2

Using the International Space Station for Engineering Research and Technology Development

Engineering research and technology development on the space station could produce many benefits but are not emphasized in NASA's current plans for the ISS. This chapter discusses (1) the capability of the ISS to provide support for different kinds of ERTD, (2) the kinds of ERTD that could most appropriately be performed on the ISS, (3) the kinds of benefits that might be gained from such activities, and (4) strategies to prioritize NASA's ERTD activities on the ISS. Chapter 3 explores in more detail nine technical areas (electric power, robotics, propulsion, thermal control, life-support, space environment and effects, structures, communications, and autonomous systems) that illustrate the range of potential ERTD on the ISS.

CAPABILITIES OF THE ISS TO SUPPORT ERTD

The ISS, as currently planned, will have many features that could make it a unique facility for engineering research and technology development:

  • Approximately 50 kW of electrical power will be available to researchers, compared to about 7 kW for a Spacelab mission on the Space Shuttle and perhaps 4.5 kW on the Russian Mir.

  • The ISS will have six external attachment points on the truss and 10 external locations on the Japanese Experiment Module (see figure 2-1) where experimental apparatuses can be exposed to the LEO environment. Electric power and data transfer capabilities will be available at these sites.

  • The pressurized volume of the ISS will be approximately 1,120 cubic meters, compared to about 160 cubic meters on a Spacelab flight and



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Engineering Research and Technology Development on the Space Station 2 Using the International Space Station for Engineering Research and Technology Development Engineering research and technology development on the space station could produce many benefits but are not emphasized in NASA's current plans for the ISS. This chapter discusses (1) the capability of the ISS to provide support for different kinds of ERTD, (2) the kinds of ERTD that could most appropriately be performed on the ISS, (3) the kinds of benefits that might be gained from such activities, and (4) strategies to prioritize NASA's ERTD activities on the ISS. Chapter 3 explores in more detail nine technical areas (electric power, robotics, propulsion, thermal control, life-support, space environment and effects, structures, communications, and autonomous systems) that illustrate the range of potential ERTD on the ISS. CAPABILITIES OF THE ISS TO SUPPORT ERTD The ISS, as currently planned, will have many features that could make it a unique facility for engineering research and technology development: Approximately 50 kW of electrical power will be available to researchers, compared to about 7 kW for a Spacelab mission on the Space Shuttle and perhaps 4.5 kW on the Russian Mir. The ISS will have six external attachment points on the truss and 10 external locations on the Japanese Experiment Module (see figure 2-1) where experimental apparatuses can be exposed to the LEO environment. Electric power and data transfer capabilities will be available at these sites. The pressurized volume of the ISS will be approximately 1,120 cubic meters, compared to about 160 cubic meters on a Spacelab flight and

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Engineering Research and Technology Development on the Space Station about 400 cubic meters on Mir. The ISS will have 33 international standard payload racks ISPRs) (see box 2-1). Figure 2-2 shows an empty ISPR. Frequent opportunities (including up to seven shuttle flights a year) will be available to deliver new experiments, experiment components, or samples to the ISS and return used items to Earth for further analysis. This ISS will have a significant communications capability, including a 50 megabits/second (Mbps) link through the tracking and data relay satellite system (TDRSS). Limits on the duration of experiments will be eliminated. Experiments on the Space Shuttle, by comparison, have a maximum duration of about two weeks, too short a time for research on the long-term behavior of materials and technologies in the space environment. The crew of six will provide at least 23 person-hours per day in support of experiments. The presence of the crew will enable the conduct of experiments that require human interaction and will allow other ERTD experiments to be monitored and reconfigured more easily. Moreover, the crew—like crews on some Space Shuttle and other missions —may be able to repair malfunctioning experiments. FIGURE 2-1 ISS attached payload sites. Source: NASA

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Engineering Research and Technology Development on the Space Station BOX 2-1 Capabilities of the ISPR The international standard payload rack (ISPR) will be the primary internal location for experiments on the ISS. Thirty-three ISPRs will be fitted into the walls of the U.S., Japanese, and European modules of the ISS. Each rack will have an internal volume of about 1.5 cubic meters and will hold up to 700 kg of equipment. ISPRs will have standard power, data, thermal control, nitrogen, waste gas, fire detection, and mechanical interfaces. Equipment for experiments is usually built into the ISPR on Earth. The ISPR is then brought to the ISS and fitted into a laboratory module. The ISS will have a robotic mobile servicing system and the capability for extravehicular activities (EVA) by crew members. These capabilities would make possible the monitoring, reconfiguring, and replacement of external experiments. The ISS itself will be available as a test structure for engineering research. NASA currently plans to reserve 40 percent of the ISS resources that will be available to U.S. researchers for ERTD and commercial activities. More detailed information on the capabilities of the ISS can be found in The Capabilities of Space Stations (NRC, 1995). The ISS, however, will also have a number of inherent limitations that will restrict ERTD. Most prominently, the presence of a crew will mean that all ISS experiments will have to be carefully screened to ensure that they do not threaten crew safety. Experience on the Space Shuttle has shown that this process increases both the cost and the time required to prepare experiments. Second, the ISS's low-altitude orbit will expose external ERTD experiments to a very different environment (in terms of atomic oxygen, thermal cycles, etc.) than the environment at higher altitudes. As a result, some of the data acquired by such experiments will not be widely applicable. Third, resupply activities, the presence of the crew, the need to rotate solar arrays to track the Sun, and the numerous other experiments conducted on the ISS will result in a more contaminated external environment and more disturbances of the microgravity environment than on an uncrewed vehicle. And finally, ERTD experiments will have to be configured to minimize interference with other ISS activities. KINDS OF ERTD THAT COULD BE PERFORMED ON THE ISS A variety of ERTD activities could be performed on the ISS, ranging from the most basic engineering research to the in-space testing of new technologies

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Engineering Research and Technology Development on the Space Station and systems. The first major opportunity to conduct in-space engineering research in the space station program will come during the assembly and operation of the ISS. Much can be learned, for example, about the stability and control of large structures in space by studying the behavior of the ISS during the assembly process. To support such research, a network of small sensors would have to be attached or built into the structure to measure its response to perturbations, including shuttle dockings, thruster firings, and inputs from vibration inducers. As each new component is added to the ISS, the station 's structural dynamics will change, in effect creating a new experiment to be monitored by these sensors. The data gathered at each stage could then be applied to the design and control of certain types of future large space structures, allowing them to be lighter and more stable. Like other experiments in which the ISS itself would be an integral element, this type of research cannot be performed on the ground and could be prohibitively costly to conduct in space if the ISS were not available. Engineering research that does not directly concern the assembly and operation of the station could also be carried out on the ISS. For example, samples of FIGURE 2-2 An ISPR. Source: NASA

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Engineering Research and Technology Development on the Space Station different materials could be attached to the outside of the station to determine how they are affected by the space environment and to determine ways to improve the durability of materials in that environment. Similar experiments on the Long Duration Exposure Facility (LDEF) showed that some materials were durable in the LEO environment and that other widely used materials deteriorated rapidly. Materials exposure experiments on the ISS not only could be performed for longer periods of time than the LDEF experiments, but also could be monitored regularly. Other engineering research that could be carried out using ISS facilities include tests of the effects of capillary action on fluids in microgravity in order to improve thermal control devices and in-orbit propellant transfer, and determining the effects of microgravity on the characteristics of battery cells in order to develop better energy storage devices for spacecraft. The continued upgrading of the ISS over the years will offer opportunities to develop new technologies that might later be used elsewhere in space or on Earth. One possibility would be the development of hardware for solar dynamic power generation that would be smaller and lighter than comparable photovoltaic systems. A prototype solar dynamic system not only would provide extra power for the ISS but also could be modified and tested in various ways to gain the knowledge required to develop a second-generation system. The second-generation system might be applicable to a future space station or to high-power communications satellites. Technology development unrelated to the station's assembly and operation also could be carried out on the ISS. For example, the ISS could be used as a communications technology test bed. Developers of communications satellites could use the ISS to test deployable antennas, to experiment with LEO optical cross-links, or to characterize the on-orbit performance of communications systems. As another example, the ISS could be the site of a testbed for advanced electric propulsion systems. Finding 1. Much valuable ERTD, including projects that use the space station as a laboratory and projects that use the space station as the test article, could be performed on the ISS. POTENTIAL BENEFITS The benefits of ERTD performed on the ISS could include (1) improving the performance of the ISS itself while reducing operational costs and improving its capacity to serve researchers, (2) developing technologies and gaining knowledge that would benefit other space activities, and (3) producing benefits on Earth in both related and unrelated fields. These benefits are not mutually exclusive. A single new technology development program might result in improvements to the ISS and commercial satellites and also contribute to the education of engineers and technicians on Earth.

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Engineering Research and Technology Development on the Space Station Improving Performance and Reducing Operating Costs of the ISS New technologies integrated into the ISS could significantly reduce the space station's operational costs and thus free resources for other space activities. Lower costs also would bring the vision of self-supporting commercial activities on the ISS closer to reality, leading to increased commercial investment in orbital research. One possibility for reducing operational costs would be to reduce the station's resupply requirement. Over 90,000 kg of water, air, and crew supplies are to be delivered to the ISS in the first five years after assembly is complete. The propellant needed to maintain the station's orbital altitude would make up about 33,600 kg of that total, and resupply will require about 30 flights of the Progress cargo vehicle. Thus, any technology that could reduce this propellant requirement would probably pay for itself rapidly. For example, advanced photovoltaic panels could replace the ISS's initial large solar arrays, thus reducing the station's drag and its propellant requirements. Advanced electric propulsion systems would also reduce propellant requirements, since they would be more efficient than the current station-keeping system. The benefits to the ISS of advanced photovoltaic arrays and electric propulsion systems would not be limited to simply reducing the demand for propellant resupply. Like other technologies, they could also be used to improve the performance and capabilities of the ISS. Advanced photovoltaic arrays, for example, could be used to provide additional power for researchers, while the use of an electric propulsion system to counteract drag losses might improve the quality of the station 's microgravity environment. Other advances in technology that could both reduce operating costs and improve capabilities include those that would enhance on-board autonomy. ERTD on advanced robotics, for example, could reduce the need for experimenters to build automation into experiments and lower demands on crew time. An experimental version of such a system, the “Charlotte” robot, was used on a 1995 Shuttle flight. Charlotte, a six-degree-of-freedom cable-supported robot, was able to perform simple tasks, such as operating switches and making video observations (Campbell et al., 1995). As more advanced robots are tested in space, the capability of robots to support ISS research will grow. Robotic systems that can operate outside the space station will be particularly valuable, since EVA time is a precious resource. Numerous advances in external robotics are already being explored for the ISS, including free-flying robots, large robot arms similar to the Space Shuttle remote manipulator system, and smaller robotic arms for precision tasks. Improving Performance and Reducing Costs of Other Space Missions A wide variety of ERTD activities using the ISS, including the technologies developed and engineering knowledge gained in construction and operation of

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Engineering Research and Technology Development on the Space Station the ISS itself, could yield valuable lessons for future commercial and government spacecraft. For example, knowledge about the effects of the space environment on the ISS's exposed thermal control surfaces and photovoltaic cells could be used to provide an invaluable database on the effects of the LEO environment on spacecraft materials. Future designers could use this database to get a better idea of which materials to use to stabilize thermal performance and power generation capability over the life of their spacecraft. Similar data from LDEF experiments showed that both the LEO environment and materials deterioration are much more complex than previous models had suggested. This data has been applied in selecting exterior paint and thermal blankets for the ISS and other spacecraft (NASA, 1995). Information gained from the ISS also could be used to verify models of structural dynamics and to investigate control-structures interactions of large flexible structures in a weightless environment. These data would play a large role in the design of the next generation of lighter-weight spacecraft structures, including solar arrays and communication antennas. New power generation technologies developed or tested on the ISS also could be applied to many other spacecraft, and advances in autonomous fault detection, isolation, and recovery (FDIR) for ISS systems would be directly relevant to long-duration planetary exploration missions. Improving Technologies and Gaining Knowledge for Use on Earth Ancillary benefits, such as terrestrial applications of ERTD on the ISS, cannot be the primary justification for building and operating the ISS. But when such ancillary benefits are tangible and can be achieved at little or no cost to the primary mission, then their pursuit is sound public policy. The nonspace uses of the ERTD conducted on the ISS can clearly enhance its value to the nation, even though their economic value is likely to be small in relation to the costs of the ISS program. Technologies developed to enhance the ISS, as well as technologies developed as a result of experiments conducted on the ISS, might be applicable to uses on Earth. For example, very sensitive accelerometers developed to monitor microgravity levels on the ISS might find uses in some research laboratories on Earth. Moreover, ERTD conducted on the ISS might shed light on a range of engineering problems. For example, research on thermal coatings for spacecraft might possibly be applicable to the development of more durable paints on Earth. Finally, the opportunity to perform experiments on the ISS can help to motivate and train a new generation of engineering students. As these students graduate and take their place in the technological infrastructure, their experience with the ISS would broaden their intellectual horizons to include using space for applied research. Steps that can be taken to increase the likelihood that space ERTD will produce benefits on Earth are discussed in chapter 5.

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Engineering Research and Technology Development on the Space Station Finding 2. ERTD on the ISS could result in new technologies and knowledge that (1) could significantly reduce the space station's operating costs and improve its performance, (2) could be applied to other spacecraft, and (3) could, in some cases, be transferred to Earth-based applications. PRIORITIZING NASA ERTD ON THE ISS The federal government (primarily NASA) and private industry will be the two principal sources of financial support for ERTD on the ISS. Although a market-based method can be used to prioritize commercial research (see chapter 5), NASA will have to develop a formal and public process to prioritize the numerous ERTD experiments that it will be asked to fund. The central elements of this process should be (1) a technology road map, (2) a process to ensure that the road map is being followed and that the quality of the projects is high, and (3) a strategic intent to use the ISS to drive technological development. The Technology Road Map: A Guide to Priorities NASA is guided by a well-articulated vision that meshes well with current fiscal and political realities. The agency realizes that the approaches that succeeded in the past are inadequate for the present era of constrained budgets and public concern that government investments in technology should produce demonstrable payoffs. NASA 's strategic concepts have changed (1) from central control of large space systems to the more decentralized approaches made possible by the information sciences, (2) from an internally oriented, self-referencing culture to one that interacts with external stakeholders, and (3) from seeking public support through spectacular accomplishments to seeking public support through cost-effective value-added operations. However, this vision remains inadequately connected to NASA's ERTD programs for the ISS. The most powerful management tool to provide that connection is a technology plan, or, more colloquially, a technology road map. A technology road map can be a highly effective instrument for managing varied and decentralized activities, such as NASA's program for ERTD on the ISS. The task of creating such a road map would force the agency to think rigorously about which goals are central to achieving its vision and which are of secondary importance. Once in place, the map would become a useful communications device linking the various program offices and centers in a set of common goals. When made public, it would inform the agency's external stakeholders of agency priorities, thus enabling them to make sound decisions regarding their own programs. Most important for times of constrained budgets, the map would allow NASA to set priorities with full knowledge of the consequences of alternative courses of action. Thus, the technology road map should be thought of as a management tool and not as an end in itself. Like any management tool, the map would be subject to modification as technology advances and priorities change.

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Engineering Research and Technology Development on the Space Station The process of developing a road map for ERTD on the ISS would begin with the creation of a set of engineering and technology goals that support those elements of NASA's vision most relevant to ERTD. NASA 's external stakeholders from universities, industry, and other government agencies would be invited to join in developing these goals. To guide NASA's program offices and centers, the goals would be organized in terms of the amount of time and funding required for their achievement, their degree of difficulty, the element of NASA's strategy they support, and the extent to which they are central to achieving NASA's vision. Some goals would be short-term goals achievable with straightforward extensions of current practices. Others should be designed so that achieving them requires significant advances in the state of the art, forcing NASA to become the moving force behind the achievement of a key set of national capabilities. Once the goals are in place, the ERTD programs in various parts of NASA would be reviewed and re-engineered to mesh with and support the goals, and hence the larger vision, of NASA. Finding 3. NASA currently has no plan for effectively prioritizing the ERTD it will fund on the ISS. A technology road map could be a powerful tool for developing such a plan. Experiment Selection Process NASA's Offices of Space Access and Technology, Life and Microgravity Sciences and Applications, Space Flight, Space Science, and Mission to Planet Earth are the main sources of funding for in-space experiments on the Space Shuttle. These offices use a number of methods to select the experiments that are flown on the shuttle—methods that for ERTD typically do not require peer review and are not clearly linked to project or program goals. In short, current NASA practices for selecting projects for in-space experiments do not guarantee that the best technical work relevant to NASA's strategic vision will be chosen. Instituting a review process that impartially assessed proposed experiments (including those proposed from within NASA itself) and was closely tied to the technology road map could reduce such problems as concentration on one technology to the exclusion of others with a potential for high payoffs, a perception by private and university researchers of bias toward in-house experiments, approval of a portfolio of experiments that do not support NASA's strategic vision or program goals, and the assignment of experiments to a particular space platform when they might more effectively be performed on a different platform or on Earth. A review process designed to avoid these potential pitfalls should include the following features: Reviewers from NASA, academia, other government agencies, and the space industry should be included to ensure a wide perspective and to

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Engineering Research and Technology Development on the Space Station allay any suspicions that the selection process was biased toward in-house experiments. The review process should be structured to support experiments that advance technology or increase knowledge in the directions laid out in the agency's road map. Periodic reevaluations of ongoing experiments should be included to ensure that resources continue to support progress along the road map. Proposed experiments should be evaluated to determine if they require the unique attributes of the ISS or could be more effectively conducted elsewhere (e.g., on the Space Shuttle, on uncrewed spacecraft, or in drop towers). Although this type of review can be a powerful tool, several caveats must be observed. First, it should not be used for commercial experiments. (A system for prioritizing commercial ERTD experiments is discussed in chapter 5.) Second, it is best suited for ERTD experiments that “push” technology and is less useful for evaluating technologies required to meet program goals, since project managers need some discretion in choosing technologies that are crucial to the success of their missions. Third, because of proprietary concerns, this type of review is sometimes difficult to apply to joint efforts that might lead to the creation of commercial products. (In general, this type of review will be inappropriate if proprietary concerns require the signing of confidentiality agreements.) These concerns, however, should not prevent the majority of NASA-funded ERTD on the ISS from being reviewed. The experiments that emerge from this review process will place a diverse set of demands on ISS resources (accommodations, power, crew time, and so forth). NASA will need a resource allocation process that is widely perceived to be fair to all experimenters and that allocates scarce ISS resources efficiently. A body of theoretical work is available to support development of an efficient resource allocation model (Banks et al., 1989; Ledyard et al., 1994). This base of theory was used successfully by NASA in the Cassini project for the design and operation of science instruments (Boudville and Porter, 1992) and could be applied in developing an efficient and transparent resource allocation model for the ISS. Finding 4. ERTD research on the Space Shuttle is not usually peer-reviewed and is not clearly linked to NASA's goals. A well crafted review process could be a powerful tool to ensure that ERTD funded and flown by NASA on the ISS supports NASA's overall strategic plan. Strategic Intent: Using the ISS to Drive Technology Development Advances in space technology will be essential to the tasks of significantly cutting costs and improving the capabilities of future space missions. Typically,

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Engineering Research and Technology Development on the Space Station however, government and private organizations in the space field have been slow to accept new space technologies. Project managers are reluctant to use new technologies on their spacecraft until the success of these technologies has been demonstrated in space—they are unwilling to accept the increased risk of mission failure that might result from the incorporation of untried technology. The current design of the ISS, for example, has been strongly influenced by this attitude (see box 2-2) and as a result incorporates relatively little new technology. Because program managers are reluctant to use new technologies, NASA and private organizations have had little incentive to invest in the high-risk but high-payoff ERTD needed to develop such technologies. One way to overcome this problem is to create programs designed to flight-test new space technologies. NASA's New Millennium program is a prominent example of such a program. Another is to set goals that can only be achieved with new technologies. The Apollo program was the most prominent example of such a challenge in the space arena. Apollo's goal of putting a man on the Moon required the development of a host of new technologies, some of which still underpin many of the nation's current space endeavors. Within a limited scope, the ISS offers NASA a platform where both approaches might work together. First, the ISS will be well-suited to flight-testing new space technologies. There will be several advantages to using the ISS over other platforms for this purpose. The failure of one technology will not jeopardize other tests, for example, because the experiments would not be vital to the operation of the ISS. On smaller technology demonstrators, the failure of one technology can end a mission. In addition, the presence of the crew on the ISS might make it possible to correct some failure modes on site. And even if a failure could not be corrected, hardware could be returned to Earth for a detailed analysis, increasing the chances that the problem could be corrected over the long term. One disadvantage of using the ISS for flight-testing new space technologies is that the safety constraints associated with the presence of a crew could increase costs and cause delays in test and development schedules. NASA also could use the ISS program to “drive” the development of new space technologies for use within or outside the space agency. As demonstrated in box 2-2, development budget constraints have led to a space station design that incorporates relatively little new technology. These continuing budget constraints also prevent NASA from setting a grand new goal (such as sending astronauts to the Moon again) to drive technology development. An aggressive mandate to reduce the operations and maintenance (O&M) costs of the ISS, however, with the incentive that savings would remain in the system for further development of relevant technologies would present an opportunity to drive the development of new technology without large increases in funding. Since the operating costs of the ISS will be shared by many countries, this type of ERTD could appropriately be supported and conducted cooperatively with other ISS international partners.

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Engineering Research and Technology Development on the Space Station BOX 2-2 Disappearing Technologies Earlier designs of the ISS envisioned a space station that would be used, among other things, as a satellite repair platform, a transportation node from LEO to geostationary orbit and other locations, and an observation platform. In the less-constrained budget environment of the 1980s, many advanced technologies were incorporated into the design to carry out these missions as well as to ensure efficient operation of the space station. As budgets became tighter and the need to maintain schedules grew more important, most of the technologies requiring high-risk development were deleted from the design. These included: solar dynamic power generation for “high power/low drag” energy generation hydrogen-oxygen propulsion to eliminate the need to transport hydrazine propellant and the related safety concerns resistojets for supplemental propulsion reclamation of water and oxygen in a closed loop environmental control system for use by the crew or as a propellant for hydrogen-oxygen propulsion advanced robotics, including a U.S.-developed fine dexterous manipulator to assist in performing external activities a software-based GPS-type traffic control management system an instrument pointing system for Earth or celestial-viewing payloads electric propulsion for drag make-up up-link television for training and conferencing high-data-rate communications to improve experiment monitoring capabilities fault-detection, isolation, and recovery technologies to improve autonomous on-orbit operations Many of these technologies, which often were studied in detail before being discarded or delayed indefinitely, could still be added to the ISS. Some could result in benefits to future space systems, and some could make a significant contribution toward reducing the operating costs and enhancing the capabilities of the ISS. Finding 5. Continued upgrading of the ISS, once assembly is complete, could be a source of new technologies for use elsewhere in space or on Earth. To date, however, ISS designers have been reluctant to incorporate revolutionary technology, which has been seen as introducing unacceptable elements of risk. If the ISS program is to serve as a technology driver, it will have to be given a strong incentive to understand, prioritize, and incorporate new technology.

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Engineering Research and Technology Development on the Space Station RECOMMENDATIONS Recommendation 1. NASA should immediately begin to develop a technology road map to ensure that ERTD on the ISS supports NASA's strategic vision. The map should be distributed widely, revisited periodically, and reviewed by external constituencies. Wherever possible, NASA-funded ERTD research should be peer-reviewed by panels composed of NASA, university, and industry experts, who would use the technology road map for guidance in deciding which experiments would be funded. NASA should consider developing an efficient and transparent approach to allocating ISS resources among the projects that pass peer review. Recommendation 2. To encourage the development of advanced technology in the ISS program, one of the road map goals should be to steadily reduce O&M costs of the ISS through the infusion of new technology. The savings from reduced O&M costs should be applied to additional technology development to further reduce O&M costs in the later years of the station. REFERENCES Banks, J.S., J.O. Ledyard, and D. Porter. 1989. Allocating uncertain and unresponsive resources: an experimental approach. RAND Journal of Economics 20(1):1–25. Boudville, W., and D. Porter. 1992. Cassini Resource Exchange Users' Guide. Pasadena, California: California Institute of Technology. Campbell, P., P. Swalm, and C. Thompson. 1995. Charlotte™ Robot Technology for Space and Terrestrial Applications. SAE Technical Series paper #951520. Warrendale, Pennsylvania: Society of Automotive Engineers. Ledyard, J.O., D. Porter, and A. Rangel. 1994. Using computerized exchange systems to solve an allocation problem in project management. Journal of Organizational Computing 4(3):271–296. NASA (National Aeronautics and Space Administration). 1995. LDEF Materials Results for Spacecraft Applications. NASA Conference Publication 3261.Proceedings of a conference held October 27–28, 1992, Huntsville, Alabama. Washington, D.C.: National Aeronautics and Space Administration. NRC (National Research Council). 1995. The Capabilities of Space Stations. Committee on Space Station. Aeronautics and Space Engineering Board. Washington, D.C.: National Academy Press.