Click for next page ( 53


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 52
4 Needed Technological Advances in Space Power Subsystems to Meet SDI Requirements IMPLICATIONS OF SDI SPACE POWER ARCHITECTURE SYSTEM STUDIES FOR ADVANCES NEEDED IN POWER SUBSYSTEMS The following seven listings indicate key features of some space power systems selected by SD! space power architecture studies as being capable of providing powerin the relatively near term at the levels indicated. At maximum power levels greater than 100 Mwe: 1. open-cycle, gas-cooled fission reactor + turbine; 2. open-cycle, H2 + O2 combustion + turbine; 3. closed-cycle, Brayton, gas-cooled fission reactor + turbine; and 4. closed-cycle, Rankine, liquid-metal cooled fission reactor + turbine. At power levels of 10MWe or less: 5. closed Rankine cycle, fission reactor; 6. closed Brayton cycle, fission reactor; and 7. thermionic conversion, fission reactor. These selections were provided to the committee in the form of prepublication results obtained from three simultaneous, indepen- dent studies of Space Power Architecture System (SPAS) options 52

OCR for page 52
NEEDED ADVANCES IN SPACE POWER SUBSYSTEMS 53 (1988~. These SPAS studies were performed under contract to the SDIO Power Program Office. It should be noted that the studies ap- parently did not make allowances for system survivability, leading the committee to Finding 3 below. Abbreviated descriptive summaries of some results of the SPAS studies are shown in Tables 4-1 and ~2, and in Figure 4-1. These exhibits make no allowance for the mass of the hydrogen used for weapons cooling or for H2-O2 combustion. The entries in Table ~1 for masses of the open-cycle systems (first two columns) do not include the masses of hydrogen needed for cooling the reactor or for burning in the turbine. Rather, the table assumes the specific direction given to the SPAS contractors: namely, that the hydrogen for these purposes is available "free" for example, with no mass penaltyfrom the weapon system. On the other hand, coolant required for the 1,800-s burst is included in the masses for the closed-cycle systems. According to the Sanda people who prepared Table ~1 (Cropp, 1988), the turbine design for the H2-O2 combustion system was optimized assuming free hydrogen, so that simply adding the required mass of hydrogen will somewhat overestimate the overall system mass because of mass tradeoffs between hydrogen mass and turbine mass. Another view of system mass comparisons is shown in Figure 4-2, from the lEG Field Support Team's critique of SPAS contrac- tor reports. This figure shows an overview of the power system masses (exclusive of power conditioning) calculated by the SPAS contractors in terms of specific mass as a function of run time. The bands labelled "Open power systems," "Closed power systems," en c! "Closed thermodynamic cycle power systems" indicate the envelopes of contractor-calculated masses for these classes of systems. The mass of hydrogen is included in all of these specific mass figures, as is seen from the increases in mass with run time for all but the thermodynamic cycle systems. In Figure 4-2, the power systems that produce effluents that are discharged into space (which are referred to as open power systems) include (where TRW means TRW, Inc.; MM means Martin-Marietta, Inc.; and GE refers to the General Electric Co.~: TRW Nerva-derivative reactor MHD (least massive) TRW gel MHD TRW Nerv~derivative reactor turboalternator TRW H2-O2 combustor turboalternator GE Nerva-derivative reactor turboalternator GE pebble bed reactor MHD

OCR for page 52
54 _~ cn o v ._ h ._ h m a' m U) . _ o ._ C: 0 o o' 1 o o - 3 o o U: m m :^ U] 3 o P~ C~ V] o a, m o m m oo o o oo ._ h .0 V 1 m :~ 3 .o ~ . ~ a) ~ 6 ~ ~ o ~ 3vo o . _ ~ ~o o >, $ ~ O U: ~ ~ ~mcn - ~ @~ O ~ O O ~ ~U2 b~ - ~ @~ O ~ ~ U: ~ U] O 0 1 1 1 1 1= 0 1 1 1 01 O O 0 1 ~ C53 1 U: ~ C~ ~ U: t- O 0 1 I. . . O o o e~ U: - e~ . 00 l 1 ~ 1 0 1 1 1 1 1 C~ 0 1 1 1 0 I ~ I I I . O U. O O 5~ ~Q ~ I ~ o3 ~ ~ ~L o C) , C~ m _. ~ _ _ O O O V 4= a' o o v l I ~ 1 e~ 0 oo . . . cO o: h 0 _ 3 ~ ._ o U, QL o b~ L' o oo oo a, Q ~ h ~; ~ O E~ V 1 1 1 1 1 1 h o C. S" ct S~ C) O CO I-' .~ O ~ ;>

OCR for page 52
55 o ~ ~ CO . . . . o o Do o ~ CO . . . . o Cal CO o ~ o - o ~ C9 o o ~ CO o ~ ~ ~ o ~ ~ Do O (D O en 0 1 00 00 0 1 Cal O o 49 ~ ~ CD 0 1 0 is_ o ~ Cot o ~ C~ 0 1 U. U3 . . . . o ~ oo o ~ ~ ~ o .= o ._ . ~ ~ o 8 o o C. ~ 3 3 bO.= O P~ ~ ~ - a o ~ c) a) Q v ~ =: Q ~ ._ s~ C >, ~ O.> ~ ~ V ~ ~ ~ _ a~ cts ~ c: ~ ~ ~ ~ S o n:5 ~ ~ ,= V O ~ ~ ~ O . ~ ~ ~ ~ ~= ~ a, ~ ~ b~ (6 ~ ._ ~ ~ ~ ~ ~ O O bO =-O S ~d ~ ~ ~ .g- v ~ ~ ~.= ~ 3 ~ ~ ~ S 3 D ~ ~ ~ .= ~ D ~ o ~ 0 ~ g a, O 3 D ~ ~ ~ ~ ~ ~ v 3 ~ 3 .= ~. C o ~ ~ g ~ ~ 30 D ~ ~ <,, S O O ~ D o: E Q E ~ E ~ E =~ ~ oo v ~ 0 0 v -= ~ ~V~ bO a, ~ ~ ~ ~ ~ ~ V ~ ~ ~6 ~ ~;, 41, 0 3 ~ ~ :^ ~ a) ~ ~ ~ ~ ~ ~ :^ ~IQI bev! ~1 ~ Co ~

OCR for page 52
56 - .m ~ a: o - ~ > ~ c o o O G.) Go c c O O -- is; A m O Y 0 ~ 0 0 ~ V V a' ~ .. O GO ~ . fin U) . ~ U) U) 6,0 ~ cn . 0.) U) :- a) so To m ~ a) te on At, ~ p4 us ~ us O cd _ ~ ~ .a) . a) ~4 Do a) _ ~ ,,, 3 0 ~ ~ ~ ~ a) ~ ._ 'A a) v O 0O ~ ~ U2 3 ~ :> 0) u, O ~4 ~ ~ ~ . - 0 0~= 0O c s" ~ 0< ~ ~ 3 V a .= ce U2 0 ~ ~ ~ a.~, U~) . ~ ~ ~ _ Cd 0 ~ ~ s~ 0 V I-. ~ O _~ ~ 0 I ~ 0 ~4 ~ _ 0!U!wJaq1 UO~eJg au!~ue o!uo!wJau UO~BJ~ OU!~UE * o!uo!u`Jau, UO~BJg au!~ue~ 0!Uo!wJag UO~eJg au!>lue~ o (M1/61) ssew o!~!oads wa~s{S u) - a' c) a au

OCR for page 52
NEEDED ADVANCES IN SPACE POWER SUBSYSTEMS TABLE 4-2 Multimegawatt Space Power System Comparison (10-MW, 1-year operation; masses are in metric tons) Component Power System Rankine 13rayton Thermionic Reactor 1.1 5.5 12.8 Shield 1.1 4.6 6.6 Turbine and generatom 7.4b,c 0.8b'C -- Compressor -- 0.5 -- Vapor separator 3.8 -- -- Power conditioning and generator radiator 1.4 1.4 2.0 Power conditioning 2.0 2.0 5.0 Radiator 1 1.4b 31.0b 30.0 Miscellaneous 2.8 4.6 5.6 TOTAL 31.0 50.4 62.0 a It is assumed that the specific mass of a standard generator is 0.05 kg/kW. The mass of a cryogenic generator may be lower by a factor of two. b No allowance is made for weapons cooling. c Although some turbine mass differences are expected between Brayton and Rankine systems, these differences may be too large. Consistent algorithms for power conversion masses are still being formulated. SOURCE: Sandia National Laboratories and NASA, Independent Evaluation Group Field Support Team, using reference models they developed prior to the Space Power Architecture System (1988) studies. GE open H2-O2 fuel cell GE H2-O2 combustor turboalternator GE Li-HC1 battery MM H2-O2 combustor MHD (most massive) MM H2-O2 combustor turboalternator MM open H2-O2 fuel cell MM combustor turboalternator (no H2O) MM Nerva-derivative reactor turboalternator 57 The power systems that generate chemical products that are retained (known as closed power systems) include TRW ice-cooled H2-O2 fuel cell (least massive); MM ice-cooled H2-O2 fuel cell with radiator; TRW closed combustor turboalternator (most massive); and TRW lithium-metal sulfide battery.

OCR for page 52
58 3.50 3.00 - Q) 2.50 - cn cn C) 11 CL co 2.00 1.50 ~ 1.00 Oh 0.50 0.00 AD VANCED PO WER SOURCES FOR SPA ClE MISSIONS Closed Thermodynamic Cycle Power Systems No Power Conditioning Mass Masses Include Hydrogen - - .' - - . . I' . ' .' ~ Closed Power Svstems - ~ . ~ _ _ _ ~~ ( Open Power Systems - ~ _ 0 200 400 600 800 1000 1200 1400 1600 RUN TIME (see) FIGURE 4-2 Mass as a function of whether the system is open or closed and of run time. SOURCE: Sandia National Laboratories, Independent Evaluation Group Field Support Team, based on inputs from Space Power Architecture System contractors (1988~. The closed thermodynamic cycle systems are Martin Marietta's reactor-powered Rankine and therm~onic systems, which use radia- tors to reject waste heat. Figure 4-3 is a block diagram showing a closed-cycle Brayton space power system energized by a nuclear reactor. Figure 4~4 is a similar diagram for an open-cycle space power system energized by the combustion of hydrogen and oxygen. Note that weapons cooling is diagrammect in Figure 4~4. The above-mentioned descriptive summaries are based on three SPAS studies performed by SDIO-supported contractors who, unfor- tunately, employed an inconsistent set of assumptions. Consequently, there are necessarily differences in the three sets of results that are difficult to interpret, a problem recognized by the SDIO Power Pro- gram Office's technical team charged with interpreting the SPAS results. This problem is especially severe in comparing estimates of system mass. In that regard, this team noted significant differences in assumptions among the contractors along with overall limitations in the assumptions pertaining to the following technological and packaging considerations, to which the mass estimates are sensitive:

OCR for page 52
59 ~ "l ~ t G O Z I 1 L CC o LLJ L l 1 l LL] Z m \ 1 l ~ _ ~1 U] z cB ._ o o ,Q c3 - o ._ Cal z ._ V] v a U) :^ to do Cal Cal so 1 or -0 CC o :^ cd m 1 o C) Cal C) o - v r ~ , O \ I) - cn llJ ~ \ 1 ~ 1 8 \ ~~ Cal 1

OCR for page 52
60 . needed. ADVANCED POWER SOURCES FOR SPACE MISSIONS . High-voltage power systems perform well with tube radiofre- quency (RF) systems. . Low-voltage power systems perform well with solid-state RF systems. High-voltage alternators save mass, since no transformers are ~ Cryo-cooled power conditioning, if realizable, saves mass, hence conductors, transformers, and other components can be less massive. . Mass estimates were based on conservative near-term or on optimistic far-term assumptions regarding technology. Masses required for thermal management and packaging were not uniformly considered. exist. The technology postulated for power conditioning does not Tables 4-1 and (2 warrant comment, in view of the fact that the information they contain became the bases for several of the committee's findings, conclusions, and recommendations. In Table 4-1, masses for several burst-mode space power systems are quoted in metric tons. For the convenience of those unaccustomed to thinking in those units, Table 4-3 shows the range of system masses from smallest to largest. Assuming typical costs per pound for development, production, and launching to orbit, and noting that the power system may range from 20 to 50 percent of the total orbital vehicle mass, these systems appear to be very large hence probably prohibitively expensive and too massive to lift into orbit with any practical launch vehicle, unless they were launched separately and assembled in orbit, thus motivating Conclusion 2 below. A further difficulty encountered in Table 4-2 is the significant difference in reactor masses between the Brayton and Rankine sys- tems. This large discrepancy resulted from the fact that the two sets of results were obtained by separate contractors who used differ- ent technical assumptions, some of which may be questionable. The range of their results for reactor and turbogenerator masses of two multimegawatt space power systems is shown in Table 4-4. These differences contributed significantly to the committee's reluctance to recommend, with any assurance, either the selection or elimination of any candidate space power systems. Figure 4-1 expresses the above results in terms of system-specific masses (in kg/kWe), for gross power levels of 1, 5, 10, and 20 MWe. In this

OCR for page 52
61 z ~ o c 6 LL ~ Z <= z z O LL 5 Z o o ~ C) 1 o C~ z IL CD ~ LL 1 LLl I LL ~- I LU \ \ LL \ Z m \~ ( 1 _ 51 :~'L .__x ~ ~ ~ i1___O,. ~ _ 1 _ _ _ O. J , z~ O IL 11 llJ ~ ~ cr LL cn 1 Z~ ~ LL 6 cn llJ O: ~ IJJ cn o ,= P4 ;' _ P4 eq o 1 _-! . _ V :^ o P4 Q 3 ~ o ~ . L. o o ~ ~o o = o 1 ~ _ o C~ o ~,, o 1 o . . U] ~ ~ u)

OCR for page 52
62 ADVANCED POWER SOURCES FOR SPACE MISSIONS TABLE 4-3 Range of System Masses of Various SPAS Burst-Mode Space Power Systems System Mass System Metric tons Kilograms Pounds Smallest 148.5 148,500 326,700 Largest 2,908.4 2,908,400 6,398,480 NOTE: For comparison, the payload capacity of the Space Shuttle is currently about 45,000 pounds, and the largest U.S. heavy-lift launch vehicle could lift less than the 326,700 pounds stated above. SPAS = Space Power Architecture System. TABLE 4-4 Range of Reactor and Turbogenerator Biasses of Two SPAS Multimegawatt Space Power Systems Reactor Mass System Metric Tons Kilograms Pounds Reactor (Rankine) 1.2 1,210 2,662 Reactor (Brayton) 5.5 5,500 12,100 Turbine and generator 7.4 7,400 16,280 (Rankine) Turbine and generator 0.8 800 1,760 (Brayton) NOTE: SPAS = Space Power Architecture System. form, the difference between Rankine and Brayton cycles is open to question. Perhaps most striking is the difference between these cycles for reactor plus shield, a factor of 5 to 6 at all gross power levels. Figure 4-3 indicates, for one particular system, the integration of power system and weaponat least to the extent of using a common source of hydrogen. In other systems studied, there is no explanation of the extent to which the power system was reviewed in the context of the entire orbital vehicle. Consequently, the committee cannot assess either penalties or advantages that might be encountered in making the power system an integrated part of the complete orbital platform. These difficulties (Findings 2, 3, and 4) lead the committee to

OCR for page 52
NEEDED ADYANOES IN SPACE POWER SUBSYSTEMS 63 the conclusion that there is still an insufficient basis for making a selection between the power architectures examined; they also motivate the committee's Recommendation 1 to carry out detailed, whole-system design studies. Despite difficulties in comparing the SPAS results, however, the dramatic mass differences between open-cycle and closed-cycle power systems, found by all of the contractors (cf. Fig. 4-2), were qual- itatively adequate to motivate Recommendation 2 regarding the question of permissible effluent. Figures 4-3 and 4-4 highlight the differences between typical closed-cycle and open-cycle systems, re- spectively. ADVANCES NEEDED IN HIGH-TEMPERATURE STRUCTURAL MATERIALS TECHNOLOGY As is evident in Tables 4-1 and 4-2, radiator masses are a large frac- tion of the mass of closed-cycle (Brayton and Rankine) space power systems. As the peak operating temperature (T. measured in K) of a space power plant increases, the heat radiated per unit radiator mass increases rapidly (as TV. Therefore, advances in current materials technology could provide high-temperature, creep-resistant materials that could greatly reduce the radiator mass required (Rosenblum et al., 1966; Buckman and Begley, 1969; Devan and Long, 1975; Klopp et al., 1980; DeVan et al., 1984; Stephens et al., 1988~. For example, if the technology for carbon-carbon composites were sufficiently advanced so as to provide a material for constructing a Brayton cycle power plant, it ~ conceivable that the turbine-inlet temperature could be raised from the 1500K stated in Table 4~1 to 2000K. This temperature increase would reduce the mass of the required radiator by about a factor of three, thereby roughly halving the total mass of such a power system, and would also increase the efficiency of power conversion. Realization of the full potential of such a material, about 2300K (National Research Council, 1988), would reduce power system mass even further. Accordingly, achiev- ing broadly based advances in high-temperature structural materials could provide a basis for dramatic potential gains in power plant performance and corresponding reductions in power plant mass. Use of such materials in space would avoid the need for the antioxidation coatings that are required for terrestrial applications in an oxidizing atmosphere.

OCR for page 52
64 ADVANCED POWER SOURCES FOR SPACE MISSIONS TABLE 4-5 Potential Department of Defense (DOD) Applications of Superconductors for Power Components Power Application Responsible DOD Organizationts) Megawatt power generation (low specific mass: 0.1 kg/kWe) Synchronous alternators Pulsed alternators DC generator exciters MHD generator magnets Megawatt propulsion motor (DC) Power conditioning and energy storage Low-mass, fast-pulsed energy storage Ground-based, slow-pulsed energy storage Low-mass inverter transformer Low-mass inductor components Power transmission lines Air Force, SDI Air Force, Army, BTI, DARPA Air Force, Army, BTI, DARPA, Nary Air Force, BTI Napery Air Force, Army, BTI, DARPA, SDI, Air Force, Army, DNA, SDI Air Force, SDI Air Force, DNA, SDI Air Force, Army, BTI, DARPA, DNA, Navy, SDI NOTE: BTI = Balanced Technology Initiative; DARPA = Defense Advanced Research Projects Agency; DC = direct current; DNA = Defense Nuclear Agency; MHD = magnetohydrodynamics; SDI = Strategic Defense Initiative ADVANCES NEEDED IN POWER-CONDITIONING AND PU[SE-GENERATING TECHNOLOGIES Superconducting Materials Superconductors are potentially useful throughout the power sys- tem/weapon system. The importance of superconductors in power applications lies in their ability to carry large current densities with essentially no resistive losses. Tables 4~5 and 4~6 list potential SDI power- and weapons-related applications, respectively. In view of their potential to operate in liquid hydrogen, the recently discovered high-critical-temperature superconductors could impact many SDI applications if they can be developed into usable forms. As a result of this potentially major impact, a more detailed discussion on these materials is provided in Chapter 5. Increased research in this area is being sponsored by industry, the Depart- ment of Defense, Department of Energy, and the National Science

OCR for page 52
NEEDED ADVANCES IN SPACE POWER SUBSYSTEMS 65 Foundation. These agencies are redirecting funding into high-critical- temperature superconducting materials and their applications. Component Technology The state of the art in power conditioning is adequate to satisfy the needs of most commercial land-b~ed power applications. For those applications, it is sufficient to improve product reliability without developing new devices. For instance, radar designs, both airborne and land-based, are rather standard and use only proven components, resulting in the availability of only a few competing designs. This is done to avoid the cost of developing new components and the subsequent need for a program to prove their reliability. The same TABLE 4-6 Potential Department of Defense (DOD) Applications of Superconductors for Weapons Components Weapon Application Responsible DOD Organizations Directed Energy Laser RF cavities Wiggler magnets Electron beam guidance magnets Particle beam RF cavities Beam-guiding magnets Focusing magnets Kinetic Energy (electromagnetic launchers) Tactical Augmentation magnets (railguns) High-current switches Coil gun accelerators Strategic Augmentation magnets (railguns) High-current switches Coil gun accelerators Air Force, Army, SDI Air Force, Army, SDI Air Force, Army, SDI Air Force, Army, SDI Air Force, Army, SDI Air Force, Army, SDI Army, BTI, DARPA, DNA Air Force, Army, BTI, DARPA, DNA Army, BTI, DARPA Air Force, DNA, SDI Air Force, SDI Air Force, SDI NOTE: BTI = Balanced Technology Initiative; DARPA = Defense Advanced Research Projects Agency; DNA = Defense Nuclear Agency; RF = radio frequency; SDI = Strategic Defense Initiative.

OCR for page 52
66 AD VANCED PO WER SO URGES FOR SPA CE MISSIONS can also be said of much of the electronics associates] with launch vehicles and satellites. Newer applications requiring very fast pulses or very high average powers have met with cli~culties, in that the present state of the art in component technology is generally inadequate to achieve the desired level of performance (Rohwein and Sarjeant, 1983~. These applications have not offered sufficient economic impact to stimulate substantial corporate investment in a new technology base required to establish the next generation of power-conditioning designs. Instead, such applications have attained their rather modest goals through modifications and extensions of existing components or techniques. Although present designs serve very well, they do not scale di- rectly into the multimegawatt range for SDI applications. This dif- ficulty is partly attributable to the emphasis placed on conservative designs in order to obtain the requisite reliability; however, in the multimegawatt range, extension of standard designs leads to imprac- tically large and massive systems. More must be known about the failure mechanisms of critical components. The mass penalty of the large design margins affordable in small systems cannot be toler- ated at high SDI power leveb. Indeed, entirely new components and concepts may be required to achieve SDI objectives. Power must be made available at specific voltage and current levels matching weapons power requirements. FINDINGS, CONCLUSION, AND R1:COMMENDATION Based on the discussions In this chapter, the committee arrived at the following findings, conclusion, and recommendation. Fm~mg 2: The space power subsystems required to power each SD! spacecraft are a significant part of a larger, complex system into which they nmet be integrated, hence the only completely valid approach is to analyze them In the system context. (see Concision 2 and Recommendation 1.) Finding 3: E:xisting space power architecture system studies do not adequately addrese questions of survivability, reliability, main- tainability, and operational readiness for example, availability on very short notice.

OCR for page 52
NEEDED ADVANCES IN SPACE POWER SUBSYSTEMS 67 Finding 4: Existing SDI space power architecture system stud- ies do not provide an adequate basis for evaluating or comparing cost or cost-effectivenese among the space power systems examined. Conclusion 2: Gross estimated masses of SD! space power sys- tems analyzed in existing studies appear unacceptably large to op- erate major space-based weapons. At these projected masses, the feasibility of space power systems needed for hig_-power SD! con- cepts appears unpractical from both cost and launch considerations. Avenues available to reduce power system costs and launch weights include (a) to substantially reduce SD! power requirements; (b) to significantly advance space power "ethnology. Recommendation 1: Using the latest available information, an in-depth *~-vehicIe-system preli~n;nary design study for two sub- stantially different candidate power systems for a common weapon platform should be performed now, in order to retreat secondary or tertiary requirements and limitations in the technology base which are not readily apparent in the current space power architecture sys- tem studies. Care should be exercised in establishing viable technical action and performance requirements, including Livability, maintainability, availability, teammate, voltage, current, torque, ef- fluents, and so on. This study shoed carefully define the available technologies, their deficiencies, and high-le~rerage areas where in- vestment win produce significant improvement. The requirement for both alert-mode and burst-mode power and energy nmet be better defined. Such an in-depth system study win improve the basis for power system selection, and could also be helpful in refining mission requirements.