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AIAA 2000-5294

Space Building a Permanent Bridge for Space Exploration and Economic Development

David V. Smitherman, Jr. Marshall Space Flight Center National Aeronautics and Space Administration

AIAA Space 2000 Conference & Exposition September 19-21,2000

For permission to copy or republish, contact the American Institute of Aeronautics and Astronautics, 370 L'Enfant Promenade, SW, Washington, D.C. 20024 SPACE ELEVATORS: BUILDING A PERMANENT BRIDGE FOR SPACE EXPLORATION AND ECONOMIC DEVELOPMENT

David V. Smitherman, Jr.* NASA Marshall Space Flight Center Mail Code: FD02 Flight Projects Directorate Huntsville, Alabama 35812 Tel. 256-544-8522 Fax. 256-544-5861 E-mail. David.Smitherman_msfc..gov

Abstract number of issues will be discussed that has helped to bring the space concept out of the realm of A is a physical connection from the science fiction and into the realm of possibility. In surface of the to a geo-stationary above conclusion, it appears that the space elevator concept the Earth approximately 35,786km in altitude. Its may well he possible in the latter part of the 21_ is at the geo-stationary point such that century if proper planning and it has a 24-hour orbit, and stays over the same point development is emphasized to resolve key issues in above the equator as the Earth rotates on its axis. The the development of this advanced space infrastructure structure is utilized as a transportation and utility concept. system for moving payloads, power, and gases between the surface of the Earth and space. It makes 1, History the physical connection from Earth to space in the The idea of building a tower from the surface of the same way a bridge connects two cities across a body Earth into space, the sky, or the heavens dates back to of water (Fig. 1). The space elevator may be an some of the very earliest known manuscripts in important concept for the future development of existence. The writings of Moses reference an earlier space in the latter part of the 21 _tcentury. It has the civilization that in about 2100 BC tried to build a potential to provide mass-transportation to space in tower to heaven out of brick and tar. About 1900 BC, the same way highways, railroads, power lines, and Jacob had a dream about a staircase or ladder built up pipelines provide mass-transportation across the to heaven, commonly called Jacob's Ladder. The idea Earth's surface. The low energy requirements for for building a structure from Earth into space, has moving payloads up and down the elevator make it been dreamed, invented, and reinvented, many times one of only a few concepts that has the potential of throughout modern civilization. lowering the cost to orbit to less than $10 per kilogram. Today, the world's tallest structure is a stayed, television-transmitting tower near Fargo, North This paper will summarize the findings from a Dakota, USA, that stands 629 m high. The CN Tower 1999 NASA workshop on Space Elevators held at the in Toronto, Ontario Canada is the world's tallest NASA Marshall Space Flight Center (MSFC). The building at 553 m in height. The world's tallest office workshop was sponsored by the Advanced Projects building is the Petronas Towers in Kuala Lumpur that Office in the Flight Projects Directorate at MSFC, stand 452 m in height, about 10 m taller than the and was organized in cooperation with the Advanced Sears Tower in Chicago, Illinois. The height of Space Transportation Program at MSFC and the existing towers and buildings today are not limited by Advanced Concepts Office in the Office of Space construction technology or by materials strength. Flight at NASA Headquarters. New concepts will be examined for space elevator construction and a 2. Key Findings

*Architect,Technical Manager, Advanced Projects Office. Several key findings were identified during the workshop that helped determine thc overall Copyright © 2000 by the American Institute of Aeronautics and feasibility of a space elevator as a possible future Astronautics, Inc. No copyright is asserted in the under Tille 17. U.S. Code. The U.S. Govermnent has a royally-free project instead of being pure science fiction. The key license t_)exercise all rigbls underlhe copyright claimed herein for findings included: government purposes. All other rights are reserved by the copyright owner. I. The materials technology needed for space elevator construction is in the development

American InstituteorAeronautics and Astronautics 1 process in laboratories today. Continued research platform works like a seaport where cargo and and development will likely produce the high- passengers make their transfers from terrestrial strength materials needed for efficient space transportation systems to the space elevator vehicles. elevator construction and for a wide variety of A sea platform was selected because it illustrates that new and improved products. a remote location in international waters would be appropriate for a project of this scope that will 2. The tallest structure today is 629 meters in probably require international cooperation and height. Buildings and towers can be constructed consensus to succeed. There was some discussion, many kilometers in height today using without resolution, over whether the base would be conventional construction materials and fixed to the ocean floor or could actually float and methods. These heights have not been attempted because there has not been a demonstrated need. move if needed. Figure 1B illustrates a concept for Advanced materials and new construction high-altitude support and control of the elevator tower through the use of inflatable platforms, which methods could make it possible to construct may also be useful during early construction phases. towers tens and perhaps hundreds of kilometers in height. From the top of the tower to the station at GEO is a long 36,000-kin ride. The best concept for traveling 3. A tether structure hanging down from GEO this structure is an electromagnetic-propelled vehicle connected to a tall tower constructed up from the that can travel thousands of kilometers per hour. Earth, through most of the Earth's atmosphere, Figure tC illustrates a concept for this vehicle, appears to be the most efficient and technically suspended in a track, with no moving parts (wheels) feasible method for space elevator construction. in contact with the elevator rails. Any other type of 4. Climatic conditions at the equatorial zone are mechanical system would require traction wheels that very mild in comparison to more northern and would be much slower and cause considerable wear southern latitudes, making construction along the on the vehicle and the elevator structure. The vehicle equator ideal from a weather hazard standpoint. is completely reusable, and returns to the base port on It is not physically possible for hurricanes and Earth, transferring passengers and cargo up and down tornadoes to form at the equator. the structure. At the GEO transfer station, Fig. ID, passengers and cargo are _ransferred into the station 5. The space elevator structure is inherently flexible or to outbound space transfer vehicles. This station is and can be designed to avoid major hazards. the center of mass for the total system, therefore large Minor hits from debris are inevitable reels are present to adjust the location of the station, and will require standard repair procedures. A tension of the structure, and the counterbalance mass. simple analogy is to think of the space elevator An inflatable habitation structure is also present for structure as a 36,000kin hmg interstate, or living and working environments. Outbound vehicles railroad, that requires ongoing maintenance and can continue on the elevator track through the repair. asteroid counterbalance mass, Fig. IE, to the end of 3. A Space Elevator Concept the structure at 47,000 km altitude. Launch to the or other deep space destinations requires A baseline concept for a space elevator was created minimal energy because the rotation of the elevator during the workshop to illustrate its purpose, scale, acts like a sling beyond GEO to throw its payloads and complexity. The concept, as shown in Figure 1A, out of orbit. is envisioned to emerge from a platform at sea. The

American Instituteor Aeronautics and Astronautics 2 _N

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Figure 1. A Space Elevator Concept

American Institute or Aeronautics and Astronautics 3 4._Technoloev space construction and space industrial development. Key components include The Earth to GEO space elevator is not feasible highly reusable systems, today, but could be an important concept for the reusable in-space transportation, and space future development of space in the latter part of the facility support from LEO to GEO. 21 _' century. The low energy requirements for moving payloads up and clown the elevator could As part of the workshop, these were make it possible to achieve cost to orbit bellow 10 organized into technology demonstration roadmaps dollars per kilogram. This makes consideration of the for input into NASA technology programs as well as technology paths required for space elevator the technology initiatives of other agencies and construction very important today. Five primary organizations. Technology demonstrations were technology thrusts were identified that are critical to identified for tethers, towers, and electromagnetic the development of space elevators in the 21 st systems as being critical to a technology progression century. The five technology areas are: towards space elevator construction capabilities during the 21 _t century. Figure 2 illustrates one 1. Materials: Develop advanced high strength logical course of events over an indefinite period of materials like the , alumina, and time leading up to the full-scale development of whiskers that exhibit laboratory Earth to GEO space elevators. The intent is to show strengths over 20 G Pa. Continue that these technology demonstrations and development of the nanotube developments can provide incremental benefits and materials that exhibit strengths 100 times are logical to pursue for their own merit in addition to stronger than steel. Introduce these new their obvious relationship to future space elevator lightweight, high-strength materials to the developments. commercial, space and military markets for new and improved product developments. One of the most common misconceptions about the space elevator concept is the assertion that 2. Tension Structures: Continue development of materials strong enough to span the 36,000-km height technologies for space from the surface of the Earth to GEO are unavailable. transportation systems to gain experience in But, it is theoretically possible to build a structure of the deployment and control of long this size out of any common structural material by structures. Utilize higher strength materials, simply increasing its thickness to compensate for the as they become available. Continue analysis high tensile or compressive loads. The problem is on momentum exchange and LEO space that for most readily available construction materials elevator facilities for low cost in-space transfer to GEO. it simply is not practical, due to the massive quantity and associated cost that would be involved. So 3. Towers: Introduce lightweight composite finding the right material in combination with the structural materials to the general right construction method is the key to success. construction industry for the development of The problem with compression structures or tall tall tower and building construction systems. towers is that thilure is usually through buckling, and Foster the development of multi-kilometer most materials are actually stronger in tension than in height towers for commercial applications compression. So, the ideal structure will likely be a (i.e., communications, science observatories, combination of a tall tower in compression connected and launch platforms). to a tension structure. Many early engineering 4. Electromagnetic Propulsion: Develop high- concepts for the Earth to GEO space elevator have speed electromagnetic propulsion systems for assumed a requirement for a -filament cable. mass transportation systems (Maglev), launch Diamond was used because it exhibited the strongest assist systems (Maglifter), and high velocity tensile and compressive material strengths available launch rails. at that time.

5. Space Infrastruq_re: Develop transportation, utility, and facility infrastructures to support

American InstituteorAeronautics and Astronautics 4 Figure 2. Technology Demonstration Roadmap (see also Fig. 4)

4.1 Materials important (see Fig. 3). At present, production of nanotubes is very expensive and limited in The lightest and strongest materials readily quantity. However, there are numerous commercial available today are the graphite epoxy composite applications for materials in existing materials that are commonly used in aerospace markets, and potentially many new applications that applications: but, the material strengths required for cannot be envisioned today. Space elevator materials space elevator development appear to be far more strength requirements is thought to be about 62.5 G demanding. Continued development of a new Pa. Carbon Nanotubes have measured strengths in material known as carbon nanotubes that has excess of 20 G Pa with theoretical strengths in excess exhibited laboratory strengths 100 times stronger of the 62.5 G Pa needed. than steel with only a fraction of the weight will be

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Strength comparison of carbon nanotubes Space Elevator requirements in comparison with (SWNT) with steel theoretical strength of carbon nanotubes

Figure 3. Space Elevator Materials Strength

American Institute or Aeronautics and Astronautics 5 4.2 Tension Structures structures are illustrated in Figure 4. The second technology area is in the continued development of tension structures for space 4.3 Towers applications. This includes a LEO space elevator as The third technology area is in the continued an intermediate version of the Earth to GEO space development of tall towers for Earth applications. elevator concept, and appears to be feasible today This requires the introduction of lightweight using existing high-strength materials and space composite structural materials to the general technology. It works by placing the system's construction industry for the development of tall midpoint station, and center of , in a relatively tower and building construction systems. However, low-Earth orbit and extending one cable down so that one of the most fundamental problems with high- it points toward the center of the Earth and a second strength materials is that they are typically stronger in cable up so that it points away form the Earth. The tension than in compression. Therefore what is bottmn end of the lower cable hangs down to just needed is a way to convert tensile strength into above the Earth's atmosphere such that a future sub- compressive strength. orbital space plane transferring a payload up fi'om the Earth's surface would require less change in velocity. By converting tensile strength into compressive A payload delivered to the lower end is then lifted to strength with a pressurized shell, PBO can be the upper end and released into a GEO transfer orbit. used to build towers many times taller than would The overall length of a LEO space elevator from the otherwise be possible. The tower would be bottom end of its lower cable to the top end of its constructed in segments with bulkheads to keep the upper cable is anywhere from 2,000 to 4,000 km, pressurized gas from migrating to the bottom of the depending on the amount of launch vehicle AV tower. A problem with a tower of this height is reduction desired. If a resonant orbit is used, the failure through buckling. Although the PBO lower end of the system will pass within range of materials in combination with a pressurized system most of the world's major airports twice a day on a can likely handle the compressive loads, some type of fixed schedule. Periodic re-boost of the LEO space active stabilization system would be required to keep elevator will be required to compensate for the drag the tower vertical. Today, many tall buildings include of the lower end in the upper atmosphere and the active control systems to control movement from capture and transfer of payloads to higher levels on high winds and earthquakes. the elevator structure. A variety of Earth orbiting and

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Figure 4. Earth Orbiting And Lunar Space Elevator Concepts.

American Institute or Aeronautics and Astronautics 5 4.4 Electromagnetic Propulsion platforms to support a growing economy in space. In support of the vehicles, stations, and platforms, a The purpose for exploring electromagnetic network of propellant production, delivery, and technology for the elevator is to develop a means by storage systems will be needed. It is likely that which vehicles can climb up and down the elevator human activities on orbit will include tourism and without contacting the structure or guide rails. This permanent residency in new -type would be very low maintenance on both the vehicles facilities called space business parks before space and the structure, and potentially very fast. High- elevator developments can be supported speed systems in the thousands of kilometers per economically. hour arc desirable due to the great length to be traveled in space. Energy is used during the initial lift Solar-powered systems in space are in common and acceleration phase up the elevator. With an use today on the ISS and most Earth-orbiting electromagnetic system there is the potential that . There is an abundance of solar energy electrical energy could be recovered in the braking available, and technology work is in progress to phase to slow the vehicle down. improve the performance of these systems. Advances in the development of solar cell films may make it There are several intermediate steps that can be possible for the surface of the space elevator to taken toward reaching this level of technology. These become a solar collector. The first Earth to GEO have been grouped into Iow-g and high-g systems. tether structure used to make the initial connection Low-g systems are for human-rated vehicles that could be used as a direct power line. Initially, power have accelerations up to =3-g (3 Earth gravity levels). would be delivered up the elevator for construction Two concepts for use of electromagnetic propulsion support at GEO, and later power would be delivered in low-g systems are the MagLev for magnetically down the elevator to ground utility systems from levitated train systems, and the MagLifter for solar powcr satellites stationed in GEO. magnetic lift to provide launch assist for space launch vehicles. High-g systems are for nonhuman payloads Developments at the Moon and could that can survive high accelerations in the hundreds have an important role to play in the overall plan to and thousands of gees. Two concepts for high-g develop and demonstrate the technology for a space propulsion systems are the rail gun and the mass elevator. Development of space resources for driver (or coil gun). It is not yet evident which of the materials and propellants will likely play an electromagnetic systems would be best suited for the important role in overall space development as well space elevator or exactly how any of them could be as support development of the space elevator. Of integrated into a space elevator structure that needs to particular interest is the concept for utilizing an be as light as possible. asteroid as a for the space elevator and mining its resources to produce some of its 4.5 Space Infrastructure construction materials.

Today, our progress in space development is The interesting thing about the space elevator restricted to single projects of limited scope in LEO. concept and space development in general is that the Significant expansion of space infrastructure will be opportunity is here to chart a course for expansion necessary to create the economic base and the that is no longer limited to the physical constraints of construction capabilities needed for major Earth resources. Through development of these devel_pments beyond LEO. A mature space technologies and infrastructures there will be many transportation system from Earth to GEO will be new benefits, products, and services that cannot needed to l'acilitatc space elevator construction. possibly be envisioned at this time from an Earth These transportation systems should not be built perspective. exclusively for space elevator construction. The space elevator concept will only be successful if it is 5. Safety Issues done in support of a growing economy in space Major issues related to the space elevator concept where people are actively working to make this new tended to focus on either environmental or safety frontier their home. The transportation systems must concerns. The environmental issues dealt primarily also be lnultipurposc and highly reusable to support with the effects the natural environment on Earth and frequent flights comparable to today's airplane. in space would have on the space elevator system. Transportation support facilities from Earth to Some of these concerns led to safety issues for people GEO would include space stations and servicing traveling on the elevator as well as for others on

American Instituteor Aeronauticsand Astronautics 5 Earth and in space in the event of a catastrophic science fiction. More detailed analysis of the system failure. indicates that it is indeed very complex, but it is comparable to other Earth-based infrastructures that The single greatest safety concern identified have been built over many years. Many benefits were centered on the hazards caused by potential collisions identified that supported the pursuit of space elevator between the elevator structure and other objects in technology, most of which centered on the potential orbit. Cleanup of orbital debris was identified as a for low-cost mass transportation capabilities to space. high priority that needed to be done to protect all However, there are many questions and problems to future spacecraft. Small debris materials <1 mm in be resolved before space elevators can be considered diameter are numerous and can cause erosion of economically feasible. The space elevator is not a spacecraft surfaces. The real problem is with debris near-term project, but can now be considered a good and incoming meteoroids in the 1 mm to 10 cm size. potential project tbr the latter part of the 212' century. They are difficult to track with current technology and can cause significant damage to spacecraft BiblioeraDhy systems. and meteoroids I mm to 10 cm New Space Industries For The New Millennium, in diameter are thought to be many times greater in NASA/CP--1998-209006, Marshall Space Flight number than the known tracked objects. Center, AL, December 1998. 6. Conclusion Space Elevators: An Advanced Earth-Space Tile massive size and complexity of the space Infrastructure for the New Millennium, elevator concept is often cited as making such a NASA/CP--2000-210429, Marshall Space Flight system impossible to conceive except in the realm of Center, August 2000.

American Institute orAeronautics and Astronautics 5