That way it can spin on its axis in the direction of travel. The international Space Station was not designed to spin. Moreover the hull needs to be fiber carbon like the stealth covered with re-entry material. Reason aluminum struck with solar wind creates secondary radiation over long periods harmful to the crew. As for nuclear energy the reactor needs to heat a liquid metal in the periodic table that is liquid at room temperature and at 2,000 degree centigrade. By heat exchanger transfers heat not radioactivity to mercury. The mercury drives the turbine that drives the superconductor power for plasma rockets. The ship that stays in orbit needs radiator vanes for the mercury. All that is not necessary in the first ships to Mars if the intent is who gets there first.