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MATERIALS SCIENCE AND ENGINEERING* INTRODUCTION This chapter explores the relationship of the materials science and engineering (MSE) community to the possible availability at some future time of an unmanned space platform or manned space station. How might it benefit the materials science and engineering community to have access to the space environment, specifically via a space platform or manned space station, for research and for eventual applications? What scientific and technological opportunities might be provided by a space station as compared with those available to materials science and engineering on the Space Shuttle and in ground-based research? The Panel addresses these and related questions. In order to assess the potential use of a space station, the Panel felt it necessary to review the past and present status of the National Aeronautics and Space Administration's (NASA) materials processing in space (MPS) program . The Panel reviewed materials experimentation in a microgravity environment (both ground- and space-based) and assessed the needs of various MSE activities for longer experiment time. In addition, the Panel considered whether the various MSE activities could utilize a space station or platform advantageously. * The Panel expresses its appreciation to the following people who participated in discussions: John R. Carruthers, Hewlett-Packard Laboratories; John A. Graham, Deere and Company; David Keaton, GTI Corporation; Fortunate J. Micale, Sinclair Laboratory; and David W. Richman, McDonnell Douglas Astronautics Company. 72
73 Finally, the Panel described the requirements for a space station that would be needed to serve the MSE community, including gravity level, power requirements, and flight hardware design. BRIEF HISTORY OF NASA'S MATERIALS PROCESSING IN SPACE PROGRAM Since scientists began to consider the space environment and especially since the early days of space flight, it has been recognized that materials, particularly in a fluid state, behave differently in space than they do on earth. As it became recognized that the behavior of materials in space might lead to useful developments, opportunities for low-gravity (low-g) experiments became available in rockets and spacecraft, both unmanned and manned. For example, exploratory experiments in crystal growth, bioprocessing, and containerless processing were conducted during the l973-74 Skylab missions. Most of these early experiments were driven by the exigencies of flight opportunities and were poorly conceived, in the sense that while most went through their prescribed duty cycles, the hoped-for results were often masked by extraneous phenomena. Nevertheless, some premature claims were made for new and possibly useful materials or materials-related processes in space. Most of these claims were not validated, usually because of the complexity of the experimental situation, the lack of sufficient ground-based science for comparison, and the lack of opportunity for additional trials in space. NASA, in an attempt to find new and broadly based purposes following the culmination of the Apollo program in the mid-l970s, heralded the potential of MPS as an activity that would eventually create new business ventures and profits for the private sector. There were hopes of exotic alloys, nearly perfect crystals, and new drugs that would cure disease, each so rare and unique that the high cost of space flight and extraterrestrial processing would be warranted. Subsequently, NASA's MPS program was studied in detail by the National Research Council's (NRC) Committee on Scientific and Technological Aspects of Materials Processing in Space (STAMPS). Among the conclusions and
74 recommendations in the STAMPS report, published in l978, were the following: NASA's MPS program had been weak; nevertheless, some work done in space has shown that valid experiments can be planned, manipulations performed, and useful samples returned to earth for study. The Committee found that "the work that gave productive results had a sound base in terrestrial research." ". . . space experimentation will have little value unless its planning is founded on substantial earth-based information and unless the results are coupled to those of complimentary terrestrial programs." The principal values of the space environment are the availability of low-gravitational acceleration for long periods of time and the effect of the space environment on phenomena related to buoyancyâe.g., containerless processing, solidification, and transport processes under conditions of reduced convection. The committee had "not discovered any examples of economically justifiable processes for producing materials in space." The program should emphasize the use of the space environment for the scientific study of materials phenomena in an effort to gain new knowledge and possibly improve materials processing on earth. Following the report of the STAMPS committee, there was significant improvement in the MPS program. Some highlights of the succeeding (post-l978) era are listed below. An extensive, ground-based program of research was developed to provide the knowledge needed to set potential materials research in space in a proper scientific context. A peer review system was instituted to select only those proposals that had true scientific and technological merit and to decouple their selection from funding stimuli or from the need merely to fill flight opportunities. A higher quality of principal investigator and of ideas was attracted to the program; conclusive scientific findings surfaced and were published in the literature. A program in joint ventures was established by which NASA and the private sector could cooperate on a
75 no-funds-exchanged basis to test systems for space processing that might someday be commercially viable. A committee of materials scientists and engineers, STAAC, was constituted to monitor the program and to help ensure that the lessons learned from the preceding era would be heeded. PRESENT STATUS OF NASA'S MATERIALS PROCESSING IN SPACE PROGRAM At present, NASA's MPS program includes ground-based experiments, reduced-gravity experiments, and commercial ventures. A discussion of each area follows. Also, in view of the importance attached to the joint venture activity on electrophoretic separation of biological materials, the Electrophoresis Operations in Space (EOS) project is discussed in a separate section. Ground-Based Experiments NASA's current MPS program involves approximately 70 principal investigators associated with 50 organizationsâ3l universities, l2 industrial organizations, and 7 national (government-supported) laboratories. Funding in FY l983 to support the science base and approved flight-support experiments totaled $l2.5 million and was distributed in the form of single-investigator contracts and institutional block funds. Approximately two-thirds of these funds supported a broad, applied-research program contributing to the improvement of the science base. The areas currently funded by the MPS program include: Containerless technology, which involves development of techniques and construction of hardware for using acoustic, electrostatic, and electromagnetic fields to position material under microgravity conditions Containerless science, which includes experimental and theoretical studies of the behavior of materials, especially in the molten state, when removed from physical contact with confining walls. Such studies include nucleation, glass formation in undercooled systems, thermophysical measurements on levitated samples, and buoyancy and convection in levitated systems.
76 Crystal growth, which involves modeling of crystal growth processes, thermodynaraic and kinetic studies of crystal formation from vapors and liquids, and growth of crystals under terrestrial and microgravity conditions Solidification and fluid processes, which includes the response of microstrueture to thermal gradients, freezing rate, and gravity vector orientation and studies aimed at understanding how convection and sedimentation affect cast and composite materials Bioprocessing, which involves separation of biologically interesting materials (cells, viruses, proteins, etc.) using techniques that can benefit from the reduction in gravity level Cloud physics, which deals with ice growth in supercooled water vapor and includes modeling of microscale transport phenomena. Extraterrestrial materials processing, which involves identification, acquisition, and processing of lunar, asteroidal, and meteoritic materials, including studies of the chemical extraction of useful materials from simulated extraterrestrial matter Combustion science, which studies kinetics of flame propagation and reactions in combustion systems under terrestrial and orbital conditions The program is monitored by a system of six science working groups (SWGs) formed under the auspices of the Universities Space Research Association (USRA). Active SWGs now include those for Containerless Processing, Solidification, Float Zoning, Bioprocessing, Fluids, and Transport and Combustion. The SWGs report to NASA through USRA and provide periodic peer reviews. In the five-year period since the STAMPS report was published, the ground-based research program has improved in breadth and quality. The portion of MPS program funds allocated for nonflight science support has increased from less than 20 percent in l976 to more than 40 percent of the total program budget. The scientific publication rate has risen from l0 papers in l976 to 35 papers in l980, with a projection of more than 70 papers published in l982. The possibility of selecting flight experiments from the ground-based research program now exists, but limited flight opportunities and hardware development remain pacing factors.
77 Reduced-Gravity Experiments The Panel notes with approval that NASA's MPS Program Office has increased access to reduced-gravity-level environments. NASA provides a variety of opportunities, including drop tubes and towers, aircraft, and sounding rockets, all of which produce relatively short-duration microgravity environmentsâseveral seconds in drop tubes and towers, a half-minute in KC-l35 and F-l04 aircraft, and five minutes in sounding rockets. In addition, the Space Processing Applications Rocket (SPAR) program has conducted some nine flights over a period of approximately five years with about 30 experiments carried out. The SPAR experiments were flown in a timely manner at relatively low cost, with few failures and with some useful scientific results. In regard to the Space Shuttle, opportunities for flying experiments have just begun, but it appears certain that these will continue to remain at a premium, even for worthy projects, and that the frequency of these opportunities will limit the program. Commercial Ventures Commercial involvement with NASA's MPS program takes two forms at present: (l) the Joint Endeavor Agreement (JEA) and (2) the Technical Exchange Agreement (TEA). The purpose of the JEA is to provide easier access to space flight opportunities for organizations that have an interest in developing commercially viable products and processes and that are willing to assume an appropriate financial risk. At present, JEAs have been negotiated with several leading national firms. TEAs are designed to permit use of the characteristics of low earth orbit and short-duration opportunities to study effects on materials in order to improve the product or process in its ordinary terrestrial setting. TEAs have been established with some six national firms. There has been very limited experience to date with commercial ventures. However, these experiences suggest that commercial materials processing activities in space might fall into three general areas: (l) utilization of space as an applied-research tool, (2) on-orbit services provided at a cost to the customer, and (3) scenarios leading to commercial production of products manufactured in space.
78 Finally, inasmuch as the flight opportunities aboard the Space Shuttle have just begun, it is premature to judge how important or advantageous a space station might be to the MPS program. However, the major concerns of the MSB community center on ease of access, turnaround time, hardware development and integration, and costâprecisely the same concerns the community faces regarding use of the Space lab/Shuttle for the current MPS flight program. Electrophoresis Operations in the Space Program In July l982, on the fourth flight of the Space Shuttle, McDonnell Douglas Corporation and Ortho Pharmaceutical Corporation (a division of Johnson & Johnson) carried out one phase of their Electrophoresis Operations in Space (EOS) program under a JEA with NASA. The objective of the electrophoresis experiment performed at that time was to verify that concentrated biological materials flow at greater volume and separate with good resolution under microgravity conditions. McDonnell Douglas reported that all test objectives were met (Yardley and Rose, l983 personal communication). Separations of rat and ovalbumin, very difficult to perform in earth's gravity, and separations of a tissue culture of commercial interest were accomplished. With regard to the quantities producible on earth and in space, the company reported its research had shown that 0.2 percent is the maximum concentration of albumins that can be separated on the ground, whereas in the l982 Shuttle experiment a 25 percent concentration was possible. Thus the l25 times greater concentration of biological materials and the 4 times greater volume resulted in throughput improvement of about 500 times (McDonnell Douglas, "Results of McDonnell Douglass Electrophoresis Experiment on STS-4," November 8, l982). Comparable success was achieved in the separation of the tissue culture media. Under this EOS program, two Shuttle flights are scheduled for l984 with the purpose of producing significant quantities of processed material for use by Ortho in clinical testing (Yardley and Rose, l983, personal communication, p. 2); plans call for testing a production prototype unit in l985, and for clinical programs leading to Food and Drug Administration (FDA) approval in l987. The production prototype unit, designed to become the first commercial production unit following FDA approval of the processed material, would be capable
79 of operating in the Shuttle orbiter bay, or as a free-flier supported in orbit by the Shuttle or a prospective space station. However, in the Panel's view, even though increased throughput was demonstrated in space, it appears that free-flow electrophoresis may become a technology with limited practical industrial application, and that some simpler, less costly alternatives are available on earth. For instance, in the last decade, rapid advances in the technology of separation (greater selectivity and throughput) and advances in biology have resulted in the commercial availability of biologically active compounds. THE ROLE OF MAN IN SPACE This section discusses the characteristics of present and possible future (e.g., manned space station) microgravity environments, in an effort to understand better the limitations and advantages of each, and to explore the advantages of real-time interactions by a human that might be provided in a manned space station. A variety of facilities are now available for conducting experiments in a microgravity environment. Following is a list of examples: Drop towers provide a microgravity environment for up to 6 seconds. Aircraft such as KC-l35s and F-l04s flying in ballistic trajectories provide a microgravity environment for up to 50 seconds. Rockets provide a microgravity environment for up to 5 minutes. Earlier one-shot orbiting facilitiesâe.g., Skylab and Apollo-Soyuzâprovided several hours of microgravity environment. The Space Shuttle and Spacelab will, for the next several years, provide a number of opportunities for conducting experiments in a microgravity environment for periods of up to 7 days. Materials scientists and engineers have used and will continue to use these facilities to conduct experiments under conditions of microgravity, albeit in some cases for a very short time. However, there are disadvantages to using these facilities, including a significant loss in versatility in the equipment that can be used, a decrease in the availability of support functions, the long time
80 required between conceiving and carrying out an experiment, and the meager real-time human attention and intervention that are available. Spacelab is expected to be manned; however, only limited interactive capabilities with materials science experiments will be possible. Highly automated equipment is being designed for Spacelab, instead of equipment that would take advantage of man's presence and unique capabilities. Hence, even if particularly careful planning and preparation are given to activities carried out in these facilities, the programs become one-shot experiments that have a high chance of failure. To meet the needs and exploit the full potential of materials research, a space station would have to provide a laboratory that significantly alleviated the restrictions of experimentation in currently available space facilities. A space station could diminish the constraints on the equipment used, could provide more electrical power and cooling capability, and make possible much longer continuous periods of experiment time in microgravity. If a space station were available for materials research, the time between conception and conduction of the experiment could be reduced significantly. And, most important, real-time human observation and intervention in an adaptive mode would be possible, which is particularly important in the event of unexpected results. However, this latter capability would require the presence of personnel with experience and skill in conducting materials experiments. The Panel believes, therefore, that the basic style or mode of the materials science and engineering experimentation that could be conducted in a manned space station is very different from most of that conducted heretofore in space. In particular, the presence of man in a space stationâpresumably a trained professional or one trained to interact by communication in real-time with a technician or scientistâwould allow for an interactive mode of materials research in space that is much closer to that now conducted in laboratories on earth. Space laboratories in a manned space station could be equipped not only to observe and collect data, but to alter and adapt experiments, based on observation and results. No longer would it be necessary to plan (or guess) in advance virtually all aspects of an experiment. Neither would it be necessary to depend so heavily on very expensive but highly restrictive general-purpose hardware. Experience so far in the MPS program has led to limited, painstaking
81 progress achieved in the noninteractive mode. Perhaps the availability of the interactive mode would make possible the rapid progress that has long been hoped for in MSE. Thus, an MSE laboratory aboard a manned space station should contain many of the same facilities as a materials science and engineering laboratory on earth, including facilities for machining, manipulation, fabrication, characterization, and so forth. Power requirements and available materials should be those needed for alteration and repair of apparatus, not just for operation and samples. Many of these facilities could be shared with other activities (e.g., satellite repair) that might be conducted in a space station. MATERIALS SCIENCE AND ENGINEERING REQUIREMENTS FOR A SPACE STATION This section describes the characteristics and requirements of a space station for optimal use to the MSE community. The development of specific MSE experiments to be conducted on a space station is hampered by rapid evolution of the materials technology industries and the limited MSE flight experience. Even an inspired selection of experiments made at this time might be obsolete before a space station could be available. Consequently, it is more prudent to assess the MSE requirements on a functional basis. Free-fall is the principal attribute of space exploited in MSE experiments; the secondary attributes are vacuum (level and pumping capacity), radiation, and solar power. To date, all MSE experiments have focused on the exploitation of continuous free-fall, or low-g, and this will continue to be the basis for these proposed requirements. Experiments in low-g (orbital and suborbital) have demonstrated that a working environment less than l0-3g can produce substantive changes in convective phenomena and in the relative significance of surface and body forces. Therefore, this l0-3 g will be accepted as a maximum working value. As critical dimensions and parameters are increased, a further reduction in g-level may be required simply to maintain the same level of convection. Similarly, more sensitive materials, processes, or geometries require further reductions in g-level. The g-level associated with the Space Shuttle is expected to be l0-4g to l0-7g. Similar g-levels would
82 be required in a space station. Moreover, the g-level must be continuous, not intermittent, for the duration of an experiment. Most materials processing experiments impose a specific thermal geometry on a sample for a period of timeâhours and sometimes days. The temperature and scale of an experiment will determine the power requirements, and the time at temperature determines the energy required. Some MSE experiments with sample sizes on the order of a few centimeters have been designed to consume less than 2 kW. A scale-up in size or in temperature would make 4 to 5 kW for each experimental system and 25 kW for the total facility realistic goals. Requisite energies would be on the order of l00 kW-hr for each experimental system. Comparable cooling capacity would have to be provided. The weight and size of the materials are negligible compared with that of furnaces, power, control, and data-acquisition hardware. Hardware dedicated to a single experiment is likely to range from l50 to 400 kg, and facility complexes of a larger scale, from l,000 to l,500 kg. These masses could be housed in volumes from as little as 0.25 m3 (for a middeck experiment) to facilities that might use 20 to 50 m3. The masses should be housed in volumes of standardized geometry in order to promote the economies of a modular approach. The data produced by an MSE experiment may be in video or digital forms, but more likely would reside in the specimens themselves. Process control and data-acquisition rates are small when compared with those of remote sensing. The MSE experiment must be free of interference from other facility usersâfor example in the forms of electromagnetic interference or vibration, particularly below l00 Hz. CONCLUSIONS NASA's MPS program is primarily a scientific program; its contribution will be through the development of better materials and processing methods. Improvements in the quality and scope of the NASA MPS program over the last five years now permit experiments for flight to be selected from a variety of ongoing ground-based programs. The program is now restricted by limited flight opportunities and hardware.
83 Present and planned electrophoretic experiments aimed at commercial purification of biologically active substances in space cannot be properly assessed by the Panel for lack of detailed information. However, it does not appear to the Panel that it will be a commercially viable enterprise. Before electrophoresis for commercial purposes is widely promoted as a justification for a space station, NASA should make an in-depth assessment of the process by soliciting the opinion of separations experts and providing them with access to the details of the process. The Panel concludes that materials science and engineering should not be used as a primary justification for a space station. However, the style or mode of MSE experiment that might be conducted aboard a manned space station would have distinct advantages over that which can be conducted in other microgravity facilities; the advantages would result largely from the potential for real-time adaptive interaction by humans.