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Executive Summary
CHARGE TO THE COMMITTEE AND REPORT ORGANIZATION
The primary charge to the Committee on Microgravity Research (CMGR) from the Microgravity Science and
Applications Division (MSAD)1 of NASA reads:
CMGR will undertake an assessment of scientific and related technological issues facing NASA's Human Explora-
tion and Development of Space (HEDS) endeavor. The committee will look specifically at mission enabling and
enhancing technologies which, for development, require an improved understanding of fluid and material behavior
in a reduced gravity environment. These might range from construction assembly techniques such as welding in
space, to chemical processing of extraterrestrially derived fuels and oxygen. The committee will identify opportuni-
ties which exist for Microgravity research to contribute to the understanding of fundamental science questions
underlying exploration technologies and make recommendations for some areas of directed research.
In addition to the above charge, which is stated in full in Appendix A, the committee was asked to give some
consideration to radiation hazards and shielding.
The committee and MSAD mutually interpreted the main thrust of the charge to be the determination of the
gravity-related physicochemical phenomena most relevant to HEDS technology needs and the recommendation of
fundamental research on those phenomena. The technologies considered were those judged to be relevant in the
next one to three decades.
The organization of this report reflects the committee's interpretation of the charge. Following the introduc-
tion and brief descriptions of relevant phenomena and concepts, the report surveys a set of selected HEDS-
enabling technologies, classified according to function. The survey is intended not to be comprehensive but to
identify those underlying scientific phenomena that are vital to the technologies, that are gravity related, and that
present a compelling need for research. The committee defines a gravity-related phenomenon as a phenomenon
that is either directly affected by reduced gravity or that becomes significant as gravity level is reduced. A
Now the Microgravity Research Division.
1
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MICROGRAVITY RESEARCH
phenomenon of the latter type may sometimes be used to compensate for the loss of gravity (an example is surface-
tension-driven flow in wicks and heat pipes in the absence of gravity-induced convection). Selected phenomena
and their dependence on, or importance in, reduced gravity are then discussed, along with the research needed to
develop predictive models and better databases for characterizing the phenomena. The remainder of the report
deals with other gravity-related features of HEDS technologies, discusses microgravity countermeasures (e.g.,
artificial gravity), and offers research and programmatic recommendations.
TECHNOLOGIES SURVEYED
The selected technologies are discussed according to their functions: (1) power generation and storage,
(2) space propulsion, (3) life support, (4) hazard control, (5) materials production and storage, and (6) construction
and maintenance. They were examined for their dependence on gravity level by considering the gravity depen-
dence of the components (subsystems) or processes of which they are made up. In many instances, the subsystems
(e.g., pumps or boilers) or processes (e.g., electrolysis) are common to many technologies, so the phenomena
underlying them were recognized as especially important for HEDS. An example of a strongly gravity-dependent
subsystem is a heat exchanger subsystem, such as a boiler or a condensation-based space radiator, that uses a two-
phase fluid (i.e., liquid and vapor). Its operation is radically affected by microgravity because the phenomena of
buoyant convection and density stratification are absent. An example of a strongly gravity-dependent process is
liquid electrolysis, common to life support and fuel production systems. The phenomenon of buoyancy-driven
migration of the gases (i.e., bubbles) in the liquid does not occur in microgravity, so phase separation of the
product gases from the liquid must be accomplished by other means.
Although the charge to the committee did not specifically include an evaluation of technologies, a remark on
this point seems in order. Now and in the past NASA has chosen not to use active, high-power-density systems
that involve heat transfer by phase change (e.g., condensation and boiling) to meet energy needs but has chosen
instead to use lower-power-density, passive systems such as solar collectors, fuel cells, and radio isotope genera-
tors. This approach has been motivated by the requirement to reduce risk and to ensure reliability, since the
performance of multiphase (two or more phases) flow and heat transfer processes in reduced gravity is not well
understood and was therefore considered risky. Unfortunately, however, the lower-power-density systems will
not be able to supply enough energy for proposed long-duration, crowed space and interplanetary missions. The
high efficiency and high power-to-weight ratio of closed-cycle multiphase systems, based on the use of the latent
heat of phase change (i.e., condensation and evaporation) to transfer energy, are so attractive that the committee
believes it is imperative for NASA to undertake a directed research program on multiphase flow and heat transfer
that will enable it to decide if systems dependent on these processes can be successfully controlled and utilized in
space. Accordingly, one of the higher-priority recommendations in this report proposes this research.
PHENOMENA IDENTIFIED AS AFFECTED BY OR DOMINANT IN REDUCED GRAVITY
Phenomena that are identified as underlying HEDS-enabling technologies and that either are directly affected
by gravity level or emerge as dominant factors in reduced gravity are generally organized in Chapter IV of this
report as follows: (1) surface and interracial phenomena, referring to effects stemming from surface wetting and
interracial tension; (2) multiphase flow and heat transfer, referring to the flow of more than one fluid phase in
pipes, pumps, and phase-change components, and flow in porous media, exemplified by the flow of fluids in the
packed and fluidized particulate beds used in chemical reactors; (3) multiphase system dynamics, which deals with
the global instabilities that may occur in multiphase systems; (4) solidification, referring to the phase change of a
liquid to a solid, as occurs in casting or welding; (5) fire phenomena and combustion, used in some power
generation and propulsion systems and occurring in accidental fires; and (6) granular mechanics, referring to such
topics as the response of granular media and soils to geotechnical loads and the flow of granular materials in chutes
and hoppers.
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EXECUTIVE SUMMARY
RECOMMENDED HIGHER-PRIORITY RESEARCH ON FUNDAMENTAL PHENOMENA
3
Of the specific areas recommended for research in this report, those discussed below were considered by the
committee to have higher priority based on their potential to affect a wide range of HEDS technologies that are
mission enabling. In each area, the technological importance of the phenomena is briefly explained first.
Surface or Interfacial Phenomena
Surface tension effects are of critical importance in such diverse HEDS technologies as welding, liquid-phase
sintering, the operation of wicks in heat pipes for thermal management, the use of capillary vanes (wet by the
liquid) in cryogenic storage tanks to control the position and movement of liquids, lubrication, and boiling/
condensation heat transfer, including the rewetting of hot surfaces. A special (Marangoni) effect occurs when the
surface tension varies over the surface of a liquid (or the interface between two liquids) because of thermal and
composition gradients. Marangoni effects can produce strong gravity-independent convection, which may be
beneficial, as in the stirring of weld pools and the enhancement of the critical heat flux in multicomponent boiling,
or detrimental, as in the migration of fluid in a thermal gradient to unwanted locations.
The committee's recommendations, which are based on the critical issues underlying the technologies, call for
research on the following topics:
· The physics of wetting, with an emphasis on hysteresis effects, the dynamics of the wetting process, and the
molecular basis of wetting, to elucidate the wetting of both solid and porous media (e.g., wicks and nanoscale
media) and to provide a basis for the choice of material combinations and conditions for optimal wetting and
wetting agents; and
· Capillary-driven;flows, with modeling of the flows induced by the Marangoni effect, which are compli-
cated because of the feedback between the flow and the surface temperature and composition gradients that drive
the flow.
Multiphase Flow and Heat Transfer
Multiphase flow and heat transfer are the fundamental processes in systems using a fluid of two or more
phases (e.g., liquid and vapor) to transport mass, momentum, and energy. They are critical to the operation of
many power production and utilization systems and other systems that require high energy-transport efficiency
and high power-to-weight ratios. Multiphase systems have these characteristics because they are able to utilize the
latent heat of evaporation/condensation to efficiently transfer energy. Their successful operation in Earth' s gravity
often depends on buoyancy-driven convection and density-induced stratification of the phases, processes that are
reduced or absent in microgravity. Moreover, new flow regimes may occur in which the spatial distribution of the
phases reflects the absence of a gravitational force. Therefore, to exploit the attractive advantages of multiphase
systems under microgravity conditions, it is imperative to determine how they can be used and controlled in the
absence of gravity.
It is recognized that there will probably be a continuing need for experimental microgravity data and appropri-
ate empirical correlations, since some physical phenomena and HEDS design issues go beyond current, and
anticipated near-term, computational capabilities. Nevertheless, the primary objective of the proposed research is
the development of a reliable, physically based,2 multidimensional two-fluid model for the computational fluid
dynamics (CFD) analysis of multiphase flow and heat transfer phenomena of importance to the HEDS program.
Indeed, the following recommended research is aimed at developing predictive models of multiphase flow and
heat transfer and testing these models against reduced-scale data taken in microgravity environments:
2In the context of this report, a physically based model is one that is developed from fundamental principles and physical mechanisms, as
opposed to an empirical model. See the glossary, Appendix C.
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MICROGRAVITY RESEARCH
· The development of physically based models to predict the;flow regimes, flow regime transitions, and the
multiphase flow and heat transfer that occur in fractional gravity and microgravity environments. These models
should include the effects of two-phase turbulence, surface-tension-induced forces, and the axial and lateral
interracial and wall forces on the flowing phases (i.e., the flow-regime-specific interracial and wall constitutive
laws). They should be suitable for use in three-dimensional CFD solvers.
· Assessment of the predictive capabilities of these models by comparing them with the results of reduced-
scale and separate-effects experiments performed under microgravity conditions. In particular, detailed data are
needed on flow-regime-specific phenomena in simple and complex geometry conduits (where gravity dependence
may occur even at high flow rates); the distribution and separation of the phases for the various flow regimes,
including the effect of phase separations induced by swirl; and the local velocity, temperature, void fraction, and
turbulence fields.
· A program parallel to the one described above for assessing the effect of gravity level on forced convective
Lows, especially the forced-flow boiling curve for different boiling regimes (e.g., nucleate and film). Such a
program is essential, since forced flow can compensate for some of the problems arising from the loss of buoy-
ancy.
Multiphase System Dynamics
In systems using multiphase flow, effects on a global scale may emerge from the interaction of the compo-
nents. In particular, phase or time lags in feedback loops can cause potentially dangerous instabilities that are
revealed only by analysis of the system as a whole. The following research is recommended:
· The development of models and the collection and analysis of stability data on boiling and condensing
systems at fractional gravity and microgravity levels. In particular, the effect of gravity level on excursive
instabilities, as well as on those dynamic instabilities that can be induced by compressibility and lags in the
propagation of density waves around closed loops in multiphase systems, needs to be investigated and analyzed.
Fire Phenomena
Accidental fires are a major hazard in the confined quarters of spacecraft. The structure and dynamics of fires
and flames are drastically altered in microgravity, primarily because there is no buoyancy-driven convection or
sedimentation (e.g., of smoke particles). Accordingly, the following research is recommended:
· Experimental, theoretical, and computational studies of;flame spread over surfaces of solid materials in
microgravity andfractional gravity. These studies should focus on generic materials, both cellulose and synthetic
polymers, and should include ignition requirements, flame-spread rates, and flame structure. Parallel studies on
the production of gaseous fuel from solid-fuel pyrolysis are needed.
· Gravity effects in smoldering, as in the case of electrical cable fires. In particular, the research should look
at the initiation and termination of smoldering, propagation rates of smoldering fronts, and the production of
hazardous or flammable products from smoldering, including conditions for transition from smoldering to flaming
combustion.
Granular Mechanics
The granular materials encountered in lunar and Martian soils will serve both as the physical foundation that
supports people, equipment, and buildings and as a raw material to be mined and used for construction and for the
extraction of valuable resources. Granular material in the form of dust is expected to be a serious environmental
problem on the Moon and Mars. The behavior of granular materials in response to loads and digging, with respect
to their flow in chutes and hoppers, or in atmospheric transport (i.e., dust) and adhesion to surfaces, is affected by
gravity level. Accordingly, the following research is recommended:
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EXECUTIVE SUMMARY
s
· The response of granular material to applied stress. The knowledge gained will allow researchers to
examine separately the effects of gravity and shearing using both analytical and experimental studies. Predictive
models of granular deformation and flow under reduced gravity need to be developed that include the effects of
particle size and shape, the effects of particle constitution, and the effects of particle agitation and of electrostatic
charge, especially at low pressures.
· Predictive models of the behavior of dust in spacecraft and extraterrestrial environments. An understand-
ing of this behavior will permit the reliable prediction of dust transport and deposition. An understanding is also
needed of the cohesion and adhesion mechanisms that control dust attachment, where the attraction mechanism
appears to be electrostatic.
OTHER CONCERNS
Reduced-Gravity Countermeasures
Because reduced or variable gravity is generally a troublesome complication of system design for HEDS (and
has harmful consequences for human health), research should be carried out on means to counter the adverse
effects. Such means would probably be mechanical in nature, involving rotation or vibration, and could be
implemented at a range of levels, from that of the whole spacecraft down to the level of small but critical
components. Design studies of structural and system problems would be required to establish technical practical-
ity and costs for large-scale artificial-gravity concepts.
Applied research looking toward economic and effective artificial gravity should emphasize solutions that
would apply to both technical and biological systems.
Research on and development of reduced-gravity countermeasures are given high priority in the report and
must obviously proceed hand in hand with the microgravity research recommended elsewhere in this report,
because the latter will establish the target gravity levels desired for various components and systems. In turn, the
specific benefits of an artificial gravity system must be understood and weighed against the penalties (e.g., weight
and cost) so that design trade-offs can be made. In other words, it is to be expected that artificial gravity will be
part of integrated system designs for HEDS.
Indirect Effects of Reduced Gravity
Reduced gravity will have indirect effects on systems and components, necessitating designs different from
the corresponding, more familiar ones on Earth. For example, seemingly mundane components such as piping,
valves, and bearings will have to be adapted to the altered structural forces and loads in reduced- and variable-
gravity environments. Then, too, products of wear and decay are presumably less easily managed in microgravity.
Such concerns are additional elements in a central HEDS issue, namely the effect of reduced and variable gravity
on system reliability and safety.
RECOMMENDED RESEARCH WITH A LOWER PRIORITY
The committee also recommends research on other fundamental phenomena in addition to those described
above. These phenomena, listed below in no particular order, were judged to be somewhat less critical to mission
success, so the research has a lower priority.
· Marangoni material parameters;
· Static equilibrium capillary shapes;
· The effect of gravity on convective condensation heat transfer;
· The effect of gravity on the heat transfer characteristics of fluid flow in porous media;
· The transport of flame suppressant to fires in reduced gravity;
· Diffusion-flame structure of fuels and flame products as affected by gravity levels;
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MICROGRAVITY RESEARCH
· Flammability and flame behavior of gaseous combustible mixtures, sprays, and dust clouds; and
· The effect of gravity on nucleation and growth of solid from the melt.
PROGRAMMATIC RECOMMENDATIONS
It should be clearly understood that the committee's research recommendations deal with fundamental phe-
nomena associated with fluid and material behavior rather than with the direct development of subsystems and
their integration into technologies operable in a reduced-gravity environment. However, the committee recognizes
that blending conceptual design needs and phenomenological research findings requires a great deal of communi-
cation, coordination, and interdisciplinary collaboration among designers and researchers. For this reason, it
makes recommendations in this report concerning the goals, research planning, and programmatic activities of
NASA that support gravity-related research for HEDS. Similar recommendations made in the phase I report
(NRC, 1997) are reflected in this more extensive study as well. It was thought then, and is still believed, that in
view of the long time scale needed for the evolution of basic scientific concepts into practical applications, the
suggested research programs will require a sustained commitment on the part of NASA to achieve an understand-
ing of gravity-related issues.
A Research Approach for the Development of Multiphase Flow
and Heat Transfer Technology
For NASA to be able to decide whether multiphase and phase-change systems can be used and controlled in
future HEDS missions, a well-focused experimental and analytical research program will be needed to develop an
understanding of how multiphase systems and processes behave in reduced gravity. Since parametric full-scale
testing in space is not feasible, NASA should consider developing a reliable three-dimensional, two-fluid CFD
model that can be used to help design and analyze multiphase systems and subsystems for HEDS missions. The
approach that has been recommended is that a reliable, physically based analytical model be developed and
qualified against appropriate terrestrial and microgravity data. The resulting computational model could then be
used to analyze and optimize final designs and to scale up the reduced-scale data obtained in space. While this is
expected to be the most reliable, least expensive, and quickest means of developing the potentially enabling
technology required by HEDS, programmatic changes would be required to accomplish this goal. In particular, it
would be necessary for NASA to refocus its multiphase fluid physics research program and to be much more
proactive than it has been in defining and managing the research needed to develop predictive capabilities for
multiphase flow and heat transfer. In this context, NASA should investigate the possibility of consulting the U.S.
Department of Energy-Naval Reactors program for help in designing research programs aimed at developing the
required multivariate, physically based computational models.
Coordination of Research and Design
The NASA office responsible for microgravity research should diligently inform NASA at large about the
issues of reduced gravity that are foreseen for space hardware design, so that such considerations may enter design
thinking at the concept stage. It should also apprise the microgravity research community of design issues relevant
to microgravity research, and NASA should encourage the blending of conceptual design and phenomenological
research. This will require active communication and coordination among basic researchers and system designers
and users, which should be specifically encouraged by such means as regular workshops and study groups in
which both mission technologists and microgravity scientists participate.
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EXECUTIVE SUMMARY
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Microgravity Research and the International Space Station
It is expected that the International Space Station (ISS) will provide a unique platform for conducting long-
duration microgravity scientific research and assessing the efficiency and long-term suitability of many of the
technical systems important to HEDS. The committee reiterates a recommendation from its phase I report (NRC,
1997, p. 39~: in addition to carrying out basic research aboard the ISS, NASA should take advantage of the station
and its subsystems, using them for testbed studies of scientific and engineering concepts applicable to HEDS
technologies. In particular, the ISS can play an important role in the multiphase flow and heat transfer research
program recommended above.
Peer Review for Reduced-Gravity Research
The NASA Research Announcement process and its peer review system have greatly enhanced the productiv-
ity and quality of NASA's gravity-related research. These mechanisms should be maintained as steps are taken to
develop areas of science affecting HEDS technologies.
REFERENCE
National Research Council (NRC), Space Studies Board. 1997. An Initial Review of Microgravity Research in Support of Human Exploration
and Development of Space. Washington, D.C.: National Academy Press.
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Representative terms from entire chapter:
microgravity research
