Editing Space Shuttle external tank (section)

==Components==
The ET has three primary structures: an LOX tank, an intertank, and an LH<sub>2</sub> tank. Both tanks are constructed of aluminium alloy skins with support or stability frames as required. The intertank aluminium structure utilizes skin stringers with stabilizing frames. The primary aluminium materials used for all three structures are 2195 and 2090 alloys. [[2195 aluminium alloy|AL 2195]] is an Al-Li alloy designed by Lockheed Martin and Reynolds for storage of cryogenics (and used for the SLW version of the ET - earlier versions used Al 2219<ref name=Nasa-SLW>[http://www.nasa.gov/sites/default/files/113020main_shuttle_lightweight.pdf Super Lightweight External Tank], NASA, retrieved December 12, 2013.</ref>). [[2090 aluminium alloy|Al 2090]] is a commercially available Al-Li alloy.
[[Image:Sts et cutaway.jpg|thumb|300px|Anatomy of the external tank]]

===Liquid oxygen tank===
The LOX tank is located at the top{{efn|name=orientation}} of the ET and has an [[ogive]] shape to reduce aerodynamic drag and aerothermodynamic heating. The ogive nose section is capped by a flat removable cover plate and a [[nose cone]]. The nose cone consists of a removable conical assembly that serves as an aerodynamic fairing for the propulsion and electrical system components. The foremost element of the nose cone functions as a cast aluminium lightning rod. The LOX tank volume is {{convert|19744|cuft|m3|abbr=on}} at {{cvt|22|psi}} and {{convert|-297|F|K C|abbr=on}} ([[cryogenic]]).

The tank feeds into a {{convert|17|in|mm|abbr=on}} diameter feed line that conveys the liquid oxygen through the intertank, then outside the ET to the aft right-hand ET/orbiter disconnect umbilical. The {{convert|17|in|mm|abbr=on}} diameter feed line permits liquid oxygen to flow at approximately {{cvt|2,787|lb/s}} with the RS-25s operating at 104% or permits a maximum flow of {{cvt|17,592|USgal/min}}.

All loads except aerodynamic loads are transferred from the LOX tank at a bolted, flange-joint interface with the intertank.

The LOX tank also includes an internal slosh baffle and a vortex baffle to dampen fluid slosh. The vortex baffle is mounted over the LOX feed outlet to reduce fluid swirl resulting from slosh and to prevent entrapment of gases in the delivered LOX.

===Intertank===<!-- This section is linked from [[STS-51-L Mission timeline]] -->
The intertank is the ET structural connection between the LOX and LH<sub>2</sub> tanks. Its primary functions are to receive and distribute all thrust loads from the SRBs and transfer loads between the tanks.

The two SRB forward attach fittings are located 180° apart on the intertank structure. A beam is extended across the intertank structure and is mechanically fastened to the attach fittings. When the SRBs are firing, the beam will flex due to high stress loads. These loads will be transferred to the fittings.

Adjoining the SRB attach fittings is a major ring frame. The loads are transferred from the fittings to the major ring frame which then distributes the tangential loads to the intertank skin. Two panels of the intertank skin, called the thrust panels, distribute the concentrated axial SRB thrust loads to the LOX and LH<sub>2</sub> tanks and to adjacent intertank skin panels. These adjacent panels are made up of six stringer-stiffened panels.

The intertank also functions as a protective compartment for housing the operational instrumentation.

===Liquid hydrogen tank===
[[Image:Sts et1.jpg|thumb|The {{convert|70|ft||adj=mid|-long}}, {{convert|17|in||adj=mid|-diameter}} liquid oxygen feedline runs externally along the right side of the liquid hydrogen tank up and into the intertank. Two {{convert|5|in|adj=on}} diameter re-pressurization lines run beside it. One supplies hydrogen gas to the liquid hydrogen tank and the other supplies oxygen gas to the liquid oxygen tank. They are used to maintain the [[Ullage (rocketry)|ullage]] pressure in each tank during the launch.]]
The LH<sub>2</sub> tank is the bottom{{efn|name=orientation}} portion of the ET. The tank is constructed of four cylindrical barrel sections, a forward dome, and an aft dome. The barrel sections are joined together by five major ring frames. These ring frames receive and distribute loads. The forward dome-to-barrel frame distributes the loads applied through the intertank structure and is also the flange for attaching the LH<sub>2</sub> tank to the intertank. The aft major ring receives orbiter-induced loads from the aft orbiter support struts and SRB-induced loads from the aft SRB support struts. The remaining three ring frames distribute orbiter thrust loads and LOX feedline support loads. Loads from the frames are then distributed through the barrel skin panels. The LH<sub>2</sub> tank has a volume of {{convert|53488|cuft}} at {{cvt|29.3|psi}} and {{cvt|-423|F}} (cryogenic).

[[File:Space Shuttle external tank assembly 01.jpg|thumb|Interior of a liquid hydrogen tank during assembly at the NASA rocket factory, with humans for scale]]

The forward and aft domes have the same modified ellipsoidal shape. For the forward dome, mounting provisions are incorporated for the LH<sub>2</sub> vent valve, the LH<sub>2</sub> pressurization line fitting, and the electrical feed-through fitting. The aft dome has a manhole fitting for access to the LH<sub>2</sub> feedline screen and a support fitting for the LH<sub>2</sub> feedline.

The LH<sub>2</sub> tank also has a vortex baffle to reduce swirl resulting from slosh and to prevent entrapment of gases in the delivered LH<sub>2</sub>. The baffle is located at the siphon outlet just above the aft dome of the LH<sub>2</sub> tank. This outlet transmits the liquid hydrogen from the tank through a {{convert|17|in}} line to the left aft umbilical. The liquid hydrogen feed line flow rate is {{cvt|465|lb/s}} with the main engines at 104% or a maximum flow of {{cvt|47,365|USgal/min}}.

===Thermal protection system===
[[Image:Sts et.jpg|thumb|upright|The Orbiter attachment hardware, liquid hydrogen umbilical connection (left), and liquid oxygen umbilical connection (right) are visible at the bottom of the tank.]]
The ET thermal protection system consists primarily of spray-on [[foam]] [[Thermal insulation|insulation]] (SOFI), plus preformed foam pieces and premolded [[ablate|ablator]] materials. The system also includes the use of [[Phenolic resin|phenolic]] thermal insulators to preclude air liquefaction. Thermal isolators are required for liquid hydrogen tank attachments to preclude the liquefaction of air on exposed metal, and to reduce heat flow into the liquid hydrogen. While the warmer liquid oxygen results in fewer thermal requirements, the aluminum of the liquid oxygen tank forward areas require protection from [[Aerodynamic heating|aeroheating]]. Meanwhile, insulation on the aft surfaces prevents liquified air from pooling in the intertank. The middle cylinder of the oxygen tank, and the propellant lines, could withstand the expected depths of frost accumulation condensed from humidity, but the orbiter could not take the damage from ice breaking free. The thermal protection system weighs {{convert|4823|lb|kg|abbr=on}}.

Development of the ETs thermal protection system was problematic. Anomalies in foam application were so frequent that they were treated as variances, not safety incidents. NASA had difficulty preventing fragments of foam from detaching during flight for the entire history of the program:

*[[STS-1]] ''[[Space Shuttle Columbia|Columbia]]'', 1981: Crew reports white material streaming past windows during orbiter-external-tank flight. Crew estimated sizes from {{convert|1/4|in|mm|sigfig=1}} to fist-sized. Post-landing report describes probable foam loss of unknown location, and 300 tiles needing outright replacement due to various causes.
*[[STS-4]] ''Columbia'', 1982: PAL ramp loss; 40 tiles require outright replacement.
*[[STS-5]] ''Columbia'', 1982: Continued high rate of tile loss.
*[[STS-7]] ''[[Space Shuttle Challenger|Challenger]]'', 1983: {{convert|50|by|30|cm|in|abbr=on}} Bipod ramp loss photographed, dozens of spot losses.<ref name="sts7">{{cite web |url=http://www.astronautix.com/flights/sts7.htm |title=STS-7 |publisher=Astronautix.com |access-date=November 25, 2010 |url-status=dead |archive-url=https://web.archive.org/web/20101129060148/http://astronautix.com/flights/sts7.htm |archive-date=November 29, 2010 }}</ref>
*[[STS-27]] ''[[Space Shuttle Atlantis|Atlantis]]'', 1988: One large loss of uncertain origin, causing one total tile loss. Hundreds of small losses.
*[[STS-32]] ''Columbia'', 1990: Bipod ramp loss photographed; five spot losses up to 70&nbsp;cm in diameter, plus tile damages.<ref name="flatoday">[http://www.floridatoday.com/columbia/shuttlechart.html Insulation problems seen before] {{webarchive |url=https://web.archive.org/web/20070715124906/http://www.floridatoday.com/columbia/shuttlechart.html |date=July 15, 2007 }}</ref>
*[[STS-50]] ''Columbia'', 1992: Bipod ramp loss. 20×10×1&nbsp;cm tile damage.<ref name="flatoday"/>
*[[STS-52]] ''Columbia'', 1992: Portion of bipod ramp, jackpad lost. 290 total tile marks, 16 greater than an inch.
*[[STS-62]] ''Columbia'', 1994: Portion of bipod ramp lost.

In 1995, [[Trichlorofluoromethane|chlorofluorocarbon-11]] (CFC-11) began to be withdrawn from large-area, machine-sprayed foams in compliance with an [[United States Environmental Protection Agency|Environmental Protection Agency]] ban on CFCs under section 610 of the [[Clean Air Act (United States)|Clean Air Act]]. In its place, a [[hydrochlorofluorocarbon]] known as [[1,1-Dichloro-1-fluoroethane|HCFC-141b]] was certified for use and phased into the shuttle program. Remaining foams, particularly detail pieces sprayed by hand, continued to use CFC-11 through the end of the program. These areas include the problematic bipod and PAL ramps, as well as some fittings and interfaces. For the bipod ramp in particular, "the process of applying foam to that part of the tank had not changed since 1993."<ref name="Deseret">Bridis, Ted. "Foam called a concern on flight before Columbia," Deseret News (Salt Lake City), March 22, 2003, pp. 1: http://findarticles.com/p/articles/mi_qn4188/is_20030322/ai_n11384413</ref> The "new" foam containing HCFC 141b was first used on the aft dome portion of ET-82 during the flight of [[STS-79]] in 1996. Use of HCFC 141b was expanded to the ETs area, or larger portions of the tank, starting with ET-88, which flew on [[STS-86]] in 1997.

During the lift-off of [[STS-107]] on January 16, 2003, a piece of foam insulation detached from one of the tank's bipod ramps and struck the leading edge of {{OV|102}}'s wing at a few hundred miles per hour. The impact is believed to have damaged one comparatively large reinforced carbon-carbon panel on the leading edge of the left wing, believed to be about the size of a basketball which then allowed super-heated gas to enter the wing superstructure several days later during re-entry. This resulted in the [[Space Shuttle Columbia disaster|destruction of ''Columbia'']] and the loss of its crew. The report determined that the external fuel tank, ET-93, "had been constructed with BX-250", a closeout foam whose blowing agent was CFC-11 and not the newer HCFC 141b.<ref name="D07">[http://www.nasa.gov/columbia/caib/PDFS/VOL2/D07.PDF Columbia Accident Investigation Board Report, Volume 2, Appendix D], Section 11.3 and figure 11-1, p222, Columbia Accident Investigation Board,</ref>

In 2005, the problem of foam shed had not been fully cured; on [[STS-114]], additional cameras mounted on the tank recorded a piece of foam separated from one of its Protuberance Air Load (PAL) ramps, which are designed to prevent unsteady air flow underneath the tank's cable trays and pressurization lines during ascent. The PAL ramps consist of manually sprayed layers of foam, and are more likely to become a source of debris. That piece of foam did not impact the orbiter.

Reports published concurrent with the [[STS-114]] mission suggest that excessive handling of the ET during modification and upgrade may have contributed to the foam loss on ''Discovery''{{'s}} Return to Flight mission. However, three shuttle missions ([[STS-121]], [[STS-115]], and [[STS-116]]) were later conducted, all with "acceptable" levels of foam loss. However, on [[STS-118]] a piece of foam (and/or ice) about {{cvt|100|mm|order=flip}} in diameter separated from a feedline attachment bracket on the tank, ricocheted off one of the aft struts and struck the underside of the wing, damaging two tiles. The damage was not considered dangerous.

===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.

===Range safety system===

Earlier tanks incorporated a [[Range safety|range safety system]] to disperse tank propellants if necessary. It included a [[Battery (electricity)|battery]] power source, a receiver/decoder, antennas and [[Explosive material|ordnance]]. Starting with [[STS-79]] this system was disabled, and was completely removed for [[STS-88]] and all subsequent flights.