External Tank

Propelling the space shuttle into orbit requires a lot of fuel—more than 2 million liters (525,000 gallons) are used during every launch—and a very large tank to hold it. The biggest and heaviest element of a fully fueled space shuttle is the rust-colored, bullet-shaped external fuel tank, which the National Aeronautics and Space Administration (NASA) calls an ET.

Stretching 46.9 meters (153.8 feet) long and spanning 8.4 meters (27.6 feet) in diameter, the external tank forms the structural backbone of the shuttle during launch, absorbing most of the 2.7 million kilograms (6 million pounds) of thrust generated during blastoff. The primary job of the external tank, however, is to feed pressurized fuel to the shuttle's three hydrogen-burning main engines during the eight-and-a-half-minute ride into space. The engines consume more than 242,000 liters (64,000 gallons) of propellants every minute.

Carrying that much fuel into space is difficult enough—about 25 percent of the shuttle's 2-million-kilogram (4.4-million-pound) launch weight is the weight of the fuel itself. But adding to the complexity is the unusual nature of the fuel, which is only remotely similar to the petrochemicals used in most automobiles.

The external tank contains liquid hydrogen and liquid oxygen, super-cold substances that have to be kept well below freezing in the usually warm weather found at the shuttle's Florida launch site. The external tank is coated with many layers of a special foam insulation to keep ice from forming on the outside of the tank during the final hours before launch. Any ice on the shuttle could break off during launch and damage the spaceship.

The external tank actually contains three tanks: one at the top for liquid oxygen, one in the middle to house electronics, and a large container in the rear to hold the liquid hydrogen. The oxygen must be kept at -183°C (-297°F), and the hydrogen at -253°C (-423°F)—just a bit shy of absolute zero, the point at which there is a complete absence of thermal energy.

The external tank is the only part of the space shuttle that is not reusable. The tank is attached to the underside of the orbiter at three locations. When the shuttle is almost in orbit and the fuel tank nearly empty, small explosives are fired to break the tank's connective bolts and jettison it from the spaceship. The tank breaks into pieces as it reenters the atmosphere, and any debris splashes down into a remote area of the Indian or Pacific Oceans.

Several proposals have been made over the years to turn the shuttles' spent fuel tanks into mini space stations and other orbital platforms, but so far the tanks, which cost NASA about $43 million apiece in 2001, have never been recycled.

see also Rocket Engines (volume 1); Rockets (volume 3); Solid Rocket Boosters (volume 3); Space Shuttle (volume 3).

Irene Brown

Bibliography

Collins, Michael. Liftoff: The Story of America's Adventure in Space. New York: Grove Press, 1988.

Internet Resources

National Aeronautics and Space Administration. National Space Transportation System Reference. 1988. <http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/et.html>.