[dvdkhlng]

If you want to use the RTG to power thrusters, then I'd think that doing the "detour" via a stirling engine to create electrical current to turn into thrust might be unnecessary.

There must be some way to directly turn radioactivity (or heat) into thrust with sufficiently high exhaust velocity!?

[wbl]

Let me introduce you to the NERVA rocket. Nuclear thermal propulsion exists, but it's a real pain to make work and they had to search for coatings that could stand the heat and fuel.