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The Mitsubishi J2M Raiden

The Mitsubishi J2M Raiden (Thunderbolt) land-based fighter used by the Japanese Navy was the first Japanese fighter to be designed from the outset for the interception role, with emphasis being placed on speed and climb rather than on maneuverability. Its good performance, powerful armament, and adequate armor protection made it perhaps the most effective bomber destroyer used against the B-29 by the Japanese during the Pacific War. However, the Raiden was persistently plagued by technical difficulties and production snags throughout its entire life, and only 476 of these excellent interceptors were built, too few and too late to affect the outcome of the war.

The design originated back in October of 1938, when Mitsubishi's chief designer Jiro Horikoshi held preliminary discussions with the Japanese Naval Air Force about a new land-based interceptor that would stress speed and climbing performance rather than maneuverability. This was a bold departure for the Japanese Navy, and since the Mitsubishi team was preoccupied with the A6M Reisen (Zero Fighter), the project was kept in limbo for a year or so.

It was not until September of 1939 that an official 14-Shi (14th year of the Showa reign, or 1939) specification was drawn up. The 14-Shi specification called for a land-based interceptor capable of reaching a maximum speed of 373 mph at an altitude of 19,685 feet. It was to be able to climb to this altitude in less than 5.5 minutes. The endurance was to be 45 minutes at full-rated power. The takeoff run at overload weight was not to exceed 985 feet. The landing speed was to be no greater than 81 mph. Armament was to be the same as that of the A6M2 Reisen (Zero Fighter) --- two 20-mm cannon and two 7.7-mm machine guns. Armor plate was to be incorporated behind the pilot's seat. There was no mention of maneuverability in the specification.

The choice of the engine was left up to the designer. Horikoshi had a choice between the Aichi Ha-60 Atsuta (a derivative of the Daimler-Benz DB 601A) inverted-vee 12-cylinder liquid cooled engine rated at 1185 hp for takeoff and the Mitsubishi Ha-32 Kasei Model 13 14-cylinder air cooled radial rated at 1440 hp for takeoff. Although the Aichi Tokei Denki K. K. (Aichi Clock and Electric Company, Ltd) promised a future increase of at least 15-20 percent in the power of the Atsuta engine, Jiro Horikoshi decided to select the more powerful Kasei radial despite its higher fuel consumption and larger frontal area.

Design work began in the early weeks of 1940. The first prototype was designated J2M1. It was powered by a 1460 hp Mitsubishi MK4C Kasei 13 radial engine driving a 3-bladed propeller. Since the Kasei radial engine had a rather large frontal area, in order to minimize drag the engine was fitted with an extension shaft to permit the use of a finely-tapered cowling. An engine-driven fan pulled in cooling air through a narrow annular intake in the front of the cowling. A low aspect ratio wing with a laminar-flow airfoil was selected, and an extremely-shallow, curved windshield was used in order to provide streamlining. Combat flaps were fitted to improve the maneuverability.

The high priority given by Mitsubishi to the development of the A6M Reisen (Zero Fighter) series of carrier-based fighters caused the completion of the first prototype J2M1 to be delayed until February of 1942. By this time, Dr. Jiro Horikoshi, suffering from overwork, had relinquished his post of chief designer to Kiro Takahashi. The prototype flew for the first time on March 20, 1942, with Mitsubishi test pilot Katsuzo Shima at the controls. A total of three J2M1 prototypes were built.

The J2M1 handled well, but test pilots complained that the view from the cockpit was totally inadequate and the curvature of the windshield severely distorted forward vision, especially during landing. The propeller pitch change mechanism proved to be unreliable, and the main undercarriage members had difficulty in retracting at speeds greater than 100 mph. The ailerons tended to stiffen up at speeds above 323 mph. In addition, the speed (357 mph at 19,685 feet) and the climb rate (19,685 feet in 7.8 min) were both below those promised in the original specification. Consequently, the fourth prototype was extensively reworked into the more-powerful J2M2, which is the subject of the next post.

The performance of the three J2M1 prototypes had been found to fall below the original 14-Shi specification. In addition, the view from the cockpit (especially in the forward direction) had been found to be totally inadequate. In order to correct these deficiencies, the fourth prototype was modified to take the Mitsubishi MK4R-A Kasei 23a radial, which could provide 1820 hp by incorporating water-methanol injection into the supercharger. It was fitted with individual exhaust stacks which provided some degree of thrust augmentation. The engine was modified to incorporate a marginally shorter extension shaft. With the new engine, the length of the nose could be reduced by about a foot, improving the pilot's view during landing. In addition, a four-bladed propeller was fitted and a taller cockpit canopy with flat panels was provided in an attempt to improve the forward view. However, the engine weight had increased by 14 percent, so the fuel tankage had to be reduced from 156 Imp gall to 120.5 Imp gall in order to maintain the position of the center of gravity. The fuel was distributed between two 46 Imp gall wing tanks and a single 28.5 Imp gall fuel tank in the fuselage immediately aft of the engine firewall. With these changes, the fourth prototype was redesignated J2M2.

The J2M2 took off on its initial flight on October 12, 1942. Armament consisted of two 7.7-mm Type 97 machine guns in the upper fuselage decking with 550 rpg and two wing-mounted Type 99 Model 2 20-mm cannon with 200 rpg. The Japanese navy was so confident that the J2M2 would meet its requirements that they ordered the design into production as the J2M2 Raiden (Thunderbolt) Interceptor Model 11 even before the aircraft could be fully evaluated. Production was to take place at Mitsubishi's No. 3 (Airframe) plant at Nagoya.

A few production J2M2s began to reach development units late in 1942. It turned out that the decision to place the Raiden in production had been premature. Almost immediately upon entry into service, severe problems were encountered with the fuel-injected Kasei 23a engine. Excessive smoke was emitted by the engine when it was being operated at its maximum power. At certain engine speeds, uncontrollable vibrations could be encountered. These problems were gradually alleviated by introducing appropriate modifications to the engine mounts and to the water-methanol injection system, but progress was difficult and delivery rates were slow. By March of 1943, only 11 J2M2 aircraft had been delivered.

The second J2M2 was lost in a takeoff accident on June 16, 1943. Shortly after takeoff, the fighter had inexplicably nosed down and crashed into a barn, the fuel exploding on impact. A month later, the tenth J2M2 encountered the same problem, but this time the pilot was able to regain control of the aircraft by simply lowering the undercarriage. Examination of the aircraft revealed that the tailwheel struts had pressed against the torque tube lever after retraction, jamming the controls in the dive position. The modifications needed to correct this problem were fairly straightforward, but they resulted in an additional delay.

In December of 1943, the first batch of Raiden Model 11 fighters was delivered to the 381st Kokutai at Toyohashi, southeast of Nagoya. The production J2M2 was armed with two 7.7 mm machine guns in the fuselage and two 20-mm cannon in the wings. Deliveries were slow, only 141 examples being produced in the fiscal year 1943- 44.

In January 1944, the 30th J2M2 disintegrated in mid-air immediately after its pilot had made a firing pass at a target streamer. The cause of the accident was unknown, but it was believed that it was possible that a violent oscillation had set up when an engine attachment point broke loose. Alternatively, it was theorized that an engine cowling panel could have broken away and hit the tail. In any case, the engine attachment points were reinforced and the cowling fasteners were strengthened in an attempt to cure the problem. However, there were other incidents in which Raiden fighters disintegrated in midair with no satisfactory explanation. These accidents were never adequately explained.

The oscillation problems thought to have been cured by the changes introduced in the early development of the J2M2 would recur on occasion in the field. In an attempt to fix these problems, a variety of high-rigidity thick propeller blades were tested, but the problem was never adequately resolved. A series of crank-pin failures occurred, which were solved by raising the oil pressure. This in turn required that the oil temperature be raised, requiring the adoption of a new and enlarged oil cooler with an external air intake.

A total of 155 J2M2 aircraft were built before production was transferred to the J2M3 version in May of 1944.

Produced in parallel with the J2M2 was the J2M3 Raiden Model 21, which was destined to be the major production version. The J2M3 was equipped with a stronger wing carrying four 20-mm cannon. The two fuselage-mounted 7.7-mm machine guns were discarded. Two of the cannon were Model 2 versions with projecting muzzles and the other two were slower-firing Model Is buried entirely inside the wing. The additional wing guns had dictated some local strengthening of the wing structure and required that some reduction be made in the size of the wing-root fuel tanks. The fuselage tank was also slightly reduced in capacity. However, a 44-Imp gall drop tank could be carried underneath the fuselage centerline as an alternative to an external load of a pair of 132-pound bombs underneath the wings. The J2M3 standardized on the enlarged oil cooler with an external air intake that had been introduced during the production run of the J2M2. The Kasei 23a engine was retained.

The J2M3 was initially produced in parallel with the J2M2, but it soon supplanted it and became the major production model of the Raiden. A new domed cockpit canopy (which had been first tried out on the J2M6) was introduced on the production line in June of 1944 in order to address the continual complaints from pilots about poor vision from the cockpit.

The differing type of cannon carried by the J2M3 resulted in different ballistic characteristics. In an attempt to address this problem, the J2M3a Model 21A version was built. The J2M3a differed from the J2M3 only in having the two wing-mounted Type 99 Model 1 20 mm cannon removed and replaced by two 20-mm Type 99 Model 2 cannon carried in pods beneath the wings. The quartet of Model 2 cannon proved more effective, but the drag of the underwing gondolas had an adverse effect on performance, and only 21 of these J2M3a versions were built.

The first J2M3 appeared in October 1943, some time before the J2M2 had been delivered to the 381st Kokutai. This model was adopted in succession to the J2M2 and placed in production at Mitsubishi's Nagoya and Suzuka factories. The first production J2M3 was delivered at the beginning of February 1944.

Unfortunately, the J2M3 was somewhat heavier than the J2M2 owing to its better armament, and the J2M3 could no longer attain the performance called for in the original specification. In addition, its protracted teething troubles and poor mechanical reliability had resulted in slow deliveries and in low availability. Consequently, in June of 1944 the Japanese Navy decided to adopt the Kawanishi N1K1-J Shiden (Allied code name George) as its primary interceptor aircraft. However, production of the Raiden was permitted to continue at a reduced pace until the A7M Reppu (Hurricane) could be placed in production.

However, within weeks of the Japanese Navy's decision to phase out the Raiden, the B-29 Superfortress begin to appear. Since the J2M3 had a good high-altitude performance and an effective armament, it was judged to be a potent B-29 interceptor and its production priority was reinstated. In addition, The Koza Kaigun Kokusho (Koza Naval Air Arsenal) was instructed to join in Raiden production.

The production of Raiden fighters by the Mitsubishi Jugogyo K K of all types totaled 476. The Raiden made its operational debut in September of 1944 over the Marianas during the Battle of the Philippine Sea, when a small number of Raidens had operated from Guam. A technical manual on the Raiden was discovered by American intelligence after the capture of Saipan, and the Raiden was assigned the Allied code name Jack. A small number of Raidens were deployed to the Philippines and were active during the invasion of these islands by the Americans.

The Raiden got its primary use during the defense of the Japanese home islands. Its good performance, powerful armament, and armor protection made it perhaps the best bomber destroyer employed by Japan in the latter stages of the war. It had a good high-altitude performance, and was one of the few Japanese fighters able to reach the high-flying B-29 Superfortress. Its armament of four 20-mm cannon was sufficiently heavy that it could do major damage against B-29s.

In February of 1945, an American technical intelligence team discovered a single Raiden abandoned among the trees alongside the Dewey Boulevard on the outskirts of Manila. It was disassembled and transferred to Clark Field, where it was repaired by the Technical Air Intelligence Command (TAIC) and test flown. A senior test pilot attached to TAIC rated the Raiden as being the best Japanese fighter he had flown, offering a good performance, good stability, good stalling characteristics, and good takeoff and landing qualities. It had a steep climbing angle and a rapid climb rate. Handling and control were good, but the ailerons became rather heavy at speeds above 325 mph. Stalling characteristics were exceptional. Even though there was relatively little stall warning, the recovery from the stall was extremely rapid, with very little altitude being lost. There was no tendency to spin, the aircraft being exceptionally stable. The maneuvering flaps were rated as being very effective. On the negative side, the brakes and rudder brake action were poor, the ailerons were heavy which made the maneuverability fall off at high speeds, the mechanical reliability was poor, and the range was short.

The Raiden was available too late and in insufficient numbers to affect the outcome of the war. It is indeed fortunate for the B-29 crews that more of these capable interceptors were not deployed by the Japanese in the last year of the war.

In an attempt to improve the high-altitude performance of the Raiden, the J2M4 Model 34 was introduced, flying for the first time in August of 1944. It had a Kasei 23c engine equipped with a turbosupercharger mounted in the side of the fuselage just behind the engine. The turbosupercharger allowed the rated power of 1420 hp to be maintained up to 30,000 feet instead of only 15,750 feet.

Two oblique-firing 20-mm cannon were fitted inside the fuselage behind the cockpit. These guns were aimed upward at an angle of 70 degrees from the horizontal in a manner similar to the German Schrage Musik installation. The four cannon in the wings were retained.

This aircraft could reach a speed of 362 mph at 30,185 feet, which would have made it useful against the B-29s which were able to fly above the combat ceilings of most Japanese fighters. However, difficulties with the turbosupercharger caused the project to be terminated after only two experimental J2M4s were built.

The J2M5 Raiden Model 33 version retained the airframe of the J2M4, but was powered by a MK4U-4 Kasei 26a engine with a mechanically-driven three-stage supercharger. It had actually preceded the J2M4, having first flown in May of 1944. This installation proved to be more reliable than the turbosupercharger used by the J2M4. The J2M5 could reach a speed of 382 mph at 22,310 feet, and was the fastest version of the Raiden series. In addition, it had a wider and roomier cockpit which offered the pilot a better view.

The appearance of the B-29 over Japan had created a sudden, urgent need for more capable interceptors, and the J2M5 was immediately ordered into production as the Model 33 at the Koza Kaigun Kokusho (Koza Naval Air Arsenal). However, shortages in supplies of Kasei 26a engines delayed the production of the J2M5, and only 34 examples were built before the war ended.

The J2M5a Model 33A was a proposal for a version with four 20-mm Type 99 Model cannon and a domed cockpit canopy. The end of the war prevented this version from ever being built.

The J2M6 (which actually chronologically preceded both the J2M4 and the J2M5) differed from the J2M3 in having a new wider cockpit and a domed cockpit canopy which improved visibility. Only one example of the J2M6 was completed, but the new domed canopy introduced on the J2M6 was fitted to some later production J2M3 and J2M3a Raidens.

The J2M6a Model 31A was a proposal for a similar modification of the J2M3a. It was never built.

The J2M7 Model 23 was a projected variant of the J2M3 with the Kasei 26a engine. The J2M7a Model 23A was basically similar, but was based on the J2M3a with four 20-mm Type 99 Model 2 cannon. The end of the war prevented either one of these variants from being built.

Parker Information Resources
Houston, Texas
E-mail: bparker@parkerinfo.com

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