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2 Space Power Requirements and Selection Criteria OVERVIEW OF SPAC~BASE:D POWER REQUIREM1 :NTS Power system requirements for U.S. space applications can be con- sidered based on the needs of three categories of users: SD! systems, military systems other than SDI, and civil missions. At this time, the requirements of these user categories have only been broadly defined. Summaries follow of these three broad sets of requirements and their commonalities. SD! Power Requirements for Housekeeping, Alert, and Burst Modes New U.S. requirements for spac~based power imposed by the SDI program greatly exceed space power system capabilities available from past civilian and military experience in spacecraft. In addi- tion, SDI systems and their power subsystems must be survivable in wartime, as discussed later in this chapter, and in the face of possible peacetime attrition attacks while traversing Soviet territory. SDI applications require electrical power for directed energy weapon (DEW) and kinetic energy weapon (KEW) systems; for 9
10 AD DANCED PO WER SO URGES FOR SPA CE MISSIONS surveillance, acquisition, tracking, and kill-assessment (SATKA) sys- tems; and for command, communications, and control (C3) SyS- tems. SDI's requirements also include the integrated and mission- coordinated power-conditioning technologies that are needed to con- vert the source power into the form required by the specific Toad being driven. One or more power systems are needed by each SDI spacecraft, potentially at multikilovolt levels, to satisfy power needs categorized by three modes of operation: . Housekeeping mode. Electric power ranging from several kilowatts to tens of kilowatts (or hundreds of kilowatts, if refrigera- tion of cryogens is necessary) is needed continuously for long periods of time durations of up to about 10 years for baseload operation of the space platform, including communication, station-keeping, and surveillance systems. A typical household consumes energy at the average rate of about 1 kW (i.e., 1 kilowatt-hour per hour). · Alert mode. In the event of a hostile threat, powers from about 100 kW to about 10 MWe might possibly be required. Cur- rently, among the three SDI power modes, the alert-mode require- ment is the least clearly defined. The total duration of power needs while in an alert status or during periodic testing might even be a year or more. The power level and duration required for the SDI alert mode appear to depend on a postulated operational cycle that is not easily defined, and may also include power for periodic status- checking. Alert-mode power requirements are likely to be higher than can be accommodated with energy storage at reasonable energy storage system masses. Otherwise, either a power system probably nuclearwould have to be provided or excessive storage capability would be required. Accordingly, unless considerable effort is made to develop SDI systems that minimize the alert-mode requirement, there may be so many kilowatt-hours of energy storage needed- especially if nonnuclear power subsystems are used- that the prime power subsystem would become a major factor in sizing the orbital platform. ~ Burst mode. For weaponry and fire control during battle, power needed for the burst mode may extend from tens to hundreds of megawatts (and beyond) for durations of a few hundred to a few thousand seconds, and these power levels must be available quickly on demand. Commercial power plants fall into about this power range. For the alert and burst modes, the unprecedented high power
SPA GE PO WAR REQUIREMENTS TABLE 2-1 Salient Features of the SDI Housekeeping, Alert, and Burst Modes SDI Mode Duration Start-Up Availability Power Level Housekeeping Many years Not critical Continuous Several to 100e of kWe Alert Burst Uncertain (up to 1 year) Minutes Seconds Minutes Sporadic loos to 1,OOOs (on demand) of kWe Sporadic lOs to lOOs (on demand) of MWe 11 levels, durations, integrated energies, and time-profiles far exceed any current experience with space power systems. Table 2-1 outlines salient features of these three SDI power modes. Power requirements need to be considered for the major potential SDI weapons and sensor systems that the SDI program is pursuing. Those systems include the following: ground-based free-electron lasers (FELs); space-based FELs; ground-based excimer lasers; neutral-particle beam (NPB) systems; charged particle beam (CPB) systems; kinetic energy weapon (KEW) systems; chemical lasers; radars (radio detecting and ranging systems); and lidars (light detecting and ranging systems). The SD! Space Power Architecture System (SPAS) studies (1988) indicate that current ground-based versions of FELs and excimer lasers would require prime power in excess of 1 GWe (perhaps as high as tens of gigawatts) at each site; power needs for space versions of these devices are yet to be determined. Space-based free-electron lasers, charged particle beams, and neutral particle beam systems may require from 50 to about 200 MWe per platform; chemical lasers may require only tens of kilowatts. As summarized in Table 2-1, space-based weapons platforms will require continuous housekeeping power of tens to hundreds of kilo-
12 AD LANCED PO WER SO URGES FOR SPA CE MISSIONS watts (perhaps into the megawatt regime for advanced radar/sensor platforms), depending on the specific system. Power will be needed for refrigeration, communications, radar, and other continuously op- erating systems. Most SDI space systems are expected to be deployed at altitudes ranging up to a few thousand kilometers, while SATKA platforms, communication/weapon relay, and other special-purpose platforms may be deployed in a range of low earth to geosynchronous orbits. System life is ultimately intended to exceed 10 years if inter- mittent servicing is feasible on an as-needed basis. These power requirements are extremely critical to the design of any orbiting platform; severe mass and cost penalties accompany un- due conservatism with respect to power level or duration. Depending on the specific SDI system, power subsystems are estimated in the SPAS studies to make up some 2~50 percent of the total mass of the space platform. The SPAS studies and related data showed that no present inte- grated technologies could satisfy these ranges of power requirements. Even for the 1990-1995 period, initial estimates of prime power re- quirements for electrically energized DEW systems tests at the White Sands Missile Range indicate that extremely high power levels hav- ing fast time-ramping capabilities must be provided during the tests. Only highly efficient prime power or power conversion technologies could qualify for space-based versions of such applications. Although the major challenges in developing power technology for SD! applications are associated with space-borne systems, SDI also has power requirements for high-power, ground-based weapons systems. In addition to sources of prime power for ground-based and space-based SDI systems, new forms of energy storage for delivery of the burst mode in space and on the ground may be needed to meet the simultaneous requirements of power level and running time. Satisfying requirements for "instant-on operation would necessitate development of new ways to switch both power sources and loacis. An experiment likely to be relevant to testing the ability for rapid start- up of both ground-based and space-based power systems is expected to be performed as part of the SDIO superconducting magnetic energy storage (SMES) project now under way. A space environment poses many problems affecting integration and feasibility that have not been previously encountered in either ground-based or space-based systems. Examples of major concerns include source-to-Ioad power transmission; the close physical prox- imity of source and load; and the large magnitude of the power being
SPA GE PO WER REQUIREMENTS 13 transrn~tted compared to current power levels (a few kilowatts) being used in space. Requirements of Military Missions Other than SD] A number of non-SDI military space missionsfor example, those of the U.S. Air Force will require advanced power sources for applica- tions that include surveillance, tracking, and communications. Such applications (Johnson, 1988) focus on hardened surveillance systems as well as electric propulsion systems for orbital transfer vehicles to position military assets in more favorable high orbits. These appli- cations are provisionally projected (USAF/DOE, 1988) to require perhaps 5 kWe in the near term, up to 40 kWe for the midterm, and up to hundreds of kilowatts in the long term; steady generation of power will be the rule. Such space power requirements are techni- cally satisfiable with nuclear reactor or solar power systems, but the size of the required solar arrays could present problems relating to detectability or maneuverability. Current U.S. activity toward devel- oping a space nuclear reactor system (known as SP-100) is directed toward achieving a nominal 100-kWe power output. Subsequent vari- ants of that design may be possible over the power range from 10 to 1,000 kWe. Power requirements for military, non-SDI space applications will probably overlap those of civil space missions. Military spacecraft requirements include power both for electric propulsion and for on- board uses. A significant additional requirement that pouter systems for military applications must satisfy will be their survivability in the presence of a hostile threat. Survivability considerations include needs for military spacecraft to be maneuverable and to have both the capability of being hardened against enemy weapons and of avoiding detection. These considera- tions impose certain constraints on candidate space power system, such as the size of solar arrays and the temperature of radiators needed to reject heat to space. Power requirements for non-SDI military missions can proba- bly be satisfied with solar dynamic or small nuclear reactor power systems. The choice between using a solar or nuclear system may depend on various factors, especially specific mass (measured in kilo- grams per kilowatt). Future use of advanced Brayton or Stirling cycles could make the solar dynamics option competitive with the nuclear option at power levels of 60 kWe or greater. On the other
14 ADVANCED POWER SOURCES FOR SPACE MISSIONS hand, using dynamic power cycles in space nuclear reactor systems could make nuclear systems more attractive from the standpoint of reducing their specific masses. Requirements of Civil Missions Among its current approved missions, NASA's largest projected near- term power need is for the Space Station. Future missions, such as establishing a lunar base and traveling to Mars, will probably require significantly greater power. Solar power systems will suffice for most NASA requirements in earth orbit, but space nuclear reactor systems will probably be needed for planetary and deep-space missions, as discussed in a survey of such needs by Mankins et al. (1987~. NASA options such as space-based materials processing facilities, located in earth orbit or on the lunar surface (ColIaday and Gabris, 1988; Ride, 1987), would have power requirements in the hundreds of kilowatts or greater. The Jet Propulsion Laboratory survey (Mankins et al., 1987) of possible NASA needs for nuclear power sources lists approximately 20 possible missions with power requirements ranging from tens to hundreds of kilowatts. For Phase 1 of the Space Station's development, 75 kWe of average power* will be available from a system of photovolta~c arrays and storage devices. The total area of the solar cell arrays needed to achieve this average power level exceeds 2,000 m2. Although the earth's atmosphere is extremely tenuous at the station's orbital altitude, atmospheric drag on this very large area of solar cells would periodically require reboosting of the station itself to maintain its orbital altitude. NASA plans to reboost the station by burning gaseous oxygen and hydrogen, obtained by electrolyzing excess water; thus, reboosting would not require fuel supplies from earth. Once started, photovoltaic space power systems used for civil applications typically operate reliably at their rated average powers for their entire useful lives. Usually the spacecraft for such missions rotate around the earth and experience day and night during each orbit. Thus the energy input into these~power systems will ramp up and down once each orbit, necessitating reliable power conditioning and on-board storage. For Phase 2 of Space Station's development to increase the average power generated to 125 kWe (300-kWe peak)NASA's 1987 *Requiring a peak power input of about 200 kWe.
SPACE POWER REQUIREMENTS 15 planning called for adding two solar-dynamic power systems* to the Phase 1 power supply. Because of the greater overall efficiency of a solar-dynamic power system compared to that of a photovoltaic space power system, a solar-dynamic system can produce a unit of power from less collection area than is required for an array of solar cells. The improvement in system efficiency results from advantages in thermal storage versus battery storage and from the increased conversion efficiency of a solar-dynamic power cycle compared to solar cells. Commonality of Requirements Among Citric and Military Missions While the most demanding space-based power requirements are those of SDI, some projected civil or NASA applications under discussion could capitalize on the SDI investment. For example, to operate an outpost on the lunar surface, a power plant suitable for the SDI housekeeping mode may suit the utility needs. Such a power- generating capability might also be applicable to providing future communications satellites with the capability for a direct-broadcast mode of operation. The burst-mode capability might be useful for powering a catapult on the lunar surface, a device that conceivably could be a factor in making mining of the lunar surface (Kuicinski and Schmitt, 1987) practical. The alert and burst modes may also be useful for spacecraft propulsion. These potential applications are speculative, pending further study. Long life and reliability are desirable qualities for all space power systems. In addition, many potential missions that have been studied will have power needs that significantly exceed the capabilities of any previous space power sources. These much higher power outputs will require the development of technologies leading to advanced power system components. APPROACHES TOWARD SELECTING SPACE POWER TECHNOLOGIES TO ME ET SDI REQUIREMENTS Studies completed to date do not provide a basis for selecting a preferred SDI power system or for ranking preferable systems, but *A solar-dynamic power system converts solar radiation into high-temperature heat, then uses the heat to drive a thermodynamic power cycle.
16 AD LANCED PO WER SO URGES FOR SPA CE MISSIONS they do point to areas of leverage. These are areas of technology, mission requirements, and program emphasis where early, careful attention is likely to be cost-effective in: achieving savings in mass, cost, and/or component development time; improving reliability; and in ultimately establishing feasibility (See Chapter 4~. SDI missions impose electrical power requirements far exceed- ~ng the state of the art; in particular, to power weapon systems in the burst mode and to supply high-power-demand sensors during the alert mode. These requirements dictate power systems having capabilities ranging from hundreds of megawatts for hundreds to thousands of seconds to supplying several megawatts for times to- taTing as much as a year to support system operations under alert conditions. Bimodal operation providing both continuous and burst power capabilities may effectively address the combined mission re- quirements. The multimegawatt technology task of the SDIO Power Program should address these needs by providing for development of an integrated power technology base that considers both nuclear and nonnuclear multimegawatt power sources and combinations of those . sources. For each concept, the following related electrical power supply subsystems should be considered: energy source (a source of heat or voltage); heat transfer and rejection (thermal management); power conversion; energy storage (if needed); power conditioning and control; power transmission; and . transient performance. During this consideration of possible multimegawatt power source concepts, a parallel program of review, analysis, and test- ing of applicable technologies should be conducted to ensure that feasibility issues associated with the systems concepts can be re- solved. In many cases, proposed power system operating designs will result in extremely stringent operating conditions, including high temperatures, high pressures, and corrosive substances. The effects of radiation, micrometeorites, space debris, and microgravity on sys- tem operating components and materials must also be considered. Research issues include demonstrating technological feasibility for such considerations as:
SPACE PO WER REQUIREMENTS 17 . long-term autonomous operational reliability of high-power systems in both natural and perturbed space environments; . minimizing system mass and size; employing higher temperatures; and . using lower-mass structural and shield materials. The technology overview should definitely include the following factors peculiar to nuclear power sources: radiation safety; reactor fuels; neutronics and control; shielding; and reactor thermal hydraulics. In addition, relevant technologies should be included that affect all power sources, such as the following: . materials; thermal management; energy storage; and . energy conversion and storage. Several critical issue areas in satisfying SDI space power require- ments are discussed below without attempting to rank them by their relative importance; Al of them may be vital. Critical Issue Areas Figure 2-1 (based on the SPAS studies), which does not include weapons coolant mass, illustrates the sensitivity of the specific mass (measured in kilograms per kilowatt) of space power systems to two critical assumptions: open versus closed cycles* and operating time. For example, an open-cycle space power system might combine hy- drogen and oxygen, then discharge the resulting water to space. There may be effluents from the spacecraft even if a closed-cycle power system ~ used, since military weapons in the spacecraft pay- load may require a coolant such as liquid hydrogen, which can then be made available to the space power plant as fuel before being dis- charged. It is tempting to conclude from Figure 2-1 that open cycles *Power systems are classified into those that utilize an open cycle or a closed cycle, according to whether they discharge or recirculate a working fluid, respectively.
18 ~- _ _ An en at ~ _ C: _ Cat _ Q Ct) ~ .2 LL] ~ ~ _ Cf) ~ AD DANCED PO WER SO URGES FOR SPA CE MISSIONS CLOSED CYCLE NUCLEAR (very high mass due to radiators; no consumables) OPEN CYCLE NUCLEAR (intermediate fixed mass plus relatively modest mass of consumable hydrogen) ~ ~ OPEN CYCLE CHEMICAL (low fixed mass plus relatively large mass of consumables such as hydrogen and oxygen) - 0 200 400 600 800 1000 1200 1400 1600 RUN TIME (see) FIGURE 2-1 Sensitivity of system-specific mass to choice of power system and to duration of power use. (Masses of consumables required for weapon cooling are not included in the calculations.) SOURCE: Space Power Architecture System studies, Sandia National Laboratories, and NASA. must be made to work at least for the burst power mode or the entire SDI concept may be very severely penalized. For the higher-power burst mode, the need for hundreds of megawatts rising from zero or near zero to full power in a few secondsfor a comparatively short period unposes drastic demands on the system designer. It is presently unclear what penalties are ex- acted as the price for achieving rapid (i.e., several seconds) start-up times; increasing these times by a factor of two or more could reduce system mass and complexity. With the exception of turbines, all of the power system components being proposed for space applica- tions are massive. Those components include magnetohydrodynam- ics (MHD) channels, radiators, fuel cells, and power-conditioning equipment. To minimize total system mass aboard a spacecraft, open-cycle systems that exhaust their working fluid into space are an attractive option. To be successful, this option must include a means to cope with possible adverse effects of releasing effluents. Both the effluent question and the rapid start-up consideration are issues that suggest
SPA GE POWER REQUIREMENTS 19 the necessity of careful review of mission requirements and of the de- sirability for emphasis on weapons concepts that require only modest power levels. System Considerations Satisfying SD! space power requirements necessitates a system ap- proach. Descriptions of nuclear and nonnuclear options sometimes overlap in the following discussion, and some hybrid systems concepts must be covered under the description of nuclear options, particularly in their relation to driving the development of electrical component technology. Three Space Power Architecture System (SPAS) studies (1988) were performed for the SDIO Power Program Office. The studies were designed to consider and analyze system factors in SD! archi- tecture that define space power requirements in scale, in state of technology, in time, in transient capability, and in reliability. The SPAS studies were also intended to provide guidance for making step improvements in system performance through integrated technology development. The SPAS studies addressed indiviclual space power system on tions. However, the spacecraft power supply needed to satisfy re- quirements for the three SDI power modes (housekeeping, alert, and burst modes) may well not be a single system, but rather an integrated set of generating and power-conditioning systems that optimize total-life performance and reliability. Such approaches may well enable power systems that would otherwise remain impractical. Life-cycle costs will likely be a major factor in the selection of all weapons systems. However, cost considerations were not included in the SPAS studies. Qualification of Power-Conditioning Subsystems and Components To qualify for meeting SDI requirements, there must be an adequate experience base for power-conditioning subsystems and components. The committee believes that projections of component performance must be developed based on: an experimental data base for component performance; analytical models that are anchored to the data base and that permit future capability to be projected; and
20 ADVANCED POWER SOURCES FOR SPACE MISSIONS . basic research that anchors the mode! to fundamental pro- cesses. The data base, models, and fundamental understanding must provide technology projections (at required reliability) to assess where additional funding is required. Considerable experience has been obtained for power-conditioning components used with pulses, including elements such as thyratrons, diodes, capacitors, inductors, and transformers. In contrast, less experience has been obtained in the areas of power conversion and conditioning components, such as high-voTtage inverters, alternators, generators, and compuisators. Experience suggests that there will be an optimum load-driven power-moduTe size, as found by designers of accelerators, radar, and electrical power systems many years ago. An analysis of this nature can be applied to SD! power needs and matched to the megawatt average power class of most of the conversion devices. The optimum module size will depend on conversion efficiency, thermal manage- ment, power flow, and voltage levels, and may be in the same power range already experienced in the very-high-power radar and fusion fields; namely, between 1 and 10 MWe. Keeping components and subsystems small and modular also enables local control of faults and minimizes development time. Local fault-control approaches are likely to be required for these very-high- power systems, since only a short period of delay in clearing a fault will destroy the power system. Influence of SD! Survivability and Vulnerability Criteria A fundamental SDI requirement is that a space power system, like any SD! system, must be technically effective, cost-effective, and survivable in the face of natural or hostile threats. These three cor- nerstone requirements are known as the Nitze criteria. Attaining SDI goals of crisis stability and arms-race stability would require satisfying these criteria before system deployment. The difficulty of simultaneously satisfying all three Nitze criteria can lead to frustra- tion, which can motivate finding a creative solution for providing power or developing weapons that require less electrical energy. Assuring a high probability of survival of each system element can be quite costly, both in economic and launch-weight terms, hence survivability is best treated as a system issue. Accordingly, the sys- tem designer must balance capabilities for maneuvering, shooting back in defense, decoying, and hardening to provide the required
SPACE PO WER REQUIREMENTS 21 system survivability at minimum cost and launch weight. System trade-offs must include consideration of uncertain parameters such as the threat and technical effectiveness of postulated weapons. These and many other uncertainties tend to lead one to delay trying to satisfy the survivability requirement of system components using ad- vanced technology until that requirement has been more definitively specified and validated. Pending such a definitive specification, SDIOin conjunction with the lEG and other advisory groups has adopted (SDIO, 1986) the interim approach of formulating a list of general guidelines for survivability. Values are listed in that publication for maneuvering, hardness against x rays, and so on, and are probably satisfactory as interim survivability guidelines except for platforms in Tow earth orbit. Although minimal, these survivability requirements are nev- ertheless very stressing, hence applying them in the meantime to evaluate the relative survivabilities of otherwise comparable candi- date technologies may promote some progress. There are differences in viewpoint as to how early in the system development cycle one should consider survivability requirements and when there should be an insistence on high levels of survivability. If the system design evolves without survivability in mind, compromises to benefit one criterion may jeopardize survivability. For example, having hydraulically interconnected parallel paths to the many pan- els of a heat exchanger improves reliability but makes the system fatally vulnerable to a single hit. Incorporating survivability consid- erations from the outset might lead to thermally interconnected but hydraulically separatedcoolant loops for both reliability and sur- vivability. Some technologists prefer to emphasize the survivability criterion from the outset, while others recommend postponing survivability is- sues. The first group argues that applying the criterion early would avoid pursuing inappropriate technologies and would also stimulate new ideas that might be able to satisfy all of the criteria simul- taneously. The other group recommends allowing initial research and development on candidate concepts to proceed unfettered by survivability constraints, in order to avoid the risk of prematurely precluding any promising but undeveloped options. Many technologists are in the first group, while many system architects, such as those who performed the SPAS studies, are in the second group. In the SPAS studies, none of the power systems the contractors exarn~ned were hardened prior to estimating masses.
22 ADVANCED POWER SOURCES FOR SPACE MISSIONS While the committee sees merit on both sides of this issue, it reached no final conclusion as to when in the development cycle survivability should be emphasized. It was apparent that the subject could be handled only at the SDI system level, not solely at the power system or component level. Rigidly applying survivability concerns to space power systems now would mean there could be comparative studies of only hardened power systems, there could be no development of the largest space- based consumers of electrical power (those requiring more than about 100 MWe), and weapons requiring minimal energy per kill would be favored. Such restrictive actions at this time are unwarranted. Accordingly, Recommendation 7 below would elevate the con- cern for survivability to acting as a stimulus to innovation in the development process, and at this stage of exploratory development the committee regards that stimulus as sufficient. Findings, Conclusions, and Recommendation Based on the preceding discussion, the committee developed the following findings, conclusions, and recommendation: Finding 1: Of the three significantly different SD! modes of operation (housekeeping, alert, and burst mode), requirements for the alert mode are inadequately (refined, yet they appear to be a major design dete~iinant. llor that mode, the unprecedented high power levels, durations, and unusual t~me-profiles as well as the associated voltages and currents that are envisioned avid usually make extrapolation from previous experience quite risky and nnreli- able. A possible exception is In the area of turbine technology, where an adequate range of power levels has been validated for terrestrial applications, although not for spacedight. Proposed space power systems will need to be space-qualified for long-term unattended use. Finding 5: Among the power systems that are candidates for SD] applications, the least massive, autonomous self-conta~ned space power systems currently being considered entail tolerance of su~ stantial amounts of effluent during system operation. The feasibility of satisfactorily operating spacecraft sensors, weapons, and power systems In the presence of effluents is still nnresol~red. Conclusion 1: Mult~megawatt space power sources (at levels of tens to hundreds of megawatts and beyond) win be a necessity if the
SPACE PO WER REQUIREMENTS 23 SDI program is to deploy electrically energized weapons systems for ballistic missile defense. Conclusion 4: The rate of rise (~ram~rate~) from zero to full burst-mode power ferret appe are to be a critical requirement. It is not apparent to the committee what relationships exist among elapsed time for power buildup and system complexity, mass, cost, and reliability. ConcI~ion 8: Survi~rabUity and ~uInerabUity concerns for SD! space power systems have not yet been adequately addressed in presently available studies relevant to SD! space power needs. Recommendation 7: After adequate evaluation of potential threats, *farther analyze the subject of vulnerability and survi~rabdity, mainly at the overall system level. Data renting from implementing Recommendation 1 would be appropriate for this analysis. Pending such analysis, candidate power systems should be screened for their potential to satisfy Veteran SDIO survivabUity requirements, re- serv~ng judgment as to when or whether t_ose requirements should constrain technology development. Convey the screening results to the advocates of those candidate power systems, to stimulate their finding ways to enhance survivability as they develop the technology.
