This was developed along time ago under the name project Pluto the engine was able to run on nuclear power how it works is that once it gets up to a certain speed, it is completely self-sufficient, requiring no fuel to work, but it first needs to get up to a speed to be functional. I believe this would be a useful part for long-term exploration of bodies with an atmosphere due to the fact that the engine needs absolutely nothing to run small thrusters couldn’t get it up to a certain speed, and then you’d be able to work indefinitely.
