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9 Applied Physical Sciences Applied physical sciences are central to many key exploration technologies. Many of the design challenges of new exploration technology systems are addressed by applied physical sciences research on fluid physics, combus - tion, and materials, as described in this chapter. This research will enable new exploration capabilities and yield new insights into a broad range of physical phenomena in space and on Earth. Applied physical sciences research will result in fundamental approaches for improved power generation, propulsion, life support, and safety. A broad range of space-related technologies are advanced by key areas of research in the applied physical sciences: • Fluid physics. Flows involving multiple phases (e.g., liquids and vapors) are present in many current and proposed space systems. Moreover, multiphase systems and thermal transport processes are enabling for proposed human exploration missions by NASA.1 Complex fluids, including the granular physics associated with the flow and compaction of soil found on planetary bodies, present engineering challenges on topics ranging from habitat design to the degradation of life support systems because of dust. • Combustion. Fire safety is integral to astronaut safety; fires behave differently on Earth than in space, in part because of differences in the ways that materials burn and gases flow. The understanding of material flam - mability in reduced gravity is incomplete. Concerns also exist about the effectiveness of fire detection and fire suppression systems designed for reduced gravity. An improved understanding of combustion in reduced gravity can lead to more efficient use of combustion processes on Earth, a cleaner environment, and better fire safety. • Materials science. Lightweight materials, self-healing materials, and other new materials tailored to NASA’s demanding missions are central to the success of future human and robotic exploration. Research in reduced gravity can lead to new insights about how various processing methods affect the internal structure and, ultimately, the properties of materials. Reduced gravity provides a unique environment for fundamental research in fluid physics, combustion, and materials science, which are central to the robust performance of current and proposed space systems. On Earth, gravity induces fluid motions that may mask important phenomena under investigation. For example, multiphase flow regimes and the performance of heat transfer phase-change systems on Earth are very different than they are in the absence of gravity. Flame ignition, propagation, and quenching are also affected by gravity. If certain combustion processes related to the presence of gravity can be excluded, then it is possible to extract important, 265
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266 RECAPTURING A FUTURE FOR SPACE EXPLORATION fundamental data and insights relevant to similar systems both on Earth and in space. This includes information on chemical reaction rates, diffusion coefficients, and radiation coefficients, as well as insights into flame struc - tures, soot formation, and droplet combustion. Furthermore, the production and processing of new materials often involve a vapor or liquid. The examination of crystal growth and solidification processes in reduced gravity can provide new insights into the manner in which a liquid or vapor transforms into a crystal and the accompanying pattern formation processes, such as dendritic and cellular growth. This chapter presents recommendations for applied physical sciences research that enables space exploration and is enabled by space exploration. These recommendations are based on the expertise of the panel, on approxi - mately 40 white papers submitted by the scientific community, on briefings presented to the panel, and on other documents reviewed by the panel. Chief among these documents were two previous reports from the National Research Council (NRC). The first, Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies,2 known as the HEDS report, contains an extensive discussion of the central role that research in the applied physical sciences plays in enabling space exploration, as well as an encyclopedic survey of important exploration technologies and the physical phenomena underlying these tech - nologies. It discusses many key technology areas, including power generation and storage, space propulsion, life support, hazard control (fire and radiation safety), and materials production and storage. Drawing on the many technologies in these broad areas, the HEDS report also presents extensive background on fluid physics, on topics such as interfacial phenomena, multiphase flow, and heat transfer; combustion and fire safety; and materials. The second report, Assessment of Directions in Microgravity and Physical Sciences Research at NASA,3 focuses on research that is enabled by reduced-gravity research platforms supplied by the National Aeronautics and Space Administration (NASA). As with the HEDS report, this assessment contains an expansive discussion of background information and important research questions in fluid physics, combustion, and materials science. Both reports contain additional supporting information that, due to space limitations, is not included in this chapter. Recommended research portfolios in fluid physics, combustion, and materials science are presented below. In general, to make the most out of each experimental opportunity, research should include a combination of reduced- gravity experiments, numerical simulation, and analysis. The timeline for each of these portfolios was constructed assuming that, over the next 10 years, the International Space Station (ISS) will be available with adequate crew time and up-mass and down-mass capabilities, current ground-based facilities will remain available, and funding will be available to support an expanded research program. As noted below, most of the recommended research should be structured to facilitate the development of related critical technologies, as detailed in Chapter 10, “Trans - lation to Space Exploration Systems” (see Tables 10.3 and 10.4). The rest of the recommended research (i.e., in the areas of complex fluid physics, numerical simulation of combustion, and fundamental materials research) is broadly applicable. Although this research is generally applicable to research topics listed in Tables 10.3 and 10.4, it is not easily focused on the specific critical technologies associated with those topics. This chapter concludes with a summary of the recommended research, a review of key facilities that will enable recommended research, and a discussion of programmatic recommendations. FLUID PHYSICS The panel considered many interesting NASA-related research issues in fluid mechanics (e.g., aerodynamics, hypersonic flows, and plasma dynamics). This section focuses on the gravity-related research issues of most cru - cial importance to NASA’s future crewed and uncrewed missions. Areas of particular interest are reduced-gravity multiphase flows, cryogenics, and heat transfer: database and modeling; interfacial flows and phenomena; dynamic granular material behavior; granular subsurface geotechnics; dust mitigation; and fundamental research in complex fluid physics. Each of these fields encompasses a myriad of individual phenomena. The targeted topics, of highest priority for NASA, are both enabling to, and enabled by, NASA’s access to reduced-gravity environments. As discussed below and in Chapter 10, advances in multiphase flow and heat transfer provide enabling tech - nology for many of NASA’s proposed crewed missions.4 A recent survey of NASA and industry identified high- priority gravity-related challenges such as the following:5 (1) storage and handling of cryogens and other liquids,* * “Other liquids” includes liquid oxygen, helium, and hydrogen, which are used for breathing, cooling, and propulsion, as well as non- cryogenic fuels such as hydrazine.
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267 APPLIED PHYSICAL SCIENCES (2) life support, (3) power generation, and (4) thermal control. Specific examples include fluid management for both crewed and robotic missions; water processing for plants and people, including reclamation, recycling, hydro - lysis, and hydration; mission-enabling phase-change technology for power production and thermal management; medical fluids; and food. Experiments in both applied and fundamental fluid physics are needed. Key fluid physics issues include reduced-gravity multiphase flow, cryogenics and heat transfer, interfacial flows† and phenomena, dynamic granular material behavior, granular subsurface geotechnics, dust mitigation, and complex fluid physics. Research into all of these issues is uniquely enabled by NASA reduced-gravity facilities. Space systems involving multiphase flows face significant design challenges due to the strong dependence of such flows on gravity.6 In particular, reduced gravity poses unique challenges when weak forces that are often masked by Earth gravity dominate fluid behavior in unexpected ways.7,8,9 For reasons detailed in the section entitled “Thermal Management” in Chapter 10, system designers have often avoided designs that use multiphase phenomena pending additional research and experimental demonstrations. 10 An improved understanding both of the key forces involved in multiphase flows and of the consequences of multiphase flows is necessary to prevent system failures and to enable designers to develop heat exchangers that employ multiphase flows (e.g., enhanced evaporation and condensing surfaces for evaporative and condensing heat exchange). Such an understanding could lead to better and more robust design principles and could dramatically enhance system performance and reliability while reducing system volume, mass, and cost. Attaining such knowledge is a high priority, as it is often mission enabling.11 In addition, important research along these lines is uniquely enabled by access to reduced gravity (e.g., on the ISS and in spacecraft, sounding rockets, aircraft, and drop towers). Robust phase separations by means of interfacial flows will be essential in numerous water-processing systems for long-duration life support in reduced gravity. A basic understanding of and the ability to predict the performance of multiphase, cryogenic flows and phase separation are essential to the design of the on-orbit propulsion fueling depots described in Chapter 10. Additionally, phase-change systems achieve competitive, if not mission-enabling, performance advantages for high power production (e.g., Rankine cycle phase separations) and thermal control systems (e.g., high-performance heat pipes), and they are inherent in space transportation systems for cryogenic propellants (e.g., ullage positioning, priming, and venting).12 Although many classifications are possible,13,14,15 the most challenging gravity-dependent vapor-liquid interfacial flows are as follows: (1) high-inertia multiphase flow and heat transfer and (2) interfacial-induced flows and phenomena. The nature of gravity-dependent interfacial flows depends on the relative importance of inertia, acceleration, and fluid and system properties.16 These flows may also be complicated by phase-change heat transfer, chemical and biological reactions, particulates, contaminates, and so on. System scale-up may be a challenge in partial gravity (on the Moon or Mars), but the microgravity environment on the ISS or other spacecraft is the most problematic. Many of these challenges may be dramatically reduced or eliminated for robotic missions (when no life support is necessary) or when an artificial-gravity countermeasure is used. In any event, the importance of research on multiphase flows and processes for space applications has been thoroughly, if not exhaustively, documented in previous studies and white papers.17-21 Fluid flow and heat transfer are interrelated processes in which gravity plays an important role. 22 During boiling heat transfer, for example, the formation, growth, and departure of a vapor bubble during nucleation on a heated surface induce motion in the host liquid in the presence of a gravitational field. The induced flow improves heat transfer from the solid surface. The relevant phase-change processes are boiling under pool and forced-flow conditions, evaporation at vapor-liquid interfaces, and condensation. Research literature, unfortunately, contains only very limited data on pool boiling in reduced gravity. Thus, available correlations and models are unable to provide reliable data on nucleate boiling and critical heat flux in reduced gravity. Flow boiling occurs when liquid is pushed over the heated surface by external means—for example, with a pump. A few studies of flow boiling under reduced gravity have been performed. 23 In reduced gravity, vapor bubbles become much larger as local coalescences occur on the heater surface, and they rarely detach from the solid. Detailed data from systematic experimental studies are needed to validate numerical simulation models for the development of the flow regimes in a heated channel and prediction of pressure drops and heat-transfer rates. † “Interfacial flows” are those in which an interface between two liquid or gas phases and the forces associated with the presence of the interface are important in determining the resulting flow.
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268 RECAPTURING A FUTURE FOR SPACE EXPLORATION During nucleate boiling, evaporation occurs from a thin liquid layer that forms between the vapor-liquid interface and the solid wall, as well as around the vapor-liquid interface of the vapor bubble located in the superheated liquid layer adjacent to the wall. Thin-film evaporation is associated with very high heat fluxes, and so an understand - ing of thin-film behavior in reduced gravity is extremely important for the development of mechanistic models for boiling and for other applications in propellant storage, life support systems, and dissipation of waste heat. Physical models for thin-film evaporation should include the impact of a wide variety of relevant forces (e.g., disjoining pressure, viscous force, inertia force, capillary force, gravity force, and recoil pressure). Capillary pres - sure gradients resulting from temperature and/or concentration gradients (Marangoni convection) and wettability of the surface also play an important role. The stability and rupture of thin films with or without external forces such as electric, magnetic, and acoustic forces should also be considered. Condensation is important to thermal management, power systems, in situ resource utilization (ISRU), and propellant management, but little data on condensation in tubes in reduced gravity exist in the literature. Such data are badly needed to validate numerical simulation tools that could be used for the design of space-based two-phase heat rejection systems. A better understanding of granular physics would have tremendous practical importance and is critical for enabling the human or robotic exploration of the Moon or Mars for the following reasons: (1) little is known about Moon and Mars regolith (soil) except that it is unlike soils found on Earth; (2) exploration of the Moon and Mars requires the direct interaction of human explorers and/or equipment with granular matter; (3) fundamental equa- tions that define the influence of gravity on the compaction and flow of granular materials are lacking; (4) granular matter is critical in life-threatening situations ranging from the collapse of structures to lung disease caused by the inhalation of dust; (5) extraterrestrial exploration is enabled by habitat construction, ISRU, mining, and surface transportation, all of which involve granular matter; (6) technologies that handle granular materials, such as heavy construction equipment and foundation construction processes, cannot be directly transferred from existing ter - restrial applications to future extraterrestrial applications; and (7) electrostatic forces can dominate the behavior of granular matter in reduced gravity and almost complete vacuum. 24,25 Research in Support of NASA’s Exploration Missions Reduced-Gravity Multiphase Flows, Cryogenics, and Heat Transfer: Database and Modeling In reduced gravity, the limitations on the empirically based predictive methods used on Earth for relatively high-speed multiphase flows do not allow NASA to exploit the advantages of using multiphase technology in space. This is because there is essentially no reliable database for flow regimes, void fraction, two-phase pressure drop, and wall heat transfer in reduced gravity. Therefore, a new predictive capability and design methodology need to be developed. In particular, physically based multiphase thermal-hydraulic models will be required to quantify accurately the effect of gravity (on Earth, Mars, the Moon, and in space). To be effective, such models must nec - essarily be developed with, and assessed against, appropriate small-scale, reduced-gravity data, and they must be capable of accurately scaling up these data to the relatively large multiphase systems and processes required on NASA’s future human exploration missions. In multiphase gas and liquid flows, the interfacial structures, flow regimes, and bubble sizes depend on the internal length scale mostly determined by the Taylor wavelength. In normal gravity flow, this length scale is on the order of several millimeters, and multiphase flow can easily reach near-equilibrium interfacial structures called flow regimes. Therefore, the use of the standard flow regime map and regime-dependent constitutive models has been quite successful. However, in reduced gravity, this length scale can be an order-of-magnitude larger than in normal gravity. Under such conditions the evolution of the interfacial structures is much more prolonged and complicated. These interfacial structures strongly affect the formation of flow regimes, phase separation, nucleation characteristics, bubble dynamics, vapor addition, and critical heat flux. A simple dynamical model of the interfacial structures would be a useful tool. This model should be compatible with standard computational multiphase fluid dynamic (CMFD) codes using two-fluid or multifield formulations. Phase separation and distribution are the key gravity-dependent multiphase fluid mechanic phenomena that
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269 APPLIED PHYSICAL SCIENCES must be understood and accurately modeled to meet the needs of future systems. For forced fast flows beyond the natural wicking rates of capillary systems, separation and stratification of vapor and liquid phases occur when a multiphase mixture is accelerated. This property can be used to advantage when designing active and passive phase separators, but it can also cause problems in devices with complex geometries, such as parallel channel arrays, outlet plenums, and conduit fittings. Significantly, many phase separators are sensitive to gravity because of the phase distribution that enters the separators. In any event, the ability to predict phase distribution (i.e., flow regimes) accurately is essential for any new three-dimensional analytical or numerical models developed by NASA. Pronounced lateral phase distribution occurs in multiphase conduit flows on Earth. 26,27 This phenomenon also occurs in microgravity,28,29,30 but it behaves quite differently, strongly influencing phase separation, pressure drop, and phase-change heat transfer. Therefore, an understanding of the evolution of interfacial structure under reduced gravity is required. Concerning heat transfer, the limited data available indicate that, for pool boiling, reduced gravity can enhance nucleate boiling heat transfer, but it can also significantly reduce critical heat flux. 31 Nevertheless, important scientific questions remain concerning the effect of gravity on boiling and condensation phenomena, including the surface nucleation of bubbles, bubble dynamics (bubble growth, merger, departure, and post-departure trajec - tory), phasic structure near a heated surface, rate of heat transfer, critical or dry-out heat flux, and both drop-wise and film-wise modes of condensation heat transfer. Also, as discussed below, related thin-film and interfacial phenomena, including surface wetting, need further study. Experiments to support model development will need access to reduced gravity. As noted in Chapter 10, two-phase forced convective heat transfer is essential for high-power thermal man - agement systems; NASA is likely to use this mode of heat transfer for energy production and utilization if it is available. In contrast to single-phase (gas or liquid) heat transfer, relatively little is currently known about the effect of gravity on heat transfer within two-phase systems during forced convective boiling and condensation. Detailed study of the effect of gravity on the forced convective condensation and boiling curves is particularly needed, especially with respect to the ebullition cycle (i.e., the formation and motion of vapor bubbles during surface boiling), critical heat flux, and the heat transfer during transition, film boiling, and quenching. In addition, the effectiveness of heat transfer enhancement devices (e.g., twisted ribbons) should be assessed in appropriate reduced-gravity experiments. The output of all of these heat transfer studies should be used to produce a new thermal design basis (i.e., models and correlations) that is valid in reduced gravity and for geometries suitable for NASA’s future missions.32,33 In order to use multiphase technology extensively in space, NASA must develop reliable, physically based predictive capabilities with respect to effect of gravity on multiphase systems and associated transport processes. These models should be based on detailed numerical simulations and/or reduced-gravity data (which currently are almost nonexistent); they should be capable of accurately scaling up so that they can be used to design and analyze hardware of interest on NASA’s spacecraft and extraterrestrial habitats. Physically based, three-dimensional, two- fluid models of multiphase flow and heat transfer have been developed by rigorously averaging the Navier-Stokes equations of fluid mechanics and the associated mass and energy conservation equations. 34,35 This results in a set of equations that can be efficiently integrated numerically. However, important physics is lost during the averaging process, and this information must be re-introduced into the two-fluid model through the use of mechanistic closure relations to describe the various interfacial and wall transfers mathematically. 36 Once this is done, the resultant multiphase model can be efficiently evaluated using a suitable CMFD solver such as NPHASE. 37 CMFD models of this type are widely used on Earth for transient and steady-state analysis of multiphase flow and heat-transfer phenomena in industrial systems and processes (e.g., nuclear reactors, chemical reactors, oil wells, etc.). However, these two-fluid models use closure relations that were developed for conditions on Earth; they are unreliable for applications in reduced gravity. Appropriate new closure relations can be developed, but this will require detailed measurements in reduced gravity along with numerical “data” from direct numerical simulation (DNS) or other numerical simulation techniques (e.g., lattice Boltzmann techniques) of multiphase flows. Fortunately, recent advances in computational hardware (e.g., high-speed massively parallel processors) have enabled the detailed simulation of multiphase flow and heat-transfer phenomena using DNS. 38,39,40 This is an important new development, which allows near-first-principle predictions to be made. For example, in recent
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270 RECAPTURING A FUTURE FOR SPACE EXPLORATION turbulent multiphase flow simulations, all interfaces were explicitly tracked, and the computational mesh was fine enough to resolve the turbulence structure as well as all significant mass, momentum, and energy transfers at the interfaces and at the wall of the conduit without the need for phenomenological closure relations. 41 Although DNS is numerically intensive, it is within the current state of the art and produces detailed three-dimensional results that quantify the effect of gravity on multiphase flow and heat transfer. Moreover, in principle, detailed numerical results can also be obtained using molecular simulation techniques, 42,43,44 but this approach has not yet been as thoroughly developed as DNS has for multiphase flow and heat-transfer applications. In any event, either DNS or molecular simulation results can be appropriately averaged and used as “data” in conjunction with physical data to help develop the closure relations needed by CMFD models.45 CMFD will be a very effective tool for NASA to perform analysis and design studies for various engineering systems and technologies. However, for CMFD to predict multiphase flow behaviors effectively in reduced gravity, a dynamical model for the evolution of interfacial structures should be developed and implemented. The conventional approach using only a flow regime map should not be applied. The coupling of a CMFD approach to heat transfer and vapor generation at the wall is essential. Significantly, CMFD models require much less computational power than do DNS and molecular simulations, and thus CMFD models can be used more readily to design and analyze relevant multiphase systems and processes. Moreover, DNS or molecular simulations can also be used to analyze directly various phenomena that may be important in reduced gravity but not on Earth. For example, disjoining pressure-induced forces (i.e., van der Waals forces) at the contact line in a horizontal conduit leads to a remarkable stratified-to-annular flow regime change when going from Earth gravity to reduced gravity.46 Thus, DNS or molecular simulations can also provide the insight needed to develop accurate three-dimensional multiscale CMFD models that are much more numerically efficient than DNS or molecular simulations and yet capture the same phenomena. This Grand Challenge type of research and development approach is consistent with the recommendations in previous NRC studies 47 and past international workshops on scientific issues in multiphase flow and heat transfer. 48,49 Interfacial Flows and Phenomena Numerous inadvertent or purposeful reduced-gravity multiphase flows are driven by gradients in surface tension caused by temperature, concentration, wetting conditions, and other factors, or by force fields involving pressure, shear forces, electric fields, magnetic fields, acoustics, acceleration, and so on. These are multiphase flows in which the influence of surface tension in the flow direction is strong, including phenomena relating to bubbles, drops, and nucleation. An extensive variety of flows and phenomena are represented here and either are present or find application in virtually all aspects of fluid systems aboard spacecraft. These systems include propel- lant management systems, phase distributions for power cycles, thermal control systems (e.g., heat pipes), water recycling for life support (for plants and crew), and other systems. Important challenges for research along these lines include the identification and assessment of key NASA application-specific phenomena. A prime example might be heat transfer due to temperature gradients near moving contact lines in cryotanks with small amounts of non-condensable gases.50 Such studies contain elements of fundamental discovery, as well as inspiration for improved concepts in which concurrent increases in technology readiness level (TRL) should be expected. In any event, this research should be able to produce verified models and tools for advanced system design and analysis. Spontaneous reduced-gravity interfacial flows, including wicking, can be exploited to control large quantities of liquids in reduced gravity. Such flows provide a passive means of liquid handling, enhancing possibilities for robust (no moving parts) primary or redundant solutions to reduced-gravity fluids management problems. Specific mission-enabling research that requires access to reduced gravity includes surface spreading (partial wetting and non-wetting), the coalescence of drops or bubbles, the rupturing of liquid films, local and global equilibriums, and stability, among other topics. Key challenges include passive phase separation and models for cryogenic fluid management and liquid handling for life support systems. Specific problems include complications associated with complex geometries and surfaces, heat and mass transfer, reactions, surfactants, contaminants, and moving contact- line boundary conditions, particularly for systems with partial wetting. With regard to the storage of cryogenic fuels, the impacts of length scale, fluid-structure interactions, and other considerations are important. Of practical concern, particularly for water processing, is the ability to predict system performance with confidence despite widely varying and perhaps poorly defined wetting conditions. 51
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271 APPLIED PHYSICAL SCIENCES Global multiphase system response phenomena are also important. In particular, it is well known that there can be global interactions among the various interconnected components in phase-change systems on Earth, which can lead to system-wide instabilities and failures. Limiting global instabilities include flow excursion instability, density-wave oscillations, and pressure-drop oscillations.52 These are relatively low-frequency instabilities that can cause large-amplitude flow changes. These instability modes are relatively well understood for conditions on Earth,53 but it is not yet clear how they will manifest themselves in multiphase systems in reduced gravity. 54 Preliminary analysis indicates that there may be a significant geometry-dependent effect of gravity, 55 but long- duration data in reduced gravity are needed to assess these results. In addition, development of new analytical models is needed for predicting and scaling up the observed system instabilities. The transient three-dimensional CMFD models previously discussed can be used for this purpose, but it appears that simpler, one-dimensional drift-flux models may be sufficient.56 Other flow-stability problems that warrant further investigation include the following: freeze-thaw cycles, flow excursions, system start-up transients, and cross-contamination. Dynamic Granular Material Behavior During the landing and launching of a spacecraft, its rocket exhaust directly interacts with the regolith, caus - ing a spray of material that can damage the spacecraft or nearby structures and reduce visibility. 57 Although the Apollo and Viking missions included considerable research concerning blast effects on planetary regolith, ques - tions remain with regard to scaling laws for erosion craters, jet plumes in a vacuum, the extent of spray, and the interaction between an impinging body or jet and a heterogeneous regolith. Rovers for future surface exploration missions would likely be designed for a range of 100 km or more. The mobility of wheeled vehicles is a direct function of the wheel and terrain interactions. Challenges include improving the understanding of rolling wheel contact behavior along with multiscale modeling of terrain and wheel interaction under reduced-gravity condi - tions, the potential for vehicles to kick up loosely packed regolith that may contaminate or damage instruments or the vehicles themselves, environmental impact of vehicles on virgin regolith, designing for wheel contacts with various loads, and detailed boundary-traction models at reduced gravity and for unusual materials (both the wheel surface and the soil). An interesting inverse problem is that of extracting geotechnical information about the soil based simply on footprints and vehicle tracks. Granular Subsurface Geotechnics Temporary and/or permanent structures on or beneath the surface of the Moon and Mars will be needed for exploration and settlement. For example, buried structures could be used to provide radiation shielding. Lunar and terrestrial soils have different geologic origins: lunar soils have been “weathered” by meteoroid bombardment instead of wind or water. As a result, lunar soil particles are much sharper than their terrestrial counterparts and are composed of agglutinates (aggregates of smaller soil particles bonded together by melting during microme - teoroid impacts). These agglutinates can easily be crushed. Further, the lunar regolith has not been characterized deeper than a few meters below the surface.58 Basic soil mechanics information for the reliable design of buried structures and safe mining for ISRU requires the sampling of soil at depth. Conventional terrestrial methods impart large material disturbances and require heavy machinery. Thus, research to develop novel in situ soil sampling and characterization techniques in the reduced gravity of the Moon and Mars is warranted. Terrestrial sands59 and simulated lunar soils60 tested on the space shuttle have demonstrated very high strength and elastic moduli for the low effective stresses expected on the Moon or Mars. Similar characterizations are needed for the soils specific to the Moon and Mars, for which the ISS is an ideal platform. The low-strain deformational characteristics of lunar and martian soils are the key to avoiding differential settling of structures, particularly because the impact of reduced gravity on compaction is not clearly understood. In particular, regolith re-compaction (after mining or excavation) may be quite different on the Moon than for materials encountered on Earth because of the jagged and brittle nature of the lunar regolith in a dry, erosion-free vacuum environment. Even on Mars, the dusty soil has slowed exploration by rovers and could present problems in providing a firm foundation for structures.61 Exploration missions that rely on ISRU will require equipment for the excavating, mining, crushing, sieving,
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272 RECAPTURING A FUTURE FOR SPACE EXPLORATION and conveying of lunar or martian soil. Currently, there is limited understanding of the behavior of granular mate - rials in rough rolling or sliding contact for irregularly shaped particles such as jagged regolith. New multiscale models are needed for developing the understanding of and predicting the tangling of particles and particle interac - tions with wheels and abrasion on handling equipment.62 Discrete element methods, in which interactions between many independent particles are modeled, may be useful in this regard, but further development to expand current capabilities beyond spherical particles to irregular-shaped particles is needed for lunar applications. 63 Upscaling to large, structural-scale systems will cause computational difficulties. Continuum models require the development of soil-specific constitutive relations, for which samples of lunar and martian soils will be needed, because simu - lated soils tested to date lack the ability to mimic accurately the effects of the crushable agglutinates. 64 Geologic variability hampers accurate prediction for even the most sophisticated terrestrial-based soil models. Thus the nature of spatial variability of lunar and martian soils needs to be assessed. Additional research would be needed to support the needs of asteroid surface missions. Applications for ISRU, excavation, landslides, and other phenomena are related to the transition from static (jammed) to dynamic (flowing) states. Often gravity plays a key role in the initiation of flow. In granular flows, particles of different sizes tend to segregate because of percolation or buoyancy. Furthermore, granular flows can be influenced by interstitial gas.65 Particle shape, electrostatic effects, frictional and shape properties, thermal cycling, and size distributions will also affect flow characteristics. A better understanding of the fundamental physical aspects of granular flow (e.g., pile, chute, and tumbler flows) is needed to enable designing for situa - tions in which it is necessary to have granular flow (ISRU) and to prevent dangerous situations (e.g., landslides), and to contribute to a better understanding of the geomorphic features of the Moon and Mars. Although some progress has been made,66 improved scaling laws that include the impact of gravity are needed to address many of the challenges described above. Dust Mitigation The fine regoliths on the surface of the Moon and Mars are essentially devoid of moisture and air. On the Moon, the dust that covers the surface is electrically charged, and thus it sticks to almost anything. The interac - tion of particles with solar radiation and the solar wind may result in dust lifting off the surface and falling back again, a process that is poorly understood and yet may be responsible for the streamers observed by Apollo-17 astronauts. Electrostatic charging of martian dust is also an open issue. 67 Dust can interfere with many aspects of both human and robotic exploration. Concerns include the health effects of inhaling micron-sized jagged particles and/or silica dust, degradation of life support systems, obscura - tion of instruments, and damage to bearings, gears, and seals.68 On Mars, dust storms with strong winds can last for weeks and can envelop the entire planet, and dust devils may create local hazards. 69,70 As with dust storms on Earth, wind-driven dust particles can damage equipment, foul filters, cause excessive wear, pose health risks, and reduce the amount of sunlight reaching solar panels. The interaction of dust with the solar wind, the thin atmosphere on Mars, and natural and artificial surfaces is unclear. Measurements of dust particle size and concentration are lacking. Surface electric fields cause dust to adhere to objects and drive dust transport, yet this process is poorly understood. Fundamental Research in Complex Fluid Physics As noted in Chapter 8, complex fluids are excellent candidates for study in reduced gravity. New experiments could greatly enhance the understanding of important reduced-gravity fluid physics phenomena and lead to new fundamental insights in fluid mechanics and transport in general. These experiments are enabled by NASA space - flight, and they would address phenomena of broad interest to the science and engineering communities. This is particularly true with critical-point phenomena, rheology, and complex fluids such as biofluids, colloids, foams, nanoslurries, granular materials, plasmas, and liquid crystals. 71 The scientific community should design specific experiments to be conducted in response to research solicitations by NASA. Citing granular physics as an example, granular flows are typically driven by gravity, and experiments with granular flows at reduced gravity could reveal
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273 APPLIED PHYSICAL SCIENCES much about the influence of gravity on such flows. Other aspects of granular physics include particle clustering, self-assembly, and dissipation, all of which are altered by gravity under terrestrial conditions but could be studied free of sedimentation or gravity-induced stresses and friction in a microgravity environment. This enables, for example, the study of electrostatic effects, interstitial fluids, and the nature of flows involving particles of multiple sizes and/or shapes without the complication of buoyancy or gravitational settling. Recommended Research in Fluid Physics Recommended research in fluid physics is summarized below and in Table 9.1 toward the end of this chapter. Reduced-Gravity Multiphase Flows, Cryogenics, and Heat Transfer: Database and Modeling A detailed reduced-gravity database is essential for the development and assessment of reliable models for multiphase flow and heat transfer, but very little data is currently available. These data should include phase sepa - ration and distribution (i.e., flow regimes), heat transfer involving phase change (i.e., boiling and condensation), and/or advanced devices (e.g., twisted ribbons), pressure drop, and multiphase system stability. These data can be best acquired using a multipurpose phase-change test loop in the fluids integrated rack aboard the ISS. Direct numerical simulation or other numerical simulations (e.g., lattice Boltzmann techniques and molecular simulation) should also be performed to allow NASA to develop an understanding of mission-enabling phase distribution, separation, liquid management, and phase-change system phenomena. In conjunction with ISS data, the results from these detailed simulations can also be used as “data” to support the development of mechanistic three- dimensional CMFD models (e.g., to describe the required interfacial and wall closure relations), which are much more efficient than DNS or molecular simulation models and can be used for most design purposes. In addition, modeling efforts should include the following: • One-dimensional drift flux models should be developed and used for phase-change system stability analysis. • Modeling of the evolution of interfacial structures in reduced gravity, which NASA has initiated, should be completed. • Bubble nucleation characteristics and interfacial structure evolution near a heating surface in terms of bubble departure size, departure frequency, and bubble motion should be analyzed and modeled for reduced-gravity boiling flow. Their relation to the occurrence of the critical heat flux also should be investigated. • The effects of particular geometries on the evolution of the interfacial structures under reduced-gravity conditions should be modeled by a simple and effective approach. All of the above models should be developed in a form that can be easily implemented into CMFD codes within the framework of the two-fluid model. This research could be performed in the next 10 years and beyond. Research in this area should support the development of the following critical technologies described in Chap - ter 10 (see Tables 10.3 and 10.4): two-phase flow thermal management technologies; technologies to enable engine start after long quiescent periods, combustion stability at all gravity conditions, and deep throttle; supersonic retro propulsion systems; cryogenic fluid management technologies, including zero-boiloff propellant storage systems; regenerative fuel cells; thermoregulation technologies for lunar habitats, rovers, and space suits; and the develop - ment of fluid and air life support subsystems (e.g., to enable closed-loop air revitalization and closed-loop water recovery for extravehicular activity [EVA] and life support systems). Interfacial Flows and Phenomena Interfacial flows, which are affected by the forces associated with the presence of an interface between two liquid or gas phases, are central to spacecraft functioning. Flows of interest include storage and handling systems for cryogens and other liquids, life support systems, power generation, thermal control systems, and others. Research could lead to advanced systems that would be significantly more capable, more reliable, and more affordable than
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274 RECAPTURING A FUTURE FOR SPACE EXPLORATION current systems. Relevant phenomena are strongly gravity-dependent and are highly enabling to and uniquely enabled by NASA missions. A wide variety of fundamental and applied fluid phenomena should be investigated, whenever possible employing multiuser facilities that develop mechanistic models for induced and/or spontane - ous multiphase flows (with or without phase change). Research should include targeted experiments that expand core knowledge, improve designer options and confidence, and increase TRLs over the next 10 years and beyond. Research in this area should support the development of the critical technologies described in Chapter 10 that are listed in the section above titled “Reduced-Gravity Multiphase Flows, Cryogenics, and Heat-Transfer: Database and Modeling.” Dynamic Granular Material Behavior and Granular Subsurface Geotechnics Improved predictive capabilities related to the behavior of lunar and martian soils on the surface and at depth would enable advanced human and robotic planetary surface exploration and habitation. Surface operations such as wheel/track-soil interaction and cratering would benefit from the development of particle-scale and multiscale models and simulations of key dynamic interactions with soil, including the crushing and compaction of agglu - tinates. ISRU mining, the design of structural foundations and anchors, and berm/trench stability analysis would benefit from improved soil-specific computational models and methods for sampling planetary soil at depth. Model development can begin in the first part of the decade, but the refinement of site-specific models will likely require ground-based and ISS testing of actual lunar soils. Research in this area should support the development of the following critical technologies described in Chapter 10 (see Tables 10.3 and 10.4): regolith- and dust-tolerant systems for planetary surface construction and teleoperated and autonomous construction. Dust Mitigation The development of fundamentals-based strategies and methods for dust mitigation would enable advanced human and robotic exploration of planetary bodies. Areas of interest include experimental methods, the under- standing of the fundamental physics of dust accumulation and electrostatic interactions, and methods for modeling dust accumulation. Issues related to dust seals, environmental hazards, solar panel obscuration, and sensor fouling should also be addressed. Much of this work can be done with ISS and ground-based studies early in this decade. Research in this area should support the development of the following critical technologies described in Chap - ter 10 (see Tables 10.3 and 10.4): dust mitigation technologies and systems for EVA and life support systems, for planetary surface construction, and for lunar water and oxygen extraction systems. Complex Fluid Physics Unique experiments for understanding complex fluid physics in microgravity are enabled by the ISS. Such experiments can unravel the behavior of complex fluids, including granular materials, colloids, foams, nanoslurries, biofluids, plasmas, non-Newtonian fluids, critical-point fluids, and liquid crystals, without the bias of gravity. The ISS microgravity environment further enables unique capabilities for fundamental experiments of complex flow systems that can explore fundamental fluid physics and geological systems with small-scale models. These stud - ies could be accomplished with a combination of ground-based and ISS efforts in the next 10 years and beyond. COMBUSTION Combustion has evolved into an extremely multidisciplinary subject, as diverse as fire safety and astrophys - ics. In the most fundamental sense, combustion deals with the process and effects of energy released into a sur - rounding medium and the response and feedback of that background. Usually, the focus of combustion is on a reaction front, where reactants are converted into combustion products. Such fronts exhibit flames, deflagrations, detonations, and myriad intermediate states. Sometimes what seems to be the most esoteric combustion question in, for example, details of a flame structure or dynamics becomes a key issue for an application that is of critical importance to safety or a developing technology.
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275 APPLIED PHYSICAL SCIENCES NASA’s reduced-gravity combustion research program has had a number of broad objectives, ranging from increasing the fundamental understanding of the basic physical processes to preventing and controlling fires on spacecraft. A better understanding of combustion itself has evolved as NASA reduced-gravity facilities have been used to eliminate buoyancy effects, which often dominate terrestrial combustion, and to compare measurements with theory and numerical simulations. In addition, the program has tried to relate fundamental combustion principles to applications such as the simulation of fire growth in spacecraft and the assessment of actual fire risks. In this area, the understanding of combustion has provided underpinning information that enables a better understanding of material flammability and fire prevention measures that can improve fire safety in the future. 72,73 NASA’s reduced-gravity combustion research has led to enabling technologies for space exploration, and it has provided new insights into fundamental combustion processes. Both areas are addressed further in the recommendations. Research in Support of NASA’s Exploration Missions Combustion research in support of NASA’s exploration missions is addressed in Chapter 10, “Translation to Space Exploration Systems.” Fundamental Combustion Research This section focuses on gravity-related combustion research issues of most crucial importance to NASA’s future crewed and uncrewed missions. In particular, NASA should support fundamental combustion research in fire safety and combustion processes. Research in both of these areas would be facilitated by more capable numerical simula - tions of combustion. Because fire safety is so important both as a topic of fundamental combustion research and as an operational element of human space exploration missions, fire safety is addressed below and in Chapter 10. Fire Safety Fire safety includes fire prevention, detection, and suppression (all of which are discussed here and in Chapter 10) and post-fire recovery (which is discussed only in Chapter 10). Wherever a fuel and an oxidizer appear in reasonable proximity, there could be a scenario in which they meet by accident in the vicinity of an ignition source. To minimize the likelihood and impact of accidental fires in spacecraft, combustion needs to be better understood in all relevant environments (that is, on Earth and in the reduced-gravity environments of the space shuttle [and replacement vehicles], the ISS, the Moon, and Mars). Dealing with accidental fires in reduced gravity is different from dealing with them on Earth in several impor - tant aspects. On Earth, buildings are designed to allow inhabitants to escape to a safe outside location. Spacecraft and habitats in reduced gravity, such as on the Moon or Mars, are enclosed by pressurized vessels with hostile outside environments, and so outside escape is generally not a viable option. Thus, every aspect of fire prevention, detection, and suppression is more critical in reduced gravity than in terrestrial scenarios. Several fires have occurred in spacecraft on the ground or in space. As a result, new approaches to high- pressure oxygen atmospheres, design standards, flammability of materials, flammability testing, operational emer- gency procedures, quality control, and suppression strategies have been developed. 74,75 From a fundamental science perspective, fluid transport in normal gravity is controlled by buoyancy. As a result, terrestrial systems are designed to deal with aspects of the combustion process that behave very differently in space, and an improved knowledge of combustion in reduced gravity is essential in order to adapt fire safety concepts and systems to the more stringent conditions of the space environment. Fire Prevention A major part of NASA’s strategy for fire prevention is to improve the design and selection processes for materials used on spacecraft. This improvement involves determining acceptable materials, techniques to reduce material flammability, and ways to monitor and control ignition sources. Material screening for flammability is now based on empirical procedures that use standard tests performed in normal gravity. 76 For example, one test used for many solid materials considers upward ignition and flame growth. If the flame spreads more than six
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288 RECAPTURING A FUTURE FOR SPACE EXPLORATION TABLE 9.1 High-Priority Research Areas and Topics, Status, Recommended Research, and Outcomes for 2020 and Beyond Research Area and Topic Current Status 2010-2020 2020 and Beyond Outcomes Fluid Physics —Only very limited, —Design and build a —Acquire —A reliable database Reduced-gravity mostly qualitative, multipurpose phase- comprehensive, detailed with which to develop multiphase flows, reduced-gravity data change test loop for the three-dimensional data and assess accurate cryogenics, and heat exist, leading to fluids integrated rack on phase distribution models for the design transfer: database and insufficient designer aboard the International and separation and and analysis of new and/ modeling: Phase separation and confidence. Space Station (ISS). phase-change heat or significantly improved distribution, phase- —Few reliable detailed —Acquire targeted transfer. systems for NASA (e.g., change heat transfer, simulations of reduced- database on phase —Develop mechanistic, for power production and pressure drop, and gravity multiphase distribution and multiscale three- utilization, waste water multiphase system phenomena exist. separation, phase- dimensional recovery, on-orbit fueling, stability. (AP1)a change heat transfer computational in situ resource utilization (e.g., boiling and multiphase fluid (ISRU) extraction of condensation), pressure dynamic models (using water from surface drop, and system a reduced-gravity materials, etc.). stability. database and DNS or —Reliable predictive —Perform detailed molecular simulation capabilities for multiphase direct numerical results). flow and heat transfer at simulations (DNSs) —Develop a one- reduced-gravity levels for or molecular dimensional drift- system design, scale-up, simulations of selected flux model based on and analysis. phase distribution, a reduced-gravity liquid management, database. cryogenics, and phase- change phenomena at reduced gravity. Reduced-gravity Perform targeted Expand breadth of A reliable database, Interfacial flows and thermo-capillary and experiments that experiments to increase models, and experience phenomena: Acquisition of data buoyancy-driven expand core knowledge technology readiness sufficient to design and development of flows are reasonably and improve designer level. and analyze mission- mechanistic models well understood. In options and confidence enabling spacecraft fluid for induced and/or contrast, there is a (multiuser facilities systems with dramatically spontaneous multiphase poor understanding of preferred). increased reliability. and cryogenic flows problems dominated by Terrestrial applications with and without heat moving contact lines also expected. transfer. (AP2) with partial wetting.
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289 APPLIED PHYSICAL SCIENCES TABLE 9.1 Continued Research Area and Topic Current Status 2010-2020 2020 and Beyond Outcomes —Computational —Develop —Improve particle- —Predictive capability Dynamic granular methods and models computational scale models for for interactions between material behavior and are limited to simple methods and models irregular-shaped vehicles and granular granular subsurface particle shapes; impact for irregular-shaped particles. materials, cratering, geotechnics: Granular flow dynamics of gravity is uncertain. particles and crushing/ —Improve multiscale excavating, and the and geotechnics —Current compaction of models that include the jamming-flow transition for Moon and Mars characterization agglutinates. effects of gravity. for complex granular environments for methods are based —Improve —Collect lunar soil systems at reduced human or robotic largely on empiricism understanding of samples for Earth-based gravity. exploration, ISRU specific to Earth; dynamic interactions characterization. —Accurate and reliable mining, and habitation. exploration and with vehicle systems —Develop methods predictive models of lunar (AP3) sampling techniques and cratering, including for excavating and and martian soil behavior are unsuitable for effects of gravity. conveying materials for for analysis and design extraterrestrial use. —Develop methods ISRU. of structural foundations, —No lunar soil data for in situ sampling at —Develop methods for berms, slopes, and are available at depths depth. the design of below- excavations. below a few meters, —Develop suitable grade structures and —Accurate and reliable where structures may models for stress-strain foundations. computational models be sited and mining for behavior of soils for for the deformational ISRU will occur. foundations, berms, etc. and strength behavior of extraterrestrial soils. —There is qualitative —Conduct ISS —Extend approaches —Fundamental Dust mitigation: Development of evidence of dust- and ground-based specific to lunar or understanding of dust fundamentals-based related challenges experimental martian environments. behavior in reduced strategies for dust from previous lunar investigation of —Develop practical gravity. mitigation on lunar and missions and observed dust accumulation, methods for mitigating —Practical approaches for martian surfaces. (AP4) atmospheric dust- specifically electrostatic adverse effects of dust mitigating impacts of dust related phenomenon on effects. on seals, sensors, and on mechanical systems Mars. —Develop models/ solar panels. and sensors. —Minimal fundamental simulations of dust understanding of in extraterrestrial the physics of dust planetary environments. accumulation exists. Many important —Conduct fundamental Continue relevant A better understanding of Complex fluid physics: Utilization of microgravity experiments on the research on the ISS. the physics of complex ISS microgravity experiments have ISS to unravel the fluids and flows. environment to study been completed, but complex behavior the fundamental physics fundamental aspects of of granular material, of complex fluids and many issues have yet to colloids, foams, flows. (AP5) be resolved. biofluids, plasmas, non-Newtonian fluids, critical-point fluids, and liquid crystals without gravitational bias. —Conduct fundamental experiments on complex flow systems otherwise biased by gravity. continued
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290 RECAPTURING A FUTURE FOR SPACE EXPLORATION TABLE 9.1 Continued Research Area and Topic Current Status 2010-2020 2020 and Beyond Outcomes Combustion Fire safety: Screening Current terrestrial Improve and Optimize and Improved fire safety for methods for material methods are inadequate. supplement current implement new astronauts. flammability and fire materials screening methods. suppression for space methods. applications. (AP6) Incomplete knowledge Complete droplet-phase, Conduct larger-scale, Deeper understanding of Combustion processes: Combustion of basic processes gas-phase, and solid longer-duration fundamental combustion experiments in reduced and their response to experiments on the experiments. phenomena. Some of the gravity to cover longer reduced gravity existed. ISS. Begin preparations fundamental knowledge durations, larger scales, The importance of and planning for large- contributes to enabling new fuels, and practical gravity in combustion scale, long-duration technologies for fire aerospace materials has been demonstrated experiment. safety. Other knowledge relevant to future and new phenomena in contributes to terrestrial missions. (AP7) reduced gravity have applications. been discovered. Many computational Develop and validate Integrate models with Validated numerical Numerical simulation tools are available; selected numerical experiment and design. models to enable of combustion: Development and input data and models with reduced- prediction, design, and validation of single and boundary conditions are gravity experiments. interpretation of data from multiphase numerical incomplete. Theoretical experiments and missions. combustion models that and numerical treatment relate reduced-gravity of solid processes in and Earth-gravity tests. combustion should be (AP8) improved. Materials Science Very little research has Provide benchmark data Employ new An increased ability Materials synthesis been done in the past for materials synthesis experimental facilities (1) to understand and and processing 8 years. Prior to this, and processing and to address questions predict the formation and the control of extensive research was microstructural control that cannot be answered of microstructure and microstructure and carried out, as outlined using reduced gravity. with existing ISS properties of a wide range properties: Study of materials in prior National facilities. of materials in terrestrial synthesis and Research Council and space environments reports.b,c processing and and (2) to create new mircrostructure materials. development that are affected by gravity. (AP9) Very little fundamental Develop novel, Develop novel materials Improved spacecraft and Advanced materials: Materials that enable research has been advanced materials that will significantly mission capabilities at the NASA mission. conducted on advanced using both experimental improve weight and reduced cost. (AP10) materials for space and computational property factors (e.g., exploration. NASA experimental techniques decreased weight, has relied on existing and methods. increased operating materials. temperature, and self- healing capabilities).
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291 APPLIED PHYSICAL SCIENCES TABLE 9.1 Continued Research Area and Topic Current Status 2010-2020 2020 and Beyond Outcomes Although the need Identify and produce Produce elements, Improved prospects In situ resource has been recognized, a selected group of materials, and/or for extended human utilization: Fundamental studies of little research has been strategic elements (e.g., components on the exploration to how to utilize in situ conducted in this area. oxygen), materials, Moon, Mars, and/or extraterrestrial bodies. minerals and materials. and components asteroids. (AP11) that enables space exploration and can be manufactured from extraterrestrial resources in both normal and reduced gravity. aRecommendation identifiers are as listed with clarifying material in the main text of this chapter and also in Tables 13.1 and 13.2. bNational Research Council, Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies, National Academy Press, Washington, D.C., 2000. cNational Research Council, Assessment of Directions in Microgravity and Physical Sciences Research at NASA, The National Academies Press, Washington, D.C., 2003. PROGRAMMATIC RECOMMENDATIONS NASA has long known (in some cases for many decades) about the need for research that enables the crewed exploration of space, but to date some needs have not been thoroughly addressed. As a consequence, for example, NASA and its contractors cannot reliably design and deploy large-scale multiphase systems and processes in space, fire safety research in reduced gravity is relatively immature, and spacecraft are designed using materials and design techniques that are generally available rather than tailored for a specific mission. To change this situation, NASA should alter the way that relevant applied research is solicited and funded. Moreover, the panel agrees with recent recommendations141 that a new long-term space technology research program with realistic objectives and stable funding is urgently needed. It appears unlikely that individual principal-investigator-driven research programs will satisfy the mission- oriented research needs of NASA. For example, in many cases well-coordinated research teams are needed. In others the direction of the research that satisfies NASA’s needs may not be represented by the proposals received in response to a broad research announcement. The panel emphasizes the following pertinent observations made in a 2004 report prepared for NASA:142 Industry and other government research agencies, such as the Defense Advanced Research Projects Agency and the Office of Naval Research, regularly engage in programmatic research. . . . These agencies typically request or invite the formation of appropriate research teams and solicit research proposals from one or more of these teams, [or they request mission-related proposals from individual principal investigators]. Research is initiated based on a careful internal or external review and assessment of the team’s capabilities and responsiveness to the sponsor’s needs of the proposed research. [Subsequent to funding, the] . . . sponsor or its designee continuously evaluates the performance of the research team or teams through programmatic meetings and the peer-reviewed technical publications that result from the research being performed. The process is a dynamic one where research directions are changed as required to accomplish [emerging] mission goals. The researchers who are involved in such programs normally find this an exciting and rewarding way to do research because they are all on the critical path, and peer pressure among team members encourages superior performance. (p. 15)
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