The External Tank, also known as the ET, was like the "gas tank" for the space shuttle orbiter. It carried propellants—liquid oxygen and liquid hydrogen—that flowed into the Space Shuttle Main Engines (SSMEs), where they combined and ignited to produce almost one and a half million pounds of thrust to help push the space shuttle to orbit. The external tank also served as the structural support for the whole shuttle stack, with attachment points for the orbiter and booster rockets.
During each shuttle mission, all the propellants in the tank were used up by the SSMEs during the shuttle's trip up to orbit. As the space shuttle orbiter had almost reached the speed it needed to stay in orbit about 8.5 minutes after launch, the empty tank detached about 70 miles (113 kilometers) above the Earth's surface. After detaching, the tank fell back towards Earth on a planned path over the Pacific or Indian Oceans, almost all of it disintegrating in the atmosphere on the way down. The external tank is the only component of the shuttle stack that was not reusable—a new one was constructed for each launch.
Three different types of external tanks were built over the course of the 30-year Space Shuttle Program. The first six space shuttle missions used standard-weight tanks (SWTs), but later, two newer types of tanks were developed—lightweight tanks (LWTs) and super lightweight tanks (SLWTs). All three types looked the same on the outside, but have differences in their internal construction and materials.