Space Shuttle external tank

The Space Shuttle external tank 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 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, away from shipping lanes and were not recovered.

Overview

The ET was the largest element of the Space Shuttle, and when loaded, it was also the heaviest. It consisted of three major components:

the forward liquid oxygen tank
an unpressurized intertank that contains most of the electrical components
the aft liquid hydrogen tank; this was the largest part, but it was relatively light, due to liquid hydrogen's very low density.

The ET was the "backbone" of the shuttle during launch, providing structural support for attachment with the Space Shuttle Solid Rocket Boosters and orbiter. The tank was connected to each SRB at one forward attachment point and one aft bracket, and it was connected to the orbiter at one forward attachment bipod and two aft bipods. In the aft attachment area, there were also umbilicals that carried fluids, gases, electrical signals and electrical power between the tank and the orbiter. Electrical signals and controls between the orbiter and the two solid rocket boosters were also routed through those umbilicals.
Although the external tanks were always discarded, it may have been possible to re-use them in orbit. Plans for re-use ranged from incorporation into a space station as extra living or research space, as rocket fuel tanks for interplanetary missions, to raw materials for orbiting factories.
Another concept was to use the ET as a cargo carrier for bulky payloads. One proposal was for the primary mirror of a 7-meter aperture telescope to be carried with the tank. Another concept was the Aft Cargo Carrier.

Versions

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.

Orange color

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 reduced weight by leaving the rust-colored spray-on insulation unpainted beginning with STS-3, saving approximately.

Standard Weight Tank

The original ET is informally known as the Standard Weight Tank 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. 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.

Lightweight Tank

Beginning with the STS-6 mission, a lightweight ET, 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, 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.

Super Lightweight Tank

The Super Lightweight Tank was first flown in 1998 on STS-91 and was used for all subsequent missions with two exceptions. The SLWT had basically the same design as the LWT except that it used an aluminium-lithium alloy for a large part of the tank structure. This alloy provided a significant reduction in tank weight 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.
Image:Pegasus barge being moved by Freedom Star and towboat American 2.jpg|right|thumb|300px|The Pegasus barge carrying ET-119 is towed to Port Canaveral.

Technical specifications

SLWT specifications

Length:
Diameter:
Empty weight:
Gross liftoff weight:

LOX tank

Length:
Diameter:
Volume :
LOX mass :
Operation pressure:

Intertank

Length:
Diameter:

LH₂ tank

Length:
Diameter:
Volume :
LH₂ mass :
Operation pressure:
Operation temperature:
Contractor

The contractor for the external tank was Lockheed Martin, New Orleans, Louisiana. The tank was manufactured at the Michoud Assembly Facility, New Orleans, and was transported to Kennedy Space Center by barge.

Components

The ET has three primary structures: an LOX tank, an intertank, and an LH₂ 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. AL 2195 is an Al-Li alloy designed by Lockheed Martin and Reynolds for storage of cryogenics. 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 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 .
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.

Intertank

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.

Liquid hydrogen tank

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 .
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.