Mitsubishi A6M Zero A6M6 Tipo 0 Modelo 53

The A6M6, designated Type 0 Carrier Fighter Model 53 by the Imperial Japanese Navy, was the most direct attempt to restore the speed advantage the Zero had lost to Allied fighters by 1944. Its origin lay in a structural weakness of the whole programme: with the exception of the late Model 54 and 64, every Zero flew with the Nakajima Sakae family of engines, and successive increases in armour and armament were never matched by a corresponding rise in power. When the Navy's new Raiden and Shiden fighters ran late, Mitsubishi was instructed to fill the gap with a Zero variant carrying heavier armament and protection and, above all, higher speed: the Model 53 Hei (A6M6c).

The heart of the project was the Sakae 31, a development of the Sakae 21 with a water-methanol injection system. With that boost, take-off power was planned at 1,300 hp, and factory estimates suggested that even with the strengthened airframe the Model 53 could reach top speeds in the 580 km/h range. English-language references give the Sakae 31 a rating of 1,130 hp, boosted to 1,210 hp with water-methanol injection; the 1,300 hp figure was the target the engine was planned around. Alongside the engine, the A6M6 introduced self-sealing wing tanks, a protection the Zero had sacrificed from the outset in favour of range and agility. Airframe and armament were essentially those of the A6M5c Model 52 Hei: one 13.2 mm Type 3 machine gun in the fuselage, two more in the wings and two 20 mm Type 99 cannon, with provision for 10 kg or 60 kg air-to-air rockets under the wings.

Engine development initially went reasonably smoothly and service evaluation was carried out with the first Model 53 Hei prototype. In the autumn of 1944, however, propeller-shedding accidents multiplied among Zeros in service, and the cause was traced to insufficient strength in the Sakae 21's planetary reduction gear. The Navy engineers responsible for evaluating the Sakae 31 were diverted to the modification of some 300 affected Model 52-series aircraft, and evaluation of the new engine was suspended. With the Philippines campaign beginning, it was decided to keep building the A6M5c with the 1,130 hp Sakae 21 rather than the still-unapproved Sakae 31; the water-methanol injection units that had been manufactured in parallel, difficult to tune and of little real effect, were left in storage unused.

The consequences were significant. The mass-produced A6M5c received the heavier armament and armour without the engine meant to offset the weight, so that its normal loaded weight approached 3,000 kg: it dived better, but lost manoeuvrability and climb, the very qualities that defined the Zero. By the time evaluation of the Sakae 31 resumed, it was early 1945. Preliminary testing of the A6M6 was judged unsatisfactory because the promised additional power failed to materialise and the injection system proved unreliable. Testing continued, but the end of the war halted further development: only one prototype was built.

The Model 53's legacy carried on in the versions that followed. The Sakae 31 eventually powered the A6M7 Model 63, while the majority of A6M7s produced from February 1945 were Model 62s with the Sakae 31 Ko/Otsu without water-methanol injection, because the complete system — roughly 100 kg of equipment plus a 70-litre tank — added more than 170 kg and penalised both manoeuvrability and serviceability. The A6M8 Model 64, for its part, was an A6M6 with the Sakae replaced by the 1,560 hp Mitsubishi Kinsei 62. In the final versions, from the Model 53 Hei to the 64, tank armouring reduced fuel capacity and the fuselage tank was given over to the water-methanol mixture, so the Zero's legendary range vanished; by 1945, with the aircraft used mainly as a local interceptor, it hardly mattered.