Each propellant tank has a vent and relief valve at its forward
end. This dual-function valve can be opened by ground support equipment
for the vent function during prelaunch and can open during flight
when the ullage (empty space) pressure of the liquid hydrogen tank
reaches 38 psig or the ullage pressure of the liquid oxygen tank
reaches 25 psig.
The liquid oxygen tank contains a separate, pyrotechnically operated,
propulsive tumble vent valve at its forward end. At separation,
the liquid oxygen tumble vent valve is opened, providing impulse
to assist in the separation maneuver and more positive control of
the entry aerodynamics of the ET.
There are eight propellant-depletion sensors, four each for fuel
and oxidizer. The fuel-depletion sensors are located in the bottom
of the fuel tank. The oxidizer sensors are mounted in the orbiter
liquid oxygen feed line manifold downstream of the feed line disconnect.
During SSME thrusting, the orbiter general-purpose computers constantly
compute the instantaneous mass of the vehicle due to the usage of
the propellants. Normally, main engine cutoff is based on a predetermined
velocity; however, if any two of the fuel or oxidizer sensors sense
a dry condition, the engines will be shut down.
The locations of the liquid oxygen sensors allow the maximum amount
of oxidizer to be consumed in the engines, while allowing sufficient
time to shut down the engines before the oxidizer pumps cavitate
(run dry). In addition, 1,100 pounds of liquid hydrogen are loaded
over and above that required by the 6-1 oxidizer/fuel engine mixture
ratio. This assures that MECO from the depletion sensors is fuel-rich;
oxidizer-rich engine shutdowns can cause burning and severe erosion
of engine components.
Four pressure transducers located at the top of the liquid oxygen
and liquid hydrogen tanks monitor the ullage pressures.
Each of the two aft external tank umbilical plates mate with a
corresponding plate on the orbiter. The plates help maintain alignment
among the umbilicals. Physical strength at the umbilical plates
is provided by bolting corresponding umbilical plates together.
When the orbiter GPCs command external tank separation, the bolts
are severed by pyrotechnic devices.
The ET has five propellant umbilical valves that interface with
orbiter umbilicals: two for the liquid oxygen tank and three for
the liquid hydrogen tank. One of the liquid oxygen tank umbilical
valves is for liquid oxygen, the other for gaseous oxygen. The liquid
hydrogen tank umbilical has two valves for liquid and one for gas.
The intermediate-diameter liquid hydrogen umbilical is a recirculation
umbilical used only during the liquid hydrogen chill-down sequence
The ET also has two electrical umbilicals that carry electrical
power from the orbiter to the tank and the two SRBs and provide
information from the SRBs and ET to the orbiter.
A swing-arm-mounted cap to the fixed service structure covers the
oxygen tank vent on top of the ET during the countdown and is retracted
about two minutes before lift- off. The cap siphons off oxygen vapor
that threatens to form large ice on the ET, thus protecting the
orbiter's thermal protection system during launch.