NASA studied this McDonnell Douglas/Martin Marietta concept for a fully reusable space transportation system during the Shuttle research effort in 1969-1972. This two-stage system featured two piloted fly-back vehicles, a swept-wing booster and a delta-wing orbiter, both with large internal liquid propellant tanks visible in the cutaway. The 12-engine rocketplane would lift the attached orbiter to a set altitude, then detach to fly home as the orbiter ignited to complete its ascent into space. The orbiter's delta wings and retractable jet engines would improve maneuverability for the return flight to landing. NASA transferred a variety of concept models to the Museum after settling on the final Space Shuttle design.