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 booster and an orbiter, both using liquid propellant. The booster would carry the orbiter to a set altitude, then detach to return to base. After separation, the orbiter's engines would ignite for final ascent into orbit. This concept had a second configuration for heavy payloads (such as large space station components) that did not need an astronaut crew; the booster, augmented by six solid rocket boosters, would launch a large disposable cargo carrier. NASA transferred a variety of concept models to the Museum after settling on the final Space Shuttle design.