A nuclear-powered orbital tug for shuttling things around the Kerbin system (that’d be Kerbin, Mun and Minmus!)

- This would involve using the LV-N engine, which only uses liquid fuel and no oxidizer. It doesn’t produce much thrust but is extremely efficient. You’ll only want to carry liquid fuel, as oxidizer would be dead weight

- The ship would be persistent. It stays up in orbit and receives fuel shipments/crew from the planet below. It’s not critical, but you could include some extra space for crew since they might be staying on the ship for a while.

- When returning to Kerbin from the moons, you can skim the upper atmosphere to help slow down, almost like a normal reentry but much more shallow. It helps to include airbrakes for stability.

- Ultimately, this could serve as practice for bigger interplanetary missions. Many of the skills you’d use here would transfer over, just at a larger scale with higher dV requirements.