How feasible would it have been in reality for the fictitious "J7W2" to actually be built and fly?

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Oct 21, 2021
I suppose I should give some context: While I don't know where or how the idea started, it seems that there was an idea that the J7W was designed to be easily swapped to a Jet engine, should one be available. I can't find any evidence for this story, and even if there was, Japan had 5 semi-production turbojet units at the time of surrender (all 400kg Ne-20, with less than 500kgf of thrust), and successful testbed operation of the Ne-130: That is to say, there was no real chance of those engines EVER being ready in time.

However, I do think it is interesting to ask, based on what little we know about this interceptor prototype, whether it would be plausible for the basic airframe to be fit with a turbojet, with relatively short redesign period (assuming either that the war continued into 1946 or 1947, OR [perhaps even more fancifully] that the US decided to reverse engineer the design for research purposes)

Here's what I can piece together, but I'm not an engineer, just someone that like to pretend to be one:

  1. The pusher, tricycle gear configuration of the J7W means that the powerplant is already located near the rear of the aircraft, and the line of thrust is parallel to the runway at rest. This means that the "default" engine location would not require extreme piping with according channel losses, and there would be no concern about the jet exhaust melting tarmac or kicking up disastrous amounts of gravel.
  2. The powerplant installation of the J7W appears to be a "long drive shaft" design. This suggests that the internal space already allocated to the engine would either be entirely sufficient for installation of a single turbojet, or nearly enough. If Wikipedia is to be trusted with dimensions, the Ha-43 without any such extended drive shaft is already 2 meters in length, and 1.2 meters in diameter. The overall length of the J7W is 9.66 meters, and somewhere between 1/2 and 1/3 of that length is behind the canopy, so around 3 to 4 meters appear to be available internally. The Ne-130 has a length of 3.9 meters, with a diameter of 0.9 meters. (The Ne-20 and theoretical 20 Kai would be even smaller, at 2.7m long and 0.62 diameter, but even a pipe dream power output of 600kgf would be too small to really be worth the effort). Supposedly, the Ne-230 would be somewhat shorter than the 130, at around 3.5 meters.
  3. The Ha-43, sans Propeller and Driveshaft, has a mass of nearly 1,000kg. All of the information I can find suggests the turbojets of interest, 130 and 230, are actually somewhat lighter than this, at around 900kg. If extension of the rear section were necessary to fit the engine, with according increase in weight to the rear, it would likely not be enough to push the CoG outside of acceptable ranges.
  4. As far as crudely predicted performance goes (assuming the weights and balances remain roughly the same, with a Ne-230 giving around 850kgf of thrust), this hypothetical J7W2 would have reduced acceleration compared to the -1 until a speed of around 600kph TRUE Airpseed (NOT Indicated, Calibrated, or Equivalent). At this speed, the effective propulsive horsepower of the propeller driven model would stagnate at roughly 2,000, while the Turbojet model would reach around 1,900hp, gaining more as speed increases. At around 750kph (the supposed maximum of the prop model), this would translate to almost 2,400 propulsive horsepower, although this admittedly does not consider the loss in thrust at high altitudes. Shallow angle, high-speed climbing maneuvers would put the "Shinden Kai" well ahead of and above allied fighters, and almost nothing would be able to maintain pace in a dive. Ironically, it seems that when this hypothetical is compared to the Me 262, the 262 would be the better low speed aircraft due to nearly double the thrust for similar weight. However, the Shinden Kai would have far lower drag due to the engine being part of the fuselage rather than separate pods, and this would pay dividends in top speed. And while armament is really not a part of this discussion, it is notable that the Shinden's Autocannons have far better ballistics than the Mk 108s, making fighter vs fighter combat more feasible

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