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:

Full-scale models with realistic engine clusters and stage separation.

Because kk.7z is a generic name often used for temporary uploads or re-hosted files on platforms like MediaFire or MEGA, it is vital to verify the source:

In the 3D modeling or "MikuMikuDance" (MMD) communities, "KK" is sometimes used for Koikatsu-related asset conversions.