[hopscotch]

KSP has gravity assist built in: it's an expected consequence of newtonian physics (Which KSP models).

Mods might be to swap to the Solar rather than Kerbol system, etc.

[skykooler]

Yep. Real-Scale Solar System provides that. There are also mods to add real-world rockets (like the Falcon Heavy) and real-world aerodynamics (Ferram Aerospace Research).

[craftyguy]

Well, it's time I fire up KSP again!