[AlexanderHowl]

Anyway, 55 mps is around the best that you can do with a TL9 fusion rocket without compromising protection or capacity. If we assume one control room, one fusion rocket, three armor, and eight tanks, that leaves us with seven systems. You can try a two year mission without armoring the entire spaceship, but I doubt that anyone will come along for the trip.

If you are transporting human passengers at TL9, I would not really use anything less than SM+12 for interplanetary travel in the Inner Sol System or less than SM+14 for interplanetary travel in the Outer Solar System (radiation protection and spin gravity are required). At TL10, medicine can fix radiation damage and counter zero-g degeneration, and the propulsion technology improves, so you can reduce the minimum size to SM+6 and SM+8 respectively.