Build a fleet of reusable deep space delivery spacecraft that are refueled and repaired in space, and remain in space.

Build reusable spacecraft/systems that are built, refueled, and serviced in space, and remain in space.
The purpose of these craft/systems is to deliver personnel or very large probes into deep space faster than current delivery systems, and to return to the Earth for reuse.
Because these craft/systems are built and serviced in space, unconventional energy sources can be utilized (ex: Nuclear). And because these craft/systems are large, more complex scientific probes can be delivered to the farthermost ends of the solar system with possible return missions. Also, if the delivery system is ferrying personnel, a more complete complement of crew and equipment can be included in deep space missions, such as a fully equipped medical bay or an entertainment center for the crew to relax.