External Tanks in Orbital and/or Sub-Orbital ApplicationsSummary White Paper, NASA Dr. Sweet

Introduction

Many studies have been completed during the past eighteen (18) years concerning various proposed applications for expended Space Shuttle External Tanks (ET) after the Shuttle has boosted the ET to (or nearly to) low Earth orbit. These proposed uses have ranged from converting the ET’s into pressurized habitats, to converting the ET’s into space telescopes, to melting the ET and using the resulting material as feedstock for in-space manufacturing. Some of these studies were conducted at only a high level, while others involved a considerably more detailed consideration of prospective applications.

Interest in ET applications peaked in the late 1980s. In a February, 1988, announcement, “Space Policy and Commercial Space Initiative to Begin the Next Century”, President Reagan stated that expended ET’s would be made available for five years “to all feasible U.S. commercial and nonprofit endeavors, for uses such as research, storage, or manufacturing in space.” This eventually resulted in a Memorandum of Agreement (MOA) with the University Corporation for Atmospheric Research (UCAR), GLOBAL OUTPOST, Inc. and an organization at the University of Colorado. Both the University Corporation for Atmospheric Research (UCAR) and GLOBAL OUTPOST, Inc. put up $ 100k in deposit money for five ET’s in orbit. GLOBAL OUTPOST, Inc. proceeded to put in place a contract with Martin Marietta (the ET’s manufacturer) to work out a salvage plan; this involved Thomas B. Mobley (504-257-2480) in Advanced programs, and Faye Baillif at the Michoud Assembly plant. GLOBAL OUTPOST has continued to examine the technical aspects of a salvage approach; results of which have been discussed with the Shuttle Program Office at NASA Johnson Space Center (JSC). A 1998 meeting with NASA indicated that the earliest an ET might be available would be after ISS assembly.

Major Issues

  • Results of various studies indicate that there are four primary technical concerns regarding on-orbit use of ETs.

    Circularization, Orbit Maintenance and Control

    From a performance and dynamics standpoint, the Shuttle system can deliver an ET to a stable orbit using the Shuttle OMS engines. The payload penalty to the Orbiter resulting from ET delivery would be a function of orbit altitude, inclination, etc. The ET, of course, has no inherent attitude control or re-boost capability. An unmodified ET released on-orbit would tumble randomly, the orbit would decay (how quickly would depend on a number of factors, such as the initial altitude, solar activity, etc.) and uncontrolled reentry would occur. Typically, if the Orbiter carried some other payload, the ET delivered would have a lifetime in orbit of only a few weeks. Engineering changes to provide these functions, such as the integration of an “Aft Cargo Carrier (ACC)” – essentially a 26’ diameter, 20’ length payload cylinder incorporated behind the aft dome of the LH2 tank – is feasible. However, such a major modification of the ET/Orbiter would have significant cost, performance, operations and safety impacts. This path leads to the ET becoming capable of operating as an independent spacecraft.

  • Power, Thermal Control, Communications

    All of these functions are necessary to maintain the ET in safe orbit. Again, these might be integrated systems as part of an Aft Cargo Carrier (e.g., with large ~22-kW deployable solar arrays and radiators, S & W-band communications). Cable runs for eventual power distribution would be integrated into the existing ET cable tray.

    Development of all of the systems described in Part 1) and Part 2) would be required just to safely deploy the ET and maintain it in a minimum altitude orbit. Once again, you have essentially built a spacecraft and bolted it to the bottom of the ET.

  • Spray-On Foam Insulation erosion and orbital debris generation

    The TPS/SOFI is applied to nearly the entire outer surface of the ET. There is major concern that this material would erode for extended periods of time on-orbit due to the effects of out-gassing, atomic oxygen, and thermal cycling (day/night transitions). While this material is lightweight and somewhat fragile, it still would pose an orbital debris hazard due to high relative velocities of any objects in orbit. Some analysis indicates that debris particles may decay and reenter relatively quickly due to their low ballistic coefficient, however, if the erosion is of long duration, the ET would be, in effect, a continuous debris generator in low earth orbit. Some proposals have examined developing anti-debris insulation or “bagging” the ET in orbit with a mylar or elastic sheath to contain the debris. One other potential solution would be to put a new coating on the ET. These have unknown cost and operation issues associated with them. Given the importance of minimizing the production of orbital debris, these issues must be dealt with conclusively before any on-orbit ET Applications could be considered.

  • Interior Access

    Presumably, one of the reasons a structure like the ET would be used on-orbit would be due to its large interior and, presumable, pressurizable volume. Deliberate attempts were made in the ET design to minimize the number of penetrations into the propellant tanks as each of these represents a Crit-1 potential leak path. New hatches, ports, etc. penetrating the LOX or LH2 tanks would have major implications on ET flight certification. On-orbit cutting, welding, etc. to gain access would represent challenging EVA scenarios, and one of the major arguments for ET-based structures is to reduce the level of on-orbit integration required. One approach would require a new cryogenic seal in the barrel section of the LOX tank, and an outer cover conformal to the ET moldline. Within the intertank would be a bladder system that would be deployed (presumably via EVA) within the LOX tank to provide a thermal and pressure barrier for atmosphere.

  • Conclusions

    Space applications of the Space Shuttle External Tank have been discussed repeated and studied at various levels of detail during the past 20 years. During that time, technical viability of ET re-use has been largely established, although by no means proven. However, no compelling, economically viable application has been identified. Moreover, the probable requirement of providing an orbital ET with attitude control and orbital reboost capability immediately after deployment has major implications to any such application. The ET is no longer a tank, it must be a highly reliable spacecraft with propulsion, power, thermal control, and communications capabilities. This is essentially the same challenge NASA space station designs have encountered in the past, resulting in spacecraft such as the Russian FGB and the Interim Control Module. To incorporate these capabilities onto the ET will most likely result in significant modifications to this element with unknown, but likely high redesign and recertification costs. The most significant of these are probably redesigned insulation to eliminate orbital debris generation, tank penetrations for interior access, and structural modifications to accommodate an Aft Cargo Carrier or similar module.

    Alternate design concepts may be less costly: for example, the packaging efficiencies inherent in inflatable or deployable structures may allow volumes consistent with current launch capabilities. A variety of alternative solutions to the challenge of low cost habitable volume in Earth orbit should be considered. The advantage of orbital ETs for potential users will depend upon who bears the burden of t
    he redesign, augmentation, recertification and launch costs.

    Supporting Material

    Attached is a list of studies and related documents concerning ET applications. This list is almost certainly not a complete history of all the studies conducted on this topic. Information on the results of these studies is available in a range of formats, from Internet searches, to science meeting abstracts, to formal studies conducted by commercial interests or NASA. Given the wealth of information available from these studies, most of which are in the public domain, it would seem prudent for anyone interested in pursuing this area, to carefully review the data already available, rather than commission an additional study on this subject at this time. If, upon a fresh look, it is deemed that some creative commercial space development application warrants a more detailed investigation, it might be more appropriate to expect support for these potential applications to be provided by the private sector.

    Space Shuttle External Tank Aplications

    References

    Technical Studies, Reports and/or Documents

    1975

    System Definition Handbook, Space Shuttle External Tank, Martin Marietta, Nov 1975

    1976

    “The High Frontier: Human Colonies in Space,” Dr. Gerard K. O’Neil, Published Book, 1976, Summarized several summer studies of space applications including the use of the ET as reaction mass for Mass Drivers, use as building materials, melting to get base aluminum, etc.

    1977

    1978

    “External Tank – a Usable Resource?,” Martin Marietta, no date

    1979

    “ET Applications in Space,” Thomas C. Taylor, Final Report* Dec 1979, Martin Marietta Corporation, Denver Aerospace, Michoud Operations Advanced Programs; Frank Williams Director of Advanced Programs, Initial Conceptual ET Contract by Mr. Taylor and detailed over 100 conceptual ideas for the use of the ET in Orbit including the Aft Cargo Carrier (ACC) and Shuttle Derived Vehicle configurations.

    Report on Space Shuttle External Tank Applications, Web Site: http://www.freemars.org/history/index .html

    Reusing today’s rockets spent fuel tanks in orbit, Web Site: http:/permanent.com/ext-tank.html

    The Space Shuttle External Tank as a Re-Entry Module, Stanley Kent, 1979

    Construction of a ET Derived Torus Facility, Thomas C. Taylor, 1979, Cal State Northridge Project in Class, B. J. Bluth, Instructor, Assembly concept for Habitation Torus from eight hydrogen tanks assembled into a ring and rotated 2 rpm. Included a 1:200 Scale Model.

    1980

    Aft Cargo Carrier (ACC)*, Thomas C. Taylor of Taylor & Associates, Inc., 1980, Martin Marietta Corporation, Denver Aerospace, Michoud Operations Advanced Programs; Nick Witek of Advanced Programs, Second Contract and assisted in the detailed applications using the ACC cargo Pod under the ET as habitation, construction base and cargo carrier.

    System Definition Handbook, Space Shuttle External Tank (Lightweight Model), Martin Marietta, Aug 1980

    Space Shuttle External Tank Habitation Study, Space Systems Development Group, John Spencer Director, T. Taylor, Advisor, 1979-1980, Created 104 page study on interior design of the 8 tank torus. Result in several years was a Space Theme Destination Resort Hotel based on the concept by SRE, Inc., which failed to attract significant funding.

    1981

    Thermal Protection System Test Mission*, Thomas C. Taylor of Taylor & Associates, Inc., 1981, Martin Marietta Corporation, Denver Aerospace, Michoud Operations Advanced Programs; Nick Witek of Advanced Programs, Conceptual designs and hardware for the detailed inspection of TPS on orbit after the ET is taken to orbit.

    External Tank Pop-Out Workshop, Gary Johnson, March 1981

    ET Residuals Recovery Concept, Rockwell International, Oct. 1981

    Summary of Studies Concerning, MSFC – PD, Dec. 1981

    Utilization of External Tank as Element of Space Station, 1981, possibly a ET Corp. study

    Future Uses of the Spacelab Module, Thomas C. Taylor of Taylor & Associates, Inc., Volume I, II & III of Final Report* Nov 1981, Aeritalia, Dr. E. Vallerani, A conceptual study exploring the future uses of the Spacelab Module, the external tank in orbit and manned station alternatives, which included ET surface track, ET Interior Volume use, and doubling the interior volume of Spacelab concepts. United States Patent 4,562,979,”Expandable Spacecraft,” Taylor, Issued Jan. 7, 1986

    1982

    External Tank Applications in Space, Martin Marietta, Aug 1982, Martin Marietta, Advanced Programs Group at Michoud Assembly Facility, NASA-Marshall Space Flight Center, Huntsville, AL., One of a series of MMA Contract Studies funded by NASA and performed by the ET manufacturer on the use of the ET in orbit.

    ET Applications in Space – Martin Marietta, Dec. 1982, Final Report, MMA, NASA-MSFC, One of a series of MMA Contract Studies funded by NASA on the use of the ET in orbit.

    “External Tank Applications in Orbit,” Thomas C. Taylor of Taylor & Associates, Inc., Final Report* 1982, Martin Marietta Corporation, Denver Aerospace, Michoud Operations Advanced Programs; Thomas Mobley and Faye Baillif of Advanced Programs, Continuing Conceptual ET Contract by Mr. Taylor and refined the Aft Cargo Carrier (ACC), TPS Inspection Mission and other application for the ET in orbit.

    User Orientation and Motivation Presentation, Taylor & Associates, Inc., 1982, Dr. E. Vallerani of Aeritalia, Study for potential Spacelab and Space Station users in Italy, Austria and Switzerland, which included some ET Applications.

    1983

    Space Station Plans Take Shape, Brian O’Leary, March 1983

    ET Applications in Space, T. Taylor of Taylor & Associates, Inc., June 1983, Martin Marietta, Pamela Mitchell, A conceptual study on various near term and long term uses for the ET in Orbit, which concentrated on the use of the ET as an evolving platform concept.

    Workshop on Possible Applications Of the STS external Tank to Life Support, and to Chemical and Life Sciences, California Space Institute, Sep. 1983

    Aft Cargo Carrier (ACC) Scale Model, Thomas C. Taylor of Taylor & Associates, Inc., 1983, Martin Marietta Corporation, Denver Aerospace, Michoud Operations Advanced Programs; Tom Mobley of Advanced Programs, Contract built a 1:20 Scale Model of the ACC as it attached to the aft of the ET. The ACC doubles the available P/L Volume of the shuttle.

    Large Diameter Reflector (LDR) in the ACC, T. Taylor of Taylor & Associates, Inc., Final Report 1983, R. Bruce Pittman, Study Manager, Advanced Studies group, NASA-Ames, Moffett Field, CA, A conceptual study increasing the LDR Panel diameter from 15’ to 25’ and exploring the transport and assembly of the LDR panels in orbit at reduced cost. The resulting cost reduction included fewer panels (19 to 7), less on orbit assembly EVA and potentially less cost.

    Performance Enhancement of the Space Transportation System, T. Taylor of Taylor & Associates, Inc., Proposal & Phase I Award, U.S.Air Force Small Business Innovation Research (SBIR), Dept. of the Air Force, Air Force Systems Command, Wright-Paterson, AFB, OH 45433, Proposed 15’ aerodynamic flow altering projection on the forward end of the ET to enhance the ascent performance and to act as a structural connection at various facilities in orbit. Resulted in ~8% performance enhancement based on Mar. 1984 U.S. Air Force Academy Tri-Sonic Wind Tunnel tests performed by Remtech, Inc. under subcontract to GOI. Evolved into two patents: United States Patent 4,650,139, Taylor, et. al., Issued Mar. 17, 1987, United States Patent 4,790,499, Taylor, Cerna, et. al., Issued Dec. 13, 1988

    1984

    External Tank/Aft Cargo Carrier – On-Orbit Applications, Martin Marietta, Advanced Programs Group at Michoud Assembly Facility, Nov. 1984, NASA-Marshall Space Flight Center, Huntsville, AL., One of ~ $8m in NASA Contract Studies performed by ET manufacturer on a aft pod for transporting cargo under the ET. Concept resulted in baseline ACC design requiring a ~$300m new start program.

    LDR Study, T. Taylor of Taylor & Associates, Inc. 1984-5, Lockheed Missiles and Space Company Inc., NASA Ames LDR Study, Assisted as a consultant on the NASA-Ames investigation of concepts using the ACC and other launch vehicles to transport LDR panels to orbit.

    Aft Cargo Carrier (ACC) Video, Taylor & Associates, Inc., 1984, Martin Marietta Corporation, Denver Aerospace, Michoud Operations, Tom Mobley was point of contact. Contract study produced a marketing video of the Aft Cargo Carrier (ACC) including four each nine minute scripted segments which each depicting one of the ACC payloads deployed on orbit and emphasizes the large design diameter of the ACC. Subcontractor Richard Dowling of Space Media in Hollywood was the director and provided both animation and stock video.

    1985

    External Tank Applications in Space Manufacturing, J. Alex Gimarc, 1/85

    1986

    The Transportation Node Platform in Orbit, Thomas C. Taylor, AFSC/NSIA Cost Reduction and Cost Credibility Workshop, Final Report, 1986, Suggested cost effective utilization of the ET in orbit with detailed cost estimates for using the ET and tethers.

    The Shroud Enhancement with the Aerospace Concept, Thomas C. Taylor, AFSC/NSIA Cost Reduction and Cost Credibility Workshop, Final Report, 1986, Suggested drag reduction techniques for the transport of the ET to orbit based on USAF SBIR Phase 1 Wind Tunnel Experiments at the Tri-Sonic Wind Tunnel, U.S. Air Force Academy and patent.

    1987

    Mission and Systems Implications of Phobos-Deimos Propellant Processing and the Use of the Shuttle External Tank, Dr. Brian O’Leary, Aug 1987

    External Tank Torus Concept, Thomas C. Taylor, 1987, Space Resort Enterprise, Inc., John Spencer Director, Expanded torus concept for use as a surface resort and training area for space tourists going to a partial gravity orbital torus resort.

    1988

    External Tank Gamma Ray Imaging Telescope Study, Martin Marietta, Feb 1988

    Preliminary Data for On-Orbit Utilization of the External Tank, MSFC – PD, Apr. 1988

    Feasibility study of an External Tank Gamma Ray Imaging Telescope, MSFC – PD, Jun 1988

    The President’s Space Policy and Commercial Space Initiative to begin the Next Century,” President Ronald Reagan, Feb. 3, 1988, fact sheet documents

    NASA Support for President’s Commercial Space initiative Involving Private Use of Space Shuttle’s External Tanks, Commerce Business Daily, Special Notice, Jun. 1, 1988, page 32, Issue No. PSA-9602, Result: 11 proposal and 3 awards by NASA with Memos of Understandings.

    NASA/UNCAR ET MOA, several revisions, Sep 1988

    The NASA-GLOBAL OUTPOST Enabling Agreement, Sept 1998, Revisions 1. 2 and 3, Signed with NASA Hqs, placed $ 100k deposit for five ET’s in orbit, Resulted in “appears technically feasible” NASA letter, dated 19 Feb 93,

    NASA/”Third Group” ET MOA, University of Colorado, 1988

    1989

    Evolution of External Tank Applications, J. Gimarc, Jan 1989, funded by Space Studies Institute, Detailed summary of the evolution of the ET in Orbit applications.

    Intertank Door Pallet Concept Definition Study, Martin Marietta, Aug 1989, External Tanks Corp., Randolph Ware, Tom Rogers, Phil Culbertson, Proposed an Access Door in the ET’s.

    1990

    External Tank Debris Monitoring Platform (ET_PMP), Martin Marietta, Jan 1990

    Outpost Platform Business Plan, T. Taylor, Global Outpost, Inc., 1990, Proposed an unmanned Commercial Service Platform in orbit derived from the external tank salvage with NASA cooperation.

    (Studies continuing with interest expressed from various California political leaders), Gene Meyers, etc
    External Tank Gamma Ray Imaging Telescope (GRIT) Study, Martin Marietta, 5/90

    1991

    Commercial Robotics Service in Orbit, Dr. Jackson C.S. Yang, Director Robotics Lab, Univ. of Maryland, Phase I $ 75k, 12 months, completed Aug 15,1991, Maryland Industrial Partnerships (MIPS), State Research MIPS Grant study to GOI for university researchers to create ET derived Platform Robotics using the S.A.T Arm to assemble a GOI Truss, nuclear test platform support hardware and video animation.

    Safety and Reliability of Space Nuclear Reactors, Commercial Space Nuclear Power in Orbit on the OUTPOST Platform, Dr. Marvin L. Roush study director, Univ. of Maryland, Phase I $ 75k, 12 months, completed Nov 1991, Maryland Industrial Partnerships (MIPS), State Research MIPS Grant to confirm the market for reliable safety testing of Space Nuclear Reactors in orbit on the ET Platform.

    1992

    Commercial Hardware Cooperation – A carrier to Transport Commercial Payloads to and from the GLOBAL OUTPOST Platform, T, Taylor of GLOBAL OUTPOST, Inc. & Dr. David Akin of the University of Maryland Space Systems Lab, June 1992, Conceptual study exploring the transport, docking and use of commercial hardware on the GOI Platform including neutral buoyancy underwater test.

    Technical Feasibility Study, Tom Mobley, 1992, Advanced Programs, Michoud Assembly Facility, Martin Marietta Aerospace, GLOBAL OUTPOST, Inc., A GOI funded contract with MMA to assess the technical details of the GOI salvage of the ET with NASA cooperation on orbit and the methods used to accomplish the technical objectives.

    GLOBAL OUTPOST Technical Feasibility Assessment Results – An Executive Summary, Jeffrey G. Williams/TC3, SSP Integration & Opns, April 30, 1992, NASA-JSC, Chuck Pace, A summary of the first GOI mission as reviewed by the Shuttle Program Office, which resulted in Feb 19,1993 NASA letter saying “appears technically feasible.”

    Commercial Robotics Service in Orbit, Dr. Jackson C.S. Yang, Director Robotics Lab, Univ. of Maryland, Phase II $ 75k, 12 months, completed Aug 15,1992, Maryland Industrial Partnerships (MIPS), State Research MIPS Grant study to GOI for university researchers to create ET derived Platform Robotics, nuclear test platform support hardware and video animation models.

    Safety and Reliability of Space Nuclear Reactors, Commercial Space Nuclear Power in Orbit on the OUTPOST Platform, Dr. Marvin L. Roush study director, Univ. of Maryland, Phase II $ 75k, 12 months, completed Nov 1991, Maryland Industrial Partnerships (MIPS), State Research MIPS Grant to confirm the reliability and safety issues related to testing the Russian TOPAZ II Space Nuclear Reactor in orbit on the ET Platform.

    1993

    What the United States Must Do to Realize the Economic Promise of Space, Who would build a Second Space Station?, T. F. Rogers, Chairman, 17 Dec 1993, A Report of the Aerospace Research and Development Policy Committee of IEEE, Recommends increased use of the external tank.

    Commercial Robotics Service in Orbit, Dr. Jackson C.S. Yang, Director Robotics Lab, Univ. of Maryland, Phase III $ 75k, 12 months, completed Aug 15,1992, Maryland Industrial Partnerships (MIPS), State Research MIPS Grant study to GOI for university researchers to complete the ET derived Platform Robotics assembling the NASA Langley truss in orbit, docking the TOPAZ II at the nuclear test platform and color video animation.

    Safety and Reliability of Space Nuclear Reactors, Commercial Space Nuclear Power in Orbit on the OUTPOST Platform, Dr. Marvin L. Roush study director, Univ. of Maryland, Phase III $ 75k, 12 months, completed Nov 1991, Maryland Industrial Partnerships (MIPS), State Research MIPS Grant to work out the docking safety issues related to testing and disposal of the Russian TOPAZ II Space Nuclear Reactor in orbit on the ET Platform.

    GOI Neutral Buoyancy Simulations, Dr. David L. Akin, Aerospace Engineering Department, University of Maryland, 1993, GOI purchased underwater simulations in the Space Systems Lab 50’ diameter, 25’ deep tank with full scale models of GOI Platform interfaces, TOPAZ II hardware, OMV and modified Soviet docking adapter including color animation and underwater videos.

    Commercial Nuclear Power Services in Orbit, GLOBAL OUTPOST, Inc. for Lyle Rutger, U.S. Department of Energy, Mar 1993, Computer and underwater simulations plus a video promoting U. S. Space Nuclear Power development in the United States. Produced an 8 minutes color video with sound.

    TOPAZ II Study, GLOBAL OUTPOST, Inc., Fred Tarantino, White House Science Office, Richard L. Verga, Strategic Defense Initiative Organization, Daniel M. Mulder, W. J. Schafer Assoc. Inc., 1993

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