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 31
4
Spacecraft Electric Power
BACKGROUND AND STATUS
A spacecraft's electrical power system generally consists of the primary power
generating unit (solar or nuclear), the power management and distribution system, and
an energy storage unit. The power system typically accounts for 25 to 35 percent of
spacecraft dry mass (Herrera and Kuck, 1992; Larson and Wertz, 19921. In solar power
systems, the energy storage unit represents about one-third of the mass of the power
generating unit. Nuclear radioisotope power systems are independent of sunlight and
require little or no energy storage. The choice of power system depends on such
considerations as power level (average and peak), mission lifetime, and operating
environment. In the vast majority of cases, solar power systems are preferred over
nuclear power ones due to their lower cost and simpler launch approval procedures.
POWER SOllRCES
Solar Arrays
For space missions that are sufficiently close to (and with an unobstructed view
of the sun, solar cell arrays can meet most near-term space power needs for small,
lightweight spacecraft by converting solar energy to electrical power. Solar cells can be
mounted directly on the external surface of the spacecraft or on panels that are deployed
once the spacecraft achieves orbit. The performance of solar arrays is quantified by (~)
the specific power, power delivered by solar array per unit weight (watts per kilogram);
(2) the power density, the power delivered by a solar array per unit area (watts per
square meter); and (3) the survivability level, the capability of an array to survive hostile
attack (for DoD missions) and the space environment.
Solar array configurations are either (~) unconcentrated, in which case they
operate on the as-received solar flux or (2) concentrated, where the solar cells' output
is increased by the use of lenses in order to focus more solar radiation onto the cells.
Unconcentrated arrays are not necessarily planar; they can also be cylindrical or spherical
31
OCR for page 32
32
Technology for Small Spacecraft
and can be subdivided into rigid and flexible arrays. The flexible arrays include blanket
arrays and inflatables. Concentrated arrays are usually planar.
Silicon and gallium arsenide cells are currently used on spacecraft arrays. Of the
two technologies, silicon solar cell technology is the more mature and has the lower cost
per watt. However, gallium arsenide, first flown in 1983, has a higher energy conversion
efficiency and is inherently more resistant to radiation Carson and Wertz, 19921.
Unfortunately, gallium arsenide costs approximately two to five times more than silicon
and is 2.2 times as dense (Chetty, 19911. Other cell types, such as amorphous silicon,
aluminum gallium arsenide, indium phosphide, copper indium diselenide, cadmium
tellur~de, and multibandgap cells, are uncler development and have been flown
experimentally, but have not yet been flight qualified for major U.S. operational
spacecraft (Cooley, 1991~. For example, although copper indium diselenide and
amorphous silicon cells are both now flying on the UPS- satellite, and a new program
has been funded by OACT for a flexible, indium phosphide, thin-fiIm solar array
experiment, neither of these systems has been qualified to an acceptable level for
deployment in major U.S. flight systems (Landis and Hepp, 1991; NASA, 1993a).
NASA and DoD, both with industry support, are developing high specific power
solar array systems for small spacecraft applications. The status of research within each
organization is summarized below.
NASA Programs
NASA's research activities on solar cell and solar array technology are currently
centered at the LeRC. Until recently, JPL also had a program for high-performance solar
array technology, which was complementary to I~eRC's program on high-eff~ciency,
radiation-resistant solar cells.
LeRC. LJeRC's power systems programs focus on Earth orbital applications. LeRC
contributed to the large area silicon cell technologies (with strong contributions in cell
technology from the European Space Agency) that were user] on the Hubble Space
Telescope and were scheduled for use on Space Station Freedom, prior to the latest
redesign (Cooley, 1991~. Flexible, roll-up arrays made from these silicon cells were
initiated at LeRC and further developed by the U.S. Air Force at the Wnght-Patterson
Air Force Base. These flexible silicon cell arrays were eventually utilized on the Hubble
Space Telescope.
LeRC is currently working on many advanced solar cells, such as copper indium
diselenide cells, amorphous silicon cells, indium phosphide on germanium cells, cadmium
telluride cells, and other multibandgap cells. LeRC is also performing research on
~ The multijunction approach utilizes the solar spectrum more efficiently by stacking several
bandgap cells (e.g., a thin gallium arsenide cell stacked on top of a silicon cell) in series such
that successive junctions convert different frequency ranges of sunlight.
OCR for page 33
Spacecraft Electric Power
ultralight flexible panels that could utilize many of these advanced cells in various
flexible array designs.
One inflatable array design under development at LeRC utilizes flexible silicon
cell panels, which can provide total power in the range of 200 to 500 watts at a specific
power of 200 watts per kilogram. In the future, substitution of silicon cells with cells of
20 percent indium phosphide on germanium could result in a more radiation-tolerant
array with approximately 1 percent degradation over 10 years and an efficiency of greater
than 17 percent. Use of arrays with indium phosphide on germanium solar cells is
expected to produce a specific power of 130 watts per kilogram and would enable
long-life missions in polar orbits and other high-radiation environments (Budinger et al.,
1993).
At a recent joint LeRC, JPL, GSFC workshop, the participants concluded that
most of LeRC's advanced solar cell work could be ready for flight qualification within
seven years (NASA/OACT, 19931.
JPL. JPL`'s research and technology programs generally focus on planetary
exploration. Most of the solar array work at JPL has been carried out under the
Advanced Photovoltaic Solar Array (APSA) program, which was cancelled due to
funcling limitations. A flexible, lightweight fold-up solar array with a mass of 1 kilogram
per square meter of collecting area was developed by JPL antler the APSA program, in
conjunction with TRW's Space and Electronics Group. The array design incorporates
both thin silicon cells and thin gallium arsenide on germanium substrate cells. The cells
are attached to a flexible Kapton polyamide blanket. A fiberglass deployment mast is
used. The complete array system produces 135 watts per kilogram (Scala, 19931.
Although a full-size flexible blanket array was not flight tested, fabrication, integration,
ground vibroacoustic testing, and ground deployment tests of a prototype 6-kilowatt
APSA array were successfully completed (Kuriand and StelIa, 19921. The JPL/TRW
APSA flexible solar cell blanket array technology is currently scheduled for use on two
future missions: the NASA Earth Observing System AM-! mission, using gallium
arsenide on germanium solar cells, and a DoD mission supported by TRW, also using
gallium arsenide on germanium solar cells.
Prior to program termination, the midterm goal of the APSA program was to
implement cell fabrication methods and array assembly procedures for thin-film solar
cells that could increase array specific power to 190 watts per kilogram. As an example,
advanced thin-f~im gallium arsenide or aluminum gallium arsenide cells produced by
Kopin Corporations's Cleaved Lateral Epitaxy Films for Transfer (CLEFT) process could
have been utilized by 1996. Just prior to termination, lightweight flexible modules
utilizing thin-fiIm gallium arsenide solar cells from this process were fabricated for JPL
and sent to LeRC for thermal cycling tests. The long-term APSA program goal was to
develop flexible solar cell blanket array designs with array specific powers of 300 watts
per kilogram (at 12 kilowatts total power) by the year 2000, and of 20 to 25 kilowatts
total power ultimately.
IPL has been working with industry and universities to improve the performance
of silicon cell arrays at distances from two to five astronomical units from the sun. These
33
OCR for page 34
34
Technology for Small Spacecraf!
low-intensity, low-temperature conditions degrade the performance of conventional
silicon cells due to metallization and silicon interactions. JPL hopes to solve the low-
intensity, low-temperature technology problem by 1995, but the program is likely to be
terminated due to budget reductions.
DoD Programs
DoD, including the U.S. Air Force, BMDO, and the U.S. Navy has performed
substantial work to increase the specific power in solar arrays for low-power, small
spacecraft applications. Prior to the availability of flexible arrays analogous to those
developed in the NASA APSA program, DoD platforms generally utilized planar silicon
arrays with specific powers ranging from 40 to 60 watts per kilogram. These arrays have
beginning-of-life efficiencies of 12 to 15 percent that drop to end-of-life efficiencies of
10 to 12 percent (Russell et al., 19921. For the same specific power, advanced
~ . ~ ~ ~ ~ ~
technology for gallium arsenide on germanium cells could offer a nominal end-of-life
efficiency as high as IS percent (Russell et al., 19921.
In the past, DoD has supported research on concentrator arrays through venous
programs, such as the now-cancelled BMDO/Martin Marietta Survivable Power
Subsystem Demonstration program and its predecessor, the Survivable Concentrator
Photovoltaic Array program. These technology programs were supported by the Strategic
Defense Initiative Organization with plans for future incorporation into the Brilliant Eyes
program.
Although the design objectives for DoD concentrator technology generally focus
on survivability rather than efficiency and high specfic power, there were several DoD
technology advancements made in these programs that may hold promise for future
NASA spacecraft. For example, the concentrator technology developed for the Survivable
Concentrator Photovolta~c Array and the Survivable Power Subsystem Demonstration
programs (mini-Cassagrainian arrays developed by TRW, minidome arrays developed by
Boeing, and slats being developed by General Dynamics) has the potential to reduce by
a factor of two the cost of planar arrays and to eventually provide specific power of
around 80 watts per kilogram, at beginning-of-life overall efficiencies of 24 percent.
However, with the formal dissolution of the Strategic Defense initiative Organization into
BMDO, and the termination of several programs such as the Survivable Power
Subsystem Demonstration program, the advancement of the concentrator technology
within DoD is uncertain.
Currently, the Air Force is trying to maintain work in multibandgap cells and
thin-fiIm cells despite the absence of funding from BMDO and overall DoD budget cuts.
Production of cells with intermediate capability levels is within a year or two of
completion.
BMDO, the Naval Research Laboratory, and NASA are jointly sponsoring the
Deep Space Program Science Experiment (Clementine) program, which will demonstrate
lightweight technology components with a lunar mapping and asteroid flyby mission that
was launched in January 1994. The spacecraft is utilizing gallium arsenide on germanium
OCR for page 35
Spacecraft Electric Power
cells that are 0. 14 centimeters thick at 40 watts per kilogram. These are the thinnest
gallium arsenide solar cells flown to date (Nozette, 19931.
Nuclear Technology
Nuclear radioisotope power systems convert the heat from a radioisotope heat
source into electricity. Current radioisotope power systems are more compact than solar
systems and are mass-competitive, but they are quite costly and require a complicated
launch approval process. Therefore, solar power is always preferred over radioisotope
power, except for deep space or sun-obscured missions, where there is too little sunlight
for efficient photovolta~c conversion, and for missions near the sun, where the solar flux
is too intense and too variable for practical solar powered systems. The use of
nonrechargeable batteries for primary power has been proposed for some missions, but
for the power levels and operating times required by those missions, batteries become
prohibitively heavy for a small spacecraft. For such missions, radioisotope systems are
enabling and are used in spite of their cost and complicated launch approval process.
In today's systems, thermoelectric unicouples are used to convert heat into
electricity. These systems, while reliable and long-lived, are inefficient. Substantial
reductions in cost and mass of radioisotope power systems can be achieved through
development of more efficient power conversion technologies. Potential conversion
technologies include advanced thermoelectric materials, Stirling engines, alkali metal
thermoelectric converters, and thermophotovoltaic systems. The last three options offer
the possibility of tripling or even quadrupling the efficiency of thermoelectric converters,
with corresponding reductions in the cost and mass of the required radioisotope fuel.
Both NASA and the Department of Energy (DOE) have invested in conversion
technologies for radioisotope power systems that could be used for small~spacecraft. The
status of research within each organization is summarized below.
NASA Programs
LeRC. LeRC and its contractors, Mechanical Technologies, Inc., Sunpower, STC,
and others, have been working on free-piston Stirling engines internally coupled to linear
alternators, to increase engine reliability and lifetime by eliminating the need for external
seals on moving shafts. Mechanical Technologies, Inc., recently completed a large
system of that type, possibly for a second-generation space station or as an alternative
conversion system for the since-cancelled SP-lL00 reactor program. The engine produced
an electrical output of 12 kilowatts at an overall system efficiency of over 23 percent.
The system gave an initial performance that was in excellent agreement with analytical
predictions, but it has not undergone life-cycle testing.
A scaled-down, Stirling engine was recently designed for possible use in NASA's
proposed Pluto Fast Flyby mission. Analytical models showed that a 75-watt engine
would have an efficiency of 23 percent.
35
OCR for page 36
36
Technology for Small Spacecraft
JPL. For nearly twenty years, JPL and Advanced Modular Power Systems have
been developing a highly efficient, static alkali metal thermal-to-electric converter for the
direct conversion of heat to electricity. Alkali metal thermal-to-electric converter cells
suitable for zero-gravity conditions were recently tested by Advanced Modular Power
Systems and yielded an efficiency of 9.6 percent at 700°C. Advanced Modular Power
Systems predicts efficiencies of 15 to 20 percent through advanced cell designs and
higher-temperature operation. Based on that prediction, JPL system studies estimate that
an alkali metal thermal-to-electnc converter-generator for the Pluto Fast Flyby mission
that uses two standard radioisotope heat source modules would have a system mass of
9.7 kilograms. Such a system has not yet been demonstrated, and one major uncertainty
about these devices is their ability to withstand launch vibration.
JPL is also working on thermophotovoltaic conversion systems, which are an
outgrowth of recent advances in photovoltaic materials developed for high-efficiency
solar cells. Instead of converting solar radiation to electricity, they convert infrared
radiation emitted by the radioisotope heat source. Since infrared radiation has a very
different spectral distribution than solar radiation, different photovoltaic conversion
materials are required. One material under test by Boeing for JPL is gallium antimonide,
whose bandgap is well matcher! to the infrared spectrum. Relatively high efficiencies
have been demonstrated with this material, and extremely high efficiencies (greater than
30 percent) may be achievable through addition of reflective filters or mirrors to return
the unconverted radiation to the heat source.
DOE Programs
DOE has extensive experience with nuclear power technology. In the late sixties
and early seventies, thermoelectric unicouples employing silicon germanium materials
were developed by RCA and General Electric for DOE. These unicouples were
successfully flown in 200- to 300-watt radioisotope thermoelectric generators (RTG) on
several NASA missions and are slated for use on the proposed Cassini mission. The
RTGs, developed by DOE laboratories and contractors, with assistance from JPL, have
rather low efficiencies (less than 7 percent) but have proved extremely reliable and long-
lived (approximately 150,000 hours). A typical small (70-watt) RIG for the Pluto Fast
Flyby mission, based on silicon germanium unicouples, has a mass of 15 kilograms, with
a cost of $51 million estimated by DOE for three fueled flight units.
During the 1980s, a modular, radioisotope heat source module (the General
Purpose Heat Source) was developed and safety qualified by DOE laboratories and
contractors. Being modular, these heat sources are adaptable to a wide range of power
levels and conversion systems. RTGs are flying on the Galileo and Ulysses missions and
are slate to be flown on the proposed Cassini mission.
Thermoelectric multicouples employing silicon germanium materials with
additives have been under development by DOE contractors for over ten years, but their
development was recently suspended by the department. The multicouples were
developed for use in modular RTGs, which are scalable over a wide range of power
OCR for page 37
Spacecraft Electric Power
levels with minimal redesign. They are only a little more efficient than unicouple RTGs,
but offer a significantly higher specific power. They also make it possible to generate
high voltages from small RTGs (28 volts DC has become the accepted industry standard
power bus voltage for small spacecraft). Multicouples have been successfully tested for
up to 15,000 hours, but their measured degradation rates were about twice as high as
those of unicouples. As of this wnting, funding for continuing development has not been
allocated.
DOE has also been involved in work on other conversion technologies, sponsoring
Fairchild Space and Defense Corporation to prepare and analyze detailed system designs
for integrating the advanced conversion systems (Stirling engines, thermophotovoltaic
systems, and alkali metal thermoelectric converters) with a radioisotope heat source and
a heat rejection radiator. A recently completed system design study showed that
replacement of the RIG with a thermophotovoltaic generator for the proposed Pluto Fast
Flyby mission would reduce the required number of costly heat source modules by 60
percent, reduce the power source mass by over 50 percent, and triple or quadruple the
system efficiency.
BATTERY TECHNOLOGY FOR ENERGY STORAGE
NASA Programs
NASA's battery technology research and development activities are located at
LeRC and JPL and have focused principally on the following systems.
Nickel Cadmium (NiCdJ Batteries. Rechargeable NiCd batteries, which have been
used in spacecraft for over 20 years, may be considered the currently available
technology, although they are gradually being phased out, due in part to government
restrictions on manufacturing processes involving cadmium and in part to the increasing
availability of superior alternatives.
Individual Pressure Vessel (IPV) Nickel Hydrogen (NiH2J Batteries. In an IPV
NiH2 battery, each cell (cathode-anode pair) is individually contained in its own pressure
vessel. (Pressure vessels are neec ed to contain the cell's hydrogen gas at high pressures
of 6.2 x lo6 pascals to 6.9 x lo6 pascals.) A NiH2 battery is typically composed of 22
cells.
The first such battery was flown in 1977 by the Naval Research Laboratory.
Today, it has replaced the NiCd battery for defense applications in geosynchronous orbit,
and it is quickly becoming the preferred battery technology in low Earth orbit as well.
IPV NiH2 batteries are more voluminous than NiCd batteries due to individual cell
containment in rounded vessels, but they offer substantially longer cycle
(charge/discharge) lifetime, greater depth of discharge, and improved tolerance of abuse
(e.g., overcharging).
37
OCR for page 38
38
Technology for Small Spacecraft
LeRC has developed a new, lightweight nickel electrode that is usable in IPV
NiH2 batteries as well as in other nickel-based ones and that will increase specific power.
Nickel Metal Hydride (NiMH) Batteries. NiMH batteries, which are being studied
by LeRC and IPL, would be about 10 percent lighter than Nigh batteries and could
provide up to about 50 watt-hours per kilogram. They also offer a longer shelf life, lower
cost, and higher power density in a reduced volume.
NiMH batteries have not yet been flown, and some development work remains
to be done, but they represent an attractive near-term option to replace NiCd batteries
on the low-power end of the small spacecraft spectrum.
Lithium Batteries. Lithium batteries are a highly promising mid- to far-term
technology. Lithium titanium disulf~cle (LiTiS2) batteries, for example, are being studied
at JPL for low-power (less than I-kilowatt-electric) applications. These batteries have a
high power density (100 watt-hours per kilogram), a lifetime of i,000 cycles at 50
percent depth of discharge, a lO-year shelf life, and low volume, all of which would
make these batteries well-suited for long-duration planetary missions.
Lithium polymer batteries are being investigated by LeRC and JPL to achieve a
specific power goal of 150 to 200 watt-hours per kilogram. Substantial effort is still
needed in electrolyte research, but the ultralight weight and small size of these batteries
would provide important benefits.
In the far term, lithium primary (i.e., nonrechargeable) batteries may present an
alternative to radioisotope power systems for outer planetary missions, but such batteries
would have much lower specific energies than RTGs or other radioisotope systems. High
energies are required not only for extended survey or exploration missions but also for
brief flybys like Pluto Fast Flyby. The mission's power requirements are determined not
by the length of the flyby but by the amount of stored data to be transmitted back to
Earth from deep space. In the case of the proposed Pluto Fast Flyby mission, the power
demand stipulated by the current design would have to be reduced by orders of
magnitude to lower the battery mass to that of the radioisotope power system. Clearly,
that would be a very different mission, with a much smaller scientific return.
Other Government Programs
DoD, particularly the U.S. Air Force Phillips Laboratory, BMDO, and the Naval
Research Laboratory, and DOE have several advanced battery concepts that complement
NASA's work.
Common Pressure Vessel (CPV) NiH2. The CPV NiH2 battery is a logical
near-term follow-on system to the individual pressure vessel NiH2 battery. The
containment of all! cells in a single pressure vessel allows for a significant reduction in
battery volume as compared with the IPV battery concept. As currently designed,
OCR for page 39
Spacecraft Electric Power
however, the CPV battery does not permit monitoring of individual cell performance and
the ability to switch defective cells off-line.
However, the CPV battery offers significantly higher specific power (nearly 50
watt-hours per kilogram), lower cost, and simplified electrical and thermal interfacing.
It is a strong candidate for use in the "larger members" of the small spacecraft family.
CPV battery technology development is centered at the Naval Research Laboratory.
Additional work is underway at U.S. Air Force Phillips Laboratory.
A CPV NiH2 battery has been scheduled to fly within a year on a DoD spacecraft
as part of a joint effort between the Naval Research Laboratory and industry under a
Cooperative Research and Development Agreement. A second CPV battery is being used
on the BMDO Clementine mission, launched in January 1994. CPV battery technology
is currently planned for use in the }~DiUM_/SM no_ A; A~..~A be.,
Lockheed (AuCIair et al., 19931.
bpd~rd1 ~ Oilily U~V~1~ by
Sodium Sulfi'r (NaS) Batteries. NaS batteries have the potential for a further
doubling of specific power up to 100 watt-hours per kilogram, but they require additional
development work and a flight experiment in order to complete qualification. This
appears to be a promising technology, but predominantly for larger spacecraft.
LeRC had planned a NaS cell flight experiment in 1995, but this has been
cancelled due to funding limitations. (This decision may be subject to reconsideration by
NASA.) The U.S. Air Force Phillips Laboratory remains committed to the technology,
but it also has only limited resources available.
Lithium Batteries. The U. S. Air Force and BMDO are conducting early work on
solid-state batteries (including lithium titanium disulfide and lithium polymer), with a
long-term performance goal of 200 watt-hours per kilogram.
DOE has produced tiny, bench-scale thin-fiIm lithium batteries to power
individual chips. Preliminary work is underway to increase production rates and to
develop larger rechargeable lithium batteries with a calculated specific Dower of more
than 300 watt-hours per kilogram.
The Lawrence Livermore National Laboratory has performed life-cycle testing on
a lithium ion battery that may be suitable for low-Earth-orbit applications in the near
term.
FINDINGS AND PRIORITIZED RECOMMENDATIONS
Improvements in power generation and storage technology would be beneficial to
virtually all classes of small spacecraft missions, reducing mass and cost, as well as
enhancing performance.
The Pane! on Small Spacecraft Technology believes that developments in low-
weight, high-efficiency solar cells and arrays would enhance not only the power
generation capability of small spacecraft but also that of solar electric propulsion. The
pane! also found that advanced, high specific power battery technologies for space
39
OCR for page 40
40
Technology for Small Spacecraft
applications have received insufficient attention and lag considerably behind developments
in terrestrial power storage. Successful development of compact, long-lived, high specific
power battery systems, coupled with improvements in power generation and
management, will significantly enhance the utility and affordability of small spacecraft
by offering reductions in mass and launch costs and improved performance.
The use of currently available radioisotope power system technology for
interplanetary missions and others where the sun is obscured results in higher-than-
desired cost or mass. Technologies that would enable more efficient, lighter-weight
systems have shown promise in research and development programs at NASA and DOE.
Development times are probably too long to permit use in near-term planned programs
such as the proposed Pluto Fast Flyby and Mars Pathfinder missions. However, for
future deep space missions and Martian planetary surface investigations with small
spacecraft and microrovers, especially at high latitudes, these technologies are enabling.
Future developments in the radioisotope power technology require the active involvement
of DOE as mandated by the Atomic Energy Act of 1954, which is still in effect.
In considering the use of radioisotope power generation systems in future
spacecraft, special attention must be paid to ensuring that there is a source of
plutonium-238. The U.S. reactors capable of its production have been shut down.
Arrangements should be made to ensure the availability of a foreign source (e.g., Russia,
France, or the United Kingdom).
Considering that investments in this technology area will produce returns across
the entire spectrum of missions, the pane! recommends the following, in priority order:
I. An advanced solar array program should be initiated at a funding level that
will allow reaching a goal of 200 watts per kilogram with 5 to 10 kilowatts of total
power within the next five years.
2. The development, characterization, and testing of NiMH batteries for low-
power small spacecraft should be completed.
3. Building on the work already completed for the Clementine mission, the
characterization and testing of CPV NiH2 batteries for mid- to high-power small
spacecraft should be completed.
4. The development of lithium alloy (LiTiS2) batteries, particularly for low-
energy-demand planetary missions, should be continued.
5. The application of lithium ion batteries developed by DOE should be
evaluated for possible use in low-Earth-orbit spacecraft. If found promising, the
technology should be adapted for small spacecraft.
6. For mid- to far-term applications, the development of lithium polymer
batteries should be accelerated.
OCR for page 41
Spacecraft Electric Power
7. In the long-term, work on other advanced solar cell and solar array
technology, including thin-fiIm cell development, inflatable arrays, ant} flexible blanket
wing APSA arrays, should continue at an increased funding level, with the goal of
achieving a specific power of 300 watts per kilogram.
8. There is a small but important subset of small spacecraft missions that
cannot use solar power or batteries and that are enabled by radioisotope power systems.
For those missions, development of more efficient conversion systems to reduce heat
source mass and cost would be beneficial. Radioisotope power system designs using
Stirling, thermophotovoTtaic, and alkali metal thermal-to-electric converter conversion
techniques should be jointly evaluated by NASA and DOE, and the ability of these
techniques to satisfy various NASA missions should be assessed. Based on the evaluation,
NASA and DOE should select one or more of these systems for experimental
demonstrations of its performance against specific pre-determinec] criteria that are
peculiar to the approach selected. NASA and DOE should then select the most promising
approach for further clevelopment. A decision about flight demonstrations should be made
contingent on future NASA planning of missions that would utilize the technology.
9. Research on concentrator arrays, with a goal of reaching power densities
in excess of 300 watts per kilogram at one-half the cost of existing arrays, should be
increased.
41
Representative terms from entire chapter:
gallium arsenide
