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, combustion, 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 flammability 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,
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 structures, 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 approximately 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 technologies. 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, “Translation 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.
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 crucial 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 technology 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.
(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, hydrolysis, 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.
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 understanding 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 pressure 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 equations 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 terrestrial 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
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 necessarily 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
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 trajectory), 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 management 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
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 studies47 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 propellant 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
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, causing 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, questions 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 conditions, 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 micrometeoroid 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,
and conveying of lunar or martian soil. Currently, there is limited understanding of the behavior of granular materials 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 interactions 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 simulated 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 situations 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.
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 interaction 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, obscuration 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.
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 spaceflight, 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
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 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 separation 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 Chapter 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 development 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
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 spontaneous 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 agglutinates. 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.
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 understanding 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 Chapter 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 studies could be accomplished with a combination of ground-based and ISS efforts in the next 10 years and beyond.
Combustion has evolved into an extremely multidisciplinary subject, as diverse as fire safety and astrophysics. In the most fundamental sense, combustion deals with the process and effects of energy released into a surrounding 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.
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.
Combustion research in support of NASA’s exploration missions is addressed in Chapter 10, “Translation to Space Exploration Systems.”
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 simulations 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 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 important 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 emergency 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.
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
inches without self-extinguishing, the material fails the test (test 1 in NASA-STD-6001A). Fire-control measures may include, for example, installing firebreaks to limit the extent of flame spread or covering flammable materials with nonflammable materials. Despite these and many other measures, data taken in reduced gravity suggest that existing screening methods could be improved to provide a more scientifically based description of the material properties that enable combustible materials to sustain and grow a fire.77,78,79 Future research should encompass existing space atmospheres and the new atmospheric conditions (enriched oxygen, reduced pressures) proposed for spacecraft cabins and surface habitats in future space missions.80 Therefore, a more fundamental understanding of processes such as pyrolysis and combustion in microgravity are urgently needed to define the limitations of current screening tests and, if necessary, to propose improved tests. Experimental and theoretical methods are needed to improve the understanding of how screening-test results in normal gravity can be used to predict performance in reduced gravity.
The earlier a fire is detected, the less the damage it can cause. At the same time, false alarms should be minimized. To be able to design and best exploit a fire detection system, it is essential to understand what is being detected and to capture adequate quantities of the effluent from the fire. The current ISS uses smoke or particle detectors that are sensitive to a given range of particle sizes. Key questions include the following:
• What should be sensed for faster and more reliable detection?
• If smoke particles are to be used, what ranges of particle sizes indicate incipient fires in reduced gravity?
• How can the detector signal be used best to initiate the fire suppression?
Early investigations suggested that smoke particles emerging from flames are larger in reduced gravity than in normal gravity. (This is attributed to the longer residence times that occur in low-speed flows in reduced gravity.81) Nonetheless, a more recent ISS experiment indicates that the particle sizes from pure pyrolysis processes (no fire) can actually be smaller than on Earth.82 Since pyrolysis from the overheating of electrical insulation is of concern in space and an accumulation of pyrolysis products is a dangerous pre-fire situation, detectors should cover the entire size range for both fire and pyrolysis products. It is important to continue current efforts to characterize the chemical products of fires and to determine the particle-size distribution produced by the combustion of spacecraft materials in reduced gravity (to help define “fire signatures”) and to understand the transport of combustion products from the fire to the detector.
Fires are typically suppressed by releasing a spray of gas or droplets. Two other methods, unique to space, are also available: cutting off air ventilation and depressurizing the spacecraft cabin. Early microgravity research suggested that most materials cannot maintain a flame when there is no convective airflow.83,84 Even when combustion is sustained in a quiescent atmosphere (such as for a small droplet or a candle), flames that are supported by purely diffusive transport processes are relatively weak; thus they can be controlled and extinguished more easily. Shutting down spacecraft ventilation is now a standard procedure after a fire is detected. The second approach unique to space is to depressurize the cabin by opening an external vent. When the pressure is reduced enough, a fire is extinguished.85,86 This approach requires astronauts to don space suits or to move to a different module. Also, the fire can briefly intensify during the depressurization process because of the induced flows caused by depressurization. Depressurization is normally considered a last resort.
Fire suppression agents used in spacecraft should be (1) extremely efficient, (2) nontoxic and causing little or no damage to the equipment, (3) easy to clean up with onboard resources, and (4) able to reach hard-to-access places, such as inside equipment racks. Types of suppression agents used in the past include portable aqueous gel or foam (Apollo, Skylab, Mir, and the ISS), bottled carbon dioxide (ISS), and Halon (space shuttle).87,88 The most effective fire suppression agents and deployment methods have yet to be determined for specific space applications. In addition to the suppression agents currently used, water mists are promising candidates; they have demonstrated the ability to extinguish pool fires89 and solid-fuel fires.90,91
There are several ways in which fire suppression agents extinguish a fire. One is by changing gas-phase pro-
cesses so as to slow key reaction rates and to increase heat losses (thereby affecting the combustion reaction). For example, at high temperatures, Halon decomposes to release halogen atoms, such as bromine and fluorine, that combine with active hydrogen atoms to quench the flame propagation reaction. Alternately, fire suppression agents may cool or block the surface of a solid fuel, which reduces the pyrolysis rate (i.e., reduction of fuel supply). A few studies have examined gas-phase mechanisms for fire suppression in reduced gravity,92,93 but little has been done to study mechanisms related directly to the fuel surface. New multidimensional simulations predict that to extinguish a flame most effectively, water mist must be injected differently under different gravity conditions. This is a multiphase combustion problem involving issues of gravity, buoyant and forced convection, and droplet sizing.
Verifying and understanding analytical and experimental results as well as better understanding the interactions among suppression agents, flames, and surfaces in reduced gravity will provide insight into the fundamental differences in balances between buoyancy and other forces and will contribute to the development of improved fire-extinguishing systems. In addition, methods for screening materials for use in spacecraft should consider factors related to fire suppression.
Studies of combustion in reduced gravity lead to a greater understanding of terrestrial combustion. Flames are controlled by energy released from exothermic chemical reactions and the interaction of this energy with the atmosphere. On Earth, the chemical reactions, energy, fluid dynamics, and gravity-induced buoyancy interact nonlinearly. Their effects are usually only separable through theoretical analysis and numerical simulation. By varying or eliminating the effects of gravity, one can extract fundamental data that are important for understanding combustion systems. Such data include parameters such as chemical reaction rates, diffusion coefficients, and radiation coefficients that strongly influence the ignition, propagation, and extinction of combustion waves. This approach has been implemented to some extent for the limited time and spatial scales in existing reduced-gravity platforms.
Besides gravity, there may be other factors specific to the space environment that differ from typical terrestrial environments. These include atmospheric conditions, local material compositions, stoichiometry, and ignition sources. The effects of these environmental changes on combustion in reduced gravity are unclear, and some range of relevant parameters should be investigated.
Data from reduced-gravity experiments, when combined with theory and computations, have “contributed to our fundamental knowledge of some of the most basic combustion phenomena, to the improvement of fire safety on present and future space missions, and to the advancement of knowledge about some of the most important practical problems in combustion on Earth.”94 These results include the discovery of new phenomena that challenged previous theories and demonstrated the actual behavior of previously proposed combustion states. The existence of theoretically predicted flame balls has been revealed, and their persistence and stability in reduced gravity have been demonstrated. Flame balls have been shown to evolve into flame strings as the combustion mixture becomes leaner, demonstrating the persistence of burning in very lean conditions. In addition, reduced-gravity experiments have demonstrated theoretically predicted pulsating flames, halos of soot rings around burning fuel droplets, greatly altered burning properties of metal wires, and the altered distribution of soot and particulate sizes in diffusion flames, which shows the inadequacy of smoke detectors used in space. A radiative quenching limit in low-straining or low-speed flows has been discovered, along with an upper size limit to spherical droplet flames. Experiments in reduced gravity have shown that flame spread over solid fuels changes because of both gravity and the nature of the background (enriched-oxygen) atmosphere, and in oxygen-starved conditions, flamelets and fingers appear before the final extinction occurs. Thin cellulose has been found capable of sustaining a flame in a lower-oxygen atmosphere in lunar gravity than in Earth gravity.
Much of the new information derived from combustion experiments in reduced gravity has been published in NASA reports, journal articles, conference proceedings,95 a special issue of Combustion and Flame,96 a special section of the AIAA Journal,97 and a dedicated summary monograph on microgravity combustion.98 The work has also left many unsolved or unexplained phenomena, as discussed below. The main point is that combustion phenomena behave differently as gravity changes from Earth gravity to zero gravity (or g-jitter). Some of this difference is related to the effects of gravity on the combustion front itself, and some of it is related to how gravity affects convection or the supply of fuel.
Gas-phase combustion, including ignition, persists as a fundamental research area that has important practical consequences in terms of fire safety. As a gaseous fuel and oxidizer mix, conditions for igniting fires (which might be detectable early) or explosions (which might not) can occur. Ignition limits for combustible gas-phase mixtures are often expressed in terms of the stoichiometry or the percentage of fuel; they are usually determined by specified laboratory tests. Other critical factors, some of which are seldom considered, are the method and rate of energy input into the reactants and the geometry and size of the entire system. Some work has been done, for example, to extend the ignition-limit concept and consider the power-energy relation required for hydrogen ignition in reduced gravity, but little has been done recently. It now appears that the ignition limits can change as a function of the size of the system. More work is required on gas-phase ignition in conditions representative of those found in specific microgravity environments and as a function of sources of input energy.99,100
It is very rare, on Earth and in space, for an area, cabin, or room to be completely filled with a stoichiometric, homogeneous mix of fuel and oxidizer. Instead, there are variations in mixture composition, stoichiometry, temperature, and/or other energy input. These result in gradients of chemical reactivity, but very little is known about the behavior of flames propagating through reactivity gradients. In any case, when obstacles are present, the situation becomes even more dangerous, as turbulence intensifies and flames accelerate. Reactivity gradients are important at all stages of a fire or an explosion, from ignition and propagation through to extinction. Reduced-gravity experiments can be used to learn more about flame ignition, propagation, and extinction in reactivity gradients. Such experiments could enable more accurate assessments of the risk and behavior of fires and explosions in space environments.
What is the flame structure near the flammability limits in microgravity? Previous studies of combustion in reduced gravity have shown that gaseous flammability limits may be extended beyond those measured on Earth.101,102 It is now speculated that such limits might not exist at all—that a diffusive or hydrodynamic mechanism may cause extinction, or that flame balls sometimes evolve into flame strings, and, as the mixture becomes leaner, flame balls promote the persistence of burning in very lean conditions. To understand these issues, future research should investigate the nature of hydrogen and methane flame structures as the flame approaches the currently accepted flammability limits with respect to dilution or stoichiometry.
Most combustors and unwanted fires involve diffusion flames. There remain significant gaps in the understanding of these flames—particularly with regard to chemical kinetics, transport, radiation, soot formation, pollutant emissions, flame stability, and extinction—in part because long residence times possible in reduced gravity can result in unusual behavior. Gravity, which has a tremendous impact on most diffusion flames, is an important parameter to vary in experiments and models. Studies of laminar and turbulent diffusion flames in reduced gravity can yield insight into fundamental turbulent-flame structure. It can also provide important understanding of the limits and validity of the multidimensional computational models now commonly used.
Some of the most persistently uncertain input data in flame modeling are the molecular diffusion coefficients of lightweight species. They are often as important as the most important chemical reaction rate. Analyses have shown these to be particularly critical near the extinction limits. Sustained experiments in reduced gravity, in conjunction with measurements on Earth, can be used to determine accurate values of diffusion coefficients required for all models of flame behavior.
Liquid fuels are normally burned as sprays or droplets. Reduced gravity enables the detailed study of the burning of individual droplets or droplet arrays to help improve the understanding of vaporization and combustion processes, including transient burning processes, multicomponent diffusion, extinction limits, and soot formation. Optimizing and controlling this form of combustion can lead to higher efficiency and cleaner combustion. Both pure liquid and multicomponent fuels, as well as synthetic fuels and biofuels, should be included.103,104
Solid Combustion and Smoldering
Gas-phase combustion from a solid fuel produces a special type of diffusion flame that is profoundly affected by gravity. Smoldering has many applications in material synthesis, but it is mostly encountered as a fire safety
problem (e.g., with insulation or cables). Local diffusion flames anchored next to a region of solid fuel can spread and grow in size as the temperature of the solid increases. Differences in the types of convection (buoyant versus forced), flow-spread directions (with flame spread opposed to or concurrent with the flow directions), sample thickness, and charring characteristics affect the spread rate and extinction limits. Currently, the level of understanding is very low with respect to flame spread and growth with common solid fuels, such as cellulose and plastics, in a low-speed forced flow in reduced gravity. For complex solids (e.g., Nomex®, which is used extensively in space), there is even less understanding. Part of the difficulty is the lack of a reduced-gravity platform over an extended period of time for testing solids with realistic thicknesses. Because flame growth is a major concern in spacecraft fire safety, fundamental studies of solid-fuel flammability (ignition, extinction, and flame-spreading processes) are urgently needed. Validated numerical models are needed for conditions representative of the environment of future spacecraft and extraterrestrial habitats. The development of these models would require studies in which physical variables such as gravity, flow velocity, pressure, and oxygen percentage are varied. A detailed numerical model for solid-fuel combustion is also needed, along with simplified, phenomenological submodels that can be included in full-scale models of fires for large volumes, representing large portions of future space habitats.105
Smoldering, which is a surface reaction, is another mode of combustion unique to solid fuels. Smoldering is greatly affected by oxygen transport and heat loss. In reduced gravity there is less heat loss, but there is also a lower rate of oxygen transport. Thus, it is not clear whether the tendency to smolder is higher or lower in reduced gravity. Transitions between the flame and smoldering modes can also occur. Smoldering is a slow process, and so long-duration experiments are required to study it.
Other Combustion Regimes and Applications
Combustible mixtures, under supercritical conditions of increased pressure and low temperatures, are now being considered for liquid rocket fuels and for possible applications to solid-waste processing in reduced gravity.106 Supercritical combustion supports many types of propagating flames, which are susceptible to a range of flame instabilities, including some influenced by gravity. Supercritical fuel droplets in background oxidizers represent another multiphase regime of combustion, and mechanisms for atomization and spray combustion may be different in these regimes.
Microscale combustion devices reduce spatial constraints and increase operational efficiency. A fundamental weakness of such devices is the large conductive heat loss resulting in low energy-conversion efficiency. By using a heat-recirculating design (i.e., a spiral counterflow or “Swiss roll” design), this liability is eliminated.107 A number of interesting new phenomena or physical mechanisms have been proposed for use in reduced gravity. One example is a propulsion device using channels with diameters on the order of a few flame thicknesses. In this geometry, very large thrust is created by the interaction of a flame with boundary layers.108 This capability presents possibilities for propulsion engines and thrusters in microgravity using almost any available fuel.
Recommended research in combustion is summarized below and in Table 9.1 toward the end of this chapter.
Improved methods for screening materials in terms of flammability in space environments will enable safer space missions. Present tests, performed in normal gravity, are not adequate for reduced-gravity scenarios. By 2020, improvements can be achieved that would supplement current screening methods. Beyond 2020, new methods could be optimized and implemented.
Research in this area should support the development of materials standards related to ignition, flame spread, and toxic or corrosive gas generation in various environments and gravitational force fields, as described in Chapter 10 (see Table 10.3).
Space exploration is enabled by and enables extended combustion experiments in reduced gravity that have longer durations and larger scales and that include current and future fuels (e.g., biofuels and synthetic fuels), as well as practical aerospace materials, that NASA may consider for future missions. Previous microgravity experiments made new discoveries and raised new questions. There are now new fuels and new space exploration missions. In addition, most of the solid materials tested to date in reduced-gravity combustion experiments have been simple materials in the form of combustion samples. Future experiments should include more complex materials and shapes. Results from such experiments, along with a deeper understanding of fundamental combustion phenomena, will contribute to technologies for improving fire safety in space and terrestrial applications. By 2020, droplet-phase, gas-phase, and solid combustion experiments on the ISS could be completed, and the preparation and planning for larger-scale, longer-duration experiments could begin. Beyond 2020, larger-scale, longer-duration experiments would be conducted.
Research in this area should support the development of materials standards related to ignition, flame spread, and toxic or corrosive gas generation in various environments and gravitational force fields, as described in Chapter 10 (see Table 10.3).
Numerical Simulation of Combustion
Numerical models are powerful and necessary tools for studying combustion processes in reduced gravity, including issues related to fire safety in spacecraft. The development and validation of detailed single and multiphase numerical combustion models are needed to relate reduced-gravity and Earth-gravity tests. Because of limited access to space for experimentation, numerical simulations are necessary enabling tools for the prediction, design, and interpretation of data from experiments and missions. By 2020, selected numerical models could be validated with reduced-gravity experiments. Beyond 2020, these models could be integrated with experiments and design.
Research in this area should support the development of the following critical technologies, described in Chapter 10 (see Table 10.3): materials standards related to ignition, flame spread, and toxic or corrosive gas generation in various environments and g fields; particle detectors; and fire suppression systems.
Materials science and engineering help shape elements of the modern world, from integrated circuits, to single-crystal turbine blades, to ceramic fuel cells. The study of seemingly disparate materials is unified by the paradigm that the synthesis and processing of a material affect its structure, which in turn governs its properties. Thus the design of materials for terrestrial or space applications, including in situ resource utilization, requires a deep understanding of the relationship between the properties of a material and its structure and of the manner in which synthesis and processing affect its structure. Materials research in reduced gravity has the potential to explore processes that govern materials production on the ground and produce new materials for use during spaceflight and the exploration of planetary bodies. These processes and materials may be critical for applications related to space exploration, and they may lead to new materials for various applications in space and on Earth.‡
Materials science and engineering are central to NASA’s exploration mission, both crewed and uncrewed. This section focuses on gravity-related research issues in the development of new materials that (1) are most critical to meeting NASA’s unique requirements and (2) would not be developed by other governmental agencies. Areas of particular interest include materials synthesis and processing, advanced materials, ISRU, and fundamental materials research. Enabling synthesis and processing techniques related to these topics are also of great importance. For
‡ Table 13.3 in this report describes the potential for all recommended research to impact space exploration and to meet terrestrial needs.
example, computational materials science would contribute significantly to the creation of new, unique materials in an efficient and cost-effective manner. Advances in the fundamental understanding of materials synthesis, processing, computational materials science, and microstructural control, coupled with the development of new advanced materials, has the potential for creating novel materials that could improve weight and/or property factors—for example by providing reduced weight and increased operating temperatures or improving structural strength by 10 to 30 percent. An example of a new, unique property is self-healing capabilities after a meteoroid impact.
Materials Synthesis and Processing
Improvements to existing synthesis processes and the development of new processes are needed for the ground-based synthesis of a wide range of enabling materials, both traditional and advanced, for space exploration and for materials repair and production during missions. Applications of interest range from the structural components of spacecraft to nanoscale sensors that can be used to determine the health of astronauts. Many synthesis techniques of interest involve vapor- or liquid-phase transport and so can be influenced by convection. Thus research in this area is also be enabled by access to reduced-gravity platforms.
Combustion synthesis, or self-propagating high-temperature synthesis (SHS), is one example of a versatile reaction synthesis process that is ideally suited to operate in reduced gravity.109,110 Past work has demonstrated that SHS can efficiently produce ceramics, intermetallics, glasses, functionally graded materials, and composites (metal-matrix composites, ceramic-matrix composites, and intermetallic-matrix composites) in a vacuum and with minimum energy input.111-114 The raw materials for these SHS reactions can be fabricated from metals, elements, and compounds extracted from planetary regolith. SHS can be used to manufacture materials as well as for joining and repairing components.
Vapor-phase reactions involve reactants in a gaseous phase that participate in a gas-gas, gas-liquid, or gas-solid reaction to synthesize a product. The products can be ceramics, metals, or semiconductors in the form of nano- to microscale particles, films, or whiskers. These can be final products, as in the case of films or crystals, which have a variety of applications such as solar cells and high-temperature, wear- and corrosion-resistant coatings and lubricants.115,116 Vapor-phase processing can produce bulk materials that are composed of nanoscale crystals. This unique structure gives rise to high-strength, low-weight materials that are important for NASA space applications. Vapor-phase processing can also produce carbon nanotubes and particulate reinforcements that can be used in other synthesis processes as growth initiation sites, raw materials for combustion reactions, and strengthening materials in metal-matrix and ceramic-matrix composites. In addition, semiconducting materials are normally produced using vapor-phase processing, and nanoscale semiconductors can be used for novel electronic devices and detectors. For example, the gas-detection properties of semiconductors in the form of nanowires and quantum dots (nanoscale cluster of atoms) can be used in sensors for liquid propellant tanks, for biomolecules of importance to monitoring astronaut health, or for toxic gases.
Previous materials science research in reduced gravity has utilized the Materials International Space Station Experiment (MISSE) in which a series of experimental trays were flown on the exterior of the ISS to measure space environment effects, such as atomic oxygen, vacuum, ultraviolet (UV) radiation, charged-particle radiation, thermal cycling, and debris and micrometeorite impact on spacecraft materials and components. These experiments have provided important information and a knowledge base on the degradation of materials, such as polymers, coatings, thin films, seals, solar cells, and paints.
With the advent of powerful computers and efficient algorithms, the virtual synthesis and processing of many materials are now possible. First-principle calculations at various levels are sufficiently developed to predict accurately, for example, crystal structure, surface reactivity, electronic band structure, and the mechanical strength of many materials using only atomic numbers of constituent elements as inputs. Thermodynamic and kinetic databases can be used to predict the phases that are present and transformation rates in complex multicomponent materials. Mesoscale simulations are used to predict solute segregation following solidification as well as precipitate size and spatial distributions in multicomponent alloys. On a larger scale, the three-dimensional computation of structural development of materials is now possible. It is thus possible to link conditions under which a material is processed and its resulting structure and properties with greater accuracy. However, despite significant improvements in these
techniques, the complexity of materials used for spaceflight applications still require that computational methods be supplemented with experiments to verify results, to provide missing data, and to provide information on processes that cannot yet be modeled. An integrated computational and experimental effort (that is, the so-called integrated computational material engineering approach) reduces the number of prototype materials required. This approach drastically decreases the time and cost of developing new materials, and it should facilitate the development of new materials for NASA’s unique applications. Research to develop new computational methods could further reduce the time required to design new materials.
Materials with novel mechanical properties and low density are needed for spaceflight components. Examples include aluminum, magnesium, and titanium alloys, as well as metal-matrix and ceramic-matrix composites. These materials may behave differently in the space environment (reduced gravity, high vacuum, atomic oxygen, etc.). Even so, very little, if any, work has been conducted on the effects of the space environment on the mechanical properties (strength, fatigue, fracture toughness, etc.) of some advanced intermetallic and composite structural materials. As new materials are developed—and as existing materials are adapted for new space applications—the effects of the space environment on these materials should be investigated.
“Smart” materials have the ability to monitor the condition of a structure and/or to adapt the structure and properties of materials to the environmental conditions. Smart materials may even have the capability of repairing (self-healing) a damaged structure. A vast array of smart materials can be employed in a variety of ways that will enable new space exploration capabilities. Shape-memory alloys and polymers are restored to their original shape or form a new shape within a certain temperature range, magnetic field, pH range, or light sensitivity.117 These shape-memory materials can be used in small devices like actuators and also in the construction of buildings that can form large structures from compact structures. Self-healing materials possess the ability to repair cracks due to impact, fatigue, or wear. Many of these materials are now being developed and designed, but further, significant research must be performed in order to fully define their properties and capabilities in the space environment.
Bulk materials and surface-engineered coatings, primarily ceramics and intermetallics, that can withstand high temperatures (up to 1,200°C) and ultrahigh temperatures (1,200°C to 2,200°C) are crucial for heat shields, rocket nozzles, and other high-temperature applications. Present research consists of the synthesis of these materials as well as processing using sintering, reactive hot pressing, and a combination of combustion synthesis and hot pressing for bulk materials,118,119,120 and vapor deposition techniques are used to produce high-temperature coatings.
New surface engineering and advanced coatings are needed to improve the ability of materials to perform in the harsh environments associated with space missions.121 Various applications will need specific properties to be enhanced and will require multifunctional, multiproperty capabilities. The materials used in heat shields and rocket nozzles require a high-temperature coating to improve performance. Increased fatigue and wear lifetimes as well as improved fracture toughness will benefit components that experience repeated loads or friction.122 New coatings could also give components multifunctional properties, such as a combination of wear and corrosion resistance, resistance to thermal fatigue and oxidation, and low coefficient of friction.123 Smart coatings that provide information on how the surface of a component is performing (strain, wear, corrosion, oxidation, etc.) and/or provide a self-healing and repair function would also be useful in the harsh space environment.
In Situ Resource Utilization
Processes to acquire minerals from lunar or martian regolith by means of in situ separation and concentration will need to be developed in order to synthesize materials in space and to manufacture and, in some cases, to repair components in space. Conventional mineral processing, metal extraction, and refining processes have been used to extract concentrated minerals, metals, and other materials from simulated regolith. Hydro-, pyro-, and electro-metallurgical processes have been used to extract metals from regolith.124-127 Fused salt electrolysis can be used to extract reactive metals such as aluminum, calcium, and silicon and also to generate oxygen from oxide regolith. Past research has demonstrated the ability to extract hydrogen from regolith using vacuum pyrolysis.128
Even so, oxygen-extraction processes need to be advanced to secure a source of oxygen for prolonged missions. In addition, each of these techniques requires additional research to develop the most energy-efficient extraction processes. Research should also investigate the possibility of extracting hydrogen from the solar wind.129
Materials synthesis methods that have been proposed can be designed to utilize materials extracted from regolith or the regolith itself.130 It may also be feasible to use materials discarded from prior missions. The separation of oxides into oxygen and metallic elements takes place during oxygen extraction; the metallic elements can be a very important by-product. Silicon can be used for solar cells. Aluminum, titanium, and iron can be used for structural components. In addition, regolith compounds can be the raw materials for combustion synthesis reactions used for the fabrication of strategic components or welding/joining of structural components. Reactive elements, such as aluminum, that are extracted from the regolith or from scrapped spacecraft bodies and components can be used as a fuel for combustion synthesis reactions to fabricate and repair materials and components on the planetary surface. Alternatively, regolith separated into appropriate minerals could be fused into structural construction blocks using a solar concentrator or solar furnace.
The most accessible and abundant source of energy on the surface of the Moon or Mars is solar power. Since there is an abundance of silicate materials in the regolith, silicon-based photovoltaics and solar concentrators will be able to utilize solar energy for power, providing that essential processes, such as extracting silicon from the regolith, can be developed.131 The efficiency of solar cells has increased rapidly in recent years, which has improved their ability to power space missions.132 The current trade-off for space applications is weight for efficiency.133 Efficient solar cells are not lightweight, and lightweight solar cells are not efficient. Investigations into organic photovoltaics are underway in an effort to achieve a better efficiency-to-weight ratio for space applications.134
It may be possible to fabricate materials for high-efficiency, low-operating-temperature solid-oxide fuel cells from the regolith, which would provide a supplementary planetary energy source. Much of the current research on fuel cells is very diverse in regard to the materials being investigated. Many of these systems may be applicable to space exploration missions, but the specific characteristics required for the rigors of space travel are not at present being investigated.135 System performance at low temperatures and pressures should be studied in greater depth in order to produce a fuel cell capable of operating efficiently in the vacuum of space.
There is also a great need to find creative technologies that use the regolith to fabricate and, in some cases, repair components needed for life support, construction, energy generation and storage, forming tools, and other purposes.
The relationship between the processing of a material and the resulting structure is, in many cases, not well understood. For example, many materials are processed using approaches that involve a change in phase, such as a transition from a liquid or vapor to a solid. Density changes and the influence of temperature and concentration on the density of a fluid or vapor lead to buoyancy-driven convection or sedimentation on Earth, which frequently masks processes under study. Thus the study of a wide variety of phenomena related to processing that involves transformation from a liquid or a vapor to a solid can benefit greatly from experiments in reduced gravity. The objective of the experiments and associated ground-based modeling would be to gain a deeper understanding of processes underlying transformations, which would thus improve processes used on Earth.136 A second objective would be to provide insight into the effects of reduced gravity on processes of importance for ISRU applications and materials repair during spaceflight.
Materials Synthesis and Processing
The properties of many materials are directly related to the phases that nucleate from a liquid during processing. Thus, nucleation is an essential, but poorly understood, aspect of materials processing. Microgravity experiments, in which liquids can be held and solidified without a container, thus removing the effects of walls, can shed new light on the nucleation process. In addition, convection due to compositional inhomogeneities that accompany the formation of nuclei can be avoided in microgravity. It is thus possible to cool liquids far below their melting points.
The ability to undercool liquids in a controlled manner allows the study of the formation of stable and metastable phases, the formation of glasses, the relationship between liquid structure and the resulting crystal structure, and the thermophysical properties of deeply undercooled liquids. Containerless processing would also allow properties of liquids such as thermal conductivity and heat capacity to be determined at the very high temperatures at which crucible materials are not readily available.
Crystals are frequently grown from a liquid or vapor. On Earth, crystal properties are affected by the density differences between the phases, the temperature and composition dependence of the density of the parent phase, and variations in the surface tension of a vapor-liquid interfacial-induced convection. This convection leads to non-uniform compositions as well as defects in the resulting crystal. Microgravity allows these crystal growth phenomena to be studied without the confounding effects of gravitationally induced (buoyant) convection. The ability to grow crystals is important to the U.S. economy and research infrastructure; NASA funding has played a central role in supporting research in this important area, but recent reductions in funding by NASA (and other agencies) have had a large negative impact on crystal growth research.137
The synthesis of metal-ceramic composites138 and ultra-lightweight porous materials139,140 is also significantly affected by convection. Removing the gravity-driven buoyancy effects can result in a more uniform distribution and shape of the ceramic phase, the pores, and the ratio of open porosity to closed porosity. These uniform porosity structures have several important applications for space exploration that range from gas and liquid filters and separation membranes to drug delivery systems for astronauts.
Microstructure and Property Control
Microstructures are central to properties of materials as diverse as nanoscale precipitates in aluminum alloys and single-crystal turbine blades. These two structures also provide excellent examples of (1) pattern formation during solidification that involves dendrites and cells and (2) self-organization, which occurs during nucleation, growth, and Ostwald ripening of multiphase materials. The dendritic and cellular patterns that are formed during solidification result from the destabilizing effects of diffusion of impurities in the liquid and the stabilizing influence of surface energy and a temperature gradient. The effects of interfacial energy and processing conditions, such as the temperature gradient and growth velocity, remain an area of active investigation. Similarly, the two-phase microstructures that form during liquid-phase sintering and phase transformations, and which are governed by diffusion, interfacial energy, and kinetics, require further investigation. Unfortunately, buoyant convection or sedimentation makes it very difficult to study the physics that underlie processes such as dendritic and cellular solidification, liquid-phase sintering, and phase separation. The effects of interactions between individual dendrites or cells on spatial distribution and structure, the evolution of dendrites during transient heating or cooling, and the effects of noise and initial conditions on resulting patterns remain unclear. Interactions between dendrites are particularly important in the development of solid-liquid mixtures, so-called mushy zones, and resulting fluid instabilities need further investigation. Fluid flow in mushy zones can lead to especially deleterious casting defects in single-crystal turbine blades, and it remains an area of active research. In some cases, the effects of complex interactions among some combinations of factors also remain difficult to predict. Given this lack of understanding, in many cases it is difficult to identify processing conditions that will produce a desired set of material properties.
Computational materials science is an enabling technology at the mesoscale. With the advent of powerful computers and efficient numerical algorithms, such as the phase-field and level-set methods, in some cases it is possible to predict the evolution of interfaces or precipitates in solids in three dimensions on experimentally accessible timescales. Further development of algorithms and methods will allow, for example, thermal treatments to be designed for a given alloy that yield a certain set of properties and ultimately will eliminate the expensive and time-consuming trial-and-error approach to determining the processing path for a given alloy. Research in this area is also necessary to support the experimental microgravity effort by providing explanations for the observed behavior and suggesting future experiments.
Recommended research in materials science is summarized below and in Table 9.1 in the next section.
Materials Synthesis and Processing and Control of Microstructure and Properties
The space environment enables the production of benchmark data for the development of microstructure, crystal growth, nucleation, and the synthesis of composites. These data, along with computational materials science, will improve the methods used for the synthesis and processing of existing and new materials on the ground. This would increase the ability to understand and predict formation of microstructures in a wide range of materials in both terrestrial and space environments. Results that will alter the fundamental understanding of these processes and improve terrestrial materials processing could be obtained by 2020, with longer-term efforts requiring new hardware.
New advanced materials would enable operations under increasingly harsh space environments and reduce the cost of human exploration. By 2020, advanced materials that meet new property requirements could be designed and developed using both current and novel materials synthesis and processing techniques and computational methods.
Research in this area should support the development of the following critical technologies described in Chapter 10 (see Table 10.4): inflatable aerodynamic decelerators, space nuclear propulsion, fission surface power, and radiation protection systems.
In Situ Resource Utilization
There is a strategic and critical need to utilize extraterrestrial resources for future space exploration and thereby extend human space exploration capabilities. Fundamental and applied research is required in developing technologies for the extraction, synthesis, and processing of minerals, metals, and other materials that are available on extraterrestrial surfaces. By 2020, a select group of strategic elements (e.g., oxygen and silicon), materials, and components could be identified and produced from simulated lunar and martian regolith both on Earth and in reduced gravity.
Research in this area should support the development of the following critical technologies described in Chapter 10 (see Table 10.4): ISRU capability planning and lunar water and oxygen extraction systems.
The following criteria were used to develop a subset of high-priority topics from those presented above in this chapter.
• Enabling research
1. The importance of the problem being addressed by this research.
2. The degree of impact that the research will have on the problem being addressed.
3. The likelihood that the research will be successful in addressing the problem. The risk of an investigation failing to reach a successful conclusion.
4. A reasonable potential that needed resources such as crew time and research platforms could become available.
5. The consumption of program resources compared to that by other potential investigations.
6. The contribution to terrestrial value (medicine, economy, education, national security, etc.).
7. The efficiency of the proposed research in terms of addressing multiple questions in a single investigation.
8. The impact on furthering fundamental knowledge in relevant fields.
• “Enabled-by” research
1. The level of impact on fundamental knowledge.
2. The breadth of impact across a number of disciplines.
3. The likelihood that the research will produce a definitive answer.
4. A reasonable potential that needed resources such as crew time and research platforms could become available.
5. The consumption of program resources compared to what might be consumed by other potential investigations.
6. The contribution to terrestrial value.
7. The efficiency of the proposed research in terms of addressing multiple questions in a single investigation.
8. The impact on enabling space exploration.
The recommended high-priority research areas in applied physical sciences are listed below and in Table 9.1, which also lists current gaps, the specific research recommendations that cover a 20-year period, and the expected research outcomes.
• Reduced-gravity multiphase flows, cryogenics, and heat transfer: database and modeling—NASA should create a detailed reduced-gravity database that includes phase separation and distribution (i.e., flow regimes), phase-change heat transfer, pressure drop, and multiphase system stability. In addition, NASA should support the development and use of direct numerical simulation and molecular simulation techniques (e.g., lattice Boltzmann methods) to improve the understanding of mission-enabling phase distribution, separation, liquid management, and phase-change system phenomena. (AP1)
• Interfacial flows and phenomena—NASA should investigate interfacial flows (including induced and spontaneous multiphase and cryogenic flows with or without phase change) relevant to storage and handling systems for cryogens and other liquids, life support systems, power generation, thermal control systems, and other important multiphase systems. (AP2)
• Dynamic granular material behavior and granular subsurface geotechnics—NASA should improve predictive capabilities related to the behavior of lunar and martian soils on the surface and at depth, with the ultimate goal of developing site-specific models. This would likely require both the ground-based and ISS testing of actual lunar soils. (AP3)
• Dust mitigation—NASA should develop fundamentals-based strategies and methods for dust mitigation during human and robotic exploration of planetary bodies. This should include experimental methods, the understanding of the fundamental physics of dust accumulation and electrostatic interactions, and methods for modeling dust accumulation. (AP4)
• Complex fluid physics—NASA should conduct experiments on the ISS leading to an understanding of complex fluid physics in microgravity, particularly with regard to the behavior of granular materials, colloids, foams, nanoslurries, biofluids, plasmas, non-Newtonian fluids, critical-point fluids, and liquid crystals. (AP5)
• Fire safety—NASA should develop improved methods for screening materials in terms of flammability and fire suppression in space environments. (AP6)
• Combustion processes—NASA should conduct droplet-phase, gas-phase, and solid combustion experiments in reduced gravity with longer durations, larger scales, new fuels (e.g., biofuels and synthetic fuels), and practical aerospace materials relevant to future missions. (AP7)
• Numerical simulation of combustion—NASA should develop and validate detailed single-phase and multiphase numerical combustion models to relate reduced-gravity and Earth-gravity tests, to interpret data from experiments and missions, and to facilitate experimental and design activities. (AP8)
• Materials synthesis and processing and the control of microstructure and properties—NASA should support research in reduced gravity on the development of materials synthesis and processing and the control of microstructures to improve the properties of existing and new materials on the ground. (AP9)
• Advanced materials—NASA should support research to develop new and advanced materials that would enable operations in increasingly harsh space environments and reduce the cost of human exploration. (AP10)
• In situ resource utilization—NASA should support fundamental and applied research to develop technologies that would facilitate the extraction, synthesis, and processing of minerals, metals, and other materials that are available on extraterrestrial surfaces. (AP11)
Currently, NASA has ground-based laboratories, drop towers, aircraft, the space shuttle, and the ISS for studying reduced-gravity phenomena. Reduced-gravity test durations range from several seconds to days.
The ISS creates significant possibilities for experimentation because of its microgravity environment and the ability to vent to an infinite high vacuum. The combustion integrated rack on the ISS has two inserts, the MDCA (Multi-user Droplet Combustion Assembly) and ACME (Advanced Combustion via Microgravity Experiments). The combustion integrated rack is currently configured for two sets of investigations using MDCA: the Flame Extinguishment Experiment (FLEX), which focuses on the efficiency of fire suppressants in microgravity; and FLEX-2, which focuses on more fundamental science issues. The fluids integrated rack is a multipurpose facility for fluid physics research in space. In addition, the microgravity glove box can be used for a wealth of experiments in the combustion, fluids, and materials areas. The Space Dynamically Responding Ultrasonic Matrix System (SpaceDRUMS) has recently been installed on the ISS to provide a facility for the containerless processing of materials using acoustic levitation.
Reduced-gravity experimental platforms are currently limited to aircraft (test duration up to 30 s with 10-2g-jitter) and NASA’s newly designed centrifuge in drop towers (test duration up to 5 s). The latter produces a very clean artificial gravitational level from 0 to 1 g, but it is subject to the Coriolis force and gravity gradients due to its small size. No reduced-gravity platforms capable of supporting longer test durations are currently available.
Other than the now-canceled ISS centrifuge, future possibilities include a rotating free-flyer (with or without a tether). This could include an emptied cargo vessel for long-duration experiments. For example, when cargo is delivered to the ISS, it comes in relatively large vessels (with volumes up to about 40 m3), which are emptied and then ejected back into Earth’s atmosphere where they burn up on re-entry. Before they are destroyed, however, these vessels could be used for relatively large-scale microgravity experiments lasting up to 2 h. The possibility of using these vessels for experiments is a potentially important opportunity for both basic science research and research in support of the exploration mission. For example, empty cargo vessels could be used for fire safety tests to assess whether the properties of a combustion system scale to larger sizes. The absence of g-jitter created by equipment and astronauts also makes them an ideal platform for crystal growth experiments that are particularly sensitive to vibrations.
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|
|Reduced-gravity multiphase flows, cryogenics, and heat transfer: database and modeling:
Phase separation and distribution, phase-change heat transfer, pressure drop, and multiphase system stability. (AP1)a
|—Only very limited, mostly qualitative, reduced-gravity data exist, leading to insufficient designer confidence.
—Few reliable detailed simulations of reduced-gravity multiphase phenomena exist.
|—Design and build a multipurpose phase-change test loop for the fluids integrated rack aboard the International Space Station (ISS).
—Acquire targeted database on phase distribution and separation, phase-change heat transfer (e.g., boiling and condensation), pressure drop, and system stability.
—Perform detailed direct numerical simulations (DNSs) or molecular simulations of selected phase distribution, liquid management, cryogenics, and phase-change phenomena at reduced gravity.
|—Acquire comprehensive, detailed three-dimensional data on phase distribution and separation and phase-change heat transfer.
—Develop mechanistic, multiscale three-dimensional computational multiphase fluid dynamic models (using a reduced-gravity database and DNS or molecular simulation results).
—Develop a one-dimensional drift-flux model based on a reduced-gravity database.
|—A reliable database with which to develop and assess accurate models for the design and analysis of new and/or significantly improved systems for NASA (e.g., for power production and utilization, waste water recovery, on-orbit fueling, in situ resource utilization (ISRU) extraction of water from surface materials, etc.).
—Reliable predictive capabilities for multiphase flow and heat transfer at reduced-gravity levels for system design, scale-up, and analysis.
|Interfacial flows and phenomena: Acquisition of data and development of mechanistic models for induced and/or spontaneous multiphase and cryogenic flows with and without heat transfer. (AP2)||Reduced-gravity thermo-capillary and buoyancy-driven flows are reasonably well understood. In contrast, there is a poor understanding of problems dominated by moving contact lines with partial wetting.||Perform targeted experiments that expand core knowledge and improve designer options and confidence (multiuser facilities preferred).||Expand breadth of experiments to increase technology readiness level.||A reliable database, models, and experience sufficient to design and analyze mission-enabling spacecraft fluid systems with dramatically increased reliability. Terrestrial applications also expected.|
|Research Area and Topic||Current Status||2010-2020||2020 and Beyond||Outcomes|
|Dynamic granular material behavior and granular subsurface geotechnics:
Granular flow dynamics and geotechnics for Moon and Mars environments for human or robotic exploration, ISRU mining, and habitation. (AP3)
|—Computational methods and models are limited to simple particle shapes; impact of gravity is uncertain.
—Current characterization methods are based largely on empiricism specific to Earth; exploration and sampling techniques are unsuitable for extraterrestrial use.
—No lunar soil data are available at depths below a few meters, where structures may be sited and mining for ISRU will occur.
|—Develop computational methods and models for irregular-shaped particles and crushing/compaction of agglutinates.
—Improve understanding of dynamic interactions with vehicle systems and cratering, including effects of gravity.
—Develop methods for in situ sampling at depth.
—Develop suitable models for stress-strain behavior of soils for foundations, berms, etc.
|—Improve particle-scale models for irregular-shaped particles.
—Improve multiscale models that include the effects of gravity.
—Collect lunar soil samples for Earth-based characterization.
—Develop methods for excavating and conveying materials for ISRU.
—Develop methods for the design of below-grade structures and foundations.
|—Predictive capability for interactions between vehicles and granular materials, cratering, excavating, and the jamming-flow transition for complex granular systems at reduced gravity.
—Accurate and reliable predictive models of lunar and martian soil behavior for analysis and design of structural foundations, berms, slopes, and excavations.
—Accurate and reliable computational models for the deformational and strength behavior of extraterrestrial soils.
|Dust mitigation: Development of fundamentals-based strategies for dust mitigation on lunar and martian surfaces. (AP4)||—There is qualitative evidence of dust-related challenges from previous lunar missions and observed atmospheric dust-related phenomenon on Mars.
—Minimal fundamental understanding of the physics of dust accumulation exists.
|—Conduct ISS and ground-based experimental investigation of dust accumulation, specifically electrostatic effects.
—Develop models/simulations of dust in extraterrestrial planetary environments.
|—Extend approaches specific to lunar or martian environments.
—Develop practical methods for mitigating adverse effects of dust on seals, sensors, and solar panels.
|—Fundamental understanding of dust behavior in reduced gravity.
—Practical approaches for mitigating impacts of dust on mechanical systems and sensors.
|Complex fluid physics:
Utilization of ISS microgravity environment to study the fundamental physics of complex fluids and flows. (AP5)
|Many important microgravity experiments have been completed, but fundamental aspects of many issues have yet to be resolved.||—Conduct fundamental experiments on the ISS to unravel the complex behavior of granular material, colloids, foams, 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.
|Continue relevant research on the ISS.||A better understanding of the physics of complex fluids and flows.|
|Research Area and Topic||Current Status||2010-2020||2020 and Beyond||Outcomes|
|Fire safety: Screening methods for material flammability and fire suppression for space applications. (AP6)||Current terrestrial methods are inadequate.||Improve and supplement current materials screening methods.||Optimize and implement new methods.||Improved fire safety for astronauts.|
|Combustion processes: Combustion experiments in reduced gravity to cover longer durations, larger scales, new fuels, and practical aerospace materials relevant to future missions. (AP7)||Incomplete knowledge of basic processes and their response to reduced gravity existed. The importance of gravity in combustion has been demonstrated and new phenomena in reduced gravity have been discovered.||Complete droplet-phase, gas-phase, and solid experiments on the ISS. Begin preparations and planning for large-scale, long-duration experiment.||Conduct larger-scale, longer-duration experiments.||Deeper understanding of fundamental combustion phenomena. Some of the fundamental knowledge contributes to enabling technologies for fire safety. Other knowledge contributes to terrestrial applications.|
|Numerical simulation of combustion: Development and validation of single and multiphase numerical combustion models that relate reduced-gravity and Earth-gravity tests. (AP8)||Many computational tools are available; input data and boundary conditions are incomplete. Theoretical and numerical treatment of solid processes in combustion should be improved.||Develop and validate selected numerical models with reduced-gravity experiments.||Integrate models with experiment and design.||Validated numerical models to enable prediction, design, and interpretation of data from experiments and missions.|
|Materials synthesis and processing and the control of microstructure and properties:
Study of materials synthesis and processing and mircrostructure development that are affected by gravity. (AP9)
|Very little research has been done in the past 8 years. Prior to this, extensive research was carried out, as outlined in prior National Research Council reports.b,c||Provide benchmark data for materials synthesis and processing and microstructural control using reduced gravity.||Employ new experimental facilities to address questions that cannot be answered with existing ISS facilities.||An increased ability (1) to understand and predict the formation of microstructure and properties of a wide range of materials in terrestrial and space environments and (2) to create new materials.|
|Advanced materials: Materials that enable the NASA mission. (AP10)||Very little fundamental research has been conducted on advanced materials for space exploration. NASA has relied on existing materials.||Develop novel, advanced materials using both experimental and computational experimental techniques and methods.||Develop novel materials that will significantly improve weight and property factors (e.g., decreased weight, increased operating temperature, and self-healing capabilities).||Improved spacecraft and mission capabilities at reduced cost.|
|Research Area and Topic||Current Status||2010-2020||2020 and Beyond||Outcomes|
|In situ resource utilization: Fundamental studies of how to utilize in situ minerals and materials. (AP11)||Although the need has been recognized, little research has been conducted in this area.||Identify and produce a selected group of strategic elements (e.g., oxygen), materials, and components that enables space exploration and can be manufactured from extraterrestrial resources in both normal and reduced gravity.||Produce elements, materials, and/or components on the Moon, Mars, and/or asteroids.||Improved prospects for extended human exploration to extraterrestrial bodies.|
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.
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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