Try Orbiter space simulator, start at orbiterwiki.org. It will have a link to the download and for add-ons. It is a free simulator written by Dr Martin Schweiger and the add-ons by a group of developer hobbyists, providing real world physics, orbital mechanics and aerodynamics. The stock sim comes with the real solar system, but you could archive the bodies and develop your own planets and moons, others have done that already. Check it out. The folks at Orbiter-forum.com are from all over the world and are very helpful to get you up and running with sim play as well as development. Once you develop new bodies, spacecraft, whatever, you could upload them to the orbiter-hangar so others can enjoy your work. Let me know what you think. Just be patient, they do this in their spare time, it’s their hobby and who knows, it could be yours, as well.