The Successor to the A6M?

I think if Kawasaki had realized the problems with the Ha-140 inline engine earlier, they could have jumped at a chance to find a replacement that involved the Mitsubishi Ha-112-II engine, which started to be available in quantity by circa late 1943. As we all know, the Kawasaki Ki-100 turned out to be one of the best Japanese fighters of World War II, with excellent maneuverability minus the mechanical problems that plagued the Ha-140 inline engine and Nakajima Ha-45 Homare radial engine.

In fact, I wonder why didn't Mitsubishi themselves design a true replacement for the A6M earlier that used the Ha-112-II engine. The IJN could have operational a true A6M successor by early 1944 that would have easily beat the F6F Hellcat and be even a serious threat to the F4U Corsair.
 
In fact, I wonder why didn't Mitsubishi themselves design a true replacement for the A6M earlier that used the Ha-112-II engine. The IJN could have operational a true A6M successor by early 1944 that would have easily beat the F6F Hellcat and be even a serious threat to the F4U Corsair.

In fact, the engine favored for use in the Zero fighter, by its designer, was the Mitsubishi Kinsei, an engine about the size of a P&W R-1830, 48" diam. This eventually found its way into the A6M8, with the cowl guns moved to the wings. By this time, Nagoya had shook and B-29s were raining bombs. The Kinsei could have powered the first Zero, instead of the last. The IJN designation for the Kinsei engine was Ha-112. By comparison, the Jack engine, the Kasei, was 52" diameter, putting it in a class with the Shvetsov M-82, the BMW 801, Bristol Hercules and Wright R-2600.
 
As others have noted, the A7M Reppu was the "official" planned successor. It was delayed, initially because it was underpowered and then by an earthquake, I believe. By the time it was ready, Japan had no surviving carriers.

The lack of fleet carriers by 1944 is the main problem, not the absence of replacement for the Zero. A carrier capable version of the N1K was developed in 1944-45, but was no longer needed because by then most if the IJN's carriers had been lost.

The most probable "what if" is the Reppu, which if Mitsubishi/IJN had pushed its development might have been ready by 1943. It would not have made much difference in the battles of 1944-45 however. After Coral Sea and Midway, the Japanese had lost the cream of their naval aviators and from then on the USN was facing poorly trained pilots.
 
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