Space Shuttle external tank
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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. The ET is jettisoned 18 seconds after the SSMEs are shut down, and re-enters the Earth's atmosphere. It breaks up before impact in the Indian Ocean (or Pacific Ocean in the case of direct-insertion launch trajectories, which are currently utilized) away from known shipping lanes. It is not reusable, unlike the Solid Rocket Boosters (SRB).
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[edit] Overview
The ET is the largest element of the space shuttle, and when loaded, it is also the heaviest. It consists 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 is the largest part, but it is relatively light.
The ET is the "backbone" of the shuttle during launch, providing structural support for attachment with the solid rocket boosters and orbiter. The tank is connected to each SRB at one forward attachment point and one aft point, and it is connected 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.
[edit] Evolution of the ET
[edit] Standard Weight Tank
The original ET is informally known as the Standard Weight Tank (SWT). The first two, 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 600 pounds of weight.<ref name="et_paint">National Aeronautics and Space Administration "NASA Takes Delivery of 100th Space Shuttle External Tank." Press Release 99-193. 16 Aug 1999.</ref>
After STS-4, 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 subsequent missions. The total length and diameter of the ET remain unchanged. The last SWT tank, flown on STS-7, weighed approximately 77,000 pounds (35 t) inert.
[edit] Lightweight Tank
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 on the ill-fated STS-107 Space Shuttle Columbia flight. Although the tanks varied slightly in weight, each weighed approximately 66,000 pounds (30 t) 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 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.
[edit] Super Lightweight Tank
The Super Lightweight Tank (SLWT) was first flown in 1998 on STS-91 and has been used since with only two exceptions (STS-99 and STS-107). The SLWT is basically the same design as the LWT except that it uses an Aluminum/Lithium alloy (Al 2195) for a large part of the tank structure. This alloy provides a significant reduction in tank weight (~7000 lb) over the LWT. The disadvantages of the SLWT are its increased cost (~$5M) and production time (~4 months) when compared to the LWT. Although all ETs currently produced are of the SLWT configuration, one LWT remains in inventory and can be used if requested.
[edit] Technical data
SLWT Specifications
- Length: 153.8 ft (46.9 m)
- Diameter: 27.6 ft (8.4 m)
- Empty Weight: 58,500 lb (26,559 kg)
- Gross Liftoff Weight: 1.680 million lb (762,136 kg)
LOX tank
- Length: 54.6 ft (16.6 m)
- Diameter: 27.6 ft (8.4 m)
- Volume (at 22 psig): 19,541.66 cubic feet; 146,181 gallons (553,355 liters)
- LOX mass (at 22 psig): 1,387,457 lb (629,340 kg)
- Operation Pressure: 20-22 psig (138-152 kPa (gauge))
Intertank
- Length: 22.6 ft (6.9 m)
- Diameter: 27.6 ft (8.4 m)
LH2 tank
- Length: 97.0 ft (29.5 m)
- Diameter: 27.6 ft (8.4 m)
- Volume (at 29.3 psig): 52,881.61 cubic feet; 395,582 gallons (1,497,440 liters)
- LH2 mass (at 29.3 psig): 234,265 lb (106,261 kg)
- Operation Pressure: 32-34 psia (221-235 kPa (absolute))
[edit] Contractor
The contractor for the external tank is Lockheed Martin (previously Martin Marietta), New Orleans, Louisiana. The tank is manufactured at the Michoud Assembly Facility, New Orleans, and is transported to Kennedy Space Center by a barge.
[edit] Components
The ET has three primary structures: an LO2 tank, an intertank, and an LH2 tank. Both tanks are constructed of aluminum alloy skins with support or stability frames as required. The intertank aluminum structure utilizes skin stringers with stabilizing frames. The primary aluminum 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
[edit] Liquid oxygen tank
The LO2 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 forward most element of the nose cone functions as a cast aluminum lightning rod. The LO2 tank volume is 19,744 cubic feet at 22 psig and -297 *F (cryogenic).
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).
All loads except aerodynamic loads are transferred from the LO2 tank at a bolted, flange-joint interface with the intertank.
The LO2 tank also includes an internal slosh baffle and a vortex baffle to dampen fluid slosh. The vortex baffle is mounted over the LO2 feed outlet to reduce fluid swirl resulting from slosh and to prevent entrapment of gases in the delivered LO2.
[edit] Intertank
The intertank is the ET structural connection which joins both the LO2 and LH2 tanks. Its primary functions are to receive and distribute all thrust loads from the SRBs and transfer loads between the tanks.
The SRB two 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 LO2 and LH2 tanks and to adjacent intertank skin panels. These adjacent panels are comprised of six stringer-stiffened panels.
The intertank also functions as a protective compartment for housing the operational instrumentation.
[edit] Liquid hydrogen tank
Image:Sts et1.jpg 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 LO2 feedline support loads. Loads from the frames are then distributed through the barrel skin panels. The LH2 tank has a volume of 53,488 cubic feet at 29.3 psig and -423 °F (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 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).
[edit] ET thermal protection system
</div>Image:Sts et.jpg 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 (2.188 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. 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 gas 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; on STS-114, 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 thick, manually sprayed layers of foam, and more likely 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. Two shuttle missions (STS-121 and STS-115) have since been conducted, both with "acceptable" levels of foam loss. The successes of the last two shuttle flights have given NASA confidence in the current ET design.
[edit] 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.
Image:Sts et ecographic.jpg 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 cutoff 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.
The ET has external cameras mounted in the brackets which attached to the shuttle along with transmitters that can continue to send video data long after the shuttle and the ET have separated.
[edit] ET range safety system
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 no longer used. The assembly was completely removed by the time STS-88 flew, and is not present on any tank since then. Subsequently, it is no longer possible to destroy the vehicle during second stage ascent.
[edit] Future use
Like the Solid Rocket Boosters, the ET will be used in the upcoming heavy-lift Ares V (CaLV) and man-rated Ares I (CLV) for the proposed Orion spacecraft that will replace the Shuttle after 2010. Unlike the current ET design, the new Ares V ET-based core stage will have five (as of May 18, 2006) RS-68 rocket engines attached to the bottom and will be 33 feet in diameter to accommodate the extra propellents needed to run the new engines (the core stage is the same diameter of the S-IC and S-II stages of the Saturn V rocket). The SRBs, which will have five segments, will be attached at the sides in the normal configuration, but with the Earth Departure Stage (EDS) and Lunar Surface Access Module (LSAM) located above the core stage, rather than piggybacked like that on the current Shuttle stack.
The Ares I second stage, which will be liquid-fueled, will be similar in internal architecture to the Ares V, but smaller in diameter size, but slightly larger than the CEV's service module, and will not feature any spray-on foam at the interstage assemblies. The Ares I second stage will be powered by a single J-2X rocket engine derived from the J-2 engine used on the S-IVB upper stage used on all manned Apollo flights. Both the Ares I and the Ares V were to use a pre-fired, throw-away version of the Space Shuttle Main Engine (SSME), but due to the complexity of developing an air-startable SSME (for the Ares I), along with the liquid-fueled stages being expendable, NASA decided to use the RS-68 and J-2X engines due to their simplicity, relability, and expense (roughly $20 million USD for each RS-68 or J-2X engine as opposed to $55 million USD for a pre-fired SSME).
[edit] See also
[edit] References
- National Aeronautics and Space Administration. Booster Systems Briefs. Basic, Rev F, PCN 1. April 27, 2005.
- National Aeronautics and Space Administration. Shuttle Systems Design Criteria. Volume I: Shuttle Performance Assessment Databook. NSTS 08209, Volume I, Revision B. March 16, 1999.
[edit] External Links
[edit] Notes
<references/>
| Space Shuttle Program | |
|---|---|
| Image:Shuttle Patch.png | Main Articles: Space Shuttle program | Space Shuttle |
| Components: Orbiter | SRB | External Tank | SSME | OMS | Crawler Transporter | |
| Orbiters: Enterprise | Columbia | Challenger | Discovery | Atlantis | Endeavour | |
| Launch Sites: Kennedy Space Center LC-39 | Vandenberg Air Force Base SLC-6 | |
| Developments: Shuttle-Derived Launch Vehicle | Shuttle-C | Ares I | Ares V | |
| Mockup Shuttles: Pathfinder | Explorer | America | |
| Misc: Missions | Cancelled Missions | Decision | Crews | Abort modes | In Fiction | |
de:Space Shuttle External Tank pl:Zbiornik zewnętrzny promu kosmicznego pt:Tanque externo
