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Space Power System Options and Selection Constraints SUMMARY OF AVAILABLE SPACE POWER SYSTEM OPTIONS The classes of space power systems that are capable of meeting the special prime power and power conditioning requirements of SDI space system architectures are based on three approaches: nonnu- clear space power systems, nuclear space power systems, and ground- based power systems for beaming power to space. The nonnuclear options refer to solar photovolta~c, solar-dynamic, and chemical (in- cluding magnetohydrodynamic tMHDJ) systems. Nuclear options include radioisotope thermoelectric generators (RTGs), dynamic iso- tope power sources (DIPS), and nuclear reactor systems (see Figures 3-1, 3-2, and 3-3~. These power sources can be utilized in "closed" or "open" thermodynamic systems. Closed and open thermodynamic systems are defined as follows: A closed-cycle system is one in which a working fluid is heated, does work, rejects heat, and is recycled. Closed thermodynamic cycles have various designs. Three important varieties are known as Bray- ton, Rankine, and Stirling cycles. A Brayton cycle is a conventional closed-cycle employing a gas turbine, in which the working fluid is gaseous throughout the power-generating loop; a Rankine cycle is like a conventional steam cycle, in which the vapor is liquefied in a condenser; and a Stirling cycle is a closed-cycle reciprocating engine 24
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POWER OPTIONS AND SELECTION CONSTRAINTS Orbital Power, Non-Nuclear open-cycle* *Open-cycle systems raise the question of effluent impacts on the spacecraft, sensors, weapon systems, and on the power system itself #The committee did not consider this option in detail FIGURE 3-1 A nonnuclear orbital power source. 25 _ ' 1 Storable hemical | r ; N2O4 & mixed amines H2 + O2 Gas Generator# Storage 1 ' ~~=I~:~ r closed-cycle Brayton Rankine Stirling AMTEC whose working fluid is a high-pressure gas, either helium or hydrogen. The alkaTi-metal thermoelectric converter (AMTEC) cycle is simi- lar thermodynamically to the Rankine cycle turbogenerator system. AMTEC utilizes high-pressure sodium vapor supplied to one side of a solid electrolyte of beta alumina, causing low-pressure sodium vapor to be removed from the other side. Sodium ions transported across this electrolytic membrane produce a voltage difference, which drives electrons through a useful load, whereupon they reunite with the sodium ions to complete the circuit. In an open-cycle system, a working fluid is heated, does work, and is discharged, carrying waste heat with it. An open-cycle system is also a "single-pass" system, since the working fluid is used only once. A variation of the open cycle is the use of chemical reactants following an exothermic chemical reaction such as combustion—to produce a pressurized vapor and liberate heat. In order to minimize the impact of the resulting efflu- ents on the overall spacecraft system, the reaction products can be separated and, conceptually, some or all of them could be retained, but, in practice, the retention option may prove to be difficult to achieve or totally unrealistic. Another category of open, or single-pass, systems is one that has no thermodynamic working fluid, or prime mover. Examples are batteries and fuel cells. Usually but not always—such devices store their effluents.
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26 r Isotope Thermoelectric Thermionic ADVANCED POWER SOURCES FOR SPACE MISSIONS | Orbital Power, Nuclear | ~:smallt!sion I _ reactors of _ the SP-100 and PER** type 1 Multi-MW | :3 fission modular | open cycle* | r I l open L cycl e* ' l closed cycle closed cycle Brayton Brayton Rankine Rankine Stirling AMTEC 1 1 [systems receiving primary committee emphasis] MAUI nuclear systems introduce complex safety requirements, (see discussion later in this chapter) *Open-cycle systems raise the question of effluent impacts on sensors, on weapon systems, and on the power system itself **PER refers to Pebble bed Reactors and Particle bed Reactors, which are distinguishable from each other by the size of their fuel elements FIGURE 3-2 A nuclear orbital power source. Although open-cycle space power systems tend to be less massive than closed-cycIe systems for operating periods of less than about an hour, they present the problem of spacecraft toleration of their effluents. The major space power options available for each SD! power mode are presented in Table 3-1 according to whether or not an effluent is produced. The committee reviewed SDIO briefing documents that sum- marized results emerging from SPAS studies (1988) that were being simultaneously conducted by three SDIO contractors while this study
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POWER OPTIONS AND SELECTION CONSTRAINTS | Ground Based Power* l 27 1 1 1 1 1 ~ . ~ | fission * | Fusion Conventional Non-Nuclear coal gas oil Chemical Non-Nuclear MHD ! 1 [Areas where there are possible applications of superconducting magnetic energy storage (SHIEST *Requires microwave beam to orbit Han added benefit might ensue, since this military requirement could--as a spinoff--lead to a true second-generation, fail-safe reactor for civil applications FIGURE 3-3 G round-based power. TABLE 3-1 Space Power Options for Each SDI Power Mode Power Mode Operational Option No Effluent Effluent Housekeeping Solar, RTG, DIPS, nuclear reactor, ground-based source Alert Burst None Solar-dynamic, None nuclear reactor Nuclear reactor Chemical, nuclear reactor . NOTE: DIPS = dynamic isotope power sources j RTG = radioisotope thermoelectric generator.
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28 AD VANCED PO WER SOURCES FOR SPACE MISSIONS was in progress. Unfortunately, during the course of this study the SPAS studies did not become available in published form. Informa- tion on the SPAS studies was supplied to the committee by the SDIO Power Program Office, by its Independent Evaluation Group (lEG), and by the lEG Technical Support Team. The committee also read the reports of other relevant SDIO-sponsored studies. Although the committee did not review all power system concepts originally con- sidered by SDIO, it did examine several power systems not treated in the IEG summaries. Figures 3-1, 3-2, and 3-3 are summaries of the power systems reviewed by the committee: (1) nonnuclear power generated in orbit, (2) nuclear power generated in orbit, and (3) power generated on the ground and beamed into orbit electromagnetically. Details on each of these options follow. Various combinations of prime power generation and storage are possible, but at present it is unclear how to make optimum use of storage whether by fuel cells, batteries, electrolysis, thermal me- dia, flywheels, superconducting magnetic energy storage (SMES), or by using a combination of those techniques. A power technol- ogy research and development program should continue to establish technical feasibility, develop the technologies and resources, and con- duct proof-of-principle testing. Applying terrestrial turbomachinery technology to high-power systems for space applications may prove viable but could pose technical obstacles. Areas of uncertainty that will need attention include mass reduction, providing capability for rapid start-up (especially nuclear reactors); operational lifetime, if high-temperature operation is contemplated; embrittlement of tur- bine blades and other components; in-orbit maintainability; and integration of the power system with its loads and with the space platform. A major concern for space power generation at the multimeg~ watt level is thermal management; in particular, the problem of rejecting heat from the power cycle. If a nuclear reactor is used in a multimegawatt space power system, then unlike a low-power system such as SP-100 the mass of the radiators, rather than the mass of the reactor and its shield, is the dominant component of the mass of the overall power system. Although chemical energy heat sources appear attractive because they offer rapid response and rea- sonable mass for limited duration, they emit effluents that may have unacceptable impacts. Another potentially attractive option, fusion
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POWER OPTIONS AND SELECTION CONSTRAINTS 29 reactors, will be unavailable even for terrestrial applications until well into the next century. Closed-cycle power conversion systems generate practically no effluents (although attitude control and weapon cooling may pros duce some additional effluents) and require much Tower storage and replenishment of expendables than do open cycles. On the other hand, compared to open-cycle systems operating for short durations or cases in which their working fluid comes from weapon cooling, closed-cycle systems tend to be more massive and require large radi- ators to reject heat. The choice of optimum temperature levels for power conversion depends on the selection of conversion cycle and materials, but can be summarized as follows: High heat-addition temperatures* aid thermodynamic performance, but pose materials problems. Low heat-rejection temperatures improve thermodynamic performance, but result in large, massive radiators, thus posing vulnerability and maneuverability problems. Some low-mass concepts for heat rejection are worthy of consid- eration for heat rejection at low temperatures (i.e., below 1000°K). These include the liquid droplet and liquid sheet radiator concepts, and various moving belt (liquid, solid, and hybrid) radiators. All of these approaches are in the advanced conceptual stages of devel- opment, and none of them have been adequately tested. Questions regarding maneuverability issues, particularly for the belt radia- tors, and contamination caused by escaping fluids have not been addressed. It is anticipated that such systems, if successful, would not be available before the year 2000. Nonetheless, the heat rejec- tion issue is sufficiently critical that such advanced concepts merit consideration for future SDI systems. A closed-cycle system typically employs a Brayton or Rankine closed loop and a turboalternator for power generation. Although activation of a chemical heat source can be rapid, it may be more difficult for a fission reactor to reach full power quickly. Consequently, substantial housekeeping power may be required to maintain the power conversion cycle components in a warmed-up condition ready for rapid start-up. Bimodal operation of a nuclear power plant to supply power for both the alert and burst modes would substantially *Currently, temperatures for metallic parts of terrestrial gas turbines can go to 1100°C (1373°K). Refractory metals, if oxygen is completely absent, offer reasonable hope of attaining 1500°K-1600°K in space (Klopp et al., 1980~.
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30 AD VANCED PO WER SO URGES FOR SPA CE MISSIONS alleviate its start-up problems. The reactor's increased power output in going from aTert-mode operation to burst mode could then be achieved with only modest change in its operating temperature, in contrast to the significant changes in temperature and heat output that would occur if the reactor must be activated just prior to the burst mode. Nonnuclear Power for Orbit a] Use Nonnuclear (largely chemical and solar) power-generating systems (Figure 3-3) offer the attractive feature of avoiding certain safety con- cerns associated with nuclear systems. Furthermore, the gross mass of open-cycle chemical systems is less than that of open-cycle nuclear systems for the short operating times typical of SD} burst-mode power requirements. In these open-cycle systems, a major effluent is hydrogen, which is typically used for weapon cooling. Chemical power generation systems may also emit water or other reaction products, although these products could conceivably be condensed and stored on board. Presently it is unclear whether any or all of these effluents can be tolerated by SDI systems; however, the effluent question is clearly an important consideration in choosing between effluent-em~tting and closed-cycle nonnuclear power systems for space applications. The several levels of power required by the housekeeping, alert, and burst modes are significant discriminators among candidate non- nuclear systems. Photovoltaic Space Power Systems The most commonly used long-lived space power systems are based on the photovoltaic conversion of solar radiation into electric power. The largest such power plant usefully applied in a space mission was aboard Skylab. For that spacecraft, 3.8 kWe of solar power was installed for the Workshop and 3.7 kWe to operate the Apollo Telescope Mount. There are still substantial problems to be solved. Means for erecting large arrays (about 25,000 m2/MWe) have yet to evolve, and the structural dynamics of these low-mass and generally flexi- ble arrays remain to be developed. In addition, when high-enough voltages are generated, some interaction will occur between the so- lar array and the space plasma, resulting in arcing or power Tosses.
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PO WAR OPTIONS AND SELECTION CONSTRAINTS 31 Since arcing can damage the array, electrical insulation in the space environment is a major issue. The arrays cause orbital drag, requir- ing make-up thrust to compensate. Any magnetic fields generated interact with the earth's magnetic field, producing torques. Photovoltaic arrays presently used in space are sensitive to hos- tile action because of their large size, their low mass per unit area, and exposure of their semiconducting cells to the threat. In contrast, in concentrator arrays now being studied, the metallic mirror and supporting structure provide a substantially smaller target and some protection to the semiconducting cells. Concentrator arrays have a very narrow cone of optical vulnerability, centered on the direction of the incorn~ng solar radiation. If the hostile threat is a beam, that orientation is difficult for the beam to achieve. Solar-Dynamic Power Solar-dynamic power generation is being considered for the Space Station as a means of reducing the area required for collection of solar radiation compared to the area that would be necessary if all the power were supplied by photovoltaic arrays. To date, the largest solar parabolic collector built for space power applications was a mirror 6 m (20 It) in diameter (English, 1978~. This solar collector and its heat receiver also require fairly accurate orientation toward the sun, an acceptable pointing error being perhaps 1 to 3 arc-minutes. Rankine, Brayton, and Stirling power conversion cycles have been proposed for use with solar energy sources. The Stirling cy- cle employs a reciprocating engine—for which a firm long-lifetime technology base is not yet available and is attractive primarily be- cause of its high cycle efficiency at moderate temperature. The Rankine and Brayton cycles utilize a turbine driving an alternator. By employing fluid-fiIm, gas-supported, or magnetic bearings, tur- boalternator wear mechanisms can be avoided, hence long lifetimes appear to be attainable. Substantial development work on both the closed mixed-gas Brayton cycle and on the organic Rankine cycle engine has been done over the past 20 years, first as candidates for 1- to lO~kWe, isotope-fueled power systems, and more recently as contenders to supply power for the Space Station. Both Brayton cycle and Rank- ine cycle power conversion systems use large multistage turbines to drive electric generators.
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32 ADVANCED POWER SOURCES FOR SPACE MISSIONS In summary, for generating solar-electric power in Tow earth orbit, solar-dynamic power plants have the potential to produce several times the power output of solar photovoltaic arrays having the same collecting area. These power plants are constructed almost entirely of metallic materials, and their semiconducting components (chiefly for power conditioning and control) are small, and thus more easily shielded from being damaged by charged particles in space or by man-made radiation. Solar-dynamic power systems to supply from 50 to 300 kWe are being considered for Phase 2 of the Space Station. Chemical Space Power Systems Chemical reactants can be stored aboard spacecraft for power gen- eration as well as propulsion. These reactants can be used to power open-cycle power systems, as summarized in Figure 3-2. Consider- able technology relevant to this application is available from the extensive technical experience derived from using stored propel- lants aboard the Titan rocket and aboard spacecraft used during the Apollo program. Nitrogen tetroxide (N2O4) and mixed amines are quite easily stored and require no separate ignition system. The associated metals and synthetic sealing materials have been amply demonstrated. The principal unknown in using chemical reactants to produce space power is the tolerability of the spacecraft systems to the im- pacts of any chemical effluents that are released. A basic shortcoming in using chemical reactants is that their mass becomes prohibitive for durations beyond about 1,000 s (see Figure 2-1~. Cooling of the weapon system would require a separate liquid hydrogen supply, and would also produce effluent, but release of hydrogen may well be tolerable. Space experiments could help resolve the relative tolera- bility of hydrogen compared to other effluents, such as water (see Recornrnendation 2~. Space power systems using stored cryogens such as liquid oxy- gen and liquid hydrogen provide an attractive source of energy for an open-cycle space power system. The Apollo and Space Shuttle programs provide a well-developed background for applying cryogens to propulsion. The turbine-driven fuel pump for the Shuttie's main engine represents a record achievement in horsepower per unit mass. Insulation for cryo-tankage is well understood. The liquid hydrogen supply could be provided by the weapon-cooling system.
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POWER OPTIONS AND SELECTION CONSTRAINTS 33 The principal penalty for a cryogenic system is the requirement for active refrigeration. There may be some trade-off between sys- tem pressure, loss due to boil-off, refrigeration system mass, and insulation mass. Figure ~4 is representative of such a system. Both storable and cryogenic space power systems are feasible. The choice between them should be based not merely on compari- son of the power subsystems, but on the basis of comparing all-up spacecraft system designs (see Recommendation 1~. Magnetohydrodynamic Space Power Systems Magnetohydrodynamic (MHD) space power systems, a variety of chemical power systems, are still in the research phase. The basic principles of operation of an MHD electrical power generation sys- tem are conceptually simple, although practical systems are difficult to realize. In an MHD system, electricity is generated directly by causing a conducting fluid to flow across magnetic field lines. Such a system operates similarly to a Faraday-disk machine, except that a conducting gas is substituted for a metallic conductor, and linear motion through a channel is utilized instead of rotational motion. Because of this substitution, an MHD generator may be less massive than a conventional generator, hence MHD generators have some prospect for reducing the mass of space power systems, especially for burst-power applications requiring peak powers measured in multi- megawatts. In practice, introducing MHD technology poses several practical problems in addition to its extremely high operating temperatures and the need to obtain adequate electrical conductivity. One category of problem relates to achieving satisfactory behavior of the fluid flow in the MHD channel during the conversion process. Another problem is the management of system effluents emerging from the channel. Mitigation of the first problem requires attaining a highly ion- ized, high-velocity gas stream having adequate uniformity. The gas flowing through the MHD channel consists of a mixture of the hot combustion products of an exothermic reaction which provide fuel seeded with an alkali metal (e.g., potassium) to improve electri- cal conductivity when ionized. Small nonuniform ties of gas density and/or ionization concentration (conductivity) can result in major flow instabilities, and the excess heating in these regions causes acoustic disturbances and flow disruptions. The effluent problem requires finding channel geometries that
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34 ADVANCED POWER SOURCES FOR SPACE MISSIONS maximize uniformity of flow and minimize excess heating and the resultant acoustic disturbances in the conversion and exhaust re- gions of the channel. The high-MHD generator-exhaust tempera- tures (about 2500°K) pose difficult materials problems and, if the escaping ionized gas is discharged to space, the glow emitted by the recombination of its ions and electrons would be visible to an enemy. The total mass flow rate of effluent for a 200 MWe MHD space power system was estimated in the General Electric SPAS study (1988~. This mass flow was 24.3 kg/s, which was considerably less than the 39.6 kg/s emitted by a conventional 20~MWe hy- drogen/oxygen chemical power system. The SDIO Power Program Office has recently funded a feasibility study to evaluate the applicability of MHD to SD] requirements. The first phase of that project is for independent feasibility assessments of two candidate concepts for a multimegawatt space power system. The second phase is to assess innovative approaches to develop such systems. These studies will address problem areas such as uniformity of the ionized gas in the MHD conversion channel, channel erosion, and dealing with substantial quantities of metallically seeded ion- ized effluents. MHD space power systems could degrade spacecraft stability or perturb orbits. Based on these considerations, the committee considers that the state of the art in MHD technology may eventually warrant demonstration in space. However, until MHD systems that might be developed for SD] are projected to be capable of modifying or trapping such effluents, it is the sense of the committee that fur- ther MHD development for SDI beyond the conceptual studies and scaling validations presently contemplated is not warranted. Nuclear Power for Use in Space Nuclear power technology can provide the capability to satisfy the power-density and power-level requirements for a variety of civil and military missions in space. The United States has used radioisotope thermoelectric generators (RTGs) in space, but has never employed nuclear reactor power systems for space applications except for a short-term test of the SNAP-IDA power system in 1965. In contrast, the USSR has continued to develop and deploy fission reactor systems that have been largely successful, although two unplanned reentries of Soviet nuclear-reactor-powered satellites have occurred, causing adverse public reaction throughout the world.
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POWER OPTIONS AND SELECTION CONSTRAINTS 41 conversion. The baseline SP-100 design provides a power output of 100 kWe. Possible future redesign options are contemplated for space power applications over the range extending from 10 kWe up to 1 MWe. In defining the baseline SP-100 system, it was necessary to select technologies that would both represent advances in the state of the art and have a reasonable likelihood of success. As a result, uranium nitride fuel, clad with PWC-ll (niobium one percent zirconium 0.1 percent carbon), was chosen as the material for operating in the range of approximately 1400° K. Considerable operational experience with N~one percent Zr had been obtained from the previous space nuclear reactor system pro- gram. Although higher operating temperatures would be desirable, the creep strength of Phone percent Zr decreases at temperatures significantly above 1450°K. Similarly, thermoelectric power conver- sion was selected because, despite its low conversion efficiency, con- siderable operating experience with RTGs is available. The major drawbacks of such a system are its net conversion efficiency of about four percent and the resulting need to reject large amounts of heat. In addition to the baseline SP-100 power system, several alter- native materials and subsystems that could substantially improve SP-100 performance are also being investigated. Materials under in- vestigation (Cooper and Horak, 1984) include other clad and struc- tural alloys, such as PWC-ll (Lundberg, 1985), and molybJenum- rhenium alloys. These alloys have significantly greater creep strength than NWone percent Zr, but little information is available on their behavior under irradiation. On the power conversion side, two al- ternatives are being examined: a high-temperature therm~onic fuel element and the Stirling engine. The therm~onic fuel element would operate at internal temperatures of 1800°K to 2000°K (external tem- peratures in the 1000°K to 1200°K range), while the Stirling engine, with its significantly higher conversion efficiency, would in the near term operate at 1050°K, permitting the use of special stainless steels or lower-alloy superalloys such as S-590, S-816, or N-155. To make use of the full capability of the SP-100 reactor, the long-range goal of the Stirling power conversion program is to develop a converter operating at 1300°K, which would result in magnum specific power density (watts per kilogram) of the system. The 1300°K Stirling converter would require the use of refectory alloys (such as PWC-ll) or composites (e.g., tungsten wire-reinforced NWone percent Zr). The SP-100 program is currently entering into development of a
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42 ADVANCED POWER- SOlJRCES FOR SPACE MISSIONS ground engineering system (GES) to demonstrate the nuclear reactor power system during a 90 day test on the ground. A reactor and pri- mary heat transport system will be combined in a full-scale system test. In addition, an out-of-pile, end-0end demonstration will be performed using 1/12th of the full system, and employing a reac- tor simulator, the power conversion system, and the heat rejection components. Another objective of the GES tests is to advance fuel- element technology through the use of both advanced ceramic fuels and refractory metal-alloy materials. If these tests are successful, a demonstration of SP-100 in space could follow. The SP-100 system design incorporates innovative components wherever appropriate, and the SP-lOO program is designed to allow opportunities for evolutionary growth. It is recognized, however, that the materials and subsystems employed must be qualified through demonstration at the operating temperatures envisioned and over the desired operating lifetime. System trade-offs for example, system responses to a reduction in operating temperatures must also be considered. In that example, system performance would be reduced but the probability of achieving -a successful system would increase. The prunary technical goals of SP-IOO development are to achieve terrestrial and space demonstrations of a nuclear reactor power sys- tem acceptable to planners of space missions that require high powers and high power densities. If the SP-100 engineering design is shown to be feasible, a wide range of civil and Unitary space missions are foreseeable where SP-100 technology could be utilized. Additional delay in implementing a space nuclear reactor power system program will of course add to the time needed for planning and deployment of an associated space mission. The dilemma is that the development time for such a system still significantly exceeds nor- mal mission-development time, yet a candidate power system must be fully demonstrated to be feasible, reliable, safe, and acceptable to the general public, legislative bodies, and regulatory agencies be- fore any space mission planner can count on utilizing the system as a power source. Hence a primary issue to be addressed by the SP-100 program is that SP-100 satisfy rn~ssion safety criteria and requirements. A rigorous review based on the earlier discussion of safety for nuclear power systems is an absolute prerequisite to mission assignment. This situation is recognized in Finding 8.
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POWER OPTIONS AND SELECTION CONST~IN~ Smaller Nuclear Space Reactor Systems 43 A number of mussions, both civil and military, have been identified with power requirements in the 5 to 40 kWe range. Studies are being conducted (USAF/DOE, 1988) to determine if an alternative nuclear fission space reactor power system would be preferable to redesigning SP-100 to meet these requirements. Several potential candidate power systems are being consiclered, including a lower- power version of SP-100. While there appear to be sufficient mission needs for a long-term energy source in this low-power range, as with SP-100, it will be nec- essary to demonstrate a small-reactor system before planners would incorporate such a design into upcoming missions. In addition, if an alternative to the SP-100 design were selected for this application, years may be added to the development schedule if new technologies are required. Again, a primary issue of concern will be safety. Multimegawatt Nuclear Space Reactor System Designs Various designs have been proposed for nuclear fission space reactor systems capable of producing power in the multimegawatt range. Candidate system concepts are discussed, for example, in the Pro- ceedings of the Symposium on Space Nuclear Power Systems. These Proceedings are available for symposia that are held each January in Albuquerque, N. Mex. Currently, candidate options exist only at the conceptual stage. In addition, multimegawatt systems for SDI wail need to satisfy requirements that include providing power for operating in the housekeeping, alert, and burst modes. Both open- and closed-cycle systems are being investigated, as well as several approaches to power conversion; however, closed-cycle sys- tems impose significant mass penalties, while open-cycle systems produce effluents that could impair overall performance (see F~nd- ings 1 and 3~. The multimegawatt power range can be divided into three regimes: (1) tens of megawatts for durations of 10 years or more, (2) bursts of about 100 MWe, and (3) bursts of hundreds of megawatts. Power sources for the two burst-power regimes probably correspond to closed- and open-cycle systems, respectively. Tests are being conducted with two candidate technologies: in- core therm~onics and the gas-cooled pebble/particle bed core. In the thermionic fuel element demonstration, power conversion is accom- plished within the nuclear fuel elements.
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44 ADVANCED POWER SOURCES FOR SPACE MISSIONS Other Advanced Nuclear Systems New concepts utilizing nuclear processes other than radioactive de- cay or nuclear fission may play a role in the future. Such systems might, for example, include magnetic fusion reactors. The overriding technical questions will be demonstration of the eventual feasibility of these processes for space power systems. Furthermore, it is not possible to project a date of availability for these advanced systems. If space power sources are to be developed to provide electric- ity to power space weapons for defense against ballistic missiles, it seems clear that multimegawatt power sources either nuclear or nonnuclear—are an absolute necessity, based on current unclerstand- ing of requirements. With current SDI emphasis on ~near-term" deployment, space power systems are being developed only for such functions as surveillance, discrimination, or detection, hence they lack the multimegawatt capabilities required for weapons (Conclu- sion 7 and Recommendation 3c). Finding, Conclusion, and Recommendation Based on the above, the corrunittee arrived at the following finding, conclusion, and recommendation: Finding 8: The time needed for the development and demon- stration of a U.S. space nuclear reactor power system currently exceeds the time required to plan and deploy a space mission depen- dent upon that power source. Conclusion 7: A space nuclear reactor power system, once available, could serve a number of applications for example, in NASA and military missions req~ir~g up to 100 kWe of power or more in addition to SDI. Recommendation 3a: Give early, careful consideration to the regulatory, safety, and National Environmental Policy Act require- ments for space nuclear power systems from manufacture through launch, orbital service, safe orbit requirements, and disposition. Ground-Based Power Beamed to Orbit Power transmission from ground-based power systems for reception and use aboard spacecraft for propulsion or power needs could con- ceivably become practicable. The ground-based portion of power- beaming systems could employ a combination (summarized in Figure
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POWER OPTIONS AND SELECTION CONST~4INTS 45 3-3) of dedicated power plants, energy storage devices, and linkages to the commercial power grid. However, in examining the relative merits of the power-beaming option, trade-offs must be made based on resolving certain problems intrinsic to this option. One such problem is the vulnerability to hostile threats of fixed ground-based transmitter facilities and in the case of microwave transrn~ssion—of the rectenna~ used for reception of power at the spacecraft. Another problem is that space platforms will "see" a given transmitter beam during only a small portion of each earth or- bit for the low earth orbit (LEO) application. Solving this problem would necessitate using some combination of multiple ground-based transmitters and spacecraft storage of electrical energy. The received electromagnetic energy would be used to charge an on-board energy storage device (e.g., batteries, superconducting magnetic energy stor- age). The need for energy storage and a rectenna is associated with significant spacecraft mass. The relative attractiveness of the power-beaming-to-spacecraft option is probably closely linked with the brief time available for line- of-sight transmission (Hoffert et al., 1987) and with the masses of the rectenna and of on-board power storage systems. Existing stor- age devices, especially batteries, are too massive, although future batteries may qualify. Even if the mass of the storage system com- bined with the rectenna were attractive, any potential mass benefits of this option must also be balanced against system vulnerability. A rectenna would make the spacecraft difficult to maneuver, and in contrast to the relatively compact SP-100 nuclear option which includes a significant mass penalty to achieve minimal hardening is so extensive and fragile that it would be difficult to camouflage or to harden as a target. The committee considered the lirruted available information (Brown, 1987; Gregorwich, 1987; Holiest et al., 1987) regarding the option of beaming power from the ground into orbit using mi- crowaves, and found that this concept has some attractive features. For example, there is the potential for keeping much of the power- generating machinery on the ground, where mass is not critical and where maintenance and refueling are simpler. On the other hand, mi- crowave power-beaming systems do have certain drawbacks: brevity of transmission periods, complex orbital mechanics, the masses of on- board energy storage systems, the vulnerability of large-area recten- nas and ground installations, and a possible need for orbiting relay reflectors. The committee regards this option both cautiously and
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46 ADVANCED POWER SOURCES FOR SPACE MISSIONS seriously, and believes that further study is warranted to evaluate this system concept. There has been only a modest evaluation of the various power- beam~ng-to-spacecraft options that have been suggested. These op- tions include (a) beaning power from earth directly to a weapon; (b) beaming power from earth to a reflector in geosynchronous orbit and thence to the weapon in low earth orbit; and (c) beaming power from earth to a converter in synchronous orbit, thence to a reflector in Tow earth orbit. A variation on these options would be to beam power downward from a space power source in higher orbit. Finding and Recommendation Based on the above, the committee arrived at the following finding and recommendation: Finding 6: Beaming power from earth to spacecraft by m~- crowaves or lasers (see Recommendation 6) has not been extensively explored as a power or propulsion option. Recommendation 6: Review again the potential for ground- based power generation (or energy storage) with subsequent electro- magnetic transmission to orbit. C - Orbiting Power Sources Power can be delivered to a spacecraft from a detached part of that spacecraft or from a co-orbiting spacecraft by the use of Tong tethers, rigid booms, or by beaming. The concept of locally beamed or tethered (i.e., via long cables) power transmission from a power source to a weapon "at some distances which is taken to mean within a distance on the order of a kilometer or so, appears possible but is probably very complex. ENVIRONMENTAL CONSTRAINTS INFLUENCING THE SELECTION OF SPACE POWER SYSTEMS The Natural Space Environment The natural space environment contains neutral gases, plasmas, ra- diation (both penetrating particles and solar electromagnetic), mag- netic fields, meteoroids, and space debris. Characteristic densities, energies, fluxes, and so on vary widely with both time and position
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POWER OPTIONS AND SELECTION CONSTRAINTS 47 (including altitude) in orbit. The lower-energy constituents of the space environment, notably neutral particles, plasmas, and fields, can be dramatically perturbed by the presence and operation of space systems, creating a local environment much different from the natural one. The impact of system interactions must be examined in the context of the local space environment, leading to results that will, of course, be system-dependent. The higher-energy constituents of the space environment, such as radiation and particulates, are less influenced by the system than are the lower-energy constituents. Accordingly, the high-energy constituents can most readily be con- sidered in terms of their direct impact on the system. However, to the extent that these constituents modify the system, their impact will also be manifested in the local space environment of lower-energy components. Orbital Env~romnental Impacts Two key factors in the operation of SDI systems in natural orbital environments drive system design in terms of both feasibility and launch weight. These are (1) tolerance of effluents, most dramati- cally those from open-cycle cooling and/or power systems; and (2) achieving satisfactory operation of very-high-power systems in the natural orbital environment. Effluents and high-power operation are clearly interrelated, and in the final analysis these factors must be considered at an overall system level, because the local space envi- ronment couples the impacts of various subsystems (e.g., the power and weapon subsystems). Furthermore, both the power levels and the effluent-expuIsion levels envisioned for SDI systems are orders of magnitude beyond present experience in space. Resolution of these factors wiD require more than simple extrapolation of existing knowledge. In evaluating possible impacts of effluent from open-cycle space power systems and space weapon systems, consideration must be given to effluent behavior under a variety of circumstances. Data from past experience in space will be of limited utility, however, since attitude-control jets produce effluent on a much smaller scale than will open-cycle space power systems. Those data do suggest that condensation may occur on cold surfaces of the spacecraft, and that there is also the possibility of creating "snow flakes,n such as those reported in the Apollo program.
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48 ADVANCED POWER SOURCES FOR SPACE MISSIONS Chemically reacting gases or ionized gases could also emit elec- tromagnetic radiation. For an SDI platform, such a radiant plume could interfere with its sensors, increase its detectability, and increase its vulnerability. High-power systems use high voltages and/or high currents, hence operation of such systems in the low-pressure, ionized space en- vironment requires great care to avoid surface arcing and "vacuum" breakdowns. The usual approach to operating high-power systems in the earth's atmosphere is to insulate the high-voltage compm Dents using oils or pressurized gas containers. Implementation of this approach for space power systems may make them prohibitively massive, especially since the mass problem is further aggravated by the need to make space systems survivable against natural and hostile threats: damage from space debris and meteoroids must be prevented over long mission lifetimes. Survivability concerns introduce an additional factor that is best considered in synergism with dealing with effluents and providing high-voltage insulation. For example, if space platforms were hard- ened to several calories per square centimeter, the substantial armor shell employed would provide an environment that may eliminate the possibility of electrical breakdown between bare electrodes and at the same time protect against effluents and space debris. Thus, re- sponding to survivability needs could also serve these two additional purposes. The prospect for simultaneously achieving these multiple objectives suggests that there should be an effort both to develop low-density, high-dielectric-strength materials for encapsulation and to formulate appropriate space experiments for testing them. Arcing on partially insulated probes (planar solar-array seg- ments) biased negatively in plasmas, both in ground test facilities and in orbit, has been observed at voltages in the range of a few hun- dred volts. Probes of other geometries—such as conducting discs on insulator and "pinhole" geometries are less prone to arcing. While arcing mechanisms are not fully understood, enough ~ known that attempts to develop arc-resistant designs could prove fruitful. An initial demonstration of this possibility is the success of the SPEAR- ~ rocket experiment, which obtained data at altitudes up to 369 km. On December 13, 1987, using specially designed 1-m Tong booms to suppress surface arcing, two 2~cm diameter spherical probes on SPEAR-T clemonstrated space vacuum insulation that withstood a maximum (pulse) voltage of 44 kV applied between the rocket body and the spherical boom terminal. The payoff, in terms of reduced
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PO WAR OPTIONS AND SELECTION CONSTRAINTS 49 power system mass, could be large if insulation requirements can be reduced or partially eliminated in pulsed systems. The basic strategy is to achieve a design solution that both elim- inates along-surface arcing and takes advantage of the space vacuum to help avoid gas breakdown when using high-voltage pulses. Steps along these lines are being taken within the current SDI program. However, it should be noted that attempts to follow this strategy may be compromised by high levels of contaminants, both because (a) surface arcing and gas-breakdown thresholds are dependent on background neutral and plasma densities, and (b) breakdown near a high-voltage surface can short out such space-vacuum-insulated conductors. Neutral-gas-breakdown threshold voltages are reduced below the Paschen levels when plasma is present (due to the availabil- ity of free charges for initiation), and under some conditions are further reduced by magnetic fields. Magnetic fields at levels that may be present in high-power equip- ment may inhibit both charge mobility and breakdown potentials. In the density regimes characteristic of the unperturbed natural space environment, pressures are so low that very high voltages are nec- essary to initiate gas breakdown. Yet an orbiting vehicle introduces local surrounding gas and plasma densities through outgasing as well as local magnetic fields characteristic of the orbital vehicle- creating conditions markedly different from those of the natural en- vironment. Again, large amounts of effluent will make the system much more susceptible to breakdown and dictate more stringent in- sfflation requirements. In addition, attaining survivability against hostile threats may complicate implementation of the exposed high- voltage approach, since direct use of the space vacuum as an external surface insulator will be precluded for those space platforms that must be hardened by encapsulation to withstand hostile threats. In the SPAS studies examined by the committee, open-cycle space power systems are tentatively regarded as attractive choices compared to closed-cycle power systems because of their potential to be significantly less massive. As this report was being finalized, the committee became aware of a study by El-Genk et al. (1987) favoring closed-cycle nuclear power systems. Open-cycle space power systems may not be practical if they liberate large amounts of effluent. Open-cycle weapon systems (e.g., chemical lasers) are also being proposed, although any resulting evolution of effluent clouds is not well understood in terms of ex- pansion, dissipation, ionization, excitation, radiation emission, and
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50 ADVANCED POWER SOURCES FOR SPACEMISSIONS interactions with surfaces, background environments, sensors, and weapons. To understand the impacts of effluent clouds, estimates are re- quired of neutral particle densities around a vehicle due to back- streaming around nozzles. Such estimates vary widely, however, de- pending on the nozzle geometry and the analytical models employed. There are order~of-magnitude differences between independent pre- dictions of densities in the region "behind" nozzles. None of the models has been fully validated. Impacts of effluents include chem- ical interactions with surfaces, condensation on cold surfaces, inter- ference with power system operation, and interference with weapons and sensors. All of these impacts depend on the kind of effluent and its mode of emission, density, and temperature. The question of what liberated effluents, if any, can be tolerated has not been resolved. Yet the mass penalties resulting from containing large quantities of efflu- ents are significant. Various possible impacts of effluents on weapons and sensors need resolution. For example, one unresolved issue is whether ef- fluent releases will interfere with propagation of a neutral particle beam. The simplest approach to estimating such interference is to approximate the effluent as a spherical cloud emanating from a point source at distance Ro from the particle beam source, then compute an effluent dump rate that will interfere with the beam. The validity of using the spherical approximation has been estimated by compar- ison to Space Shuttle data via the calculations shown in Appendix D. These calculations suggest both that the approximation is rea- sonable and that the issue of neutral-particle-beam stripping must be addressed. To complement these estimates, a program of space experiments is needed, as suggested in Recommendation 2. From the space power system perspective, two strategies appear to have high payoffs in terms of reducing system mass. One is to ex- plore development of effluent-tolerant systems; the other is to explore using space as a "vacuum insulator," if this approach is consistent with achieving survivability against hostile threats. The two are an- tithetical, as an effluent-tolerant system must be carefully insulated for the long term, while a space-vacuum-insulation approach may be intolerant of effluents. Given the prohibitive mass estimates gener- ated in systems studies to date, it is imperative to quantify both of these potentially high-payoff mass-reduction approaches. Because effluents can affect power systems, sensors, and weapons, analysis of the total space system must be significantly refined
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POWER OPTIONS AND SELECTION CONSTRAINTS 51 before a better understanding of the effluent issue can be reached. Meanwhile, aggressive pursuit of both the above strategies and space experimentation (Recommendation 2) appear to be indicated. Resolution of both the high-power and effluent issues will require modeling and validation of models through comparison both with ground test data and with data from spaceflight testing. Meanwhile, the Space Plasma Experiments Aboard Rockets (SPEAR) program is obtaining flight data to address some of the high-voltage puise- insulation issues. However, the SPEAR results may be experiment- specific, hence their generalized application requires caution. Relationships among efflux density, background density, and ve- locity are such that a high-altitude, rocketborne experiment may be a good vehicle to obtain early data on the evolution of effluent clouds. These data would be applicable to high-altitude (greater than about 1,000 km) orbital vehicles. Because these issues are inherently total- system dependent, close cooperation among SDI's power, weapon, and sensor programs is indicated. Experience with the Space Shuttle (see Appendix D) can cast some light on impacts of effluent dump rates projected for SDI systems. Conclusion and Recommendation Based on the preceding, the committee arrived at the following con- clusion and recommendation: Conclusion 3: The amount of effluent tolerable is a critical dis- criminator in the ultimate selection of an SD! space power system. Pending resolution of effluent tolerability, open-cycle power systems appear to be the most mass-effective solution to burst-mode elec- trical power needs m the multimegawatt regime. If an open-cycle system cannot be developed, or if its interactions with the spacecraft, weapons, and sensors procure Unacceptable, the entire SD! concept wild be severely penalized Tom the standpoints of cost and launch weight (absent one of the avenues stated in Conclusion 2, Chapter 4~. Recommendation 2: To remove a major obstacle to achiev- ing SD! burst-mode objectives, estimate as soon as practical the tolerable on-orbit concentrations of effluents. These estimates should be based to the maxinmm extent potable- on the results of space Ferments, and should take into account the impact of effluents on high-voltage insulation, space-platfomn sensors and weapons, the orbital environment, and power generation and distribution.
Representative terms from entire chapter: