Kerbal Space Program is described as 'Create and manage your own space program, build and fly spacecraft, and try to conquer space' and is a simulation games in the education & reference category. There are more than 10 alternatives to Kerbal Space Program for a variety of platforms, including Windows, Steam, Mac, iPad and Linux apps. The best Kerbal Space Program alternative is Orbiter, which is both free and Open Source. Other great apps like Kerbal Space Program are Juno: New Origins, Children of a Dead Earth, Spaceflight Simulator and Reentry.
Kerbal Space Program alternatives are mainly Simulation Gamess, but if you're looking for
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In no way is orbiter a KSP clone... it's a realistic simulator in continuous development for 21 years and counting. Orbiter is a flight simulator, not a game. When KSP just doesn't satisfy you anymore and you can calculate Hohmann transfers in your head, try this.