NASA adopted the idea of an expendable propellant tank in the course of shuttle development in the first half of the 1970s. At the time, the agency realized that it could not afford a fully reusable shuttle that held all its fuel inside. The introduction of the external tank came as a compromise that allowed engineers to significantly reduce the shuttle development costs, while at the same time preserving all the expensive hardware, such as engines and flight-control computers, aboard the reusable orbiter.

In return for their sacrifice, shuttle developers were allowed to increase the estimated payload of the orbiter from 10 to almost 30 tons.

The 153.8-foot- (47-meter-) tall external tank is divided inside into two sections. The forward section contains 541,000 liters (143,000 gallons) of liquid oxygen, which serves as an oxidizer for the shuttle's main engines. The rear section holds 1.45 million liters (383,000 gallons) of hydrogen, which is the fuel for the shuttle's main engines.

A special intersection connects two propellant sections of the tank. During launch, the propellant from the external tank flows under pressure through 17-inch ducts into the three main engines of the orbiter.

NASA awarded contract to Martin Marietta, now Lockheed Martin, to build the external tank. The company uses the facility in Michuide, Louisiana, to construct the tanks. A giant barge, previously used to ship the Saturn 5 moon rocket's stages, transports the ET from Louisiana's southern coast to Cape Canaveral.

At the Cape, the external tank is transported into the Vertical Assembly Building where it is attached in a suspended position between two solid-fuel rocket boosters previously assembled on a movable launch platform. The shuttle orbiter itself later attached to the external tank.

Originally, the ET was covered with a layer of white protective paint, however after the first two test flights this largely cosmetic cover was abandoned to save weight. During the sixth flight of the program, which was also an inaugural mission of Space Shuttle Challenger, a new lightweight version of the tank was introduced.

Since only a small additional push would be required to insert the ET into Earth orbit during a shuttle launch, many ideas have emerged over the years for possible uses of the tank in space.

One proposal envisioned a small habitable module attached to the nose of the external tank by the shuttle. An initial cleanup crew would then enter the expansive interior of the tank, pumping air inside and outfitting it for the long-term habitation. The giant structure, which would easily rival the largest objects ever-deployed or planned to be deployed in space, would serve as a platform for external payloads, an orbiting warehouse or a habitable outpost.