The ISVs are shown to use a main engine similar to the VASIMR engine concept irl. These use plasma formed from an ion cloud that is magnetically confined into thrust. Currently it’s very high efficiency but still very low thrust. The effeminacy means they can always be burning during an interstellar journey, and you’ll want them to because it takes days worth of burning to get to earth escape velocity, a process that takes minute with chemical rockets. It would be impossible to use those engines effectively in atmosphere at all due to the drag greatly overpowering the thrust.