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II Research in the Microgravity Environment THE NATURE OF MICROGRAVITY RESEARCH Microgravity sciences and applications comprise a broad range of research and development activities that are less than 20 years old. As basic and applied scientific research conducted in space, this field is gaining recognition as a legitimate, cohesive, scientific endeavor. Microgravity applications are similarly new, and collectively constitute an immature technology without demonstrable commercial successes as yet, but with potential practical importance. The nearer term practical impact of microgravity research likely will be on the terrestrial processing of materials, enhancement of some biotechnology, and the improvement of industrial processes. An important aspect of microgravity research to be considered is its inherent breadth and interdisciplinary nature. The field of microgravity science encompasses a number of subfields including: fluid, thermal, and transport sciences; condensed matter and gravitational physics; materials science and materials processing; combustion science; biotechnology and separation science; and life sciences. The scientific constituency for microgravity research is dispersed over a number of contributing disciplines, although a unifying, almost ubiquitous feature of microgravity research is the study of gravitationally modified physicochemical transport phenomena. Included among the phenomena of interest are: (1) reduction of gravitational sedimentation, which is the spatial separation of heavy and light objects immersed in a fluid medium; (2) elimination of hydrostatic pressure, which is the internal pressure of a fluid resulting from its weight; and (3) reduction of buoyancy-driven fluid flows, which normally arise from local density differences due to variations in temperature or chemical
composition within a fluid body. These fundamental fluid effects interact with ordinary chemical, physical, and biological processes to produce both quantitatively and qualitatively altered states displaying novel physicochemical behavior. For example, surface tension forces, normally so weak as to be generally unimportant under terrestrial conditions, can become dominant under microgravity conditions, suggesting the possibility of containerless confinement of fluids under their own molecular forces for a variety of basic experiments and practical applications. A spectrum of space- and ground-based experiments will be required to advance microgravity research. This spectrum will make use of facilities ranging from drop towers to suborbital and orbiting spacecraft. The complexity of research in the transport, materials, and life sciences disciplines usually requires, in the terrestrial laboratory, human interaction with experiments in order to observe nuances and unexpected phenomena and to adjust experimental parameters in real time. Many space-based materials experiments will require similar human interaction, including communication with principal investigators on the ground. To date, few resources and limited focused efforts have been invested in developing microgravity research hardware that would be capable of semiautonomous or teleoperational modes, although a broad range of robotic and telescience technology is available. Clearly, further effort is required in this area. In addition, microgravity sciences are highly reliant on the return to Earth of processed materials and biologicals. KEY PARAMETERS IN MICROGRAVITY RESEARCH A number of parameters characterize types of microgravity research and applications activity: the gravitational acceleration environment, the energy intensiveness of the process, the duration of the process, and the degree of experimenter understanding of the phenomena under study. These requirements dictate which type of experimental facility is preferable for particular research projects. As noted above, however, exhaustive experimentation on Earth must precede experimentation in space. Research conducted in space is too expensive to allow trial and error experiments. Gravitational Acceleration Environment The microgravity environment in Earth orbit is characterized by several components. The first is the set of quasi-steady accelerations on a vehicle due to atmospheric drag and gravity gradient effects. The second is the set of random, broadband accelerations (referred to as "g-jitter") that time-average to zero, but that might detrimentally influence certain processes with relatively short characteristic times. Sources of g-jitter include crew motion, thruster firings, and mechanical 10
vibrations. In general, the net effect of the above-mentioned accelerations on an experiment can be either minimized or exacerbated by the spacecraft's orientation, frequency of thruster firings, placement of the experiment relative to the spacecraft's center of gravity, degree of experiment isolation, overall flexibility of the spacecraft's structure, and so forth. The limited experience of U.S. microgravity investigators in orbital processing and the lack of well-documented experimental observations backed by accurate timelined microgravity accelerometer data make it difficult to assess how the acceleration power spectrum (in effect, the "g" level) really affects the outcome of an experiment. The greater Soviet experience in microgravity science has shown that some classes of experiments can be successfully executed below 10 g (at frequencies < 1 Hz), but the true influence of the full spectrum remains uncertain, as do such critical issues as the effect of the orientation of the net acceleration vector with respect to the thermal and solutal gradients developed during processing. Recent results by the Soviets seem to indicate that there is a strong correlation between increased crew activity and degraded crystal quality obtained from orbital processing. It is thus apparent that the trade-offs needed to achieve a cleaner g-spectrum must be carefully evaluated. For example, what is the trade-off between having crew intervention during an experiment and accepting more g-jitter? Which experiments degrade sufficiently because of human presence as to be inappropriate on a manned platform such as the Shuttle or the Space Station? When would a free-flyer mode, with its greater reliance on teleoperation, prove to be a better compromise than a fully manned vehicle? Clearly, a thorough assessment of the gravitational acceleration power spectrum must be available for any microgravity platform in order to decide these issues. Although such information is not presently available at the level of detail required, NASA is supporting computational fluid dynamics research that addresses the theoretical aspects of these issues and, in parallel, is developing a Space Acceleration Measurement System (SAMS) capable of microgravity measurements over the relevant frequency range. Energy Intensiveness of Processes The energy requirements of microgravity experiments vary greatly, arid it is not possible to specify a unique value range. Peak power required for some experiments involving use of furnace facilities can range up to several kilowatts. Other experiments require lower power levels but involve processes that require energy input over a long duration. Researchers generally agree that in the available as well as in most planned space facilities, power limitations will impose restrictions on some experiments. 11
Duration of Processes As with energy, the required time durations of processes of interest to microgravity researchers vary widely. Some experimental processes reach completion within a fraction of a second; others, notably those involving vapor-phase and solution crystal growth, ideally could make use of experimental run times on the order of several days or weeks. However, the committee did not find substantial interest in long-duration microgravity research at present. For example, the responses to a recent NASA Announcement of Opportunity for microgravity experiments showed that only 13 percent of the proposals required a mission duration in excess of 16 days. (The committee recognizes that the proposers may have been influenced by their knowledge of the duration capability planned for the Shuttle.) Degree of Experimenter Understanding of Phenomena Under Study The microgravity phenomena of interest to researchers differ greatly in terms of the degree to which they are understood. Typically, experiments and applications activities involving processes for which the underlying phenomena are reasonably well understood are likely to require little human interaction on a real-time basis and could be automated. The converse is likely to hold when novel phenomena are under study. In general, however, microgravity research on materials, fluids, and processes is an embryonic science. Ground research will not only help develop more meaningful experiments that are likely to succeed, but will also insure the identification and assessment of reduced gravity effects. Large amounts of experimental and analytic work will be required before comprehensive research strategies can be mapped and before the potential advantages of a human-tended free-flyer can be optimized. These parameters for microgravity research (the gravity environment, energy requirements, duration, and degree of experimenter understanding of phenomena under study) determine an experimenter's choice of the type of access to space that is appropriate for his or her research. NOTES 1. More detailed discussions of microgravity phenomena are contained in Slichter, pp. 7-20, and in Ostrach, pp. 313-345. 2. Naumann, June 8, 1988. 3. For example, recent computations for Bridgman crystal growth from the melt show that alignment of the quasi-steady state gravity vector with the crystal growth direction is desirable. Components of the gravity vector orthogonal to the crystal growth axis are an order of magnitude more effective than the axially aligned component in inducing fluid flow and causing dopant inhomogeneities in the resulting crystal. Similar studies 12
are needed for other high-priority microgravity experiments such as protein crystal growth, float zone growth, solution crystal growth, and vapor-phase crystal growth. 13