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
September 7, 2000
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.
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.
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.