There is an engine design that uses a small unshielded nuclear reactor to heat atmospheric air as a working fluid in a RAM jet, giving several months of flight

There is an engine design that uses a small unshielded nuclear reactor to heat atmospheric air as a working fluid in a RAM jet, giving several months of flight time before the fuel runs out: https://en.wikipedia.org/wiki/Supersonic_Low_Altitude_Missile https://en.wikipedia.org/wiki/Supersonic_Low_Altitude_Missil... There are some significant drawbacks to this approach (assuming you managed to overcome the technical limitation of making everything work at the temperature and radiation involved) such as massive radiation leakage, which is why this was intended to be part of a weapon system that could drop nuclear bombs and then just spend its time flying around your enemies cities irradiating them until they shoot it down or it runs out of fuel, causing yet another nuclear disaster to deal with as it crashes. The successful test run shows the design can work, so an external microwave heat source could do the same if you could transfer enough power.