Solar Power System for Lunar ISRU Applications
Takashi Nakamura1 and Benjamin K. Smith2
Physical Sciences Inc., Pleasanton, CA 94588
This paper discusses the development of a solar power system for lunar ISRU
applications. In this solar thermal system, solar radiation is collected by the concentrator
array which transfers the concentrated solar radiation to the optical waveguide (OW)
transmission line made of low loss optical fibers. The OW transmission line directs the solar
radiation to the thermal receiver for thermo-chemical processing of lunar regolith for
oxygen production on the lunar surface. Physical Sciences Inc. has been developing an
engineering solar thermal power system which provides thermal power to a thermochemical oxygen production system and PAR radiation for plant growth. We report the
results of the program pertaining to: (1) component improvement status; and
(2) development of the ground-based engineering system for ISRU applications.
I. Introduction
U
SING native lunar materials for production of propellant, construction blocks or other building materials is
attractive because it can significantly reduce the cost for exploring and living on the Moon. For in-situ
resource utilization (ISRU) on the moon, solar power is a readily available heat source. For this reason, the solar
furnace for materials processing has been widely studied in the past. For most of the past solar furnace experiments,
high intensity solar radiation, concentrated by parabolic reflectors, was applied to the materials in the furnace
directly or through a window. In such an arrangement it was often difficult to achieve ideal heating of the raw
materials because solar power is concentrated in a high temperature spot which can cause uneven heating and
vaporization of some material components [1,2]. Because of these difficulties in controlling the process
environment, some processing cycles must employ electric heating at the expense of significant power inefficiency.
In order to realize viable in-situ resource utilization, we need an innovative solar thermal system which can
effectively supply solar thermal power to material processing plants on the moon.
This paper discusses development of the solar thermal power system at Physical Sciences Inc. (PSI). In this
solar thermal system, as schematically shown in Fig. 1, solar radiation is collected by the concentrator array which
transfers the concentrated solar radiation to the optical waveguide (OW) transmission line made of low loss optical
fibers. The OW transmission line directs the solar radiation to the thermal receiver for thermochemical processing
of lunar regolith for oxygen production on the lunar surface. Key features of the proposed system are:
1. Highly concentrated solar radiation (~ 4 ×103) can be transmitted via the flexible OW transmission line directly
to the thermal receiver for oxygen production from lunar regolith;
2. Power scale-up of the system can be achieved by incremental increase of the number of concentrator units;
3. The system can be autonomous, stationary or mobile, and easily transported and deployed on the lunar surface;
and
4. The system can be applied to a variety of oxygen production processes.
1
Area Manager, Space Exploration Technologies, Applied Sciences, 6652 Owens Drive, Pleasanton, CA 94588, and
AIAA Associate Fellow.
2
Senior Scientist, Applied Sciences, 6652 Owens Drive, Pleasanton, CA 94588, and AIAA Member.
1
American Institute of Aeronautics and Astronautics
Concentrator Array
II. The Optical Waveguide Solar
Thermal System
The OW solar thermal system was
originally developed for lunar materials
processing with NASA/JSC funding
support from 1994 to 1996.3,4 Figure 2
shows the photo of the ground test model
developed in this program. The system
consists of three major components: the
concentrator,
the
solar
power
transmission
line,
and
the
thermal
reactor.
High Intensity Solar Thermal Power
Optical Waveguide Cable
The concentrator consists of multiplefacet parabolic concentrators with
nonimaging secondary concentrators
Lunar
Oxygen
Regolith
attached to optical fiber cables. Four
Thermochemical Processing
parabolic
aluminum
concentrators
(50 cm) were machined by a diamond
Figure 1. Solar Thermal System for Oxygen Production from tool to an accuracy of 17 nm. The
Lunar Regolith.
reflectivity of the concentrators over the
entire solar spectra was 0.85.
The OW solar thermal system shown
in Fig. 2 was used for hydrogen reduction
of lunar regolith during 1996. In this
program, hydrogen reduction of JSC-1
was demonstrated at 832°C. A summary
report on the oxygen production process
is given in the final report of the
program,3 and in a recent conference
paper.5 A more complete analysis of the
experiment is given in a recent
publication.6
II.1 Component Improvement
In recent years we made an effort to
improve
each component of the system.
Figure 2. The Ground Test Model of the OW Solar Thermal Power
Among
the system components, we
System.
focused our attention on the following
key
components:
(i) secondary
concentrator; and (ii) receiver interface with oxygen production process. The reason for choosing the secondary
concentrator and the receiver interface with the thermochemical process as key components is that the operation
requirements for these components will dictate the system configuration for the lunar-based oxygen production
plant.
The secondary concentrator attached to the inlet of the optical fiber cable acts as the funnel to inject the
concentrated solar radiation into the optical fiber cable. It plays two roles: (i) further concentrates the solar radiation
collected by the primary dish concentrator; and (ii) injects the focused solar radiation into individual optical fibers
with minimal loss. In previous programs PSI used the Quartz Secondary Concentrator in which the solar ray from
the primary concentrator is further concentrated by the conical quartz secondary concentrator. The conical quartz
concentrator is highly effective as it depends on the total internal reflection at the boundary. However, as the conical
quartz secondary concentrator is coupled with the bundled fibers, a significant amount of solar power falls into the
inter-fiber gaps. In a recent program we developed and tested a precision machined reflective matrix secondary
concentrator in which a matrix of miniature reflective non-imaging concentrators injects the solar flux into
individual optical fibers.
2
American Institute of Aeronautics and Astronautics
We prepared a test optical fiber cable with
the reflective matrix secondary concentrator
and tested it with the PSI 20-inch concentrator.
Figure 3 shows the photo of the cable outlet
emitting the concentrated solar radiation. An
overall transmission efficiency of 69% was
measured. In our previous tests conducted
with conical quartz secondary concentrator,
transmission efficiency was in the range
45~50%. By developing the new reflective
matrix secondary concentrator, we have
improved the transmission efficiency by 20%.
Based on the results we obtained in recent
programs, we can make a realistic prediction
for the improvement of the component
technology in the future. Table 1 lists a
summary of the component efficiencies in our
previous programs, those achieved in this
program and those expected in the future. The
method of how we achieve the projected
improvement is also given in the table.
Figure 3. Solar Concentrator Test of the Sample Cable with
the Reflective Matrix Secondary Concentrator.
Table 1. Pathway to Component Efficiency Improvement.
Component
Concentrator
Reflectivity
Intercept factor
19962005
0.722
0.82
0.88
May
2007
0.722
0.82
0.88
SpaceBased
Operational
System
0.912
0.950*
0.96
Improvement Measures
 Apply protected silver coating.
 Use high slope accuracy reflector. Absence of
atmospheric scattering helps.
Optical Fiber Cable
0.52
0.691
0.812
Front Fresnel refl.
0.965
0.965
0.983
 AR coating (400~850 nm)
Fiber fill factor
0.734
1.0
1.0
 Already accomplished
Integral fiber trans.
0.77
0.742
0.84
 Improve inlet optics and use high purity fibers
Back Fresnel refl.
0.965
0.965
0.983
 AR Coating (400~850 nm)
System Efficiency
0.38
0.499
0.740
* Reflectivity of the protected silver = 0.975. Reflectivity of the Cassegrain concentrator : 0.975 × 0.975 =
0.950 The concentrator efficiency will become 0.95 × 0.96 = 0.912.
II.2. Regolith Heating Capability
We conducted several JSC-1 melting experiments with
the Xe-Arc source starting in January 2007. Figure 4
shows melting of the regolith (JSC-1) with non-imaging
optics. Figure 5 shows the vitrified JSC-1 melt after the
experiment.
A series of solar powered JSC-1 melting experiments
was conducted in 2007. Figure 6 shows two PSI solar
concentrators melting JSC-1. The non-imaging optics
arrangement is shown in Fig. 7. Figure 8 shows three
quartz rods focusing solar radiation on the JSC-1 surface.
The JSC-1 melt zone is visible in the figure. In these
regolith melting experiments, the temperature of the melt
was measured by Type-C (W5%Re–W26%Re) Figure 4. Melting JSC-1 with Non-Imaging Optics.
3
American Institute of Aeronautics and Astronautics
Figure 5. Vitrified JSC-1 Melt (dia.= 14 mm; depth
= 6 mm).
Figure 7. Two Non-Imaging Optics Focusing
Solar Power on the JSC-1 Surface.
Figure 6. Two PSI Solar Concentrators for Melting
JSC-1.
Figure 8. Three Quartz Rods Focusing Solar
Power on the JSC-1.
thermocouples. During the thermocouple
measurement we conducted, we experienced
Table 2. Surface Temperature of the JSC-1 Melt vs. Heat Flux.
very large temperature differences within
the melt zone. Because of this steep
Heat Flux
Temperature
temperature gradient, temperature recording
Power Source (Date)
(W/cm2)
(C)
was time-consuming. We sampled the
PSI Xe Arc (1/25/07)
63.4
1368
thermocouple readings at various locations
PSI Xe Arc (10/19/06)
71.8
1450
within the melt, and then selected the
PSI Solar Concentrator (4/13/07)
84.39
1556
highest temperature as the surface
PSI Solar Concentrator (5/8/07)
117.4
1728, 1800
temperature. Table 2 summarizes results of
PSI Solar Concentrator (5/8/07)
~ 127
1900 (unsteady)
the temperature measurement.
In Fig. 9 we plot the melt surface
temperature we measured. It is shown that we have achieved 1800 C, the temperature necessary for the Carbothermal
process. Note that these temperature readings (except for the lowest temperature: 1368 C) were taken without the
radiation shield. With the radiation shield, the melt zone becomes thermally more homogeneous and creates a larger
melt. It is important to note that Fig. 9 conclusively demonstrated the capability of the OW solar thermal system to
melt the lunar regolith at temperatures necessary for the carbothermal reduction process which requires a very high
temperature.
4
American Institute of Aeronautics and Astronautics
2500
III. The Ground-based Engineering System
Temperature (°C)
2000
1500
1000
PSI Solar
PSI Solar (unsteady)
PSI Xe Arc
500
0
0
50
100
150
Flux (W/cm2)
200
250
J-3836a
Figure 9. Surface Temperature of the Molten JSC-1 vs.
Heat Flux.
During the past two years, PSI has developed a
ground-based engineering model of the solar thermal
system for oxygen production from lunar regolith [7].
An illustration of the ground-based engineering unit is
shown in Fig. 12. The system consists of:
(1) A single solar concentrator array equipped
with seven 27-inch concentrators and a solar
tracking system;
(2) Optical fiber cable which transmits the solar
radiation from the concentrator array to the
oxygen production reactor; and
(3) Reactor interface optics to inject the high
concentration solar radiation into the oxygen
reactor.
III.1 System Components
Concentrator Array
The concentrator array consists of seven 27-inch parabolic reflectors mounted on a single platform. The
concentrators use the secondary reflectors to focus the solar image at the inlet of the optical fiber cable. A schematic
of the concentration method is given in Fig. 10 and the dimension of the array is given in Fig. 11.
1.0 m
Figure 10.
The concentrator array dimensions.
Fiber
Cable
Inlet
Extension
Inlet
Optics
Flat
Reflector
J-4393a
Solar Tracking System
Side View
The concentrator array is equipped with a sun sensor and a twoaxis tracking system. PSI procured the commercial sun sensor and Figure 11. Schematic of the concentration
the two-axis tracking system and improved the sensitivity of the sun method.
sensor to achieve the tracking accuracy within 0.04 degree.
Figure 12 shows a photo of the concentrator array tracking the sun.
Optical Fiber Cable
The optical fiber used for this program is the hard polymer-clad fused silica fiber manufactured by CeramOptec
Industries, Inc. Figure 13 shows the construction and attenuation characteristics of the optical fiber. PSI has used
this fiber for several previous programs. PSI assembled seven 5-m long optical fiber cables each containing 55
optical fibers.
5
American Institute of Aeronautics and Astronautics
Oxygen Production Reactor Interface
A photo of the reactor interface hardware is
given in Fig. 14.
The seven cables are
accommodated by the conical section which
connects to the quartz rod holder. The quartz rod
injects the high concentration solar flux on the
regolith surface within the carbothermal reactor to
create high temperature molten regolith.
III.2. Initial Performance Tests
The first system performance test was
conducted in March 2009. Figure 15 shows the
front view of the integrated solar concentrator
system. The array structure tracks the sun within
0.04 degree. There are seven reflectors, each
connected to a single optical fiber cable. Figure 16
shows the back of the concentrator array with the
reactor interface optics emitting solar flux.
Hard Polymer Cladding
Tefzel Jacket
(-40˚ to +150˚C)
Nd:YAG
Nd:YAG
HeNe
1000
Alexandrite
Silica Core
Optran HWF
Kr Ion
Optran HUV/HWF
Attenuation (dB/km)
Figure 12. Solar concentrator array tracking the sun
within 0.04 degree accuracy.
100
Transmission/m 99%
10
99.9%
1
400 500 600 700 800 900 1000 1100 12001300140015001600 1700
Wavelength (nm)
Figure 13.
J-6614
Construction and attenuation characteristics of the hard polymer-clad optical fiber.
Figure 14. Oxygen reactor interface for seven
optical fiber cables integrated with a single quartz
rod.
Figure 15. Solar concentration array tracking the
sun within 0.04 degree accuracy.
6
American Institute of Aeronautics and Astronautics
Measurement of the power output was made as
the solar power input to the system was
incrementally increased by adding one concentrator
at a time. Figure 17 shows the termination of the
seven optical fiber cable assembled to one outlet
(left), and the power delivered at the cable
termination (right). In this photo, one of the seven
cables located at the center is emitting the solar
power. The optical fibers in the integration
hardware are cooled by the water and solar flux
goes through a quartz plate and the cooling water
film between the optical fiber termination and the
quartz plate.
Figure 18 shows the power output data
measured at the termination of the cable (Fig. 17).
The data were taken on March 20, 2009. The
Figure 16. Quartz rod emitting solar power.
ambient direct solar flux intensity at the time of the
measurement was 880 W/m2, a typical solar flux intensity at the PSI solar lab site (San Ramon, CA). Figure 18
shows that with the seven concentrator mirrors the power output from the optical fiber cables is about 800 W.
Figure 17.
(right).
Cable outlet for seven optical fiber cables (left) and solar power emitted by the center cable
900
800
700
600
500
400
300
200
Measured Power at 880
W/sqm (3/20/09)
100
0
0
1
Figure 18.
2
3
4
5
# of Mirror/Cable
6
7
8
After
the
cable
output
measurement, the quartz rod was
attached to the cable. The quartz rod
delivers the solar power into the
carbothermal reactor to melt the
regolith inside the rector (see Fig. 14).
The power output from the quartz rod
was measured and is shown in
Fig. 19. The data was taken on
March 23, 2009 when the solar flux
was 800 W/m2, about 10% lower than
the solar flux observed in March 20.
With the seven concentrators the
power output from the quartz rod was
700 W. Figure 19 also shows the
power output adjusted to the typical
solar flux (880 W/m2).
K-1274
Solar power output from the cables.
7
American Institute of Aeronautics and Astronautics
900
800
700
600
500
400
300
200
Measured Power at 800 W/sqm
(3/23/09)
Adjusted Power for 880 W/sqm
100
0
0
1
2
3
4
5
6
7
# of Mirror/Cable
Figure 19.
8
K-12
Solar power output from the quartz rod.
900
800
700
It is instructive to compare the output from
the optical fiber cables and the output from the
quartz rod for the same solar flux condition.
Figure 20 shows the comparison for the same
solar flux intensity (880 W/m2). The figure
shows that the two outputs are almost identical
indicating that there is no appreciable loss
mechanism within the long quartz rod.
The power output from the quartz rod as
projected on a flat metal screen is shown in
Fig. 21. The flux intensity of the power output
was measured using an aperture (1 cm dia.)
placed 4 cm away from the quartz rod tip. The
intensity within the 1 cm aperture was
137 W/cm2. The previous experimental data
shown in Fig. 9 indicate that the measured flux
intensity will bring the regolith temperature to
1800C. Note that the quartz output flux
intensity was measured at the low solar flux
value of 800 W/m2. With the typical solar flux
at the test site (880 W/cm2), the solar output
flux intensity should have been 150 W/cm2.
According to Fig. 9, this flux intensity
corresponds to 2000C.
600
500
400
300
200
Measured Cable Output at 880W/sqm
(3/20/09)
100
Measured Quartz Output Data (3/23/09)
adjusted for 880W/sqm
0
0
Figure 20.
output.
1
2
3
4
5
Number of Mirror/Cable
6
7
8
K-1276
Comparison of the cable output and the quartz
Figure 21. Solar output from the quartz rod as projected
on the metal screen.
III.3. Performance Test Results Summary
Based on the results of the measurement
conducted to date, we have determined the
performance characterized of the ground test
model of the OW solar power system. Table 3
summarizes the performance. A discussion of
the performance characteristics summarized in
Table 3 is given below.
The
concentrator
performance
was
determined by the measured reflectivities of the
primary and the secondary mirrors, and the
intercept factor.
The intercept factor is an
important measure for the focusing capability
of the concentrator. The intercept factor of the
current system, 0.72, is lower than that of the
PSI laboratory concentrator (0.88, see Table 1)
and the major contributing factor for the
relatively low efficiency of the concentrator
system (62%). We employed this concentrator
primarily because of the affordable cost.
The transmission efficiency of the cable
(5-m) was determined to be 70% based on the
measurement conducted during the development
phase. The cable efficiency includes the
efficiency of the inlet optics and that of the
optical fiber. The efficiency of the solar power
system, consisting of the concentrator and the
optical fiber cable, is determined to be 43.6%.
This value is lower than that of the PSI
laboratory system (49.9%, see Table 1) mainly
because of the low intercept factor.
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American Institute of Aeronautics and Astronautics
Table 3. Summary of Performance Characteristics
System/Component
Values
Solar Flux at Test Site (San Ramon, CA)
880 W/m2
Concentrator Area (Effective): note (i)
0.341 × 7 = 2.387 m2
Concentrator (Cassegrain configuration)
Primary Mirror Reflectivity (measured): note (ii)
0.91
Secondary Mirror Reflectivity (manufacturer data): note (iii)
0.95
Intercept Factor (measurement data): note (iv)
0.72
Concentrator Efficiency
0.91 × 0.95 × 0.72 = 0.6224
Optical Fiber Cable (Inlet Optics and Optical Fiber)
Cable Transmission (based on measurement data)
0.70
Solar Power System (Concentrator and Cable)
Solar Power System Efficiency
0.6224 × 0.70 = 0.4357
Reactor Interface Optics
Transmission of Interface Layer (calculated): note (v)
0.875
Solar Power System for Carbothermal Reactor
Overall System Efficiency
0.4357 × 0.875 = 0.3812
Solar Power Delivery to Reactor (calculated)
880 × 2.387 × 0.3812 = 801 W
Solar Power Delivery to Reactor (measured)
795 W
(i) excluding shadowed area
(ii) Aluminum mirror
(iii) protected Silver mirror
(iv) a measure of focusing capability
(v) cooling water film between the fiber outlet and the quartz window/rod
Application to Oxygen Production by the Carbothermal Process
The solar power system connects with the carbothermal oxygen reactor through the reactor interface optics. The
boundary between the optical fiber cable and the reactor interface optics is cooled by water to provide safety heat
sink. We estimated that heat loss of 12.5% will result, due to absorption of solar power by a 1-mm thick water film
between the cable outlet and the quartz rod (or quartz window). The thickness of the cooling water film can be
reduced for higher transmission efficiency.
The overall efficiency of the solar power system for carbothermal reactor is calculated to be 38.1%. Based on
the ambient solar flux at the test site and the concentrator area, we calculated the power delivered by the quartz rod
(or quartz window) to be 801 W in good agreement with the actual measured value of 795 W.
Having determined the performance characteristics of the ground based engineering system, we considered
potential system improvement for the system. The current system is the first generation model. As such we can use
the results of the current system for implementing a near-term improvement. In Table 4 we tabulated options that
can be implemented with the current technology. A summary discussion is given below.
The primary mirror reflective surface can be made of protected silver for 95% reflectivity. The most important
improvement for the overall system can be achieved simply by using a high surface accuracy reflector. The
intercept factor of 0.88 can be easily attained by diamond turning. Improvement of cable transmission can be
achieved by improving the inlet optics. PSI is currently working on the improvement measures. The reactor
interface optics efficiency can be improved by reducing the cooling water film thickness. Table 4 shows that the
overall system efficiency can be improved from 38% to 59% by: (1) procuring a high quality concentrator (intercept
factor): (2) improving component manufacturing technology (inlet optics); and (3) modifying the component design
(reactor interface optics).
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American Institute of Aeronautics and Astronautics
Table 4. Improvement Options
Current
Improvement for
Ground-based Engineering Ground-based Engineering
System
System
Concentrator (Cassegrain Configuration)
Primary Mirror Reflectivity
Secondary Mirror Reflectivity
Intercept Factor
Concentrator Efficiency
Optical Fiber Cable (Inlet Optics and Optical Fiber)
Cable Transmission
Solar Power System (Concentrator and Cable)
Solar Power System Efficiency
Reactor Interface Optics
Transmission of Interface Layer
Solar Power System for Carbothermal Reactor
Overall System Efficiency
0.91
0.95
0.72
0.91 × 0.95 × 0.72 = 0.6224
0.95
0.95
0.88
0.95 × 0.95 × 0.88 = 0.7942
0.70
0.8
0.6224 × 0.70 = 0.4357
0.7942 × 0.8 = 0.6354
0.875
0.938
0.4357 × 0.875 = 0.3812
0.6354 × 0.938 = 0.596
Application to Surface Stabilization by Sintering Lunar Regolith
For robotic or human exploration on the Moon, lunar surface stabilization is an important issue in reducing the
risks of human extra-vehicular activity (EVA) and robotic operation. Just as on the terrestrial surface, a stable lunar
surface will enhance robotic and human mobility and will reduce dust generation on the lunar surface. Lunar dust
effects have been recognized in large sectors of lunar surface activities. One obvious way to stabilize the lunar
surface is to thermally process the regolith to form a stable surface layer with higher load bearing and dust
containing capabilities.
Stabilization of the lunar surface can be accomplished by sintering which takes place at lower temperature than
regolith melting. The sintering process takes place at a temperature around ~1000C. Based on the data given in
Fig. 9, the heat flux intensity necessary to bring the regolith surface to temperature necessary for sintering process
will be less than 50 W/cm2. Sintering temperature and time to form a strong sintered sample vary depending on the
chemical composition and physical properties. Sintering experiments for lunar surface stabilization have been
conducted at NASA/KSC and detailed characterization of processed samples has been made [8].
At the writing of this paper we are preparing for a series of experiments to be conducted as part of NASA’s
ISRU demonstration experiments scheduled during February 2010.
IV.
Conclusion
In light of the results reviewed in this paper, we may conclude that the solar thermal system based on the optical
waveguide (OW) technology is viable and effective for oxygen production from lunar regolith. We demonstrated a
significant increase in system efficiency. One other important achievement is that we conclusively demonstrated
that the OW solar thermal system is capable of heating the lunar regolith to the temperatures necessary for thermochemical processing of lunar regolith. The regolith stimulant (JSC-1) was heated to 1800~1900°C by the solar
thermal power delivered by the PSI concentrator system.
The ground-based engineering solar thermal system was designed, manufactured, integrated and tested at PSI.
Based on the test results, it is projected that the solar thermal system will deliver to the carbothermal reactor 800 W
of solar power with the solar flux intensity at 880 W/m2. The measurement made at PSI during a cloudy day with
low solar flux (direct flux ~800 W/m2) yielded 700 W. At a higher solar flux (880 W/m2) the output was about
800 W. At a high altitude location, the direct solar flux intensity will be ~1000 W/m2. In this case the power output
from the system will be 900 W.
The efficiency of the solar power system was measured to be about 44%. With the reactor interface optics
attached to the solar power system, the overall solar power system was measured to 38%. It was shown that the
ground-based system efficiency can be improved by 20% with currently available technology.
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American Institute of Aeronautics and Astronautics
The ground-based engineering system is integrated with the carbothermal reactor and going through a series of
oxygen production tests at Orbitec Corporation, Madison, Wisconsin. In early 2010, the solar power system will be
transported to the NASA ISRU demonstration test site in Hawaii for field demonstration tests.
Acknowledgments
The programs reviewed in this paper were supported by NASA/JSC through SBIR Phase I and II programs
(NNJ07JB26C, NNJ08JD44C, COTR: Mr. A. Paz), and by NASA/GRC through SBIR Phase III program
(NNC08CA59C, COTR: Dr. A. Hepp). The Phase I and II programs were conducted in collaboration with
Lockheed Martin Space Systems Company (LMCO, Mr. L. Clark) and Orbital Technologies Corporation
(ORBITEC, Mr. R. Gustafson).
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American Institute of Aeronautics and Astronautics