Fullerton helped test the shuttle before it ever reached orbit. In 1977 he flew as pilot with Fred Haise during the Enterprise Approach and Landing Tests, including free flights that showed the orbiter — open definition could glide to a runway landing after separation from the Shuttle Carrier Aircraft.

He flew to orbit as pilot of STS-3 in 1982 with Jack Lousma. STS-3 was the third Columbia test flight and expanded the orbiter's operating envelope while NASA continued learning how the reusable spacecraft behaved in space and during landing.

Fullerton later commanded STS-51-F, the Spacelab 2 mission. That flight suffered an abort-to-orbit after a main engine shutdown but still completed significant astronomy, solar physics, plasma physics, and life-science work.

After leaving the astronaut office, Fullerton remained a NASA research pilot at Dryden Flight Research Center, flying a wide range of experimental aircraft and propulsion-control programs. His career is a bridge between military test piloting, shuttle development, and NASA aeronautics research.