Apollo–Soyuz Test Project
During his first years at NASA, Zwieg flew in support of the Apollo program, Skylab, and the Apollo–Soyuz Test Project. Later, he became the primary flight instructor for astronauts training in the Shuttle Training Aircraft, a Gulfstream II modified to mimic the flight characteristics of the Space Shuttle. He trained every astronaut who would ever pilot a shuttle. Roger held a world's record for flight time in the T-38 of over 10,498 hours.