Space Shuttle external tank

From Wikipedia, the free encyclopedia.

Jump to: navigation, search
The Space Shuttle External Tank (ET) on its way to the Vehicle Assembly Building. (Image Credit: NASA/KSC)
Enlarge
The Space Shuttle External Tank (ET) on its way to the Vehicle Assembly Building. (Image Credit: NASA/KSC)

The Space Shuttle External Tank (ET) contains the liquid hydrogen fuel and liquid oxygen oxidizer and supplies them under pressure to the three space shuttle main engines (SSME) in the orbiter during lift-off and ascent. When the SSMEs are shut down, the ET is jettisoned, enters the Earth's atmosphere, breaks up, and hits the ocean somewhere far from land, usually in the Indian Ocean or Pacific Ocean away from known shipping lanes. It is not recovered. The ET is constructed by Lockheed Martin for NASA at the Michoud Assembly Facility.

Contents

Overview

STS-1 at liftoff.  The External Tank was painted white for the first two Space Shuttle launches.  From STS-3 on, it was left unpainted.
Enlarge
STS-1 at liftoff. The External Tank was painted white for the first two Space Shuttle launches. From STS-3 on, it was left unpainted.

The ET is the largest element of the space shuttle, and when loaded, it is also the heaviest. It is 153.8 feet (47 m) long and has a diameter of 27.6 feet (8.4 m) and has three major components:

  • the forward liquid oxygen tank
  • an unpressurized intertank that contains most of the electrical components
  • the aft liquid hydrogen tank; this is the largest part, but it is relatively light.

The first two external tanks, used in STS-1 and STS-2, were painted white. As a weight-saving measure, Lockheed Martin ceased painting the external tanks beginning with STS-3, leaving only the clear primer over the now-trademark rust-colored insulation, saving approximately 1,000 pounds (0.5 t) of weight.

Beginning with the STS-6 mission, a lightweight ET (called the LWT by NASA), was introduced. This tank was used for the majority of the Shuttle flights, and was last used on the ill-fated STS-107 flight. Since 1998, NASA started using, for International Space Station flights, a so-called "Super Lightweight Tank" (SWLT), which reduces the weight even further, using new construction techniques. Although future tanks may vary slightly, each will weigh approximately 66,000 pounds (30 t) inert. The last heavyweight tank, flown on STS-7, weighed approximately 77,000 pounds (35 t) inert. For each unit of weight reduced from the ET, the cargo-carrying capability of the space shuttle is increased almost the equivalent amount. The weight reduction 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 are milled differently to reduce thickness, and the weight of the ET's aft solid rocket booster attachments were reduced by using a stronger, yet lighter and less expensive titanium alloy. Earlier several hundred pounds were eliminated by deleting the anti-geyser line. The line paralleled the oxygen feed line and provided a circulation path for liquid oxygen to reduce accumulation of gaseous oxygen in the feed line while the oxygen tank was being filled before launch. After propellant loading data from ground tests and the first few space shuttle missions was assessed, the anti-geyser line was removed for STS-5 and subsequent missions. The total length and diameter of the ET remain unchanged.

The ET is attached to the orbiter at one forward attachment point and two aft points. In the aft attachment area, there are also umbilicals that carry 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 also are routed through those umbilicals.

Components

Diagram of Space Shuttle External Tank showing internal structure
Enlarge
Diagram of Space Shuttle External Tank showing internal structure

Liquid oxygen tank

The liquid oxygen tank is an aluminum monocoque structure composed of a fusion-welded assembly of preformed, chem-milled gores, panels, machined fittings and ring chords. It operates in a pressure range of 20 to 22 psig (240 to 250 kPa absolute pressure). The tank contains anti-slosh and anti-vortex provisions to minimize liquid residuals and damp fluid motion. The tank feeds into a 17 inch (430 mm) 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 17 inch (430 mm) diameter feed line permits liquid oxygen to flow at approximately 2,787 lb/s (1264 kg/s) with the SSMEs operating at 104 % or permits a maximum flow of 17,592 gal/min (1.1099 m³/s). The liquid oxygen tank's double-wedge nose cone reduces drag and heating, contains the vehicle's ascent air data system (for nine tanks only) and serves as a lightning rod. The liquid oxygen tank's volume is 554.0 m³ (19,560 ft³). It is 27 feet 7 inches (8.41 m) in diameter, 49 feet 4 inches (15.0 m) long and weighs 12,000 pounds (5.4 t) empty.

Intertank

The intertank is a steel / aluminum semimonocoque cylindrical structure with flanges on each end for joining the liquid oxygen and liquid hydrogen tanks. The intertank houses ET instrumentation components and provides an umbilical plate that interfaces with the ground facility arm for purge gas supply, hazardous gas detection and hydrogen gas boiloff during ground operations. It consists of mechanically joined skin, stringers and machined panels of aluminum alloy. The intertank is vented during flight. The intertank contains the forward SRB-ET attach thrust beam and fittings that distribute the SRB loads to the liquid oxygen and liquid hydrogen tanks. The intertank is 270 inches (6.9 m) long, 331 inches (8.4 m) in diameter and weighs 12,100 pounds (5.5 t).

Liquid hydrogen tank

The liquid hydrogen tank is an aluminum semimonocoque structure of fusion-welded barrel sections, five major ring frames, and forward and aft ellipsoidal domes. Its operating pressure range is 32 to 34 psia (220 to 230 kPa absolute). The tank contains an anti-vortex baffle and siphon outlet to transmit the liquid hydrogen from the tank through a 17 inch (430 mm) line to the left aft umbilical. The liquid hydrogen feed line flow rate is 465 lb/s (211 kg/s) with the SSMEs at 104 % or a maximum flow of 47,365 US gal/min (2.988 m³/s). At the forward end of the liquid hydrogen tank is the ET / orbiter forward attachment pod strut, and at its aft end are the two ET / orbiter aft attachment ball fittings as well as the aft SRB-ET stabilizing strut attachments. The liquid hydrogen tank is 331 inches (8.4 m) in diameter, 1,160 inches (29.46 m) long, and has a volume of 53,518 ft³ (1,515.5 m³) and a dry weight of 29,000 pounds (13 t).

ET thermal protection system

The ET thermal protection system consists of sprayed-on foam insulation 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-exposed metallic attachments and to reduce heat flow into the liquid hydrogen. The thermal protection system weighs 4,823 pounds (2.2 t).

The thermal protection system has been problematic, and has proven a fatal weakness to shuttle mission safety. NASA has had difficulty preventing fragments of foam from detaching during flight. Prior to 1997, the foam insulation was made using freon, a chemical known for its destructive effect on the ozone layer. Although NASA was exempted from legislation that called for a reduction in freon use, and the amount of freon used in the tank contributed minimally to total freon use, the composition of the foam was changed in response. The new foam has proven to be much more likely to peel or flake off, causing over a tenfold increase in impacts with shuttle tiles over the old foam. Additionally, ice often forms on the outside of the tank after it has been fueled, which also poses a hazard to the shuttle during flight. During the lift-off of STS-107, a piece of foam insulation detached from the tank and struck the leading edge of Space Shuttle Columbia's wing at a very high velocity. The impact destroyed several reinforced carbon-carbon thermal tiles on the leading edge of the wing, which allowed super-heated plasma to enter the wing superstructure several days later during re-entry. This resulted in the destruction of Columbia and the death of her crew. As of 2005 the problem of foam shed has not been fully cured; cameras mounted on the shuttle recorded a piece of foam blowing away from the ET on STS-114. This foam did not strike the spacecraft.

While 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, NASA has elected to postpone all further launches until the problem is understood and resolved.

ET hardware

The external hardware, ET / orbiter attachment fittings, umbilical fittings, electrical and range safety system weigh 9,100 pounds (4.1 t).

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 38 psig (360 kPa absolute) or the ullage pressure of the liquid oxygen tank reaches 25 psig (270 kPa absolute).

The liquid oxygen tank contains a separate, pyrotechnically operated, propulsive tumble vent valve at its forward end. At separation, the liquid oxygen tumble vent valve is opened, providing impulse to assist in the separation maneuver and more positive control of the entry aerodynamics of the ET.

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 SSME 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, 1,100 pounds (500 kg) of liquid hydrogen are loaded over and above that required by the 6-1 oxidizer / fuel engine mixture ratio. This assures that MECO from the depletion sensors is fuel-rich; oxidizer-rich engine shutdowns can cause burning and severe erosion of engine components.

Four pressure transducers located at the top of the liquid oxygen and liquid hydrogen tanks monitor the ullage pressures.

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.

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.

A swing-arm-mounted cap to 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 on the ET, thus protecting the orbiter's thermal protection system during launch.

ET range safety system

Prior to the mid-1990's, a range safety system provided for dispersing tank propellants if necessary. It included a battery power source, a receiver/decoder, antennas and ordnance. This was removed from later-production lightweight tanks and is not a production feature in the current super lightweight tanks.

Various parameters are monitored and displayed on the flight deck display and control panel and are transmitted to the ground.

The contractor for the external tank is Lockheed Martin (previously Martin Marietta), New Orleans, Louisiana. The tank is manufactured at Michoud, La. Motorola, Inc., Scottsdale, Arizona, was the contractor for range safety receivers.

See also

Personal tools
In other languages