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...