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Component of the Space Shuttle launch vehicle

The Space Shuttle external tank (ET) was the component of the Space Shuttle launch vehicle that contained the liquid hydrogen fuel and liquid oxygen oxidizer. During lift off and ascent it supplied the fuel and oxidizer under pressure to the three RS 25 main engines in the orbiter. The ET was jettisoned just over 10 seconds after main engine cut off (MECO) and it re entered the Earth's atmosphere. Unlike the Solid Rocket Boosters, external tanks were not re used. They broke up before impact in the Indian Ocean (or Pacific Ocean in the case of direct insertion launch trajectories), away from shipping lanes and were not recovered.

Over the years, NASA worked to reduce the weight of the ET to increase overall efficiency. The weight reduced from the ET resulted in an almost equal increase of the cargo carrying capability of the Space Shuttle.

The external tank's orange color is the color of the spray on foam insulation. The first two tanks, used for STS 1 and STS 2, were painted white to protect the tanks from ultraviolet light during the extended time that the shuttle spent on the launch pad prior to launch. NASA engineer Farouk Huneidi told the agency that the paint did not actually protect the foam. Martin Marietta (now part of Lockheed Martin) reduced weight by leaving the rust colored spray on insulation unpainted beginning with STS 3, saving approximately .

The original ET is informally known as the Standard Weight Tank (SWT) and was fabricated from 2219 aluminum alloy, a high strength aluminum copper alloy used for many aerospace applications. After STS 4, several hundred pounds were eliminated by deleting the anti geyser line. This line paralleled the oxygen feed line, providing a circulation path for liquid oxygen. This reduces accumulation of gaseous oxygen in the feed line during prelaunch tanking (loading of the LOX). After propellant loading data from ground tests and the first few Space Shuttle missions were assessed, the anti geyser line was removed for subsequent missions. The total length and diameter of the ET remain unchanged. The last SWT, flown on STS 7, weighed approximately inert.

Beginning with the STS 6 mission, a lightweight ET (LWT), was introduced. This tank was used for the majority of the Shuttle flights, and was last used during the launch of the ill fated STS 107 mission. Although tanks vary slightly in weight, each weighed approximately inert. The weight reduction from the SWT 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 were milled differently so as to reduce thickness, and the weight of the ET's aft solid rocket booster attachments was reduced by using a stronger, yet lighter and less expensive titanium alloy.

The Super Lightweight Tank (SLWT) was first flown in 1998 on STS 91 and was used for all subsequent missions with two exceptions (STS 99 and STS 107). The SLWT had basically the same design as the LWT except that it used an aluminium lithium alloy (Al 2195) for a large part of the tank structure. This alloy provided a significant reduction in tank weight (about ) over the LWT. Manufacture also included friction stir welding technology. Although all ETs produced after the introduction of the SLWT were of this configuration, one LWT remained in inventory to be used if requested until the end of the shuttle era. The SLWT provided 50% of the performance increase required for the shuttle to reach the International Space Station. The reduction in weight allowed the Orbiter to carry more payload to the highly inclined orbit of the ISS.

SLWT specifications). Al 2090 is a commercially available Al Li alloy.

The LOX tank is located at the top 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 at and (cryogenic). The tank feeds into a 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 diameter feed line permits liquid oxygen to flow at approximately with the RS 25s operating at 104% or permits a maximum flow of
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.

The intertank is the ET structural connection between the LOX and LH2 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 LH2 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.

The LH2 tank is the bottom 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 LH2 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 LH2 tank has a volume of at and (cryogenic). The forward and aft domes have the same modified ellipsoidal shape. For the forward dome, mounting provisions are incorporated for the LH2 vent valve, the LH2 pressurization line fitting, and the electrical feed through fitting. The aft dome has a manhole fitting for access to the LH2 feedline screen and a support fitting for the LH2 feedline. The LH2 tank also has a vortex baffle to reduce swirl resulting from slosh and to prevent entrapment of gases in the delivered LH2. The baffle is located at the siphon outlet just above the aft dome of the LH2 tank. This outlet transmits the liquid hydrogen from the tank through a line to the left aft umbilical. The liquid hydrogen feed line flow rate is with the main engines at 104% or a maximum flow of .

The ET thermal protection system consists primarily of spray on foam insulation (SOFI), plus preformed foam pieces and premolded ablator materials. The system also includes the use of 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 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
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 Columbia, 1981: Crew reports white material streaming past windows during orbiter external tank flight. Crew estimated sizes from 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 Challenger, 1983: Bipod ramp loss photographed, dozens of spot losses. STS 27 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 cm in diameter, plus tile damages. STS 50 Columbia, 1992: Bipod ramp loss. 20×10×1 cm tile damage. 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 '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 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. 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 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 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.

The external hardware, ET–orbiter attachment fittings, umbilical fittings, and electrical and range safety system weigh .

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 or the ullage pressure of the liquid oxygen tank reaches
On early flights, the liquid oxygen tank contained a separate, pyrotechnically 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. 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. 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.

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 cavitate (run dry). In addition, 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. Four pressure transducers 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.

Earlier tanks incorporated a range safety system to disperse tank propellants if necessary. It included a battery power source, a receiver/decoder, antennas and ordnance. Starting with STS 79 this system was disabled, and was completely removed for STS 88 and all subsequent flights.

In 1990, it was suggested that the external tank would be used as a lunar habitat or as an orbital station. These proposals did not come to fruition.

With the retirement of the Space Shuttle in 2011, NASA, with its canceled Constellation program, which featured the Orion spacecraft, would also have featured the debut of two Shuttle derived launch vehicles, the human rated Ares I crew launch vehicle and the heavy lift Ares V cargo launch vehicle. While both the Ares I and Ares V would have utilized a modified five segment Solid Rocket Booster for its first stage, the ET would have served as a baseline technology for the first stage of the Ares V and the second stage of the Ares I; as a comparison, the Ares I second stage would have held approximately of LOX, versus the ET holding , more than 5 times that amount. The Ares V first stage, which would have been fitted with five RS 68 rocket engines (the same engine used on the Delta IV rocket), would be 33 feet (10 m) in diameter, as wide as the S IC and S II stages on the Saturn V rocket. It would have utilized the same internal ET configuration (separate LH2 and LOX tanks separated with an intertank structure), but would have been configured to directly accept LH2 and LOX fill and drain, along with LOX venting on a retractable arm like that used on the Shuttle for LH2. The Ares I second stage, on the other hand, would have only used the spray on insulation foam currently used on the current ET. Originally configured like that of the Ares V and the Shuttle ET, NASA, upon completing its design review in 2006, decided, in order to save weight and costs, to reconfigure the internal structure of the second stage by using a combined LH2/LOX tank with the propellants separated by a common bulkhead, a configuration successfully used on the S II and S IVB stages of the Saturn V rocket. Unlike the Ares V, which would have used the same fill/drain/vent configuration used on the Shuttle, the Ares I system would have utilized a traditional fill/drain/vent system used on the Saturn IB and Saturn V rockets, but with quick retracting arms due to the "leap frog" speed the Ares I would expect upon SRB ignition. As originally envisioned, both the Ares I and Ares V would have used a modified "throw away" version of the RS 25 engine, but in due course, because of the need to keep R&D costs down and to maintain a schedule set by NASA Administration Michael D. Griffin to launch the Ares and Orion by 2011, NASA decided (after the 2006 review) to switch to the cheaper RS 68 engine for the Ares V and to an uprated J 2 engine for the Ares I. Because of the switch to the less efficient RS 68, the Ares V was widened from to accommodate the extra propellants, while the Ares I was reconfigured to incorporate a fifth solid rocket segment with the J 2X upper stage, as the new engine has less thrust than the original RS 25. Because of the trade off, NASA would save an estimated USD $35 million by using simplified, higher thrust RS 68 engines (reconfigured to fire and perform like the SSME), while at the same time, eliminate the costly tests needed for an air startable RS 25 for the Ares I.

The DIRECT project, a proposed alternative shuttle derived vehicle, would have used a modified, standard diameter, external tank with three RS 25 engines, with two standard SRBM, as a Crew Launch Vehicle. The same vehicle, with one extra RS 25, and an EDS upper stage, would have served as the Cargo Launch Vehicle. It was planned to save $16 billion, eliminate NASA job losses, and reduce the post shuttle, crewed spaceflight gap from five plus years to two or less.

The Space Launch System (SLS) is a US super heavy lift expendable launch vehicle, which first flew on Artemis 1 in November 2022. The core stage of the rocket is in diameter and mount a Main Propulsion System (MPS) incorporating four RS 25 engines. The core stage is structurally similar to the Space Shuttle external tank, and initial flights will use modified RS 25D engines left over from the Space Shuttle program. Later flights will switch to a cheaper version of the engine not intended for reuse.

MPTA ET is on display with the Space Shuttle Pathfinder at the U. S. Space & Rocket Center in Huntsville, Alabama. ET 94 (older version LWT) is in Los Angeles and in 2019 was scheduled to be displayed with Space Shuttle Endeavour at the California Science Center when the Samuel Oschin Air and Space Center opened. On January 15, 2024 it was announced in a press release that ET 94, 2 solid rocket boosters, and the Space Shuttle Endeavour had been linked together and expected them to be moved by the end of the month to their new exhibit location. Three other external tanks were in preparation when the manufacturing stopped. ET 139 is at advanced stage of manufacturing; ET 140 and ET 141 are in early stages of manufacturing.