The External Tank (ET) is the only non-reusable major component of the Space Shuttle system that consists of the ET, the Orbiter and the two Solid Rocket Boosters. The ET is the single largest element at 154 feet long and 27.6 feet in diameter, and during launch it serves as the structural backbone of the shuttle, absorbing most of the six million pounds of thrust generated during flight and providing propellant to the Orbiter’s three main engines.
A new version of the ET, the Super Lightweight Tank (SLWT), is 7,500 pounds lighter than the previous Lightweight Tank design. The reduced weight is a result of component redesign and the use of Weldalite™, an aluminum-lithium (Al-Li) alloy developed by Lockheed Martin. The Al-Li alloy is 30 percent stronger and five percent less dense than the aluminum alloy previously used in manufacturing ETs.
The first SLWT flew as part of STS-91 on June 2, 1998.
Weighing 58,500 pounds empty and 1.6 million pounds when filled with cryogenic propellants, the ET supplies over 535,000 gallons of liquid oxygen and liquid hydrogen to the Orbiter's engines. The 7,500-pound weight savings resulting from the SLWT increases shuttle payload capacity by a similar amount. The performance increase is vital to the building and supplying of the International Space Station.
Lockheed Martin is under contract to the NASA Marshall Space Flight Center to assemble ETs at Michoud Assembly Facility in New Orleans through 2001. Negotiations are currently underway for the production of additional ETs to support the Space Shuttle program.
External Tank Components 
The liquid oxygen tank is an aluminum monocoque structure composed of a fusion-welded assembly of preformed, chem-milled gores, panels, machined fittings and ring chords. It operates in a pressure range of 20 to 22 pounds per square inch. The tank contains anti-slosh and anti-vortex provisions to minimize liquid residuals and damp fluid motion. The tank feeds into a 17-inch-diameter feed line that conveys the liquid oxygen through the intertank, then outside the ET to the aft right-hand ET / orbiter disconnect umbilical. The 17-inch-diameter feed line permits liquid oxygen to flow at approximately 2,787 pounds per second with the Space Shuttle Main Engines operating at 104 percent or permits a maximum flow of 17,592 gallons per minute. The liquid oxygen tank's nose cone reduces drag and heating and serves as a lightning rod. The liquid oxygen tank's volume is 19,563 cubic feet. It is 331 inches in diameter and 592 inches long.
The liquid hydrogen tank is an aluminum semimonocoque structure of fusion-welded barrel sections, five major ring frames, and forward and aft ellipsoidal domes. Its operating pressure range is 32 to 34 pounds per square inch. The tank contains an anti-vortex baffle and siphon outlet to transmit the liquid hydrogen from the tank through a 17-inch line to the left aft umbilical. The liquid hydrogen feed line flow rate is 465 pounds per second with the Space Shuttle Main Engines at 104 percent or a maximum flow of 47,365 gallons per minute. At the forward end of the liquid hydrogen tank is the ET / orbiter forward attachment pod strut, and at its aft end are the two ET / orbiter aft attachment ball fittings as well as the aft solid rocket booster-ET stabilizing strut attachments. The liquid hydrogen tank is 331 inches in diameter, 1,160 inches long, and has a volume of 53,518 cubic feet.
The intertank is a steel / aluminum semimonocoque cylindrical structure with flanges on each end for joining the forward liquid oxygen and aft liquid hydrogen tanks. The intertank houses ET instrumentation components and provides an umbilical plate that interfaces with the ground facility arm for purge gas supply, hazardous gas detection and hydrogen gas boiloff during pre-launch operations. It consists of mechanically joined skin, stringers and machined panels of aluminum alloy. The intertank is vented during flight. The intertank contains the forward solid rocket booster (SRB)-ET attach thrust beam and fittings that distribute the SRB loads to the liquid oxygen and liquid hydrogen tanks. The intertank is 270 inches long and 331 inches in diameter.
To prevent the super cold liquid oxygen (-297 degrees F) and liquid hydrogen (-423 degrees F) from forming ice on the outside surfaces of the ET, a multi-layered thermal protection coating approximately one inch thick is applied. The insulation allows the ET to withstand the extreme internal and external temperatures generated during prelaunch, launch and flight.
At launch, propellants are pressure-fed at a combined rate of 1,035 gallons per second through 17-inch diameter feed lines to the Orbiter’s engines. Eight and one-half minutes into flight, with the Orbiter and ET at an altitude of about 71 nautical miles, the main engines are cut off and the ET is jettisoned. The tank slowly tumbles, reenters the atmosphere and burns up, with small surviving parts safely falling into remote areas of the Pacific or Indian Oceans.
ET Weight Savings Means More Payload
The first Space Shuttle powered by an External Tank flew on April 12, 1981. The version of the ET used on the initial launches weighed 76,000 pounds. A subsequent redesign program netted a 10,000-pound weight savings on the ET. Because the ET and orbiter have virtually reached gravitational escape velocity when the Et is jettisoned, every pound reduced from the ET results in another pound that can be taken to orbit. Thus the 66,000-pound Lightweight Tank, introduced on the sixth Space Shuttle mission in 1983, resulted in substantial improvements to shuttle payload performance.
The weight-savings continued in 1998 with the Super Lightweight Tank weighing another 7,500 pounds less. This opened the door for the Space Shuttle to carry the heavier components of the International Space Station.
Related NASA's Fact Sheet site: A Walk Around the Space Shuttle for information on the other components (Orbiter and Solid Rocket Boosters) of the Space Shuttle.