NASA studied this Martin Marietta concept for a fully reusable space transportation system during the Shuttle research effort in 1969-1972. It featured two piloted fly-back vehicles - a twin-fuselage booster craft and a delta-wing orbiter - in a two-stage configuration. The liquid-propellant booster would carry the orbiter to a set altitude, then detach and be piloted back to land. After separation the orbiter would ignite its own engines to reach orbit. Both vehicles had retractable air-breathing jet engines for powered airplane-like flight during descent to landing. NASA transferred a variety of concept models to the Museum after settling on the final Space Shuttle design.