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KSP's spiritual successor Kitten Space Agency just released a free public pre-alpha
(www.youtube.com)
Tabletop, DnD, board games, and minecraft. Also Animal Crossing.
Rules
Orbiting is pretty straightforward, so is transferring to a moon or something. Planets are tougher because both are moving instead of one orbiting the other. You need to speed up to go somewhere and you need to slow down to stop there. That's about the gist.