The best free alternative to Kerbal Space Program is Orbiter, which is also Open Source. If that doesn't suit you, our users have ranked more than 10 alternatives to Kerbal Space Program and six of them is free so hopefully you can find a suitable replacement. Other interesting free alternatives to Kerbal Space Program are Juno: New Origins, Spaceflight Simulator, Kitten Space Agency and Pioneer Scout Plus.
Orbiter is a realistic 3D real-time space flight simulator program.
Fed up with space games that insult your intelligence and violate every law of physics? Orbiter is a simulator that gives you an idea what space flight really feels like - today and in the not so distant future.
Bring to life anything you can imagine from rockets and airplanes to cars and more. Automate them with a visual programming language. Explore 3D planets and download more from the community. Share your creations online.
In Spaceflight Simulator, you will get to build your rockets, plan launches and flights, attempt landings, deploy payloads and explore new worlds.
Built from the ground up with the spirit of discovery, Kitten Space Agency invites players and supporters to join a community pushing the boundaries of what a spaceflight program can be.
A game of missions, trade and space combat in the 31st century using the Pioneer Space Sim engine with many additions and improvements.
Pioneer is a space adventure game set in the Milky Way galaxy at the turn of the 31st century. The game is open-ended, and you are free to explore the millions of star systems in the game. You can land on planets, slingshot past gas giants, and burn yourself to a crisp flying...
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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.