The external tank contains
the liquid hydrogen fuel and liquid oxygen oxidizer and supplies
them under pressure to the three space shuttle main engines in the
orbiter during lift-off and ascent. When the SSMEs are shut down,
the ET is jettisoned, enters the Earth's atmosphere, breaks up,
and impacts in a remote ocean area. It is not recovered.
The largest and heaviest (when loaded) element of the space shuttle,
the ET has three major components: the forward liquid oxygen tank,
an unpressurized intertank that contains most of the electrical
components, and the aft liquid hydrogen tank. The ET is 153.8 feet
long and has a diameter of 27.6 feet.
Beginning with the STS-6 mission, a lightweight ET was introduced.
Although future tanks may vary slightly, each will weigh approximately
66,000 pounds inert. The last heavyweight tank, flown on STS-7,
weighed approximately 77,000 pounds inert. For each pound of weight
reduced from the ET, the cargo-carrying capability of the space
shuttle spacecraft is increased almost one pound. The weight reduction
was accomplished by eliminating portions of stringers (structural
stiffeners running the length of the hydrogen tank), using fewer
stiffener rings and by modifying major frames in the hydrogen tank.
Also, significant portions of the tank are milled differently to
reduce thickness, and the weight of the ET's aft solid rocket booster
attachments were reduced by using a stronger, yet lighter and less
expensive titanium alloy. Earlier several hundred pounds were eliminated
by deleting the anti-geyser line. The line paralleled the oxygen
feed line and provided a circulation path for liquid oxygen to reduce
accumulation of gaseous oxygen in the feed line while the oxygen
tank was being filled before launch. After propellant loading data
from ground tests and the first few space shuttle missions was assessed,
the anti-geyser line was removed for STS-5 and subsequent missions.
The total length and diameter of the ET remain unchanged.
The ET is attached to the orbiter at one forward attachment point
and two aft points. In the aft attachment area, there are also umbilicals
that carry fluids, gases, electrical signals and electrical power
between the tank and the orbiter. Electrical signals and controls
between the orbiter and the two solid rocket boosters also are routed
through those umbilicals.