Kk.7z -

Early and updated iterations of SpaceX’s next-generation Mars vehicle.

While the SpaceX mod is the most likely candidate, "KK" can occasionally refer to other niche projects:

Functional capsules for cargo and crew transport to the International Space Station (ISS) analogs.

To give you the most accurate guide, could you tell me or what game/software you were looking for when you came across it? This will help me confirm if it's the SpaceX mod or something else entirely.