External Tank (ET)

The External Tank (ET) is 154.2 feet (47 meters) long and has a diameter of 27.5 feet (8.37 meters). The External Tank is the rust colored piece of the STS and it is the largest component that comprises the Shuttle Transportation System (STS); the rust colored material is actually one inch of insulated foam (poly-iso-cyanate). In fact, the ET used to be painted white, however, it was discovered that hundreds of pounds of weight savings would be achieved if the paint was not applied. The ET contains liquid hydrogen fuel and liquid oxidizer and puts them under pressure to the Orbiter's three main engines during the launch and ascent phases. The ET holds 143,000 gallons (540,000 liters) of liquid oxygen in its smaller upper tank and it holds 383,000 gallons (1.5 million liters) of liquid hydrogen in its larger lower tank. The ET is the only piece on the STS that is not reused and it burns up in the atmosphere when it is jettisoned from the Orbiter. It is the heaviest component and contains the most fuel. The External Tank has to be really strong due to the weight and stress that the Solid Rocket Boosters and the Orbiter put on it during launch and ascent. Therefore, the External Tank acts as the backbone for the entire assembly.