Kerbal Space Program (v1.12.5.03190) [gog] Here

Instead of building massive, expensive rockets, learn to use planets as "free" engines.

: To reach the outer planets (Jool, Eeloo) with minimal fuel, launch toward Eve first. Passing behind Eve in its orbit will "pull" your craft, boosting your velocity and raising your apoapsis significantly for almost zero delta-V. Kerbal Space Program (v1.12.5.03190) [GOG]

Since KSP 1 development has ended, the mod scene is incredibly stable. For an "interesting" upgrade, consider: Instead of building massive, expensive rockets, learn to

: Attempt to visit three of Jool's moons in a single mission using gravity assists from Tylo and Laythe to move between them without burning fuel. Since KSP 1 development has ended, the mod

: Always perform your major ejection burns at the lowest possible point of your orbit.

: Look for the icon on the bottom left (the transfer window icon). It will calculate the exact date and angle needed to reach Duna or Jool so you don't have to guess when to launch.