c . d 8# CRITICAL TECHNOLOGIES FOR THE DEVELOPMENT OF FUTURE SPACE ELEVATOR SYSTEMS David V. Smitherman, Jr., Architect Technical Manager, Advanced Concepts Office NASA Marshall Space Flight Center, Huntsville, Alabama, USA E-mail: David.Smitherman@,nasa.gov ABSTRACT: A space elevator is a tether structure extending through geosynchronous earth orbit (GEO) to the surface of the earth. Its center of mass is in GEO such that it orbits the earth in sync with the earth’s rotation. In 2004 and 2005, the NASA Marshall Space Flight Center and the Institute for Scientific Research, Inc. worked under a cooperative agreement to research the feasibility of space elevator systems, and to advance the critical technologies required for the future development of space elevators for earth to orbit transportation. The discovery of carbon nanotubes in the early 1990’s was the first indication that it might be possible to develop materials strong enough to make space elevator construction feasible. a This report presents an overview of some of the latest NASA sponsored research on space elevator design, and the systems and materials that will be required to make space elevator construction possible. In conclusion, the most critical technology for earth-based space elevators is the successful development of ultra high strength carbon nanotube reinforced composites for ribbon construction in the lOOGPa range. In addition, many intermediate technology goals and demonstration missions for the space elevator can provide significant advancements to other spaceflight and terrestrial applications. INTRODUCTION length of about 100,OOOkm. Its center of mass would be in GEO such that it would orbit the During the past year, through a cooperative earth in sync with the earth’s rotation. Once in agreement, the Institute for Scientific Research, place, the structure would be used to deliver Inc., (ISR) in Fairmont, West Virginia, and the payloads from earth to orbit at a cost that could u NASA Marshall Space Flight Center in be many times lower than conventional rocket Huntsville, Alabama, have been studying a launch systems, and could provide many other concept for space elevator construction and benefits by having a much larger capacity, and a operations first proposed by Dr. Bradley more benign launch environment. Edwards through a study grant from the NASA Institute for Advanced Concepts (NIAC) as The space elevator, being of interest to NASA, described in his book The Space Elevator’. The has been examined over the years to determine concept proposes deployment of a ribbon, from the technologies required, their current state of GEO to the Earth’s surface, that would be readiness, and overall feasibility. In 1984, several meters in width, thinner than a piece of Georg von Tiesenhausen referenced several paper, and because of the counter balance mass space elevator concepts in a NASA technical required, would extend beyond GEO to a total manual, “Tethers in Space: Birth and Growth of a New Avenue to Space Utilization’,” speculating what the future of tethers might be beyond the Tethered Satellite System missions that were under development at that time; see Figure 1. 1 F ‘I c r’ Figure 1: Early orbital tower and skyhook concepts’ In 1999, after many reports on the possible be developed, it appears that the other applications of carbon nanotubes, the NASA challenges have reasonable technological or Marshall Space Flight Center conducted a operational solutions that can be pursued to workshop3 to examine the feasibility of the make space elevators feasible. space elevator concept if a suitable carbon nanotube tether . could be developed, and identified many other technologies required and TECHNOLOGY READINESS some general design and operational considerations. In the 2000-2002 timefiame, NASA often uses a system for ranking NIAC fimded Dr. Bradley Edwards to further technologies called the Technology Readiness examine a new simpler concept for construction Level4 (TRL) to help managers make decisions of a space elevator utilizing a carbon nanotube on whether a particular technology of interest composite ribbon and robotic climbers, which should be fknded for further development, or is represents the baseline concept under study ready for inclusion in a flight program, Figure 2. today. And finally, following the NIAC study, There are 9 TRL levels, and in general they can NASA and the Institute for Scientific Research, be described as TRL 1-2, basic technology Inc., have conducted this study of the concept. research; TRL 3-5, technology development in In general, each of these examinations had laboratory experiments and prototypes, and TRL similar conclusions, in that the overall concept is 6-9, integrated systems that are ready for flight. quite large with many challenges, but if the Program managers consider the TRL ranking carbon nanotube composite ribbon material can when examining program risk because they . 2 E c 1’ ~ System Test. Laun Actual system “flight proven”through successful mission & Operations operations Actual system completed and “flightqualified” through test SystemEubsystem and demonstration (Ground or Flight) System prototype demonstration in a space environment Technology DemOnStration Systemlsubsystem model or prototype demonstration in a relevant environment (Ground or Space) Component andlor breadboard validation in relevant environment Component and/or breadboard validation in laboratory environment Research to Prwe FeaSarUty Analytical and experimental critical function and/or characteristic proof-of-concept Basic Tachnology Technology concept and/or application formulated Basic principles observed and reported Figure 2: Technology Readiness Level know that any technology at TRZ. 5, and lower, importance to the development of future space brings with it the potential for cost increases and elevator systems. It is important to remember schedule delays if the technology is on the that many of these systems are either already critical path for their program. under development in other programs, or could be developed for other purposes that would have TRL Evaluation: broad applications to many space and terrestrial The space elevator is of interest to NASA as a systems in addition to future space elevator potential future system, but is not a funded developments. program or project because of its low ranking on the TRL scale. In particular, the ribbon material Carbon Nanotube (CNn Development: is perhaps the single most critical technology CNT research and development has been in development, which will need to be made of progress for over a decade at many research what we now refer to as an ultra high strength institutions around the world. Of particular carbon nanotube reinforced composite. Stresses interest is the development of single wall carbon in the ribbon have been calculated to be in the nanotubes because of their ultra high strength 60GPa to 80GPa range with an anticipated characteristics in the 15OGPa range. Lacking in strength requirement of at least 100GPa. The this field of research are controlled growth carbon nanotube is the primary candidate methods that will yield long, aligned, or material that might possibly provide composite continuous length tubes, testing and constructions in this range with a reported measurement standards for reporting of research strength above 15OGPa. There are many findings, and ultimate predictions on potential systems in the space elevator that have been production rates and cost. These developments examined and ranked on the TRL scale’. But, will have significant impact on what can or the lowest ranking and perhaps most critical cannot be accomplished in the development of systems that need developing are as follows in the ultra high strength carbon nanotube the next section. reinforced composite ribbon for the space elevator. Because of the successful laboratory Low TRL Systems (TRL 1-51: experiments to date, CNT technology can be The following systems have been examined and safely labeled at a TRL 4 rating. determined to be at a low TRL and of critical 3 CNT Composite Fiber Development: cross section and thereby help reduce wind Ultra high strength CNT reinforced composites loading. development consists of incorporating large 2) Tape Sandwich Construction: Several quantities of CNT into a suitable composite fiber ribbon fabrication methods have been that will provide overall strength to weight examined, each with particular advantages characteristics in the lOOGPa range. This will or disadvantages. The tapped sandwich require development of methods for CNT construction consists of a cross member that dispersion and alignment in the fiber, bonding keeps vertical fibers in the ribbon properly between the CNT and the fiber matrix, and a aligned and will transfer the load from a combination of these two that will yield high broken fiber to an adjacent fiber. CNT loading. In addition, the process must be Alternatives to this approach include woven scalable for mass production, be adaptable to methods that will transfer loads between surface coatings for protection from the space fibers as needed. environment, and be a fiber that is adaptable to 3) Environmental Coatings: The space fabrication techniques for ribbon construction, environment introduces atomic oxygen and splicing, damage repair, and maintenance of radiation that catl deteriorate fibers and the fibers and coatings. This technology is perhaps composite matrix that binds them together. the most important leap for future space