IIRC in Feynman' Surely You're Joking, Mr. Feynman, some government folks surveyed the people working on the Manhattan Project for prospective usages of nuclear power to patent and a nuclear powered airplane was one of his first ideas, using the heat from the nuclear plant to heat air for a jet turbine.

To a very simple approximation, heating air expands it. If you control the shape of the space around it, you can expand it more in one direction than the other. That generates thrust. This is the idea behind ramjets.

Meanwhile, nuclear reactors are hot, and this can be used to heat up air.

I'm no expert, but this has come up in previous discussions: https://en.wikipedia.org/wiki/Ramjet#Nuclear-powered_ramjets.

This article is also quite interesting. On the US version of this temporarily developed in the 60s. More info on this project will be under “Project Pluto”

https://jalopnik.com/the-flying-crowbar-the-insane-doomsday-...