They are... interesting
Basically you have a second craft on the inside of a hollow object (eg. fairings) that have (in my case) dual stayputniks clipped into them, it allows them to spin with minimum glitching, there are 8 rtgs clipped inside, with 8-10 small reaction wheels, you part clip the decoupler which its mounted on out of the craft, and then you move the craft inside, then you switch to the propeller, and you force it to trim spin with alt-q or alt-e.
Warning, they are still glitchy. And never phys warp with them running.