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1993-02-04
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"6_2_15_5.TXT" (3575 bytes) was created on 11-02-87
EXTERNAL TANK
The Space Shuttle external tank has two major roles in the
Space Shuttle Program -- to contain and deliver propellants to the
main engines and to serve as the structural backbone of the Space
Shuttle during launch operations.
The external tank is composed of two tanks -- a large liquid
hydrogen tank and a small oxygen tank -- joined together by an
intertank section to form one large propellant storage container.
The liquid oxygen tank is the forward portion of the
external tank. The forward end of the oxygen tank curves to a
point to reduce aerodynamic drag. It also provides lightning
protection for the entire Shuttle vehicle. The oxygen tank holds
approximately 542,000 liters (143,000) gallons) of oxygen.
The liquid hydrogen tank is located aft of the oxygen tank
and is about two-and-one-half times larger than the smaller
oxygen tank. In this tank is stored liquid hydrogen at a
temperature of about 251 degrees below zero centigrade (420
degrees below zero fahrenheit). The hydrogen tank holds
approximately 1,458,000 liters (385,000 gallons) of hydrogen.
The intertank joins the two tanks. Its primary functions are
to receive and distribute all thrust loads from the solid rocket
boosters and transfer loads between tanks for structural
continuity.
For launch, the external tank is attached to the solid
rocket boosters. The Shuttle orbiter is attached to the external
tank. Since thrust is generated by the main engines on the
orbiter and the two solid rocket boosters, the external tank must
absorb the thrust loads for the Shuttle vehicle during launch.
Much of the outer surface of the tank is protected
thermally. Spray-on foam insulation is applied over the forward
portion of the oxygen tank, the intertank, and the sides and
bottom of the hydrogen tank. The foam insulation is needed to
reduce ice or frost formation on the tank during launch and to
minimize heat leaks into the tank that would cause boiling of the
liquid propellants. Ice build-up on the tank would increase
significantly the Shuttle lift-off weight.
An ablating material -- a material that chars away -- is
applied to the external tank bulges and projections to protect
them from aerodynamic heating during flight through the
atmosphere.
The external tank contains systems that are necessary to
support its operation. These include a propellant feed system to
pipe the propellants to the orbiter; a pressurization and vent
system to regulate the tank pressure; an environmental
conditioning system to regulate the temperature and humidity in
the intertank area; and an electrial system to distribute
electrical power and instrumentation signals and to provide
lightning protection.
All fluid control and valves, except for vent valves, for
operation of the engines are located in the orbiter. This is
done to minimize throw-away costs since the external tank is not
reused.
External Tank
Length 46.9 meters (153.8 feet)
Diameter 8.4 meters (27.6 feet)