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Principia has n-body
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Stream
Stream - Scalable APIs for Chat, Feeds, Moderation, & Video. Stream helps developers build engaging apps that scale to millions with performant and flexible Chat, Feeds, Moderation, and Video APIs and SDKs powered by a global edge network and enterprise-grade infrastructure.
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RP-0
Discontinued Realistic Progression One - Career mode for Realism Overhaul [Moved to: https://github.com/KSP-RO/RP-1]
As others stated, try RSS/RO; it's a great way to learn about many more realistic aspects of space vehicle design while still being fun. See here to install: https://github.com/KSP-RO/RP-0/wiki
NOTE:
The number of mentions on this list indicates mentions on common posts plus user suggested alternatives.
Hence, a higher number means a more popular project.
Related posts
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Mods to add after RP-1 express install?
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Communication coming out today
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One week before the release of Kerbal Space Program 2, some kerbonauts are beginning to have doubts
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Eli5, Why are planets orbits elliptical and not circular?
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Just a friendly PSA that kerbal space program is free on the epic game store this week. Build rockets and figure out how to get to space!