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The giant, cylindrical, external fuel tank, with a length of 47 m (154 ft) and a diameter of 8.4 m (27.5 ft), is the largest single piece of the space shuttle. It fuels the orbiter's three main engines. During launch, the external tank also acts as a support for the orbiter and SRBs to which it is attached.

Inside separate pressurized tanks, the external tank holds the liquid hydrogen fuel and liquid oxygen oxidizer (which reacts with the hydrogen to produce combustion) that runs the shuttle's three main engines. During launch, the external tank feeds the fuel under pressure through small ducts that branch off into smaller lines that feed directly into the main engines. Some 450 kg (1,000 lb) of fuel are consumed by each of the main engines each second.

The space shuttle’s external fuel tank is the only part of the launch vehicle that currently is not reused. After its 1.99 million liters (526,000 gallons) of fuel are consumed during the first 8.5 minutes of flight, the external tank is jettisoned from the orbiter and breaks up in the upper atmosphere, its pieces falling into remote ocean waters.

During the first 17 years of shuttle flights, the external fuel tanks were made of aluminum alloys. The tanks that the first five shuttle missions used weighed about 35,000 kg (about 77,000 lb) when empty. A design change in 1983 reduced the weight to 30,000 kg (66,000 lb) when empty. In 1998, anticipating the extra power that the shuttle will need to get to the International Space Station (ISS), which will orbit at a higher altitude than the space shuttle usually uses, NASA announced the introduction of a new tank design. The new tanks, first used in May 1998, are made of aluminum lithium, which is significantly lighter than the aluminum alloys used for previous tanks. The new tanks weigh about 27,000 kg (about 59,000 lb) when empty.


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