THE EVOLUTION OF EXTERNAL TANK APPLICATIONS
27th Aerospace Sciences Meeting
Jan 9-12, 1989, Reno NV
J. Alex Gimarc
Colorado Springs, Colo. **
The External Tank (ET) of the Space Shuttle offers unique opportunities for orbital applications. These applications fall far outside of the original design criteria of the tank. A large number of applications have been proposed over the last twelve years. Early interest was oriented toward the on-orbit storage and use of the tank as various habitats. Work during the early 1980s detailed on-orbit applications as tethered bodies, life sciences experiments, cargo carriers, materials resources in space, and the basis of a wide variety of manned platforms. Current interest is primarily in the areas of manned or man tended platforms and the construction of a large telescope for gamma ray observation. The external tank may facilitate early American commercial manned space efforts in the near future.
Early External Tank Applications:
Initial study of possible applications of the external tank surfaced in the middle 1970s in three locations. First was the proposed use of tanks as propellant for free flying mass driver reaction engines proposed by O’Neill.(12) This proposal made use of the potential large number of tanks available for use in space. The tanks were to be ground into powder and used as reaction mass for the transport of large masses to the lunar surface and high cislunar orbits. An additional proposal that fell out of the early O’Neill Space Industrialization work was the construction of a habitat out of tethered hydrogen tank sections. A rotating construct of fourteen hydrogen tanks was to be constructed for the purpose of housing a crew of between 230 – 250. The exterior of the tanks was to be covered by a 42+ inch thick radiation shield made out of lunar soils transported from the lunar surface.
Parallel interest surfaced at Marshall Space Flight Center and internally at the Martin Marietta Michoud Aerospace Corporation. While little has been written about the early Martin work, it was apparently conducted on an informal basis in conjunction with the work at Marshall. It may be pointed out that the early work on the potential use of the tank occurred shortly after the successful Skylab missions of the early 1970s. Results of this early interest led to a number of papers and drawings detailing the use of the tanks as various manned platforms with the oxygen tank manned and pressurized. The hydrogen tank was typically turned into a hangar in these drawings. An orbiting hangar is still being discussed as part of the ongoing Space Station effort. Early tethered applications were also proposed at this time.
Interestingly enough, while several concepts were proposed for the use of expended tanks in orbit, most discussions did not detail engineering designs of the proposals. The conclusions typically ended with some sort of discussion of the importance of storing large numbers of tanks on orbit until decisions were made exactly how to use them. The interest in storing tanks on orbit and providing them to users has persisted to this day.
Tank Applications – Early 1980s:
In the early 1980s, interest underwent a rapid expansion. The thrust of this was fueled by the continuing work by Martin Marietta and rapidly growing interest by a number of non- aerospace groups. Entrepreneurial interest in the use of the tank and tank based platforms surfaced during this period. This time also saw the initial work toward the use of the tank as a telescope constructed on orbit. During this period, ET Applications became a legitimate topic for study outside NASA.
Corporate and entrepreneurial interest in the application of the tank in orbit came together in a series of contracts between Martin Marietta Michoud Aerospace and Tom Taylor.(4, 5, 21) These contracts provided the funding for Taylor to research and conduct first order engineering on a wide set of future applications of the external tank. The studies surfaced a large number of potential applications in five areas. These areas included tin can uses, tether applications, structural uses, scientific applications, and propellant resources. Additional applications utilizing tank modifications for more shuttle capability were also proposed at this time.(9, 18) Results of the studies were widely published by Taylor in the years to follow.
New areas mentioned were the scientific applications and the propellants resources. The majority of the potential uses took advantage of the inherent advantages of the external tank in orbit. These advantages included the availability of a 69,000 pound aerospace structure with two pressurized volumes in orbit at little to no cost, and the availability of an average of 11,000 pounds of residual cryogenics.(23)
At the same time, a second group working out of the California Space Institute (CALSPACE) investigated the tank and its applications. These scientists and engineers held a number of workshops, wrote a large number of papers, and greatly added to the expanding number of ideas detailing the various applications of the tank in orbit. This work was significant in that it greatly expanded concepts associated with biology, astronomy, life sciences, metallurgy, and tether applications.(10, 14, 15, 27)
One of the members of this group, Joe Carroll, added to the earlier work by Dr. Giuseppe Colombo on the establishment and growth of large tethered structures based on the ET. Results of these studies indicate that tether applications of the tank can gain the user some outstanding capability for minimal dollars and minimal risk.(2)
During this period, private interest in the tank continued to grow. One of the groups was headed by a gentleman with an extensive background in government, space, and industrial operations. Dr. Tom Rogers, as the head of the Sophron Foundation, had researched the problems associated with privately financed spaceflight for a number of years.(16, 17) His conclusion was to keep purchase and operations costs to the absolute minimum. The overall goal is to get the maximum number of people into space for the minimum cost. After a thorough inspection of the problems, it was determined that the tank and its application as an orbiting manned or man-tended platform was the preferred solution.(8) The platform is intended for use as a corporate, private, and/or university research facility. A company called the External Tanks Corporation (ETCO) was formed in 1986 under the auspices of the University Consortium for Atmospheric Research (UCAR) for the purpose of obtaining and constructing a manned facility for use by university research scientists.(1) The effort is not funded or supported by the government and uses no taxpayer dollars.
Tank Applications – Middle 1980s:
Dr. David Koch, from the Smithsonian Astrophysical Observatory, was at this time working a project to construct a gamma ray imaging telescope. Initial work in the early 1980s has verified the attractiveness of this concept. The idea had matured greatly through the years. Support for the concept came from both Marshall and Martin for a number of reasons. Martin was interested from the standpoint of future tank applications and their role as manufacturer and future supplier. Marshall interest came from the overall responsibility for the external tank within NASA. This project is important because it allows the study of most of the basic operations necessary use a tank on orbit. A tank that is to be converted into a pressurized telescope by astronauts requires most of the basic operations necessary to convert it into a habitat, a workshop, a hangar, or whatever else is desired by the eventual user. Additionally, construction of a telescope of this nature will provide an outstanding scientific opportunity. The projected surface area of a tank based gamma ray imaging telescope is 40 times larger than the largest that has been proposed to date.(6)
The Space Studies Institute exercised their continuing interest in ET applications with the SSI REPORT ON SPACE SHUTTLE EXTERNAL TANK APPLICATIONS published in late 1985. This report was prepared for presentation to the National Commission on Space (NCOS) during that year. It was prepared as a catalog of all of the potential uses for the tank at that time and was one of only two presentations made to the Commission on possible future uses of the ET. The commission made a recommendation to take tanks into orbit, stockpile them, and make them available to private customers.(13)
During 1988, there have been significant changes in the status of the tank and its potential for use in space. First was the President’s space policy of 1988. It has directed NASA to make available to interested parties the expended external tanks of the space shuttle once it has entered orbit. After the announcement, there was immediate interest by three companies. To date, there have been ten proposals made to NASA for tanks. These companies include ETCO and Global Outposts.(26)
Work has started at Marshall for a Pathfinder mission.(6) The concept is to take a tank into orbit, conduct a series of experiments and measurements, and reenter it in a controlled manner. Pathfinder is intended to provide the first actual information on dynamics, residual propellant outgassing, stability and control, impact and debris problems, and controlled reentry. This mission will require most shuttle and astronaut activities necessary to take a tank to orbit for actual use.(7)
ETCO is conducting a series of discussions with university experimenters wanting to conduct near term space experiments.(1) The concept is to mount experiments on the interior wall of the intertank and fly a suborbital mission. Data is gathered and transmitted back to ground stations. The experiment is destroyed as the tank reenters. The significance of this concept is twofold. First, the intertank is being used. Second, the experiments do not require a significant wait in the current queue and can be quickly and cheaply integrated into the tank. This concept will help alleviate the large number of middeck lockers and getaway specials waiting shuttle flight assignments.
External Tank Applications – Future:
Future interest in the use of the external tank in space will continue to grow. The flight of a Pathfinder mission will allow actual measurement and study of a number of potential problems raised through the years. Commercial interest in manned and man-tended platforms will grow commensurate with the inability to freely participate at the Space Station. Flight in private tank based platforms should provide an attractive and affordable alternative to current and planned space platforms.
1. 1987 Summer Symposium on Scientific Uses of Orbiting External Tanks, Final Report, UCAR Foundation, Boulder, Colo., Aug 1987.
2. Carroll, J.A., Guidebook For Analysis of Tether Applications, Final Report on Contract RH4-39409 with Martin Marietta, San Diego, Calif., March 1985.
3. The Enumeration and Evaluation of Optional Uses of the Space Shuttle External Tank, Report, Wilhelm, M., MSFC, Ala., Aug 1977.
4. ET Applications In Space – Final Report, Vol II, Martin Marietta Advanced Programs, by Taylor, T.C., Dec 1982.
5. External Tank Applications, Martin Marietta, 1982.
6. External Tank Gamma Ray Imaging Telescope Study, Final Review Phase II, NAS8-36394, Martin Marietta, Feb 1988.
7. External Tank On-Orbit Inspection, Martin Marietta MMC-ET- SE05-98, done under NAS8-30300, May 1982.
8. The Feasibility of Using the Shuttle External Tank as a Low Cost Warehouse – Workshop – Residence in Low Earth Orbit, Final Report, unpublished or the Sophron Foundation, McLean, Va., Aug 1984.
9. General Purpose Cft Cargo Carrier Study – Final Report, Vol I, Technical, Martin Marietta, May 1985.
10. Identification of New Potential Scientific and Technology Areas for DOD Application, Summary of Activities, Final Report, California Space Institute, N84-17050, La Jolla, Calif., Aug 1983.
11. Monitor,D.S., Studies of an External Tank Tethered to an Orbiter in Orbit, AIAA GNOS 83-024, New Orleans, La., Sept 1983.
12. O’Neill, G.K., The High Frontier, Bantam Books, Jan 1978.
13. Pioneering the Space Frontier, The Report of the National Commission on Space, Bantam Books, May 1986.
14. The Process of Space Station Development Using External Tanks, Report by the External Tank Working Group of the California Space Institute, Scripps Institute of Oceanography, University of California at San Diego, La Jolla, Calif., March 1983.
15. Report on the Utilization of the External Tanks of the Space Shuttle Transportation System, NAS8-35037, California Space Institute, University of California at San Diego, Workshop held Aug 1982, April 1983.
16. Rogers, T.F., Homesteading the New Frontier, Spaceworld, pp. 4-7, Jun 1985.
17. Rogers, T.F., A University Private Research Laboratory in Space, unpublished comments presented at the AIAA 23rd Aerospace Sciences Meeting, Reno, Nev., Feb 1985.
18. Shuttle Performance Enhancement – Option 4 (External Tank Rocket Assist), NAS8-30300, Martin Marietta Denver, Jun 1978.
19. Space Resources and Space Settlements, NASA SP-428, NASA, 1979.
20. Space Shuttle External Tank (Lightweight Model) Systems Definition Handbook, Vol I and II, Martin Marietta, NAS8-30300, NASA, April 1983.
21. Spencer, J.S., Space Shuttle Habitability Study, Space Systems Development Group, Los Angeles, Aug 1980.
22. SSI Report on Space Shuttle External Tank Applications, Space Studies Institute, Gimarc, J.A., Dec 1985.
23. STS Propellant Scavenging Systems Study – Final Report, Vol I and II, Part II, Executive Summary and Study Results, Cost and WBS/Dictionary, Martin Marietta, Jun 1987.
24. Summary of Studies Concerning – Utilization of External Tank as Element of Space Station, Brady, H., MSFC-PD, Briefing, Mar 1982.
25. Svoboda, T., et al, Space Shuttle External Tank Used as a Space Station Study Project PERUN, IAF-79-ST-05, 30th IAF Congress, Munich, Sep 79.
26. White Paper – The Space Based Support Platform (SBSP) Derived From External Tank Technology Using OUTPOST, Global Outpost, Inc., by Taylor, T.C., Alexandria, Va., Dec 1987.
27. Workshop in Possible Applications of the STS External Tank to Life Support and to Chemical and Life Sciences, California Space Institute, CSI 82-3, Scripps Institute of Oceanography, La Jolla, Calif, Feb 1983.