This concept for a partially reusable space transportation system emerged near the end of the Shuttle research effort in 1971-1972. It featured a piloted orbiter with a disposable external fuel tank attached to two liquid propellant booster rockets. In this stage-and-a-half system all engines (four per booster and four on the orbiter) would ignite for lift-off. After the boosters burned out and were jettisoned, the orbiter, fed from the disposable tank, would continue into orbit. The final Shuttle concept was quite similar to this but had two reusable solid-propellant boosters and only three orbiter engines. NASA transferred a variety of concept models to the Museum after settling on the final Space Shuttle design.