This is the hardest of the hard science fiction campaign. I think the Traveller combat system is fairly realistic, but what if you go one step further by substituting fusion rockets for manuever drives. There is no artificial gravity and no jump drives, but fusion rockets are just as good as the manuever drives for going places in the Solar System, the occupants in the spaceships would feel the full accelerations though. If they are accelerating at 6-G they would be pinned to an acceleration couch. If the cabin was filled with water during this acceleration they could move about with their increased weight thusly distributed. With 5/6th of their bodies immersed in water they would only feel their full body weight instead of 6 time that. There would need to be political divisions within the Solar System to make it interesting. Most people would live under domes or in free orbiting space colonies.