Subcommittee on Space and Aeronautics
Committee on Science
U.S. House of Representatives
On
The Technical Feasibility of Space Solar
Power
By Jerry Grey
Director, Aerospace and Science Policy
American Institute of Aeronautics and
Astronautics
1801 Alexander Bell Drive, Suite 500
Reston Virginia 20191-4344
September 7, 2000
Introduction
My
name is Jerry Grey. I am Director of Aerospace and Science Policy for the
American Institute of Aeronautics (AIAA) and Visiting Professor of Mechanical
and Aerospace Engineering at Princeton University. Although the views I express
here on space solar power are consistent with those appearing in the AIAA’s
publications, they are my own and do not necessarily reflect the formal position
of the AIAA.Thank you for this
opportunity to offer my comments on this important subject.
Background
The
AIAA has conducted independent assessments of NASA’s recent research efforts in
space solar power (SSP): the Concept Definition Study (1997 – 1998) and the SSP
Exploratory Research and Technology (SERT) program (1999 – 2000). The final
report on the most recent of these assessments will not be issued until
October, but because the results of that assessment bear directly on the
subject of this hearing, I will summarize them here today.
The
AIAA assessment covered three separate facets of the SSP field: (1) the work
being conducted outside the U.S. and the opportunities for international
coordination and/or cooperation; (2) the prospects for multiple use of the
SSP-enabling technologies in terrestrial and in civil, commercial, or military
space applications; and (3) a technical assessment of the SSP work conducted in
NASA’s SERT program. Note that our assessment specifically excluded economic and
environmental considerations [except as they impact the technical aspects of
SSP], since other groups were conducting studies in these areas.
International SSP Activities
There
are three formal mechanisms for international coordination and/or cooperation
among SSP research groups worldwide: the International Astronautical
Federation’s Committee on Space Power, the Sunsat Energy Council, and Japan’s
SPS 2000 Equatorial Countries Alliance. These organizations offer an excellent
avenue for the promotion and coordination of U.S. SSP programs with those of
the other nations of the world.
Major
efforts in both system aspects and technology advancement for SSP are currently
taking place in Japan, Europe, and the former Soviet Union. Prior to the recent
intensification of restrictions placed on technology interchanges with non-U.S.
engineers and scientists, NASA had invited non-U.S. researchers to contribute
to the SSP Technical Interchange Meetings. This interchange had resulted in the
incorporation of several non-U.S. technical concepts in the NASA program, and
if current restrictions under the International Traffic in Arms Regulations
(ITAR) can be mitigated, interchanges with non-U.S. SSP researchers should
certainly be continued and expanded.
The AIAA found the following
specific areas worthy of consideration for international coordination and/or
cooperation:
(1) Computer modeling. Much work in this
area has been conducted in Germany and Japan.(2) Solar array technology development.
Again, Germany and Japan have extensive research and development programs in
this area.(3) Wireless powertransmission. Japan, France, and Canada are
currently developing demonstrations of microwave power transmission, both on
the ground and in space.(4) Research facilities. Japan, in
particular, has built a Long Duration Microwave Exposure Facility (LDMEF) that
could be evaluated for joint use in
radiation exposure studies over a range of frequencies.(5) Innovative concepts and technologies.
Japan’s “sandwich” SSP design concept has already been included in NASA’s
studies. Germany, Russia, and Ukraine have conducted studies of other
innovative SSP concepts. Japan’s Laser Energy Network could supplement U.S.
laser power transmission studies.(6) Multiple-use applications.
Space-to-space “power-plug” concepts developed in France, Japan, and Russia
could be of interest in current U.S. evaluations of multiple-use prospects.(7) Demonstrations. Japan has conducted a
number of demonstrations of SSP-enabling technologies and concepts on the
ground, in the air, and in space. France, the European Space Agency, the
International Space University, Ukraine, Russia, and Canada are all currently
developing demonstration projects, some of which would use the international
space station. Japan is engaged in a major demonstration of a prototype SSP
station, SPS-2000, that involves ten equatorial nations.
You had asked specifically for
information on “efforts among the international community to develop and
coordinate implementation of an agreement that would permit a space solar power
satellite system to proceed.” Our assessment addressed that issue.
The most important recent effort in this
direction was the Workshop on Clean and Inexhaustible Space Solar Power, held
at UNISPACE 3 in Vienna in July 1999. The premise of the Workshop was that SSP cannot be realized without international cooperation
and worldwide public acceptance. They recommended that:
(1) Organizations around the
world should be encouraged to investigate further the technical and economic
feasibility of SSP, and especially to perform demonstrations to validate needed
technologies and engender global familiarization with SSP.(2) Countries around the
world should be encouraged to examine ways in which SSP might be uniquely
suited to meeting a portion of their energy needs.(3) The value of SSP in
improving the quality of life worldwide should be identified (e.g., clean air,
clean water, communications, and standard of living).(4) International
collaboration, cooperation, and data-sharing on SSP should be encouraged.(5) A dialogue should be set
up with the appropriate national and international organizations responsible
for standards and regulation, to assure due consideration of SSP matters as
they affect, for example, health, the environment, spectrum management, and orbit
allocations.(6) A series of international
conferences on SSP should be organized, involving both developing and developed
countries.(7) A standing committee should
be formed under the auspices of the United Nations for long-term consideration
of SSP issues, building on the ad hoc
efforts now being conducted by the IAF Power Committee and the SunSat Energy
Council.
The UNISPACE-3 Committee
reviewed the above list of recommendations, and accepted the following, which
appear in the UNISPACE-3 report:
Organizations around the
world are encouraged:
(a) to investigate further
the technical and economic feasibility of space solar power over the next few
years;(b) to stimulate
international cooperation and data-sharing regarding space solar power; and(c) to give due consideration
to space solar power matters, for example, as they concern health, the
environment, spectrum management, orbit allocations and other topics.
The AIAA assessment further
identified specific global concerns that require near-term consideration and
resolution under an international umbrella such as the UN. They include the
following:
(1) Allocation of
microwave frequencies for wireless power transmission (e.g., by the World
Radiotelecommunications Conference)(2) Allocation of acceptable
orbits and locations within those orbits (e.g., by the International
Telecommunications Union)(3) Formulation of land-use
policies for rectenna locations(4) Definition of the
environmental and climate impact studies required(5) Definition of health and
safety requirements (perhaps under the auspices of the World Health
Organization’s Magnetic Field Project)(6) Identification and
formulation of key demonstration projects(7) Definition and resolution
of economic and market issues
Actions suggested in the AIAA
assessment were as follows:
(1) ITAR constraints on SSP
technical interchange need to be mitigated. This could be best accomplished by creating an umbrella list of SSP
technologies, and submitting to the U.S. Department of State (DOS) a rationale
for DOD approval of technical SSP interchange as a research activity.(2) In addition to the three existing
mechanisms for international coordination, an International Working Group (IWG)
on SSP Systems should be created, perhaps as a subcommittee of the IAF’s Power
Committee. Typical subjects for the IWG might include:– Identification of
demonstration projects, some of which may require formal international
agreements– Setting up a standard
mechanism for companies seeking joint SSP efforts to apply for Technical
Assistance Agreements– Seeking methods to
mitigate current U.S. ITAR constraints– Coordination with the
UNESCO World Solar Program; e.g., (a) by identifying specific needs of developing
countries (such as in Japan’s SPS-2000), (b) by promoting SSP as a supplement
to terrestrial solar systems.– Developing long-term
energy demand scenarios from all sources.– Addressing
non-technical international issues; e.g., those identified at Unispace-3(3) An organized international
information exchange mechanism was identified as being needed to publish and
review the work being done in all countries. This was set up in May 2000 in the
form an International SSP Wing of NASA’s Virtual Research Center. It requires
access badges but does not involve ITAR-sensitive information.(4) Avenues for involving the public
should be explored. These might include demonstrations having general public
and/or educational interest and public participation.
Multiple Use of SSP-Enabling Technologies
The key SSP technologies
could find broad applications in other areas of space science, exploration, and
development. The AIAA assessment identified a wide range of examples of such
applications. The following were considered to be high-priority suggestions:
(1) Solar power
generation. High-efficiency solar cells for small stand-alone terrestrial
powerplants, for individual dwellings, and for supplemental power in
automobiles; thin-film Fresnel concentrator lenses for space telescopes;
efficient thin-film flexible solar arrays for a variety of low-power consumer
products; and ultralight solar arrays for a wide range of satellite
applications.(2) Wireless power
transmission. Efficient, mass-produced, low-cost, clean-spectrum,
microwave-oven and plasma-lamp exciters using frequency-locked magnetrons;
bistatic radar illuminators for both planetary mappers and orbital debris
detection; high-temperature radio-frequency (RF) phased-array systems for
spacecraft communications; and optical amplifiers for both power transmission
and high data-rate communications(3) Power management and distribution.
High-voltage switches (e.g., 600 V); high-temperature silicon carbide power
semiconductors; and modular converters.(4) In-space transportation.
High-efficiency solar-electric propulsion systems; and beamed laser propulsion
systems, using a laser based at a fixed power station to transmit power to a
spacecraft continuously.
The AIAA assessment suggested a number of
opportunities for multiple-use of the SSP-enabling technologies in terrestrial
and space endeavors Of these, the following high-priority areas were
identified:
(1) Human space exploration.
(a) Power systems for the Martian
surface. If nuclear systems turn out not be available for use, large
photovoltaic arrays in the 100 – 200 kWe range, coupled with wireless power
transmission (WPT), become highly promising. These solar power systems are
especially attractive if they can be combined with an Earth-Mars transportation
system using solar-electric propulsion (SEP).(b) In-space transportation. SEP
is generally considered a viable alternative to nuclear thermal propulsion for
human Mars exploration.(c) Beamed power. WPT could be
used for mobile extraction systems deployed in permanently-shadowed cold traps
at the lunar poles and for in-situ
resource utilization at various locations on Mars. Other applications include
beamed power to communications and information-gathering stations on planetary
surfaces or in orbit; e.g., high-power radar mappers; mobile robotic systems;
remote sensing stations; dispersed habitation modules; human-occupied field
stations; and supplementary power to surface solar power systems during periods
when they are shadowed.(2) Science and robotic space exploration
(a) Multi-asteroid sample return.
Visit a significant number of belt asteroids in a 2-5 year period, collecting
samples for return to Earth.(b) Asteroid/comet analysis.
Determine the chemical content of comets and asteroids on rendezvous missions
(enabled by solar-electric propulsion) by using deep-penetration imaging radar
and by beaming laser and/or microwave power down to the surface to vaporize
material for spectrographic analysis.(c) Orbital debris removal. Use
beamed energy to rendezvous and grapple with a piece of space junk. Space-based
lasers could also be used to vaporize smaller debris or to redirect the orbits
of larger pieces to atmospheric reentry trajectories.(d) Weapons-oriented demonstrations. Fire a high-energy laser
from a lunar orbiter at the lunar surface to vaporize and excite surface
materials, determining their chemical composition with a spectrometer aboard
the orbiter.(e) In-space transportation. Use
SEPS for a wide variety of science missions, also using WPT for sensor
deployment via laser sails, laser-thermal propulsion, and laser-electric
propulsion.(f) International space station (ISS).
Replace ISS solar arrays using advanced
SSP technologies, and use WPT for co-orbiting experiment platforms.(g) Radar and radiometer mappers.
Use high-power planetary probes to conduct radar mapping of planetary surfaces
and high-power radiometer surveys for comprehensive scientific studies of
planetary environments.(h) Rovers. Deploy many small
rovers on lunar and planetary surfaces using WPT.(i) Networked sensor systems. Use
hundreds of tiny WPT-powered sensors to conduct detailed four-dimensional
surveys of interplanetary and other space regions.(3)
National security missions(a) Military surveillance. Use WPT
for very small military surveillance satellites.(b) Radar satellites. Use advanced
SSP-type solar arrays to power 100-200-kW radar sensors.(c) Maneuverability. Use
WPT-driven electric propulsion to increase maneuvering reserves.(d) Unmanned aerial vehicles (UAVs).
Use WPT-powered UAVs for a number of military purposes, especially long-term
surveillance and battlefield communications.(4) Commercial space development
(a) Power for communication satellites.
Use WPT from dedicated space-based powerplants.(b) High power for the International
Space Station (ISS). Beam supplementary power to the ISS to extend the
scope and breadth of commercially oriented research and experiments, allow
additional crew members, and increase the station’s self-sufficiency.(c) High-efficiency solar arrays.
Use high-performance SSP power and structure technologies to provide power
growth (e.g., up to 35 – 50 kWe) for communication satellites.(5) Terrestrial applications
(a) Aerial vehicles. Use
WPT-powered aircraft for surveillance with indefinite loiter capability,
meteorological observations, field communications between
line-of-sight-obstructed mobile stations, measurement of high-altitude
Sun-Earth interactions, upper-atmosphere sampling without contamination by
onboard combustion, pollution monitoring and other Earth-observation
applications, and support of Mars surface operations.(b) Offshore oil platforms. Use
WPT to transmit to land the power converted from now-wasted natural gas via
onboard turbine-generators.(c) Tornado mitigation. Use WPT from a
space-based satellite to heat raindrops in cold downdraft regions of
mesocyclones in large thunderstorms.
From among these multiple-use
opportunities, the AIAA assessment selected the following prospects for
near-term demonstrations:
(1) System flight demonstration. Use a solar array mounted
in the Shuttle’s payload bay to demonstrate power transmission to nearby
(co-orbiting) targets.(2) Tether demonstration. Use the
Shuttle to demonstrate a static tether by releasing a mass to a higher orbit
(tether up) and releasing a mass to de-orbit it (tether down).(3) Robotic operations. Use robot
platforms to demonstrate end-to-end transport of cargo and installation on the
international space station.(4) Ground power conversion comparison.
Demonstrate WPT using threeadjacent
ground-based power systems employing (a) ground-based photovolaic arrays, (b)
ground-based arrays supplemented by laser power at approximately one-sun
brightness, and (c) ground-based arrays supplemented by microwave power.(5) Combined power/communications
systems. Demonstrate microwave power transmission containing high data-rate
information.(6) Power beaming to aerial platforms.
Use magnetron directional amplifiers to transmit power to aircraft and/or
airships for telecommunications, observation, and stratospheric/tropospheric
science demonstrations.(7) High-power Mars-orbiting
communication relay satellite. Demonstrate
SSP technologies aboard a Mars-orbiting high-power communications satelliterelaying Mars probe information directly to
Earth at very high data rates.(8) Orbital debris removal
Maneuver a Shuttle-based or ISS-based small satellite, using beamed energy, to
rendezvous and grapple with a piece of space junk and lower its orbit.(9) Lunar surface spectroscopy.
Fire a laser spectrometer, aboard a lunar orbiter, at the lunar surface with
enough energy flux to vaporize and excite the surface materials to determine
their chemical composition.(10) Transport of energy from offshore
oil platforms. Transport energy obtained from natural gas combustion to
land via WPT.(11) Comet Rendezvous. Use a
high-power (150 kWe) spacecraft to
rendezvous with a comet and (a) conduct deep-penetration studies using
imaging radar, (b) use laser drilling to perform high-resolution mass
spectrometry and chromatography of volatilized material from the comet’s core,
and (c) transmit to Earth high-definition television (HDTV) images of the
comet.
Technology
Assessment
Overall
Conclusions. This section of my testimony addresses directly the subject of
the hearing: “The Technical Feasibility of Space Solar Power.” The overall
conclusions of the AIAA assessment in this area are as follows:
Although implementation of a viable SSP system for
commercial delivery of terrestrial power is still far in the future, NASA’s
SERT program has identified and defined the key technologies and has laid out
rational roadmaps leading to ultimate demonstration of those technologies.
Moreover, the quality and quantity of technical advancement achieved by the
SERT program in virtually all the key SSP-enabling technologies were excellent,
demonstrating a level of accomplishment far in excess of what would be expected
from the modest funding committed by NASA to the SERT program.
Especially noteworthy were the
formulation of new system configurations which substantially reduce SSP
technical and economic risk, remarkable improvements in solar power generation
technologies (including actual measurements and flight demonstrations), and
significant advancements in structural, robotic, power management, and
materials technologies.
Perhaps most important was the emergence
and validation of a viable alternative to microwave power transmission: laser
power beaming, at intensities that comply with current health regulations and
at acceptable projected overall system efficiencies. Although the terrestrial
environmental issues are being addressed by the SERT studies, the space
environmental concerns may turn out to preclude the use of microwave-based
concepts.
The major barrier to eventual
implementation of terrestrial power delivery from space, as with all large
space systems, is the lack of a national commitment to develop a viable path to
low-cost, reliable space transportation.
Comparisons need
to be developed of environmental, health, and safety issues of SSP (for
terrestrial markets) with other competing power sources and with hybrid
(terrestrial + space) supply scenarios. In addition
to small-scale demonstrations of key technologies, scale models or pilot-plant
demonstrations of SSP concepts would provide unique information in virtually
all areas. Also, how much of the required demonstration work would be possible
on the ground is an important question that should be addressed.
Specific
Conclusions. The specific conclusions of the SSP technology assessment were
as follows:
Solar power generation. Progress in identifying and
demonstration advanced power generation technologies has been significant.
Among the most promising of these are ultra-low-mass, high-efficiency thin-film
photovoltaic (PV) arrays; solar dynamic power generation using Brayton-cycle
power conversion; a stretched Fresnel lens concentrator design which has
already demonstrated 378 W/kg and over 300 W/sq.m in conjunction with
triple-junction PV arrays; a “rainbow” assembly of five or more different PV
cells covering a broad range of the solar spectrum, illuminated through prisms
to split the spectrum appropriately, which has achieved a net efficiency of 39%;
and quantum-dot (Q-Dot) PV arrays which, when developed, will employ quantum
transport to cover 80% of the available solar spectrum. Prospects are also
being explored for combining the stretched lens, rainbow, and Q-dot PV
technologies to achieve near-100% coverage of the solar spectrum.
Wireless power
transmission (WPT).
(1)
Microwaves. The technology in this area is mature and well defined.Technical progress made during the SERT
program has been primarily in defining the key interference issues and devising
commensurate filter designs, and in developing phased-array antennas employing
solid-state amplifiers based on high-temperature gallium nitride integrated
circuit technology. Even if these developments are successful, however, the
prospects are dim for obtaining regulatory approval of a high-power
transmission system which has any possibility of interference with
communications satellites and position-location systems operating from both
geostationary and low Earth orbits.(2)
Lasers.One of the major new
developments of the SERT program has been the emergence of laser power
transmission as a viable option that complies with existing health regulations.
Viable system concept designs have been proposed and are being pursued. The
most promising of these employs a number of satellites in halo orbits equipped
with PV panels feeding incoherent arrays of thousands of solid-state yttterbium
lasers. Other laser concepts being explored include directly solar-pumped
iodine laser sources, solar-thermal conversion to provide the electric power
feeding the laser diodes, and beaming from the satellites to the ground via
circling high-altitude airships or aircraft instead of directly. Laser
technology is advancing rapidly, the technical risk does not appear excessive,
and the use of lasers resolves many of the concerns with the more mature
microwave power transmission technology. However, despite the benign nature of
the actual power transmission mechanism and its full compliance with existing
standards and regulations for laser exposure, the “weapon perspective” will
impose serious public-image barriers.
Power management and
distribution (PMAD).
The sheer size of the SSP concept using microwave power transmission presents a
formidable PMAD challenge, requiring high voltage, high currents, complex
command and control, and protection against plasma discharge and arcing. PMAD
components such as switchgear, cabling, and power converters all require
significant technological improvements in performance. GPS-based systems for
command and control have been devised and need to be demonstrated.Transmission and distribution systems using
low-temperature superconductors appear practical for some SSP configurations,
using a low-voltage DC architecture that minimizes the need for power
converters, sharply reduces arcing and breakdown risk, and therefore
significantly cuts SSP mass, cost, and risk of failure. In converters, much
progress has been made in developing and validating high-power,
high-temperature silicon carbide components and electronic control circuits,
which have been demonstrated to be far more reliable than silicon circuitry.
However, their mass penalty when used with conventional magnetrons, klystrons,
or solid-state phased arrays severely compromises them as compared with either
lasers or low-voltage PMAD systems using superconductors.
Structures, materials,
and controls
(1) Structures. A number
of innovative structural concepts have been proposed to reduce the mass of the
basic SSP structure needed to support the solar panels and transmitter. The
most promising of these employ deployable composite or inflatable tetrahedral
or prismatic trusses used in conjunction with tension members and stretched
graphite tension webs.(2) Structural Stability.
Non-planar solar-array truss configurations (prismatic or tetrahedral) increase
the fundamental vibration-mode frequency of some SSP structures by factors of 3
or 4. However, it remains to be seen whether or not such large, massive systems
(e.g., 2,800 metric tons) with low stiffness (e.g., fundamental-mode frequency
of 0.01 Hz) are indeed controllable.(3) Materials. Graphite epoxy composites and thin-film polymers
are the materials of choice, the former for stiff structures and the latter for
inflatables. Graphite epoxy composites have low mass and high stiffness, and
can be fabricated by automatic beam-builders. They could also be used for
inflatable trusses by employing thermosetting composites cured by ultraviolet
light. Thin-film-polymer inflatable structures have been identified as one of
the primary options for SSP, but there are serious issues regarding their
long-term durability. Packing and deployment processes can exacerbate their
degradation in space by imparting stress concentrations that create sites for
incipient failure. Two highly promising advanced materials are carbon
microtruss fabric, which has been demonstrated for use in high-temperature,
high-acceleration laser sails, and carbon nanotubes, which have been
demonstrated to be 100 times stronger than steel at one-sixth the mass. Both
would require significant development.(4) Controls. In addition
to shape control, primary structure control elements involve attitude sensing,
attitude control, and command and data-handling. Systems involving combinations
of sun-sensors and electric thrusters (also used for orbit-raising and
stationkeeping) have been devised, but involve very high data rates to assure
reliability (e.g., 1.5 Mbps). The major control issues are defining control
laws for large, loosely coupled structures and developing sufficiently
reliable, low-cost sensors, ion thrusters, transmitters, and transponders.
Thermal materials and
management. The
range of potential requirements for the thermal management system is very wide.
Although several deployable radiator concepts (with experimental hardware) have
been developed, thermal modeling of the SSP concepts is not yet adequate.Little effort has been devoted to the
competing requirements of clear fields of view for sunlight input, radio-frequency
(RF) output, and thermal radiator output.
Some thermal
architecture progress has been made, but much more needs to be done,
particularly for the "sandwich" concepts that marry photovoltaic
arrays on one side with radio-frequency arrays on the other.
The thermal architecture
of laser systems is about in the same state. Because of the lower overall
system efficiency, heat rejection loads are higher than for microwave systems.
Also, laser performance is temperature-dependent, so thermal management is
likely to be the major design driver for SSP concepts employing laser power
transmission. Considerable effort will be required to evaluate the various
trades.
Robotic assembly,
maintenance, and servicing
(1) Fabrication, Assembly
and Integration. Once the SSP elements are transported to GEO, they would have
to be assembled and integrated into an overall SSP system capable of providing
gigawatts of power. The SERT program is exploring a number of different
autonomous robot concepts for these functions, and rudimentary operating models
have been built and tested. A robotic workforce analysis for SSP systems is
also being conducted, and rough estimates of the mass and cost requirements of
such systems are being prepared. However, a great deal of work still needs to
be done, especially in the architecture needed to coordinate and integrate the
work of many robot devices simultaneously.(2)
Fault Detection and Maintenance. The SSP requires an elaborate on-board
diagnostic system to ensure reliable assembly and operation. The same classes
of robots developed for fabrication, assembly, and integration could be used or
adapted for routine inspection, fault detection, and maintenance tasks. Again,
however, a great deal of work remains to be done.
Platform systems. Reliability analyses are addressing
risk-mitigation approaches for mission assurance; failure modes; lifetime
predictions; and maintenance requirements. System monitoring and health
management functions are also being defined and evaluated; i.e., communications
and data-handling; command and control algorithms and linkages; and the
communications, data-processing, and command infrastructure needed for robotic
systems and operations. The tools required for these functions have been
identified, along with the required autonomy capabilities. All these tools and
functions require significant technology advancement beyond
state-of-the-art.
Ground power systems
(1)
Microwave Receiving Antenna. Rectenna technology is mature and poses little or
no technical risk. Because of the high cost of the large mass of hardware
required, however, efforts to improve operating efficiency and reduce cost were
implemented during the SERT program.(2) Laser Power
Conversion. The ground receiver for laser power transmission is, simply, a
ground-based photovoltaic solar power plant, but with much higher conversion
efficiency than for sunlight. The beam spot for a 500 MW powerplant is small –
less than 1 km – and rain attenuation of the laser is dealt with by beaming to
several alternative PV fields, all of which have dual use as ground-based solar
powerplants. Other benefits include matching of power supply to low-demand
consumers and combination with ground-based solar power suppliers to meet
typical daily demand profiles.(3) Energy Storage.
Storage options being evaluated include pumped hydro, compressed-air storage,
electrochemical (batteries, fuel cells), inertial (flywheels), and
superconducting magnetic energy storage. There is as yet no clear “winner” of
the various trade studies.
Earth-to-orbit
transportation and infrastructure. The Earth-to-orbit cost goal of $400 per kg. in 10 years
is critical to making the SSP concept economically feasible. The current
reusable-vehicle technology demonstrator program (X-33), which is based on a
single-stage-to-orbit concept, appears to be foundering. But even if we do
achieve a lower-risk two-stage reusable system, the achievement of $400 per kg
in ten years does not seem reasonable. It is
possible that, with substantial effort, costs of $1,000 to $2,000 per kg could
be reached in perhaps 15 years. Hence launch cost will continue be the major
economic barrier to any SSP system within the next two decades.
Some of the heavy-lift
EELV configurations currently being developed are projected to achieve launch
costs less than $4,000 per kg for 40-metric-ton payloads, and hence might be
considered in the relatively near term (i.e., 5 to 15 years) for placing
demonstration or pilot SSP plants in orbit, especially those employing laser power
transmission, which can be built in the 40-metric-ton size.
The SERT program did not
attempt to develop new Earth-to-orbit (ETO) space transportation technologies,
and properly so, since the subject is far too ambitious for the minimal
resources available to SERT.
In-space transportation
and infrastructure
(1) Orbit Transfer. The
SSP system will require transportation from its delivery point in the Low Earth
Orbit (LEO) to its operational orbit in GEO. This requires an efficient,
high-specific-impulse propulsion system which, ideally, should also have high
thrust. Hall-effect thrusters would be well suited for this application, as
would a solar thermal propulsion system.Propulsion technologies
explored during the SERT program included ion propulsion, Hall-effect
electromagnetic thrusters, various forms of magnetoplasmadynamic (MPD)
thrusters, arcjets, electrothermal thrusters, tethers, solar-thermal power (for
either ion or Hall-effect thrusters), nuclear thermal propulsion, and laser
propulsion.Orbit transfer analyses
for the four system configurations studied during the SERT program resulted in
a wide range of propulsion mass conclusions. Although the technologies for both
ion and Hall-effect electromagnetic propulsion of spacecraft are mature (both are
now in use onboard commercial satellites), much technology-related development
will be needed for the SSP’s very large distributed payload and the multiple
orbit-transfer operations scenarios required.(2) Stationkeeping. In
addition to the conventional north-south and east-west stationkeeping
functions, the SSP requires attitude stabilization of the large structure to
counteract gravity gradient forces, countering solar pressure, and maintaining
shape control of the very large, flexible structure. This will require an
onboard propulsion system consuming several hundred kilowatts to megawatts of
power and a substantial amount of onboard propellants. However, the engines
used for orbit raising provide far greater thrust than is needed for all these
stationkeeping functions; for example, the amount of propellant needed for
orbit raising would provide over 40 years of station-keeping.Because of the extreme
size and flexibility of the SSP (all configurations), the application of
station-keeping thrust must be carefully coordinated with forces and moments
applied for attitude control and shape control, possibly by a combination of
thrusters and control-moment gyros. The attitude-control requirements must be
carefully integrated with the station-keeping thrust to maintain stable
attitude during thrust maneuvers, as well as minimizing excitation of the
flexible structure. These problems do not have simple solutions, but are
certainly amenable to analysis and eventual resolution.
Environmental and
safety factors
(1)
Interference. The effects of SSP microwave radiation sidelobes and harmonics on
other spacecraft in both GEO and LEO may well preclude the deployment of any
large-scale SSP employing RF power transmission. In any case, these power
satellites will require considerable filtering, which means substantial mass
and insertion loss. To resolve these and other wireless power transmission
issues, a test and verification facility is needed to develop techniques and
make actual measurements of the various spectrum sidelobe and grating lobe
levels from a large power-transmitting phased array.A similar facility should be constructed and operated for laser
beaming.(2) Orbital
Debris.Although the SSP configurations
are large, their diaphanous nature and location in geostationary or
geosynchronous halo orbits imply low susceptibility to serious damage by either
natural or anthropogenic orbital debris. Moreover, since all the proposed
concepts employ robotic inspection and maintenance, repairs of any such damage
should be able to be accomplished.(3) Effects on Terrestrial
Environment, Health, and Safety.These
effects are being evaluated by a separate study and were not covered by the
AIAA assessment.
Systems integration
(analysis, engineering, modeling)
(1) General. Two of the
most important accomplishments of the SERT program were (a) the formulation and
refinement of several system concepts that significantly reduce both technical
and economic risk; and (b) the definition of a number of small-scale demonstrations
involving applications for SSP technologies and capabilities in other areas of
both space and terrestrial activity.(2) System Requirements.
Insufficient attention has been given to the system requirements and interfaces
for a fleet of SSP spacecraft; e.g., safety control for the many multiple
beams, the Earth electrical grid interfaces for gigawatt-level beam outages,
and fast-acting energy storage and switching. The primary requirements issues
should be defined and generic paths formulated to resolve them.(3) Systems Analysis.
Coordinated systems analysis of the various SSP concepts, the model system
categories, and the point-of-departure designs have been extremely effective in
helping guide and systematize the course of SSP research. Point-of-departure
designs for the following SSP system concepts have been especially useful:
gravity-gradient abacus (SunTower-derived); reflector abacus; integrated
symmetrical concentrator; and Halo orbit concept. The system analyses for these
concepts included power train (efficiency) analysis; PMAD design concepts;
launch packaging and deployment concepts for the solar arrays, reflectors, PMAD
systems, and transmitters; robot assembly procedures; and full mass and cost
breakdowns, plus a number of sensitivity studies.
Point-of-departure designs for early
demonstration projects included space-station free-flyer demos using the
Spartan payload, a cargo delivery and power beaming vehicle, a low-Earth-orbit
propellant conversion and cryogenic storage facility, a Mars transfer vehicle,
a lunar crater ice-mining mission, a high-power commercial communication
satellite, a Mars cargo mission, a Mars human-crew sprint mission, and various
laser power transmission applications, both large and small scale.
Preliminary results of the system
analysis appear to be well supported by the analysis. They include the
following:
(1) Solar cell efficiency is a major
mass and size driver.(2) Solar-thermal power generation
using a Brayton (gas-turbine) cycle offers the highest overall system
efficiency, followed by Q-dot PV systems.(3) Increasing power density via the
Stretched Lens Array (SLA) concentrators also has a major effect on mass and
size reduction of PV-based power generation concepts.(4) Technology for the small
assembly robots, and especially control issues for multiple coordinated robot
families, is highly immature, and imposes a major technical risk.(5) The high voltage required for
microwave-system PMAD poses significant technical risk.(6) Structural and PMAD mass of the
SunTower and SunTower-derived concepts has grown significantly since the 1998
Concept Definition Study. However, the new Integrated Symmetrical concentrator
concept reduces both structural and PMAD mass significantly.(7) The filtering required to preclude
interference with communications satellites will be very costly in overall
system efficiency, and hence in both mass and cost.(7)
There is little cost sensitivity among the three microwave power
transmission devices (klystrons, magnetrons, or phased-array solid-state
devices).(8) Reflector flatness is a key
factor in the ISC and transmitter-reflector configurations.(9) PMAD systems employing ac are
much lighter and more efficient than those employing dc.
Feasibility
of Power Relay Concepts
You
had asked specifically for an assessment of “the deployment of satellites
designed to relay power from Earth-based power generation facilities.”This subject was studied in some detail
beginning in the 1960s, but recent technology developments certainly warrant a
“fresh look” at the concept.
There
are three possible mechanisms for relaying power via satellite from one
terrestrial location to another: reflected sunlight; transmission and
reflection (or conversion and retransmission) of microwaves; and reflection of
laser power beams. Because of difficult control problems associated with the
use of low-Earth-orbit satellites, it is probable that only geostationary-orbit
satellites could be used.
Previous
studies had examined the use of reflected sunlight and reflected (or
retransmitted) microwaves, and had concluded that excessive dispersion made
both impractical for geostationary-orbit ranges. In the case of reflected
sunlight, the dispersion results from the broad spectrum of even concentrated
sunlight beams. In the case of microwaves, the diffraction due to the low
frequency of the power beam would require enormous receiving antennas or
reflectors in orbit.
The
now-promising use of laser power transmission, however, as indicated earlier in
my testimony, opens a new vista on the satellite relay concept. The collimated
narrowband laser transmission mode significantly reduces beam dispersion, and
laser reflectors have been shown to be highly efficient. The major technical
problem, aside from the still-low (but improving) efficiency of laser
power-beaming technology and the need for alternative sites to allow for heavy
clouds or rain, is the need to comply with current health regulations on laser
power density. As with the above-described SSP systems, this constraint
requires the use of thousands of low-intensity laser beams, substantially
increasing the minimum size of the photovoltaic receiver field. As far as I
know, this approach has not been analyzed in detail, and would certainly be
worthy of scrutiny. In addition to assessing the purely technical aspects of
the reflected system, possible means might be explored for increasing
permissible beam density without abrogating safety regulations. For example,
beam density of the uplink could conceivably be increased significantly without
violating the regulations.
Thank
you for the opportunity to address the Subcommittee on this important subject.
I will be pleased to respond to any questions you may have.
AIAA
American Institute of Aeronautics and Astronautics
BRIEF
BIOGRAPHY: JERRY GREY, PhD
DIRECTOR, AEROSPACE AND SCIENCE POLICY
Dr.
Grey received his Bachelor’s degree in Mechanical Engineering and his Master’s
in Engineering Physics from Cornell University; his PhD in Aeronautics and
Mathematics from the California Institute of Technology.
He
was Instructor in thermodynamics at Cornell, engine development engineer at
Fairchild, Senior Engineer at Marquardt, and hypersonic aerodynamicist at the
GALCIT 5-inch hypersonic wind tunnel. He was a professor in Princeton
University’s Department of Aerospace and Mechanical Sciences for 17 years,
where he taught courses in fluid dynamics, jet and rocket propulsion, and
nuclear powerplants and served as Director of the Nuclear Propulsion Research
Laboratory. He was President of the Greyrad Corporation from 1959 to 1971,
Adjunct Professor of Environmental Science at Long Island University from 1976
to 1982, and Publisher of Aerospace
America from 1982 to 1987. He is now Director, Aerospace and Science Policy
for the American Institute of Aeronautics and Astronautics, Editor-at-Large of Aerospace America, member of the
Universities Space Research Association’s Science Advisory Panel for the NASA
Institute for Advanced Concepts, consultant to a number of government and
commercial organizations, and Visiting Professor of Mechanical and Aerospace
Engineering at Princeton.
Dr. Grey is the author of twenty
books and over 300 technical papers in the fields of space technology, space
transportation, fluid dynamics, aerospace policy, solar and nuclear energy,
spacecraft and aircraft propulsion, power generation and conversion, plasma
diagnostics, instrumentation, and the applications of technology. He has served
as consultant to the U.S. Congress (as Chairman of the Office of Technology
Assessment’s Solar Advisory Panel and several space advisory panels), the
United Nations (as Deputy Secretary-General of the Second UN Conference on the
Exploration and Peaceful Uses of Outer Space in 1982), NASA (as a member of the
NASA Advisory Council), the Department of Transportation (as Vice-Chairman of
the Commercial Space Transportation Advisory Committee), the Department of
Energy (as a member of the Secretary of Energy Advisory Board), and the U.S.
Air Force, as well as over thirty industrial organizations and laboratories. He
was Vice-President, Publications of the AIAA, Chairman of the Coordinating
Committee on Energy of the American Association of Engineering Societies, a
Director of the Scientists Institute for Public Information, Vice-President of
the International Academy of Astronautics, and President of the International
Astronautical Federation.
He
is listed in over twenty biographical publications, and has received national
awards from the Aviation/Space Writers Association and the American
Astronautical Society.
