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The Only Throwaway Component on the Space Shuttle: The External Fuel Tank
 
Dave R. Hunt
 
Abstract

The Space Shuttle has three major components: the orbiter, the solid rocket boosters, and the external fuel tank. The external fuel tank is the only piece of hardware that is not reused and burns up in the atmosphere and falls into the ocean after it is jettisoned from the orbiter. The external tank is made up of a liquid oxygen tank, a liquid hydrogen tank, and an intertank. These components have been improved since the inception of the Space Shuttle and now the materials are lighter weight and stronger. There have also been ideas of placing the external tank into orbit and using them as a space station for research, living quarters, and even entertainment. The challenge of converting these external tanks has caused some problems and this idea is still only perceived in theory.

Table of Contents
 
Abstract
Introduction - The Only Throwaway Component on the Space Shuttle: The External Fuel Tank
History
Components
Liquid Oxygen Tank
Intertank
Liquid Hydrogen Tank
Thermal Protection System
ET Hardware
Super Lightweight Tank (SLWT)
Structural Verification Test
Operation
Future Uses
Converting the ET
Problems With Placing an ET in Orbit
Conclusions
References
Appendix A – Picture of all Components
Appendix B – Picture of Liquid Oxygen Structure
Appendix C – Picture of Intertank Structure
Appendix D – Picture of Space Station

The Only Throwaway Component on the Space Shuttle: The External Fuel Tank
Introduction

    The external fuel tank on the Space Shuttle is the largest single integral part of the Shuttle system. According to Damon (1995), “The external tank (ET) serves two purposes: it carries the propellants for the orbiter’s three main rocket engines and it is the support structure that connects the orbiter and solid rocket boosters (SRBs) together during ascent to orbit” (p. 133). This paper will be an in-depth analysis of the history and development of the components that make up the ET and also the new structure that is being designed to decrease its weight. There have also been ideas of placing the ET into an orbit and using them for space or refueling stations. These issues and more will be discussed in detail in the following report.

History

     The concept of a reusable spacecraft is nearly as old as that of flying machines itself. As the development of rocket propulsion systems increased, the idea of entering space became a reality. With this technology, the United States begun its research to construct a vehicle that could be reused over and over again. On January 5, 1972, President Nixon approved the three-element Space Shuttle consisting of an Orbiter, rocket boosters, and a disposable propellant-tank (Gatland, 1981). This was the answer that has worked since the first test flight on April 12, 1981 (Damon, 1995).
 In 1975, the prime contractor for the ET was Martin Marietta Aerospace. The first ET was assembled at the Michoud Assembly Facility (MAF) in New Orleans, Louisiana in 1976. In July 1977, the fabrications for the first flight ET began. The intertank structural test program was completed in November 1977, and the first ET tanking test was conducted in December 1977. After all the testing was completed, the first flight ET (External Tank-1) was delivered to Kennedy Space Center in July 1979 (The External Tank, 1997).

Components

     “The ET has three major components: the forward liquid oxygen tank, an unpressurized intertank that contains most of the electrical components, and the aft liquid hydrogen tank” (Dumoulin, 1988, p. 1). See Appendix A for picture of all the components. It is 154 feet long and 27.6 feet in diameter and carries more than 535,000 gallons of cryogenic propellants that are fed to the orbiter’s three main engines (LaNasa, 1997). “Prior to propellant loading, the ET weighs approximately 66,0000 pounds. But once liquid oxygen and liquid hydrogen are loaded into the vehicle beginning eight hours prior to Shuttle launch, the ET weighs 1.65 million pounds” (LaNasa, 1997, p. 1). The first five ET’s weighed approximately 77,000 pounds inert, which made it a heavyweight tank compared to the 66,000-pound lightweight tank.

  Liquid Oxygen Tank

    The upper tank carries 1.36 million pounds of liquid oxygen at minus 297 degrees Fahrenheit (F) at liftoff (Damon, 1995). It is 331 inches in diameter, 592 inches long, and weighs 12,000 pounds empty with a volume of 19,563 cubic feet (143,000 gallons). See Appendix B for a picture of the liquid oxygen tank. Dumoulin (1988) describes its construction as follows:

Intertank

    “An intertank collar connects the two propellant tanks together and provides space for most of the electrical components” (Damon, 1995, p. 134). The intertank is 270 inches long, 331 inches in diameter, and weighs 12,100 pounds. See Appendix C for a picture of the intertank. Dumoulin (1988) better describes the configuration of the intertank as follows:

Liquid Hydrogen Tank

    “The lower tank is about 2.5 times larger (383,000 gallons) and carries about a quarter of a million pounds of liquid hydrogen at minus 423 degrees F” (Damon, 1995, p. 133). It is 331 inches in diameter, 1,160 inches long, and 53,518 cubic feet of volume and weighs 29,000 pounds empty. The liquid hydrogen tank’s composition is specified below:

 Thermal Protection System

    “The entire outer surface of the external tank is insulated with a half inch thick cork/epoxy layer covered with 1 to 2 inches of spray-on foam” (Damon, 1995, p. 134). “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” (Dumoulin, 1988, p. 4) The two reasons protection is essential are because both propellants are very cold and they boil at very low temperatures. The following are problems that could happen if there was no insulation (Damon, 1995):

 ET Hardware

    The external hardware, ET / orbiter attachment fittings, umbilical fittings, electrical, and range
safety system weigh 9,100 pounds. 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 or the ullage pressure of the liquid oxygen tank reaches 25 psig. 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 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 (Dumoulin, 1988).
    The range safety system provides for dispersing tank propellants if necessary. It includes a battery power source, a receiver/decoder, antennas, and ordnance. Various parameters are monitored and displayed on the flight deck and control panel. These parameters are then transmitted to the ground (Dumoulin, 1988).

Super Lightweight Tank (SLWT)

    The first weight reduction of 10,000 pounds in April 1983 resulted in increased payload. Now a new design will weigh another 7,500 pounds less. This lighter weight will allow the Space Shuttle to carry heavier cargo into orbit, which is a key element in building the international space station (Cabbage, 1995).

The formulation for the aluminum-lithium, A1 2195, is one percent lithium, four percent copper, 0.4 percent silver, 0.4 percent magnesium, with the remainder being aluminum (Williams, 1997). “This alloy is weldable, 30 percent stronger and five percent less dense than the A1 2219 alloy previously used in the ET. The new alloy also provides higher fracture toughness at cryogenic temperatures, as low as minus 423 degrees F, versus room temperature fracture toughness” (Williams, 1997, p. 1). Besides the new material, the tank’s structure design has improved. “The walls of the redesigned hydrogen tank are manufactured in an orthogonal waffle-like pattern, providing more strength and stability than the previous design” (NASA, 1997, p. 1). “Manufactures at NASA’s MAF will also try to keep the tank’s weight down with a new, more precise way of applying the insulating foam coating to the exterior” (Cabbage, 1995, p. 1). With the use of the new alloy, about 2.5 million dollars will be added to the ET’s 50 million dollar cost now (Cabbage, 1995).

Structural Verification Test

    Parker Counts, manager of the External Tank Project at the Marshall Space Flight Center said, “The new external tank has passed one of the most innovative structural verification test programs ever designed, culminating with these proof tests” (Rahn & Malone, 1997, p. 1). The following is a description of the sate of the art test technology:

In October 1997, a notable production milestone happened when the mating of the major components was completed. “The SLWT, designated ET-96, is currently in Final Assembly at MAF for completion of mechanical, electrical, and thermal protection system installations, and final acceptance tests. The tank is on schedule for delivery to NASA in January 1998 in support of the May 1998 launch of Space Shuttle mission STS-91, the final scheduled Shuttle/Mir docking mission concluding the joint U.S./Russian Phase 1 Program” (Nead, 1997, p.1).

Operation

    The following is a schedule of events that happen to the ET during a Space Shuttle launch. The time is displayed first, followed by the event (Damon, 1995):

Basically the ET supplies the liquid hydrogen fuel and liquid oxygen oxidizer to the SSME’s during liftoff and ascent. During the first part of the flight, the emphasis is gaining altitude. After getting through the dense part of the atmosphere, the orbiter increases horizontal velocity to reach orbital speed (Damon, 1995). The following is a description of when the ET’s propellants are depleted and what happens to the ET once it leaves the orbiter: Mark Prado describes what happens to the ET and why it cannot be reused. “When more than 97 percent of orbital speed is attained, the ET is detached from the Shuttle Orbiter and directed to cross Earth’s atmosphere to burn up Skylab-like with remnants falling into a remote section of the Indian Ocean. The ET cannot be returned to Earth for reuse on later launches because it cannot be returned without burning up in Earth’s atmosphere, unlike the Boosters which detach themselves early before high speeds are attained. Currently, the ET is just thrown away” (Prado, 1997, p. 1).
 
Future Uses

    There has been many ideas brought forward to use the ET as a space station or even a refueling station. “Martin Marietta has proposed modifying one tank to serve as a pressure vessel to house a gamma ray imaging telescope. Another possible use which has been proposed is as an orbital fuel storage facility to support on-orbit operations” (Bridwell, 1997, p. 2). “Some planners envision them clustered together as a space station, fitted with rockets and launched to the Moon for a lunar colony, or refitted a little at a time and used as orbiting gas stations for vehicles heading to the outer reaches of the Solar System” (Damon, 1995, p. 146). See Appendix D for a picture of the Space Station concept. The following is a simple idea for a space station:

“This has been dubbed a wet launch of a habitat. It solves the most of the problems and expense of needing lots of robotic or human extravehicular activity in space to outfit the tank with its desired contents. However, since NASA has said that any use of the ET can’t have any effect on launch performance, and this design has a more massive tank with a resultant loss in payload capacity, it doesn’t look as if NASA will accept this. NASA doesn’t like any redesign of the manned Shuttle system due to the potentially lowering the safety to the crew by any mistakes due to redesign, e.g., structural dynamics” (Prado, 1997, pp. 3-4).
    Gene Meyers, an industrial engineer and entrepreneur, is promoting a privately funded plan to use the ET’s via the Space Island Project. His envision is also a giant ring formation that would allow for the production of gravity sensitive materials on a large scale. Other uses would be increased ease of cell farming for medical research, repairing and constructing satellites in space, and eventually for entertainment (Ball, DeFilippo, Ritter, Skryd, and Ball, 1997).

Converting the ET

    The following is a portrayal from Tom Abbott, an ET enthusiast, of how an ET would be outfitted in orbit:

Problems With Placing an ET in Orbit

    The technical challenges of placing an ET into orbit include the circularization and maintenance of orbit, the cleanup and evacuation of residual liquid oxygen and liquid hydrogen, and dealing with the foam insulation (Fitch, 1997). There have been many different ideas of how to solve these problems and they will be discussed further in the following paragraphs.
    The first problem is to get the ET in a circular orbit and keep it there. “If left in a very low Earth orbit, the tanks would have to be periodically boosted to higher altitude to keep them from becoming a hazard to traffic and from eventually burning in. A costly alternative is to strap on rockets and boot them to a higher stable parking orbit” (Damon, 1995, p. 146). With that, some form of attitude jets would need to be attached, plus a way to remotely control them. Tom Abbott said, “In all on-orbit ET space station conversion proposals, a propulsion system is installed just as soon as is practical and the ET remains attached to the Space Shuttle until the is accomplished. Positive control of the ET at all times is the only acceptable way to operate” (Fitch, 1997, p. 1).
    When the Space Shuttle jettisons the ET, there are from 5 to 20 tons of residual fuels remaining in the tank, and something has to be done with them (Fitch, 1997). Tom Abbott said, “According to a study undertaken at the direction of the ET Project Office, there are three ways to accomplish this: (a) through the Orbiter’s fill and drain valves, (b) through the Orbiter’s engines, and (c) through the ET vent and relief valves. The first method is recommended. The second method has the disadvantage that the vented hydrogen could affect the engine unfavorably, and the third method requires modifications to the ET” (Fitch, 1997, p. 3).
    There is concern that the Spray-On-Foam Insulation (SOFI) could erode in orbit and cause annoying and potentially dangerous debris (Fitch, 1997).  Tom Abbott suggests his solution. “After the ET reaches orbit, it can be held in a 170 mile high orbit while the SOFI is scrapped off. One study predicts it would take less than a week to strip the SOFI, and debris would deorbit in from hours to a couple of days, depending on the size of the piece. Another is to leave the ET in a 160 mile orbit for about a month and all the SOFI would oxidize off of it” (Fitch, 1997, p. 4).

Conclusions

    The ET is an important piece of equipment and without it, the Space Shuttle would never make it into space. There have been many improvements since the first ET was constructed, resulting in more payload for the Shuttle to carry into space. These lighter weight designs have even proved to be stronger and they provide more stability. As technology expands, the reality of placing an ET into orbit and using them as a space station could possibly happen. I feel this will happen in the near future, basically because of the efforts by Gene Meyers and his Space Island Project. It is best said by Bridwell (1997), “The ET is a proven, reliable piece of hardware. The recently completed reassessment has only reinforced my conviction that the tank will provide reliable service for many years to come and will be the basis for many innovative adaptations” (p. 2).
 

References

    Abbott, T. (1997). Outfitting an external tank in orbit. [On-line]. Available: http://www.vswap.com/fitch/text/et_tom.htm
    Ball, N., DeFilippo, R., Ritter, M., Skryd, K., and Ball, J. (1997). Space manufacturing and processing. [On-line]. Available: http://cher.eda.doc.gov/oasc/spcmfg.html
    Bridwell, P. (1997). External tank. [On-line]. Available: http://spacelink.nasa.gov/NASA.Projects/….to.Flight/External.Tank-Porter.Bridwell
    Cabbage, M. (1995, December 24). NASA working to decrease weight of shuttle fuel tanks. Gannett news service, p. 1
    Damon, T.D. (1995). Introduction to space. Malabar, FL: Krieger Publishing Company
    Dumoulin, J. (1988). External tank. [On-line]. Available: http://www.ksc.nasa.gov/shuttle/technology/sts-newsref/et.html
    Fitch, C.A. (1997). External tanks in orbit. [On-line]. Available: http://www.vswap.com/fitch/text/et_orb3.htm
    Gatland, K. (1981). The illustrated encyclopedia of space technology. New York, NY: Harmony Books
    LaNasa, M. (1997). Space shuttle external tank. [On-line]. Available: http://www.lmco.com/michoud/etfact1.html
    NASA: Shuttle’s new lighter, stronger external tank completes major pressure tests. [On-line]. Available: http://www.elibrary.com/getdoc.cgi?id=87…ydocid=522000@library_e&dtype=0~0&dinst=
    Nead, A. (1997, November 12). First super lightweight tank achieves major production milestone. [On-line]. Available: http://www.lmco.com/michoud/Slight.htm
    Prado, M. (1997). Shuttles throwaway external tank. [On-line]. Available: http://www.permanent.com/ext-tank.htm
    Rahn, D. & Malone, J. (1997). Shuttle’s new lighter, stronger external tank completes major pressure tests. [On-line]. Available: http://nexus.nasa.gov/Now/News/PAOArchive\97-058.html
    The external tank. (1997). [On-line]. Available: http://www.primenet.com/multimedia/space/rings.htm
    Williams, N.P. (1997). Space shuttle super lightweight tank. [On-line]. Available: http://www.lmco.com/michoud/slwtank.html
 

Appendix A

 

 
 
Back to Components

Source: (Damon, 1995)

Appendix B
 
 
 
 Back to Liquid Oxygen Tank
 
Source: (Dumoulin, 1988)
Appendix C
 
 
 
 
 Back to Intertank

Source: (Dumoulin, 1988)

Appendix D
 
 
 
 
 Back to Future Uses
 
Source: (Ball et al., 1997)
 
Back to Table of Contents
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