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Length: 153 feet, 10 inches

Diameter: 27 feet, 7 inches

The External Tank (ET) is the only non-reusable element of the Space Shuttle. It is also the largest element of the Space Shuttle, and provides the structural backbone of the entire system. The chief purpose of the ET is to carry the liquid fuels necessary to provide power for the Orbiter's three main engines.

Martin Marietta (later Lockheed Martin) won a competitive contract to design and manufacture the Space Shuttle ET in 1973. The contract specified that the ET be constructed at the NASA Michoud Assembly Facility in New Orleans, Louisiana.

The Michoud plant afforded ample space to support construction of ET's, plus deep-water port access to allow transportation of ET's by enclosed barge to the Kennedy Space Center. Completed ET's are individually barged to the Kennedy Space Center in a voyage that typically takes 4.5 days.

For many years, the ET barge was towed to and from the Kennedy Space Center by leased tug. In 1998, however, NASA opted to save money by introducing a policy of towing the ET barge to and from the Kennedy Space Center by one of the Solid Rocket Booster (SRB) retrieval ships, whose crews would otherwise have remained idle.

The first ET was delivered to NASA on September 7, 1977. This was not a flight-ready ET, but rather was used for tests in association with the Space Shuttle Main Propulsion Test Article (MPTA-098) assembled at the Rockwell plant at Palmdale, California. Three test ET's were ultimately delivered to NASA.

The first flight-ready ET was delivered to NASA on June 29, 1979. The first six ET's delivered to NASA were called the Standard Weight Tank (SWT), with each tank weighing 75,500 pounds. SWT's were flown on Space Shuttle Missions STS-1 through STS-5 and STS-7.

During Space Shuttle Missions STS-1 and STS-2, the ET's were painted white. NASA quickly determined that hundreds of pounds of weight and thousands of dollars in preparation work would be saved if the ET's remained unpainted, and so all ET's flown from STS-3 onward remained unpainted, sporting an orange-brown color.

In 1979, even before a Space Shuttle had completed a space flight, NASA issued a directive that ET's be lightened by at least 6,000 pounds so that Space Shuttles would be able to carry proportionally heavier payloads.

The resulting ET was called the Lightweight Tank (LWT) which was made 10,000 pounds lighter than the SWT through several methods, including materials and design changes and the use of new fabrication techniques. The 65,500-pound LWT was first flown on Space Shuttle Mission STS-6, and then from STS-8 through STS-90.

Beginning with Space Shuttle Mission STS-91, a new ET called the Super Lightweight Tank (SLWT) has been flown. At a weight of 58,000 pounds, the SLWT is 7,500 pounds lighter than the LWT. The SLWT was introduced primarily to allow NASA to carry heavier payloads aboard the Space Shuttle in support of International Space Station construction.

Weight savings in the SLWT were created primarily by constructing the ET from an aluminum-lithium alloy. Previous versions of the ET were constructed using an aluminum-steel alloy and titanium. In general terms, however, much about all three versions of the ET are identical.

Each ET is comprised of a liquid oxygen tank located at the top and a liquid hydrogen tank located at the bottom. The liquid oxygen tank is connected to the liquid hydrogen tank by an intertank, which is located in between the other two.

The intertank is designed to receive and distribute all thrust loads that the SRB's produce during launch and flight. The intertank is able to absorb and transfer these loads evenly, providing vital structural integrity for the Space Shuttle. The intertank is 22 feet, 6 inches long by 27 feet, 7 inches wide.

The liquid oxygen tank is 54 feet, 7 inches long by 27 feet, 7 inches wide and carries 145,138 gallons of liquid oxygen weighing 1,380,000 pounds. The liquid hydrogen tank is 96 feet, 8 inches long by 27 feet, 7 inches wide and carries 390,139 gallons of liquid hydrogen weighing 230,000 pounds.

Fully loaded with a total of 1,610,000 pounds of liquid fuel, the SLWT version of the ET in current use weighs 1,668,000 pounds at liftoff. Liquid oxygen is stored aboard the ET at a temperature of -297 degrees Fahrenheit, while the liquid hydrogen is stored aboard the ET at a temperature of -423 degrees Fahrenheit.

Since the liquid fuels stored inside the ET are this cold, the outer surface of the ET requires thermal protection. This is provided by a one inch layer of spray-on foam insulation applied over the forward portion of the liquid oxygen tank, the intertank and the sides and bottom of the liquid hydrogen tank.

This foam insulation is intended to reduce ice or frost formation on the tank, which could both increase the weight of the ET and become hazardous to the Space Shuttle during launch. The foam insulation also serves to protect the ET from the effects of engine and aerodynamic heating, which could potentially cause the liquid fuel to boil.

An ablative coating made of material that flakes off as it becomes hot is applied to the ET external bulges and projections to protect them from aerodynamic heating that develops during flight.

The ET also contains systems that are necessary to support its operation. These include a pressurization and vent system to regulate the tank pressure, an environmental conditioning system to regulate the temperature and humidity of the intertank area and an electrical system to distribute power and provide lightning protection.

Much of this equipment is housed in the intertank. All fluid control mechanisms and valves that regulate performance of the ET, with the exception of the vent control valves, are located in the Orbiter. This is due to the cost savings over replacing this equipment after every Space Shuttle mission, since the ET cannot be recovered.

The ET supplies liquid oxygen and liquid hydrogen to the Orbiter's three main engines through 17-inch diameter feed lines. The liquid oxygen is fed at a rate of 159,480 pounds, or 16,800 gallons, per minute while the liquid hydrogen is fed at a rate of 26,640 pounds, or 45,283 gallons, per minute.

At launch, the ET is attached to both the Orbiter and SRB's. The SRB's are jettisoned from the ET about two minutes after launch. The ET typically empties about 8.5 minutes after launch, at which time it is detached from the Orbiter, breaks up and falls into a predetermined area of either the Pacific or Indian Ocean.

Copyright © 1998 by Spaceline, Inc.