This isnt really an aerospace problem, just basic physics. Been thinking about hardSF spaceship designs. Could someone comment on my logic and maths? The basic plan has a nuclear powerplant and a habitable section connected by a long tether. the whole contraption spins to provide cetrifugal artificial gravity and the length of the tether provides radiation protection. The direction of thrust is the same as the axis of rotation. Originally I had an ion drive at the centre of mass, but more recently Ive preferred the idea of a nuclear thermal engine. However for a nuclear thermal engine, the thrust would come from the power plant, not the center of mass. This seems ok to me(although bizarre) because the engine is only meant to give a weak thrust compared to gravity. Think of spinning a frisbee on your finger. The force will average out over a full rotation. The only effect will be to change the angle of the tether slightly off 90 degrees to the direction of travel, against the restoring force from the spin. Suppose we assume the power plant and habitable sections are both the same mass, so both are under 1g force, and we decide we only want to allow 1 degree twisting. I think the restoring force (as a fraction of the centrifugal force) is very nearly sine(1 degree) or 0.017, implying the engine could provide almost 2% of 1g acceleration and change the tether angle by only 1 degree. Thats plenty for travel within the solar system if we could maintain it for long periods. In conclusion, an off center thrust is perfectly reasonable for my spinning interplanetary craft.