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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 power—in 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 53
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 penalty—from 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 54
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OCR for page 57
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 58
58
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AD VANCED PO WER SOURCES FOR SPA ClE MISSIONS
Closed Thermodynamic
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No Power Conditioning Mass
Masses Include Hydrogen
-
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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 59
59
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OCR for page 60
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 61
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OCR for page 62
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 weapon—at 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 63
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 1500°K stated in Table 4~1
to 2000°K. 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 2300°K (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 64
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 65
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 66
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 67
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.
Representative terms from entire chapter:
sdi air
