Funnily enough, there is an n-body mod for ksp1, which makes interplanet interactions more realistic (in fact, the mod has to slightly change the default system to stop the moons of Jool from slingshotting each other out) and allows advanced maneuvers like ballistic capture and lagrange points
Other games i know that do this are factorio (you are able to download the game as a zip, and it doesnt stop you from making as many copies as you desire)