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