NASA studied this North American Rockwell concept for a partially reusable space transportation system during the Shuttle research effort in 1969-1972. This two-stage system featured two piloted fly-back vehicles, a booster and an orbiter, both using liquid propellant. The orbiter was mounted on a large external tank above the booster. Powered by five F-1 engines, the booster would carry the orbiter to a set altitude, then detach to return to base. After separation, the orbiter's engines, fed from the tank, would ignite for final ascent into orbit. Moving all the orbiter's propellants into a disposable external tank made the orbiter smaller and lighter-weight. NASA transferred a variety of concept models to the Museum after settling on the final Space Shuttle design.