery, etc.

At last, on December 25th, an official plan request was drafted by the Navy at the Kuugishou, and the contents of this draft are written below.

Experimental "Kikka" Plan Request Draft

December 25 1944

1. Purpose

To obtain a land-based attack plane suitable for attacking enemy warships

coming in close range, and suitable for large quantities of production.

2. Form-Type

Turbine Rocket Twin-Engine Monoplane Model

3. Main


As small as possible, dimensions at the time of [wing] folding

5.3 meters width, 9.5 meters length, and 3.1 meters height, or less.

4. Equipped


TR12 Model x2

5. Crew


1 person

6. Performance

Flight Performance 

(with a No. 50 bomb

unless specified).

1. Top Speed

275 knots (579km/h) at sea level

2. Cruising


No. 50 bomb condition,

maximum 110 nautical miles (204km) sea level 

No. 25 bomb condition,

maximum 150 nautical miles (278km) sea level

3. Climbing


No particular numbers shown, but not too little at the

time of retracting undercarriage immediately after takeoff.

4. Takeoff

   Run Distance

Within 350m in no wind,

when using takeoff acceleration devices.

5. Landing


Within 80 knots (148km/h) at light load.

Stability and


To be able to turn easily, and have basic aerobatic potential.

7. Strength

Category III

8. Armament

Bomb Armament

Type2 Large Bomb Suspension Rack

Type97 Twin-Drop Controller

Tentative Name Type4 No. 50 Mk. 8 Bomb or Type3 No. 25 Mk. 8 Bomb

Radio Armament

Type3 Air Mk. 1 Radiotelephone (Reciever)

9. Defense

1. Pilot

Equipped with 70mm thick bulletproof glass at the front of the cockpit,

and 12mm thick bulletproof steel plates below and behind the cockpit.

2. Fuel Tank

To be 22mm thick inner-bag type bulletproof tank.

10. Instruments



Tachometer, Fuel temperature gauge, Fuel pressure gauge,

Oil pressure gauge, Oil temperature gauge.



Speedometer, Pitot tube electric heater, Altimeter,

Artificial horizon, Type0 High Altitude Compass Model 1.

Others to be determined at mockup inspection.

11. General


Type0 Parachute, Automatic fire extinguisher, Mk. 3 Dry Storage Battery,

Lifesaving Raft Model 1, Reserved weight 30kg.

12.  If the characteristic performances are satisfied, aim to extend the cruising power as much as possible.

This plane, once known as the 'Prosperous Country Mk. 2 Weapon' was now officially designated by the Navy as the 'Kikka'(橘花). Kikka is a name literally translating to 'Tachibana Blossom'. Tachibana is a type of orange with deep cultural significance in Japan, being associated with immortality in ancient mythology, and famously having a symbolic place on the western flank of the Shishinden at the Kyoto Imperial Palace.

In terms of designation nomenclature, the name 'Kikka' denotes the plane as a special attacker of the same purpose as the "Ouka", "Touka", and "Baika" special ramming planes. However, it is noteworthy that the purpose statement of this plane does not deliberately mention ramming attacks. In addition, the 'Type 3 No. 25 Mk. 8' and 'Tentative Name Type 4 No. 50 Mk. 8' bombs are bouncing bombs which skip over the surface of the sea, and also do not necessitate a ramming attack. So, there was a disconnect between the design of this plane and its planned application by the Japanese Navy.

On the same day of its creation, the plan draft was handed over to the project lead Engineer Kenichi Matsumura at the Kuugishou, and he brought it to the Koizumi factory. With the delivery of this draft, after almost 4 months of unofficial development encouraged by the enthusiasm of Vice Admiral Wada alone, the plan for ‘Kikka’ was finally officially promoted by the Navy.

A few days later the year changed. On January 4th, a "'Kikka' Plan Request Council" was held at the Kuugishou, and the contents of this request draft were finalized. At this time, the air battles over the Philippines which the Kikka was initially conceptualized for were long decided.

While the 'Prosperous Country Mk. 2 Weapon' was far too late for a decisive air battle at the Philippines, it was now to be further promoted as the 'Kikka' for the next decisive battle of the Japanese home islands.


  1. muy buen articulo, aunque soy mas de tanques, si puedes podrias hacer algo sobre los tanques de posguerra, y añadir las diferencias de los tanques americanos que tenia japon con sus homologos de EEUU, si lo hubiese. gracias

  2. Hi! I know you are a bussy person and that this probably isn't the place.
    But i had a doubts regarding wartime japan produccion.
    In many sources the "army technical bureau its alluded to create tanks on their own and then chossing a company to produce it.
    While in others the bureau approach a company directly with especifications and then the company its the one that creates the prototypes that get selected and produced.
    Did the bureau had the capability of producing tanks on their own? could they even be considered a manufacturer aswell? Or its just a case of word mishap?