The tank is also the "backbone" of the Shuttle during the launch, providing structural support for attachment with the solid rocket boosters and orbiter.
The tank is the only component of the Space Shuttle that is not reused. Approximately 8.5 minutes into the flight, with its propellant used, the tank is jettisoned.
At liftoff, the External Tank absorbs the total (7.8 million pounds) thrust loads of the three main engines and the two solid rocket motors.
When the Solid Rocket Boosters separate at an altitude of approximately 45 kilometers (28 miles), the orbiter, with the main engines still burning, carries the external tank piggyback to near orbital velocity, approximately 113 kilometers (70 miles) above the Earth. The now nearly empty tank separates and falls in a preplanned trajectory with the majority of it disintegrating in the atmosphere and the rest falling into the ocean.
The three main components of the External Tank are an oxygen tank, located in the forward position, an aft-positioned hydrogen tank, and a collar-like intertank, which connects the two propellant tanks, houses instrumentation and processing equipment, and provides the attachment structure for the forward end of the solid rocket boosters.
The hydrogen tank is 2.5 times larger than the oxygen tank but weighs only one-third as much when filled to capacity. The reason for the difference in weight is that liquid oxygen is 16 times heavier than liquid hydrogen.
The skin of the External Tank is covered with a thermal protection system that is a 2.5-centimeter (1-inch) thick coating of spray-on polyisocyanurate foam. The purpose of the thermal protection system is to maintain the propellants at an acceptable temperature, to protect the skin surface from aerodynamic heat and to minimize ice formation.
The External Tank includes a propellant feed system to duct the propellants to the Orbiter engines, a pressurization and vent system to regulate the tank pressure, an environmental conditioning system to regulate the temperature and render the atmosphere in the intertank area inert, and an electrical system to distribute power and instrumentation signals and provide lightning protection.
The tank's propellants are fed to the Orbiter through a 17-inch diameter connection that branches inside the orbiter to feed each main engine.