- Mitsubishi J2M Raiden
Aircraft manufacturers competed for the few in-line engines available in 1939. With the potentially deteriorating situation that a long war could create, some aerodynamic essays were performed to reduce the radial engines drag in Germany, Belgium and the USA. The Renard R-37 was introduced to the public in July 1939 as an emergency solution, in face of the prospect that France and the United Kingdom might cancel the exports of the Hispano-Suiza and Merlin in-line engines. The R-37 was propelled by a large Gnome-Rhône engine of 1,100 hp with a large propeller spinner which completely covered the engine front air intake.
In Germany, the Focke-Wulf company had the same problem after realising that the whole production of Daimler Benz in-line engines had been assigned to Messerschmitt. The design team of Kurt Tank was impelled to use the BMW radial engine for the new Fw 190 fighter, a prototype of which flew in June 1939 with an extremely aerodynamic cowling and a large ducted spinner to reduce drag.
In USA the NACA published a report at the beginning of 1939 about the methods to improve the volume of cooling air flowing over radial engines. In February 1939, Seversky performed the first flight tests with the NX2597 AP-4 Lancer prototype, equipped with ducted and non-ducted large propeller spinners. In March, the Curtiss Company decided to experiment with the new technology transforming the P-36A s/n 38-004 into the XP-42 prototype by adding a large propeller spinner and extension shaft with air intake under the engine.
In May, the Japanese flight tested the third prototype of the Kawasaki Ki.45 with a spinner that was almost identical to that of the Fw 190 V1. The NX21755 c/n 142 Vultee 48 fighter flew for the first time in September with an equipment similar to that of the Curtiss XP-42.
By mid-1939 the Northrop Company tested the A-17A s/n 36-184 bomber with ducted and non-ducted large propeller spinners and several nose and side-mounted blowers configurations. In United Kingdom, the Hawker company performed several late experiments by the beginning of 1945, modifying the Tempest Mk.V NV768 with several types of ducted spinners.
As a result of all this research, however, there was not any significant aerodynamic achievement. The R-37 was captured by the Germans before starting its flight tests and the Fw 190 V1, the Curtiss XP-42, the Vultee 48, the Kawasaki Ki.45-03 and the Seversky Lancer all encountered insurmountable cooling problems and had to be refitted with conventional cowlings. The A-17 A was turned back to its original configuration and back to operations.
When the team of Jiro Horikoshi started the design of the Raiden, by the end of 1939, the result of previously described experiments that were understandably kept secret, was unknown at Mitsubishi. It was also ignored that the XP-42 extension shaft had suffered serious problems of vibrations. At this time the main worry of the IJN was the possibility that the North American B-17, able to fly at 11,000 m, would be operational.
Every naval fighter of the 30s was designed to protect the fleet against low altitude attacks by torpedoes and dive bombers, but a four-engine heavy bomber could attack the Japanese naval bases from a high altitude without any opposition. The whole Zero manufacturing programme might be modified or even cancelled if the B-17 was mass produced and operated from the air bases of Philippines and Central China.
And so, the IJN published a specification for the design of a fighter able to intercept a B-17 flying at a speed of 600 km/h and an altitude of 6,000 m with a climbing time of 5 minutes. However, when the enemy bombers first appeared over Tokyo they were of the B-29 type, flying at a speed of 575 km/h and an altitude of 7,600 m with an absolute ceiling of 9,700 m. They were so large, heavy and powerful that the turbulence they created in the air could produce a total loss of control to the small Ki.44 fighters of the IJA that tried to intercept them. The key was in the superchargers of their huge engines. Their manufacturing techniques were considered top secret up to an extent that not even the Australian of British Allies had access to them.
Japan lacked the experience or the technology to duplicate the samples obtained from the airplanes that were shot down and their attempts with the Ki.100 and the J2M4 did not achieve practical results. It was also impossible for the IJN to have access to the German Daimler Benz in-line engines, given their rivalry with IJA that have acquired the manufacturing licence of the DB 601A for the Ki.60 fighter. Like the Italians, they could only pursue the aerodynamic solution.
The Raiden was designed with a fuse shaped fuselage with a propeller extended shaft, like that in the XP-42, to permit a finely tapered cowling, a narrow annular air intake like that in the Renard R-37 and the engine-driven fan like that in the Focke-Wulf Fw 190 V1. The cockpit was extremely shallow with curved windscreen and the wings were shorter than those used by the Zero with laminar flow aerofoil section.
The selected radial engine was the most powerful available with water-methanol injection system. The armament was the same than that of the Zero, two Type 97 machine guns of 7.7 mm and two Type 99 cannons of 20 mm. Real combat experience showed that all this was wrong; pilots complained of lack of visibility and the curved windscreen cockpit was replaced by a flat panel windscreen to avoid optical distortion. The extended shaft caused serious problems of vibrations and, as a consequence, the engine attachment points and the cowling fasteners had to be reinforced, the number of fan blades reduced, and the propeller replaced by a more rigid one with a hydraulically controlled system.
The water-methanol injection system behaved hazardously and, although capable of providing extra power during take-off, it was useless above 8,000 m of altitude. At higher altitude they should have used the German GM-1 system that generated extra power boost for nitrous-oxide injection. However, this technology was apparently either not shared or not properly used. The short wings were not right for combat at high altitude; controls did not work well at high speed and manoeuvrability was poor at any altitude. The Raiden could not face the P-47 and P-51 escort airplanes. The machine guns – weight of 35 kg excluding the synchronization system – were practically useless against the American well armoured fighters and bombers. The Type 99-I cannons were the lightest 20 mm weapon in the world; they had short barrel and low muzzle velocity and were fed by a 60 rounds drum.
The J2M3 replaced the machine guns by a pair of Type 99-II cannons with major muzzle velocity and belt feed, but the simultaneous usage of both types of guns with different ballistic hindered the aiming. Some units of the J2M3-Ko with four Type 99-II cannons were manufactured to solve this issue, which led to increasing the structural resistance of the wing and reducing fuel capacity even further. The J2M2 was able to transport two bombs of 30 kg under the wings, and two of 60 kg in the case of the J2M3.
Considering that the Taki 18 warning radars of the time could not accurately estimate the flight altitude of hostile airplanes and that the alert was issued with an advance of 30 minutes at its best, the Raiden interceptors had just the time to scramble up to 10,000 m and wait for the arrival of the bombers. Reaching that altitude, however, took them 19.5 minutes and fuel did not last more than 40 minutes under those conditions, given that they could not afford the additional weight of a detachable fuel tank. If the B-29 airplanes did not arrive on time or deviated towards another target, the Raiden had to abort interception due to lack of fuel. Otherwise, they could only make a ‘hit and run’ type frontal attack to avoid the threat posed by the 20 mm guns on the tail of the B-29 bombers.
The attack was performed at an altitude between 6,000 and 9,000 m and required that both airplanes crossed at a compound speed between 970 and 1,000 kph. The practical range of the Type 99-I was of 1,000 m and of 400 m in the case of the Type 97 machine guns. If the Raiden J2M2 started shooting within the lethal area of 1,000 yards – under the fire of all the heavy dorsal machine guns of the group that would not be less than 60 per box – and should start deviating at least 100 m before the impact, it could only shoot 58 rounds of 20 mm. In the case of the J2M3, it could shoot 108 rounds of 20 mm and around 100 rounds in the case of the J2M3-Ko.
The Luftwaffe statistics on the probability to shot down a B-29 with a 95 per cent of certainty, from a range of 1,000 m, stated that at least 203 projectiles of 20 mm should be shot to make 36 impacts, equivalent to approximately 500 gr of HE or to an impact of a 55 mm gun. The J2M2 just carried 120 projectiles of 20 mm and the J2M3-Ko around 400 projectiles. The Raiden was difficult to control at 600 kph, with its heavy ailerons and lack of manoeuvrability, and therefore the probability to shot down a B-29 in a single attack was below 50 percent.
And a unique attack is all that the Japanese could afford with just 40 minutes of fuel. In the way back to the base, the Raiden airplanes were pursued by the P-51 that were superior in manoeuvrability. Many were shot down and those that could escape in vertical dive suffered compressibility buffeting when reaching Mach 0.75. The bigger structural resistance of the J2M3 wing gave them some advantage under 5,000 m with at least one Mustang losing its tail when trying to reach them over Yokohama on 29 March 1945.
The Raiden were scarce and the IJN had forbidden their usage for ramming both to them and to the valuable Shiden KAI. In September 1944 they used small Type 99, Number 3, Mark 3 cluster bombs of 33.7 kg to shoot down 17 B-24 bombers over Menado. This tactic was used by the J2M2 of the 381st Kokutai that had been specially modified to carry small weight under the wings. Only the 303rd, 332nd, 343rd and 352nd Kokutais actually ever fought against the B-29 airplanes.
From April 1945 onwards the J2M3 started to use the new Type 3, Number 6, Mark 27, Model I rocket-bombs of 60 kg that were shot from the rails installed under the wings. They reached a speed of 270 m/s, exploding at a predefined distance by means of a clockwork fuse, disseminating 140 iron pellets and 4 kg of white phosphorus in a conical pattern of 60 degrees. The rocket-bombs were shot from 1,200 m and before the attack with guns started.
However, the B-29 was an airplane difficult to shoot down. Some bombers could even return to base after being rammed by two fighters! The 302nd Kokutai claimed several damaged B-29, as well as the 42-24735 downed over Tokyo on 3 December 1944 and during some fighting at high altitude against B-29 and P-51 over Yokohama on 29 May 1945. Several damaged bombers were reported, along with the 42-24735 downed over Kyushu at the beginning of April, the 44-69966 over Nagoya on 14 May and the 42-63567 over Omiya, on 10 June. The 302nd Kokutai could also shoot down the 42-65295 B-29 over Kyushu using the burst bombs on 29 April.
The 352 nd Kokutai claimed the downing of the 42-93848 over Omura on 21 November 1944. The 332 nd Kokutai reported another shot down over Hansin on 22 December 1944. The Yokosuka Kokutai had some success against the naval airplanes of the Task Force 58 during their attack against Tokyo on 16 February 1945 when several F6F-5 fighters were destroyed.
Between the end of 1943 and the end of 1945, a small number of Raiden fighters were sent for operational testing to the Yokosuka, Yatabe, Genzan, Tainan, Konoike and Chusi Kokutais, as well as to the 256th, 301st, 381st, 1001st and 1081st Kokutais. They were used for the specific defence of the Chosen, Genzan, Ranan, Funei, Rashin, Konan, Guam, Saipan and Philippines airfields. Although in the great battles over Okinawa and Kyushu the pilots preferred to replace the Raiden by Zero airplanes whenever possible.
By the end of 1942, Mitsubishi produced the mechanically driven Ru-302 supercharger, with two-stages and two-speeds, para el Ki.46-IV high-altitude reconnaissance airplane. The Ru-302 did not perform well during the tests, conducted in May 1944 with a J2M4 Raiden fighter.
In August 1944, the Mitsubishi J2M5 was flown with one Ru-303 exhaust-driven turbo-supercharger mounted in the starboard side of the fuselage, just behind the Kasei 23c engine. The new supercharger also didn't work properly, causing fires during testing, and never became operational.
In the face of such lacklustre results, the IJN decided to manufacture the Kawanishi N1K2-J Shiden KAI on a mass-scale and to assign the remaining Shiden and Raiden to the Kokoku Heiki Go.1 programme of modifications that would transform them into suicide airplanes.
Mitsubishi engineers tried to develop an exhaust-driven turbo-supercharger based on the study of North American P-43 and B-17 aircraft captured in China and Philippines. By mid-1944 turbo-superchargers were indispensable to fight the B-29 bombers over 10,000 m, but the Japanese industry was unable to duplicate the captured units. The General Electric turbo-supercharger was a product of enormous technical and manufacturing resources that was not available in Japan.
The high temperatures achieved by exhaust gas and the high rotation speeds of turbines (26,000 rpm) required the use of austenitic stainless-steel chrome-molybdenum alloys and the development of work-hardening techniques that enabled the turbocharger to withstand stresses for centrifugal forces. The precision machining of turbines and impellers was made possible by sophisticated machine tools and surplus of raw materials.
In August 1944 the Mitsubishi J2M4 Raiden 34 was flown with one Ru-303 exhaust-driven turbo-supercharger mounted in the starboard side of the fuselage, just behind the Kasei 23c engine. The new supercharger did not work properly, causing fires during testing, and never became operational. The exhaust-driven turbo-superchargers were larger, involved extra piping and increasing an aircraft size, weight, complexity and cost. It is not possible to install them in a conventional single engine fighter and its use requires the design of special airplanes with enough room to install the turbo, the intercooler and the heavy tubing system.
With the entry into service of the first Focke-Wulf Fw 190 fighters with the Luftwaffe, in late 1941, it was necessary to upgrade the Merlin Mk 60, which used the Mk. IX Spitfires, giving them with a two-stage, two-speed supercharger. Each stage had its own impeller, diffuser and horn, the stages were placed in series, the first stage feeding into the second. At the end of the war, the Merlin engines had two-stage superchargers, each with three-speeds and the German BMW 801R radial engine, that powered the latest versions of the Fw 190, had two-stage, four-speed supercharger.
The study of a Merlin XX British engine, from a Hawker Hurricane Mk.II captured in Singapore, allowed Kawasaki engineers to build their first two-stage supercharger for the Ha-140 engine. The British system used two impellers that rotated at different speeds selected by the pilot by means of a gearbox. The use of two impellers in engines of more than 1,500 hp requires the installation of an intercooler (sits between the supercharger and the engine) to avoid the premature detonation problems that occur when air is injected into the carburetor at an excessive temperature.
Availability of 100 to 130-octane type 44-1 fuel let the American engines run hotter without detonation problems, the Japanese only had 87-octane fuel and should use forced air-cooling fans to avoid the overheating of their supercharged engines. By acquiring the manufacturing licence of the German engine Daimler Benz DB 601, the Japanese gained access to the Vulkan coupling technology, a supercharger fitted with continuously variable transmission device that automatically regulated the rotation speed of the impeller by means of a barometric control. With the use of the new mechanically driven superchargers Ru-302, it was expected to get a service ceiling of 14,000 m.
By the end of 1942, Mitsubishi produced the mechanically driven Ru-302 supercharger, with two-stages and two-speeds, for the Ki.46-IV high-altitude reconnaissance airplane. The Ru-302 did not perform well during the tests, conducted in
May 1944 with a J2M4 Raiden 33 fighter. Mitsubishi began working on a new type of supercharger, for the 2,130 hp Ha-43-42 engine, which combined the British technology (two-stage, three speed) technology with the Vulkan coupling German device. The first stage was mechanically driven by the engine and the continuously variable speed second stage was driven using the Vulkan hydraulic transmission.
Kokoku Heiki
In August 1944 the Japanese Technical Office Kaigun Koku Hombu published the specifications for the project Kokoku Heiki (Empire Weapon). It required the manufacturing of three classes of suicide airplanes that carrying a bomb of 800 kg would be capable of attacking the invasion fleets.
Kokoku Heiki Go.1 was a transformation program of existing naval airplanes to optimize their use in suicide missions. Based on the experience acquired with the Zero and the Val, the proposal was to modify the Kawanishi N1K1-J Shiden, the Mitsubishi J2M Raiden, the Yokosuka D4Y Suisei, the Nakajima B6N Tenzan and the Aichi B7A Ryusei.
Kokoku Heiki Go.2 should be a suicide bomber propelled by two centrifugal turbojets TsU-11, Ne-12 or Ne-30. Nakajima’s proposal was to manufacture the Maru-Ten whereas Yokosuka/Kugisho proposed the R2Y2 Keiun and the Tenga.
Kokoku Heiki Go.3 should be a conventional and easy to manufacture airplane, using non-strategic materials and designed to allow the installation of several types of reconditioned surplus engines. Kawanishi proposed the Tokko-Ki, Nakajima the Tsurugi, Sowa the Toka and Tachikawa the Ta-Go.
In the face of such lacklustre results, the IJN decided to manufacture the Kawanishi N1K2-J Shiden KAI on a mass-scale and to assign the remaining Shiden and Raiden to the Kokoku Heiki Go.1 programme of modifications that would transform them into suicide airplanes. The undercarriage legs of the Raiden were short and sturdy, leaving insufficient space between the airplane belly and the ground to install a rocket booster package like the one used in the transformation of the Kawanishi Shiden. It could however transport a Number 80, Model 2 bomb of 807.5 kg (without tailfins) under the central section of the wing, instead of the detachable fuel tank of 400 lt.
Although no illustrations were kept describing the transformation, it may be expected that there would be three Toku-Ro.1 Type 2 rockets fitted to the lower fuselage, following a similar scheme than in the suicide version of the Yokosuka D4Y Suisei, two on the sides in RATO mode and one central for the terminal dive. The procedure required the installation of two I.S.R. 12 cm rocket launcher tubes under the wings and the removal of the cannons keeping the Type 97 machine guns like those in the Shiden.
