Editing Space Shuttle external tank (section)

====Vents and relief valves====
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 {{cvt|38|psi}} or the ullage pressure of the liquid oxygen tank reaches {{cvt|25|psi}}.

On early flights, the liquid oxygen tank contained a separate, [[pyrotechnic]]ally operated, propulsive tumble vent valve at its forward end. At separation, the liquid oxygen tumble vent valve was opened, providing impulse to assist in the separation maneuver and more positive control of the entry aerodynamics of the ET. The last flight with the tumble valve active was STS-36.

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 during prelaunch.

[[File:Technicians inspecting the Ground Umbilical Carrier Plate on Space Shuttle Endeavour's external fuel tank during STS-127 - 20090624.jpg|thumb|Technicians inspect the GUCP following a scrub of [[STS-127]] due to elevated hydrogen levels at this connector.]]
As the ET is filled, excess gaseous hydrogen is vented through umbilical connections over a large diameter pipe on an arm extended from the fixed service structure. The connection for this pipe between the ET and service structure is made at the ground umbilical carrier plate (GUCP). Sensors are also installed at the GUCP to measure hydrogen levels. Countdowns of [[STS-80]], [[STS-119]], [[STS-127]] and [[STS-133]] have been halted and resulted in several week delays in the later cases due to hydrogen leaks at this connection. This requires complete draining of the tanks and removal of all hydrogen via helium gas purge, a 20-hour process, before technicians can inspect and repair problems.<ref>{{cite web|title=The Ground Umbilical Carrier Plate|url=http://www1.nasa.gov/multimedia/podcasting/gucp_prt.htm|publisher=NASA|url-status=dead|archive-url=https://web.archive.org/web/20101124043253/http://www.nasa.gov/multimedia/podcasting/gucp_prt.htm|archive-date=November 24, 2010}}</ref>

A cap mounted to the swing-arm on 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 accumulations on the ET, thus protecting the orbiter's thermal protection system during launch.