Wednesday, May 27, 2020

Japan's First Jet Plane [2/3]: Until the Completion of 'Kikka Kai'

Black and white photo of three multi-engined aircraft flying in formation while dropping a large number of bombs
1945: B-29s drop incendiaries on Yokohama.
  As the year changed into 1945, the state of the war in Japan became increasingly tragic. US strategic bombings increased heavily, and B-29s trailing across the sky, mainly from the Saipan air base, became a daily occurrence. While cities were decimated by mass-bombing, goods and resources dwindled, and the production ability vital to the existence of the war effort was chiseled away. The reality of the situation became painfully clear to the public. The Army and Navy struggled to continue fighting at the end of the rope.

As described in the previous article, the 'Mk. 2 Weapon' became formalized as the special attacker 'Kikka' at the end of 1944, and on the 4th of the following January, the contents of the Navy's request draft were confirmed. In response to this plan request, Nakajima submitted projected performance calculations for the 'Kikka' which can be viewed here.

However, the development of the powerplant for Kikka - that is, the 'Ne-12' turbojet (*name changed from 'TR12' around this time), was extremely slow. Although it was being prepared for further prototyping as the lightened and improved version 'Ne-12B' at the start of this year, these indigenous Japanese turbojets were already recognized by this time as fundamentally flawed.

In fact, due to the incessant plague of technical problems, and the recognition of the 'BMW 003A' diagram that had been received during the last July, Tokiyasu Tanegashima, the lead of jet engine development in the Navy, had expressed at the end of December 1944 that the indigenous turbojet engines should be abandoned. Work was instead to be consolidated on a Japanese model engine based on the format of the successful German BMW 003A.

Ne-12B (TR12 Kai).
*The naming change, affecting all Japanese turbojets,
was due to the terms of Army-Navy development
unification decided months prior.
The concept of this novel engine dated back to at least October 1944, and was called the 'Ne-15'. The Ne-15 was a plan-only engine modeled after the BMW 003A, and it was intended to produce the same thrust as the prior Ne-12 engine (320kgf). This target was subsequently increased to a goal of 475kgf, and it finally became known as the 'Ne-20'.

Design work on the Ne-20 officially began on Christmas day of 1944. Personnel attached to the project lived in the Kuugishou, working day and night, and development launched at an incredible pace. On January 5th, 1945, the same day (and presumably, the same meeting) that the Kikka plan request was verified, data on the Ne-20 was presented, and the performance if it was equipped to the 'Kikka' was estimated.

However, the Ne-20 was still an unfinished design, so the Kikka plan proceeded with the Ne-12B for the time being. Nonetheless, it becomes evident that the engine for the Kikka was still uncertain. On January 13th, a meeting was held at Nakajima concerning the progress of the 'Kikka Model 10'. In the naming conventions of the Japanese Navy, 'Model 11' is the first production model of a plane. The first number after 'Model' refers to number of airframe changes, and the second number, engine changes. 'Model 10' therefore indicates that the engine was undecided.

A meeting which discussed the installation of the Ne-12 engine to the airframe was held the following 15th, while the development of the prospective Ne-20 engine soared in the background. Around this time of mid-January, parts for the Ne-20 were already being manufactured, at the extremely impressive pace of only half a month since the start of design.

Solidification of the Current 'Kikka' Plan

On January 20th, Commander Eiichi Iwaya, the personal deliverer of the BMW 003A drawing 7 months prior, called the Nakajima Koizumi Factory and instructed to create a Kikka prototype schedule for planning the Kuugishou's production assistance.

By January 28th, the long-awaited initial mockup of the Kikka airframe was completed, and the mockup review was held on this day at Koizumi Factory Building 9. Vice Admiral Wada and the Nakajima Design Department Chief, Engineer Yasuo Fukuda, were present. Lead of the Kikka design team, Chief Engineer Kenichi Matsumura, and Engineer Kazuo Ohno gave explanations about the mockup to Vice Admiral Wada.

At the end of the inspection, the modification requests from the Navy were to improve visibility from the cockpit, change the windshield front to a flat glass panel, and change the canopy mechanism to a sliding-type (this implies that the canopy was at first, a folding-type...). On the same day, an official construction directive from the Ministry of Munitions with directives for production of 1, 3, and 10 airframes was delivered to Koizumi. The directed delivery date was 'as soon as possible', number 'undecided', and the delivery location was the Yokosuka Air Corps.

Commander Iwaya and Commander Matsuura of the Navy Aviation HQ (Koku Hombu) went to the Koizumi factory on the 31st and surveyed the status of the Kikka project. A meeting on the progress towards a prototype of the Kikka was held on February 4th, and the schedule at that point was complete Unit 0 (strength tester) in May, prototype unit 1 in June along with 2 more units, 3 units in July, 5 units in August, and 3 units in September. However, at this meeting, there were discussions to hasten the schedule to complete the strength tester and first prototype in May, 2 units in June, and 8 units in July.

A general examination of the Kikka project was held at the Koizumi factory on the 9th, and the plan was judged from Nakajima's explanations. Later that day, a Kikka construction schedule conference occured at Ashikaga (about 10 km away), where it was confirmed that the Kikka strength tester (unit 0) would be completed between May 15th and May 31st, and the first prototype (unit 1) would be completed from June 5th to June 20th.

Work started on producing the official final design drawings on February 9th. By February 10th the second mockup of the Kikka was completed, and the examination was conducted by Rear Admiral Sawa, Commander Itsukuya, and Lieutenant Commander Abura Takaoka from February 10th to 11th. As a result of these examinations, it was decided that the design was now satisfactory, and construction could begin immediately. To increase the speed of manufacturing the prototypes, it was instructed that up to unit 2 would not be equipped with armament, and up to unit 5 would not be equipped with bulletproof plates or self-sealing fuel tanks. By February 15th, there were 380 official design drawings produced, out of the 500 required.

Constructing the 'Kikka'

As air raids over the Japanese mainland strengthened, it became apparent that the large Nakajima Koizumi factory was an easy and attractive target. As such, it was necessary to evacuate the factory to another location, and the staff of Kikka relocated to Sano city between February 17th and 18th. The staff responsible for the airframe and stressing settled at Sano Technical School, the staff responsible for the engine settled at Sano Manufacturing Industry, and the staff responsible for electronic installations settled at Sano Textile.

On the 18th, Nakajima Engineers Ohno and Komine went to the Kuugishou to report the construction schedule decided at Ashikaga on February 9th. The Kuugishou agreed to assist the completion of the Kikka strength tester (unit 0) and unit 1 within April, by using the production facilities of the Kuugishou to assist in manufacturing parts. It was confirmed on the 20th that the Kuugishou would assist Nakajima by producing six types of parts by March 8th.

Nakajima Engineers Komine and Ohno went to the Kuugishou on February 22nd with the draft of the Kuugishou's allocated construction of Kikka parts, and held a meeting concerning the construction assistance in detail. A request for the acquisition of essential parts such as bulletproof steel plates, bulletproof fuel tanks, wheels, undercarriage parts, oil coolers, and hydraulic actuators, sufficient for the production of 12 planes, was submitted to the Aviation Weapon General Office of the Ministry of Munitions, with an acquisition date of March 20th to May 20th. A meeting on February 26th between Nakajima, the Ministry of Munitions, and the Kuugishou discussed the transition of Kikka's production.

The rear-fuselage and tails of the Kikka strength tester (unit 0) and first prototype (unit 1) were completed on March 1st. The third Kikka wooden mockup, for examining the engine installation of Ne-12, had been brought to Sano Technical School on the 4th, and was examined on the 10th. This model consisted of the port side, with a wing, nacelle, and engine fitting. 

March was also very busy month for planning the production of the Kikka between Nakajima and the Kuugishou. Meetings concerning the simplification of production were held on the 2nd, 5th and a meeting to increase production on the 7th. On March 10th, another meeting about the production of Kikka was held at the Koizumi City Hall. This meeting was attended by members of the Kuugishou, Ministry of Munitions, and Navy Aviation HQ.

Shaded areas = Kuugishou allocation. This simple sketch is
the only known drawing of Kikka when it mounted Ne-12.
Here, the production of the steel parts used in the manufacture of Kikka airframes was considered. As a late-war, simplified attack plane, it was natural that steel parts were introduced even in the structure of Kikka to save on resources. However, steel aircraft parts used a different, more lengthy method of processing compared to duralumin, so these parts were becoming a bottleneck in the construction of Kikka.

Also, because the aircraft production plans of FY1944 were not met, there was actually a surplus of aluminium stockpile for aircraft production. Special steels which were supposed to be a substitute for the former were comparatively in shortage. To build the first airframe faster, it was decided to replace the steel parts with duralumin, which was estimated to reduce the overall weight of the empty craft to 83.6%.

It was verified at this meeting that the Kuugishou would construct the outer wings, front and aft fuselage, and tail, while Nakajima would construct the inner wing, central fuselage, and gear, as well as perform full assembly. This was to split the production of steel parts to Nakajima and Duralumin parts to the Kuugishou. A simple representation of this production split between the Kuugishou and Nakajima was produced in another meeting on the 13th.

A notice of Kikka construction was transmitted from the head of the Ministry of Munitions Aviation Weapons General Office to the head of Nakajima Koizumi Factory on March 23rd. The subject matter of this notice was as follows:

Experimental Kikka Annotation

1. Note Destination: Nakajima Airplane Co., Ltd
2. Construction Number: 
            Test Airframes: 1
            Complete Airframes: 12
3. Completion Dates:
            Test Airframe: May 10 1945
            Unit 1: May 1945
            Unit 2, 3, 4: June 1945
            Unit 5, 6, 7, 8: July 1945
            Unit 9, 10, 11, 12: August 1945
4. Delivery Location: Tokyo Navy Shipbuilding and Military Inspection Office
5. Planned Construction: Separate plan requests regarding planned construction are based on the aircraft plan as well as the Navy regulations related to construction and repair.
6. Examination: It is subject to the aircraft examination regulations, and the implementation of this is based on the decision of the head of the Kuugishou.
7. Designation: Aircraft construction and repair regulations designated sign, for aircraft model types: "Experimental Kikka".

Finally, on March 31st, the production drawings of Kikka were almost complete.

Emergence of Ne-20 as the Definitive 'Kikka' Engine

The complete parts of the first Ne-20 engine had been completed on March 20th at the Kuugishou, less than 4 months after the start of design. The engine was fully assembled on the 26th, and test run on the same day at a makeshift test-cell inside a cave at Yokosuka.

This testing was immediately successful, and the Ne-20 seemed to be superior in not only output (490kgf vs. 320kgf), but also reliability, to the long-developed Ne-10 family (Ne-10, 12) at the very outset. Even after the nearly 2 year development cycle of the Ne-10 family, the most refined model, Ne-12B, proved to only be capable of about 30 minutes of full-power operation. Because the Kikka was a special attack plane with a one-way trip in all likelihood, the Navy considered this to be 'good enough for the job', but it was still disappointing.

Although Ne-12B was still the 'official' powerplant of Kikka for the time being, the lead developer of both engines, Tokiyasu Tanegashima, considered the Ne-12 to be a dead end months prior. It became inevitable that Kikka, if it were to succeed in operation, should be converted to the Ne-20 engine even if the precious development time was lost.

On April 1st, Nakajima Airplane Company was taken over by the government, and the company name was changed to the '1st Munitions Arsenal'. The Koizumi Factory, directly intertwined with the development of Kikka, became the '2nd Manufacturing Plant'. Furthermore, on this day a special weapons servicing schedule for the first half of FY1945 was produced by the Navy General Staff, which scheduled for a highly demanding 300 Kikka planes to be serviced during this period.

The 2nd Manufacturing Plant was hit by a large scale B-29 raid for the first time on April 3rd. As such, it was no longer safe to manufacture Kikka parts at the factory, and it was necessary to disperse construction using what could be called the Japanese version of the 'shadow factory' system. Accordingly, the manufacture of parts for the first unit of Kikka were dispersed from the former Koizumi Factory to many civilian sericulture huts scattered throughout the area of Serada and Kasukawa Village in Gunma Prefecture. A production commencement ceremony for the Kikka was held in Serada Village on April 18th.

On April 19th, a meeting regarding the mounting of Ne-20 onto the Kikka was held at the Kuugishou Jet Department. The performance, method of fitting, and necessary gauges were examined. The next day, an engine equipment improvement meeting was held at the 2nd Manufacturing Plant. On the same day, an engine exchange meeting was held at the Kuugishou, attended by the head Vice Admiral Wada, as well as the head of the planning and jet departments, several engineers, and the head of the Aviation HQ. At this meeting, the following was decided:

1. The prototype Kikka will be completed with Ne-20 from the first plane.
2. The drawings for [Kikka with Ne-20] will be completed by May 10th.
3. The targeted completion date of the first unit is May 15th, and if there is need for the Kuugishou's assistance, please report it.
4. Inspect the center of gravity position if the motor is exchanged along with [equipment of] MG's as a fighter.
5. Conduct mid-air testing of Ne-20.

Mr. Nozaki from the Aviation HQ then called the 2nd Manufacturing Plant with an important notification.

It was decided that Kikka will equip Ne-20. Carry forward the prototype according to that point.

In this way, the engine of Kikka quickly changed from the indigenous 'Ne-12' to the German-based 'Ne-20'. Although the plan was slightly delayed, due to the performance increase expected from Ne-20, the consideration of a gun-equipped fighter model was being conceived for the first time.

A meeting for construction work to equip Ne-20 was conducted at the Kuugishou on the 21st. Then, on the next day, the Ministry of Munitions Aviation Weapons General Office issued a new 1st quarter production schedule, which updated the production schedule of Kikka to one plane in May, and two in June. The Ne-20 equipment plan was deliberated at the Kuugishou on the 26th, and at this meeting, the Kikka was reborn with a new official plan request. In line with the engine change to Ne-20, and the creation of this plan request, the name of Kikka was tentatively changed to 'Kikka Kai'.

April 26 1945
Experimental Kikka Plan Request Deliberation Meeting Decision Matters
1. Required
(with a No. 50 bomb
unless specified).
a. Top Speed
335 knots (620km/h) at sea level or more
b. Cruising
190 nautical miles (352km) sea level or more
c. Takeoff
   Run Distance
Within 500m in no wind, when using takeoff acceleration devices.
d. Landing
Within 90 knots (167km/h) at light load.
e. Consider equipment of defensive plates.
2. Strength
Category III
a. Standard weight is the weight in the No. 50 bomb condition with half fuel, take weight with bomb.
b. Descent limit speed will be 450 knots (indicated value).
c. d. (Omitted)
e. (Loading coefficient polygon)
3. Armament
Bomb Armament
Type3 Large Bomb Suspension Rack
No. 50 Ordinary Bomb Model 2
Radio Armament
Type3 Air Mk. 1 Radiotelephone (Reciever)
Loading a No. 80 bomb is possible
4. Defense
a. Equipped with 55mm thick bulletproof glass at the front of the cockpit,
and 12mm thick bulletproof steel plates in front and behind the cockpit.
b. The fuel tank must be equipped with an automatic fire extinguisher (single use).
5. Other items are the same as the original plan.

The calculated detailed performance of Kikka Kai in response to this plan can be found here.

Structural Composition of the 'Kikka (Kai)' Design

At this point it is necessary to interject the explanation of the Kikka or Kikka Kai design in some detail. Kikka has a triangular fuselage and conventional 'underwing nacelle' engine-mounting method which make it reminiscent to the Me 262, but a size smaller, often earning it the common nickname of 'Japanese Me 262'. In reality, despite the rough silhouette, 'Kikka' owes almost nothing to the 'Me 262', and in a detailed sense these planes are worlds apart.

Wing diagram of Kikka.
The main wing of Kikka has a sweepback angle of 13 degrees and a strong taper ratio. It consists of a joined central wing, outer wings, and wingtips. The wing has a twin-spar construction, with 9 ribs arranged between them per wing. The skin of the inner wing is steel-sheet, while the outer wing is duralumin, and the wingtips were wooden. The central wing has a dihedral of 5 degrees, while the outer wings have a dihedral of 2 degrees, creating a gentle gull shape.

The airfoil is the laminar flow K-series designed by Nakajima. K125 was used for the main wings, and K309 for the wingtips. A small slot is present at the leading edge of the wingtip to counteract wingtip stall. A simple split flap is present inboard of the engine nacelle. The aileron is located outboard of the nacelle, and is also operable as a simple flap (flaperon). The full wingspan of the aircraft was a small 10 meters, and the outer wings folded upward on hinges in order to fit inside air raid shelters and caves.

Fuselage diagram of Kikka.
The fuselage of Kikka has a triangular shaped cross-section with rounded corners, more gentle than Me 262. It is of all-metal monocoque construction, with 24 frames. The 5th, 8th, and 17th frames are back-to-back L beams joined by 6mm bolts, which made the airframe convenient for rail transportation. The central fuselage (8th to 17th frame) was steel-skinned, while the rest of the fuselage is duralumin skinned. The maximum width is 1.20m, and the height including the canopy is 1.43m.

Side profiles of 'Kikka Kai' and 'Tenzan'.
The tail of Kikka is very similar to that of the 'Tenzan' which was developed by the same design staff. The horizontal stabilizer is placed higher in order to avoid the zone of the jet exhaust. It is all-metal with a structure familiar to many other conventional Japanese aircraft.

Parts of Kikka were diverted from existing aircraft in order to facilitate low production cost and high efficiency. The hinges of the flaps were diverted from the Type0 Carrier Fighter, along with the main landing gear. The front wheel was instead diverted from the 'Ginga' bomber. It was also questioned if even more parts could be substituted, such as the oleo struts of the J2M's landing gear, or the front windshield of the Ouka two-seat trainer 'K2', which had a stockpile of 40. These parts were not implemented into the design.

The final engine of the Kikka Kai, the Ne-20, was based on the format of the 'BMW 003', the diagram of which had arrived in Japan during July 1944. The length is 2.7m, about 3/4ths the size of BMW 003. The static thrust output is 490kgf, and the weight is 470kg. The maximum rotational speed is 11,000rpm. The first prototype was finished on March 26th 1945, and further testing with 5 prototypes confirmed it as a highly satisfactory engine. It was formalized during June with the completion of the first production model engine.

Special Attacker Kikka Kai Specification and Performance Summary



Top Speed

w/ 500 kg


@ 0 m

622 km/h


10.00 m

@ 6,000 m

680 km/h


9.25 m

@ 10,000 m

670 km/h

Wing Area

13.21 m2

Climb Rate

w/ 500 kg


to 6,000 m



Empty: 2,300 kg

to 10,000 m


Normal: 3,550 kg


10,700 m

Overload: 4,312 kg


@ 6,000 m

583 km

Wing Loading

269 kg/m2

@ 10,000 m

889 km

Fuel Capacity

Normal: 725 l




4,200 kg

Overload: 1,450 l


504 m

Oil Capacity

Normal: 30 l




3,950 kg

Overload: 60 l


1,363 m


Ne-20 turbine rocket (x2)



2,570 kg

490 kgf @ 11,000 rpm


159 km/h

Japan's First Jet was Born Under Thatched Roof

The first unit of Kikka had reached a state of roughly 60% construction progress by May 3rd, and a structural examination was conducted at Kasukawa. In the 2nd quarter FY1945 production plan created on June 1st, it was instructed to complete 24 units (excluding the prototype) of Kikka in June, 25 units in August, and 45 units in September, with a monthly output of 60 units during and dafter October. Meanwhile, the first production model of Ne-20, 'Ne-20A' was completed, and displayed good performance.

However, the deterioration of the war situation furthered every day, and the development of Kikka missed schedule after schedule. On June 13th, at a meeting for prototype service preparation held at the Ota Factory Nakajima Club, Navy Aviation Headquarters head Vice Admiral Wada stated the following.

Production of ‘Renzan’ will be discontinued. From now on, it is necessary to place emphasis on aircraft that carry out special-attack-like sorties, and radical attack planes. Due to the extreme shortage of gasoline, it must be used exclusively for 'Homare' engines, while others must use alternative fuels. I expect turbine rockets will be the latter.

The production of the 4-engine attacker 'Renzan' was suspended to foster the production of Kikka. Furthermore, it was concluded at this meeting that aluminium stock would be depleted by September, or at least by the end of the year. It would be necessary for all aircraft from FY1946 to be constructed of steel and wood. In such a terrible situation, it must have seemed hopeless to continue the development of Kikka as a special weapon, and this reality could not have been lost on the project staff who continued to toil in it. Despite the futility of the war situation and the unfavourable prospects of Kikka within it, each member persevered only with their pride and stubbornness to see the first Japanese jet fly as soon as possible.

The first prototype of Kikka (Kai), painted in Navy prototype yellow/orange, at the 2nd Manufacturing Plant.

On June 25th, the first Kikka Kai unit was finally completed in its thatched sericulture hut at Kasukawa Village, painted in prototype yellow/orange. It was transported to the 2nd Manufacturing Plant for reassembly. The assembly completed on June 27th, and weight and center of gravity measurements were taken. A formal completion examination was started on the 29th, attended by the head of the Saito Examination Department, 50 others, and Lieutenant Fujiwara as a representative from the Kuugishou. As Lieutenant Fujiwara looked upon the plane, he heard a voice saying that the wing was not correct, and felt a sudden sense of dread. However, it seems that they were mistaken, and there were no issues with the prototype.

The next day, the twin jet engines of Kikka Kai blew for the first time under the watch of the observation team. From the 1st of July to the 3rd, vibration and control stiffness tests were completed, and finally, on July 8th, the airframe was disassembled and shipped to the 2nd Aviation Arsenal, Kisarazu, for further testing.

Testing at Kisarazu commenced on the 13th of July. However, the very next day there was an accident in which a loose nut jumped into the intake port of the left engine, damaging the compressor blades. This accident delayed testing of the Kikka Kai by almost half of a month. Being aware, retrospectively, that the war ended only a month later, an accident like this could have easily prevented the Kikka Kai from flying at all.

The damaged engine was repaired by the 27th of July, and on this day the first ground taxi run was conducted by Leiutenant Wada in preparation of the test flight. On the 29th, the second and third taxi tests were performed by Lieutenant Commander Takaoka. A brake test was conducted after accelerating to 70 knots (130km/h), and the effectiveness of the brakes were subpar. Given the situation, there was no time to spare for improvement, and the ineffectiveness of the brakes was not poor enough to warrant a halt of testing.

At last, on August 6th, the taxi tests completed as what was considered 'satisfactory', and the Kikka Kai was at last cleared for its initial flight test.

Kikka Kai rolls out for ground testing at Kisarazu Airfield.


  1. daam boy i love this page

    in no other place i can find all this information

    thanks for all this :3

  2. Love your work! Im doing some college work on the subject of japanese aeronautical engines during ww2, this was very useful!