Turbocharged Zero Fighter

A turbocharged Zero-Sen A6M3 was built in 1942.
Article written by Bunzou Komine, (translated by Shinichiro Miura)

Importing a Turbocharger The history of the Japanese Navy developing turbochargers is surprisingly long, and goes all the way back to Showa 12 (1937).

Major Jikyu Tanegashima, who was in France at the time, successfully contracted to import a turbocharger from Brown Boveri & Cie AG in Switzerland (BBC), and the turbocharger came to Japan. This was recorded in Koukuu Gijyutsu Jouhou Tekiroku (Aviation Technology Information).

BBC's turbocharger was developed for diesel aero engines, which many countries were researching at the time. Those that were imported were designed for 500 hp diesel motors.

Using this BBC turbocharger as an example, Mitsubishi, Nakajima, Hitachi, and Ishikawajima were ordered to research and develop aircraft turbochargers. Nakajima was not able to do so, as that firm was concentrating on developing mechanical superchargers instead.

Turbochargers developed by the three companies each came to yield results. Mitsubishi's turbocharger was installed on the J2M4 Raiden Model 32, and Hitachi's turbocharger was installed on Nakajima's C6N2 Saiun. What, then, happened to the turbocharger developed by Ishikawajima Airplanes? Our investigation revealed that it was installed on Nakajima's Sakae, the Zero Fighter's engine.

Navy High-Altitude Fighter Project
The Navy’s Aerial Headquarters' report, Matter Regarding the Experimental Research After Showa 17 (1942), states the following about the turbochargers:
Completion of the turbocharger is essential to the success of high-altitude fighters. Therefore, it was prototyped and durability was tested by Ishikawajima, Hitachi, and Mitsubishi since Showa 15 (1940). However, it has not yet been tested in an aircraft or in flight. In order to proceed with testing, it is necessary to prepare a mass production facility upon the decision of the power and type of the exhaust turbine supercharger that is to be installed on adopted airplane.
Clearly, at the time, the Navy's turbocharger development was moving from a research phase to an operational phase. Then, Kuugishou Shouhou (The Naval Technical Air Arsenal Journal) printed on February 9, 1942, mentions the testing of a wooden mockup of a Nakajima Sakae Model 11 engine equipped with a turbocharger.

It writes, "It is projected to be fitted to the Zero Fighter", so this could be the first official writing in which a turbocharger for the Zero Fighter is mentioned. The Kuugishou Shouhou from 10 days later, February 19, mentions that "Initial research meeting for the turbocharged Zero Fighter" to be held. This proves, in writing, the existence of a Zero Fighter equipped with a turbocharger.

Ishikawajima Aerial Industries' Turbocharger
Ishikawajima Aerial Industries was founded on Showa 16 (1941) as part of the Tokyo Ishikawajima Shipyard. Ishikawajima's Aero Engine Factory, as it came to be known, became a separate subsidiary and established its headquarters close to Kuugishou (Naval Technical Air Arsenal) in the Kanazawa area of Yokohama. There, Ishikawajima continued developing aero engines as they did on Ishikawa Island. During the war, aside from research and development of turbochargers and turbo-compound engines, they concentrated on Sakae motor conversion production and contributed greatly toward the supply of engines for Zero Fighters. Sakae production was assigned in 1940 and the first conversion of the Sakae Model 11 was shipped out at the end of 1941.

Hiroshi Yoshikuni, the designer of the Ishikawajima Aerial Industries' turbocharger, stated that Ishikawajima made the Sakae Model 11 that was used by Kuugishou for the wooden mock-up turbocharger review. Considering Ishikawajima Aerial Industries’ Sakae production situation, we speculate that they chose the Sakae Model 11 for the wooden mock-up review instead of the Model 12 or 21. The turbocharger installed on the Zero Fighter was Ishikawajima's IET Model 4 series, evolved from its 500-hp turbocharger, which supported 1000-hp class motors. As turbocharger development continued, the IET model 5 for 2000-hp class motors was completed, but never made it to the actual airplanes. As for the turbine blades, Ishikawajima and Mitsubishi used a stud-type; Hitachi used a welded type.

Problems with the Turbocharger
Pictures show that this turbocharged Sakae engine has had the turbocharger directly attached, without an intercooler, and it has a very simple installation. Japanese turbochargers had problems with materials since BBC's sample turbocharger was made for diesel motors. There were problems with BBC's turbocharger’s materials, which were designed to withstand 500 degrees Celsius for diesel motors; in order to be used on a gasoline engine, the turbocharger needed to withstand more than 700 degrees Celsius of exhaust heat. Ishikawajima's turbochargers were made of high quality materials, capable of withstanding the heat, such as nickel-chromium-tungsten steel (much like the material used for the B-17), but accidents still occurred, such as the exhaust release butterfly valve exploding, and development did not proceed smoothly. The problem with choosing material for heat resistant steel seemed to be a difficult obstacle in developing turbochargers.

In spite of all these problems, an A6M3 Zero Fighter was modified to use a turbocharger, and was reported to be complete in 1942. But due to problems, the testing did not proceed as planned, and finally the project was abandoned before the first flight test. That the Zero Fighter was the first Japanese fighter to use a turbocharger is now known, but it is a real shame that it never flew.