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III Survey of Technologies for the Human Exploration and Development of Space This chapter provides the essential foundation for subsequent discussion of microgravity phenomena and the determination of related research needs important for HEDS. That foundation consists of rather detailed descrip- tions and assessments of the various technologies that seem most important for HEDS systems. In the committee's view, these are the types of technologies that must operate reliably and efficiently in the various space environ- ments of interest. Two important considerations were involved in the selection of technologies for discussion. First, it was clear that this report could not usefully embark on studies of systems and mission architectures, with their specific design problems, based on premature mission assumptions. Secondly, the range of technologies identified needed to be broad enough to cover reasonable possibilities for incorporation into future systems but did not have to cover all conceivable possibilities. Therefore, this report emphasizes technologies that have a wide range of potential applications and that are expected to be significantly influenced by gravity level. In this chapter, the technologies selected for discussion are grouped according to their probable functions in the HEDS program. Since some are quite well developed already, while others exist only as concepts, the level of detail varies considerably. Systems to serve HEDS functions are identified first, followed by their components or subsystems. Especially at the subsystem level, microgravity concerns are identified and summarized in a table for each function. Tables III.G.1 to III.G.3 at the end of this chapter relate microgravity phenomena to the various subsystems and processes. However, the phenomena are not treated in detail, nor are relevant research issues described. Rather, in Chapter IV, the identified microgravity concerns are related to physical phenomena, and physical research areas are identified that may provide the knowledge base needed to design systems and components that will be reliable and effective in the microgravity environments of interest. III.A POWER GENERATION AND STORAGE Introduction Future HEDS missions for the exploration and colonization of the solar system will require enabling technolo- gies for adequate, reliable electrical power generation and storage. Advanced, high-efficiency power generation and storage will be required for deep-space missions, lunar and planetary bases, and extended human exploration. 21

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22 MICROGRAVITY RESEARCH Extensive up-to-date discussions are available (NRC, 1987, 1998; Bennett et al., 1996; Brandhorst et al., 1996; Detwiler et al., 1996; Bennett, 1998) and there is no need to repeat them here. Because the electrical power technology requirements for spacecraft are similar to the requirements for the extended human occupation of the Moon or Mars (when energy demand is not constant) they are discussed together. Space propulsion is, of course, the dominant and limiting power-generation requirement for HEDS. However, due to the wide range of systems that must be considered, propulsion is discussed separately in Section III.B. Many of the means of power generation applicable to spacecraft and station power discussed in this section are also applicable to propulsion. For the purpose of the present discussion, the primary energy sources for conversion to electrical power on a spacecraft are the following: (1) solar radiation, (2) chemical and electrochemical, and (3) nuclear (radioisotope thermoelectric generators (RTGs), dynamic isotope power (DIP) sources and fission and fusion power). The choice of energy source and power-generation system and subsystem is dictated largely by the mission require- ments. These energy sources can be utilized in open or closed thermodynamic systems. A closed-cycle system is one in which a working fluid is heated, does work, and is recycled (Figure III.A.1~; an open-cycle system is one in which a working fluid is heated, does work, and is discharged, carrying waste heat with it. The electric power generated requires a power management and distribution system that includes regulators, converters, control circuits, etc. (Figure III.A.2~. Energy storage devices may also be required, since some energy sources (e.g., solar radiation) are not continuous. The power-to-mass ratio (in kWe/kg) of the power system is an important consideration for space missions. Small versus large power needs and autonomous versus manual control are additional factors. The fact that electrical power generation and onboard propulsion subsystems can account for one-half to three-fourths of the mass of the typical Earth-orbiting satellite or planetary spacecraft provides the motivation to reduce their mass, which would allow more of the spacecraft's mass to be devoted to payload. The desire to reduce costs and maintain reliable performance has led to the consideration of both some old and some new technologies for electric power generation; these technologies are reviewed and discussed in detail in Brandhorst et al. (1996), Detwiler et al. (1996), and Landis et al. (1996~. A= OR PUMA ~ r TURBINE tENERATOR | POWER HEAT SINK ~ MANAGEMENT | LOAD l FIGURE III.A. 1 Generic diagram of a simplified closed-cycle space power system. For a Brayton cycle, the heat source is a heat exchanger where heat from the source is added to the working gas, and the heat sink is also a heat exchanger (i.e., space radiator) where the working fluid is cooled. For a Rankine cycle, the heat source is a boiler where the working fluid is boiled, and the heat sink is a heat exchanger (i.e., space radiator) where the working fluid is condensed.

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SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE ENERGY SOURCE SOLAR NUCLEAR CHEMICAL CONVERTER SOLAR CELL ARRAY SOLAR DYNAMIC NUCLEAR ETC. POWER MANAGEMENT AND DISTRIBUTION REGULATORS CONVERTERS CONTROL CIRCUITS ETC. T 1 ENERGY STORAGE RECHARGEABLE BATTERIES REGENERATIVE FUEL CELLS FLYWHEELS CAPACITORS, ETC. FIGURE III.A.2 Schematic of a generic electric power system. Based on Bennett (1998~. 23 SPACECRAFT SYSTEMS OADS Gravity is an important consideration in active (thermal) subsystems for power generation. Many of these subsystems involve single and/or multiphase fluid and thermal management. Such important subsystems as boilers, condensers, evaporators, heat exchangers, normal and cryogenic fluid storage units, fuel cells, radiators, and heat pipes involve fluid flow and/or transport phenomena, including heat and mass transfer, phase separation, and others. Because fluid flow and transport phenomena are affected by gravity, a full understanding of the phenomena is needed for the design of the systems and for their safe and efficient operation in microgravity or reduced-gravity environments. Power Generation Systems Solar Power Systems The principal solar/electric power systems are of two types: passive (i.e., photovoltaic or photoelectric) and active (thermal). In the literature (Bennett, 1998) the former is referred to as static and the latter as dynamic. The solar cell arrays used on most spacecraft usually consist of a large number of cells that convert a fraction of the solar radiation incident on them to electricity by means of the photoelectric effect. The solar cells are connected into appropriate series/parallel circuits to produce needed power at required voltages and currents. On the recent Mars Pathfinder mission, the lander, the Sojourner, and the cruise system were all powered by gallium-arsenide solar cells. This was the first use by NASA of solar/photovoltaic power on the surface of Mars. Recent discussions of the advances in solar cell technology are available (Landis et al., 1996; Bennett et al., 1996; Bennett, 1998~. There are two ways of using solar energy: directly, by thermoelectric means, or indirectly, using solar radiation to heat a working fluid, which then drives a turbine/alternator (generator). The former method simply requires thermoelectric elements placed at the focus of a concentrator. While simple to construct, the power density is low and the system has not been used by NASA to power a spacecraft. Moreover, solar power will be of limited value for deep-space missions and may be unreliable at extraterrestrial sites, e.g., Mars. Indeed the use of photovoltaic solar cell arrays will be restricted to spacecraft that do not travel beyond the Mars orbit. This is the case primarily because the solar irradiation (insolation) decreases as a square of the distance from the Sun. The collectors/concentrators would have to be larger, and this would prohibitively increase the mass of the propulsion subsystem. Ionizing radiation, low intensity, and low- and high-temperature (for the solar probe) degradation effects are other problems that must be addressed in the use of solar cell arrays for photovoltaic electric power generation (Bennett, 1998~. Solar/thermal dynamic systems can overcome some of the problems,

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24 MICROGRAVITY RESEARCH such as radiation damage to solar cells and the limited life of chemical energy storage systems, but there are issues with the technology that have not yet been solved. The need for moving components and the reliability of parts of solar dynamic systems are some of the viability issues that have not yet been addressed. The active method of using solar radiation to produce electric power is to heat a working fluid that can drive a turbine/generator in much the same way as is done in the electric power industry. To this end, a large number of solar collectors can be used to convert a fraction of the incident solar radiation to sensible and/or latent energy of the fluid. Heat is transferred from the structural collector elements to the fluid by forced convection and/or flow boiling. The energy collected can then be stored in a thermal energy storage device for use when insolation is not available. The component technologies for active power conversion for both the Brayton and Rankine cycles are fully mature. However, waste heat rejection in Brayton and Rankine thermodynamic power-generation cycles is of major concern, because space radiators represent a significant portion of the weight of the system. NASA's Glenn Research Center has performed the first full-scale demonstration of a complete space-configured 2-kW solar active system based on the Brayton cycle in a relevant space environment. However, one of the criticisms that has been leveled at solar/thermal active systems is that the conversion systems depend on moving parts that are considered to be intrinsically less reliable and shorter-lived than those in photovoltaic conversion systems. In summary, solar power systems may not be feasible for many deep-space missions, lunar and planetary bases, and extended human exploration missions or for powering high-thrust, high-efficiency propulsion systems (NRC, 1998). Chemical Power Systems Power systems based on chemical energy sources include batteries and fuel cells. Storable chemical reactants (e.g., nitrogen tetroxide, mixed amines, hydrogen, oxygen, and other chemicals) can be stored aboard spacecraft or on the surface of Mars for power generation using a mass open or closed thermodynamic system. Considerable technology relevant to this application is available from the Apollo program. The principal unknown in using chemical reactants to produce electric power is the ability of the spacecraft systems to tolerate the effects of any chemical effluents that are released. Although chemical energy sources appear attractive because they offer rapid response, as Figure III.A.3 illustrates, chemical energy sources for electric power generation are suitable only for short-duration functions and/or missions. Also, as can be seen from Figure III.A.3, a fundamental shortcoming of the chemical energy sources for power generation is that the mass of the chemical reactants becomes prohibitive for burns and/or missions of long durations (NRC, 1987; Bennett, 1998~. Stored chemical energy could be used to meet short-duration peak or emergency power demand, but for long-duration missions, solar or nuclear energy sources with energy conversion technologies based on a Brayton or Rankine cycle power-generation system would be the most suitable. Fuel cells are electrochemical devices that convert the chemical energy of a fuel directly into electrical and thermal energy (Blomen and Mugerwa, 1993~. In a typical fuel cell, gaseous fuels are fed continuously to the anode (negative electrode) compartment and an oxidant, e.g., oxygen or air, is fed continuously to the cathode (positive electrode) compartment. The electrochemical reactions take place at the electrodes to produce an electric (direct) current. The fuel cell theoretically has the capability of producing electrical energy for as long as the fuel and oxidant are fed to the electrodes. In reality, degradation or malfunction of components limits the practical operating life of fuel cells. Besides directly producing electricity and having the capacity to serve as energy storage devices, fuel cells also produce heat and water. The heat can be utilized effectively for the generation of additional electricity or for other purposes, depending on the temperature. A practical consideration for fuel cells is their compatibility with the available fuels and oxidants. For HEDS missions, at least four applications of fuel cells are possible: (1) electric power generation in a space vehicle or at an extraterrestrial site, (2) surface transport on Mars or the Moon, (3) production of oxygen (O2) from carbon dioxide (CO2) on Mars, and (4) production of potable water for life support. One of the main attractive features of fuel cell systems is the expected high fuel-to-electricity efficiency and the fact that they can also be used as storage devices. This efficiency, which runs from 40 to 60 percent based on

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SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE 105 104 - a, 103 UJ > ~ 102 o ~ 10 C: UJ 10 1o-l ,1 ~ ~ DYNAMIC ISOTOPE POWER SYSTEMS // // TRI C 1 HOUR 1 DAY 1 MONTH 1 YEAR 10 YEARS DURATION OF USE 25 FIGURE III.A.3 Qualitative diagram illustrating the regimes of applicability of various space power systems. Courtesy of Gary L. Bennett, Metaspace Enterprises. the lower heating value (LHV) of the fuel, is higher than that of almost all other energy conversion systems. In addition, high-temperature fuel cells produce high-grade heat, which is available for cogeneration applications. If waste heat is utilized, the theoretical efficiency can reach 80 percent (Klaiber, 1996~. Because fuel cells operate at near-constant efficiency, independent of size, small fuel cells are nearly as efficient as large ones. Thus, fuel cell power plants can be configured in a wide range of electrical power levels from watts to megawatts. Fuel cells are quiet and operate with virtually no noxious emissions, but they are sensitive to certain fuel contaminants, e.g., carbon monoxide (CO), hydrogen sulfide (H2S), ammonia (NH3), and halides, depending on the type of fuel cell. Thus, the contaminants must be minimized in the fuel gas. Fuel cells have been identified by the National Critical Technologies Panel as one of the 22 key technologies the United States must develop and implement in order to achieve economic prosperity and maintain national security (National Critical Technologies Panel, 1993~. A variety of fuel cells have been developed for terrestrial and space applications (Blomen and Mugerwa, 1993~. Fuel cells are usually classified according to the type of electrolyte used in the cell: alkaline fuel cells (AFCs), phosphoric acid fuel cells (PAFCs), molten carbonate fuel cells (MCFCs), solid oxide fuel cells (SOFCs), and proton-exchange membrane fuel cells (PEMFCs). The operating temperature ranges from ~80 C for PEMFCs to ~1000 C for SOFCs (Kroschwitz and Bickford, 1994~. The physicochemical and thermomechanical properties of materials used for the cell components (e.g., electrodes, electrolyte, bipolar separator, and current collector) determine the practical operating temperature and useful life of the cells. The properties of the electrolyte are especially important. Solid polymer and aqueous electrolytes can be used only at ~200 C or lower because of high water-vapor pressure and/or rapid degradation at higher temperatures. The operating temperature of high-temperature fuel cells is determined by the melting point for MCFCs or the ionic-conductivity requirements for SOFCs of the electrolyte. The operating temperature dictates the type of fuel that can be utilized. Interfacial and transport (flow, heat, mass, charge) phenomena in the membranes (porous media) under reduced or microgravity conditions are also important issues in the design and safe operation of fuel cell systems.

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26 Nuclear Power Systems MICROGRAVITY RESEARCH Nuclear energy sources for power generation come in three types: radioisotope, fission reactor, and fusion reactor. Since sustained fusion has not yet been demonstrated in a laboratory and no reactors are likely to be available even for terrestrial applications until well into the twenty-first century, this type of nuclear reactor is not considered. An up-to-date discussion of nuclear power technology for spacecraft applications is available (Bennett, 1998; NRC, 1987~. Suffice it to summarize that since 1961, the United States has flown 44 radioisotope thermoelectric generators (RTGs) and one nuclear fission reactor (see below) using thermoelectric conversion to provide power for 25 space systems. The Galileo mission to Jupiter, the Ulysses mission to explore the polar regions of the Sun, and the Saturn-bound Cassini mission are powered by RTGs operating at 1000 C (Bennett et al., 1995; Bennett, 1998~. For example, the Cassini spacecraft was developed and launched in October 1997 on a mission to investigate Saturn and its rings, satellites, and magnetosphere. It is powered by three RTGs. RTGs have been used by NASA for many years, and this technology is mature and reliable. It is not sensitive to gravity; however, it is currently limited to relatively low power levels (see Figure III.A.3~. The United States has flown one space nuclear fission reactor (SNAP-1OA), which was launched in 1965 and provided 500 W of electric power. SNAP-1OA was a liquid-metal-cooled nuclear reactor with a thermoelectric conversion. A ground-test twin of the flight version of SNAP-I OA operated unattended for over a year, demon- strating the feasibility of the fission nuclear reactor. According to reports (Bennett, 1998), the former Soviet Union launched perhaps as many as 33 low-power (~1 to 2 kWe) nuclear fission reactors from 1967 to 1988 to power its radar ocean reconnaissance satellites. All of the reactors used thermoelectric elements to convert thermal energy to electricity. Fission nuclear reactors can be characterized as having a very good power-to-weight ratio. An example of a reactor designed for space use is the reactor that was being worked on for SP-100. Jointly undertaken by the Department of Energy (DOE), the Department of Defense (DOD), and NASA, the SP-100 program had the goal of developing a space nuclear reactor technology that could support a range of projected missions, including nuclear electric propulsion and planetary surface operations. Several systems were designed for a power output of 100 kWe and incorporated a high-temperature, liquid-metal-cooled reactor. One design concept used an inert-gas Brayton cycle with a turbine generator, while another was designed for use with an advanced thermoelectric converter. Most of the nuclear component development had been completed on SP-100 before the project was cancelled in 1994. Currently the United States has no useful space nuclear reactor program, even though recent studies continue to show (AIAA, 1995; NRC, 1998; Friedensen, 1998) that human exploration of the Moon and Mars will require this technology. To quote a recent National Research Council report (1998, p. 19~: "The committee is well aware that political constraints may make R&D on advanced space nuclear power systems unpopular. However, the committee could not ignore the fact that space nuclear power will be a key enabling technology for future space activities that will not be able to rely on solar power." Energy Storage Reliance on intermittent (i.e., solar) energy sources as a primary means of electrical power generation requires some method of energy storage. The storage methods may be chemical (primary and secondary rechargeable- batteries, and primary and regenerative fuel cells), electrical (capacitors), mechanical (flywheels, gravitational liquid or solid), or thermal (latent or sensible heat). The design and performance of storage systems are judged on lifetime, reliability, safety, efficiency, and specific energy. For example, the two principal systems that are being used or being considered are nickel-based and lithium-based batteries. An up-to-date review of storage systems used or under development for spacecraft applications is available (Bennett, 1998~. In summary, great progress is being made by both NASA and DOD on a range of battery technologies that promise improvements by a factor of 10 in specific energy over the old nickel-cadmium batteries. Lithium-based batteries (i.e., rechargeable lithium ion batteries) have a potential to achieve a specific storage of up to 200 We-h/kg. Fuel cells have been used by NASA to power the internal systems of Gemini and Apollo spacecraft and are currently used to power the space shuttle. The lower operating temperatures and higher efficiencies (which

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SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE If '''''' - ~~.~ ' 'a ~ ^ Off- H2'f'0 err...... __ Fuel Electro- Cell Iysis Cell . ~ Lo ~ t _ ~~ L]~^ ~ 27 aaan.~a.ea.~aa. Sad -aa.~eaaea aaa- .aa.~'aaaea an aa.~-~eal ~.aa.~eaaaaae .aae.~-al~aaaaa ~-~-~ea.~-a ~-~--~.aaa. alaaaaa-aa.~a a I.. Solar l: a Panel '.a la --.-- aa.~ea.~aae Baa ~ea.~ea.~.aaa. sea ae---~ea'--- ~ala.~. Al ~aa.~-~eala.~a ~eeaa-aaa.~ea ~.aa.~-----~-e A------ - - - --- -------------- ---------~-~e ---~-~----~--- -------------- -----------a- -- - - - - -eae - - - - -~~~ me- -~- -~a. - -------e-~e -.- -.a..~ e~-~-a-a --~-a---~----e -~~- - -aa -------------- FIGURE III.A.4 Schematic diagram of the fuel cell-electrolysis cycle. SOURCE: Mayer (1992~. Reproduced with permis- sion of the American Society of Civil Engineers. translate to reduced weight) of PEMFCsimake them attractive options for planetary missions. Their superior performance and longer life have led NASA to look closely at PEMFCs. The first PEMFC used in an operational space system was the GE-built, l-kW Gemini power plant. Two 1-kW modules provided primary power for each of the seven Gemini spacecraft in the early 1960s. Each module could provide the full mission power require- ments. The performance and life of the Gemini fuel cells were limited by the polystyrene sulfonic acid membrane used at that time. In 1968 an improved Nation membrane was introduced, significantly improving performance and life. New fuel cell technologies for electric power generation continue to be developed, and there are about 200 units being used for Earth applications in about 15 countries (Hirschenhofer,1996~. As discussed previously, four types of fuel cells (classified primarily on the basis of electrolyte and ranging in operating temperature from about 100 to 1000 C) are being developed for terrestrial applications in North America. To reduce risk due to unreliability of fuel cells, NASA has decided to use batteries on the International Space Station (ISS), but these batteries are heavy and may not be the most efficient or cost-effective means of power generation or storage on the surfaces of the Moon or Mars. An attractive alternative for power storage is the regenerative fuel cell or, even more simply, separate electrolysis and fuel cell systems (Figure III.A.4~. During the day, excess solar power may be used to electrolyze water and generate hydrogen and oxygen. At night these gases are recombined in a fuel cell to produce electricity and water. Electrolysis and fuel cell systems require a supply of oxygen and hydrogen, and platinum (wire) is used for electrochemically active surfaces. Potassium salts serve as the electrolyte. Fuel cell technology is mature and the efficiency of electrolysis is also high. One very significant advantage of the regenerative H2-O2 fuel cell is that it can have a dual function it can be used not only for energy storage but also for life support on a spacecraft (Eckart, 1996~. The electrochemical reactions involving hydrogen and oxygen are the only practical ones at the present time. The oxygen is usually derived from air, but it can be produced on Mars using solid oxide electrolysis (Sridhar and Vaniman, 1997~. Hydrogen may be obtained from several fuel sources, e.g., steam-reformed Earth fossil fuels. Other fuels such as methanol can also be used (Klaiber, 1996~. {Proton-exchange membrane fuel cells (PEMFCs) are also referred to as solid polymer fuel cells (SPFCs). By virtue of its intrinsic simplicity and high power density, the PEMFC/SPFC has a distinct advantage over other fuel cell technologies. In this discussion the name PEMFC is used.

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28 MICROGRAVITY RESEARCH The intermittent nature of solar energy availability (when, for example, spacecraft or satellites enter into planetary shadow) for low-Earth-orbit or other applications presents a particular challenge for space power man- agement schemes. One alternative to photovoltaic (PV) cells with battery storage is a solar dynamic system with latent heat thermal energy storage (LHTES) via solar heat receivers. During the charging phase heat is stored in the phase-change material while it is melted; during discharge the latent heat is released as the material is solidified. Solar receivers with integral LHTES are needed for generating electric power in space when using solar energy in conjunction with a Brayton cycle (Shaltens and Mason, 1996~. A eutectic mixture of LiF-CaF2 salts, which has a melting temperature of 1413 OF (767 C), is used as a phase-change material in this application. Electrical (capacitor, superconducting magnet), mechanical (flywheel), and thermal (latent and sensible heat) storage systems also have potential for use on specific missions but have not yet been developed for spacecraft applications (Bennett, 1998~. For example, studies have shown that a solar active system can produce a unit of power from a smaller collector area than is required for an array of solar cells. The improvement in system efficiency results from the increased conversion efficiency of a solar active power cycle compared with solar cells and from the higher specific (kWe-h/kg) energy storage capacity compared with batteries. Some Selected Subsystem Technologies There are many passive and active electric power-generation systems and subsystems. The conversion system that changes the thermal power into electric power distinguishes passive from active power-generation systems. If the conversion system does not use a working fluid it is considered to be passive, but if it employs a working fluid then it is considered to be active. For example, the alkali metal thermal-to-electric conversion (AMTEC), which utilizes high-pressure sodium vapor supplied to one side of a solid electrolyte of beta-alumina, causing sodium vapor to be removed from the other side, is considered to be an active system in spite of the fact that it does not have any rotating parts. The more important subsystems are identified in Table III.A. 1, along with the potential of reduced gravity to affect their operation. It is beyond the scope of this report to identify and discuss all of the subsystems in detail; rather, some selected ones are mentioned. Since some of the components of space propulsion systems are the same as those of power-generation and storage systems, reference is made to Section III.B of this report for a discussion of the subsystems common to both. Passive power systems can use solar (photovoltaic), nuclear-radioisotope (thermoelectric, thermoionic, and thermophotovoltaic), and chemical (fuel cells) energy sources. In the past, static electric power generation systems have enabled, or enhanced, some of the most challenging and exciting space missions, including NASA missions such as the Pioneer flights to Jupiter and Saturn, the Voyager flights to Jupiter, Saturn, Uranus, and Neptune, and the Galileo mission to orbit Jupiter (Bennett et al., 1996~. The main disadvantage of the passive power generation systems is that they are much less efficient than the active systems and therefore have a significant weight disadvantage vis-a-vis those systems. There are various designs for closed-cycle, active power-generation systems. The three most important types are the Brayton, Rankine, and Stirling cycles. It should be mentioned that solar, nuclear, and chemical energy can be used as a source of heat for all cycles. A Brayton cycle is a conventional closed-cycle system that employs a gas turbine and in which the working fluid is a gas flowing throughout the power-generating loop. A Rankine cycle is like a conventional steam cycle in which the vapor is produced in a boiler, does work in a turbine, and condenses in a condenser (radiator). A Stirling cycle is a closed-cycle reciprocating engine whose working fluid is a high- pressure gas, either helium (He) or hydrogen (Hat. For space power systems the use of both solar and nuclear energy conversion systems based on the Stirling cycle engines has been considered, and a recent discussion that cites a large number of relevant references is available (de Monte and Benvenuto, 1998~. A schematic of a closed-cycle Brayton space power generation system is shown in Figure III.A. 1. The energy source could be solar, nuclear, or chemical. The technology for the conventional Brayton cycle is well established. The main advantages of the cycle are its high efficiency and very good specific power (in kWe/kg). As already discussed, a 100 kWe power unit based on nuclear energy (SP-100 program) was designed but never tested (Bennett, 1998~. The problem of rejecting heat from the Brayton cycle space power-generation system is the main concern. The large mass and size of the radiator make it a dominant component of the overall power system. The

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SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE 29 choice of optimum temperature level for power conversion depends on the compromise between materials limita- tions and thermal performance. Low heat rejection (radiator) temperature improves thermal performance but results in large, massive radiators. The block diagram for a closed-cycle Rankine system is the same as that illustrated for a Brayton cycle (see Figure III.A.1~. The main difference between the two is that the Rankine cycle involves liquid/vapor mixtures. Boiling takes place in the heat source (boiler, nuclear reactor core) and condensation of the vapor occurs in the radiator. A dynamic system based on the Rankine cycle, which is expected to be more efficient and lighter in weight per unit power generated, has not been designed and operated. The primary reason for NASA's lack of interest in the Rankine cycle is that it involves two-phase flow and boiling/condensation heat transfer in some of its components (i.e., boiler, condenser, separator piping, etc.) and these processes are not sufficiently well under- stood in microgravity or fractional gravity environments to allow for designing active electric power-generation systems for spacecraft. Nevertheless, the Rankine cycle for space power generation is very attractive because of its relatively high efficiency and the lower mass of the conversion system compared with the Brayton cycle (Gilland and George, 1992~. It should be noted that for use in space the boiler, condenser, piping, valves, pump, and thermal management systems need to be designed for safe, efficient, and long-life operation. However, until there is a better understanding of how multiphase systems behave in space, NASA will not be in a position to utilize them. Boiler for the Rankine Cycle As noted above, the Rankine cycle is quite efficient for electric power generation and has a higher power-to- weight ratio than a Brayton cycle. However, as noted previously, the cycle (see Figure III.A.1) has components (heat exchanger-boiler, condenser-radiator, phase separator, etc.) in which the working fluid has both liquid and vapor phases. A boiler is an essential subsystem for electric power generation using a Rankine cycle for either spacecraft or stationary power at extraterrestrial sites. However, the operation of a heat exchanger in which the working fluid is boiled (evaporated) will be greatly affected by gravity. The two-phase flow and heat transfer processes and the flow separation processes in microgravity (near zero) environments are significantly different from those on Earth or on the Moon or Mars. Predictive models for two- phase transport developed for Earth applications are often empirically based and are inadequate for a microgravity environment. Thus, designers of space power-generation systems will be challenged to develop reliable sub- systems and technologies that involve two-phase flow and transport phenomena in reduced-gravity environments. The theoretical models and computer codes need to be capable of modeling two-phase flow, boiling and conden- sation heat transfer, and flow separation and distribution phenomena for all gravity levels. This would permit simulation of microgravity in a continuously variable manner and would not only lead to an increased understand- ing of, and insight into, the fundamental multiphase phenomena but would also allow NASA engineers to design and evaluate the performance of multiphase systems for use in HEDS missions. Radiators Radiators are the only effective means of rejecting heat in space without altering the mass of the spacecraft. Of course, heat can be stored in mass that is ejected from the spacecraft, but this method is not practical for missions of long duration. A comparison of different space power systems has been made (NRC, 1990), and it was found that depending on the type of system and power capability, the radiator can account for between 35 and 60 percent of the total system mass. Radiators that take advantage of a two-phase working fluid are more efficient and are relatively lightweight, but they are subject to possible two-phase flow instabilities, freezing, structural damage caused by oscillatory forces due to periodic or condensation-induced loads, damage by meteorites, and other phenomena. Innovative radiator systems based on moving-belt, liquid-droplet, liquid-sheet, bubble membrane, heat pipes, and other concepts have been proposed (Massardo et al., 1997; Ohtani et al., 1998), but neither the fundamental physical processes of two-phase flow and heat transfer nor the proposed concepts appear to have been studied in sufficient detail to determine their practical feasibility. Space system designers continue to demand

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30 MICROGRAVITY RESEARCH design-specific data because they do not understand two-phase flow and phase-change heat-transfer phenomena in microgravity. In addition, the radiators must be robust and reliable, since it is difficult to repair them in space and their impact on life support, mission success, and cost can be very large. Liquid-droplet radiators and liquid-sheet radiators are among the most promising technologies for achieving lightweight heat exchangers for space applications. In such radiator concepts, neither flow affected by surface tension and thermocapillary forces, nor radiation heat transfer from, say, a cloud of small droplets to the ambient surroundings, has been studied in long-duration microgravity environments, so neither is fully understood. Opti- mization of the liquid-droplet radiator has revealed that the minimum specific mass is estimated to be 27 percent less than the specific mass of the system with a heat-pipe radiator (Massardo et al., 1997~. A recent experimental study of the liquid-droplet radiator has been performed, and the rate at which energy is radiated by a cloud of droplets as a function of droplet velocity and frequency has been measured (Ohtani et al., 1998~. At the droplet velocities being considered, there do not appear to be any major microgravity issues associated with fluid dynam- ics for the system, but heat transfer may be affected by atomic oxygen, and in the space environment micromete- oroids and space debris are of concern. Proton-Exchange Membrane Fuel Cells Of the various existing fuel cell systems, the proton-exchange membrane fuel cell (PEMFC) is the most promising, especially for space power-generation and transportation, because of the simplicity of its design and its low-temperature operations. Today' s PEMFC membranes are solid, hydrated sheets of a sulfonated fluoropolymer similar to Teflon. The acid concentration of the membrane is fixed and cannot be diluted by product or process water. The acid concentration of a particular membrane is characterized by equivalent weight, EW (grams dry polymer/mole ion exchange sites). This number is the reciprocal of the ion-exchange capacity in moles per gram. Generally, a lower EW and thinner membranes result in higher cell performance. However, thinner membranes also result in higher parasitic cross-diffusion of reactant gas. As noted above, fuel cells have been used and tested in microgravity. PEMFCs have limited life, and the STS- 84 mission in April 1997 was terminated after only 3 days due to problems with an onboard PEMFC. The cell showed low voltage output, and there was concern on the part of the space shuttle crew and NASA ground personnel that the H2 and O2 in the cell could cause an explosion. No formal report on the cause for the low cell voltage appears to have been released by NASA or its contractors. NASA has decided to use batteries on the ISS to reduce risk from potentially unreliable fuel cells, but the batteries are relatively heavy and may not be the most cost-effective means of power generation and transportation on the surface of the Moon or Mars. Like all fuel cells, PEMFCs can serve the dual purpose of generating electric power as well as producing water for human consumption. A PEMFC typically consists of a membrane sandwiched between two gas-diffusion electrodes, which are porous composites made of electrically conductive material. The assembly is pressed between two current collectors. The electrodes are hydrophobic so that gaseous reactants can be transported through the electrodes during cell operation. The outer faces of the electrodes are exposed to the reactant gases that enter and exit the gas chamber. Heat generated during the electrochemical reaction must be removed from the system, and proper water and heat management is essential for obtaining high power density at high energy efficiency. A heat pipe, replacing a coolant pump, heat exchanger, and thermal and other controls, can remove waste heat from the system. Effective removal of the liquid water product is required to prevent flooding of the electrode, which would prevent gas from reaching the catalyst-membrane interface, where the reactions take place. The water is liquid because of the low operating temperature (<100 C) of the PEMFC. Some PEMFC designs have used a wicking arrangement to remove the liquid water. At the cathode, or air electrode, this process is a countercurrent and competing process. The oxygen flows toward the interface and product water moves away. Two-phase (gas- liquid) countercurrent flow in microgravity is a critical phenomenon that can greatly affect the performance of a PEMFC. Flooding hampers the rate of mass transfer and results in poor cell performance, which is characterized by the cell's inability to maintain high current at a given cell voltage. Full hydration of the cell membrane is required for the fuel cell to perform well and reliably. The membrane' s

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SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE 31 requirement for water is a function of the conditions of operation, the amount of water required for hydration, and how and where the water should be added to maintain a fully saturated membrane. Water management schemes that allow for complete membrane hydration have been and are commonly used, and for a broad range of practical current densities there are no external water requirements since enough water is produced at the cathode to adequately hydrate the membrane. There are, however, very limited data for PEMFC performance under reduced or microgravity conditions. In summary, there are a large number of gravity-related issues that are not fully understood and that could affect PEMFC performance and safety (e.g., explosion caused by the sudden reaction of H2 and O2 in the cell). These issues include, but are not limited to, the following: (1) the effect of gravity on capillary flow in a porous membrane; (2) the effect of gravity on membrane hydraulic permeability; (3) membrane dehydration due to boiling in the porous structure, which could occur if the temperature exceeds ~100 C due to some problem with the thermal control; and (4) transport of species and mass through porous electrodes in the noncontinuum regime, including the conjugated heat transfer through the structures. NASA is considering whether a flight experiment is needed to qualify the PEMFC and to resolve the issue of two-phase flow in microgravity. According to a recent report,2 the Jet Propulsion Laboratory (JPL), the Johnson Space Center (JSC), and the Glenn Research Center (GRC) are working to resolve some of these issues, but no specifics were provided. Capillary-Driven, Two-Phase Devices Thermal management is relevant not only to power-generation systems but also to life-support fluid and thermal systems during long-duration HEDS missions. Put simply, during space travel waste heat must be rejected to space. Fluids can be circulated through the spacecraft components, collected, and then eventually transferred to the radiator, where the heat is rejected to space. Alternatively, capillary-driven, two-phase devices (heat pipes, capillary pumped loops, loop heat pipes, rotating heat pipes, etc.) may be used as key subsystems in thermal control systems of space platforms (Faghri, 1995; Andrew s et al., 1997; Vasiliev, 1998~. Such devices are characterized by capillary-driven flow of the liquid in a wick structure or in axial grooves. The pure liquid working fluid flows to the heat input section where it evaporates and carries awaY thermal eneraY as latent heat (see Figure III.A.5, for example). ~ _ . . The working fluid changes phase from vapor to liquid in the heat sink tconcrenser) section as energy Is rejected. The working fluid is then transported back to the evaporator by means of the capillary forces in a wick. Heat pipes with different designs and operating in different temperature ranges are two-phase devices that have been recognized as key elements in the thermal management and control systems of space platforms (Faghri, 1995~. A heat pipe is an evaporator-condenser system in which the liquid is returned to the evaporator by capillary action. In its simplest form, it is a hollow tube with a few layers of a porous material (e.g., wire screen) along the wall to serve as a wick, as shown in Figure III.A.5. Typical working fluids are sodium or lithium for high- temperature applications, and water, ammonia, or methanol for moderate-temperature applications. If one end of the heat pipe is heated and the other end is cooled, the liquid evaporates at the hot end and condenses at the cold end. As the liquid is depleted in the evaporator section, cavities form in the liquid surface as the liquid clings to the wick. In the condenser section, meanwhile, the wick becomes flooded. The surface tension acting on the concave liquid/vapor interface in the evaporator section causes the pressure to be higher in the vapor than in the liquid. This pressure differential causes the vapor to flow to the condenser section, where the vapor and liquid pressures are nearly equal. Heat removal from the condenser causes the vapor to condense, releasing the heat of vaporization. The condensate is then pumped back to the evaporator section by the capillary force generated at the liquid/vapor interfaces of the pores in the wick. Since heat pipes rely on surface tension to return the condensate to the evaporator section, they can operate in 2Singh, B.S., Glenn Research Center. Multiphase flow and phase change in space power systems. Presentation to the Committee on Microgravity Research on October 14, 1997, Washington, D.C.

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100 MICROGRAVITY RESEARCH directed by the computer numerical control program fuses and consolidates individual powder particles in selected regions. Only the particle surfaces are fused, so that complex geometry control is maintained. The interior of each shell is fused in a second heating to achieve full density, with the sintered shell acting as a mold or forming die. Promising results have been achieved with titanium, Inconel 625, and mild steel/nickel alloys. In another example (W.H. Hofmeister, private communication, 1998; Keicher, 1999), powders between 50 and 100,um are entrained in a gas flow. The powders are delivered coaxially with a laser beam to a molten pool on the workpiece. The laser and powder feed traverse the workpiece to build parts in layers. Beam diameters from 0.25 to 0.5 mm have been used with layer depths from 0.2 to 1.00 mm. Linear traverse speeds from 15 to 45 mm/ s have been demonstrated. The metal volumes deposited have been on the order of cubic centimeters per minute. The cooling rates in this process are as high as 105 C/s, so that highly refined microstructures are produced, comparable to those made by other rapid solidification processes. A number of materials, including stainless steels, tool steels, nickel-based superalloys, and titanium alloys, have been successfully processed. Multiaxis laser control has been used to form complex geometries with this process. Complex ceramic parts have been successfully produced using deposits of ceramic-loaded polymers. For example (Danforth et al., 1998), the molten polymer is extruded out of a 250 to 635,um diameter nozzle, directed by a computer program, to a platform where the polymer freezes. Another process (Brady and Halloran, 1998) to achieve ceramic structures uses successive layers of ceramic-loaded polymers that are ultraviolet-curable so that layer patterns can be defined by stereolithography. In both cases, the polymer is removed and the ceramic is densified in subsequent healings. Though these processes are in their infancy, the potential advantages to the NASA program are obvious. In the area of fabricating a prototype part or parts in limited numbers, successful implementation could drastically reduce the cost and lead time for procurement. In remote locations such as the Moon or Mars, direct fabrication from computer numerical control programs could be used to produce items on location, reducing reliance on spare parts inventories. One of the original applications of this technology was in aircraft turbine repair (W.H. Hofmeister, private communication, 1998), demonstrating that the equipment is also capable of laser welding and repair of critical structural items. Further technology development and actual implementation of the technology require that considerable re- search be done in the area of microstructural control. The research is necessary to learn how to control the process to allow tailoring the microstructure of each part manufactured, thus ensuring that the resulting properties are appropriate for the desired application. The weld pools involved in the buildup of layers contain very large thermal gradients. As a result, both surface-tension-driven flows and gravity-driven flows can be large, leading to significant effects on the microstructure, which must be understood. Also, the powder feed is delivered by forced convection, and the powder particles not captured by incorporation into the process must be recovered for reuse. Current recovery methods depend on gravitational settling, so alternative methods will be necessary in reduced gravity. Fabrication of Components and Structural Elements from Raw or Processed Materials The success of a mission to unexplored destinations can depend on the ingenuity with which local resources are utilized to meet unexpected challenges. Such a challenge could come from the unanticipated failure of a mechanical component, which would require the fabrication of a replacement part to repair the problem. As discussed above, it would not be practical to carry a complete spare parts inventory, nor would an extensive collection of spares necessarily fulfill every emergency need. A different approach has been described whereby a universal, compact machine shop with a very broad capability that might even extend to repairing itself would be included in the spacecraft or at the base (Stryker, 1987~. This machine "shop" would be able to generate replacement parts as small as a wristwatch gear or as heavy as an antenna mount. Such a machine was developed by a mechanical engineer in the 1950s for personal use and was first described in 1974 by Urwick. Its operation is based on the common geometries of three machines: the lathe, the horizontal milling machine, and the

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SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE 101 horizontal boring machine. The machine is commercially available and has been used by the Royal Navy and various research institutes as a general-purpose machine tool. It is completely modular and can be disassembled and reassembled. The design allows for working on a wide range of part sizes. Tiny parts can be manufactured by bringing the movable machine components close together. The largest parts may require partial disassembly of the machine, and their handling can be assisted by use of robotic positioning. In kit form, the machine weighs 300 kg and occupies a volume of 1 m3. Operation requires minimal on-site skill since the machine' s activities can be controlled by an onboard computer or remotely by an operator elsewhere on the space station or on Earth. The latter approach offers the advantage that the machining procedure can be measured, evaluated, and planned by an expert stationed at an identical machine. The feedstock would initially be aluminum or a suitable alloy from unneeded external vehicle tanks. In situ mining and refining activities could also generate iron-nickel-titanium alloys and other suitable feedstock metals and ceramics. Such a universal machine could provide the wide range of capabilities needed to sustain a space station. It would be able to mill, shape, saw, and grind metal parts and even to generate special-purpose vises, collets, chucks, faceplates, and clamps that were not available in the kit of spare replacement parts. The mechanical and thermal stability of the machining operations present no unusual problems, but operation in a hard vacuum would require appropriate lubrication of sliding and mating surfaces to prevent cold welding. The machine's operation is not sensitive to gravity, but the generated filings, cuttings, etc., must be collected at reduced gravity so that a clean environment can be maintained. At zero gravity, the manufacturing should occur in an isolated atmosphere to prevent contamination of the adjacent spaces. In addition to machining, it is useful to note here some other common metal-working processes that may eventually be needed on HEDS missions, including extrusion, rolling, drawing, forging, bending, and pressing. None of these processes are expected to be gravity-dependent and so are not discussed further. Casting in Reduced Gravity Casting is a process in which a molten material is allowed to freeze or solidify, usually in a mold, to produce a solid object of the desired shape. The liquid is introduced into the mold by pouring or by pressure, as in die casting (Scully,1988~. Containerless solidification is also possible if a uniform crystal structure is desired, free of contamination from a container, but the resulting object will probably require machining as this process allows only a limited range of shapes (Strong et al., 1987; Hofmeister et al., 1987; Naumann and Elleman, 1986~. Molds or dyes of complicated shapes would usually have to be made in situ, as it would be impossible to anticipate or transport all the ones that might be needed. In some cases, this could be done using sand molds and lost wax or similar techniques. The performance of the product is directly determined by the microstructure resulting from the casting process. The large body of knowledge on casting metals in terrestrial gravity is treated extensively in a handbook by ASM (1988~. There is also a substantial body of work demonstrating that the microstructure of castings in microgravity differs from that in terrestrial gravity (Curreri and Stefanescu, 1988~. Finally, there is considerable research in progress (e.g., Glicksman et al., 1987, 1995a-c; Abbaschian, 1996; Bassler et al., 1995) exploring the fundamentals of solidification in a microgravity environment. At present, however, there is insufficient under- standing of these fundamentals to allow predictions of the detailed effect of gravity level on the microstructure of a casting. Microgravity experiments continue to yield surprises. For example, solidification of eutectic alloys in microgravity (Larson and Pirich, 1982) shows a closer spacing of finer rods than in Earth gravity. In general, the solidification process and the resulting microstructure are affected by gravity levels. The effect is ultimately due to differences in the strength of density-induced convection in the liquid phase. These differences affect the distribution of temperature, solute, and suspended particles or bubbles, which in turn affect the solidified microstructure. Moreover, casting operations may perform differently in reduced gravity. For example, many 1OAnthony Croucher Ltd., Alton, Hampshire, England.

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102 MICROGRAVITY RESEARCH casting operations depend on the gravity feed of liquid by way of risers as part of the design of the mold. At reduced gravity, such feeds would be less effective. Conduction/convection furnaces may also have different operating characteristics in microgravity, as treated in a study by Lenski and Filler (1987~. Sintering Sintering is an important manufacturing process for making near-net-shape parts from powder in the solid state. It is discussed, along with the reduction of a material to powder, in Metals Handbook (ASM, 1984~. The powder is poured or injected into a mold or dye, with or without the aid of a binder. Filling the dye is extremely important, particularly for parts with complicated geometry, and it relies on gravity feed unless injection molding is used. If injection molding is used, the powder may be introduced as a slurry or paste, in which case the part is usually subject to light machining after sintering. The powder compact is then heated to a temperature below the melting point of the solid, with or without pressure, to produce the consolidated part. The process may take place entirely in the solid state of the material or may be facilitated by the presence of a liquid phase in the solid particle interstices; in the latter case, it is called liquid-phase sintering (LPS). Sintering offers advantages over casting that include its capability to (1) use high-melting-point materials, (2) produce porous materials as used in self-lubricating bearings, and (3) use mixed powders, whose separate liquids are immiscible, to produce materials that cannot be formed by casting. In all three cases, however, molds or dyes are required. As with casting, molds or dyes of complicated shapes would usually have to be made in situ, as it would be impossible to anticipate and carry all the ones that might be needed. During the sintering process, densification of the aggregate of solid particles takes place by the formation of connecting necks between particles and the concomitant reduction of pore volume during heating below the melting point of the solid. The driving force is surface energy reduction and if the aggregate has been compacted or is under pressure plastic and elastic energy reduction. In the case of solid-phase sintering, material transport occurs by diffusion on the surface of the solid particles, volume diffusion occurs in the interior of the solid particles (including high diffusivity paths), and vapor transport occurs in the pores; in LPS, there are additional processes of flow of the interparticle liquid phase, diffusive transport in the liquid phase, and local melting/ freezing and dissolution/precipitation at the liquid/solid interface. Particle reorientation and plastic deformation or viscous flow of the solid phase may also play a role. Each of these processes is dependent on the temperature and the particle size; for example, surface diffusion dominates at smaller scales and lower temperatures and volume diffusion dominates under the opposite conditions. Sintering is not only an important technique for making precision parts but it is also considered to be potentially important for fabricating building material brick from lunar regolith (Allen et al., 1992, 1994; Pletka, 1993~. Solid-state sintering is slow and leads to very uneven heating owing to the low thermal conductivity of the regolith. Pletka (1993) has described an LPS process in which the liquid phase may derive from the glassy silicates in the regolith itself or from reactions that occur in the material when heated and which thus requires no additive. The advantage of sintering over casting is that lower temperatures are sufficient. The disadvantage is that the material must be comminuted and/or sieved to small particle sizes (typically ~100,um) for the sintering rates to be reasonable. The diffusion transport processes that occur during solid-state sintering are not affected to any significant degree by the level of gravity. The spatial distribution of particles, however, is affected by it. In Earth's gravity, particles settle, forming a skeleton characterized by an average coordination number (Yang and German, l991~. In microgravity, an aggregate of independent particles would not form a compact unless pressure is applied, with the effective coordination number depending on the pressure. Similarly, the distribution of particles in LPS is affected by gravity level. If the volume fraction of particles is low for LPS under microgravity, the particles tend to agglomerate toward the center, surrounded by liquid (Kohara, 1994; German, 1995), rather than settling toward the bottom as they do in Earth's gravity. This agglomeration has been interpreted as being driven by the reduction of surface and interface energy that can occur

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SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE 103 when particles coalesce to form grain boundaries at their junctions (German, 1995~; the process is analogous to the coalescence of liquid drops brought into contact. On a microstructural level, LPS involves solidification and is therefore affected by gravity level by virtue of density-induced convection and sedimentation in the freezing liquid, as was discussed in the section on casting. For example, it is found that materials formed by LPS in microgravity may be more porous (German et al., 1995) than those formed in Earth's gravity, presumably because the bubbles formed by outgassing are not eliminated by buoyancy migration; the migration of liquid-filled pores is also affected by gravity level (Heaney et al., 1995~. The coarsening of particle sizes that occurs during sintering has been studied in microgravity, mostly to test theoretical models that do not include gravitational effects. Again, there are some surprises related to the behavior of pores in LPS (German et al., 1995~. The sintering operation requires a mold or dye with equipment (e.g., injection equipment) to fill it, a furnace capable of operating at sintering temperatures (typically above 1000 C), and an atmosphere regulating system. As pointed out, molds would usually have to be made in situ. This poses no problem for simple shapes like bricks but is a serious limitation for complicated shapes unless they can be made by a lost wax or similar technique. Composite Materials On Earth, increasing use is being made of composite materials. These materials are combinations of a matrix and a dispersion. Broadly, they are classified as one of three basic types, depending on their matrix: polymer matrix composites (PMCs), metal matrix composites (MMCs), and ceramic matrix composites (CMCs). The utility of these materials stems from the synergism achieved by combining different materials into a single entity (Eckold, 1994; Schaffer et al., 1995; Callister, 1997~. It is not anticipated that exploration plans over the next few decades would include the extraterrestrial manufacture of matrix materials and particulate/fiber materials because of its complexity and the drain on re- sources that such activities would entail, including the demand for manufacturing capability. Instead, if compos- ites were needed, judiciously chosen materials could be part of the cargo. These raw materials could then be used for repair, maintenance, or replacement. The main manufacturing methodologies for polymer matrix composites are hand lay-up, filament winding, and pultrusion. These do not appear to involve gravity effects in a major way, although in hand lay-up, spray processes typically rely on free fall to distribute fiber in the mold for making the final product. Therefore, development of techniques for use in reduced gravity would be needed. For example, confinement of sprayed material such as resin, catalyst, and particulate would be necessary. Metal matrix composites can be cast to shape using an intermediate feedstock but, alternatively, these com- posites can be forged to shape. Other options are production by hot processing or casting to shape using pressur- ized feeding of liquid metal into a mold cavity containing fiber preform. Of these alternatives, casting to shape could be affected considerably by reduced gravity through its effects on flow, convection, buoyancy, and sedimen- tation. The process would be a likely candidate for experimental work in reduced gravity and microgravity. Products from ceramic matrix composites can be fabricated by pressing, hot or cold, and by sintering of prepregs as composite feedstock. In these cases, gravity level is not a factor. Joining Methods in Space Joining structural members in space is important for both construction and repair. Methods usually consid- ered are mechanical joining, adhesive bonding, and welding or soldering (including brazing). However, mechani- cal joining requires special design (e.g., provisions for O rings) to assure pressured seals, and the high polymers used for adhesive bonding are subject to degradation in space owing to outgassing and radiation damage. We therefore focus here on welding, which may be used for repair (e.g., to patch holes caused by micrometeors) as well as for construction. Welding entails the fusion of the base metals at the junction. It may be done either with or without a welding

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104 MICROGRAVITY RESEARCH rod; in the latter case, it is called autogenous welding. In brazing and soldering, only the solder and not the base metal is melted; above 450 C the process is called brazing, below 450 C, soldering. The history of welding in space is described in AWS (1991~. It starts with Russian experiments in 1969 on Soyuz 6, followed by a number of subsequent Russian experiments and tests. The first American trials occurred in 1973 on Skylab with the welding of three metals (stainless steel, an aluminum alloy, and high purity tantalum); the results were examined at Battelle and NASA. In 1984, Russian cosmonauts spent 3 hours welding outside Salyut 7 using a handheld electron beam gun designed by the E.O. Paton Electric Welding Institute in Kiev. In 1986, two cosmonauts constructed a large truss in EVA off Salyut 7. This showed that quality welds could be done with little prior training and that the VHT (versatile handgun tool) performed well in space; however, there were dangers to the welder from emitted X rays. Numerous underwater welding experiments have been carried out at Marshall Space Flight Center in a neutral buoyancy tank. Welding experiments scheduled for the October 1997 shuttle flight STS-87 were postponed and have still not taken place. The EVA welding environment is characterized by microgravity (10-6 go) with jitter, hard vacuum (modified by outgassing from the vehicle), meteoroids and debris, sunlight and ionizing radiation, atomic oxygen, and large thermal gradients (near the Sun/shade boundary). Technical conditions or limitations for welding are limited power sources (a few kilovolts for a few minutes) and the paucity of nondestructive testing methods for space. For some welding methods the associated health hazards make robotic welding very desirable. Gravity is not a dominant factor in the welding process itself, even in Earth's gravity, as indicated by the fact that welding is routinely done upside down. Under microgravity conditions, the weld pool dynamics are com- pletely dominated by capillary and electromagnetic forces. Thus, even though gravity-induced convection and sedimentation are absent, Marangoni-induced convection (due to the dependence of surface tension on tempera- ture and composition) may be strong; added to this are electromagnetic stirring forces due to the welding current. The shape of the weld pool that moves in concert with the welding rod is determined by the interplay among these forces in a way that is not entirely understood. The shape in turn affects the weld quality. A cusp shape at the trailing edge produces a seam that is generally detrimental to the material properties since impurities tend to segregate there. Since welding involves continuous solidification of the trailing edge of the moving molten zone, gravity level will have some effect on the resulting microstructure, as it does in all solidification processes. Typical microstruc- tural variables in welded material that are affected are the grain size, distribution of phases, distribution of inclusions, and porosity and cracks. Some characteristics of the microstructure of welds conducted in microgravity as opposed to terrestrial gravity (Nance and Jones, 1993) are smaller grain size despite the slower cooling rates (possibly due to nucleation on suspended particles), increased porosity in some cases (possibly due to lack of buoyancy forces on bubbles), and a more uniform distribution of inclusions throughout the weld (again possibly due to lack of sedimentation forces). References Abbaschian, R. 1996. In-situ monitoring of crystal growth using MEPHISTO. Pp. 45-87 in Second United States Microgravity Payload: One Year Report. P.A. Curreri and D.E. McCauley, eds. Huntsville, Ala.: NASA Marshall Space Flight Center. Allen, C.C., J.A. Hines, D.S. McKay, and R.V. Morris. 1992. Sintering of lunar glass and basalt. Pp. 1209-1218 in Engineering, Construction, and Operations in Space III: Space '92, Proceedings of the Third International Conference, Vol. II. W.Z. Sadeh, S. Sture, and R.J. Miller, eds. New York: American Society of Civil Engineers. Allen, C.C., J.C. Graf, and D.S. McKay. 1994. Sintering bricks on the moon. Pp. 1220-1229 in Engineering, Construction, and Operations in Space IV: Proceedings of Space '94. R.G. Galloway and S. Lokaj, eds. New York: American Society of Civil Engineers. American Society for Metals (ASM). 1984. Metals Handbook, 9th Ed. Metals Park, Ohio: American Society for Metals. American Society for Metals (ASM). 1988. Metals Handbook, 9th Ed., Vol. 15. Metals Park, Ohio: ASM International. American Welding Society (AWS). 1991. Proceedings of Welding in Space and the Construction of Space Vehicles by Welding, cosponsored by the American Welding Society-USA and the E.O. Paton Electric Welding Institute-USSR. Bassler, B.T., W.H. Hofmeister, and R.J. Bayuzick. 1995. Examination of solidification velocity determination in bulk undercooled nickel. Proceedings of the 1994 Materials Research Society (MRS) Fall Meeting. Warrendale, Pa.: Materials Research Society. Bergan, P. 1998. Potential Navy applications for selective laser sintering. P. 5 in Naval Research Reviews, Vol. L. Washington, D.C.: Government Printing Office.

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SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE 105 Boles, W., W. Scott, and J. Connolly. 1997. Excavation forces in reduced gravity environment. J. Aerospace Eng. 10(2):99- 103. Brady, G.A., and J.W. Halloran. 1998. Solid freeform fabrication of ceramics via stereolithography. P. 39 in Naval Research Reviews, Vol. L. Washington, D.C.: Government Printing Office. Callister, W.D., Jr. 1997. Materials Science and Engineering An Introduction, 4th Ed. New York: John Wiley & Sons. Carrier, W.D., G.R. Olhoeft, and W. Mendell. 1991. Physical properties of the lunar surface. Pp. 476-567 of Lunar Sourcebook. G. Heiken, D. Vaniman, and B.M. French, eds. New York: Cambridge University Press. Colwell, J.E., M. Horanyi, A. Sickafoose, S. Roberston, and R. Walch. 1998. Dynamic dust in photoelectron layers near surfaces in space. Proceedings of the Fourth Microgravity Fluid Physics and Transport Phenomena Conference, NASA Lewis Research Center. 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106 MICROGRAVITY RESEARCH Lin, T.D., and S. Bhattacharja. 1998. Lunar and Martian resources utilization: Cement. Pp. 592-600 in Space 98: The Sixth International Conference and Exposition on Engineenng, Construction, and Operations in Space. R.G. Galloway and S. Lokaj, eds. Reston, Va.: Amencan Society of Civil Engineers. Mindess, S., and J.F. Young. 1981. Concrete. Englewood Cliffs, N.J.: Prentice-Hall. Nance, M., and J.E. Jones. 1993. Welding in space and low-gravity environments. P. 1020 in Metals Handbook, Vol. 6, Welding, Brazing and Soldenng. Metals Park, Ohio: ASM International. National Research Council (NRC), Space Studies Board. 1992. Toward a Microgravity Research Strategy. Washington, D.C.: National Academy Press. Naumann, R.J., and D.D. Elleman. 1986. Containerless processing technology. P. 294 in Matenal Science in Space. B. Feuerbacher, H. Hamacher, and R.J. Naumann, eds. New York: Spnnger-Verlag. Perkins, S.W., and C.R. Madson. 1996a. Mechanical and load-settlement characteristics of two lunar soil simulants. J. Aerospace Eng. 9(1): 1- Perkins, S.W., and C.R. Madsen. 1996b. Scale effects of shallow foundations on lunar regolith. Pp. 963-972 in Engineenng, Construction, and Operations in Space V: Proceedings of the Fifth International Conference on Space 96, Vol. 2. S.W. Johnson, ed. New York: Amencan Society of Civil Engineers. Ple~a, B.J. 1993. Processing of lunar basalt matenals. P. 325 in Resources of Near-Earth Space. J.S. Lewis, M.S. Matthews, and M.L. Guernen, eds. Tucson and London: University of Arizona Press. Schaffer, J.P., A. Saxena, S.D. Antolovich, T.H. Sanders, Jr., and S.B. Warner. 1995. The Science and Design of Engineering Materials. Chicago: Richard D. hwin. Scully, L.J.D. 1988. Die casting. P. 286 in Metals Handbook, 9th Ed., Vol. 15. Metals Park, Ohio: ASM International. Shong, D.S., J.A. Graves, Y. Ujiie, and J.H. Perepezko. 1987. Containerless processing of undercooled melts. P. 17 in Matenals Processing in the Reduced Gravity Environment of Space: Proceedings of the 1986 Fall Matenals Research Society (MRS) Meeting. R.H. Doremus and P.C. Nordine, eds. Warrendale, Pa.: Matenals Research Society. Stryker, J.M. 1987. A job shop for space manufactunng. Pp. 158-163 in Proceedings of the Eighth Princeton/AIAA/SSI Conference. B. Faughnan and G. Maryniak, eds. Washington, D.C.: American Institute of Aeronautics and Astronautics. Szabo, B., F. Barnes, H.-Y. Ko. 1994. Effectiveness of vibrating bulldozer and plow blades on draft force reduction. Proceedings of the Winter Meeting of the Amencan Society of Agncultural Engineers. No. 941535. St. Joseph, Mich.: American Society of Agncultural Engineers. Yang, S.-C., and R.M. German. 1991. Gravitational limit of particle volume fraction in liquid-phase sintenng. Met. Mater. Trans. A 22:786. III.G MATRICES OF SUBSYSTEMS, PROCESSES, AND PHENOMENA Tables III.G.1 through III.G.3 summarize information from the preceding sections regarding which common subsystems and processes are likely to be affected by changes in gravity level. The tables identify the specific phenomena that are most likely to play an important role in the operation of a given subsystem when the gravity level is altered. These phenomena are discussed in greater detail in Chapter IV.

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SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE 107 TABLE III.G. 1 Phenomena Associated with the Common Subsystems Likely to Be Affected by Gravity Level Phenomenon Subsystem/ Variant Storage tanks Gas Liquid Cryogenic Pumps Condensate Liquid line Microdevices Compressors Rotary Adsorption Piping Gas-phase Liquid-phase Two-phase . Radiators Solid-state Gas-phase Two-phase Heat pipes Capillary pumped loop Simple Fans and blowers Evaporators Boilers Vaporizers Liquifiers Condensers Distillations units

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108 TABLE III.G.1 Continued Subsystem/ Variant Filters/separators Gas/solid Gas/liquid Liquid/liquid Liquid/solid Vortex separators 1 _ Rotating drum separators Spargers Valves and actuators MICROGRAVITY RESEARCH Phenomenon ~ ~ i genii 6~ Heaters - Catalyst beds Seals Heat exchangers Gaslgas Gas/liquid Gas/solid Flu id ized-bed Fire extinguishers Smoke detectors

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SURVEY OF TECHNOLOGIES FOR THE HUMAN EXPLORATION AND DEVELOPMENT OF SPACE TABLE III.G.2 Phenomena Associated with the Matenals Handling Equipment Likely to Be Affected by Gravity Level Equipment - Screens Hoppers Excavators Conveyers Drillers Bulldozer Anchor Trucks Cranes Bucket scoop Winch Rotating drum or slide charging unit Electrostatic generator Gravity collection bins 109 Phenomenon ., ~ : , ~ . ' ' ' 3 ' ! ~

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0 MICROGRAVITY RESEARCH TABLE III.G.3 Phenomena Associated with Various Material Processes Likely to Be Affected by Gravity Level Phenomenon Process Crushing/grinding Settling Sieving Transporta Sintering (LPS) Casting Welding - aIncludes such bulk material transport processes as ore transport and slurry flow in pipes. i: t .