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The Assembly and Production of the External Tank
 
by
Dave R. Hunt
 
Abstract

The External Tank used on the Space Shuttle is assembled and produced by Lockheed Martin at the Michoud Assembly Facility under direction of the Marshall Space Flight Center. The production and assembly of the External Tank is very extensive and precise and it must go through some vigorous testing to ensure a flawless Space Shuttle launch. Since the External Tanks inception, many changes have been made to reduce the weight and improve the strength of the tank. A new Super Lightweight Tank has just been built and will be used in the May, 1998 launch of Space Shuttle mission STS-91. Now that the tank is stronger and lighter, many ideas have been brought up to reuse this piece of space hardware, considering it now burns up in the atmosphere and lands in a remote part of the Indian Ocean. Lockheed Martin is involved in many new innovative programs and the idea of reusing the External Tank must be taken seriously because this could open the door to commercial industries in the development of space and technology.

Table of Contents
 
Abstract
Introduction
History of MAF
History of MSFC
History of ET Development and Testing
Components
Liquid Oxygen Tank
Intertank
Liquid Hydrogen Tank
MAF Production Capabilities
Super Lightweight Tank (SLWT)
Structural Verification Test
Other Products Made by Lockheed Martin
Future Uses
Conclusion
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 Assembly and Production of the External Tank
Introduction

    The External Tank (ET) used on the Space Shuttle is assembled by Lockheed Martin Michoud Assembly Facility (MAF) under direction from Marshall Space Flight Center (MSFC). The ET is assembled by combining three major components: the forward liquid oxygen tank, an unpressurized intertank that houses most of the electrical components, and the aft liquid hydrogen tank. According to Damon (1995), “The 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 together during ascent to orbit” (p. 133). This paper will cover the history of the Lockheed Martin MAF and the MSFC, the components of the ET, the assembly and production capabilities at MAF, the production and testing of the new Super Lightweight Tank (SLWT), other products made by the MAF, and future uses of the ET.

History of MAF

    The MAF is located in New Orleans, Louisiana on 832 acres. It is 24 miles from the New Orleans International Airport and 15 miles from the sounds of Dixieland jazz in the French Quarter. LaNasa (1996) describes why the plant was built and what it was originally used for:

History of MSFC

    “The Marshall Center is one of NASA’s largest centers, occupying 1,800 acres in Huntsville, Alabama. Its primary mission is to lead the Agency to develop and maintain space transportation and propulsion systems and conduct microgravity research” (NASA, 1998, p.1).

History of ET Development and Testing

    It was on January 5, 1972, that President Nixon approved the three-element Space Shuttle consisting of an Orbiter, rocket boosters, and a disposable propellant-tank (Gatland, 1981). On September 1, 1973, NASA announced that it had signed a contract with Martin Marietta Corporation for the design, development, and test of the ET (NASA SpaceLink, 1998). The first ET was assembled at the MAF 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 66,000 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 was heavy 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:

MAF Production Capabilities

    The following is an overview of what the MAF is capable of producing and what tools are used throughout the facility (Ferrari, 1996):

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 state 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 was delivered to NASA on January, 16 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).

Other Products Made by Lockheed Martin

    “Lockheed Martin Michoud Space Systems designs and assembles welded and composite
pressurized tanks for aerospace applications including the Space Shuttle External Tank, the
X-33/VentureStar™ Reusable Launch Vehicle, the A2100 advanced communications satellite and
the Kistler K-1 reusable launch vehicle” (X-33, 1997, p.1). The A2100 satellite will be capable of accommodating many different payloads to carry out a wide variety of missions and is described as follows (Seal, 1996):

Also, the thermal protection material used on the ET is being made available commercially for fire protection. “Today, Lockheed Martin insulating materials are being used for aircraft engine nacelle protection and thrust reverser fire protection, with a variety of other applications in review” (Baty, 1997, p. 1). Another technology Michoud Space Systems is studying is hybrid propulsion.
Future Uses

    The problem with the ET is that they are made for a one-time use. This is good for Lockheed Martin because they will always have to produce a new ET for all future Space Shuttle flights. There is also an opportunity for Lockheed Martin to use these tanks after the Space Shuttle gets into orbit. There have 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 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).
    Even though NASA seems to be against the idea of reusing the ET, many ideas of converting the ET’s safely have been conducted. “Mark Holderman, a NASA engineer at the Johnson Space Center in Houston has conceived of a Commercial/Industrial Process and Applications Platform (CIPAP) called GEODE. The purpose of the GEODE project is to provide an opportunity for the commercial and academic sectors to participate in space activities while maximizing the potential for profit. He stated that GEODE is not a research platform…GEODE is meant to be the vanguard space production platform for commercial manufacturing work” (Ball, DeFilippo, Ritter, Skryd, and Ball, 1997, p. 5).
Conclusion

    The production and use of the ET are very important to space exploration. Currently, the ET burns up in the atmosphere and lands in a remote part of the Indian Ocean. Since the assembly, production, and testing of the ET are so extensive, a new idea for reusing them shouldn’t be overlooked. Lockheed Martin, MSFC, and NASA, along with commercial backing could realistically use these ET’s for research in space. As a manager for either one of these organizations, I would seriously consider what advantage this idea would have compared to the other ideas to explore space at a cost savings.

This all could be done without wasting the valuable resource we now throw away, the ET.  The technology to complete such a project is well within our reach and by using what the experts know the ET could be a huge part in learning what space has to offer us. 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

    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
    Baty, K. F. (1997, June 11). Thermal protection materials, [On-line]. Available: http://www.lmco.com/michoud/thermal.htm
    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
    Ferrari, D. (1996, February). Michoud assembly facility production capabilities, [On-line]. Available: http://www.lmco.com/michoud/facilit.html
    Gatland, K. (1981). The illustrated encyclopedia of space technology. New York, NY: Harmony Books
    Holderman, M. L. (1998). GEODE: Commercial space production facility, [On-line]. Available: http://www.spacefuture.com/archive/geode_commercial_space_production_facility.shtml
    LaNasa, M. (1996, May 17). NASA michoud assembly facility, [On-line]. Available: http://www.lmco.com/michoud/maf_site.html
    LaNasa, M. (1997). Space shuttle external tank. [On-line]. Available: http://www.lmco.com/michoud/etfact1.html
    Mitchell, P. (1997, July 11). Hybrid propulsion demonstration program, [On-line]. Available: http://www.lmco.com/michoud/hybrid.html
    NASA/Marshall space flight center. (1998, March 9). [On-line]. Available: http://www.msfc.nasa.gov/paohome.gif.html
    NASA: Shuttle’s new lighter, stronger external tank completes major pressure tests. (1997, April 1). [On-line]. Available: http://www.elibrary.com/getdoc.cgi?id=87…ydocid=522000@library_e&dtype=0~0&dinst=
    NASA Spacelink. (1994, Sepetember 14). George C. Marshall space flight center, [On-line]. Available: http://www.msfc.nasa.gov/omline/msfc/marshall.overview.html
    NASA Spacelink. (1998, February 13). Early work on the space shuttle, [On-line]. Available: http://www.msfc.nasa.gov/online/msfc/spacelink4.html
    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
    Seal, E. (1996, January 9), Helium tanks for the A2100 satellite, [On-line]. Available: http://www.lmco.com/michoud/helium.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
    X-33 aluminum liquid oxygen tank successfully complets proof test. (1997, November 17). [On-line]. Available: http://www.lmco.com/michoud/x33NR.htm

Appendix A
 
Source: Damon, 1995)
Back to Components

Appendix B

Source: (Dumoulin, 1988)
Back to Liquid Oxygen Tank

Appendix C

Source: (Dumoulin, 1988)
Back to Intertank

Appendix D

Source: (Ball et al, 1997)
Back to Future Uses

Back to the Table of Contents