Space Power Resources, Manufacturing and Development Volume 13 Number 1 &2 1994
SPACE POWER Published under the auspices of the Council for Economic and Social Studies on behalf of the SUNSAT Energy Council. Editor: Dr. Gay E. Canough, ETM Solar Works, Inc. Associate Editors: Fred Koomanoff, Dept, of Energy, USA Andrew Hall Cutler, Minerva Labs, USA Richard Boudreault, Consultant, Montreal, Canada Lars Broman, SERC, Sweden William C. Brown, Massachusetts, USA Lucien Deschamps, Paris, France Ben Finney, U of Hawaii, USA Peter Glaser, Aurther D. Little, Inc. USA Dieter Kassing, ESTEC, The Netherlands Mikhail Ya. Marov, U of North Carolina, USA Gregg Maryniak, International Space Power Program, USA Makoto Nagatomo, ISAS, Japan Viorel Badescu, Polytechnic University of Bucharest John R. Page, U of New South Wales, Australia Tanya Sienko, NASDA, Tsukuba, Japan Space Power is a quarterly, international journal for the presentation, discussion and analysis of advanced concepts, initial treatments and ground-breaking basic research on the technical, economic and societal aspects of: large-scale space-based solar power, space resources utilization, space manufacturing, space colonization, and other areas related to the development and use of space for the benefit of humanity. Recent subject coverage: • history and status of national space power programs • technologies for large-scale space power e.g. solar power satellites • systems aspects of large-scale power, e.g. SPS and central space power utilities • potential extraterrestrial resources for use in space-based manufacturing • lunar and planetary science for understanding space resource location and availability • plasma and other space environment interactions with large space structures • medical, psychological, sociological and cultural aspects of human presence in space • forms of advanced space propulsion and power technologies and systems. Space Power is published four times per year. These four issues constitute one volume. An annual index and titlepage is bound in the December issue. 1994 is volume 13 ISSN = 0883-6272 Editorial Correspondence: Dr. Gay E. Canough, Space Power c/o ETM Solar Works, Inc., PO Box 67, Endicott, NY 13761, phone/fax = (607) 785-6499 e-mail (Internet): CANOUGH@BINGVAXA.CC.BINGHAMTON.EDU radio call sign: KB2OXA. Business Correspondence including orders, subscriptions, advertisements, back issues and off prints should be addressed to the publisher: Council for Economic and Social Studies, 1133 13th NW, suite 2-C, Washington DC 20005, 202 371 2700, fax 1523 Subscriptions: libraries: $288/year, individuals: $155/yr., additional $25 for airmail Cover: An artist's conception of a solar power satellite constructed of lunar materials, on station over the Earth's equator. Reproduced by courtesy of Charles L. Owen, Institute of Design, Ulinois Institute of Technology, 10 W. 35th St. Chicago IL 60616.
The Power Relay Satellite DR. PETER E. GLASER* Summary: The availability and use of renewable energy sources compatible with reducing risks to the global environment are key to sustainable development. Large- scale, renewable energy resources at undeveloped or underutilized sites are potentially available on several continents. The Power Relay Satellite (PRS) concept has the potential to access these remote energy resources by uncoupling primary electricity generation from terrestrial transmission lines. A global PRS network can be envisioned to provide a high degree of flexibility for supplying energy demands worldwide with wireless power transmitted from sites on Earth to geosynchronous orbit and then reflected to receivers interfacing with terrestrial power transmission networks. Past developments in wireless power transmission (WPT) are reviewed and recent successful results are noted. The origins of the PRS concept, and a possible configuration are discussed, principles of WPT at microwave frequencies, functional requirements, and system design constraints are outlined, and space transportation concepts presented. PRS assessments including applicable technologies, economic projections, and societal issues are highlighted. It is concluded that the PRS provides a promising option to access renewable resources at great distances from major markets, and represents an important stage in the future development of solar power satellites. Introduction The availability and use of energy in all of its varied forms have been key determinants in the evolution of life on Earth; they will continue to have a dominant influence on human affairs on Earth and the evolution of human activities in space. The realizations that the Earth system is closed in a material sense and that it virtually remains in a steady-state with respect to matter have strengthened the belief that the availability of renewable or inexhaustible resources will be essential to the continued progress of humanity. Technology always has had a strong influence on the nature and development of industrial activities. These capabilities already demonstrated during the preceding three decades of space exploration have created a foundation for the evolutionary steps that can be taken to achieve the promise of the space frontier. Now that this new frontier has been opened, there is no turning back. Space activities are expected to influence the 21st Century's international, political and commercial relationships as significantly as airplanes, electronics, computers and communications shaped the global economy of the 20th Century. The development of the space infrastructure must be viewed within the context of large-scale engineering endeavors continuing over an extended period. These * Vice President Arthur D. Little, Inc. Cambridge, MA 02140
endeavors should be treated as part of a new direction for industrial space development - not as an exceptional occasion for daring feats or mere entertainment. The evolution of the space infrastructure follows in the tradition of pushing forward frontiers and productively using new areas made accessible for settlement, cultivation and industrialization. Rationale For Long Distance Power Transmission There are both economic and environmental pressures requiring the development of alternative, renewable energy sources worldwide. The rationale for these efforts are the continued growth of the global population, the disparities in living standards between developed and developing countries, and the movement of populations from rural communities to urban centers. From a longer term perspective, preferred technologies for generating electricity will be those that do not place the global environment at risk and are acceptable to society. Among the technologies of interest are the utilization of large-scale, renewable energy resources at undeveloped or under-utilized sites to generate electricity. Renewable resources, for example, include hydroelectric power on several continents. These resources are significant: hydroelectric power is already making a 20% contribution to global electricity generation. However, many of the most promising sites for the utilization of renewable energy resources are located at great distances (in excess of 3,000 miles) from major markets. Therefore, effective means for electrical power transmission are required. The currently available technologies for power transmission include high voltage transmission lines, underwater cables, and wireless power transmission (WPT). Transmission of AC electric power using overhead lines operating at 230 kV and higher is standard for most generation and transmission equipment. Direct current transmission lines are used when the savings in line cost outweigh the cost of conversion to ac. The longest de transmission line in operation transmits 2 GW at 530 kV over a distance of 895 miles from the Zambesi River hydroelectric complex to South Africa. A 500-mile de transmission line with a capacity of 6.3 GW is being planned to connect the Itaipu hydroelectric station in Brazil to Sao Paulo. Underwater cables although used for short transmission links to islands or over limited distances (e.g., a 2 GW cable crosses the English Channel) are not being seriously considered for intercontinental power transmission. Super-conducting transmission lines may be of practical interest when higher temperature superconductors will be developed. However, the installation of reliable super-conducting transmission lines must await the results of further research. Past Developments In Wireless Power Transmission The beginning of efforts to develop wireless power transmission (WPT) can be traced to Nikola Tesla's experiments with wireless transmission of electricity on Long Island, New York, early in the 1900s. The development of WPT was not pursued until a microwave-powered helicopter platform was successfully demonstrated in 1964. [1]
This demonstration proved that WPT system could be constructed and that microwave generators and dipole rectifiers could be developed for efficient conversion of microwaves into de. A microwave WPT demonstration was successfully completed in 1975 at the NASA Deep Space Antenna facility at Goldstone, California. In this demonstration, a microwave beam at a frequency of 2.45 GHz transmitted 30 kW over a distance of one mile to a receiving antenna. The microwaves were converted directly into de at an average efficiency of 82%. As part of the Stationary High-Altitude Relay Platform (SHARP) Program, the Canadian Department of Communications in 1987 demonstrated that an aircraft can be maintained at an altitude indefinitely when powered by a controlled microwave beam. Several test projects including WPT to an aircraft in 1992, and from a rocket to a daughter satellite in 1993, were successfully performed in Japan. WPT in the microwave and laser portions of the electromagnetic spectrum was investigated as part of the Space Power System (SPS) Concept Development and Evaluation Program, by NASA and the U.S. Department of Energy. [2] This program evolved a SPS reference system with 5 GW delivered to the terrestrial transmission network, and provided detailed information on the technical and economic feasibility and societal acceptability of WPT. The results of this program were noted by organizations in Europe, Japan, and the former USSR, leading them to investigate power beaming. International organizations such as the Institute of Space and Astronautical Science, Japan, the European Space Agency and Eurospace, and several institutes of the former USSR Academy of Sciences have been and continue to be involved in power beaming studies. Global Power Relay Satellite Network The Power Relay Satellite (PRS) was proposed by Kraft Ehricke [3] as a means to transmit energy to specific locations distant from remote energy resources. A PRS can be located in geosynchronous orbit (GEO) to reflect a microwave beam to a selected receiving site. The energy sources should be located within less than ± 30 degree latitude to reduce system antennae dimensions A global PRS network with each transmitter beaming to a sector of GEO containing one or more PRSs can be envisaged. A global PRS network would provide a high degree of flexibility for supplying energy with attendant socio-economic advantages because renewable energy sources, e.g. hydropower could be used. Renewable Resources A scenario for an assessment of the feasibility of a PRS included hydroelectric power beamed from South America to Spain to supply the European power grid. The
hydroelectric power potential of the Caroni River, Guayana, Venezuela represents more than half of Venezuela's installed electric capacity, and is part of the world's second largest hydroelectric complex (10 GW) completed in 1986. With further additions about 17 GW capacity, will be completed by 2000 providing abundant electrical energy at a very competitive price. The existence of a transmission system provides several options for the location of a microwave beam transmitting antenna. Specific locations considered are in areas where the terrain is flat, rainfall is moderate, so as not to reduce beam transmission efficiency, environmental effects of construction are minimized and societal cooperation is likely. In 1989, Brazil had a total capacity of 54 GW with 96% supplied by hydroelectric power. Brazil's capacity expansion efforts have centered around large hydroelectric projects, such as the 12 GW Itaipu Binacional station located at the Paraguyan border. Other large projects include the 7.3 GW Tucurui station in the Amazon Basin. The upper Amazon has the potential for the development of very large hydroelectric capacity in the future. The Power Relay Satellite Concept The configuration of a PRS is shown in Figure 1. In this system, electricity is fed to microwave generators operating at a frequency of 2.45 GHz or 5.8 GHz. The microwave generators are incorporated in a phased-array antenna for transmitting the microwave energy in a controlled beam that is focused onto the microwave reflector in the PRS. The reflector redirects the beam to a receiving antenna on Earth in the desired location, where the microwave beam is directly converted safely and efficiently into electricity. The de output of the receiving antenna is fed to the power grid. The receiving antenna can be located at a preferred location. The functional systems requirements as they relate to the implementation of a PRS are indicative of the appropriate combination of technologies as shown in Table 1. The majority of technology developed for WPT systems has been at microwave frequencies, e.g., 2.45 GHz and at 5.8 GHz. Both frequencies are located in the Industrial - Scientific - Medical (ISM) band, thus permission to transmit at these frequencies is not needed as long as interference with other systems outside this band is avoided. Atmospheric attenuation is nominal up to 3 GHz. The 5.8 GHz frequency has a slightly higher loss caused by atmospheric attenuation. Transmission windows also exist in the atmosphere at 35 GHz and 94 GHz. Systems developed to operate at these frequencies enjoy the advantages of smaller antenna apertures. However, additional power margin must be factored into the system design to compensate for increased atmospheric attenuation, making these frequencies unattractive for a PRS. Microwave PRS System Description The PRS comprises three, stand-alone subsystems: • Transmitting antenna (transmitter) • Receiving antenna (rectenna) • Relay satellite reflector
Functional Requirements and System Design Constraints Functional Requirement Design Constraint(s) Power level to be transmitted Availability of prime power, power density in beam Distance of transmission Location of PRS transmitter, receiver and satellite Allowable aperture sizes. Dictated by the available real estate of space for transmitter and receiver apertures and acceptable power densities. Transmission path qualities Attenuation and defocusing effects Beam width control Tolerances on transmitting and reflector surfaces. Beam power density Peak power densities in beam, personnel safety, power density at the edge of the beam, fence lines for access control, safety and health effects, effects on biota. Available technology Source technology, receiver technology, ability to adapt or compensate for transmission medium. Transmitter/Receiver site for Power Relay Satellite applications Latitude and longitude effects on beam transmission. Orbit maintenance for PRS. Radiation pressure on PRS from sun and power beam. Frequency selection. Effects on sizing, system efficiency, all weather operation, rectenna technology availability and efficiency.
These subsystems can be designed to interact in an optimal manner to deliver electric power for transmission to distribution grids. The function of the transmitter is to transform the available electrical energy from a renewable and environmentally compatible energy source with high efficiency into a microwave beam with required characteristics needed for transmission through the atmosphere with minimum losses. Conversely, the rectenna is designed to transform the incoming microwave energy into a form (ac or de) suitable for interfacing with a terrestrial distribution grid. The role of the reflector forming part of the relay satellite in GEO is to intercept the incoming microwave beam from the transmitter and re-direct the beam towards the rectenna. These subsystem functions must be performed with acceptable environmental impacts on the ecology, on human health and safety, and be compatible with terrestrial electricity transmission systems. The key feature of such a system is the capability to transmit significant amounts of electric power over long distances. The relationships between the magnitude of delivered power, the transmission distance, and the life-cycle economic competitiveness with other energy delivery means over intercontinental distances, are highly interdependent. The geometrical relationship between the transmitter and rectenna at the Earth-based sites to the relay satellite reflector will influence the design of the microwave system. The geometrical factors associated with the two potential transmitter sites, and a receiver site in the Almeria region of Spain are shown in Table 2. Transmitter The first step in this conversion is to take the available energy, which is delivered to the system in various formats such as HV ac at 60 Hz or de, and transform it into microwave energy. This can be accomplished using suitable microwave generators. One of the common microwave generators for beam transmission applications is the magnetron. An attractive feature of the magnetron is its low noise output. Noise measurements were performed on a magnetron over a frequency range of 50 MHz from 2.45 GHz. The results of the measurements indicated that the spectral power density noise level at 10 MHz from the carrier (noise in one Hertz bandwidth) is more than 180
dB below the fundamental carrier. Furthermore, the magnetron warm-up time is only 2 to 3 seconds before CW operations can begin. Once the energy has been converted to the desired frequency, it can be transformed into a "beam" for transmission to the relay satellite reflector. This is accomplished with a transmitting or a phased array antenna. A phased-array antenna is comprised of a large number of smaller antenna elements which are controlled so that each element's beam will constructively combine to produce a beam with the same characteristics as that of a single-dish antenna. This beam can then be steered electronically, by controlling the phase of each element's output. A phased-array antenna can be used with a single high-power microwave generator by dividing the power output of the generator into the different radiating elements, and then recombining the power by controlling the individual radiators. However, it may be advantageous to use many, smaller microwave generators which will feed the radiating elements directly, and form part of an active phased array. Control must be exercised over the microwave generators as well as the radiating elements to ensure that the transmitted beam will conform to system requirements. With this configuration, the reliability of the transmitter will be high as there will not be a single point of failure. The configuration of the active phased-array design features graceful degradation as large number of sub-elements could fail without causing the system to be shut down. A magnetron microwave module can be replicated and combined with a slotted wave guide antenna assembly into a large transmitter structure. Rectenna The role of the rectenna is to transform the incoming microwave beam energy into a format suitable for interfacing with the distribution grid. The received microwave energy is converted into de using a rectification process. In an efficient rectenna, the reception and rectification functions are combined into a single device, a dipole rectifier. The dipole-type antenna is used for its omni-directional features, i.e. the efficiency of its reception is not strongly dependent upon the angle of incidence of the microwave beam. The rectification function is accomplished with a solid state diode. The parameters of the diode's design can be optimized for high efficiency, using gallium arsenide as the material of choice for the diode. Other functions are designed into the rectenna, such as de filtering to purify the output voltage, and RF filtering to limit the amount of reradiation from the diodes through the dipoles. The receiver is comprised of many thousands of rectenna elements. The de output of each rectenna element is combined with the others to produce a high power output from the rectenna system. The high power is conditioned and properly formatted for its intended use with standard power conditioning equipment. Reflector The design of the PRS must be both functional and practical. A frequency selective surface can be used to design a reflector structure that is highly reflective at the desired frequency, 2.45 or 5.8 GHz, and transparent at frequencies outside of the chosen frequency. A frequency selective surface allows the deployment of a PRS with minimum
impact on other users of the EM spectrum that may be operating at nearby frequencies. Both copper and silver exhibit attractive features for use in the reflector. To implement a frequency selective surface, a metallized pattern is deposited on the reflector substrate material. This pattern, in conjunction with a substrate material that appears transparent to microwaves at the operating frequency constitutes a spatial filter, so that only the desired beam frequency' will be reflected. Doping the substrate material with a high resistivity substance to provide a de path will counter any potential spacecraft charging that may occur. Resistive materials can be selected that will have no influence on the performance of the reflector while providing a sufficient path for bleeding off any potential charges. Beaming Efficiency Beaming efficiency depends on the sizes of the transmitting antenna, the orbiting reflector, and the receiving antenna. The larger the antenna, the smaller the beam width it produces. For efficient beaming, each antenna and reflector must be sufficiently large to focus the beam into its associated reflector or rectenna at the beaming distance. If one antenna is more costly than the other (e.g., the rectenna in orbit compared to the transmitting antenna on the ground), the diameter of one antenna can be increased and the other decreased as long as their product is not changed. For equal transmitting and receiving antennae apertures, energy will be beamed from one antenna to the other with only about 60% efficiency, partly because the receiving antenna will only catch the energy out to the half-power beam width of the transmitted beam, and partly because some of the transmitted energy falls outside the main beam as sidelobes. 100% microwave beaming efficiency can be approached by making the antennas sufficiently large. However, 95% efficiency requires increasing each antenna diameter by 34% and their areas by 80%. Therefore, size-versus-efficiency is clearly an important tradeoff. In PRS applications where a reflector is used to relay the power from the transmitting antenna to the receiving antenna, the efficiency between the transmitting antenna and the reflector (the "up-path"), and again between the reflector and rectenna (the "down-path") must be evaluated. The North-South dimensions of the transmitting antenna and the rectenna must be increased because the line-of-sight for each of these antennas to the orbiting reflector is tilted from the vertical. Although this size increase requires more area on the ground (by the inverse of the sine of the elevation angle), it does not necessarily increase the active area of the antennas, which could be built so that individual sections of the rectenna are facing the beam.
Antenna Surface Tolerance Losses A loss factor that will occur depends on how accurately the surfaces of the transmitting antenna and the reflector are controlled. Maintaining the required mechanical (and/or microwave beam phase) tolerances over such large antenna areas is challenging, and inadequate tolerances will reduce overall PRS efficiency. Since the receiving antenna rectifies the power from each element separately, it is not critically dependent on mechanical or phase tolerances, as long as the beam fills the receiving antenna. Transmitting antenna tolerances will affect beaming efficiency. Phase errors from one microwave power source to another will similarly affect efficiency. However, because both mechanical and phase tolerances can occur on the transmitting antenna, the tolerances on each must be held tighter by the square root of two to maintain performance. A tight reflector mechanical tolerance is required because a reflector surface error affects the path length of a ray both arriving and departing, compared to the path length of a ray reflected off a neighboring point. To limit defocusing losses on each path to 5%, the following tolerances must be held: The highest antenna gains that have been achieved in radar and radio-astronomy antennas, are limited by surface accuracy and tolerance. At 2.45 GHz, the highest gain achieved is about 62 dB. At 5.8 GHz, the highest gain is about 67 dB. For comparison, the required gains of the antennas for PRS applications will be about 100 dB. To achieve these significantly higher gains, an active error-correction system will be required to reduce antenna surface tolerance to the required accuracy, in a fashion similar to the "adaptive optics" now widely used in astronomical telescopes. To accomplish this correction, the transmitting antenna and the reflector in orbit will each be divided into subarrays or subsections, within which the required tolerances can be met. Each of these subarrays or subsections will be actively aligned by trimming the microwave beam phase of mechanical position to maintain the overall antenna gain. The active correction system will be controlled by additional microwave signals sent back and forth between the reflector and the ground antennas. Even though this system is operating at microwave frequencies, this arrangement is equivalent to "adaptive optics." The reflector is divided into subarrays which will hold the required phase and mechanical tolerances. The retrodirective control technique trims the phase (or position) of each subarray significantly reducing the number of control loops. To handle
a low-level retrodirective control signal in conjunction with the microwave power beam the "bit wiggle" technique used in larger phased array radar antennas can be applied. This approach reduces the amount of control electronics required while being selfcalibrating. The transmitting antenna's tolerance losses after correction by the adaptive optics will be 4% at 2.45 GHz or 6% at 5.80 GHz because the mechanical accuracy requirement is tighter at 5.80 GHz. As reflector tolerances are even tighter, reflector tolerance losses of 5% at 2.45 GHz and 7% at 5.80 GHz are expected. Local variations in atmospheric density and moisture content affect the dielectric constant of the atmosphere, which in turn bends microwaves that pass through it. For very large antennas with very narrow microwave beam widths, this defocusing effect can be very significant. If uncorrected, defocusing will degrade efficiency, in a fashion similar to poor surface tolerances. This atmospheric defocusing effect changes quite slowly, so the same "adaptive optics" that correct for mechanical or phasing tolerances in the antenna can also correct for atmospheric defocusing. Nevertheless, as this effect is a separate source of error, it cannot be corrected perfectly. An average loss of 2% for this factor at 2.45 GHz or 5% average at 5.80 GHz is expected. Extensive work has been done on improving rectenna efficiency. With the proper power level per diode in the rectenna array, a rectenna efficiency of 88% can be achieved at 2.45 GHz. At 5.8 GHz, the efficiency is reduced to about 85%. The conversion efficiencies at the preferred beaming frequencies are as follows: Based on these values, the nominal power drawn from the hydroelectric power station to deliver 1 GW to the power grid in the Almaria region of Spain will be as follows:
The dimensions for the transmitting and receiving antennas at 2.45 and 5.8 GHz frequencies are shown in Tables 3 and 4. The beamed power will be limited by the allowable energy density in the microwave beam at the transmitter and rectenna. A Gaussian energy distribution across the beam, if 4 GW were to be transmitted by the reflector, would reach about 56 mW/cm at the transmitter, 100 mW/cm at the rectenna and 120 mW/cm at the reflector not taking into account the beam slant angles that would lower the energy density across the beam. A Gaussian energy distribution results in a higher maximum energy density compared to shaping the beam to approximate a more uniform energy distribution across the beam.
Superdirectivity Technology As the design and development of the PRS progresses, the implementation of the system will benefit from technology developments being pursued in this and other related technical areas. Of particular interest is the use of antennas exhibiting superdirectivity. Superdirectivity exists when the antenna's directivity is greater than that normally defined by its geometrical considerations. This would benefit the overall efficiency of the PRS system. A superdirective transmitter aperture would provide a more tightly focused beam, and therefore, also increase the beaming efficiency for a PRS reflector of a given size. Further developments will be required before practical applications of this technology are realized. Considering the advances being made in the use of high temperature superconducting materials, this technology may be mature enough for application to the development of a PRS system.
Microwave Beam Reflector There are four challenges that have to be faced in deploying a large reflector in GEO: • A feasible structural design approach capable of producing an ultra-lightweight space structure, packaging for delivery to orbit, and high surface precision after deployment. • A concept for telerobotic assembly of the reflector in space. • Economic space transportation within the available state-of-the-art. • On-orbit station keeping, attitude control and pointing requirements and means of meeting the requirements. The active reflector size is about 2.2 km in diameter. The surface area is 4.15 km2. The actual design is slightly larger as dictated by the need to have attitude control systems at its periphery that do not interfere with the beam. At 0.175 kg/m2, a unit mass calculated to be attainable, the total mass is 722-t (metric tons). At present-day transportation costs to GEO, the cost of delivering a 1000-t. reflector would be prohibitive. Economies of scale can reduce cost, because the large mass delivery requirement can justify- development of a tailored space transportation system. The reflector must maintain its reflecting surface within 1/20 wavelength, or approximately 0.6 cm. This precision is about 1 in 3 x 10^, not exceptional for an optical surface but challenging for an ultra-lightweight space structure. The active reflector is made up of resonant conductive rings of copper or silver plated on an insulating substrate such as Kapton film. The rings are about one wavelength (e.g., 12.24 cm) diameter and are laid in an hexagonal pattern with about 1/4 wave edge separation. The reflector must maintain its assigned location in GEO. While the ground- based transmitter could be designed for electronic steering to track the reflector, minimizing electromagnetic interference with other space assets will require that the uplink power beam be directed to a specific location in GEO. The cost of the rectenna can be minimized if it is fixed-aimed at a particular point in orbit, and is required to maintain its latitude and longitude within 1/4 degree. The reflector must maintain accurate attitude control to keep the downlink power beam centered on the rectenna. Retrodirective phase control can be used to keep the uplink beam centered on the reflector, but this cannot be extended to centering the downlink beam since the reflector is electronically passive. A centering requirement of about 1% of the estimated receiver diameter was assumed. The angle subtended at GEO by 100 m on Earth is about 0.5 arc second. The reflector attitude error must be held to half this value. An on board attitude reference system can use inertial reference and star
trackers. A slow feedback loop can use the detected downlink pattern on the receiver to null out error between the reflector surface and the on-board attitude reference. Space Transportation and Operations Requirements Space transportation and operations are required to take the reflector as manufactured and delivered to the launch site, and deliver it disassembled to GEO, assemble it there, place it into operation, and maintain it for an estimated operational lifetime of 30 years or more. The reflector is too large to be delivered to GEO in one unit, assuming a launch requirement of 50-100 t. and a volume envelope of 10 m diameter by 30 m length. The 100 t. mass in this volume represents a density of about 40 kg/nA The densest deployable large space structures are somewhat less dense than this, but the available volume is potentially large enough. • Selection of the best launch payload size and mass is a tradeoff among: • Launch vehicle development cost, which increases with increasing mass capability; • Launch delivery cost per ton, which decreases with increasing mass; • Mass per unit area of the delivered reflector structure, which tends to decrease with greater payload mass and volume capability; • Assembly complexity, which also tends to decrease with greater payload mass; • Size of the (assumed) electric propulsion system for delivery from low-Earth orbit to GEO - large and cumbersome arrays are implied at 100 t. payload; and • Complexity of launch operations, which tends to increase with the greater launch rate dictated by a smaller launch vehicle capacity. Figure 2 shows the functional representation of transportation and operations requirements for the space transportation and operations systems. Cost Targets Since the reflector is part of a utility network energy delivery system, rough- order-of-magnitude allowable costs can be derived from historical and projected utility costs. The power to be delivered to the European Grid from renewable resources is assumed to be 20 GW. This can be beamed with five PRS systems so that loss of functions in one PRS would not have an excessive impact. This leads to power beamed with one PRS system of about 4 GW. Each reflector is estimated to cost between $1000 to $2000 per kW. ($4 to 8 billion). Therefore, a target cost of $4 billion for each of five reflectors was adopted. If the target cost is divided evenly between the costs of space transportation and reflector, and if the mass of a reflector is 722 t., the specific costs are $2770/kg each for the reflector and for space transportation cost to GEO.
The reflector design concentrated on structural design approaches that have the potential to reach mass, flatness, and packaging targets, and on a scheme for deployment and assembly in GEO. The structural design approach is central to the technical and economic feasibility of the reflector concept. Consideration was also given to attitude control and station keeping and other subsystems. In addition to low mass per unit area, packaging for delivery is a primary design driver for a structure as large as the microwave reflector. A 100-t launch vehicle and an efficient electric-propulsion orbital transfer delivery system will be required. The number of launches, if mass-limited, is expected to be between 15 and 20 for each reflector. A typical shroud size for such a launcher is 10 m usable diameter and 30 m usable length; these were adopted as a reference capability. The usable volume is 2356 m^ per launch, a total no greater than 235,600 m^ The volume of the deployed reflector is hundred times greater than the available packaging volume.
Reflector Structural Design The following guidelines were employed in selecting a preferred structural concept: • High specific stiffness and strength • Statistically determinate design • Easy to erect, deploy or assemble • Can accommodate imperfections without increasing stresses • Passive damping through damped joint design • Active damping through memory metals and adaptive structure • Ability to operate at very low fundamental frequencies • Veiy low to zero coefficient of thermal expansion • Low volume for packaging Loading and design conditions include solar radiation pressure, microwave radiation pressure, thermal cycling, ultra-violet and ionizing space environment radiation, gravity gradient loads, maneuvering and pointing control loads, deployment and erection loads, and launch loads. An erectable structural concept requires the least amount of new technology development. It will be of low mass, could be developed in the near future, and deployed with a limited number of launches. Deployable tetrahedral and cubic truss structures have been developed and proven in support of several space programs. The Kapton film-type reflector was selected for use with this structure. A completely deployable reflector would require significant new technology development. Several concepts for large diameter up to 1 km. dia., reflector spacecraft have been proposed in the past. One of the concepts uses a deployable truss rim structure made of composite tubular members. The reflector materials proposed for this concept are either a Kapton film or a compliant mesh type. Mechanical deployment has been one of the most failure-prone aspects of spacecraft operations. Many missions have failed or been severely degraded by deployment failures. This disadvantage can be overcome by selecting a deployable design, incorporating motor-powered deployment involving jack-screw mechanisms of the type often used in jet aircraft for flap deployment. This is a proven and robust method that is highly reliable in aircraft service. Previous spacecraft deployment failures have been associated with spring-deployed systems in which the deployment force must be reduced to prevent build-up of the deployment velocity to prevent causing structural damage when the deployment motion encounters limit stops. Rendezvous and berthing are routine operations. Rendezvous has never failed in the history of U.S. space operations. Berthing or docking has never failed in the U.S. program except for one Gemini mission in which the target vehicle nose cone failed to separate properly and the docking port was not usable. Several missions to retrieve satellites not designed for retrieval, have been performed by the space shuttle.
The rendezvous and berthing of the reflector is more easily accomplished in GEO because gravity gradients are much reduced. Also, both elements are active and cooperative. A human operator in the control loop can be considered, if desired, because the delay time for direct round-trip communications to GEO is about 0.2 seconds. The advancing state-of-the-art in robot mechanisms, perceptions, planning, and task control, coupled with demonstrated capabilities in C3I, are applicable to all phases of reflector deployment, rendezvous and berthing. An alternate concept, the "maypole" or spoked wheel fully deployable concept, is shown in Figure 3. In this concept, the Kapton film reflector is stretched across the entire diameter of the structure. The only rigid members are the center mast and the rim truss. Stay tapes or guys extend from the ends of the center mast to the rim to stabilize and shape the rim. This system is somewhat like a bicycle wheel in that the overall shape and flatness are controlled by tension of the stays. Attitude control will be accomplished by electric thrusters. Attitude control can be accomplished by performing station keeping with a low level of continuous thrust and allocating the station keeping thrust to generate the necessary attitude control torques. Although the attitude control requirement is moderately stringent, the station keeping thrust level will induce maximum angular accelerations on the order of 0.01 arc second per sec^; these are small enough to maintain adequate attitude precision. Since the structural dynamic response of the reflector will be slow, the attitude control system will have to include dynamic compensation. At GEO, station keeping involves compensating for irregularities in the Earth's gravitational field, Sun-Moon effects, and solar and microwave radiation pressure. For a large low-density object such as the reflector, radiation pressure is a significant part of the requirement. The microwave pressure can be partially compensated for by operating at slightly less than GEO altitude. The integrated change in velocity (delta V) requirements for station keeping at GEO would require an annual delta V requirement for a 4000-t. reflector and a 1200-t. reflector of 47.6 m/sec per year for the more massive object and 54 m/sec for the less massive one. For this annual delta V, the propellant mass fraction for 30 years station keeping at 3000 seconds Isp is about 6%. The power requirement is less than 150 kW. Reflector Assembly in Space Gravity gradients, atmosphere drag, the debris environment, frequent sun occultation, transportation considerations, and communications all favor assembly in GEO orbit. The only factor that favors low-Earth orbit is human access. Therefore, all- robotic assembly is preferred. Human access would only be required in the event of a breakdown that could not be repaired robotically but this would be a very infrequent occurrence. In the time period of a microwave reflector project, an advanced crew transportation system is expected to be available to transport people to GEO if needed. Therefore, GEO assembly, has been chosen.
The advanced structural design concept is based on a deployable space structure such that the payload for any one Earth launch extends to a large-area structural section of the reflector after delivery to GEO. The assembly process includes two main parts: deployment of the sections and assembly of the sections into the complete reflector. Only a few of these large sections must be assembled to complete the reflector. Therefore, an approach was chosen where each payload is independently deployed and then maneuvered under its own power to the vicinity of the reflector where manipulator arms similar to the ones used on the space shuttles complete the berthing process. The Space Station program is expected to provide the needed experience base for manipulator berthing of such masses.
Space Transportation Past and contemporary studies of space transportation provide a broad spectrum of concepts to consider. The key to understanding the potential transportation cost for the reflector is understanding that the size and technology level for most economic transportation systems varies with traffic level. There are two reasons: (1) The amount of research and development which can be amortized is clearly a function of traffic level; and (2) Economies of production and operations improve with increasing traffic. Review of the costs of present-day GEO transportation systems indicates that roughly 5/6 of the cost to GEO is in launch to LEO, the remainder in transport from LEO to GEO. Cost reductions must occur in both segments of the transportation function. Also, increasing the fraction of LEO delivery' mass that ends up as useful payload in GEO has important leverage on cost. An increase of the net useful mass to GEO to 1/2 or more of that delivered to LEO reduces the cost to GEO by almost a factor of 3, other things being equal. This clearly indicates electric propulsion for LEO to GEO transfer. Use of high-power free-electron lasers based on Earth is a promising source of radiation for electric power in cislunar space. Preliminary systems analyses indicate greater economy than either solar or nuclear power, because laser beams can reach intensities of several suns and yield photovoltaic conversion efficiencies greater than 50% at the receiver. Solar characteristics of an electric propulsion system for orbit transfer were estimated based on delivery to LEO of 14 payloads of 50 t. each, along with the propellant needed to deliver it to GEO by electric transfer vehicle. If an equatoral launch site were used, the electric propulsion delta V would be reduced to about 4700 m/sec. The savings could be applied in some combination to shorter trip time and less power for the transfer vehicle. The optimum trip time is about 75 days. The overall transportation cost is estimated as $1700/kg for Earth to LEO and $50 million per trip for orbit transfer. The total launch mass is 1050 t. (including transfer propellant, and there are 14 transfer trips. The grand total transportation cost for one reflector is $1,785 billion for Earth launch and $700 million for transfer operations, resulting in a cost of $2,485 billion. This transport cost presumes very modest advances in launch vehicle technology, but significant advances in electric propulsion technology, primarily in size and power. Performance of the components of the electric propulsion system is in the range of performance demonstrated in the laboratory. Most of the technology advances are assumed to take place in electric propulsion technology. The relatively small size of the electric propulsion system compared to the launch system (20 t. vs. roughly 150 t.) indicates a lower cost.
PRS Environmental Issues The SPS CDEP assessments of the environmental impacts of power beaming are applicable to the PRS [2], Examples include land use for the transmitting and receiving antennas, the aesthetic effects of such use. potential deleterious effects on human health, safety considerations, destruction of valued resources, and the intangible effects that may influence the quality of life. Economic Considerations Because the PRS will not be fully developed for 15 years it is difficult to project the cost of commercial PRSs and their global market. However, costs are equally difficult to provide for other advanced energy technologies. Cost estimates, uncertain as they may be, are indicators of the competitive cost of a PRS system compared with other long distance transmission, e.g., high-voltage transmission lines. Table 5 shows rough order of magnitude cost estimates for a 2 and 4 GW PRS. These estimates were prepared based on past economic studies such as the SPC CDEP, and current estimates of key system elements. The cost estimates indicate that they are in a range to justify more detailed economic assessments of the PRS design approaches as applicable technologies evolve. Increasing the microwave flux density from 23 mW/cm^ to 50 mW/cm’ and using a more uniform rather than a Gaussian distribution across the microwave beam
would provide the capacity to increase the beamed power for the same system dimensions. An increase to 50 mW/cm^ is not expected to result in heating of the ionosphere based on the results of the ionosphere-microwave beam interaction tests performed at the Arecibo. P.R. facility. PRS Development Factors The PRS development will have to include comparative technical, economic, and societal assessments to ensure that the most effective technologies are chosen for specific projects. Many factors will need to be evaluated including: • Selection of the optimum microwave frequencies based on the location of the sites for the transmission and reception of beams; • Variations in atmospheric attenuation and weather conditions; • Magnitude of beamed energy determined by the beam cross section, the maximum allowable flux density at the center and perimeter of the beam, and health, safety, environmental and other societal requirements; • Type of service to be provided (baseload, intermediate, or peak); • Required system reliability; and • Economic and market considerations. The implementation of the PRS will occur as the space infrastructure is developed and space transportation, space operations, and assembly using more mature and advanced systems are demonstrated. Thus the competitive cost structure required for the PRS will benefit from the experience gained in the construction and operation of elements of the space infrastructure that are directly applicable to the PRS development program. Conclusions The PRS is a passive device that can be designed to meet the PRS system requirements. The microwave beam transmitter and rectenna are based on a legacy of over 40 years of development, manufacture and application of a wide range of products for commercial and defense markets. Specific findings in this study are: • The microwave - based WPT technologies can apply demonstrated performance efficiency and significant existing design, development and manufacturing capabilities to meet PRS system requirements; • The engineering and cost challenges of the power beam reflector are achievable with technical advances normally expected in a development program;
• The preferred reflector design concept of a coated plastic reflector stretched over a deployable space structure can be delivered in discrete elements to GEO orbit, and utilize telerobotic deployment, berthing and assembly in orbit; • The required flatness of the reflector can be achieved with an adaptive structure already proven on other technology programs and science missions; • A completely deployable reflector concept based on a spoked wheel and a "Maypole" structure with a thin film reflector stretched across the entire diameter of the structure, is contained in a compact package that can be deployed with a single launch. This concept will require significant new technology development and extensive testing; • The attitude precision requirement is routinely achieved on present spacecraft; • The preferred reflector concept will require about 15 launches of a cargo launch vehicle such as the space shuttle with an achievable launch cost target of about $1500/kg. Orbit-to-orbit transportation will use solar electric propulsion • Several space transportation options are being developed that can meet PRS requirements and economic goals; • A phased program development sequence indicates that an operational PRS system could be available in 15 years; • The PRS concept provides a means to access renewable energy sources located at great distances from major markets; • The mechanical and power beaming technologies required for a PRS appear to be technically feasible based on the existing state-of-the-art; • 2.45 or 5.8 GHz frequency is preferred to reduce prime power requirements, to obviate higher atmospheric defocusing losses, and to draw upon the extensive experience in microwave beam transmission and reception; • Reflector design can proceed based on known principles of structural design and surface flatness control; • The current and planned space programs, systems, and technologies of space-faring countries in support of the evolving space infrastructure are advancing the feasibility of the PRS; and • The development of the PRS can provide access to renewable resources available on Earth and represents an important stage in achieving the goal of obtaining power from space for use on Earth.
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