The External Tank is, in a sense, the center of the Space Shuttle
during launch. It performs a number of roles:
Physically, it's pretty darn impressive. Here's some stats, taken from NASA and Martin Marietta:
- Length: Approx. 50 meters (158.3 ft)
- Diameter: Approx. 9 meters (27.6 ft)
- Weight (empty): 35,425 kg (78,100 lbs.)
- Weight (at liftoff): 756,445 kg (1,667,677 lbs.)
- Volume, Oxygen storage: 541,541 liters (143,351 gal.)
- Volume, Hydrogen storage: 1,045,063 liters (383,066 gal.)
- Weight, Oxygen (full load): 616,496 kg (1,359,142 lbs.)
- Weight, Hydrogen (full): 102,619 kg (226,237 lbs.)
The orbiter is attached to the external tank at two points; a frame aft and an attach point near the underside of the orbiter's nose. During launch, the external tank bears the full stress of the total 7.8 million pounds of thrust generated by the three main engines and the two SRBs. The SRBs are attached to the sides of the External Tank, not the orbiter. Fuel and oxidizer are transferred to the orbiter through 17-inch diameter feed connections.
This is the only non-reusable component of the Space Transportation System. The SRBs are recovered, refurbished and reused, and the orbiter of course returns to Earth in a controlled landing. The ET, however, is jettisoned at around T plus 8 minutes 30 seconds. At this point, the Shuttle is at approximately 113 km (70 miles) altitude. On a normal flight, the external tank is partially destroyed by re-entry into the atmosphere; its trajectory is plotted so that remnants (if any) will land in the Indian Ocean. This has happened; sightings of the external tank floating (nearly intact) have been reported. In response, NASA lightened the construction of the tank for mission STS-5. This not only increased the likelihood of the tank burning up, but also saved around 11,000 pounds of liftoff vehicle weight with no performance decrease. This led to a nearly 10,500 pound increase in the useful payload capacity of the Shuttle.
Built in New Orleans, LA by Martin Marietta Aerospace (probably now part of Lock-Mart), the tank also contains a range safety package whose receivers are built by Motorola. At launch, it is coated with a thermal protectant to survive atmospheric heating and discourage ice formation on the tank.
The tank has some 'smarts' as well - there are numerous sensors to keep the GPCs aboard the orbiter informed of remaining propellant and oxidizer, as well as the 'health' of all tank subsystems. There is a vent valve in the nose which is opened at separation from the orbiter; remaining oxygen escaping through this valve helps 'push' the nose of the tank upward and away from the orbiter.
In order to power all this, there is an electrical system which also serves to ground lightning strikes to the tank should they occur.
Since Main Engine Cut-Off can be triggered by either tank running dry as well as by the planned flight profile, the ET is slightly overloaded with hydrogen. This is because a 'fuel rich' MECO is safe, whereas an 'oxygen-rich' MECO causes hotter burnout and can erode engine components.