Conspiracy, but I don't think the US ever stopped researching nuclear propulsion. There's too many advantages. They built a few working and miniaturized test engines in the 60's but never put them on planes? I don't believe that. They put nuclear weapons on planes all the time and thats not much safer. More likely, they never told anybody they put them on planes, for obvious radioactive reasons.

An aside, the strange craft reported recently with the famous jet fighter videos had "impossible performance" and were all filmed over the ocean. The ocean would be the only safe place to test nuclear drones. And their performance would be quite unmatched by anything else. The pilots even reported that they submerged, sounds like a great failsafe if your super secret black project gets seen. I doubt you would submerge a jet turbine, but nuclear propulsion could easily work under water