Nakajima Kitsuka (Kikka) by Piotr Forkasiewicz
Even though the genesis of the turbojet began long before World War 2, it would take the war to accelerate the development of this new powerplant to the point that by the close of hostilities jet aircraft had been blooded in battle. Germany can, by some, be considered the leader in turbojet technology during the war, but the US and Britain were not far behind. Japan, too, was not idle in producing its own turbojet but it would take German knowledge to give their industries a boost. One such results was was an historic aircraft in the annals of Japanese aviation history: the Nakajima Kitsuka.
Because the Kitsuka (which in Japanese means ‘Wild Orange Blossom’) was probably the most important Japanese aircraft to use a jet engine as its powerplant, it seems apt to provide a general overview of Japanese turbojet development in this section. The first axial-flow turbojet was patented in 1921 by Frenchman Maxime Guillaume. However, the technology of his day was not enough to realise a working model. In 1930, Englishman Frank Whittle designed a turbojet using a centrifugal compressor and, despite relatively little interest being shown in it, he patented his concept. In 1933, German Hans von Ohain designed a turbojet similar to Whittle’s but it would not be until 1936 that Ernst Heinkel took an interest in the engine and hired von Ohain to continue his work. By March 1937, this resulted in the Heinkel HeS 1, the first German jet engine although in fact a hydrogen demonstrator. The following month Whittle tested his first jet engine, the WU or Whittle Unit.
Around this time, Rear Admiral Kōichi Hanajima became aware of Whittle’s work as well as that of Secondo Campini, an Italian who began work on a thermojet and an aircraft to use it: the Campini Caproni N.1 in 1934. This rekindled his interest in jet propulsion and using his position as head of the engine division of the Dai-Ichi Kaigun Kōkū Gijutsu-shō, saw to it that studies were conducted in such engines. Hanajima reached out to the Tōkyō Imperial University and Mitsubishi Jūkōgyō K.K. and together all manner of rocket and jet engines were investigated such as ramjets. To Hanajima’s disappointment, little official interest was generated from the results.
1938 saw German firm BMW begin their research into turbojets and the Heinkel He 178 V1 prototype was built to test the HeS 3 turbojet that was being developed from the earlier HeS 1. In late 1938, Messerschmitt started work on what would become the world’s first jet fighter to enter squadron service, the Me 262. In Japan, and despite the lack of interest being shown in jet propulsion, Captain Tokiyasu Tanegashima was appointed as the head of the Engine Test and Field Support Shop of Kūgishō. He was issued with a meager sum to fund jet engine research although, with the assistance of Professor Fukusaburō Numachi, he would initially focus his efforts on turboprops. Both men were able to source the Ishikawajima-Shibaura Turbine Company and Ebara Seizō K.K. to help build a number of test engines that used compressors and gas turbines, but these labours did not bear fruit.
By 1939, BMW had tested its first axial-flow turbojet design and on 27 August of that year the He 178 V1 made its first flight, the first turbojet powered aircraft to fly. In February 1940, the British Air Ministry ordered two examples of the E.28/39 research aircraft from the Gloster Aircraft Company to serve as testbed aircraft for Whittle’s engines. 1940 also saw the Italian N.1 fly for the first time and Heinkel began gliding tests of the He 280 jet fighter prototype as it waited for its two HeS 8 turbojets now under development (the He 280 did not enter production). November would see Junkers test the Jumo 004 turbojet and Gloster’s jet fighter proposal, the Meteor, was ordered in February 1941. Also in November, Lockheed commenced work on the L-1000 axial-flow turbojet, the first American jet. Finally, in December, Whittle’s W.1X turbojet, a flight ready engine, was tested for the first time.
Japan though, was not idle in 1940. Early in the year, Tanegashima, with the help of the Mitsui Seiki Kogyo K.K., created a free piston compressor for a gas turbine based on a Junkers design, but it was not a success as a means for aircraft propulsion. Another attempt was tried by a different department. Under the leadership of Lieutenant Commander Osamu Nagano, head of the Kūgishō aircraft engine division, and Masanori Miyata, who led the Kūgishō electric parts section, built a tiny free piston compressor gas turbine, generating one tenth of a horsepower at 12,000rpm that drove a magneto that lit a lamp. Despite this measure of success, apathy on the part of the IJN continued to stymie progress. Tanegashima soon realised that the Japanese industry was not capable of constructing a free piston engine and switched his studies to axial flow jets.
On 15 May 1941, the Gloster E.28/39 flew for the first time, but previously in April, Heinkel’s He 280 V1 had flown under jet power on its maiden flight, the first jet fighter to fly. 1942 saw the Junkers Jumo 004 under test while BMW focused efforts on the BMW 003 Sturm. Heinkel was instructed to concentrate on developing the HeS 011, a turbojet that was to power the second generation of German jets. On 18 July, the Messerschmitt Me 262 flew under turbojet power, becoming the second jet fighter to fly, and on 2 October, the American Bell XP-59 Airacomet jet fighter made its maiden flight. By this time, Japanese engineers and scientists had learned of the flight of the He 178 as proof that an aircraft powered by a jet engine was feasible. This was just the boost the flagging Japanese jet engine research desperately needed.
As a result two different paths were taken with renewed vigour. The first employed the principle of the thermojet (as used by Secondo Campini) and was called the Tsu-11. While this engine was to be selected for use in the Kūgishō Ōka Model 22, it was found to be unsuited as a powerplant for a jet aircraft. The second route, that of a pure jet engine, was pursued further. Kūgishō’s Vice Admiral Misao Wada was the man who oversaw the development of a turbojet and the first result was the TR-10. This had a single stage, centrifugal compressor with a single stage turbine and was, in essence, built by adapting a turbosupercharger. The engine was constructed by Ebara Seizō K.K. When the TR-10 was first tested in the summer of 1943 its performance did not meet expectations. The TR-10 was renamed the Ne 10 and the engine was further developed by adding four axial stages in front of the engine inlet. This reduced the load on the centrifugal compressor, lowered the engine RPM and produced more thrust. The revised jet engine was designated the Ne 12. The problem with the Ne 12, however, was its great weight and so steps were taken to lighten the engine, which resulted in the Ne 12B.
1944 was an ominous year for Japan. When the Mariana Islands of Saipan and Tinian were wrestled from the Japanese by US forces in July and August, Japan found herself well within striking distance of the Boeing B-29 Superfortresses. Prior to this, B-29 raids had to fly from remote bases in China and India and so the bombing of Japanese targets was relatively rare. Staging from Saipan and Tinian, B-29s were far closer, could be more active and the Japanese were only too aware of this. In addition, it was surmised that it would only be a matter of time before the main Japanese islands were targeted for invasion. In August 1944, the Kaigun Koku Hombu called for a meeting to discuss changes in air strategy to combat the air and land threat as well as to consider the aircraft that would be used. The Kaigun Koku Hombu invited aircraft designers from both Nakajima and Kawanishi to attend and the outcome of this meeting was the proposal for three classes of aircraft termed Kōkoku Heiki (one literal translation being ‘Empire Weapon’). The first class, or Kōkoku Heiki No.1, was the adaptation of current aircraft to accept a 800kg (1,760 lb) bomb with which their pilots would undertake shimpū missions and target enemy invasion ships. If the bomb overloaded the carrying capacity of the aircraft, then RATO (Rocket Assisted Take-Off) units would be used to get them airborne. Kōkoku Heiki No.3 was to be a conventional, radial engine aircraft designed by Kawanishi as the Tokkō-ki, which would be used for shimpū missions, but this project was soon abandoned (perhaps because the IJN was to build the similar Nakajima Ki-115 as the Showa Toka). It would be Kōkoku Heiki No.2 which provided the seed for the Nakajima Kitsuka. This ‘Empire Weapon’ was to be an aircraft that used the Tsu-11 and, when available, the Ne 12 turbojet.
However, three months prior to the meeting, efforts were underway to obtain the Me 262 from Germany. In May 1944, the Japanese negotiated for the manufacturing rights to the Me 262 and the Germans initially agreed to the release. However, the deal was not concluded due to the large number of modifications that the design was found to require after its flight testing. It was not until July 1944 that orders were given to provide the Japanese with blueprints of the Me 262 fighter and the Junkers Jumo 004 and BMW 003 turbojets.
On 22 July 1944, Reichsmarschall Hermann Göring authorised the licensing of the Me 262 to Japan and the delivery of one sample aircraft. However, the Japanese submarine I-29 had left Lorient, France, on 16 April with a sample Junkers Jumo 004 turbojet and plans for the Me 262 and BMW 003 turbojet among its cargo. Also aboard the submarine was Technical Commander Eiichi Iwaya who carried on his person a portion of the documentation on the German fighter and turbojets. By 14 July, the I-29 had arrived in Singapore. Iwaya, seeking to reach Japan as soon as possible, disembarked from I-29 and took only a portion of the German documentation. From Singapore, Iwaya flew to Tōkyō. On 26 July, Allied code intercepts pinpointed the location of I-29 and the USS Sawfish sent her to the bottom near the Balintang Channel in the Luzon Strait, taking the precious cargo with her.
When Iwaya arrived in Japan, all he possessed of the German files with regards to the Me 262 and turbojets was a single copy of a cross-section of the BMW 003A turbojet. The subsequent news of the loss of I-29 was a crushing blow, but not a fatal one by any means. In studying the BMW 003A document, the Japanese found it to be of a similar design to the Ne 12 but instead of the centrifugal compressor the German engine used an eight stage axial-flow compressor. It was adjudged that this method was superior to the Ne 12 and as such, efforts should be concentrated on building the Japanese equivalent to the BMW 003A. Despite the decision against it, work on the Ne 12B continued. Four companies were involved in the development of the new turbojet. Each was to be provided with a copy of the BMW 003A cross-section and other available data and to build their own versions. Ishikawajima-Shibaura Turbine Company was to develop the Ne 130, Nakajima Hikōki K.K. the Ne 230, Mitsubishi Jūkōgyō K.K. the Ne 330, and Kūgishō would move forwards with the Ne 20.
Following the August conference with the Kaigun Koku Hombu, Ken’ichi Matsumura, chief designer for Nakajima and with the assistance of Kazuo Ōno, produced a number of concept drawings for the Kōkoku Heiki No.2. Within Nakajima, the new aircraft was given the codename Maru-Ten. On 14 September 1944, IJN representatives met with Nakajima at their Koizumi plant to discuss the concepts which had been put forward. The design that stood out was based on a description of the Me 262 as provided by Technical Commander Eiichi Iwaya who, while in Germany, was able to view and study the German jet. Thus, Matsumura’s drawing bore an outward resemblance to the Me 262. After reviewing the concept, the design was approved as the Kōkoku Heiki No.2. In keeping with the shimpū mission of the aircraft, the initial design had no landing gear and was to be launched from catapult ramps, boosted with RATO units. The calculated range was a mere 204km (127 miles) due to the designated engine, the Ne 12, which burned fuel at a rapid rate. At sea level the estimated speed was 639km/h (397mph). A single bomb fixed to the aircraft was the only armament. Another feature was the inclusion of folding wings to allow the aircraft to be hidden in caves and tunnels and protected from bombing attacks.
On 8 October, Kūgishō ordered Kazuo Yoshida, plant director for Nakajima, to have a wooden mock-up of the aircraft completed and ready for inspection by the end of the month. In addition Nakajima was told to have the initial structural plans finished by the same date. This was ordered so that production of the aircraft could begin without delay. Unfortunately, delays would be a major problem. The IJN promised that the Ne 12 would be ready for testing by November 1944 and in short order thereafter, production engines would be available. Based on this assumption, Nakajima was to construct thirty aeroplanes by the end of December 1944. Because of the rush to produce the aircraft, a myriad of problems arose with the design which necessitated changes. A major issue was the lack of critical war materials which required the use of substitutes and brought additional delays. To compound the problem, Nakajima was concerned that the Ne 12B would not be ready despite the IJN’s promises.
Meanwhile, Kūgishō proceeded with the Ne 20. The engineers were forced to use alloys which were not to the standards of the German engine and would be a source of problems during testing. The design of the Ne 20 was smaller than the BMW 003A but it retained the combustion chamber shape of the German engine. While it used the same size of burner as the BMW 003A, it only used twelve instead of sixteen due to the smaller size. Kūgishō would draft and refine the design of the Ne 20 through December.
On 9 December 1944, the IJN called a meeting to discuss the progress and outlook of the Kōkoku Heiki No.2. Based on the problems Nakajima were having with the aircraft, not to mention the doubts about the Ne 12, the production schedule was revised. Nakajima were requested to produce the first prototype by February 1945 for use in static testing. It was also during this meeting that the aircraft’s specifications underwent a revision. Instead of a fixed bomb, the bomb could now be released by the pilot. The role of the aircraft was also changed. No longer was it to be used for a shimpū mission but instead for close air support, the aircraft acting as a fast attack bomber. As a consequence of these changes, the design had to incorporate a landing gear. The IJN issued its specifications for the new jet, which was now called the Kitsuka, and the documents requested:
Span: no more than 5.3m (17.3ft) with the wings folded
Length: no more than 9.5m (31.1ft)
Height: no more than 3.1m (10.1ft)
Powerplant: Two Ne 12 jet engines
Maximum Speed: 513km/h (319mph) with 500kg (1,102 lb) bomb
Range: 204km (127 miles) with a 500kg (1,102 lb) bomb or 278km (173 miles) with a 250kg (551 lb) bomb
Landing Speed: 148km/h (92mph)
Take-off Run: 350m (1,148ft) using two 450kg (992 lb) RATO bottles
Manoeuvrability: The aircraft had to be highly manoeuvrable, have a short turn radius and be stable at speed to facilitate target tracking
Protection: Shatter proof glass for the canopy. Front windscreen to have 70mm of bullet proof glass. 12mm of steel armour plate below and behind the pilot. Fuel tanks to be 22mm sandwich types
Basic Instrumentation: Tachometer, altimeter, artificial horizon, airspeed indicator, Model O Type 1 flux gate compass, fuel pressure gauge, oil pressure gauge, oil temperature gauge, tail pipe temperature gauge and a pitot tube electric heater
Basic Equipment: Type O parachute, automatic fire extinguisher, Type 3 dry battery, Type 3 radio receiver, Type 1 life raft and a reserve weight of 30kg (66.1 lb)
1945 would open with more misfortune for the Japanese war machine. Japanese troops were pushed out of Burma from 5 January and B-29s would bomb Tōkyō the next day. Two days earlier, Matsumura and Ōno, along with others involved in the Kitsuka project, discussed the possibility of using the Ne 20 turbojet in place of the Ne 12. In the debate, some suggested that the Ne 20 was not as far in development than the Ne 12 and would delay progress if used. On the other hand, some argued that the Ne 12 was not achieving significant results. In the end, the consensus was that the Ne 12 should remain as the powerplant only because it was projected to be ready before the Ne 20.
On 28 January 1945, the wooden mock-up of the Kitsuka was finally ready for inspection at Nakajima’s Koizumi plant. Vice Admiral Misao Wada and his staff visited the plant and inspected the mock-up with both Matsumura and Ōno in attendance. It was made clear to the Kūgishō inspectors that the Kitsuka was a very simple aircraft that could be constructed in 7,500 man-hours. By comparison, it took 15,000 man-hours to build a Mitsubishi A6M Reisen. Following the inspection, Nakajima was told to make two slight adjustments to the Kitsuka. The first involved the windscreen. Originally, the front windscreen was rounded but now it was desired that it should be flat panelled. This change may have been suggested to allow for the future installation of a reflector gun sight because such a sight requires flat panels to avoid sighting problems due to canopy distortion. The second alteration was to make the canopy slide to the rear instead of opening to the side. At the conclusion of the meeting, Nakajima was told to cease all work on the Nakajima J5N1 Tenrai and the company was also informed that they could expect the Nakajima G8N1 Renzan to be terminated as well. These changes in production and development were done to speed the coming production of the Kitsuka. The close of January also saw the final design draft of the Ne 20 completed and almost immediately work began to build the first engine. Kūgishō’s Aero Engine Division provided 400 machine tools and engineers and labourers began to toil day and night to realise the Ne 20.
February 1945 opened with the Japanese naval docks in Singapore targeted and destroyed by B-29 bombers along with continued fighting in the Philippines. A second inspection of the Kitsuka was called for on 10 February. Present at the inspection, among the other engineers and Kūgishō personnel, were Technical Commander Iwaya and the man who was destined to fly the Kitsuka, Lieutenant Commander Susumu Takaoka. The Kitsuka was given final approval and production was to commence at once, even before the Kitsuka had been flight tested. The first five Kitsuka aircraft, No.1 through No.5, were to serve as prototypes and none would be fitted with armour plating or self-sealing fuel tanks. In addition the first two aircraft would not to be equipped with the bomb carrying apparatus. February would also see the Ne 12B tested for the first time.
Unfortunately for the Kitsuka, US bombing ensured that production did not go smoothly. Due to the ever increasing number of strikes against the industrial centres of Japan, it was felt that it was only a matter of time before the Nakajima Koizumi plant would attract the attention of US bombers. Therefore, on 17 February, engineering staff for the Kitsuka was moved to Sano in Tochigi Prefecture. Despite the move, a sizable portion of the Kitsuka component construction remained at Koizumi while the wings, tail assembly and the centre and aft portion of the fuselage were constructed by Kūgishō in Yokosuka. In the face of further bombing attacks, production was dispersed among silkworm factories and buildings in Gunma Prefecture (northwest of Tōkyō).
March arrived in a blaze of smoke and fire as the US ramped up their incendiary bomb campaign against Japan’s cities. Tōkyō and Nagoya were particular targets, the burning cities lighting the night sky. On March 26, the first Ne 20 engine was successfully test run from a cave set into a cliff in Yokosuka. With the success of the Ne 20, the Kitsuka engineering team began to seriously consider replacing the Ne 12B with the Ne 20. It was clear that the Ne 20 outperformed the Ne 12B and, based on the higher thrust potential, it was decided that the Kitsuka should use the Ne 20 even if it meant a longer delay while the engine became available. Although the current Kitsuka production did not yet involve the engine mountings, a revision of the aircraft design plans was required to accommodate the Ne 20. By March 31, these revisions were complete and the Kitsuka program entered a stage of finality.
With the revised Kitsuka, some of the specifications were adjusted as follows:
Maximum Speed: 620km/h (385mph) with a 500kg (1,102 lb) bomb at sea level
Range: 351km (218 miles), at sea level, at full power
Take-off Run: 500m (1,640ft) with two 450kg (992 lb) RATO bottles
Landing Speed: 92km/h (57mph)
Bomb Load: 500kg (1,102 lb) as normal with the ability to carry a 800kg (1,763 lb) bomb; a Type 3 rack would be used for the larger bomb
Protection: Reduce the bullet proof glass thickness to 50mm and add 12mm of armour to the front of the cockpit, while the fuel tanks would incorporate an automatic fire suppression system
Engineers working on the Ne 20 found that, although the initial test of the engine was a success, there were many issues to solve. At first, the blades were prone to cracking but this was soon overcome. An electric starter was fitted into the compressor spinner that could spin the engine at 2,250rpm; the engine would reach maximum RPM within 10-15 seconds of engine start. Gasoline was used to start the engine and once running the fuel was switched to a pine root distillate using 20-30 per cent gasoline. What was becoming a problem was how to position the tail cone. Lieutenant Commander Osamu Nagano and his team, along with Captain Tokiyasu Tanegashima, laboured to refine the Ne 20. The worsening bombing situation saw the Ne 20 team moving to Hadano in Kanagawa Prefecture, a three hour drive from Yokosuka.