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5 Approaches Toward Achieving Advances in Critical Power Technologies In the discussion of space-based power requirements (Chapter 2), the committee pointed out the advantage of pursuing high-leverage areas; sirn~lar approaches can yield some very useful results in advancing critical power technologies. In this chapter, the following subjects are discussed: advancing thermal-management techniques, advanc- ing power-conditioning components and technologies, and materials advances required for developing power component technologies. ADVANCING THERMAL-MANAG1:MENT TECHNOLOGIES The thermal-management problem is that all heat generated on a space platform must be (a) converted into another form of energy (with the associated thermodynamic constraints); (b) absorbed as temperature rise in components or thermal storage elements; (c) absorbed by a coolant that Is vented; or (~) radiated to space either directly from the component or by use of a higher-temperature, more efficient radiator. The last option requires a heat-pump (refrigeration) cycle, in which heat is absorbed at a low temperature and rejected by the radiator at a higher temperature. Only the first three options are available for heat rejection from the space power system itself. The space power system defined for this purpose to include 68
A CHIEVING AD VOICES IN P O HER TECHNOL O GINS 69 a heat source, power conversion devices, and its loads is the pri- mary source of spacecraft-generated thermal energy that must be disposed of. Thus the efficiencies and losses of the overall power system including those of its subsystems and componentsare ma- jor factors determining how much heat is generated and thus must subsequently be absorbed or rejected. Availability of survivable, cost-effective technology to store, pump to higher temperatures, and radiate thermal energy effectively with low mass penalties is an im- portant ingredient of space power system design. The problem of thermal management is very important for space- craft of any size, to say nothing of spacecraft power systems ranging from hundreds of kilowatts to multimegawatts. The primary means for heat rejection currently employed is to use heat radiators. This method is basically the only Tong-term means of rejecting heat in space without spacecraft mass alteration. Obviously, heat can be stored in a mass that is then ejected from the spacecraft. The practi- caTity of this method is limited (to about 30 min) by the rapid increase of the mass required with increasing duration of operation. Further, heat storage (in a heat sink) IS a very useful method of point cooling and has considerable potential for SDI utilization. These methods will be discussed separately. Heat-Rejection Considerations As is well known, the amount of heat radiated from a surface is proportional to the fourth power of the surface temperature (mea- sured in °K) and to the emissivity of the surface material. For these reasons, reductions in radiator size and mass can be realized if the operating temperatures and emissivities of space power radiators can be increased. Because of the high sensitivity to temperature, dra- matic mass reductions can be achieved, as discussed in Chapter 4, whereas there is less sensitivity to ern~ssivity improvements. Significant innovation in this area has the potential to alter conventional views of power-system design trade-offs and should be examined in connection with the preliminary vehicle design proposed in Recommendation 1 of this report. Innovative radiating systems based on liquid-droplet radiators, moving-belt radiators, heat pipes, or on radiators that are deployed on power demand have been pro- posed. Although there is no assurance that any of these concepts will prove feasible, such approaches might produce significant reductions in radiator size and specific mass, and hence warrant exploratory
70 AD VANCED PO WER SO URGES FOR SPA CE MISSIONS research. An unarmored deployable radiator would be less massive than an armored radiator, yet still be survivable to attrition attacks by ground-based or space-based lasers. For high-power systems, needs for heat rejection and mass mini- mization cause system designers to favor power systems that operate at high temperatures, thereby reducing the size of power-conversion equipment (through higher conversion efficiencies), the amount of heat that needs to be rejected, and the size of the radiators. For low- temperature power systems, low-density materials (e.g., aluminum, beryllium, or titanium) can be employed as radiators, thereby pro- viding a means for reducing mass. Unfortunately, most highly devel- oped heat-rejection technology was optimized based on cost factors, rather than on considerations such as survivability, efficiency, or high-temperature capabilities. Consequently there is only a limited available technology base applicable to the problem at hand. Heat rejection is essential, and for closed-cycle (noneffluent) space power systems at the multimegawatt level, the heat rejec- tion subsystem (see Chapter 4, Figure 4-1) can easily account for half the mass of the overall power system. The SP-100 power system has large, massive radiators because of the low conversion efficiency of its thermoelectric converters. These radiators are made even more massive by the imposed survivability requirements. Two other heat rejection options are discussed below which avoid using radiators but are mass-intensive, hence they become impractical as the duration of power usage increases beyond about 1,000 s. One of these options is to use heat storage aboard the spacecraft for thermal management of multimegawatt systems that are operated for only short periods of time (i.e., in the burst mode). Most of the heat storage needed can be accomplished through endothermic chemical reactions, use of specific heat capacity, and phase changes. The other option that avoids radiators is gross heat rejection from a thermal engine, where the waste heat is simply thrown over- board with the effluent. This is a viable concept if the effluent does not unduly interfere with friendly weapon, sensor, spacecraft, or power systems. On the other hand, liberating effluents may hinder hostile action. From a strategic standpoint, duration constraints on the use of the above mass-intensive options make them ineffective against a counter-strategy of prolonging the period of combat to an hour or longer. Power system working fluid can typically be used for weapon
ACHIEVING AD DANCES IN POWER TECHNOLOGIES 71 cooling prior to entering the power-generation system. The ejected effluent will thus have served the dual function of disposing of waste heat from both weapon and power-generation systems. Resolution of the question of whether or not the release of effluents is tolerable is addressed in Recommendation 2. Survivability Consideration The survivability of space radiators is a major design problem, ow- ing to the ease of detection of such localized thermal sources. This problem is especially serious at the high rejection temperatures that might be used for nuclear reactor systems. Radiators at those tem- peratures could act as infrared homing beacons for hostile detection anti action. Early in the process of advancing candidate space power systems, thermal rejection techniques need to be identified that mit- igate the risk of detection and attack but do not impose excessive mass penalties for hardening. Candidate heat-rejection techniques showing promise should then be subjected to feasibility studies and scalability validation. ADVANCING POWE:~-CONDITIONING COMPONENTS AND TECHNOLOGIES Advances in power system components, materials, and technology are necessary to meet envisioned SDI requirements, as discussed below. Advancing the Design of Conductors Conductors usually make up a significant fraction of the overall mass of a power device and also determine its characteristics. Conductor mass is generally traded off against electrical losses, device efficiency, conductor temperature rise, complexity of cooling, and the amount of coolant required. Normal Conductors Practical conductors at ambient temperatures generally consist of copper or aluminum or their alloys. The materials are selected to give the appropriate combination of low resistivity, mechanical strength, and ease of fabrication required for specific applications.
72 ADVANCED POWER SOURCES FOR SPACE MISSIONS High-strength conductors are important in applications (gener- ally circular or solenoidal windings) where the conductor is also the only part (or a major part) of the necessary structure. In applica- tions where the structure is separate or is not of major concern, the resistivity of the conductor becomes the major factor. It is desirable to have conductors that can operate at high current density consistent with achieving structural, dielectric, and thermal requirements of the winding. Essentially all pure metallic elements of interest as conductors exhibit decreasing resistance with decreasing operating temperature. This decrease is limited at the low-temperature extreme by impu- rities, magneto-resistance, mechanical stress level, work hardening. size, and so on. High-conductivity, high-strength, wide-temperature-range me- tallic conductors are distinctly possible, but do not appear to have been examined over the temperature ranges of interest. The two best metallic conductors (commercial, at practical cost levels, formable, ductile, and tough) are copper and aluminum and oxide-dispersion- stabilized (ODS) alloys of Cu and Al. Dissolved impurities have major negative effects, especially on electrical conductivity, but also on thermal conductivity. The purer the Cu and Al, the higher the conductivity values. Fortunately it is possible (commercially) to produce Cu at purity levels of 99.9 percent or better, and Al at 99.99 percent or better. Each dissolved impurity element has a different effect (percent conductivity loss per unit of impurity content) on conductivity. The potential availability of liquid hydrogen as a conductor coolant can have a major effect on system operating temperatures. Conductors capable of operating with liquid hydrogen should be de- veloped either separately or as part of the component development. Of existing conductors, high-purity aluminum in a composite with an aluminum alloy is a promising candidate for use in a liquid- hydrogen high-power alternator. If estimates of conductor perfor- mance hold up for this option, it could be important in many direct- current and alternating-current applications. Superconductors Superconductors exhibit zero reset ance only below a certain critical temperature. They are poor electrical conductors above this value. The metallic superconductors NbTi and Nb3Sn, which require
A CHIEVING AD VANCES IN P O WER TECHNOL O GINS 73 liquid helium for their operation, are capable of achieving operational winding current densities of 50,000 A/cm2, but are not suitable for operation at temperatures other than in the liquid helium range (about 4°K). Because of their potentially high current density, these alloys will continue to be major technological contenders for SDI applications. For radiofrequency (RF) operation at low magnetic fields, su- perconductors exhibit low surface resistivities the lower the tem- perature, the lower the Tosses. Nb cavities are being used at 2°K for accelerators in the gigahertz range. Since operation at radio frequen- cies is a surface phenomenon, a layer of superconducting material of the appropriate thickness must be carefully applied to a suitable substrate. The use of superconductors in power systems generally leads to high-efficiency, compact components and subsystems. High efficien- cies of power generation, power transmission, and power condition- ing have direct beneficial effects on the ratings and masses of prime power sources and, in addition, a low-Ioss RF cavity reduces mass requirements for the entire system. At present, most superconductor applications are either direct current (DC) or quasi-DC, where the currents change slowly. It is only during DC operation that superconductors have zero resistance to the flow of electrical current. The limits of this region of zero resistance are a function of operating temperature, current density, and magnetic field. For alternating current (AC) operation at 6~Hz power frequen- cies, superconductors exhibit Tosses above a threshold magnetic field. These losses decrease with decreasing filament size one of the rea- sons for the multifilamentary configuration currently being used in Nb-based superconductors. Until now, because of the penalty of hav- ing to operate at liquid-helium temperature, applications at power system frequencies have been limited by the unavailability of con- ductors having micron-size filaments. Achieving higher operating temperatures for these materials means reduced refrigeration re- quirements, and the prospect of doing so suggests that it is timely to review potential space power applications (at frequencies much greater than 60 Hz), such as transmission, transformers, inductors, armature windings, and so on. Superconductor transition temperatures up to 125°K have now been reported by several institutions. There is no reason to be- lieve that the optimum materials have been discovered, and further
74 ADVANCED POWER SOURCES FOR SPACE MISSIONS progress is expected. Reports of achieving critical temperatures of room temperature or above have been erratic, unconfirmed, or have used inadequate measurement techniques. To summarize the present status of superconducting materials: 1. High-critical-temperature superconducting materials have the potential of carrying high critical currents. 2. The superconducting materials are ceramics and, at this stage of their development, they have poor mechanical properties. 3. Indications of transition temperatures above room tempera- ture have been reported (Materials Research Society, 1987), but have not been confirmed and may not be reproducible. 4. Applications depend on the availability of superconductors or superconductor sections with consistent properties that can be fabricated into reliable windings for magnetic components or struc- tures resembling permanent magnets. This committee concludes that high-critical-temperature super- conductors may well play a major role in SDI power applications someday. Nevertheless, because of their early stage of development, such superconductors are not currently available nor will they likely be available for many years to come- to replace present or improved power technology. Accordingly, the development of other SD! power technology should not be curtailed until these superconductors begin to become a viable option. Superconducting Magnetic Energy Storage Storage of energy in a magnetic field occurs when electricity flows through one or more coils. Since any electrical resistance in the circuit causes energy loss, the use of superconducting coils" which have no DC resistance is a very efficient approach to storing electrical energy for any length of time. A major application of energy storage is to allow energy sources to be sized for average or low power. In the case of superconducting magnetic energy storage (SMES), the coin are energized at tow power levels and then discharged at a higher power level. LJow-critical-temperature superconductor technology has been demonstrated on several large-scale projects primarily in magnetic fusion, high-energy physics, and magnetic resonance imaging appli- cations. Technology for these applications usually operates reliably, and even larger-scale applications of superconductivity in these areas are planned.
ACHIEVING ADVANCES IN POWER TECHNOLOGIES 75 Studies aimed at providing ground-based power of limited du- ration at gigawatt power levels with rapid rise-times have indicated that SMES is a very attractive approach for SDI applications, es- peciaIly in view of the possibility of time-sharing the facility with a utility during peacetime. Two large contracts for independent con- ceptual design studies of SMES systems were awarded by SDIO late in 1987. ADVANCEMENT POTENTIAL OF TECHNOLOGY FOR DYNAMIC POWE:R-CONVERSION CYCLES* The existing gas-turbine industry builds gas turbines for propelling aircraft and builds both gas and stemn turbines for terrestrial power generation. The largest gas turbines generate 100 to 200 MWe per module, and specialized gas turbines operating on stored compressed air produce up to 290 MWe from a single machine (Gas Turbine World, 1987~. About 20 separate models of gas turbine power plaints currently marketed have power ratings exceeding 100 MWe. Advances in the gas turbine for solar-dynamic power generation aboard the Space Station and for use with nuclear reactors such as SP-100 could occur (English, 1987) in the following ways: . Using a taut alum-based refractory metal alloy (ASTAR-811C) for the hot components of the power plant would permit operation at peak temperatures up to 1500°K. That alloy has been creep-tested for over 300,000 hours at temperatures from Il44°K to 1972°K (Klopp et al., 1980~. Refractory alloys based on molybdenum and niobium- having considerably lower density may in the future prove to be applicable at these temperatures; specifically, the M>HfC alloy has been tested for only a few hundred hours at temperatures up to 1800°K in ~ inert gas atmosphere, much less than the testing (over 22,000 hours) to which ASTAR-811C was subjected at temperatures above 1800°K (Klopp et al., 1980~. However, both molybdenum and niobium alloys must still undergo a very considerable testing program before final conclusions can be drawn. *The committee has discussed the Brayton cycle in considerable detail. Many of the advances described in this section are also applicable to the Rankine cycle. The committee believes additional study of both cycles is warranted in view of unexplained or inconsistent SPAS analysis results, which were unavailable in published form during the course of this study.
76 AD VANCED PO WER SO URGES FOR SPA CE MISSIONS . By using the Brayton cycle combined with molten-lithium heat storage, since the sensible heat capacity of the molten lithium is higher by a factor of two or more than the latent heat capacity of the fusible salts now contemplated for the Space Station. Use of lithium, because of its extremely favorable heat-transfer properties, would also permit a significant reduction in the size and mass of the solar heat receiver. . Inasmuch as molten lithium is not tied to any given working temperature (as Is the melting and freezing of a salt), using lithium in a Brayton cycle would peanut the gradual evolution of a given power plant by first operating it at, say, 1200°K and then gradually raising the operating temperature toward the potential of the power plant, 1500°K in this case. . This rise in peak temperature would increase not only the power generated but also the efficiency of power generation; the sizes of the solar collector and waste-heat radiator could therefore remain constant with up to a 50 percent increase in generated power. . Finally, for application to nuclear power, the solar mirror and solar heat receiver of the solar Brayton power plant could be replaced by a lithium-cooled nuclear reactor, such as the SP-100. By virtue of their high efficiency, closed Brayton and Rankine cycles could generate about 500 kWe using the same reactor from which the present SP-100 thermoelectric conversion design generates 100 kWe. Similarly, from a 2~MW reactor required for thermoelec- tric generation of 1 MWe, these power cycles would generate up to about 5 MWe. For generating very high power in the burst mode, use of molten lithium as the heat sink for a high-power closed-cycle system would provide a low-mass power plant that discharges no effluent during operating periods of 1,000-2,000 s. This same technology could also provide the megawatts of power needed for long periods in the alert mode. Advancement Potential for Alternator Technology Alternators are electrical rotating machines that convert shaft en- ergy into AC electrical power that can then be used as generated or transformed and/or rectified as required by the load. A field windingusually DC-energized is rotated, with the power being generated in the stationary armature. The power for a given-size machine generally increases with
ACHIEYING AD VANCES IN POWER TECHNOLOGIES 77 increasing speed and current density in the field and armature windings within the limits of structural integrityconsistent with the requirements for high rotational speeds and rapid start-up some- times imposed. The alternator technology relies heavily on the avail- able prime mover, the available conductors, and thermal management of the Tosses in the rotor and stator. For an ambient-temperature application, the U.S. Army is de- veloping a 3-MWe, gas-turbine-driven, oil-cooled machine. The al- ternator for that device has a specific mass of about 0.1 kg/kWe (for the generator alone) and rotates at 10,500 to 15,000 rpm. The output power, which has a frequency of about 1 kHz, is fed into transformers. Because superconductors have losses when subjected to time- varying currents or magnetic fields, the use of superconducting tech- nology has been limited up to now to the field windings of the alternator, where they are exposed essentially to DC operation. An example of this technology Is a machine using a liquid-helium-cooled superconducting rotating field and an ambient-temperature arma- ture, being developed by General Electric for the U.S. Air Force The machine is undergoing Preliminary testing. It has a rating of 20 MWe at 6,000 rpm and is capable of starting up in Is from a cooled-standby condition. The machine has a specific mass of 0.045 kg/kWe, and is designed with several system-oriented unique features, such as a rectified 40-kV DC output, potentially eliminating the need for ad- ditional transformers. It also has an ambient-temperature aluminum shield that reduces external time-varying magnetic fields, which is an important design feature for space applications. Because of its Tower speeds and high-voTtage winding, the 0.045 kg/kWe machine is not directly comparable to the 3-MWe army machine. An experimental air-core alternator with a disc rotor is being de- signed by ARDE~KAMAN with a continuous rating of 0.1 kg/kWe at about 5,500 rpm. This rating is projected to decrease to 0.03 kg/kWe for a future cryogenic machine with counterrotating discs and a 2~MWe rating. An approach using liquid-hydrogen-cooled, high-purity alumi- num conductors for both field and armature is being undertaken for SDI by Westinghouse and Alcoa. Recent measurements of the resistivity of high-purity aluminum samples by these organizations (Biliman, 1987; Eckels, 1987) are lower than previously attained. If such resistivity can be maintained in finished windings, these results indicate that high-purity aluminum may be an even better conductor
78 AD DANCED PO WER SO URGES FOR SPA CE MISSIONS than previously thought for high-current-density operation in the liquid hydrogen range. Estimated specific masses are of the order of 0.03 kg/kWe for a 3() MWe machine with output in the 50 to 100 kV range. In the United States there is essentially no operating experi- ence with alternators other than at ambient temperatures. Cryo- genic and superconducting techniques have been successfully demon- strated in homopolar types of machines and in other stationary ap- plications, such as magnets for high-energy physics, magnetic fusion experiments, and magnetic resonance imaging. While experimen- tal developmental hardware does exist, the successful application of these techniques to high-power alternators still remains to be demon- strated. An alternator configuration for use in space- because of its inter- face with power conditioning/Ioad, thermal management, the prime mover/energy source, and the torques, magnetic fields, high voTt- ages, and currents it generates must be the result of a thorough, interactive systems approach. The basic advantages of the alternator in being able to generate high voltages without transformers must be traded off against the loss of flexibility in initially developing a general purpose alternator that must then be connected to a power system with transformers to provide load-specific voltage levels. Note that all loads may not operate at the same voltage level. Direct generation of high voltages requires either placement of the alternator near the load or the transmission of power at high voltages, and has the attendant problems of high voltages in space and fault management, as cliscussed elsewhere in this report. Advancing the State of the Art In Power System Components Funding that has been available for component development has gen- erally been used within a program to unprove existing manufacturing techniques, evaluate new materials developed for other applications, and to make ~rnprovements in techniques for manufacturing compo- nents. In view of the limited resources available in the past, that was the only logical approach. However, this strategy is at best capable of achieving only modest gains. A more cost-effective approach is illustrated by the example of the recent joint SDI/DNA capacitor program. This program has been very successful, in large measure because it made maximum use of new theory and computer modeling power. This program
ACHIEVING AD VANCES IN POWER TECHNOLOGIES 79 makes use of 1980s technology rather than simply extending the stanclard approach typical of the 1960s. This approach can be ap- plied to other component technologies as well. Examples of the rapid advances achieved to date in representative power technology components when their development was aggressively funded are il- lustrated in Table 5-1. In contrast, note the dismal evolutionary advance rates of 1.5 per decade in surface-voltage withstand-level for resistors. While the comrn~ttee recognizes that technical progress is often nonlinear, use is made here of average rates of advance in order to focus attention where it is needed. The SDI power program should continue an aggressive, coor- dinated base technology program to parallel and complement its weapons platform/systems efforts. To enable the multidecades of advances needed for SD! power, program focus should be on areas such as: . high-temperature materials for nuclear reactors and power generation high-temperature radiators; advanced, high-temperature instrumentation and reactor con- trol; tw~phase flow evaporation and condensation in reduced grav- itational fields; electrical and thermal insulators; Tow-mass electrical conductors, including superconductors; thermal conductors; ferromagnetic and magnetic materials; survivable devices for switching, power conditioning, and gen- eration; techniques for managing/containing high voltages, currents, and electrical and magnetic fields; and improvements In inverters, which are not presently being de- veloped for weapons power. The comrn~ttee recognizes a clear need to make progress in ma- terials for increasing the efficiency and compactness of power compo- nents. There may also be benefit in coupling industry to university research groups via the SDIO directorates responsible for basic re- search (DOD category 6.1) and technology base development in the power area. As an example, mass reduction in high-power thyra- trons could be substantial if the ceramic insulators could either be
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~2 ADVANCED POWER SOURCES FOR SPACE MISSIONS eliminated or made of less dense insulating materials capable of high- temperature operation. Progress toward advances in the state of the art of components used in power-conditioning and puIsed-power systems could be ef- fectively achieved through initiatives anchored in materials tech- nologies. In view of the major successes achieved in applying ba- sic science to materials programs in high-energy-density capacitors, similar approaches should be applied to other areas of power devel- opment. This committee recommends a development strategy of this nature, pursued aggressively and funded adequately, to develop scal- able power technology, particularly if success would enable selection of one weapon system over a less desirable one by removing power considerations as the principal constraint. The following four areas of development form an integrated pro- gram ensemble in both prime power and power-conditioning technol- ogy: . Technology feasibility projects to demonstrate that a required capability is possible. . Scaled experiments to give high confidence in the ability to design a full-size system. . Limited near-full-scale demonstrations of advanced-develop- ment models, for technology validation and to clarify integration and compatibility problems associated with production devices. . A continuous effort to understand fundamental mechanisms as applied to component technology feasibility and scalability. In summary, development of subscale (i.e., at about 10 percent of full power level), scalable, high-performance power components and associated technology to provide a broad range of system op- tions is a prudent investment strategy. Emphasis on component development for generating, conditioning, and transmitting electri- cal power is required. The issue of high-temperature superconductivity as it affects scaling feasibility must be addressed. Furthermore, the Tonger-term and nearer-term technology base development programs must be brought into balance. A technology-based-option investment strategy for the longer-term options in SD] is needed by periodically targeting superior technologies among existing candidates as a means of achieving future needs through down-selection. Such an increased emphasis is needed on the technology base for space power system components, as the existing base is grossly inadequate to meet the mission challenge.
ACHIEVING ADVANCES IN POWER TECHNOLOGIES MATERIALS ADVANCES REQUI1lE:D FOR THE EVOLVING SPACE POWER TECHNOLOGIES 83 There are vast differences in the materials requirements for the range of space power cycles and power systems examined by this commit- tee. These systems typically demand high temperatures (with little else specified) ranging from 1300°K to 2500°K. Although the lower temperatures in this range can be met In reasonable time and at reasonable cost, the higher ones will necessitate the development of totally new or different materials, requiring a dedicated effort in or- der to achieve success in some "short-term period such as 10 years. The development of SDI space power component technologies will require significant advances of materials technology in the follow- ing areas of magnetic materials, insulators, and the development of high-temperature structural materials. Magnetic MateriaLs Magnetic materials are important for induction accelerators, low- mass, high-frequency inverters, and so on. However, data are cur- rently being obtained that indicate that FeNdB magnets can be fabricated for a variety of magnetic applications with outstanding results. Metallic glasses of selected compositions are soft magnetic materials. Being free of grains, grain boundaries, and secondary phases, these materials can be used for making soft magnetic alloys that are ent*ely free of orientation effects. Apparently Metglas~R has not yet approached the potential desirable properties achievable in magnetic materials by applying rapid quenching techniques to create new alloys. During the past year, General Electric- using ADied Signal Company MetglassR compositions built and tested a large number of commercial AC power transformers that exhibited the outstanding performance pre- viously predicted. Sweat ore Newly developed products far superior to classic baked clay ceram- ics are available for making feedthroughs, standoffs, interfaces, and other insulators. Numerous new classes of polymers and ceram- ics, processing techniques, and forming techniques can now offer
84 ADVANCED POWER SOURCES FOR SPACE MISSIONS major improvements in insulators. Such improvements include high- strength materials that can be used at high and low temperatures and that can produce intricate shapes. High-Temperature Structural Materials Because materials are almost always and properly viewed as de- sign limiters, support for the development of advanced materials has received reasonable backing since the early 1950s. Unfortu- nately, performance specifications all too frequently come fairly late in systems development programs. furthermore, almost every new application unfortunately requires new or different combinations of properties and performance: temperature, time at temperature, per- missible deformation, structural stability (i.e., changes of properties under operating conditions), surface degradation, joining problems, and so on. For new or different applications, these requirements em- phasize the need to define a proposed system so that materials can be tailored to such needs. It is rare that the more critical materials can be obtained "off the shelf." For SDI power systems, radiation hardening is a requirement for power semiconductor switches and other electrical components. There are significant opportunities for exploiting new materials such as gallium arsenide and silicon carbide for this purpose. Before the use of ceramic materials or carbon-carbon compos- ites for rotating blades or the use of filament-reinforced ceramics for temperatures between 1200°K and 1500°K can be seriously pro- posed, considerable time will be needed to develop and test such ma- terials for use in a specific power system. This is because only limited data are available on long-term performance in highly cyclical tem- perature and stress systems. A few such systems are making excellent progress, but results for these applications are emerging slowly, hence careful development of these materials for meeting specific needs wiD continue to be required. The preferred cycles and systems must be selected, and all op- erating conditions must be integrated. Such integration will permit selection of the alloy systems, if not of the alloys, for preliminary con- sideration and planning for alloy modification. Thus the committee notes the following three partial bases for arriving at its Conclusion 5 and Recommendation 5 stated below. 1. Selection of operating temperatures up to about 1500°C (1773°K) (National Research Council, 1988) may permit preliminary
ACHIEVING ADVANCES IN POWER TECHNOLOGIES 85 selection of materials already in existence for specified life cycles and environments. Usually, and fairly obviously, the lower the planned operating temperatures, the greater is the number of available appli- cable alloys. The refractory metal alloys are reasonably well known and perform well at the right temperatures and atmospheric pres- sures, but must be carefully selected for ductility. 2. For temperature applications above about 1100°C-1200°C (1373°K-1473°K) regardless of alloy type (metallic base, ceramic base, carbon base), only limited data are available for lifetimes in excess of 100 h or even for lifetimes in excess of only a few hours although there are important exceptions. Obviously, low-mass struc- tures should be emphasized. 3. Coatings may be required for advanced materials operating at high temperatures for significant periods. This area has received very little funding, yet it is critical for the selection of appropriate materials. CONCLUSION AND RECOMMENDATION Based on the discussion in this chapter, the committee arrived at the following conclusion and recommendation. Conclusion 5: Major advances in materiab, components, and power system technology wiB be determining factors In making SDI space power systems viable. Achieving such advances wig require skins, time, money, and significant technological innovation. The development of adequate power supplies may well pace the entire SDI program. Recommendation 5: Male additional and effective investments now in technology and demonstrations leading to advanced compo- nents, including but not limited to: thermal management, including radiators; materials structural, theImal, environmental, and super- conducting; electrical generation, conditioning, twitting, transmission, and storage; and long-term cryostorage of H2 and O2. Advances in these areas will reduce power system mass and environmental impacts, improve power system reliability, and, in the long term, reduce life-cycle power system cost.
