KSP: Apollo - Saturn 1B
Apollo is a manned spacecraft used to reach Moon and to land on it. So the idea was to bring to moon two ships: command module (CM), where you can store all austronauts and food for them for the journey, and the lunar module (LM or LEM) with descend (landing) and ascend (take off) stages.
But you can't just "go to Moon!" - you need to test everything. So Saturn-1B was used to test Apollo CM and LEM in flight on earth orbit.
Mods: https://github.com/KSP-RO
Wiki:
https://en.wikipedia.org/wiki/Apollo_...
https://en.wikipedia.org/wiki/Saturn_IB
https://en.wikipedia.org/wiki/Apollo_7