Thermal Insulation

Lore & Background

The ET was the "backbone" of the shuttle during launch, providing structural support for attachment with the Space Shuttle Solid Rocket Boosters (SRBs) and orbiter. The tank was connected to each SRB at one forward attachment point (using a crossbeam through the intertank) 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 (e.g. Mars), 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 (ACC).

In Their Own Story

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 272 kg (600 lb).

Reader's Guide

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 77,000 pounds (35,000 kg) 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 66,000 pounds (30,000 kg) 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 7,000 pounds or 3,175 kg) 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.

Did You Know?

• The external tank's orange color is the color of the spray-on foam insulation; the first two tanks (STS-1 and STS-2) were painted white, but NASA engineer Farouk Huneidi said the paint did not actually protect the foam.
• The last Standard Weight Tank flew on STS-7 and weighed approximately 77,000 pounds (35,000 kg) inert.
• The Super Lightweight Tank (SLWT) used an aluminium-lithium alloy (Al 2195) that reduced tank weight by about 7,000 pounds (3,175 kg) over the LWT.
• The SLWT provided 50% of the performance increase required for the shuttle to reach the International Space Station.
• The contractor for the external tank was Lockheed Martin (previously Martin Marietta), New Orleans, Louisiana, and the tank was manufactured at the Michoud Assembly Facility.