Space Studies Institute Report on Space Shuttle External Tank Applications By J. Alex Gimarc December 1, 1985
I. INTRODUCTION................................................... 1-1 Why We Need the External Tank................................ 1-1 Current Interest in the External Tank........................ 1-2 History...................................................... 1-3 ET Applications.............................................. 1-3 Costs and Ramifications.................................... 1-3 Problems and Solutions .................................. 1-4 Conclusions.................................................. 1-5 II. TANK INFORMATION............................................... II-l ET Description................................................ II-l Current ET Operations ........................................ II-l ET On Orbit - Problems and Solutions........................... II-3 Aft Cargo Carrier............................................ II-9 Additional ET Enhancements.................................... 11-19 III. ET TIN CAN USES.............................................. III-l Storage Containers............................................ III-l Cryogenic Storage.......................................... III-l Gas and Water Storage...................................... III-3 Salvage Container.......................................... III-5 Storage of Orbital Assets.................................. HI-5 SS Applications.............................................. III-6 Habitations - ACC, L02 Tank and LH2 Tank..................HI-6 LH2 Tank Hangar or Servicing Platform........................ HI-19 Corporate Space Stations/Platforms ........................ III-20 Miscellaneous Applications.......................................III-29 Inflatables...................................................HI-29 Landing and Reentry Modules...................................III-29 Spacecraft ................................................ III-32 Wake Shield...................................................III-32 IV. ET AS PROPELLANT RESOURCE..................................... IV-1 Scavenging of Residual Cryogenic Propellant .................. IV-1 Reaction Mass................................................ IV-9 Aluminum Fueled Rocket ................................... IV-9 Mass Driver Fuel.......................................... IV-11 Reaction Engines .......................................... IV-11 V. STRUCTURES..................................................... V-l Disassembly of ET on Orbit.................................... V-l SOFI...................................................... V-l Major Pieces.............................................. V-6 Minor Pieces.............................................. V-6 Large Rigid Constructs........................................ V-6 Cutting the ET................................................ V-7 Melting Facilities ........................................ V-16 Factory Machinery.......................................... V-20 Strongback.................................................... V-22
VI. TETHERS....................................................... VI-1 Introduction.................................................. VI-1 Tether History................................................ VI-2 Tether Physics................................................ VI-2 Gravity - Actual and Artificial............................ VI-2 Electromagnetic Effects.................................... VI-9 Shuttle Mission Enhancements.................................. VI-11 Orbit Raising and Lowering................................ VI-11 Shuttle to Station Advantages.............................. VI-13 Space Station Mission Enhancements............................ VI-15 Liquid Storage ............................................ VI-15 Tank Storage.............................................. VI-15 Space Station Architecture ................................ VI-21 Miscellaneous ................................................ VI-21 VII. MISCELLANEOUS................................................. VII-1 Observational Science ........................................ VII-1 Large Deployable Reflector ................................ VII-1 High Energy Observations .................................. VII-5 Low Energy Observations.................................... VII-5 Additional Observations.................................... VII-7 Biology and Life Sciences.................................... VII-7 Waste Management.......................................... VII-7 Biology.................................................... VII-7 Life Support.............................................. VII-9 Military Applications ........................................ VII-9 Military Space Station .................................... VII-13 Miscellaneous.............................................. VII-15 Orbital Cleanup .............................................. VII-15 Future Resource .............................................. VII-15 VIII. CONCLUSIONS AND RECOMMENDATIONS ............................... VIII-1 IX. REFERENCES..................................................... IX-1 APPENDIX I: CONTACTS
ET Project - Executive Summary This report will review possible applications of the External Tank (ET) of the Space Transportation System (STS) in orbit. Enhancements of the space program through ET utilization in orbit will be covered in depth. Problems will be reviewed. Recommendations will be made. This report is intended to make a coherent case for the use of the External Tank in the American space program. I. Tank Introduction The ET is carried almost to orbit with the orbiter and jettisoned with approximately 98% of the energy necessary to insert it in orbit. When jettisoned, each ET carries internally an average of 15,000 pounds of residual cryogenic fuels. These residuals are available for scavenging from the tank in a variety of scenarios. The availability of cryogenics already in orbit can potentially fuel planned OTV operations at a cost far lower than if the cryogenics are carried aloft in a tanker version of the orbiter. The ET mass is over 69,000 pounds. Of this mass, there are approximately 53,000 pounds of aerospace grade aluminium. This aluminium can be cut, melted, powdered, welded, and manipulated to suit any number of present and future structural needs. If the tanks are partially disassembled in orbit, the pieces can also be reassembled in the construction of large structures. The oxygen and hydrogen tanks making up the ET provide two factory tested pressure vessels that are two to five times larger in volume than any space station yet flown or planned for the future. These large volumes are clean and able to be entered through inspection manholes in the respective tank domes. The on-orbit adaptation of the respective tank interiors for habitation, storage, or maintenence facilities will require minimal time and effort. There are two major problems with the use of the ET in orbit. The first and most critical is orbital maintenance. This is a result of the desire not to randomly drop large bodies on the surface of the earth from space. At typical STS orbits (160 - 220 nautical miles), the orbital lifetime of a tank inserted into a parking orbit can be measured in days to months. A plan
to use the ET in orbit must address this problem. A quick solution would be to install small thrusters that use the boiloff of residual cryogenics to insert the ET in a very long lived (200 - 500 nm higher) orbit. Other solutions to this problem are possible and vary depending on the planned on-orbit use of the ET. They are also not particularly expensive. The second problem is possible contamination due to outgassing of the Spray-On Foam Insulation (SOFI). This may prove to be a pollution problem for a small number of proposed space based operations. However, it will require further study. There are several relatively inexpensive enhancements to the ET that can be purchased that will enhance STS operations. The most important of these is the AFT Cargo Carrier (ACC). The ACC is constructed using ET tooling and attaches to the aft end of the hydrogen tank. Cost of the ACC is between $150 - 250 million and it can fly three years after the go-ahead is given. The ACC is designed for minimal impact to the ET, orbiter, and operations. It provides an additional cargo volume measuring 27.5 feet by 20 feet for payloads. This is valuable to operations because it deals with the volume restriction imposed by the orbiter payload bay. In other words, there is normally additional mass to orbit capability available in each STS launch. A typical example would be a Spacelab flight. The ACC can carry additional payload or primary payload to orbit. This gives the orbiter additional payload capability that can be sold to paying customers for minimal cost. II. Tin Can Uses The ET can be used as a ’tin can’ in orbit for a number of applications. These include storage facilities for liquids, gases, and prepositioned vehicles. The ET/ACC combination can be launched with the ACC as a fully functioning manned space station. This type vehicle has an enormous expansion space inside and outside the ET itself for a variety of orbital applications. The ET/ACC as a space station can also be flown with a single shuttle launch. This capability will allow any number of potential customers to purchase independent space stations for the cost of two or three generic commmunication staellites. Potential customers for this capability include DOD, corporations, foreign nations, and private consortiums.
The ET can also be used as a part of any space station. It can be partially disassembled to make a hangar or easily turned into a space station habitation module of a far larger volume than any past, present, or future space station module. The oxygen tank can be turned into a liquid or gas storage reservoir. The cost savings by ET utilization in these operations are unspecified at this time. However, any specially designed space station module which will fill the needs addressed above must be compared against the ET in two ways. The first comparison is launch cost. With the ET, you get a large rigid body already in orbit. You must lift anything else at $2000 per pound. The second comparison concerns possible future expansion of the structure. If a future expansion is being planned, then the costs of R&D and on-orbit construction from the STS pay load bay during an EVA must be compared to the cost of on-orbit modification of a body that is already in space. This analysis should show in most cases, that the adaptation of an orbiting ET will provide enormous cost savings to the program. In addition, the use of the ET as part of a manned space effort will give the program a new perspective. As soon as the ET is inserted into orbit, the program has made large, massive, structurally strong bodies available to prospective users at a very low cost. The volume restrictions for manned habitations are removed. The storage limitations for liquids and gases are removed. This means that a specially designed structure does not need to be planned, sold to uninterested congressmen, launched, and constructed over a period of several flights. The planning turns to an emphasis on the adaptation of structures already in orbit. This adaptation will take a bit more EVA time, (at over $40,000 per hour), but the savings in launch cost alone will more than cover the difference. III. ET as a Propellent Resource Analysis of future requirements for space based operations show the largest mass requirement is for fuels. These include OTV operations, satellite launch and recovery, and space station orbital maintenance. NASA has a requirement for 2.5 million pounds of propellents over the next ten years. Analysis based on this requirement show the scavenging of residual cryogenics from the ET can fill up to 92X of the requirement at a total cost
savings of $3.5 Billion over the period. The savings comes primarily from the launch cost savings. The scavenging operation can be performed in a variety of ways. These inlcude scavenging into the orbiter after MECO, scavenging into an ACC, scavenging into the space station after rendezvous, and scavenging into a free flyer. Each tank will provide an average of $30 million worth of residual cryogenics available for scavenging. Another use of the ET as propellant is to powder the aluminium and use it as reaction mass in a Aluminium - Oxygen - Hydrogen rocket for OTV applications. Analysis of OTV traffic models show that current technology engines (RL-10) are sufficient if the OTV fuel requirements do not exceed the availability of scavenged cryogenics. However an aluminium/oxygen/hydrogen engine rather than an advanced cryogenic oxygen/hydrogen engine appears to be the most cost effective choice. This is once again due to the savings in launch costs because 40-50% of the reaction mass is already in orbit as tanks. The analysis of the aluminium rocket engine includes the $1-2 billion R&D costs, the high production costs, and the cost of flying a processing plant to grind the tanks into powder. Even with the inclusion of all these additional costs, the aluminium engine is potentially far cheaper than an advanced cryogenic engine because the mass requirements to orbit are far lower. Each tank used in this application is worth about $107 million in powdered aluminium (computed at $2000 per pound to LEO). The problems associated with these type engines are known. It is not clear at this time why the choice not to develop these engines in the 1960s was made. There were fairly serious problems with propellent transport that may be solved by zero 'G* conditions in orbit. There was also a problem with the time constraints of the Apollo program. The scientists investigating the Aluminium engine are well aware of the past history of the engine. They feel that the problems are solvable and that the aluminium engine has great potential for OTV applications. IV. Structures The ET can be used in any number of structural applications. These range from partial disassembly to complete melting and refining opreations in orbit. The ET can be partially disassembled and reassembled into a variety of rigid structures. The tank domes can be removed and the hydrogen tank barrel
can be reattached end-to-end to construct long rigid tubes. The tank can be cut into 5 feet by 60-80 feet long strips using known cutting technologies and welded into any desired shape. Once again, welding is a known technology that has been tested in space. The ET can be completely melted using electrical or solar methods. The melt can then be used to extrude structural members such as channels, I Beams, or rods. It can be used to make thin metal films by vapor deposition processes. It can also be used to make thin metal shells by an inflation technique. The shells and other manufactured structural members can be used for construction. It can also be powdered and used for casting and forming operations. Another use of the ET is as a strongback or a testbed for the construction or anchoring of large structures. The advantage in doing this is that the ET is far more massive and structurally sound than planned space structures. This is because it is the structural heart of the STS during launch. A typical strongback use would be the construction of large antennas on the ET. The mass and stability of the tank is also an advantage. Due to gravity gradient effects, the ET will tend to stabilize with the long axis pointing to the center of the earth. Any structure that can use the ET as a base or an anchor will require less active attitude control systems and thus be cheaper to build, fly, and operate. V. Tethers The use of tethers with the ET also provide significant advantages to the future space program. These advantages include artificial gravity, momentum exchange, electrical power generation, electric propulsion, and significant enhancements in shuttle and space station missions. The physics of tethers allows artificial gravity to be generated in two ways. First, two tanks can be attached to each other by a tether and stored in a gravity gradient mode. This is useful in liquid storage applications. Second, the system can be made to rotate. Artificial gravity levels of 1 ’G’ can be induced by a system 200 meters in diameter rotating at 2-3 rpm. This artificial gravity will negate the undersirable effects of long term weightlessness on the body and lengthen crew stays on station.
Momentum exchange is also useful. This can be done with either a static or a rotating system. Release of an object from the end of a long tether will insert it into a much higher or lower orbit. This could lead to the use of a tethered release from a space station for a shuttle deorbit without an OMS burn. A swinging tethered release could also be used to insert the ET into a high orbit that is long lived and drop an orbiter to a reentry. This exchanges momentum only and uses minimal fuel. A swinging release can also enhance the payload carrying capability of vehicles to higher orbits (including escape orbits). A swinging release 'steals' momentum from the system and can be used to launch a far more massive payload away from LEO than could otherwise be launched with the same onboard propellents. Savings is in fuels and the advantage is that a more massive and more capable payload is possible. Electrical uses of a conducting tether include the generation of station elctrical power and the orbital raising and lowering of the station. A properly designed tether can generate electricity by interacting with the magnetic field. This induces drag and will lower the station. A current can be forced through the tether with excess electricity and the tether can generate a net thrust. This Alfven Engine can be used for orbital maintenance, orbit changing, and energy storage through momentum. Efficiencies are higher than Ion engines and no propellents are required. Shuttle and station mission enhancements with a tether include additional payload capability to orbit, the saving of OMS fuel by a tether mediated rendezvous with a station, and storage of tanks and liquids. The tether mediated rendezvous can enhance the payload to station capability of the orbiter. It can also allow the scavenging of excess orbiter OMS fuel to the station for station requirements. Storage of liquid in a tethered tank takes advantage of the gravity gradient to store liquids where they can be pumped using conventional methods. VI. Miscellaneous The ET in orbit can also be used in a variety of scientific and military applications. The scientific uses include the use of the ET/ACC combination as a way to launch large mirror arrays for telescopes. The hydrogen tank can be used as an affordable high energy observatory of a far larger size than
planned. The exterior of the tank can be used as the structural base for large antenna arrays. The tank itself can also be used to study the interaction of plasmas with bodies in orbit. The two tanks of the ET can be used to perform biological and life science experiments in space. The large volume of each tank can be used to perform experiments in farming, genetics, and waste management. The large size is advantageous because it provides significant biological inertia. This means that in the event of a problem, the biological system can be changed before the entire system dies. Farming becomes possible in the large volume available. Cost savings here are based on the launch cost of food and consumables produced in space as compared against the cost of launching these consumables. An orbiting ’truck farm* becomes possible. Additional life support advantages are the use of the ET as a passive lifeboat. If ETs are inserted to long lived orbits while pressurized with oxygen or an air mixture, they can be entered and used by a crew in an emergency. The large volume of air can be used for weeks to months for life support without active equipment. Military uses are related to the use of the ET as a cryogenic storage facility, space base, or ’Coast Guard’ type operation in space. ETs can also serve as decoys, battle stations, ’junk* ASATs, and military space stations. The ET/ACC based space station for military purposes is something that would be affordable and attractive to those interested in manned military operations. VII. Conclusions The use of the external tank in the American space program is potentially an enhancement that will have an impact greater than the decision to go to the moon. The reason is that the decision to insert the ET into orbit will make resources available for purchase at a cost far below that of the basic launch cost $2000 per pound. The volume available internally is great. The metals available for use are areospace grade aluminiums. The technical drawings are all in the public domain.
Actual overall return based on the decision to regularly insert the ET are unknown. There are two numbers that may hint at the overall value. They are presented on the graphs to follow. The first number is the value of scavenged residual cryogenics at a rate of 12 and 24 launches per year for ten years starting in 1986. This number is based on the $2000 per pound launch cost. The second number is the value of raw aluminium at 53,000 pounds per ET at the same two launch rates and cost per pound to orbit. Note that each flight which inserts an ET into LEO is worth $30M in residual cryogenies available for scavenging and $106M in aluminium. Additional returns from the decision to use the ET in space depend on the actual application. Intangibles such as increased commercial interest and business expansion into space due to relativley inexpensive facilities are very difficult to measure. The increased capabilities of a program that extensively uses the ET are also difficult to measure monitarily, but they certainly are extremely valuable in the long run. There is also no way to measure the positive value to the space program that suddenly becomes rich in terms of mass in orbit, structures in orbit, and reaction mass. The two analyses of actual tank applications involving hardware - the ACC and cryogenic scavenging - have not presented any unpleasant suprises. Both applications appear to be not only possible but are potentially very valuable to those interested in affordable space based operations. At this time there appear to be no unknowns. Other than the materials processing facilities and the aluminium engine, there are no applications that require any great investment of R&D funds to accomplish. With few exceptions, every suggested tank application appears to be possible. These proposed applications also appear to be significant improvements in the capabilities of this nation in space. The figure that follows lists hardware R&D, and questions to be answered for the future use of the ET in orbit. Most of the major questions have been answered. Most of the R&D is already being done as part of the space station program. Most of the necessary hardware either already exists or is in development for other space-based operations.
Total Value of Scavenged Residual Cryogenics
Total Value of ET AIuminum
The use of the ET in space is limited only by the imagination. Making large massive objects available to customers will open space based operations to every interested party at a reasonable cost. The overall possibilities presented by flying the ET are diverse and extremely valuable to all concerned. VIII. Recommendations This report concludes that it is in the best interest of the United States to take the ET into orbit and store it there permanenetly as soon as possible. This will make large scale space operations affordable. Three general recommendations follow: 1. Quickly insert the ET into a permanent storage facility in a relatively high earth orbit. 2. Arrive at a pricing policy that covers the cost of orbital storage and maintenance. Sales cost should cover only the cost of storage. The purchase agreement should simply define ownership issues. The government should not attempt to recover all costs of the last 25 years of spending on the space program through sales of the ET. The intent is to expand space capability. This is best done commercially. It can not be done if the sales costs are set arbitrarily high. 3. Design and fly the ACC as an enhancement of the STS. This will provide additional cargo space at a very low costs. The external tank is an extremely flexible and potentially valuable enhancement of the space program. There is no other action that can be taken today to expand space capability that will cost so little and provide so much potential. The choice to use the ET will be a welcome addition to the American space program that will pay for itself many times over in the years to come. The tank should be flown to permanent orbital storages for future use early and often.
TABLE OF KNOWNS Item Status 1. STS direct insertion Trajectory Already flown 2. Low pressure thrusters Under development by JPL and Martin Marietta 3. ACC Phase 'B* Study complete Needs launch software 4. Additional ET Add-Ons Preliminary study complete 5. Mylar Blankets Already flown Sunshade installed on Skylab 6. Tank Habitation Preliminary studies complete for Hydrogen and Oxygen Tanks 7. Tank as a hangar Preliminary work complete 8. Inflatables Preliminary work complete Flown in early 1960s 9. ET as Spacecraft Needs further study 10. ET disassembly tools Most already flown in STS as satellite repair equipment 11. Conventional Welding Experiments in space conducted Actual welding for repairs conducted on Salyut 12. Conventional Cutting Experiments in space conducted by American and Soviet
Item Status 13. Solar Cutting Preliminary work in progress 14. SOFI Removal 'Cheese cutter' ground tested on SOFI 15. Induction Furnace Preliminary work in progress 16. Grinding/Powdering Preliminary work in progress 17. Tether Materials Short tether already flown Long tether materials under study for Italian subsatellite 18. Tether Winch Under study for subsatellite 19. Tether Dynamics Under study 20. Tether Electrodynamics Under study 21. Liquid Storage Experiments already flown 22. Liquid Handling Experiments already flown 23. Military Applications Under study 24. Orbital Cleanup Preliminary proposal only 25. Market Study Under study 26. Contamination/Outgassing Preliminary study complete 27. Residual Cryogenics Preliminary scavenging study complete 28. Aluminium Rocket Initial proposal only Likely to involve substantial investment and R&D
Item Status 29. Mass Driver Under development - three models already built and operated on the ground 30. Railgun Under development and testing
ET Project - Introduction I. Introduction This report was written under a grant from the Space Studies Institute of Princeton, N.J. The scope of the project is to research all the proposed on-orbit applications of the External Tank (ET) and write a report detailing the results of the research. The report is intended first for presentation to the National Commission on Space and second to provide an overview of current and past External Tank applications studies. The list of references in the back of the report, with the listing of those contacted during the research, will hopefully provide a source of additional information to those interested in the ET and its applications in space. I wish to thank all those listed on the pages following for their time, help, patience, and comments in completing this work. II. Why do we need the External Tank? The ET is the only portion of the Space Transportation System (STS) currently expended on each flight. A typical launch will retain the ET for the SSME burn of over eight minutes and then jettison it for a controlled reentry in either the Indian or Pacific Ocean. An alternate launch trajectory called a direct injection can allow a shuttle to take an ET, an average of 15,000 pounds of residual cryogenics (16), and up to 2,000 pounds additional payload into a typical space station orbit (68). In other words, it costs nothing to deliver a 69,000 pound, factory tested, aluminium pressure vessel into low earth orbit (LEO). In an era of launch costs running about $2,000 per pound to LEO(69), this is clearly a resource worth utilizing in future space based operations. Each and every shuttle launch can deliver a tank to orbit. This can amount to several hundred tanks over a ten year period. The economic analysis of tank utilization on-orbit typically compares costs between a ground built and launched structure and a similar structure built on orbit out of the ET or parts supplied by the ET. These studies alone make ET applications on-orbit extremely attractive. There are additional benefits that the ET can provide in orbit which can not be provided by a ground based item. These make the ET a better choice for almost every manned space operation currently envisioned. Why do we need the ET? We need it because it
will provide the leverage necessary to make future space based operations,industry, and science far cheaper and easier than would be possible without the ET. III. Current interest in the External Tank There are several groups interested in possible orbit applications of the External Tank. The interest spans the entire spectrum of all those interested in doing anything in space from hard science to DOD to private industry. The interest in the tank is also international, with papers proposing ET based space stations written by British and Czechzslovakian authors in the last six years (43, 70). Aeritalia has retained the services of a consultant working on the possibilities of ET applications on-orbit for a few years (75). A typical scientific interest is the Smithsonian Observatory which is looking into the possibilities of constructing a variety of ET based telescopes (32). DOD is interested in the ET due to its capability to carry large diameter payloads and serve as targets for SDI experiments. NASA is interested in the ET because they have to answer the question "Why don’t you use the External Tank in space?" posed by members of congress frequently. They are also interested in it as a way to get more "bang for the buck" out of money spent in space. In other words, the ET is a way to cheaply conduct a very large expansion of a space based operation with minimal cost to the public. Commercial interests are looking at the tank because it provides a way to conduct possible future manned private space operations at about the same cost as two or three generic communcations satellites (17,57, 58). It becomes possible for a company to fly their own large platform cheaply. This increases national interest in space and involves a larger segment of industry in space. It should create jobs and tax revenues due to the business expansion. Companies such as 3-M, Johnson & Johnson, Wyle Labs (17), Ford Aerospace (88), and Martin Marietta (25) all either have a present interest or possible future interest in the ET on-orbit. The Space Studies Institute is interested in the tank as a way to make the construction of Solar Power Satellites (SPS) and space-based habitats come about. There are additional private groups and consortiums interested in flying the tank. The interest in the ET continues to grow. This interest should continue to grow as long as launch costs remain high and payloads remain constrained by the dimensions imposed by the Shuttle cargo bay.
IV. History Interest in the spent stages of launch vehicles has been a part of the American manned space program since the days of Gemini. The USAF proposed the Manned Orbiting Lab (MOL) program in the early 1960s as a DOD space station. This utilized the upper stage of a Titan booster and a modified Gemini capsule as the space platform. This program had crews selected before it was cancelled. The Skylab program following Apollo utilized the Saturn V third stage as the heart of a space station. The program used Apollo hardware intended for lunar flight as a space platform. In the planning stage, there was serious consideration given to actual on orbit construction of the Skylab. This did not happen because a Saturn V booster came available to launch a ground constructed platform and because there was concern that Hydrogen leaking out of the spend third stage insulation (inside the tank - not outside, like the ET) would pose a fire hazard. The American space program is historically very good at adapting hardware from the intended to a new use. On orbit use of the ET is nothing new from this perspective. V. Costs and ramifications of ET applications As mentioned previously, the ET is attractive primarily due to launch costs and capabilities that do not exist in other launch systems. For example, the scavenging of residual cryogenics from the ET alone can give a program savings of $3.5 billion when compared to the cost of launching the same cryogenics in the payload bay to support an Orbital Transfer Vehicle (OTV) operation (29, 32, 69). A partial disassembly of the ET on-orbit and construction of a hangar out of the hydrogen tank will eliminate the need to design and launch a maintenance structure in the orbiter. The savings here is the difference between on-orbit construction using the ET and the design, launching, and on-orbit construction of a specialized structure. The capabilities that do not exist with current or planned operations that the ET can provide are limited only by the imagination. These have been broken down into the following areas: 1. External Tank Description - What properties of the ET can we take advantage of? How is it put together?
2. Tin Can Uses - What can you use a 33 ton, 150 feet long, 27.5 feet diameter, pressure tested, aerospace grade, aluminium can for? 3. ET as a Fuels Resource - The use of residual cryogenics available in the ET. What value is a rocket engine that burns powdered aluminium? Can you make the ET itself into reaction mass for OTVs? 4. Structures - What can you do with a structure that carries all the loads imposed by two Solid Rocket Boosters plus an Orbiter and carries over 1.5 million pounds of cryogenics at liftoff? 5. Tethers - How can a tether be used in concert with an ET to expand capabilities in space? 6. Miscellaneous - What science can you do with a very large bottle for biology? What advantage can be taken of a potential payload diameter of 27.5 feet? The capabilities provided by the ET that are not available elsewhere include large relatively inexpensive masses of aluminium (in excess of 53,000 pounds per tank (56)) in earth orbit, large factory tested pressure vessels, large diameter payload capabilities with small enhancements of the ET, and large enclosed volumes ready for use in orbital storage until needed. The ramifications that tank utilization can have on the space program in the future are significant. At one stroke, the program changes from one based on limits imposed by high launch costs and relatively small launch and orbital volumes to a program which has started to make use of resources found in space. In this context, the ET is a space based resource which is available for use at at very low cost. This resource can be adapted to a variety of specialized uses depending on the needs and imagination of the user. This resource is also something that has finely detailed engineering drawings available to the general public and is constructed out of Aerospace/MILSPEC grade materials (20). This is clearly a very attractive resource. VI. Problems and Solutions At the present time, there are several obstacles to the proposed use of the ET in orbit. The most significant problems are orbital lifetime (Skylab syndrome), and potential contamination (foam outgassing). The Skylab syndrome, which is tied to the orbital lifetime of the tank, is driven by the desire not to drop large massive objects in the earth in an uncontrolled
manner. This is perceived as the primary problem in the use of the tank on-orbit. As a result of the altitude of delivery and its size, the ET has a finite orbital lifetime due to aerodynamic and solar drag. Any program which will rely on the tank has to plan to expend reaction mass of some sort in order to keep the tank in orbit. Orbital lifetimes can be as long as years or as short as days depending on the altitude the ET is released, the attitude at which it is stored, and the activity of the sun (56). There are several tradeoffs which can be utilized to solve this problem. First, the residual cryogenics can be used to power low thrust gas burning hydrogen/oxygen thrusters which can boost the ET up to a very long lived orbit over the period of a few days (46). If this is done, little to no recoverable cryogenic residuals will remain in the ET, but the ET will be in a very long lived orbit. Second, the orbiter can attach a tether, set up a libration with the tank, release at the proper point, and send the ET into a much higher orbit and deorbit the shuttle. There are two problems with this technique. First, it rotates a billion dollar spacecraft and a massive ET around one another on the end of a rope. Second, it has never been done on this scale before. Contamination is the second major problem with the ET. The entire ET is coated with Spray-On Foam Insulation (SOFI) which will outgas on orbit in space. This pollution may not be tolerable in the evironment inhabited by the space station. A potential fix is to strip the foam off the ET after release in orbit. Most of the foam can be stripped by the equivalent of a hot-wire cheese slicer. The remainder can be polished off by leaving the ET 'hanging in the breeze’ over the period of a month or two exposed to the molecular oxygen present in the proposed orbits (14). There are other problems with the ET on orbit. Having more mass and volume available than is required for currently planned operations in orbit are factors. The partial disassembly of a tank has never been attempted in space and needs to be demonstrated. These problems are all solvable and in some cases may not be problems at all but advantages. VII. Conclusions It is clearly in the national interest to do whatever is reasonable to better utilize equipment already purchased. With the External Tank, we have the potential to provide for an enormous expansion in space capabilities for minimal expenditures - Extremely Low Cost and Extremely High Return. With this in mind, the following suggestions are made:
A. Initiate immediately a program to deliver the ET into orbit. This program should plan for on-orbit maintenance of the delivered tanks and the possible inclusion of ET based structures and modules in the US Space Station. B. Arrive quickly at an ownership and sales policy. Suggest sales price be set to reflect ONLY the cost of operating the ET orbital storage facility, not an attempt to recover the costs of the entire STS. The rationale here is twofold. First, the ET is essentially a salvaged article. R&D funds expanded were spent in making the STS operational, NOT in using the ET. Second, the ownership issue should be solved quickly and early. This will encourage private investors who want to purchase inexpensive space capability and not feed lawyers. The precedent for this issue comes from the experience with the KC-135. This aircraft was constructed to fill a defense need. The sale of aircraft of the same or a similar design was not constrained by any attempt to recover the cost of monies already spent. Boeing and other American aerospace companies sold no small number of these KC-135 class aircraft to airline companies worldwide. The net result was an unexpected creation of new jobs, new industries, and the generation of more tax revenues. The utilization of the ET will have a similar impact. The External Tank is a very important item in the future of this nation in space. It provides the leverage necessary to make large scale space based operations of almost any kind affordable and therefore able to be done privately. The possible applications of the tank on-orbit are constrained only by the imagination. The ET should be stored in orbit for future use as soon as possible and the sales and ownership questions settled. This action will likely have an impact on this nation's future in space comparable to Apollo.
ET Project - Tank Information I. External Tank Description The External Tank of the STS is designed to fill two primary needs. These are to safely carry sufficient cryogenics to deliver the orbiter into Low Earth Orbit (LEO) and to serve as the structural heart - the strongback - of the STS during the launch phase. The basic empty weights, volumes, and dimensions are detailed in the first figure. The tank is constructed in three primary segments which are the hydrogen tank, the intertank, and the oxygen tank. The oxygen tank rests on top of the stack. It is constructed of aluminium in four sections and includes internal slosh baffles. It is bolted to the intertank section which carries its’ loads in compression during flight. The intertank is the primary strength member of the tank. It is unpressurized and constructed using a skin stringer and ringframe arrangement. It also carries a beam which carries the thrust loads of both solid rocket boosters (SRBs) during flight. The intertank is bolted to the top of the hydrogen tank, the largest section of the external tank. The hydrogen tank is designed to carry the required liquid hydrogen during launch. It also carries the loads imposed on the stack by the orbiter engines (SSME) during launch. The entire tank is covered with Spray-On Foam Insulation (SOFI) which serves primarily to prevent ice buildup on the tank before launch and to prevent excessive boiloff of the cryogenics. Typical depth of the insulation is between one and one and one half inches (1-1 1/2"). There are also additional items attached including range safety hardware, feedlines, sensors, electrical lines, tumble valve, and venting systems (63). II. Current ET Operations After construction and testing, the ET, with low pressure in both the hydrogen and oxygen tanks, is shipped by barge to the launch site. The hydrogen tank is pressure tested to over 36 psi. The oxygen is pressure tested by filling it with water at the factory. Neither can tolerate an outside pressure differential greater than .2 psi. (48) Shipping it
LENGTH = 54.6 FT MAX DIA = 27.5 FT WT = 12,400 LB VOL = 19,500 CU FT LENGTH = 22.5 FT DIA = 27.5 FT WT = 12,100 LB INTERTANK EXTERNAL TANK STRUCTURE LENGTH = 96.7 FT DIA = 27.5 FT WT = 28,900 LB VOL = 53,500 CU FT LH2 TANK LO2 TANK
pressurized also keeps the interior clean. The tank is then checked out, attached to the appropriate stack and launched in normal sequence. A typical launch inserts the orbiter and ET into a very low earth orbit where the tank is jettisoned to reenter somewhere over the Indian or Pacific Oceans (56). The orbiter then conducts an OMS burn to loft itself into the desired orbit. The tank is jettisoned with over 98% of the energy required to keep it in orbit. It is important to control the reentry of the tank to insure that it comes down in an unpopulated area. There is an alternate trajectory for launch called a direct insertion. This trajectory has been flown successfully with the ET splashing down near the Hawiian Islands. If the tank is retained even longer and taken into orbit, the orbiter can deliver the tank, orbiter payload, and additional orbiter payload into all possible STS orbits. It attains this capability by flying a more efficient trajectory and using the more efficient SSMEs to put it all the way into orbit. The following figure is a comparison of Shuttle performance to orbit without the ET, with the ET, and with an ET enhancement called the AFT Cargo Carrier (ACC) which will be discussed later. It is important to note that if the ET is taken to orbit, this action alone improves the STS payload capability by as much as 2,000 pounds to orbit (56). At an average launch cost of $2,000 per pound (69), this is a substantial enhancement of the STS capability. III. The ET on Orbit - Problems and Solutions Taking the ET into orbit presents several opportunities and capabilities that we do not yet have. It also produces some additional problems. The opportunities available are related to the mass and size of the tank itself. The capabilities are related to things that can be done with the tank on-orbit. The primary problem is related to safety considerations. All will be discussed in the following section. The size and mass of the external tank on-orbit compare favorably with past and proposed future orbital facilities. The 53,500 cubic foot volume of the hydrogen tank alone is far larger than any facility flown or planned. It is more than twice as large as the Skylab at 18,300 cubic feet (70). It is more than five times the volume of the proposed Space Operations Center (SOC) module of 7,100 cubic feet (75). The volume available in the oxygen tank at 19,500 cubic feet is also larger than all the above mentioned facilities.
ALTERNATE TRAJECTORY ORBITER WITH ET & RESIDUALS MISSION PROFILE ALTERNATE t TRAJECTORY WITH ET BASELINE TRAJECTORY SPACE SHUTTLE ORBITAL PERFORMANCE DATA (160NM 28° INC) CONFIGURATION PAYLOAD (LBS) ORBITER ONLY 54,000 ORBITER WITH ET 56,000 ORBITER WITH ET/ACC 44,000 ENHANCED STS WITH ET/ACC 62,000
These volumes can be accessed through three foot diameter inspection hatches in the respective tank domes. The domes can also be completely removed by pulling the attachment bolts if the desire is to open the entire end of the tank. The mass of the tank is also an advantage. At an average mass of 69,000 pounds apiece, the tank can provide the inertia and strength required for large scale operations in space. Once released, the tank will assume a gravity gradient stabilized attitude with the long axis pointing at the center of the earth (56). A telescope mounted on this platform could rely partialy on the mass and inertia of the tank itself rather than an active three axis stabilization system. The tank can be used as a workbench or a strongback in orbit. This concept utilizes the actual structural strength of the tank itself as a construction platform. The construction of large space structures need not be based on specially designed lightweight members that could not support their own weight on the surface. Due to reserve, pressurization, and ullage requirements, the tank should contain an average of 15,000 pounds (5,000 to 40,000 pounds) of residual cryogenics when it arrives in orbit (16). This liquid hydrogen and liquid oxygen can be scavenged from the tank shortly after launch for a variety of uses on-orbit (69). These residuals also pose a safety problem which must be addressed. The current use of the liquid oxygen tank pressurization gas is to tumble the tank after it is jettisoned for reentry. The on-orbit problem is that there is concern about the boiloff rate and possible overpressurization and rupture of the tank. There are several ways to deal with this problem. The first is to scavenge the residuals relatively early with a set of catch tanks located on the aft end of the tank. This can also be done with the required equipment located in the payload bay of the orbiter but is less desirable because it uses payload bay space that can be better sold to customers. A second method would be to install a heat reflector to keep the sunlight off the tank. The first Skylab crew did this to the Skylab to make if habitable. The ET could be made a better storage container for cryogenics by wrapping it with mylar blankets to retard boiloff (46). The next figures detail boiloff rates for residual cryogenic propellants.
ETR Performance - ET to 28.5* Residual cryogenic Boiloff
Orbital Lifetime Decay Rate for a Single ET / —'
A third method involves trading the cryogenics as they boil off for orbital altitude. There have been several proposals for low thrust, gas burning hydrogen - oxygen rocket engines for station keeping and orbit raising. The engines proposed by Martin Marietta are 1,500 Ibf thrust with a specific impulse (Isp) of 375 seconds (46). A proposed arrangement of four of these engines mounted on the intertank as thrusters using the residuals left. The tradeoff here is between orbital lifetime desired and the amount of scavenged cryogenics desired. As was demonstrated by the reentry of Skylab, the orbital lifetime of large space objects is often far less than predicted. One of the primary considerations with taking the tank into orbit is keeping it there. The orbit of the tank will decay over time due to aerodynamic drag and effects of the solar activity. The desire is to put the tank into the highest possible orbit. Cross section ’into the wind* is also a factor. The gravity gradient stabilized tank with the 'nose down’ attitude is in the worst possible attitude for long orbital lifetimes because it presents the largest area to the wind. The best attitude is either end into the direction of motion. The following graphs detail calculated orbital lifetimes for a single external tank in three different storage modes (56). It is particularly important to control the orbital altitude of the tank if brought into orbit. The reentry point and impact footprint are extremely difficult to predict for an uncontrolled reentry (56). The leads to a requirement to reenter the tank after launch, reenter the tank after it has served its pupose on orbit, or to keep it in orbit using active measures. Thus, any plan which proposes external tank applications on-orbit must address this issue. Some form of active propulsion like small thrusters or an alternate form of orbital maintenance such as momentum transfers involving tethers or the low pressure cryogenic boiloff thruster needs to be used. This is not a particularly difficult problem to solve. It is however, a fact of life for every structure in earth orbit. There are current studies of appropriate propulsion for keeping the NASA space station in orbit. The last problem with the External Tank in orbit concerns outgassing from the SOFI on orbit. This amounts to an unwanted fouling of the environment and the equipment in the vicinity of the tank. Outgassing has been called the fatal problem with the use of the tank on-orbit (14). Like most problems in
the past that have been refered to as fatal flaws, this is not insoluble. Preliminary calculations suggest that the SOFI will deteoriate at a rate of about 0.8 grams per second for the entire ET (3). This rate is purely an order of magnitude prediction subject to change with further investigation. Outgassing may be a problem to vehicles and structures in the vicinity of the ET. Possible solutions include bagging the tank in a mylar blanket which will retard the rate, applying metals by vapor or liquid deposition to the outside of the tank on orbit, removing the SOFI from the tank, or keeping the tank based structures lower in orbital altitude than the rest of the operation by the use of tethers (14). The ramifications and the magnitude of this problem warrant further investigation as tanks are proposed for orbital applications. IV. The Aft Cargo Carrier The first actual enhancement of the STS is likely to be the Aft Cargo Carrier (ACC) (4). The ACC is detailed in the figure below. It is basically a cargo volume constructed similar to the ET components using the same tooling and bolted onto the bottom flange of the hydrogen tank. There are three basic advantages to the ACC. The first is that it provides a volume nearly equivalent to the cargo bay of the shuttle that is not constrained by a diameter of 15 feet. Its volume of 9,100 cubic feet compares well with the orbiter cargo bay of 10,600 cubic feet. The ACC dimensions of 27.5 feet in diameter and 20 feet in height allow large diameter relatively low mass payloads to be flown without the constraints imposed by the 15 feet diameter orbiter cargo bay. A typical ACC weighs near 14,000 pounds (4). It consists of a skirt which connects to the ET, a payload support structure and a shroud and is insulated for protection from the SRB plumes and blast effects. Payload penalty to orbit for an ACC is from 9,800 - 11,400 pounds (68). This is below the actual ACC weight as a result of shroud jettison after SRB separation. Center of gravity problems during flight have been addressed and do not appear to be a problem. The beauty of the ACC is that it provides another cargo volume at a minimal cost. This is important to the STS program because it helps remove a payload constraint. The orbiter imposes two constraints on payloads. These are volume and mass limitations. The mass limitation can not be changed without changing actual orbiter performance. The volume constraint can be
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