Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies (2000) / Chapter Skim
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III Survey of Technologies for the Human Exploration and Development of Space
III Survey of Technologies for the Human Exploration and Development of Space
Pages 21-110
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From page 21... ...
III.A POWER GENERATION AND STORAGE Introduction Future HEDS missions for the exploration and colonization of the solar system will require enabling technologies 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.
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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)
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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.
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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~.
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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)
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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 rechargeablebatteries, and primary and regenerative fuel cells)
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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.
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A schematic of a closed-cycle Brayton space power generation system is shown in Figure III.A.
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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.
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From page 30... ...
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.
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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.
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Industry has developed an LHP system that uses deployable radiators to increase the heat rejection from inside the spacecraft to the space environment (Parker et al., 1999~. As noted above, capillary-driven, two-phase flow devices can operate reliably in a microgravity environment because the liquid flow is driven by surface tension.
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that uses differences in the vapor pressure between an evaporator and condenser to drive the convective heat transfer process. This device is shown schematically in Figure III.A.6.
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energy source and is classified as an active electric power-generation system since thermal energy TABLE III.A.1 Selected Subsystems for Passive and Active Power Generation and the Potential Impact of Microgravity on Their Operation Representative Passive Active Subsystem PV TE TI TPV BR RA ST AMTEC Batteries L L L L Boiler H Capillary pumped loop H H H Compressor L L Condenser H H Converter L L L L Controls L L L L L L L L Evaporator H Heat exchanger M H M M Heat pipes H H H Phase separator H Pipes L M L Pumps M Radiators M H M Regenerative heat exchangers L H Regulators L L L L L L L L Solar array L L Solar collector M H M Storage M M M Turbine/alternator L L L Valves L M L NOTE: PV, photovoltaic; TE, thermoelectric; TI, thermoionic; TPV, thermophotovoltaic; BR, Brayton; RA, Rankine; ST, Stirling; and AMTEC, alkali metal thermal-to-electric conversion. The letters H
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1990. Human Exploration of Space: A Review of NASA's 90-Day Study and Alternatives.
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Rather, the aim is to include a range of systems sufficiently wide to bring out the significance of gravity level; any emphasis of one system over another reflects only its interest for microgravity research. After the potential systems are described, major subsystems, or components, are identified by the functions they fulfill (e.g., a boiler)
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These systems are described and discussed below in varying degrees of detail, depending on their perceived significance for microgravity research. Besides the capability issues already mentioned, a number of specific technical themes appear, many of which, such as method of energy conversion and heat transfer, were thoroughly discussed in Section III.A.
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For this report, which focuses on issues of reduced gravity, a dominant technical feature of the hydrogenoxygen system is the cryogenic storage apparatus; liquid gases must be refrigerated and kept cold in large tanks for long periods of time before use. Fuel replenishment, or transfer between tanks in space, is not a fully developed process.
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This thermal energy is then converted to kinetic energy, and hence thrust, in an exhaust nozzle (Dearian and Whitbeck, 1990; NASA, 1991; Rosen et al., 1993~. The nozzle is cooled by the propellant on its way to the reactor (Figure III.B.2~.
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However, start-up and shutdown in space will be required for nuclear propulsion systems (Dearian and Whitbeck, 1990) , and fluid flow and heat transfer processes will generally be affected by gravity level during those operations, depending on reactor design.
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It has many components, however, and its advantages depend on phase change, making it inherently more sensitive to gravity. In a typical Rankine cycle NEP (Figure III.B.3)
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Condenser ~ I ~ ~ redo I ~ I l/ , J 1, ,<. - __ hi_ ~ I ~ 1 r—- —J I l ~~ a_ Space Radiator Vapor Separation System FIGURE III.B.3 Schematic showing major elements of a nuclear electric propulsion system.
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Solar Sail A propulsion system deriving momentum change directly from solar radiation pressure and the solar wind would require a very large solar "sail" (Staehle,1981~. This sail would be subject to severe problems of structural dynamics and control in a microgravity environment but would of course be free of fluid-handling problems.
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Nuclear Fission Reactor A large nuclear reactor would provide heat for NTR, or a small reactor would serve an NEP system or provide for electric power generation. Coolant flow and heat transfer to a working fluid or propellant in such a reactor could be sensitive to gravity level, particularly during the transient operations of start-up or shutdown, as the coolant changes phase from a solid to a liquid.
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Boiler for a Rankine Cycle ~ 1 ——— —— ~ —_ _~ Gas or Vapor Turbines If a Rankine cycle is adopted, a heat exchanger for evaporation and separation and collection devices for gas and liquid phases are needed. As pointed out in Section III.A, such a boiler would be an essential part of any method of electric power generation by means of the Rankine cycle, whether the energy source is chemical, nuclear, or solar, and whether the purpose is to make electric power for propulsion or for spacecraft or station power.
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In reduced gravity or microgravity, other mechanisms, such as surface tension in a heat pipe, centrifugal phase separators, or direct contact condensation, must be used. Significantly, these condenser designs may incur size and weight penalties because of microgravity.
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, would be problematic in reduced gravity. An interesting multiphase heat-transfer device, the vanor-nressure numbed loon (Section III.AN.
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General Concerns Regarding Propulsion and Power in Reduced Gravity Certain general issues affecting power and propulsion component design and performance, seemingly important in microgravity, merit more thorough study in the HEDS context than they have received so far. Touched on in Section III.A on power and above in this section on propulsion, they deserve further emphasis here to round out the committee's discussion of the key topics of power and propulsion.
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System dynamics and system instabilities are very important issues for power generation generally; the generation system and its load must be managed together. Some issues of this kind, especially nuclear start-up, have been carefully studied (Kirpich et al., 1990)
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Since it appears that nuclear fission power will be essential for the success of the long-range goals of HEDS, there is a need for NASA to maintain a steady effort in this field, with attention paid both to newer reactor types and to the many advanced components needed to ensure desired performance at various gravity levels. Summary of the Effect of Reduced Gravity on Selected Subsystems Summarized in Table III.B.1 are the various subsystems and components discussed in this section and the various propulsion systems where they are found.
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Pp. 69-80 in Thermal-Hydraulics for Space Power, Propulsion and Thermal Management System Design.
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Neither document identified scientific or technical issues associated with possible failure modes arising from reduced gravity. This section emphasizes these issues as it looks at selected technologies that are likely to be both important to life-support design and significantly affected by gravity levels.
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Such biologically based systems, or bioreactors, will probably be involved in both nutrient production and waste management. Crucial to the successful development of suitable closed ecological life support systems (CELSS)
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Subsequent Resorption with steam requires a boiler, which is highly vulnerable to the gravity level and adds to the loading of heat rejection systems. Newer, carbon-based molecular sieves have been developed that are insensitive to water vapor and that offer more efficient trapping and can be cycled through Resorption at closer to ambient pressure and temperature.
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Subsystems involved in steam generation involve two-phase fluid management, so they would operate much differently in a microgravity environment. Oxygen Generation Water Electrolysis A regenerative CELSS will almost certainly require a water electrolysis system.
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Distillation requires paired phase changes and separations and is strongly affected by gravity. The four distillation schemes in Table III.C.3 incorporate different solutions to the problems posed by a microgravity environment.
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As noted, the solid waste recycling systems have not been operated in space and are not currently justified for Earth-orbiting systems like the International Space Station. Because of the inherent variability and the low immediate value of the recycled materials that can be recovered, waste recovery systems should probably not be developed as stand-alone systems but eventually should be integrated into bioregenerative systems that produce food and oxygen from plants while recycling solid waste at some level.
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impact of reduced gravity on the operation of the subsystem. Where no letter is given, the subsystem is not applicable to the system listed.
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Summary of the Impact of Reduced Gravity on Selected Subsystems Microgravity has pervasive impacts on the subsystems found in each of the major systems involved in life support (Table III.C.4~: · Oxygen and carbon dioxide generation and recovery procedures use a variety of differential liquefaction processes, together with separations of liquid and gas phases. · Solid waste processing involves complex and ill-defined multiphase systems requiring the separation of dispersed solids from the continuous liquid phase.
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For each subsystem that appears in a given system, the impact of reduced gravity on its operation is estimated as either high, medium, or low (little or none)
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As a result, International Space Station plans call for carbon dioxide extinguishers. While these may be generally effective, a better understanding of the techniques for their application in reduced gravity needs to be developed.
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Spills of solid and liquid materials behave differently and therefore need to be considered separately. Both types of materials will behave differently in reduced gravity from the way they behave on Earth.
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Despite the common perception, due to their use in terrestrial applications, that materials like lead provide optimal shielding, lighter elements and their compounds are more effective passive shields for the HZE particles than are heavier elements. For instance, polyethylene sheet, with a surface density of 0.19 g/cm2, provides far better shielding efficiency per unit mass than do lead or other metals (Eckart, 1996~.
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Knowledge of the types of contaminants that can be of concern is needed to design suitable detection systems. Much knowledge of this kind is obtainable from existing experience in the space program and more should result from operation of the International Space Station.
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Consideration can be given also to more-general removal procedures that do not require the detection or identification of specific contaminants. Summary of the Effect of Reduced Gravity on Hazard Protection Systems Table III.D.1 lists the subsystems that make up the hazard protection systems and indicates which ones may be affected by gravity levels, as discussed in the preceding sections.
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From page 66... ...
The production of hydrogen and oxygen by electrolysis of water is a critical enabling technology for long-duration life support systems and for a wide range of ISRU applications on a majority of planetary bodies found in the solar system. Water electrolysis has a critical dependency on gravity since the generated gaseous products require liquid-vapor separation, a process that fails at zero gravity.
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From page 67... ...
A similar set of problems will be encountered on other planetary bodies, which have not yet been characterized as accurately as the Moon. Before such a wide range of surface operations can be carried out, a better understanding of the behavior of soil in reduced gravity environments will be needed.
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One of the potential substances to be mined is water ice, which would be a valuable and essential resource; the extraction problems for this case are discussed in the following pages. In conclusion, the reduced gravity issues relevant to mining are the design of mining equipment for excavating, bulldozing, transport, etc.
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From page 69... ...
Because of the contrast between extracting adsorbed volatiles from small particles such as lunar fines and extracting water from extraterrestrial ice, this discussion will examine those two cases as a basis for identifying the microgravity research issues. Lunar Volatiles Adsorbed volatiles are found in the surface layer of lunar fines, with the highest concentrations on the Sunfacing side.
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70 Extraction of Water Ice on Low-Gravity Surfaces MICROGRAVITY RESEARCH Material Handling and Transport Material handling requires the design of lightweight, temperature- and dust-insensitive equipment such as bulldozers, bucket scoops, cranes, winches, conveyer belts, and trucks that can operate at reduced gravity over a large ambient temperature excursion and in a very abrasive, fine, silty soil that could extend to depths of between 3 and 10 meters. As discussed by Chamberlain et al.
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From page 71... ...
Reduced gravity levels will generally increase the difficulties of handling a particulate stream, including the generally diminished flow out of hoppers. A specific possible negative effect of reduced gravity levels on the beneficiation process would be a reduction in the force acting to separate the particulate stream from the charging drum or belt into the free-fall zone, where deflection and enrichment occur.
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From page 72... ...
Other components include heat exchangers, temperature-controlled valves, and pipes and valves. At the Martian gravity level of 0.36 go, none of the processes discussed above are expected to be greatly affected compared with operation at 1 go.
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Typical terrestrial particle size ranges are presented in Figure III.E. 1, where it is noted that particle sizes encountered on Earth, and hence particle sizes that could be encountered within a spacecraft environment, span eight orders of magnitude.
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THIS DIMENSION REPRESENTS THE DIAMETER OF ~ HU - AN HAIR, 1 - MICRONS >| THIS REPRESENTS A 0 3 MICRON DIAMETER PARTICLE, ABSOLUTE FILTERS P'EMOYE OVER 99.97a/0 OF THIS SIZE. 1 MICRON - 1 11dIICROIIdIETER - 1 IIIILLIONTH OF A DIETER FIGURE III.E.1 Sizes and characteristics of atmospheric contaminants.
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FIGURE III.E.2 Sizes and characteristics of atmospheric solids. NOTE: The values for particle surface, settling rate, and number and area per cubic meter of air are based on the following: particle specific gravity = 1 (density = 1000 kg/m3~; mass concentration = 70 ,ug/m3, typical of urban concentrations; particles are smooth spheres, all of equal size; and gas is air, with a density of 1.29 kg/m3, temperature is 21 °C, pressure is 1 atm, and viscosity is 1.86 x 10-5 kglmls.
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From page 76... ...
Reduced gravity will alter the gas-liquid interface behavior associated with the production of gaseous oxygen and hydrogen in lunar and Martian electrolysis systems, but those effects should be scalable. However, the design and operation of water electrolysis cells on board spacecraft or on very-lowgravity surfaces, such as on ice-containing asteroids or on inactive comet cores, will be determined by the microgravity environment and will require fundamental knowledge of multiphase processes in these microgravity environments to achieve necessary levels of system reliability and autonomy (see Humphries et al., 1991~.
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From page 77... ...
Both types of water electrolysis are influenced strongly by gravity level. So-called water vapor electrolysis operates directly on cabin air and serves both to dehumidify the air and to produce hydrogen and oxygen.
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From page 78... ...
Thus they can be modified for use as electrochemical processors, but the microgravity issues for fuel cell applications are essentially the same. Hence, it is more efficient to describe the operation of solid-electrolyte oxygen-extraction systems, which are currently scheduled to be demonstrated on the Mars 2001 mission, in terms of possible reduced gravity issues, noting that these devices can be used as fuel cells.
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From page 79... ...
The systems being considered here act like reverse fuel cells, consuming electrical energy to remove and then pump a specific ion across a solid membrane for collection. Because of their ability to extract oxygen directly from the Martian atmosphere for near-term space missions, solid electrolytes that conduct oxygen ions but offer very high electronic resistance are presently the most thoroughly studied systems for extraterrestrial resource processing.
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From page 80... ...
Using direct current, oxygen would be produced at the anode and various metals at the cathode. Systems of this type are difficult to operate reliably on Earth, and fundamental understanding of the physical chemistry pertaining to the entire system is needed, particularly when the basic unit operations are altered by reduced gravity.
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From page 81... ...
Hence, removal of dust from the Martian atmosphere becomes an even more important requirement, and the RF approach relies completely on detailed knowledge of the influence of reduced gravity on particle behavior. The RF plasma resides in a gas mixture that is no longer in thermodynamic equilibrium since the processing is a continuous flow process and the entering Martian atmosphere molecules are relatively cold.
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From page 82... ...
. Oxygen Production Oxygen requirements for life support and for most liquid chemical rocket propellants are so large for longduration human missions that the ability to produce oxygen mass away from Earth's surface could enable nearterm HEDS missions to be commissioned for planetary objects virtually across the solar system.
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From page 83... ...
, an atmospheric filter, a carbon dioxide adsorption/desorption compressor or a Mars atmosphere compressor, a catalyst bed reactor and thermal management system, regenerative heat exchangers, a radiator and water collector, a methane-water separator, a methane dryer, a water collection and storage unit, a water electrolysis unit, hydrogen and oxygen dryers, methane and oxygen precooler~s) , oxygen storage, methane storage, and cryogenic refrigerators)
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From page 84... ...
A Mars-based system using RWGS for oxygen production can incorporate a hydrogen storage and supply system or a system for extracting water (from polar ice, permafrost, or other sources, including the atmosphere) , an atmospheric filter, a carbon dioxide adsorption/desorption compressor or a Mars atmosphere compressor, a catalyst bed reactor and thermal management system, regenerative heat exchangers, a radiator and water collector, a water condenser, a carbon monoxide dryer and exhaust, a water electrolysis unit, an oxygen dryer, radiators, an oxygen liquefaction system, an oxygen storage system, and cryogenic refrigerators)
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From page 85... ...
The spent ore is discharged alternately into one of two hoppers, which may be locked and pumped out to recover adsorbed hydrogen. The components most affected by reduced gravity would be the fluidized-bed reactor (Gibson et al., 1990; Ness et al., 1990)
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From page 86... ...
The unit operations associated with this process will be altered by reduced lunar gravity, as will the manipulation and transport of lunar materials. Cryogenic Storage Reduced-gravity cryogenic fluid management issues fall into two distinct areas depending on whether a fractional gravity or microgravity environment is being considered.
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From page 87... ...
, but the project was never completed. Summary of the Effect of Reduced Gravity on Selected Subsystems Summarized in Table III.E.
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given system, the impact of reduced gravity on the operation of these subsystems is estimated as high, medium, or low (little or no impact)
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From page 89... ...
In the case of water, it is not possible to extract water ice and separate it from other materials without subjecting it to a phase change. Gradual collection of released (sublimated)
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From page 90... ...
needed for terrestrial power generation. Wittenberg et al.
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From page 91... ...
1991. A system for oxygen generation from water electrolysis aboard the manned space station Mir.
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1991. Life support and internal thermal control system design for space station Freedom.
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1995. Oxygen production on Mars using solid oxide electrolysis, 25th International Conference on Environmental Systems.
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Construction will be facilitated by the ability to use local materials to fabricate concrete (tin and Bhattacharja, 1998) , which will be useful for many applications such as securing tethers at reduced gravity and general construction.
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From page 95... ...
Even the simple task of covering a structure with regolith involves a number of unresolved engineering issues. Because of our more detailed knowledge of the lunar surface, it is instructive to frame the reduced gravity research issues in that context.
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, it is probably more desirable to construct 1/6 scale models of lunar equipment and operate them in Earth's gravity than to attempt to conduct tests in reduced gravity on a KC 135 flight. Future excavation research should focus on properly scaling the direct and indirect gravitational parameters, which control both the characteristics of the in situ material being excavated and the forces required to effect excavation.
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From page 97... ...
Dust intrusion must be prevented, particularly since in reduced gravity, dust will not settle as rapidly as it does on Earth. Inside atmosphere must be maintained across a wall or membrane separating it from extreme conditions on the outside where verY low pressure and verY high or verY low (perhaps variable)
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From page 98... ...
50°C MICROGRAVITY RESEARCH Formation of _~_ ~ C;= 600° C Limestone initial Compounds decomposes t Formation I initial formation of melt Of C^S / Formation of / C3S 7 - - 1 ~ lY 20n~'ng By_ 20n' 1 200° C 1 350° C 1 550° C -- t—- -- t- - 1000°C 1350°C 1450°C Clinker out Cooling grate FIGURE III.F.1 Schematic outline of conditions and reactions in a typical cement rotary kiln (dry process)
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From page 99... ...
First, it is most important to have a feed that is thoroughly pulverized and homogeneous. Since grinding and blending operations on Earth are basically driven by gravity, in reduced gravity they will need considerable modification.
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From page 100... ...
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.
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From page 101... ...
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.
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From page 102... ...
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)
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From page 103... ...
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.
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From page 104... ...
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.
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From page 105... ...
P 149 in Materials Processing in the Reduced Gravity Environment of Space: Proceedings of the 1986 Fall Materials Research Society (MRS)
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From page 106... ...
P 17 in Matenals Processing in the Reduced Gravity Environment of Space: Proceedings of the 1986 Fall Matenals Research Society (MRS)
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From page 107... ...
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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From page 108... ...
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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From page 109... ...
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 ., ~ : , ~ .
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From page 110... ...
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.
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