Editing Space Shuttle external tank (section)

===Hardware===
[[File:STS-116 PreLaunch (NASA KSC-06PD-2670).jpg|thumb|{{OV|103}} before the launch of [[STS-116]] in December 2006. Beneath ''Discovery'''s wings are the tail masts, which provide several umbilical connections to the orbiter, including a liquid-oxygen line through one and a liquid-hydrogen line through another. Seen above the golden external tank is the vent hood (known as the "[[Beanie (seamed cap)|beanie cap]]") at the end of the gaseous oxygen vent arm, extending from the fixed service structure. Vapor boils off the liquid oxygen in the external tank. The hood vents the oxygen vapor away from the Space Shuttle vehicle.]]

The external hardware, ET–orbiter attachment fittings, umbilical fittings, and electrical and range safety system weigh {{convert|9100|lb}}.

====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.

====Sensors====
[[File:Sts et ecographic.jpg|thumb|300px|The location of ECO sensors in the LH<sub>2</sub> tank]]
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 RS-25 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 [[cavitation|cavitate]] (run dry). In addition, {{convert|1100|lb|kg|abbr=on}} of liquid hydrogen are loaded over and above that required by the 6:1 oxidizer–fuel engine mixture ratio. This assures that cutoff from the depletion sensors is fuel-rich; oxidizer-rich engine shutdowns can cause burning and severe erosion of engine components, potentially leading to loss of the vehicle and crew.

Unexplained, erroneous readings from fuel depletion sensors have delayed several shuttle launch attempts, most notably [[STS-122]]. On December 18, 2007, a tanking test determined the cause of the errors to be a fault in a wiring connector, rather than a failure of the sensors themselves.<ref>
{{cite web | url = http://afp.google.com/article/ALeqM5hwoRZdYGkG1X3lZOF3x4GduJuKNQ | title = NASA eyes faulty gauge wires as source of shuttle problems | date = December 18, 2007 | publisher = AFP | url-status = dead | archive-url = https://web.archive.org/web/20080218101202/http://afp.google.com/article/ALeqM5hwoRZdYGkG1X3lZOF3x4GduJuKNQ | archive-date = February 18, 2008
}}</ref>

Four [[pressure transducer]]s located at the top of the liquid oxygen and liquid hydrogen tanks monitor the ullage pressures.

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.

The ET has external cameras mounted in the brackets attached to the shuttle along with transmitters that can continue to send video data long after the shuttle and the ET have separated.