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.

    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.

     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).

     “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.

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. For each pound of weight reduced from the ET, the cargo-carrying capability of the space shuttle spacecraft is increased almost one pound (Dumoulin, 1988, p. 1).

    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:

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. The tank contains anti-slosh and anti-vortex provisions to minimize liquid residuals and damp fluid motion. The tank feeds into a 17-inch-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-diameter feed lines permits liquid oxygen to flow at approximately 2,787 pounds per second with the Space Shuttle Main Engines (SSMEs) operating at 104 percent or permits a maximum flow of 17, 592 gallons per minute. 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 lighting rod (p. 2).

    “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:

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 (p. 3).

    “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:

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. The tank contains an anti-vortex baffle and siphon outlet to transmit the liquid hydrogen from the tank through a 17-inch line to the aft umbilical. The liquid hydrogen feed line flow rate is 465 pounds per second with the SSMEs at 104 percent or a maximum flow of 47,365 gallons per minute. At the forward end of the liquid hydrogen tank is the ET/orbiter forward attachment pod strut, and its aft end are the two ET/orbiter aft attachment ball fittings as well as the aft SRB-ET stabilizing strut attachments (Dumoulin, 1988, p. 3).

    “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):

This poses two problems: excessive loss of hydrogen and oxygen through vent valves and buildup of excessive pressure in the tanks. Controlled boiling is necessary on the launch platform to keep the tanks pressurized for structural strength and also to assist the pumps in moving the propellants out of the engines. During flight, the tanks are pressurized by gases from the engines. In addition, because of the cold temperatures, if the tank were not insulated, water vapor in the air would readily condense as ice on the sides. At liftoff, the ice would break loose and damage the Shuttle (p. 134).

    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).

    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).

Lockheed Martin Michoud Space System is assembling a super lightweight version of the ET. By substituting Weldalite, an aluminum-lithium alloy developed by Lockheed Martin, and incorporting weight-saving design changes and other efficiences, the SLWT will weigh 7,500 pounds less than the current design and thus improve Shuttle payload capacity by an equal amount. Because the ET has almost reached orbital velocity with the orbiter at main engine cut-off, every pound removed from the ET equals approximately one pound of increased payload capability (Williams, 1997, p. 1).

    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:

The proof test for the liquid oxygen tank was a hydrostatic, or water pressure test. The tank was placed vertically on the test stand at NASA's Michoud Assembly Facility in New Orleans, LA, and filled with water, which has similar density to liquid oxygen. The tests simulated conditions encountered during flights and validated the design changes. The liquid hydrogen tank was pressurized with gaseous nitrogen and subjected to conditions simulating the thrust of the orbiter's main engines and solid rocket boosters. Tests checked the new design by exposing the tank to harsher conditions than it will encounter in flight. After the tests, comprehensive X-ray and dye penetrant inspections will be performed to further verify the tank's flight worthiness. The proof tests completed March 25 were the final in a series of rigorous certification and structural verification tests (Rahn & Malone, 1997, p. 1).

    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):

T - 4 hours 30 minutes: Begin filling liquid oxygen tank.
T - 2 hours 50 minutes: Begin filling liquid hydrogen tank.
T – 2 minutes 55 seconds: Oxygen tank at flight pressure.
T – 1 minute 57 seconds: Hydrogen tank at flight pressure.
T + 8 minutes 50 seconds: External tank separation (p. 143).

When the hydrogen and oxygen in the ET are nearly consumed and the vehicle is just short of velocity, the orbiter’s main engines cut off (MECO). A vent valve at the top of the external tank opens and oxygen escapes through the nose cap. A few seconds later the tank is disconnected, the venting oxygen causing it ti pitch away from the orbiter and start to tumble. Tumbling assures the atmospheric drag will cause it ti break up as it falls back to Earth and lands in the Indian Ocean. There is some uncertainty as to where it will land because of the way it tumbles. The designated impact area is an oval 2,100 miles long by 62 miles wide (Damon, 1995, p. 146).

    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:

Dozens of Station designs have been proposed, and the engineering for each has been worked out is considerable detail. The simplest would be to attach a habitable section at the base of the ET during the ET’s construction. This Aft Cargo Carrier could be outfitted as living quarters for a half dozen astronauts. It would be roughly the size of a 2-story house. The crew would ride up in the shuttle, move to the habitable section once they'd reached orbit and begin converting the ET upper 16 stories into laboratory, production, and living areas. Once 2 or 3 of these single ET’s are in orbit their crews could begin assembling additional ET’s into a ring, or other design. A 12-ET ring would provide living quarters under one-quarter to full gravity conditions. The ring would be one-third mile round with a three deck interior holding living/working quarters and life support systems for up to 400 people (The External Tank, 1997, pp. 1-2).

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

First the leftover hydrogen and oxygen must be vented from their respective tanks. The easiest and simplest way seems to be to use helium gas, from a tank mounted in the intertank section, to purge the tanks. After the tanks are purged, an astronaut would open the access hatch on the bottom of the hydrogen tank (opening the access hatch and astronauts passing through it in spacesuits has already been demonstrated in NASA's neutral buoyancy facilities). A docking adapter/airlock/propulsion unit would then be attached to the access hatch. All our ET outfitting hardware would be passed through the hatch into the ET's interior (all this would be done while the space shuttle is still attached, for support) and then we would seal it up and fill it with a breathable atmosphere. Let's go inside and take off our spacesuits. Once inside we can look down the length of the hydrogen tank. The other end is 96 ft away. Just inside the access hatch a few feet is the tank's main ringframe, which is more or less a circular girder. This main ringframe carries the lower attachments for the shuttle and solid rocket boosters, and the tanks walls are welded to it (and the others). We see this main ringframe protruding into the interior above the tank walls about four inches, all the way around the 27 ft diameter of the tank, and as you look farther into the tank you see that there are other ringframes at intervals (about 20 ft apart) all the way to the other end of the tank. Now, we take a 20-ft-long girder (part of our outfitting hardware) and we attach it so it spans the distance between the main ringframe and the next ringframe into the tank. We then take another girder and attach it to the same two ringframes, 180 degrees from the first girder. Both of these girders can be attached to the ringframes without pentrating the outer wall of the tank. We then take our 27-ft-long, 3-ft-wide flooring material and use it to span the distance between the two girders attached to the ringframes. The flooring attaches to the girders. Once this is done, the length of the hydrogen tank, we now have a floor 27 ft wide and 90 ft long, right down the middle of the tank (which I like to call the Main Deck), with a ceiling 13 ft above the main deck's floor. It's BIG! This whole process could be duplicated and another floor could be installed 8 ft below the Main Deck, and above for that matter, but my preference would be for a 13 ft ceiling. And the oxygen tank can be similarly outfitted. Now how hard would it be to put these basic floors together? Not very. It's just like tinkertoys. A couple of people could do it in a few weeks, especially since they will be working in shirtsleeves and have plenty of room to move around in. And how hard would it be to bolt all your other equipment to this basic deck arrangement. Like the astronaut said, All we have to do is tie our feet down and we can do anything in space. With two decks in an ET, it would have about three times more laboratory floor space than the international space station (Abbott, 1997, pp. 1-2).

    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).

    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).
 

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Source: (Damon, 1995)

 

 

Source: (Dumoulin, 1988)