even if you could get the regulatory approval to risk it, the thermal conductivity of air is probably too low to make it viable. Any high surface area heat exchanger is going to cause too much drag inside the engine.

@JonathanRay direct bombardment of the air by neutrons in the reactor core would heat it quickly enough without drag problems, but no regulator would ever allow that

using high-grade plutonium with a beryllium hydride moderator you could make the a very compact and lightweight 500MWe reactor. Perhaps with titanium wings functioning as radiators.

@JonathanRay This paper says you can make a 5kg reactor using Am-242m and light water. There is no question that light hydrogen is the best moderator by miles for this purpose because it has a much higher cross section than any other viable moderator, in addition to removing far more kinetic energy per collision.

https://www.researchgate.net/publication/239521070_The_Smallest_Thermal_Nuclear_Reactor