Mitsubishi J2M "Raiden" (Jack) #4 From J2M1 to J2M2

We continue today with the fourth instalment of our "Raiden" series (previous parts: 1, 2, 3).

As we saw the first J2M1 prototype was delivered in February 1942 and during testing although overall satisfactory with few minor problems, like forward visibility, which could be rectified, the type unfortunately did not deliver the expected by the Navy performance. As a result the IJNAF put forward the idea to install a more powerful engine, the Kasei 23a MK4R-A and incorporate various modifications.

Main differences with the J2M1 were:

1. A four blade propeller of 3.3m diameter Sumitomo VDM constant speed. In the J2M1 the pitch variation mechanism was electric but since it was unreliable it was changed to hydraulic*.

2. The cowling was shortened by 20.5cm

3. Large size wind shield

4. Eight exhaust pipes

5. Flap width was 50cm bigger

6. 20mm wing cannons were changed from 1Go Model 3 to longer and more powerful 2Go Model 3

7. Cannon magazine changed from 60 rounds to a bigger with 100 rounds. As a result there are projections on top and bottom of the wing

8. The blades of the cooling fan were reduced from 22 to 14.

9.  The heavier Kasei 23a was heavier and it was installed 100mm further to the rear.

10.  Pilot’s seat was placed 70mm forward and 80mm higher to improve forward visibility

11. The fuselage fuel tanks were separated to front with 120l water methanol and rear of 280l. The two wing tanks were of 90l each for a total of 590l. Including the 250l of fuel carried by the drop tank, total fuel capacity could reach 840l.

IJNAF requirements called for a maximum speed of 340kt (629.6km/h) and reaching a height of 6,000m in less than 5min 10sec. The first prototype, designated J2M2, was completed in early October 1942 and the first flight took place on October 13, 1942 at Suzuka. During tests the J2M2 was able to do only 322kt (596.3km/h) and reached the height in 5min 38sec.

From the get go there were problems with the water-methanol injection system resulting in a lot of black smoke coming out of the engine but more serious was the excessive engine and propeller vibration as a result of the installation of a bigger engine. The mechanics of Mitsubishi constantly tried to rectify the problems by modifying and improving the engine mounting shock absorbers and the propeller. But none of these issues were satisfactorily resolved leading to the crash of the second prototype during testing in Suzuka on June 16, 1943 and the death of the test pilot Lt Hoashi Takumi.

In September 13, 1943 another accident took place with the 10th prototype when Mitsubishi test pilot Shibayama was in the controls.

In Autumn 1943 the J2M2 was officially named “Shisei Raiden” (Test Raiden) and although not all the problems had been rectified it was decided to be placed in production due to the worsening war situation. Mass production started in September 1943 and in total 155 J2M2s were produced (other sources mention 131).

The January 1964 issue of "Maru" magazine features an article by Toguchi Yuzaburo, a member of the Navy Test Department. He mentions that during ground testing the "Raiden" was different than all other aircraft. During taxing in full power it had the tendency to overturn because the engine was too strong. During this kind of testing one ground crew member had to ride on each horizontal tail surface or otherwise dead weights had to be placed. Taxing was particularly difficult with almost no forward visibility so the pilot had no other option but to zigzag on the airfield trying to avoid bumps and other aircraft. During take-off and landing the nose had the tendency to change direction and it was very difficult to retain a straight course. Toguchi-san makes clear that since "Raiden" units were organised he had never heard of in-flight accidents. There were many accidents during take-off and landing due to young pilot’s inexperience with the peculiarities of the type.

After take-off the "Raiden" had exceptional climbing speed, much better than the Zero. It was like the engine was pulling the whole aircraft and it could climb to 6,000meters in 5min. Nevertheless fuel consumption was high and therefore cruising time was short. Without a drop tank the aircraft had only 40 min to an hour flight time with only 15min combat time. In the air it had limited visibility like when on the ground and a rather wide turning circle much wider than the Zero-sen so the manoeuvres performed by the Zero were impossible to be done with a "Raiden". But apart from these two it had no other problems and using the type’s advantages like climbing power and speed a pilot could use this plane as a good interceptor.

He also mentions that since February 1945 the US bombers were escorted by scores of fighters and during battles a lot of "Raiden" were shot down. This was not due to the type's design but because the numbers were unfavourable towards the Japanese planes. In the beginning it was one "Raiden" to ten US fighters and gradually one "Raiden" to 20 US fighters.

At this point we would like to introduce one more invaluable source to the history of the type. A small book written by noted aviation historian Ikari Yoshio (born 1925 and a former member of the IJAAF Aviation Technical Department) titled "Raiden"  and published by Kojinsha in 2006. Ikari-san is using a number of official documents and other sources including the diary of Mitsubishi engineer and designer Sone Yoshitoshi who was next to Horikoshi and eventually replaced him. The publication is so full of details regarding the development of the "Raiden", none mentioned by other authors, that we will mention only a small number in this research.

As noted before there are no official Mitsubishi diaries or records detailing the exact production process of the various "Raiden" types. There are also only a small number of photos available. Therefore different authors have drawn different conclusions depending on the material available to them at the time.

Francillon and Nohara clearly mention that the fourth J2M1 prototype was modified and became the first J2M2 prototype, while Akimoto mentions that "one of the J2M1 prototypes" was modified.  

Ikari-san shows in great detail that the developments of the J2M1 and J2M2 were in parallel. Actually Mitsubishi at the time were extremely busy working on improving the Zero, on the "Raiden" and on the "Reppu". The installation of a larger engine caused many problems which Mitsubishi spent a lot of time and effort to rectify. These problems resulting in a disappointing performance of the first J2M2, made Mitsubishi and the Navy to go back and forth; Mitsubishi wanted to improve the J2M1, IJNAF promoted the J2M2. The result of this was that eventually all the J2M1 prototypes except the first were modified by Mitsubishi (see part 3) while they were working at the same time on more J2M2 prototypes. The J2M1 prototypes were actually used to test various techniques incorporated in the J2M2.

Various official documents and especially the diary of Sone confirm that not only the fourth J2M1 prototype did not become the first J2M2 but that actually none of the J2M1 prototypes became J2M2. From Ikari-san's book we will mention only a few facts that support this.

On February 19, 1942, there were meetings in Mitsubishi’s Nagoya factory regarding the design of the new J2M2 and it was decided that by the end of August, seven J2M1 were to be built and the design of J2M2 should be over by the end of May. One prototype was to be built by July and two in August and September respectively, totalling five J2M2 prototypes.

On April 22 there was another meeting discussing the painting process of the prototypes. It was decided that the first and fourth J2M1 prototypes as well as the first J2M2 prototype were to be polished, the second and fifth J2M1 as well as the second J2M2 were to have patty applied and then painted and finally the third, sixth, seventh, eighth J2M1 and the all the J2M2 following the third prototype were to be painted like the Zero-sen, gray.

The modified J2M1s were called J2M1-kai.

In the Sone diary it is mentioned that on October 8, 1942, the first and fourth J2M1-kai prototypes were to be test flown by the Navy.

The entry of the next day in Sone’s diary mentions that the second J2M2 prototype was to be completed by October 20 and the fifth by the end of November. These two entries clearly differentiate the J2M1-kai and the J2M2.

Ikari-san mentions that on January 4, 1943, the eighth J2M1 prototype and on January 13 the fourth J2M1 prototype had sustained minor damage.

On February 14, 1943, the Mitsubishi "Raiden" were as follows:

The first J2M2 returned to the factory.

The fuselage of the second J2M2 was to arrive in Suzuka by February 15.

The engine mount and the vibration dumpers of the fourth were to be tested.

The fifth was been test flown in Suzuka.

The second J2M1-kai was to be completed on February 18.

  

The first unit to receive the "Raiden" was 381ku organised on October 1, 1943, in Tateyama base, Chiba Prefecture. The unit was to be assigned to the protection of the Balikpapan oil fields of Borneo so it was deemed a suitable unit to receive an interceptor fighter. The unit relocated to a bigger base in Toyohashi, Aichi Prefecture, but "Raiden" were barely delivered! In February 1944, the unit had only ten "Raiden" even though the unit was to be equipped with at least 36. On January 5, 1944, during shooting training the engine mount of the 30th production J2M2 lost its strength and dissolved in the air resulting in the death of the pilot. Due to such accident further modifications to the type were required, causing further delays in the production. So the 381ku quit to be a "Raiden" unit and changed to Zero 52s and relocated to Borneo from March of the same year.
In the photo below taken after the end of the war at the Mitsubishi factory in Nagoya, a rare 381Ku J2M2, serial number 2024, can be seen in the back.

Photo: "Asahi Shimbun" collection

The second Raiden unit was the 301ku organised in Yokosuka base, Kanagawa Prefecture on November 5, 1943. Again there were not enough "Raiden" as their delivery started from March 1944. In February 1944 the unit was split into the "Raiden" equipped Sento 601 Hikotai and the Zero equipped Sento 306 Hikotai, each with 48 aircraft.

Various air battle techniques were discussed and tested including one which the "Raiden" was to be positioned above the enemy bomber, then roll over and attack while diving. Although there were constant problems with their "Raiden" the unit gradually reached operational level. The unit was to relocate to Truk to participate in the Marianas fight but the orders changed and were assigned the air defence of Iwojima. But due to bad weather season and the inability to fly for long distance over the sea, the mission was cancelled and eventually the unit changed their "Raiden" to Zero-sen and finally was able to take part in battle.

The experience with the two units led to serious doubts about the abilities of the "Raiden" and the original production estimates of 3,695 a/c per year were changed to about 30-40 per month. In return, the idea was to augment the production of "Shiden/Shiden-kai". Pilots too became reluctant to fly the plane preferring the much easier to fly Zero-sen.

There are less than half a dozen J2M2 photos. One featured in most publications shows a later type J2M2 belonging to the 302 Ku with tail marking "ヨD-1171". The unit was organised on March 1, 1944 at Kisarazu base, in Chiba prefecture and due to slow delivery of "Raiden" it was largely equipped with Zero-sen and "Gekko". Artwork by our friend Devlin Chouinard.

*from Wiki: Constant-speed propellers allow the pilot to select a rotational speed for maximum engine power or maximum efficiency, and a propeller governor acts as a closed-loop controller to vary propeller pitch angle as required to maintain the selected engine speed. In most aircraft this system is hydraulic, with engine oil serving as the hydraulic fluid. However, electrically controlled propellers were developed during World War II and saw extensive use on military aircraft...