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An Initial Review of Microgravity Research in Support of Human Exploration and Development of Space 3 Science and Technology Needed for HEDS: Examples from an Initial Appraisal In this chapter, all of the goals of HEDS are considered implicitly, but only the objectives of Goal 2, the human exploration and settlement of the solar system, are dealt with directly, in part because they provide the widest range of technical challenges. This chapter introduces a series of technological systems that can easily be appreciated as important to HEDS goals. Each, in turn, is discussed in terms of the impact that microgravity is expected to have and the challenges a microgravity environment is expected to present. These discussions contain implicit parallel considerations for environments intermediate in gravitational strength between microgravity and terrestrial gravity, such as those encountered on Mars or the Moon. Implicit also is the presence of humans. Through discussions, an initial survey and appraisal can be made of the challenges that the HEDS enterprise presents to NASA and the microgravity research community. At the same time, however, it should also become clear how crucial the scientific contributions required of MRD will be to meeting the overall goals of the HEDS enterprise. FLUID MANAGEMENT SYSTEMS Fundamental Effects of Microgravity In microgravity, the gravitational body force is reduced by about six orders of magnitude relative to that encountered on Earth. Perhaps the most important result of this reduction is that even the most commonplace processes in fluids no longer occur in the expected manner. Quite simply, denser portions of fluids (including both liquids and gases) no longer sink beneath less dense portions when the mixture is disturbed. In multiphase materials, such as gas-liquid, solid-liquid, or solid-gas, or in other labile materials such as fine powders or granular mixtures, buoyancy-driven forces and weight-induced stresses are reduced or virtually eliminated. In the absence of any significant gravitational force, there is of course no longer an obvious vertical reference or "down direction." Thus, liquids no longer come to rest in the bottom of a container by filling the vessel's interior shape, nor do they escape from a tipped container and spill to the floor. Inasmuch as a given volume of liquid tends to assume a minimum energy configuration (shape), the idea of what
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On Earth the motions of objects and substances often involve a balance between the force of gravity and various short-range forces such as the van der Waals attraction between particles. Since gravity is a long-range force, when it is removed from the balance, radical changes in flow patterns may result. Hence, microgravity will often affect the dynamics, statics, and stability of engineering devices and systems in ways that may be overlooked, or not understood, during the system design process. Moreover, certain forces may sometimes be masked on Earth by a strong buoyancy force but then become dominant in a reduced gravity or microgravity environment. Examples of such forces include surface or interfacial tension at fluid-fluid interfaces, colloidal or osmotic forces, electromagnetic forces, and acoustic forces. Surface capillary forces are among those that often become elevated in relative importance, and it is common in microgravity that, rather than buoyancy pressure gradients, surface tension gradients associated with temperature or concentration gradients will drive convection in liquids.1 For processes that depend critically on having buoyancy forces present, one might consider using a surrogate force, such as electromagnetic force, to compensate for the absence of gravitational body force. A reduction of gravitational forces also suppresses flows dependent on natural convection, such as boiling, or on relative motions (sedimentation) in multiphase materials (such as gas-liquid, solid-liquid, or solid-gas systems) where the densities of the phases are unequal. In processes involving the motion of powders or other granular materials, qualitatively different regimes of motion may be expected in such systems when they occur in a microgravity environment. In any system that contains a material in a fluid state, the elimination (in microgravity) or partial reduction (in, say, a lunar or Martian environment) of the gravitational force implies a weakening of buoyancy-driven flows and sedimentation processes. While many ground-based systems involving fluids or materials transport processes are not critically sensitive to gravity, it is nevertheless true that many technological and biological processes are profoundly affected. Microgravity Challenges Many of the issues in fluid management are not unique to HEDS missions but have been of concern to engineers of orbital spacecraft for decades. Some of the engineering solutions implemented for short-duration and near-Earth missions could prove feasible for HEDS missions as well. However, a better understanding of fluid behavior in low gravity will allow a more sophisticated approach to the development of technology for fluid management. In addition to new technologies, this may result in improvements in the total mass, energy efficiency, and performance of fluid management systems currently in use. Bulk Fluid Management Fluid behavior in microgravity has broad implications for HEDS mission systems. These include storage systems for liquids and gases used for chemical processing and propulsion and those for human consumables; circulation systems for liquids and gases; gas-liquid mixtures, such as cooling and heating systems; systems for water and air
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purification and for waste recycling; supply systems for drinking water (such as distillation); and, finally, systems using the transfer of fluids for the purposes of thermal management. Many of these fluid management systems involve phase changes and two-phase flows, which behave differently under reduced gravity. Thus, for example, consideration is needed of the expected modification of pressure drops through piping (Figure 3.1), the management of trapped gases in liquids, suppression of cavitation in pumps, the use of capillary effects in aiding and controlling fluid transport, and the coupled management of heat flow and fluid flow for achieving efficient and reliable systems operation. Power requirements during spaceflight missions are likely to exceed those directly obtainable from solar or thermoelectric generators. Therefore, efficient thermodynamic power cycles will be required, employing either chemical or nuclear energy. The propulsion system, the major consumer of spacecraft energy, requires propellant storage and transport systems and thermal protection and management systems—all of which will be affected by microgravity—although propulsive flows in rocket chambers and exhaust nozzles are dominated by inertial effects that overwhelm effects from reduced gravity. For example, the lack of substantial gravity means that a stored propellant is not necessarily positioned just above the tank outlet, waiting to be transferred out through the piping system. As is well known, the liquid fuel or oxidizer instead forms free surface shapes, the equilibrium configuration and motions of which depend on the fuel tank geometry, the placement of any baffles, and the location of the vapor in the tank. During station-keeping, trajectory modifications, orbital insertions, and so forth, it is crucial that all elements of the propulsion system perform well under many combinations of attitude and acceleration. Transient influences resulting from changes in the direction and magnitude of the acceleration vectors must be understood and controlled. There is also the possibility that vibrations or accelerations arising in the course of a mission will cause sloshing modes to develop in fuel tanks, unless care is taken to prevent them. The ability to control and manipulate fluids by use of acoustic or magnetic forces will no doubt be important for management of bulk fluids under microgravity conditions.
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Figure 3.1 Two-phase flow of air and water through a pipe at normal gravity (top), fractional gravity (middle), and very low gravity (bottom). (The gravity vector is directed toward the bottom of the photos.) The flow distribution of gas and liquid in two-phase systems is strongly affected by the gravity level. Currently the impact of that altered distribution on flow dynamics, heat transfer rates, and pressure drop characteristics is poorly understood. SOURCE: NASA. Two-phase Instabilities In microgravity, two-phase flows may exhibit technically important dynamic instabilities that would be absent in normal gravity. Nucleate boiling, propagation of density waves, and bubble and droplet dynamics are all quite different in microgravity as compared with normal gravity. For example, the behavior of multiphase systems during start-up or other transient operations clearly must depend on body force (i.e., gravity) level. The movement of particulates (important for in situ resource recovery), which might be suspended in flowing gases or liquids, will similarly show dynamic effects dependent on the presence or absence of gravity or other body forces. Heat Transfer A thermal management system is necessary to stabilize spacecraft environments during long-duration HEDS missions. During space transit, waste heat must be radiated to space. Fluids like Freon can be circulated through instrument panels, cabin walls, and elsewhere in the spacecraft to serve as an intermediate heat sink, collecting and then transferring heat to the space radiator. Research to extend the capabilities of "heat pipes" is clearly needed for this purpose as well. However, for high levels of heat flux, two-phase thermal control systems are generally favored, because they have lower total mass and volume than do comparable single-phase systems.2 If the heat-carrying fluid comprises two phases, then many of the multiphase flow issues mentioned in the preceding sections apply. Of fundamental concern for long-duration spaceflight under microgravity conditions are the freezing and thawing processes in stagnant fluid lines. Fluid lines may be either intentionally or inadvertently subjected to prolonged radiative heat loss in deep space (when shaded from Earth or the Sun) and then subjected to extended periods of solar heating. Exchange and Separation Special consideration of gas exchange systems is needed for oxygen recycling and for air purification. Provisions must be made for biocontainment and for quarantine in the presence of potentially hazardous or infectious agents. Separation of particulates from circulating gases and the establishment and maintenance of controlled microenvironments may be needed for many purposes aboard spacecraft and in nonterrestrial habitats. During Earth-Mars transit (100 to 200 days each way), water and oxygen must be recycled by recovery from human waste (gas, liquid, and solid) and from fuel cells. Most terrestrial systems—biological and man-made—use buoyancy and sedimentation processes to separate gas, liquid, and particulate materials. In the microgravity environment of space, systems are needed to isolate and separate particles, liquids, and
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gases reliably and efficiently, taking into account possible alteration of biological processes. This need is a key element in the overall HEDS program, because such systems are required for environmental control, energy management, chemical processing, biological separation and isolation, and distillation and purification, and all of them must operate effectively for long periods in microgravity. Spills In a leak, spill, or explosion involving a liquid on Earth, the action of gravity restricts the effects of the accident to a confined area, where cleanup and repairs can be done. In space, however, a spilled liquid could be devastating, because of its unimpeded spreading along surfaces by surface tension. To restrict the spreading of liquids in arbitrary directions in reduced gravity environments (critically important for fire suppression), surface tension barriers consisting of polymeric coatings of low-surface-energy solids such as Teflon may be effective. Flows in Fractional Gravity Fluid flows at fractional gravity clearly require more study, with such environments as the Moon and Mars in view, and in recognition that the body force environment in HEDS missions may range from microgravity far from the Sun or planets to full Earth-gravity equivalent or greater, under conditions of acceleration. Intermediate levels of 0.16 or 0.37 Earth gravity will be experienced on the surfaces of the Moon or Mars, respectively. During space travel, fractional gravity (relative to Earth levels) may need to be supplied deliberately by centrifuge or by general rotation of the spacecraft. Such "designed" gravity could introduce Coriolis force and gravity-gradient problems.3 These in turn can interact with the effects of low gravity in ways that require understanding through research. For example, Coriolis forces due to spacecraft rotation could cause rotation of fluids relative to a container, and such effects should be studied in combination with anomalous fluid behavior resulting from reduced gravity alone. MATERIALS AND STRUCTURAL SYSTEMS Fundamental Effects of Low Gravity on Materials Processing NASA's microgravity research on materials and materials processing attempts to explore and exploit the relationships existing among the structure, properties, and processing parameters of metals and alloys, ceramics and glasses, polymers, composites, and semiconductors. Figure 3.2 shows an example of changes in structural composition that take place in a solidifying alloy as the gravity level changes. Of particular relevance to HEDS goals are those processes in which the resultant materials properties and behavior exhibit sensitivity to, or modification by, the magnitude and direction of gravity during processing. The response to gravity in materials processing usually arises from the presence of a fluid phase, the transport properties of which become modified by flows induced by the presence of internal density gradients interacting with the molecular and gravitational body forces. Specifically, it is known4 that solidification, crystal growth, casting,
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fusion welding, liquid-phase sintering, and containerless processing of molten materials are some of the important examples of commonplace materials processes that are influenced in major ways by gravity and would therefore be changed if carried out under nonterrestrial or microgravity conditions. For example, the energy transfer from a welding heat source to the material being welded depends on the flow state of the molten welding pool. Gravity affects the flow patterns in a welding pool and consequently alters the solidification process and changes the metallurgical structure and mechanical properties of the weld. Another example of a materials processing technique that has been shown to be altered by microgravity processing is liquid phase sintering (LPS).5 During the LPS of metallic alloys under microgravity conditions, the spreading of liquid along the grain boundaries and the resultant microstructural evolution have been shown to be altered.6 Such microstructural changes are expected to affect final mechanical properties.7 Additionally, crystal growth of compound semiconductors in microgravity has resulted in improved chemical homogeneity of the grown crystals. It has been suggested that this is due to the damping of gravitationally dependent thermosolutal convection and the resultant achievement of diffusion-controlled growth. Yet another example of the unique processing environment afforded by microgravity is in the solidification of immiscible alloys.8,9,10,11 Under terrestrial conditions, these alloys separate upon solidification into two immiscible phases due to the density difference between phases. However, under microgravity conditions it is thought that steady-state coupled growth can be achieved, thus eliminating the undesirable phase separation in favor of an aligned microstructure. Attainment of such an aligned microstructure holds great promise for many new technologically important materials for applications such as magnetic materials, catalysts, and electrical contacts. Figure 3.2 Sample of hypereutectic cast iron alloy (iron mixed with graphite carbon) directionally solidified during parabolic flight of a KC-135 aircraft. During the high-g portion of the trajectory, the solid forms at the eutectic composition (about 5% graphite) with a uniform distribution of graphite flakes or nodules, and the excess graphite floats to the top. This excess is captured during the low-g portion of the trajectory, demonstrating that it is possible to greatly increase the volume fraction of the second phase material by processing off-eutectic compositions in microgravity. The mechanical properties of the alloy are determined
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largely by the morphology of the graphite particles. SOURCE: NASA. Microgravity Challenges The construction and deployment of materials for safe human habitats and space platforms require (1) the processing of materials under nonterrestrial gravity into structural elements, such as beams, columns, trusses, and shells, and (2) integrating these elements into useful structural systems. Welding Welding is an example of a materials processing technique that will be critical to the creation of reliable joints in space. Fusion arc welding, in particular, is an important technique for joining metals and alloys into useful structures and machines. This technique competes well with riveting and other mechanical fasteners for construction from steel on Earth. Moreover, fusion welding provides a particularly good example of a mature joining process, because it is used in virtually every sector of terrestrial manufacturing and construction. Also, fusion welds permit retention of good thermal and electrical conductivity through seams and joints. However, parameters for achieving optimal properties of a fusion weldment are expected to change when the process is conducted in space or on extraterrestrial bodies, where gravity and atmospheric pressure and composition are different from those on Earth. Arc or electron-beam fusion welding in space may prove especially desirable when permanent joints of high strength are desired between identical or compatible alloys. In addition, competing methods for creating joints—for example, preassembled self-erecting joints, mechanical fasteners, solid-state welding, or gluing, all of which are insensitive to gravity—should be assessed and compared for relative cost, reliability, and safety. Experience gained during the orbital construction of the International Space Station (ISS) over the next 5 to 10 years should prove invaluable in this regard. Electron-beam welding is a sophisticated joining method, used primarily for making high-quality metallic fusion bonds. When it is conducted in space, however, several novel problems arise for which our considerable terrestrial experience is of little value. For example, the altitude and attitude of the orbiter affect the atmospheric pressure around the welding platform. Safe and efficient operation of the electron guns used to make these welds requires ambient pressures that do not greatly dissipate the kinetic energy of the beam. In space, electrical grounding of the welding beam through its power supply must be assured to prevent buildup of a space charge, or perhaps the accidental looping of the welding current into sensitive areas of the shuttle, or worse, through the body of the welder. Also, in the vacuum of space one must contend with uncontained rapid evaporation and sputtering of certain metallic components released during welding, such as magnesium and zinc from aerostructural alloys and chromium from stainless steels. The evaporated metal atoms tend to follow line-of-flight trajectories and redeposit upon striking a cool surface. Such redeposited metal films or coatings must be controlled carefully by physical shielding to prevent inadvertent damage to optical windows, electrical insulators, sensitive mechanical devices, antennas, and, of course, the astronauts themselves. Finally, while in orbit, without Earth's gravity acting to provide normal hydrostatic forces and buoyancy, a solidifying weld pool can become sensitive to thermocapillary effects induced by the severe
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thermal gradients attending fusion welding, to environmental vibrations, and perhaps even to other mechanical disturbances such as sudden releases of gas. All these microgravity effects can influence the quality and performance of welds in ways that are not currently predictable. The interactions of metallurgical variables with the appropriate welding parameters needed for successful welding in space are still poorly understood in general and are not known for specific cases of interest. Microgravity research directed toward this important joining technology would be of value to future HEDS missions that might rely on fusion welding methods for cutting and joining of metallic materials. Structures Nonmetallic materials that cannot be welded into useful structures will also be used in HEDS missions. Systems using composite members are likely to be fabricated using fasteners, polymeric adhesives, or both. These joining methods for nonmetallics, though less sensitive to the gravitational level, still respond sensitively to environmental factors encountered in space or on extraterrestrial bodies. To be specific, the durability, viscoelastic aging, and overall mechanical reliability of curable, organic-polymer joints must be assessed before serious consideration can be given to their use for long-term structural service in space or in nonterrestrial applications. These concerns arise for organics, especially in service applications where intense ultraviolet radiation is encountered or where energetic fluxes of atomic oxygen occur. Reliability assessments based on exposure to accelerated high-fluence radiation and oxygen or on natural long-duration exposure to the actual space environment should be contemplated early enough in the development of HEDS missions to construct engineering databases on joint performance and aging. Erecting structures in space or in extraterrestrial settings might also involve the use of new technologies for creating the materials themselves, either terrestrially or from in situ sources. Examples could include construction from lunar materials and use of recycled metallic materials. Techniques must be developed, or adapted, for positioning these materials as structural components and finally joining them in space. (See also the section "In Situ Resource Utilization" below.) BIOTECHNOLOGY ASPECTS OF LIFE SUPPORT In a preliminary survey of the goals of the HEDS enterprise, the committee identified two areas currently supported in MRD's biotechnology discipline that have special relevance to space exploration and settlement. These are cell cultures and bioseparations. In the discussions below, culturing of cells is further divided into two categories, reflecting different areas of impact: (1) mammalian cell and tissue culturing and (2) microbial and plant cell culturing. Fundamental Effects of Microgravity The many and varied effects of microgravity on biological systems and biological processes are incompletely known. For certain microgravity effects that have been identified, it is suspected that fluid physics and transport processes are the root causes. For
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example, the culturing of mammalian cellular aggregates is enhanced in the low-shear- force environments possible in microgravity and emulated in bioreactor devices. In another example, the resolution of electrophoretic separations is limited by density-driven thermal convection and sedimentation. But many effects of microgravity on biological systems are not well-enough characterized to be understood in terms of fundamental gravitational properties. As the challenges presented by microgravity are appraised below, some of the hypotheses relating to influences of fundamental effects are described further. Microgravity Challenges Mammalian Cell and Tissue Culture Biological systems exhibit many variations in function in microgravity environments. Well-documented effects on the human body from relatively short-term exposure to microgravity include such problems as loss of muscle mass, reduction of bone density, fluid shift to the upper body, and diminished immune responses. Tissues and organs, which are composed of multiple cell types, have evolved mechanisms for adapting their activity to gravity, for example, using gravity to help manage the distribution and movement of fluid required to maintain proper physiological function. It is expected that, in the absence of gravity, these systems can become imbalanced and suffer the deleterious effects that have been observed. Microgravity effects that alter fluid properties may possibly influence such processes as the interaction of cells with extracellular matrices, cell attachment and adherence, cell-to-cell communication, and the efficiency of transport of molecules through cell membranes. A number of effects of short-term exposure on cells and cell processes have been noted, such as the over expression of proto-oncogenes12 and decreased mitogenic response of lymphocytes,13 and have raised concerns; the effects of long-term exposure to microgravity or to fractional gravity, expressed through many generations of cells in which the gravitational environment may act as a selection pressure, are largely unknown. Experimental studies of the effects on cell cultures of long-term exposure to microgravity represent a potentially important new area of microgravity research. The medical complications that face astronauts will be caused in large part by alterations in cell and tissue function as a result of the microgravity environment. Some of these processes may be elucidated by analysis of basic cell and tissue function in microgravity or in tissue-culturing devices, such as the bioreactor, that mimic some aspects of microgravity. Endothelial cell proliferation, platelet adherence, and secretory function are just a few of the cellular processes that may become altered by shear stress effects in the bioreactor and thus need to be examined in microgravity. Without a more complete understanding of these basic processes in reduced gravity, anticipation of life-support needs, and the ability to design systems to meet those needs, will be exceedingly difficult. While the Life Sciences Division (LSD) maintains the primary responsibility for medical research within NASA, it is likely that some of the mechanisms and techniques employed to protect astronauts' health in the future will be the direct results and developments of research sponsored, perhaps jointly with the LSD, by MRD. A component of that research should be targeted to characterize the effects of gravity, and the lack thereof, on basic cell and tissue systems and on organelles. In the broadest sense, with the knowledge provided by this research, the biological effects of microgravity can at least be anticipated and perhaps even countered at the multicellular or tissue level.
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Microbial and Plant Cell Culture Another area of biotechnology important to the success of NASA's HEDS goals is the culturing of useful microorganisms in microgravity and extraterrestrial environments. This specialized area of cell culturing may be important to the production of nutrients, waste recycling, and maintenance of closed or partially closed environments. Yeasts and bacteria rely for growth on the gas exchange that occurs at liquid-air interfaces. Microgravity- induced effects can alter the nature of these interfaces, and, as a result, systems used for the culturing of microorganisms and the recovery of their products are likely to function differently. A recent NRC report provides a more detailed discussion of cell culture challenges in the context of advanced life support.14 The use of microorganisms as sources for food remains largely unexplored at present. Fail-safe operations could entail storage of desiccated yeast and other products capable of reestablishment of proliferating organisms at the time of use. This storage and regeneration technology has not been sufficiently investigated to be relied on for fail-safe operations on a spacecraft. A similar reservation applies, to a lesser extent, to plant culture technology, where seed stocks may obviate concerns about reestablishment if cultures fail at any point. Finally, in addition to sources for production of food, nonnutrient biotechnical products such as therapeutic agents with limited shelf life may eventually be required during extended space travel. A significant role for microgravity research may exist in helping to provide the scientific understanding required to learn how to maintain an environment suitable for on- board human activities over the course of extended excursions into deep space. Technology for maintenance of the atmosphere is key, involving recycling of atmospheric gases, removal or dilution of waste gases and contaminants, and replenishment with appropriate levels of vital components. Generation of oxygen, removal of carbon dioxide, and removal of contaminants, for example, are vital processes, some of which could be carried out by appropriate microbial, cell, and plant cultures. Such uses of cell cultures will require the development of appropriate closed culture vessels with controlled atmospheric conditions that can guarantee the long-term survival and functioning of microorganisms. In such culture systems, gravity can no longer drive the separation and coalescence of liquids and gases to form an interface between phases. Solving the problems of fluid phase separations and fluid handling required to support such culture systems represents a significant technological challenge. Safety issues relating to the containment of microbial cultures must also be addressed. Because some of the microorganisms might themselves have adverse effects if not isolated from the human habitat, biocontainment must be maintained in a fail-safe manner. A variety of related questions must be answered if microbial culture systems are to be relied on as a technical means for life support under microgravity conditions, and many of these questions concern the direct and indirect influences of microgravity. Knowledge and experience are lacking about the use of microorganisms and other cells in culture for recycling, for biological product formation, and as sources of food for humans. More effort to design an integrated environment for recycling of waste materials and generation of needed gases and nutrients would be worthwhile as a step toward designing for the relatively more closed operations needed in challenging environments such as those in space, where reliable exchange of nutrients and waste with an outside environment is difficult or impossible. Some of the needed knowledge and experience may already have been accumulated in the design and operation of large enclosed systems,
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such as submarines and Biosphere II, and may be of help in the design and operation of closed systems required for space travel and habitation. Clearly, maintenance of an active space station will add to the knowledge and experience applicable to operation and maintenance of a relatively closed system in near-Earth orbit. But these experiments with relatively closed systems can only approximate the rigors envisioned for deep-space travel, and experience with closed systems that better simulate the challenges of space travel is also needed. Bioseparations The ability to separate desired nutrients or other components from the bulk secretions of microorganisms in culture is also likely to be a requirement for successful waste recycling and the reliable production of food and nutrients. The design and assembly of reliable, fail-safe separation systems, will, for example, involve containment and transport of fluids and solids, a kind of microgravity biochemical engineering that provides an integration of cell culture and the biochemical separation techniques for the manufacture of nutrients. It is possible to do this kind of integration relatively routinely at 1 g, but not necessarily in microgravity, where flows are significantly different. Nevertheless, such processes may be relied on to separate nutrients from waste or toxics and should be considered an important technological area for support of crewed space missions of extended duration. Essentially all of the prior research on separations in microgravity had as a goal the harnessing of the unique attributes of microgravity to achieve separations that appeared impossible and of high value on Earth. These efforts support HEDS Goal 1 but provide little help in accomplishing other HEDS goals. Bioseparation applications in support of the exploration goals of HEDS would require the reliable functioning in space of those separation techniques that are now dependable and predictable on Earth. This redefinition of bioseparation needs represents a change of mind-set for research on microgravity effects on biological separations and, it should be noted, is a redefinition that characterizes research on microgravity effects on cell culture before and after HEDS as well. In sum, although there has been considerable past research on cell culture and bioseparations in microgravity, some key research areas remain to be explored in support of NASA's HEDS goals. Current MRD Biotechnology Research Applicable to HEDS MRD supports biotechnology research in several of the areas identified in the previous section as having benefits for HEDS. Currently, cell culture studies are being carried out in terrestrial bioreactor devices to examine basic mammalian cell function. These research programs address cellular processes and growth requirements under rotating gravity-vector conditions.15,16 However, much remains unknown, and the differences between bioreactor-emulated and true microgravity conditions have yet to be explored. Support for development of the rotating-wall, perfused-vessel bioreactor continues at MRD. Current studies of cultured cells in the bioreactor system have shown that the size of cellular aggregates, which may form by a process analogous to the multicellular aggregation that forms tissues and organs, is limited by the ability of nutrients and gases to reach cells near the center of the cell mass. Whether this phenomenon will occur in true microgravity is unknown.
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Figure 3.4 Viking photograph of water ice cap at Mars' North Pole. SOURCE: NASA. Relevance of Current MRD Research Current MRD materials-related research is directed toward understanding fundamental processes associated with the formation of crystals and alloys in microgravity, and this experience will be useful in developing ISRU programs. In fact, MRD has recently recognized the importance of this area by calling for proposals on in situ resource development as part of the materials science NRA.47 Some elements of that research, especially those associated with solidification and transport processes, could be used directly in the design of in situ resource processing systems associated with microgravity environments—particularly for the production of materials for use in orbit. Near-term ISRU systems are also being addressed, to a limited extent, via the systematic study of fundamental fluid synthesis and transport processes in fractional and microgravity environments. Issues associated with phase separation and particle removal are being addressed, but they will probably require more direct connections with those practical processes, such as waste recycling and generation of water and energy, that will become critical for lunar and Martian ISRU missions. DYNAMICS AND MACHINES The elaborate infrastructure needed for space travel and for establishing extraterrestrial human habitation will include many complex machines to carry out mission tasks. Some of these machines will be robotic, while others will carry out fluid handling operations, employing pumps, turbines, motors, and articulated structures. In many instances, although the basic physical operations of particular devices may not be affected
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by microgravity, their designs must be flexible enough to function within a system that may, in fact, depend sensitively on the presence or absence of gravity. The longer and more adaptable missions become, the more complex their systems will tend to be, in part because the absence of gravity must be compensated for by a proliferation of specially engineered devices, such as pumps to help distribute, separate, and control fluids. Since a spacecraft is, for practical purposes, also a closed system, with highly interactive components having limited redundancies, critical components of any HEDS mission systems must remain failure-free for long periods—perhaps up to a decade or more. Design for reliability of this magnitude will require development of new design disciplines and approaches, probably quite different from those now in general use. Microgravity remains central to this issue and is generally a source of uncertainty (e.g., fluid interfaces float, load directions fluctuate, screws loosen), and uncertainty is the enemy of reliability. Therefore, research on microgravity should address the control of such uncertainties. Perhaps, paradoxically, the human presence on long missions may well prove to be the most effective way to provide the redundancy required for extremely high system reliability, because the human, perhaps within narrow limits, is the only capable "system" for efficiently making unanticipated repairs and adjustments. The following paragraphs explore how the absence of gravity would affect certain devices having obvious importance for HEDS. Film Bearings for HEDS Applications Most pumps, turbines, motors, and other rotational elements needed for HEDS technology will require bearings. It is plausible that film bearings might generally be preferred for long space missions over rotating-element bearings, because contact wear is eliminated, greatly extending life. The bearing loads themselves in the absence of gravity will be different—no doubt lower—but such loads might behave in fluctuating, unstable, or unfamiliar ways. Film bearings will often be chosen because they can be light and efficient.48,49 The lubricating films of interest for space might employ available cryogenic substances such as liquid hydrogen. Research will be needed to explore the effectiveness of such substances in microgravity environments. The use of magnetic bearings may often be preferable for use in low-gravity environments, and their use will need to be explored as a research issue. Liquid flows, associated with the collection and recirculation of lubricants for bearings and their seals, conventionally depend on gravity. In the absence of gravity, film bearings would presumably need to be entirely encapsulated, or "flooded," a difficult matter for bearings operating at high speed and high pressure. Possible phase changes or "flashing" of liquids used for bearing films could trigger problems associated with multiphase flow in microgravity. Analysis and experiments concerning the thermal behavior in realistic microgravity environments of bearing designs will clearly be needed. The matter of bearings for space machinery provides an example of research and development that is needed because of microgravity effects, even though the device using the bearing itself may not require microgravity research. In effect, the needed microgravity research pertains to the whole spacecraft system and the choices to be made in evolving its design.
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Multibody Dynamics and Space Robotics The dynamical behavior of mechanical devices in microgravity certainly merits consideration along with fluid mechanics and heat transfer. The extreme structural flexibility of low-mass components of space robots, for example, means that dynamics are likely to be complex and perhaps difficult to analyze with precision. Structural damping of unwanted oscillations is anticipated to be correspondingly weak. Without gravity, joints of articulated structures may fail to position members precisely, and joint hysteresis may become a problem. In addition, other mechanical devices such as beams, chains, and tethers can be expected to exhibit complex dynamic behavior in microgravity, and their suitability for specific HEDS systems will require careful study. These topics, which are generally discussed under the heading of "multibody dynamics" (Part 4 of Teleoperation and Robotics in Space 50), may represent an area where fundamental microgravity research can play a role. The problem of general system integration of robotic and human capabilities is discussed at length in Part 2 of Teleoperation and Robotics in Space,51 balancing issues of cost, weight, reliability, power, crew safety, and health. Of course, the mission goals and tasks must be specified before any optimization can be attempted. Further, although human design parameters are fixed, robot design is not. Presumably, robots will be especially well adapted for operations in microgravity, and they should be designed for the longest possible endurance. Therefore, how to design failure-proof joint bearings for robots and how to provide appropriate dynamic controls for robots are important topics for future microgravity research. Propulsion and Power in Microgravity Propulsion systems, both as a means for space travel and for use in energy- conversion systems for long-term colonization, pose many problems of fluid handling and heat transfer in microgravity. The exact nature of the problems, and their degree of severity, depend on which propulsion and power systems are adopted. For the purposes of this discussion, it is assumed that electric propulsion and power, possibly with a nuclear energy source,52 constitute the basic system of choice. Indeed, studies have indicated that electric propulsion could offer travel time advantages for missions to Mars and, especially, beyond Mars. Also, an electric power system is most desirable for power in permanent space settlements. For the explicit purposes of this report, electric propulsion and power pose a particularly wide range of microgravity problems, especially if a Rankine cycle is adopted for reasons of thermal efficiency. While previous studies53 have recognized that nuclear-electric propulsion and power generation provide highly desirable technology for such missions, they do not obviate the need to continue research into alternate technologies. An electric power plant, perhaps as small as 10 megawatts, could accelerate the propellant electromagnetically to high velocity in a thruster, in order to maintain a steady, low-level thrust with high specific impulse for most of the travel time. The power plant could also provide station power to support a human colony. Energy storage would be a requirement for any long-duration HEDS mission. During the full-thrust phase of the space
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journey, which might last for years, the thrust could be on the order of 0.1 g; thus, the system would need to operate without failure for a long time in severely reduced, but not zero, gravity equivalent. At other times, when providing only vehicle or station power, the power system might need to operate in microgravity. In the paragraphs that follow, the major elements of a hypothetical electric propulsion and power system are briefly discussed, with emphasis on the microgravity issues. The Thruster Various electromagnetic thruster concepts have been studied54 in which an ionized gas is accelerated by electric fields. Proposed propellant gases include hydrogen, oxygen, ammonia, noble gases, and metal vapors. These presumably are to be stored in liquid or solid form prior to use and must therefore change phase in a vaporizer.55 The vaporizer might be a low-pressure flash boiler, but, in any case, vapor with a low liquid content must be formed in fractional gravity or microgravity. Control of this vaporization process in microgravity will certainly require continued research and testing. The Power Cycle If a thermoelectric generator is used, then there appears not to be an obvious microgravity issue associated with the power generation per se. However, for efficiency and high power, the closed Rankine (condensing) thermodynamic cycle is commonly proposed, with, for example, potassium as the working fluid. This system would doubtless encounter many microgravity-related problems, including evaporation of the working fluid in a boiler, its passage through a vapor turbine, and its condensation back to a liquid metal, with heat rejection, to complete the cycle. In the evaporator56 and associated pumps and piping operating in a reduced or microgravity environment, capillary forces will become important in the process of liquid-vapor separation. Such a vapor turbine will also encounter problems of bearing design for microgravity discussed elsewhere (so too will any electric generator that it may drive). The condensation step poses problems of droplet flow in microgravity. A condenser on the Earth collects liquid by gravity. In microgravity, by contrast, condensation is expected to occur in a shear zone established in a channel where vapor is brought into contact with a liquid film. Condensation may not be completed, and vapor bubbles may persist in the flow system, altering heat transfer efficiency. Thermal Control Systems The waste heat from the various sources on the spacecraft or station will be collected and conveyed to a space radiator for rejection to space.57 The largest source would probably be the power plant condenser, but there would be contributions from a myriad of other sources, including life-support systems. Typically, heat will be absorbed in a circulating fluid, most efficiently involving phase change (ammonia would be a typical heat- transfer fluid). The fluid would then be conveyed to the radiator system through a potentially elaborate thermal network. The pumping power to accomplish this distribution would be supplied by mechanical pumps sized for the largest flows required, but also by electromagnetic pumps or capillary pumps (heat pipes) where appropriate.
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Details governing such a thermal control system will clearly depend on mission requirements, but in general, one can expect the system to have a large and flexible thermal capacity and to be able to deal with expected or accidental load changes. Furthermore, because it is likely that efficiency and weight will dictate a two-phase circulating system, microgravity issues will undoubtedly be important. Pertinent data are available for individual components of interest, but collective, or systemwide, behavior of thermal control in microgravity requires evaluation on the ISS, if possible. (See Chapter 1 of Thermal Hydraulics for Space Power, Propulsion, and Thermal Management System Design,58 especially the articles by Alario and by Braun, for a discussion of planning for ISS thermal control system evaluation.) Need for Design Simplicity The foregoing discussion for HEDS propulsion and power technology makes it clear that system definition must precede the listing of specific problems entailing microgravity. Accommodation to microgravity is one among many issues yet to be settled. This discussion should also make clear the importance of simplicity of design; the complexities introduced by microgravity, together with the need for an unprecedented high level of reliability, should establish simplicity as a central requirement for HEDS technology. Relevance of Current MRD Research The current MRD program includes many research projects that are highly relevant to the heat-transfer and fluid-handling issues discussed above, especially those of concern in the propulsion field. However, additional research of an applied (system focused) nature may be needed to exploit efficiently the applicability of fundamental scientific results to the design of systems and components for HEDS missions. In addition, an active research community is currently studying structural dynamics and robotics for space,59 and while NASA is heavily involved in supporting that work, MRD does not have a large role in this at present. Research on film-lubricated bearings for space applications is being supported by NASA; the interest is quite applied at present. A review by MRD of current activities in structural dynamics and film lubrication might uncover new opportunities for MRD to do HEDS-related research in this field.60 Artificial Gravity Previous sections have indicated how the absence of gravitational body force generally tends to complicate equipment and system designs and to be harmful to human health. Therefore, it would seem to be a matter of basic importance for HEDS that NASA explore ways to supply appropriate degrees of artificial gravity. As of 198761 and for several years thereafter,62 NASA recognized the potential value of such measures. However, the subject seems to have been neglected in recent years. With the new HEDS goals in mind, NASA should allow for the possibility that provision of a body force substitute for gravity may prove to be an absolute necessity for ambitious manned space missions of the future.
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In principle, this may be done by imparting and maintaining rotation of the spacecraft. Quite apart from the technological means that might be employed for this purpose, only the MRD research program can provide the research basis for evaluating the needs and merits involved. For example, above what gravity level will a two-phase heat- transfer loop be preferred over a single-phase arrangement? For a given desired gravity equivalent, the needed angular velocity is inversely proportional to the square root of the radius of rotation. An undesirable by-product of rotation is the Coriolis force, which is proportional to the angular velocity. Thus, a large radius would be desired, to the degree that costs are balanced by the benefits of avoiding Coriolis effects. A similar issue is posed by body force gradient. How, then, should the undesirable Coriolis and gravity-gradient effects of spacecraft rotation be evaluated against the benefits of rotation? Little is known about such effects, especially in the various combinations that seem possible. The MRD research program should include studies that will answer the questions posed above, to enable the scientific evaluation of artificial, fractional gravity schemes that may be proposed in the future. 1. Ostrach, S. 1982. Low-gravity fluid flows. Ann. Rev. Fluid Mech. 14:313-345. 2. Krotiuk, W.J., ed. 1990. Thermal Hydraulics for Space Power, Propulsion, and Thermal Management System Design, Progress in Astronautics and Aeronautics, Vol. 122, Chapter 1. American Institute of Aeronautics and Astronautics, Washington, D.C. 3. Aeronautics and Space Engineering Board, National Research Council. 1987. Space Technology to Meet Future Needs. National Academy Press, Washington, D.C. 4. Space Studies Board, National Research Council. 1995. Microgravity Research Opportunities for the 1990s. National Academy Press, Washington, D.C. 5. Liu, Y., Heaney, D.F., and German, R.M. 1995. Gravity induced solid grain packing during liquid phase sintering. Acta Metall. Mater. 43:1587-1592. 6. German, R.M. 1995. Space study of gravitational role in liquid phase sintering. Ind. Heat. 62:52-54; see also Raman, R., and German, R.M. 1995. A mathematical model for gravity-induced distortion during liquid-phase sintering. Metall. Mater. Trans. 26A:653- 659. 7. German, R.M., Iacocca, R.G., Johnson, J.L., Liu, Y., and Upadhyaya, A. 1995. Liquid-phase sintering under microgravity conditions. J. Metals 47:46-48. 8. Andrews, J.B. 1993. Solidification of immiscible alloys. Pp. 199-222 in Immiscible Alloys and Organics, L. Ratke, ed. DGM-Informationsgesellschaft, Verlag Press, Oberursel, FRG. 9. Andrews, J.B., Hayes, L.J., and Coriell, S.R. 1995. Instabilities during coupled growth of hypermonotectics. Adv. Space Res. 16:177-180.
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10. Andrews, J.B., Hayes, L.J., Arikawa, Y., O'Dell, J.S., and Cheney, A.B. 1996. Microgravity solidification of immiscible alloys. Pp. 59-66 in Solidification and Gravity, Vols. 215-216. Transtec Publications, Bülach, Switzerland. 11. Coriell, S.R., Mitchell, W.F., Murray, B., Andrews, J.B., and Arikawa, Y. In press, 1997. Analysis of monotectic growth: Infinite diffusion in the L2 phase. To appear in J. Crystal Growth. 12. Cogoli, A., and Cogoli-Greuter, M. 1997. Activation and proliferation of lymphocytes and other mammalian cells in microgravity. Adv. Space Biol. Med. 6:33-79. 13. Morrison, D.R., Chapes, S.K., Guikema, J.A., Spooner, B.S., and Lewis, M.L. 1992. Experiments with suspended cells on the Space Shuttle. Physiologist 35:S31-34. 14. Aeronautics and Space Engineering Board, National Research Council. 1997. Advanced Technology for Human Support in Space. National Academy Press, Washington, D.C. 15. National Aeronautics and Space Administration (NASA). 1996. Microgravity Science and Applications: Program Tasks and Bibliography for FY 1995. NASA-TM-4735. NASA, Washington, D.C. 16. NASA/NIH Center for Three-dimensional Tissue Culture. 1996. Annual Report FY 1996. NASA Headquarters, Code U, Washington, D.C. 17. Space Studies Board, National Research Council. 1995. Microgravity Research Opportunities for the 1990s. National Academy Press, Washington, D.C. 18. Law, C.K. 1990. Combustion in Microgravity: Opportunities, Challenges and Progress. AIAA-90-0120, American Institute of Aeronautics and Astronautics, Reston, Va. 19. Sacksteder, K.R. 1990. The implications of experimentally controlled gravitational accelerations for combustion science. Pp. 1589-1596 in Twenty-third Symposium (International) on Combustion. The Combustion Institute, Pittsburgh, Pa. 20. Space Studies Board, National Research Council. 1995. Microgravity Research Opportunities for the 1990s. National Academy Press, Washington, D.C. 21. Law, C.K. 1990. Combustion in Microgravity: Opportunities, Challenges and Progress. AIAA-90-0120, American Institute of Aeronautics and Astronautics, Reston, Va. 22. Sacksteder, K.R. 1990. The implications of experimentally controlled gravitational accelerations for combustion science. Pp. 1589-1596 in Twenty-third Symposium (International) on Combustion. The Combustion Institute, Pittsburgh, Pa. 23. Law, C.K. 1990. Combustion in Microgravity: Opportunities, Challenges and Progress. AIAA-90-0120, American Institute of Aeronautics and Astronautics, Reston, Va.
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24. Ronney, P.D. 1985. Effects of gravity on laminar premixed gas combustion. II: Ignition and extinction phenomenon. Combust. Flame 62:121-133; see also Ronney, P.D. 1990. Near-limit flame structures at low Lewis numbers. Combust. Flame 82:1-14. 25. Law, C.K. 1988. Dynamics of stretched flames. Pp. 1381-1402 in Twenty- second Symposium on Combustion. The Combustion Institute, Pittsburgh, Pa. 26. Law, C.K. 1990. Combustion in Microgravity: Opportunities, Challenges and Progress. AIAA-90-0120, American Institute of Aeronautics and Astronautics, Reston, Va. 27. Sacksteder, K.R. 1990. The implications of experimentally controlled gravitational accelerations for combustion science. Pp. 1589-1596 in Twenty-third Symposium (International) on Combustion. The Combustion Institute, Pittsburgh, Pa. 28. Law, C.K. 1990. Combustion in Microgravity: Opportunities, Challenges and Progress. AIAA-90-0120, American Institute of Aeronautics and Astronautics, Reston, Va. 29. Sacksteder, K.R. 1990. The implications of experimentally controlled gravitational accelerations for combustion science. Pp. 1589-1596 in Twenty-third Symposium (International) on Combustion. The Combustion Institute, Pittsburgh, Pa. 30. Marchese, A.J., Dryer, F.L., Colantonio, R.O., and Nayagam, V. 1996. Microgravity combustion of methanol and methanol/water droplets: Drop tower experiments and model predictions. Pp. 1209-1217 in Twenty-sixth Symposium (International) on Combustion. The Combustion Institute, Pittsburgh, Pa. 31. Space Studies Board, National Research Council. 1995. Microgravity Research Opportunities for the 1990s. National Academy Press, Washington, D.C. 32. West, J., Tang, L., Altenkirch, R.A., Bhattacharjee, S., Sacksteder, K., and Delichatsios, M.A. 1996. Quiescent flame spread over thick fuels in microgravity. In Twenty- sixth Symposium (International) on Combustion. The Combustion Institute, Pittsburgh, Pa. 33. Bhattachaujee, S., and Altenkirch, R.A. 1990. Radiation-controlled opposed-flow flame spread in a microgravity environment. Pp. 1627-1633 in Twenty-third Symposium (International) on Combustion. The Combustion Institute, Pittsburgh, Pa. 34. Ash, R.L., O'Donoghue, P.J., Chambers, E.J., and Raney, J.P. 1993. Methodology for selective removal of orbital debris. Adv. Space Res. 13(8):243-247. 35. Ramohalli, K., and Sridhar, K.R. 1992. Space processing of indigenous resources: Oxygen production, an overview. Paper No. 92-0476, presented at AIAA 30th Aerospace Sciences Meeting, Reno, Nev. 36. Aeronautics and Space Engineering Board, National Research Council. 1997. Advanced Technology for Human Support in Space. National Academy Press, Washington, D.C. 37. Haskin, L.A. 1985. Toward a spartan scenario for use of lunar materials. Pp.
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42. Nozette, S., Lichtenberg, C.L., Spudis, P., Bonner, R., Ort, W., Malaret, E., Robinson, M., and Shoemaker, E.M. 1996. The Clementine bistatic radar experiment. Science 274 (5292):1495. 43. Christiansen, E.L., Euker, H., Maples, K., Simonds, C.H., Zimprich, S., Dowman, M.W., and Stovall, M. 1988. Conceptual Design of a Lunar Oxygen Pilot Plant. NASA-CR-172082, NASA Center for AeroSpace Information, Linthicum Heights, Md. 44. Owen, T., Biemann, K., Rushneck, D.R., Biller, J.E., Howarth, D.W., and LaFleur, A.L. 1977. The composition of the atmosphere at the surface of Mars. J. Geophys. Res. 82:4635-4639. 45. Kieffer, H.H., Chase, S.C., Jr., Martin, T.Z., Miner, E.D., and Palluconi, F.D. 1976. Martian north pole summer temperatures: Dirty water ice. Science 194:1341-1344. 46. Owen, T., Biemann, K., Rushneck, D.R., Biller, J.E., Howarth, D.W., and LaFleur, A.L. 1977. The composition of the atmosphere at the surface of Mars. J. Geophys. Res. 82:4635-4639. 47. National Aeronautics and Space Administration (NASA). 1996. Microgravity Materials Science: Research and Flight Experiment Opportunities. SOL NRA-96-HEDS-02. NASA, Washington, D.C. 48. San Andres, L. 1995. Thermohydrodynamic analysis of fluid film bearings for cryogenic applications. J. Prop. Power 11(5):964-972. 49. San Andres, L. 1996. Thermohydrodynamic Analysis of Cryogenic Liquid Turbulent Flow Fluid Film Bearings. NASA-CR-202663. NASA Center for AeroSpace Information, Linthicum Heights, Md. 50. Sklaar, S.B., and Ruoff, C.F., eds. 1994. Teleoperation and Robotics in Space, Progress in Astronautics and Aeronautics, Vol. 161. American Institute of Aeronautics and Astronautics, Reston, Va. 51. Sklaar, S.B., and Ruoff, C.F., eds. 1994. Teleoperation and Robotics in Space, Progress in Astronautics and Aeronautics, Vol. 161. American Institute of Aeronautics and Astronautics, Reston, Va. 52. Barnett, J.W. 1991. Nuclear electric propulsion technologies: Overview of the NASA/DOE/DOD Nuclear Electric Propulsion Workshop. In Proceedings of the 8th Symposium on Space Nuclear Power Systems. CONF-910116, American Institute of Physics, College Park, Md. 53. Aeronautics and Space Engineering Board, National Research Council. 1987. Space Technology to Meet Future Needs. National Academy Press, Washington, D.C. 54. Barnett, J.W. 1991. Nuclear electric propulsion technologies: Overview of the NASA/DOE/DOD Nuclear Electric Propulsion Workshop. In Proceedings of the 8th Symposium on Space Nuclear Power Systems. CONF-910116, American Institute of
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