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v Other Concerns V.A INDIRECT EFFECTS OF REDUCED GRAVITY ON DESIGN It has been pointed out that reduced gravity can have indirect effects on systems and components by setting design requirements that differ from the corresponding requirements on Earth. Often, these indirect effects have to do with reduced structural forces or loads under reduced gravity. Another such indirect effect concerns the products of wear and decay, which are presumably less easily collected and managed in reduced gravity than in Earth gravity. There can be no confidence in the success and safety of long-duration crowed missions unless such indirect effects are well identified, understood, and managed. The following paragraphs address some of these issues. Piping Systems Pipe flows of fluids or particulates often contain a number of phases distributed in various ways that are not necessarily uniform or steady and that are greatly influenced by the presence or absence of gravity. Pipes are not necessarily straight; they incorporate bends, elbows, and other fittings. Therefore, any nonuniform density of flowing fluids or particulates will cause fluctuating forces on the piping structure. If the piping system (always more elaborate and massive than supposed in the original system conception) is designed to minimize the mass delivered to space, it will presumably be very flexible and will therefore respond to fluctuating forces in extreme and possibly destructive ways. Even in robust terrestrial systems, the familiar phenomena of "surge" and "liquid hammer" are always of concern to the designer (Streeter and Wylie, 1985~. Either single- or multiphase flow through a piping system can cause potentially undesirable flow-induced loads aboard a spacecraft. Moreover, the potential for flow-induced vibration is great. The structural dynamics of piping systems in microgravity is an interesting subject that has received little attention to date. Fluid networks are generally controlled by valves, and valve actions are obviously important for the dynamics of such systems, initiating or aggravating unwanted pressure waves. Cavitation is sometimes involved in such behavior. It may be that gradual, slow-acting valves will be wanted for space systems. Certainly, magnetically controlled valves could be helpful and would also eliminate the need for seals. Since pipe bends and fittings impart transverse accelerations to the fluids passing through them, phase mixing 167

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168 MICROGRAVITY RESEARCH or phase separation might be achieved by suitable design of the piping layout. This idea will be discussed later, in connection with artificial gravity. One may conclude that careful analysis should be made of the system dynamics of entire fluid networks for HEDS applications in reduced gravity, including structural effects as well as the multiphase fluid flow phenomena treated in Section IV.C. Bearings It has been pointed out that many HEDS systems and components will have rotating machinery, and these will require bearings to maintain shaft positions and to bear loads under various speed requirements. A variety of bearing types can be used, depending on the nature of the loads to be borne, and these loads obviously will depend on the gravity level or its equivalent. By way of example, one may recall that cooled rotating-element bearings have been used in fuel pumps on the Shuttle, but they require replacement after each mission because of rapid wear. These will presumably be replaced in the future by film bearings designed to be lighter, more efficient, and more durable. Especially if bearings are to last for years, as HEDS will require, film bearings will be preferred, running at very high speeds with no contact wear (San Andres, 1996~. Cryogenic fluids such as hydrogen or oxygen, if already available on the spacecraft for propulsion or other purposes, are suitable for the films as they generally have a low viscosity. Cryogenic film bearings have been extensively studied for Shuttle applications (San Andres, 1995, 1996~. One concern about such bearings is the stability of the film. The cryogenic liquid will often be supplied near its critical point and so would be subject to bubble formation, flashing, or cavitation, both because heat generated in the bearing may raise the film's tempera- ture and because pressures may drop sufficiently to cause phase change. Should such phase change occur, then gravity may affect the bearing film itself and will certainly affect the processes of collection and recirculation of the film liquid. The foregoing concerns emphasize the importance of maintaining thermal balance between a bearing and its surroundings, by effectively collecting and dispersing the heat generated by friction. Analysis and experiments that look at the behavior of bearings with different proposed designs need to be carried out in environments that realistically simulate microgravity. Furthermore, bearings will clearly be important collection sites for the heat- transmission network of a spacecraft, which we know is subject to microgravity concerns. It should be noted that cryoturbopumps have not yet been tested under realistic space conditions. Another stability concern arises because of the response of the film to load. When a film bearing provides support for a rotating shaft, the manner in which the film works is not affected by gravity (unless the film is a multiphase fluid, as discussed above). However, for journal bearings, the bearing loads are conventionally transverse to the horizontal shaft and are due to weight (gravity). Unloaded bearings, that is, with no transverse force, can be dynamically unstable (Pincus and Sternlicht, 1961~. If gravity is absent, the load is gone, the film behavior is changed, and the liquid film can be expected to suffer severe stability problems. Thus, the dynamic behavior of film journal bearings in microgravity or variable gravity will need study. In certain applications, magnetic bearings may offer a means of preventing surface friction and erosion, and their use certainly merits study, on the assumption that HEDS missions will have ample onboard or station power. Magnetic bearings would seem to be especially useful in microgravity, where their purpose would be more to position machine elements than to bear a large load. _ Seals c ~ Seals are needed whenever it is necessary to prevent or control flow through a region where machine elements slide with respect to each other. In a typical film bearing, fluid is introduced but then must leave, perhaps along the

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OTHER CONCERNS 169 shaft, perhaps through a concentric "damper seal." In terrestrial applications, film fluid may be collected by gravity, reprocessed, and then returned to the bearing. In microgravity, where this kind of collection is impossible, the solution might be to "flood" the bearing, encapsulating the entire machine in film fluid. Seals, like bearings, are subject to wear and failure. HEDS technology will also require devices intended to rotate or slip relative to each other, but only intermit- tently (robots, for instance). Therefore, starting friction in joints can be a problem. If there is a need to lubricate or to protect such surfaces in space, the joints may need to be encapsulated in a seal. In the interest of simplicity and the avoidance of occasions for wear, fatigue, erosion, and failure during long HEDS missions, bearings, seals, and valves should be avoided when possible. That would mean eliminating shafted devices in favor of more passive elements that might adequately serve the same function. An example would be the production of rotation by eccentric fluid jets rather than by shaft rotation, for the purpose of liquid/ vapor separation. This is discussed in Section B of this chapter. While some shafted pumps and turbines will probably still be needed for high-performance systems, surely the list of troublesome elements can be greatly reduced by clever use of fluid mechanics and electromagnetics. Of course, when purely fluid mechanisms are substituted for solid-surface motions, a degree of control over kinematics is lost, and stability difficulties can be expected as a consequence. Robots and Articulated Structures For HEDS missions, robots will presumably be used to assist the crew, and they should be designed with an endurance measured in years. As mechanical devices, they will experience wear, decay, and fatigue, and provision will need to be made to collect the waste products of wear and decay generated over long periods of time. How robots should be designed to include long-lasting joint bearings and appropriate dynamic controls are important topics for HEDS, requiring microgravity research. The extreme structural flexibility of low-mass components of space robots that will function over large volumes of space means that movements will be complex and of large amplitudes. The structural damping of oscillations is typically weak, and a substantial fraction of the time allotted for an operation can be spent waiting for unwanted vibrations to subside (Longman, 1994~. In fact, robot movements can cause rotational displacement, or even the tumbling of a spacecraft (Longman, 1994~. In microgravity, joints of articulated structures may fail to position members precisely, and joint hysteresis may become a factor in the dynamic behavior of such structures. Tanks and Antennas Antennas for communication, and also solar collectors and space radiators, will need to be very large and precisely positioned for distant HEDS missions of large scale. Also, tanks for storage of liquids and gases will no doubt be very large. As is always true for space applications, these devices will be designed to have the lowest mass feasible. Such large tanks and antennas may therefore be expected to have complex and technically important dynamical behavior in microgravity, just as will the elaborate piping systems postulated for many power- or environmental-control systems. The larger, simpler, and thinner these "plates and shells," then the greater the number of modes, and their amplitudes, of the oscillations that can be amplified following excitation by the various vibrations and transient forces inherent in space operations. Such oscillations, even if not immediately catastrophic, could cause fatigue failure if maintained over the time of a HEDS mission. It is well understood that structural design and fabrication are quite different in the absence of gravity from design and fabrication on Earth (Swaim et al., 1994), and it is not necessary to review this topic in this report. However, the dynamic behavior of mechanical devices in microgravity certainly merits ongoing, device-specific research for HEDS. It must be borne in mind that HEDS missions will typically impose a significant range of gravity levels, and all spacecraft elements will be expected to function in variable gravity.

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170 MICROGRAVITY RESEARCH Summary of Concerns Prompted by Considering the Indirect Effects of Reduced Gravity The following concerns arise in connection with the indirect effects of reduced gravity: Phase change in bearing films; Thermal balance in film bearings; Stability of bearing films in microgravity applications; Magnetic bearings and valves (whether they are useful); Flooding, encapsulation of lubricated bearings in microgravity; Starting friction of bearings and joints, including "cold welding" effects; Joint hysteresis; Endurance, wear, and decay; Avoidance of bearings and the like in the interest of simplicity and reliability; Dynamics of plates and shells (antennas and tanks); Surge and liquid hammer in flexible piping as a result of flow-control operations; Structural dynamics of piping networks and similar components; Dynamics of robotic and other structures; and Device performance in variable gravity. References Longman, R.W. 1994. Tutorial overview of the dynamics and control of satellite-mounted robots. Pp. 237-258 in Teleoperation and Robotics in Space: AIAA Progress in Astronautics and Aeronautics, Vol. 161. S.B. Skaar and C.F. Ruoff, eds. New York: American Institute of Aeronautics arid Astronautics. Pincus, O., arid B. Sternlicht. 1961. Theory of Hydrodynamic Lubrication. New York: McGraw-Hill. San Andres, L. 1995. Thermohydrodynam~c analysis of fluid film bearings for cryogenic applications. J. Propulsion Power 11(5):964-972. San Andres, L. 1996. Thermohydrodynam~c Analysis of Cryogenic Liquid Turbulent Flow Fluid Film Bearings. Cleveland, Ohio: NASA Lewis Research Center. Streeter, V.L., and E.B. Wylie. 1985. Pp. 521-543 in Fluid Mechanics, 8th Ed. New York: McGraw-Hill. Swarm, P.M., et al. 1994. Use of manipulators in assembly of space station freedom. Pp. 443-473 in Teleoperation arid Robotics in Space: AIAA Progress in Astronautics and Aeronautics, Vol. 161. S.B. Skaar and C.F. Ruoff, eds. New York: American Institute of Aeronau- tics arid Astronautics. V.B MI CRO GRAVITY COUNTERMEASURES Previous chapters of this report show that low gravity generally complicates the design and operation of systems for HEDS. Low gravity is also harmful to biological systems, leading to the crew's loss of bone and other tissue. A recent report (NRC, 1998) reviews the relevant results and discusses the various mechanisms that may underlie such effects. Such mechanisms are not yet fully understood, and that 1998 report urged research to "provide mechanistic insights into the development of effective countermeasures for preventing bone and muscle deterioration during and after spaceflight" (p. 232~. Thus, while microgravity poses a wealth of interesting research questions, it would seem to be a matter of basic interest for HEDS that NASA learn how to supply appropriate degrees of artificial gravity as a microgravity "countermeasure." In the past the potential importance of artificial gravity was emphasized (NRC, 1987), but subsequent plan- ning documents (NASA, 1991) have given this topic a low priority, and concepts for creating artificial gravity for spacecraft have been, and remain, hypothetical and theoretical. During a typical HEDS mission, the gravity level will vary from zero to near-Earth values; this variability of gravity is itself a problem, because a design suited to one gravity level may be quite inappropriate for other levels that are encountered in the course of the mission. Artificial gravity could eliminate this variability by making possible the continuous adjustment of effective gravity level during the course of a mission. Concerns about gravity level will not be equal for all parts of a system; some components will be unaffected, while for others, especially those involving multiphase fluids, gravity level will be vitally important. Therefore,

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OTHER CONCERNS 171 artificial gravity might be thought important for an entire spacecraft or perhaps only for certain critical compo- nents. Even for such systems as unmanned, low-thrust "freighters," artificial gravity may be useful. Earlier, it was pointed out that for most processes, scale and gravity are grouped together; in effect, a sufficiently small-scale device will be operating in a m~crogravity regime whatever the actual level of gravity. Thus, the effects of varying gravity can in principle be avoided by scale reduction, if it can be accepted that the device operates entirely in the m~crogravity regime. However, the limitations of small scale and of m~crogravity operation would generally not be desired, and artificial gravity would generally be the appropriate "countermea- sure" for reduced and variable gravity. In pnnciple, artificial gravity can be supplied by any means that introduces a "body force," and magnetic fields could do that. The most familiar and, presumably, reliable method is to impose rotation in order to provide a centrifugal force that mimics gravity, subject to certain limitations (to be discussed). There are many ways that rotation can be imposed. The mechanical rotation of a spacecraft or component is the most obvious, but introduc- tion of swirl by fluid-mechanical means is an equally important, if more subtle, method, which may be especially important for dealing with components. The following paragraphs describe some possible methods of supplying rotation at venous scales from the whole spacecraft to the level of the components. It should also be realized that on a transient or local basis, any means that produces acceleration will mimic gravity. Possibilities among this ^~^ extended class of gravity substitution are al~u u;~u~ou. Spacecraft Rotation _ i~~ J. _ V;~;~lU111~1~ ~lilvil~ L111~ Artificial gravity can be supplied by imparting rotation to a spacecraft ,and low-thrust, station-keeping kinds of rockets can easily do this. A rotating spacecraft might look like a dumbbell, or twirler's baton, with a very long, low-mass straight section connecting two or more major masses of the spacecraft (Figure V.B.1~. _ G ! W - G FIGURE V.B. 1 Schematic of spacecraft rotation about axis. G /~ y ~1 /// /~ in,: ~ Y = radius of rotation (m) W = angular velocity (rad/s) = effective "gravity" experienced in spacecraft at Y from axis ~

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172 MICROGRAVITY RESEARCH It has been proposed that the connecting section might be a tether (in principle, a long flexible cable) (Hoyt and Forward, 1995~. There may well be other structural devices of equal or greater usefulness. How long should the radius of rotation be? The longer the connecting section, the lower the angular velocity required to achieve a given effective gravity G at the ends where the masses are. For example, if two equal main masses are separated by 1,800 m, then G = go (Earth gravity) will be achieved with a rotation rate of only about 1 rpm. For 500 m, that same rotation rate will provide G = 0.28 go, and for 200 m, the same G = 0.28 go will require about 1.6 rpm rotation. The significance of angular velocity is perhaps made clear by considering motion experienced in a plane normal to an axis of general rotation at an angular velocity W. The motions are supposed to occur in a region of the plane centered at a radial distance Y from the assumed center of general rotation. In such a case, the effective gravity provided by centrifugal force is G = W2Y. The centrifugal force (G) is the desired artificial gravity effect the reason for applying rotation. There are unwelcome side effects of rotation, however: the Coriolis force and a gravity gradient. The gravity gradient is the less troublesome of these side effects; if the radius of rotation (Y) is hundreds of meters, displacements of a few meters cause little change in the effective gravity. Coriolis force is a greater problem; it is the force that needs to be supplied if a velocity is to be experienced as rectilinear motion in the rotating plane (Goldstein, 1959), and it is proportional to both that velocity and to the angular velocity of rotation (W). This Coriolis effect, if strong, could presumably be disorienting and even nauseating for an astronaut trying to move in a rotating frame, because it entails a force perpendicular to his intended movement. For physical systems and components, Coriolis force would represent a complication, but not necessarily a harmful one. For the cited example of separation distance 500 m (Y = 250 m) and G = 0.28 go, the Coriolis force is less than 0.1 G if motion in the rotating frame is at a velocity of less than 1.3 m/s. For the same G and the same velocity of motion, if the radius is longer (say, Y = 900 m), the Coriolis term will be only half as important. Of course, an increase of rotational velocity (W) will, in itself, also diminish the relative importance of the Coriolis effect (which is proportional to W) in comparison with the centrifugal force, which is proportional to w2. In other words, Coriolis effects associated with movements at the ends of the connecting section (separation member) would be higher for shorter separations and quite substantial for separation distances of the order of 100 m (which might be typical of separations used for radiation protection of crews in nuclear-powered space- craft). One may then ask, How serious are the consequences of high Coriolis effects, both for crew and for technical systems? Thus a research topic would be the study of Coriolis effects and reduced-gravity effects in combination. Similarly, shorter separations imply greater gravity gradients within the spacecraft, the significance of which also requires evaluation. Because an adjustable level of effective gravity achievable by spacecraft rotation could be beneficial for both crew health and component performance, microgravity research should take account of the following: Physical effects should be studied as functions of gravity level. That is, it should be recognized that gravity level is not necessarily "given" but might be "designed." For example, above what gravity level will a two-phase heat transfer loop be preferred to a single-phase device? Clearly, the catalog of regime-change zones discussed in Section IV.A, to be achieved through the research recommended in this report, will be essential to guide the design of any artificial-gravity system, of whatever scale. Prediction of component behavior and subsequent optimization of the system would both become more complex, because gravity would be a new free parameter a design option. The engineering virtues of simnlicitv. reliability, and safety could be pursued as functions of gravity. Coriolis effects should be studied in combination with reduced gravity. For example, in ordinary fluid mechanics, Coriolis forces cause secondary flows. Such effects could combine in interesting ways with the effects of reduced gravity. The effects of gravity gradients in reduced gravity should also be studied.

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OTHER CONCERNS 173 Spacecraft rotation has not yet been developed or designed in practical terms; neither the difficulties nor the costs have been explored. Certain practical issues are obvious: Any tether system with its attachments must be strong enough to bear a large centrifugal load, and the system must be secure from destruction by meteorite impact. The rotation rate must be well controlled, including starting and stopping. Also, it is clear that the rotation of a spacecraft would raise concerns about the orientation of antennas for communications or perhaps of arrays for the collection of solar or laser energy. Liquid/Vapor Separators Any power system that uses a Rankine cycle requires a condenser and means of ensuring that, after the condenser, there is no carry-under of vapor with the condensate (see Section III.B). On Earth, vapor carry-under is prevented by gravitational separation in the hot well of the condenser, but this technique is ineffective in space. Moreover, noncondensable gases may infiltrate the system, and they also must be separated and removed. NASA has envisioned that artificial gravity in the form of centrifugal acceleration can be used to accomplish liquidlvapor separation downstream of a condenser. The approach used may be the direct one of mechanical rotation (the so-called rotary fluid management device), or a more indirect, passive one that induces swirl by tangential flow injection. These two approaches are discussed below, and possible extensions of the passive approach are mentioned as well. Rotary Fluid Management Device The rotary fluid management device (RFMD) developed by Sundstrand, and more recently at the Johnson Space Center, is discussed in pages 98-100 of Brown and Alano (1990) and shown schematically in Figure V.B.2. It is essentially a rotating drum, intended to concentrate liquid at the outer surface. The complexity of the sketched system is associated with the recirculation to make the ultimate separation more complete and to provide thermal IQUlO SATURATION EQUlElBRIUM V APO R AT ~ NTE R F. AC E EIQUlDJVAPOR / TWO PHASE RETURN TO EVAPO RATO RS PHASES SEPARATED / IN OEMISTER / _ i~_~ PI (a . ~ _ W DRY VAPOR OUT | NIC GAS OUT ACCUMULATOR ABSORBS EXCESS LIQUID ENTERING DRUM OR VICE VE RSA W7s?7 _ . p. , 7 ~ iLIQU ID LEVE L MAINTAINED CONSTANT BY PUMPIN G F EU ID IN AND OUT OF FIGURE V.B.2 Rotary fluid management device. P2 ~ P3 3 ~ P2 CENTRAL PITOT PREFERENTIALLY PUMPS COED ElDl)ID SPRAYS TH R O U G H VAPO R SPA C E I NT O EVAPORATOR PITOT PUMP C HAM BE R VAPO R C O N D EN SES AND RESATURATES LIOU ID SU BCOO LED RETU RN FLOW FROM CONDENSER ~ N/C GAS TRAPPED AT CORE (CANNOT SINK BUBBLES) VENTED AS REQUIRED - 200 9 FIEED SEPA RATES LIQUID AND VAPOR PHASES

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74 MICROGRAVITY RESEARCH balance, since heat is released in condensation. The rotational speed of the drum required to provide on the order of 100 go is quite modest. Assuming the drum radius is about 0.1 m, rotation would be at 1,000 rpm, and the tip speed would be about 10 m/s. Apart from problems of operational stability, this device, which depends on forced rotation, requires a power supply, an electric motor or turbine, and bearings to support the shaft. The problems of bearings in microgravity are discussed in Section V.A. If device simplicity is required for maintenance-free reliability, it would seem that this approach will have to be much refined if it is to be a part of long-range HEDS missions. Free Vortex Separator The RFMD just described is considered an active device, in that rotation is imposed by mechanically spinning a tank body about a shaft. In NASA's concept of a free vortex separator (FVS), the tank is stationary and rotation is produced by eccentric injection of the mixture to be separated (Shoemaker and Schrage, 1997) (Figure V.B.3~. A free vortex, or cyclone, is thereby established in a stationary container, and the device's operation is thought of as indirect, or passive. This method of establishing rotation requires little power and is simple; it needs no shaft, bearings, or motor hardware. On the other hand, the free-vortex method lacks the definite kinematic control that the rotating tank of the RFMD provides and so is more subject to instability or breakdown of the flow pattern or failure of the vortex to form as desired. The FVS has been studied at NASA's Glenn Research Center in the laboratory and on the center's DC-9 aircraft. Results usually showed a strong, stable gas core on the axis (like a bathtub drain), into which injected bubbles are swept, as illustrated in Figure V.B.3. Under some circumstances, however, the vortex did not form or was unstable. At low through-flow or for extremely fine droplet size, the performance was not satisfactory. While work on the project has been very successful and is continually yielding design improvements to extend the operating range of the device, it is not yet clear that the FVS in its present form will be applicable to the power- cycle duty for which the RFMD is being developed. Perhaps more-radical changes of concept are needed. For example, injection at a much larger radius, followed by a funnel-like convergence to something like the present tank diameter, might yield stronger free vortices, with little size or weight penalty. In any event, the principle of passive rotation by means of free-vortex arrangements should be aggressively pursued, exploring as wide a variety of potential configurations and applications as ideas permit. Oscillation as a Microgravity Countermeasure Rotation can provide a steady and quite uniform substitute for gravity. However, other means, unsteady or nonuniform, might also be useful. Oscillations can obviously provide accelerations that intermittently mimic gravity (circular motion can be represented as the superposition of two linear oscillations that are orthogonal and 90 degrees out of phase.) Oscillations of interest could be very slow or very fast; acoustic vibrations could be useful. The oscillation parameters (frequency, amplitude, wave form) would depend on the purpose to be served and the nature of the medium. In microgravity, the purpose might be to separate phases, as in the case of the RFMD, or to mix phases. The reversal of force direction implicit in oscillations suggests that oscillations would be especially useful when mixing is the purpose, perhaps to enhance heat transfer or increase combustion rates. Ultrasonic vibration is a well-known mixing technique. Conversely, oscillations of a mixture can concentrate heavier particles at anti- nodes, depending on design parameters (panning for gold is an example). Possibilities for mixing or unmixing of phases by oscillations or wave processes should be studied for application to HEDS systems, and it should be noted that machinery complications such as the need for bearings can be avoided this way, although fatigue and erosion could still be limiting problems. Regarding the biological problem of bone loss in microgravity, the National Research Council (1998) has suggested the possible significance of unsteady or pulsating mechanisms for changes of bone mass. Analysis

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OTHER CONCERNS 175 diffuser discrete bubbles injected at inlet vortex gas liquid exit ~= ~ ~ / If buildil 19 of \ gas separation separator cylinder O/ . 1 .. .J \ gas exit FIGURE V.B.3 Free vortex separator showing bubble distribution (model 5~. SOURCE: Shoemaker and Schrage (1997~. Courtesy of NASA. reported in Weinbaum et al. (1994) suggests that the favorable effect of mechanical loading of bone is proportional both to strain amplitude due to load and to frequency of load application, for frequencies below about 25 Hz. If this is correct, then low forces imposed at rather high frequency might be just as beneficial as a much higher load statically imposed. Conceivably, then, an unsteady or oscillatory system could provide microgravity countermea- sures for the benefit of both technical devices and crew. Flow Deflection as a Microgravity Countermeasure Sinuous steady flow can also be a means of phase management. A natural example is river meander, which increases with time by continuous transport of soil from the outer bank of a river bend to the inner bank, as a result of secondary (viscous) flow in the boundary layer of the river bottom (Callander, 1978~. Another example is wing icing, caused by supercooled water droplets failing to be deflected along with the air flow as they approach the wing leading edge; in effect, while the air "misses" the wing, the more massive water droplets hit and freeze. This, of course, is a bulk effect, not a viscous boundary-layer one. Piping and fitting layouts in spacecraft could be designed to take advantage of such effects, with due account taken of the interplay between bulk and boundary-layer phenomena, in order to manage phase distribution in such systems, as mentioned in Section III.B. NASA has studied phase separation in step-diffusers,] furnishing one example of this class of possibilities. Research on Countermeasures Various research needs and suggestions for the provision of artificial gravity for HEDS appear in the forego- ing paragraphs. They underscore the importance of a theme that has appeared throughout this report, namely that knowledge of physical behaviors as functions of gravity level must be developed through research. This theme C7 ~ ~ ~ 1Singh, B., Lewis Research Center. Multiphase Flow and Phase Change in Space Power Systems. Presentation to the Committee on Micro- gravity Research, October 14, 1997, at NASA Lewis Research Center, Cleveland.

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176 MICROGRAVITY RESEARCH extends to such performance qualities as simplicity, reliability, and safety and is exemplified by the need to study Coriolis effects in combination with reduced gravity. The principle of passive rotation by means of free-vortex arrangements should be aggressively pursued, and swirl generation at large radii merits consideration. More generally, flow deflection should be studied as a general m~crogravity countermeasure for piping and fittings. Possibilities for mixing or unmaking of phases by oscillations or wave processes should also be studied. The importance of m~crogravity countermeasures for systems and components of a spacecraft has been shown, especially for the management of multiphase fluids, and a variety of potential means have been identified, ranging from spacecraft rotation to localized vibrations. For HEDS, it is clear that the human body is also in need of m~crogravity countermeasures, and a range of means will probably be feasible for both systems and crews. It seems logical that m~crogravity scientists will be able to help in the conception and development of systems that protect bone and muscle by means acceptable to astronauts. Indeed, economy and simplicity demand that m~crogravity countermeasures for all these technical devices and systems be coordinated, along with those for biological purposes, through liaison activities in the various technical and biological communities. It would be extremely wasteful to develop and deploy redundant measures to counter the effects of m~crogravity on physical systems and crew members, when a single countermeasure might effec- tively serve both purposes. Developments of m~crogravity countermeasures to maintain both the technical and the human components of a spacecraft and its long-duration HEDS mission, should be guided by results obtained in m~crogravity research. It should also be made clear to the HEDS community how m~crogravity research can explain the need for, and the value of, artificial gravity, especially for long missions; in other words, the m~crogravity research community should provide the motivation for NASA to solve perceived problems, using spacecraft rotation or other appropriate and feasible means. NASA should consider planning in-space research on partial gravity using rotational or other means to achieve desired gravity levels for HEDS systems or their components. Research on ways to counter the effects of reduced or variable gravity on HEDS technology should encourage a wide variety of ideas, even if doing so delays engineering development of intended mission hardware, because new ideas will be the basis for devices of the future. And a trip to Mars or beyond is indeed in mankind's future; it will surely require engineering developments well beyond those now already under way or planned. References Brown, R., and J. Alario. 1990. Space station mechanical pumped loop. Pp. 83-130 in Thermal-Hydraulics for Space Power, Propulsion and Thermal Management System Design: AIAA Progress in Astronautics and Aeronautics, Vol. 122. S.B. Skaar and C.F. Ruoff, eds. New York: American Institute of Aeronautics and Astronautics. Callander, R.A. 1978. River meandering. Annul Rev. Fluid Mech. 10: 129-158. Goldstein, H. 1959. Classical Mechanics. Reading, Mass.: Addison-Wesley. Hoyt, R.P., and R.L. Forward. 1995. Failsafe multistrand tether SEDS technology. Fourth International Conference on Tethers in Space, April 1995, Washington, D.C. National Aeronautics and Space Administration (NASA). 1991. Integrated Technology Plan for the Civil Space Program: Strategic Plan. Washington, D.C.: NASA. National Research Council (NRC), Aeronautics and Space Engineering Board. 1987. Space Technology to Meet Future Needs. Washington, D.C.: National Academy Press. NRC, Space Studies Board. 1998. A Strategy for Research in Space Biology and Medicine in the New Century. Washington, D.C.: National Academy Press. Shoemaker, J.M., and D.S. Schrage. 1997. Microgravity fluid separation physics: Experimental and analytical results. AIAA Paper 97-0886. New York: American Institute of Aeronautics and Astronautics. Weinbaum, S., S.C. Cowin, and Y. Zeng. 1994. A model for the excitation of osteocytes by mechanical loading-induced bone fluid shear stresses. J. Biomech. 27:339-360.

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OTHER CONCERNS V.C PREDICTIVE MODELS, RELIABILITY, AND PROBABILISTIC RISK ASSESSMENT 177 Earlier in this report, the importance for HEDS of system reliability was emphasized. To address reliability with appropriate rigor and completeness, probabilistic risk assessment (PRA) techniques would be very helpful. PRA is a proven methodology that has been widely and successfully used in the aircraft and nuclear industries. It employs fault and event trees to determine the relative likelihoods of the failure modes of specific designs (Green and Bourne, 1972; Shooman, 1968~. Moreover, it highlights weaknesses in designs and components and can therefore be used to promote design reliability, simplicity, redundancy, and maintainability. However, successful use of PRA depends on having physically based predictive models of system and component behavior. Such models are currently lacking for reduced-gravity effects and must be developed on the basis of NASA-supported research. In particular, a better understanding of scaling-law boundaries and multiphase flow and heat transfer phenomena is needed. There is, clearly, a logical path leading from microgravity research to modeling, to risk assessment, and, finally, to reliability. This is a path that, if followed, should provide an integrated, consistent design process for HEDS systems and components. References Green, A.E., and A.J. Bourne. 1972. Reliability Theory. New York: Wiley-Interscience. Shooman, M.L. 1968. Probabilistic Reliability: An Engineering Approach. New York: McGraw-Hill.

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