FEASIBILITY STUDY OF A SATELLITE SOLAR POWER STATION ----------------------------------------------- -—-— II - I II NASA CONTRACTOR REPORT NASA CR-2357 by Peter E. Glaser, Owen E. Maynard, John Mackovciak, Jr., and Eugene L. Ralph Prepared by ARTHUR D. LITTLE, INC. Cambridge, Mass. 02140 for Lewii Research Center
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ERRATA NASA Contractor Report CR-2357 FEASIBILITY STUDY OF A SATELLITE SOLAR POWER STATION by Peter E. Glaser, Owen E. Maynard, John Mackovciak, Jr., and Eugene L. Ralph February 1974 4 Page 8, Table 1, line 1: The value for Gravity Gradient should be 12.1x10 newton- m sec. Page 8, Table 1, line 8: The value in parentheses following RCS should be I 800 sec sp
1. Report No. NASA CR-2357 2. Government Accession No. 3. Recipient's Catalog No. 4. Title and Subtitle FEASIBILITY STUDY OF A SATELLITE SOLAR POWER STATION 5. Report Date February 197^ 6. Performing Organization Code 7. Author(s) Peter E. Glaser, Owen E. Maynard, John Mackovciak, Jr., and Eugene L. Ralph (see following page for affiliation) 8. Performing Organization Report No. ADL-C-74830 10. Work Unit No. 11. Contract or Grant No. NAS 3-16804 13. Type of Report and Period Covered Contractor Report 14. Sponsoring Agency Code 9. Performing Organization Name and Address Arthur D. Little, Inc. 20 Acorn Park Cambridge, Massachusetts 02140 12. Sponsoring Agency Name and Address National Aeronautics and Space Administration Washington, D. C. 20546 15. Supplementary Notes Final Report. Project Manager, Ronald L. Thomas, Power Systems Division, NASA Lewis Research Center, Cleveland, Ohio 16. Abstract A feasibility study of a satellite solar power station (SSPS) was conducted to (1) explore how an SSPS could be ''flown” and controlled in orbit; (2) determine the techniques needed to avoid radio* frequency interference (RFI); and (3) determine the key environmental, technological, and economic issues involved. Structural and dynamic analyses of the SSPS structure were performed, and deflections and internal member loads were determined. Desirable material characteristics were assessed and technology developments identified. Flight control performance of the SSPS baseline design was evaluated and parametric sizing studies were performed. The study of RFI avoidance techniques covered (1) optimization of the microwave transmission system; (2) device design and expected RFI; and (3) SSPS RFI effects. The identification of key issues involved (1) microwave generation, transmission, and rectification and solar energy conversion; (2) environmental-ecological impact and biological effects; and (3) economic issues, i. e., costs and benefits associated with the SSPS, The feasibility of the SSPS based on the parameters of the study was established. 17. Key Words (Suggested by Author(s)) Solar cells; Satellite power system; Microwave power; Solar terrestrial power 18. Distribution Statement Unclassified - unlimited 19. Security Classif. (of this report) Unclassified 20. Security Classif. (of this page) Unclassified 21. No. of Pages 199 22. Price* Domestic, $5.25 Foreign, $7.75
Authors: Peter E. Glaser, Arthur D. Little, Inc., Cambridge, Massachusetts; Owen E. Maynard, Raytheon Company, Sudbury, Massachusetts; John Mockovciak, Jr., Grumman Aerospace Corporation, Bethpage, New York; and Eugene L. Ralph, Spectro- lab (Textron Inc.), Sylmar, California
TABLE OF CONTENTS SUMMARY 1 Structure and Control Techniques 1 RFI Avoidance Techniques 1 System Optimization 1 Device Design 1 Effects of SSPS RFI on Other Users 1 Identification of Key Issues 2 Technological Issues 2 Environmental Issues 2 Economic Issues 2 GLOSSARY OF SYMBOLS 3 INTRODUCTION 5 BASELINE DESIGN 6 Principles of a Satellite Solar Power Station 6 Location of Orbit 6. Solar Energy Conversion 8 Microwave Power Generation, Transmission, and Rectification 11 SSPS Flight Control 17 Earth-to-Orbit Transportation 17 Potential SSPS Power Levels 18 STRUCTURE AND CONTROL TECHNIQUES 19 Structural and Dynamic Analysis of the SSPS 19 Summary 19 Baseline Configuration 19 Structural Mathematical Model 24 Dynamic Mathematical Model 26 Input of Elastic Body Characteristics into Attitude Control System Analysis 27 Establishment of Flight Loading Conditions 30 Input of Flight and Control System Loadings into Structural Model to Determine Internal Loadsand Structural Deflections 32 Assessment of Baseline Configuration for Internal Loads and Deflections 32 Assessment of Candidate Material Characteristics 35
STRUCTURE AND CONTROL TECHNIQUES (Continued) Identification of Areas Requiring Further Structure and Dynamic Analysis 36 Flight Control Performance Evaluation of the Baseline SSPS 39 Summary 39 Ground Rules and Assumptions 39 Discussion 40 Identification of Areas Requiring Further Flight Control Performance Analysis 74 Conclusions 75 RFI AVOIDANCE TECHNIQUES 76 Optimization of Microwave Transmission System 76 Background 76 Discussion 77 Device Design and Expected RFI 79 Device Design 79 RF Spectrum Considerations for the SSPS Amplitron 90 Effects of SSPS RFI on Other Users 96 Transmitting Antenna and Nature of the Transmitted Beam 96 Noise from the SSPS 101 IDENTIFICATION OF KEY ISSUES 112 Key Technological Issues 112 Microwave Generation, Transmission, and Rectification 112 Solar Energy Conversion 120 Earth-to-Orbit Transportation 129 Orbital Assembly 132 Key Environmental Issues 132 Resource Use 132 Effects at the Receiving Antenna Site 133 Microwave Biological Effects 133 Ecological and Environmental Effects of Added Heat 140 Land Usage in the Receiving Antenna 148 Stratospheric Pollution with Shuttle Vehicle Exhaust Products 149 Other Booster Emissions 152 Tropospheric Pollution 453 Key Economic Issues 153 Key Cost Considerations 153 Key Economic Considerations 164
CONCLUDING REMARKS AND RECOMMENDATIONS 177 Structure and Control Techniques 177 Conclusions 177 Recommendations 177 RFI Avoidance Techniques 178 Conclusions 178 Recommendations 178 Identification of Key Issues 179 Technological Issues 179 Environmental Issues 180 Economic Issues 182 Program Phasing 182 REFERENCES 184
LIST OF FIGURES Figure No. Page 1 Design Principles for a Satellite Solar Power Station 6 2 Nominal Orbit Perturbations 7 3 SSPS Dimensions 10 4 Atmospheric Attenuation of Microwaves in Two United States Locations 13 5 Hypothetical Distributions of Microwave Power Density from the Beam Center 15 6 Potential SSPS Power Levels 18 7 SSPS Characteristics 20 8 Solar Collector Configuration 21 9 SSPS Baseline Configuration 22 10 Construction of Compression Strut 25 11 SSPS Disturbance Forces and Torques 31 12 SSPS Deflections and Member Loads for Unit Control Forces Acting in the Y Direction 33 13 SSPS Deflections and Member Loads for Unit Control Forces Acting in the Z Direction 34 14 SSPS Natural Frequencies Versus Modulus of Elasticity of Non-Conductive Struts 37 15 SSPS Natural Frequencies Versus Modulus of Elasticity of Non-Conductive Struts 37 16 Pitch Axis 40 17 First Symmetric Pitch Axis Bending Mode Shape 43 18 First Anti-Symmetric Pitch Axis Bending Mode Shape 43 19 Second Symmetric Pitch Axis Bending Mode Shape 43 20 Second Anti-Symmetric Pitch Axis Bending Mode Shape 43 21 Digital Simulation of Pitch Axis Flexible Body Dynamics for a 4100 Ft.-Lb. Constant Disturbance Torque 46 22 Digital Simulation of Pitch-Axis, Rigid-Body Dynamics for a 4100 Ft.-Lb. Constant Disturbance Torque 46 23 Variation of the 1st Anti-Symmetric Pitch Mode Frequency with Attitude Error 47 24 Variation of the 1st Anti-Symmetric Pitch Mode Frequency with Attitude Error 47 25a Variation of the 1st Anti-Symmetric Pitch Mode Frequency with Response Time 48 25b Variation of the 1st Anti-Symmetric Pitch Mode Frequency with Response Time 48 26 Variation of the 1st Anti-Symmetric Pitch Mode Frequency with Steady-State Control Force 49 27 Variation of the 1st Anti-Symmetric Pitch Mode Frequency with Control System Gain 49
28a Variation of the 1st Anti-Symmetric Pitch Mode Frequency with Rate Feedback Gain 50 28b Variation of the 1st Anti-Symmetric Pitch Mode Frequency with Rate Feedback Gain 50 29 Variation of the 1st Anti-Symmetric Pitch Mode Frequency with Structural Weight and Attitude Error 51 30 Roll Axis 52 31 First Symmetric Roll Axis Bending Mode Shape 53 32 First Anti-Symmetric Roll Axis Bending Mode Shape 53 33 Digital Simulation of Roll Axis Flexible Body Dynamics for a 89,700 Ft.-Lb. Constant Disturbance Torque 57 34 Digital Simulation of Roll-Axis, Rigid-Body Dynamics for a 89,700 Ft.-Lb Constant Disturbance Torque 57 35 Variation of the 1st Anti-Symmetric-Roll Mode Frequency with Attitude Error 58 36 Variation of the 1st Anti-Symmetric Roll Mode Frequency with Attitude Error 58 37a Variation of the 1st Anti-Symmetric Roll Mode Frequency with Response Time 59 37b Variation of the 1st Anti-Symmetric Roll Mode Frequency with Response Time 59 38 Variation of the 1st Anti-Symmetric Mode Frequency with Steady-State Control Force 60 39 Variation of the 1st Anti-Symmetric Roll Frequency with Control System Gain 60 40a Variation of the 1st Anti-Symmetric Roll Mode with Rate Feedback Gain 61 40b Variation of the 1st Anti-Symmetric Roll Mode with Rate Feedback Gain 61 41 Variation of the 1st Anti-Symmetric Roll Mode Frequency with Structural Weight and Attitude Error 62 42 Yaw Axis. 63 43 1st Symmetric Yaw-Axis Bending Mode 64 44 1st Anti-Symmetric Yaw-Axis Bending Mode 64 45 2nd Symmetric Yaw-Axis Bending Mode 64 46 2nd Anti-Symmetric Yaw-Axis Bending Mode 64 47 Digital Simulation of the Flexible Body Dynamics for a 0.001 Rad-Initial Attitude 67 48 Digital Simulation of the Yaw Axis Rigid Body Dynamics for a 0.001-Rad-lnitial Attitude 68
49 Variation of the 1st Anti-Symmetric Yaw Mode Frequency with Attitude Error 69 50 Variation of the 1st Anti-Symmetric Yaw Mode Frequency with Response Time 70 51 Variation of the 1st Anti-Symmetric Yaw Mode Frequency with Response Time 70 52 Variation of the 1st Anti-Symmetric Yaw Mode Frequency with Control Force 71 53 Variation of the 1st Anti-Symmetric Yaw Mode Frequency with Control System Gain 71 54 Variation of the 1st Anti-Symmetric Yaw Mode Frequency with Rate Feedback Gain 7 2 55 Variation of the 1st Anti-Symmetric Yaw Mode Frequency with Rate Feedback Gain 7 2 56 Variation of the 1st Anti-Symmetric Yaw Mode Frequency with Structural Weight and Attitude Error 73 57 SSPS Amplitron 80 58 Vane Temperature Rise Versus Frequency in SSPS Amplitron 81 59 Vane Temperature Versus Frequency of SSPS Amplitron 82 60 Anode Dissipation Versus Frequency 82 61 Magnetic Field Versus Frequency 83 62 Size Comparison Between the Radially Gaussed Sm-Co Magnet and the Alnico V Magnet for 8129 Type CFA 84 63 Magnetic Circuit Amplitron at 2.0 GHz 85 64 Magnetic Circuit of Amplitron at 2.45 GHz 87 65 Magnetic Circuit of Amplitron at 3.3 GHz 88 66 Magnetic Circuit of Amplitron at 3.3 GHz 89 67 Schematic of ITT Gilfillan-Built Radar System 91 68 Fixed Frequency Harmonic Output — Normal Waveguide System 92 69 QKS1646 No. 2 impedance Match 92 70 Spectrum Analyzer Presentation of Tum-On/Shut-Off Type Energy 93 71 RF Spectrum Assumptions for SSPS Investigations Close to Main Beam 95 72 Antenna-Earth Surface Geometry 97 73 Rectifying Antenna Reference Plane Normal to Beam C 97 74 Transmitting Antenna Exponent = % Power Distribution 98 75 Rectifying Antenna Power Distribution for Exponent = % Distribution at Transmitting Antenna 98 76 Noise Temperature Profile 101 77 Gain of SSPS Antenna Associated with Each Amplitron 103 78 SSPS Noise Associated with Basic Device and Filter 103
79 SSPS Noise Associated with Basic Device and Filter 105 80 Transmitting/Receiving System 105 81 SSPS Noise Associated with Tropospheric Scatter 107 82 SSPS Noise Associated with Tropospheric Scatter 107 83 SSPS Noise Associated with Radio Astronomy 109 84 SSPS Noise Associated with Radio Astronomy 109 85 SSPS Noise Associated with Radar (10'Diameter) HO 86 SSPS Noise Associated with Radar (10' Diameter) 110 87 Microwave Power Density Distribution on Ground 118 88 Advanced Shuttle Requirements Definition 130 89 Crude Model of Thunderstorm 137 90 Atmospheric Attenuation 139 91 Silicon Solar Cell Cost Projections for SSPS Solar Collector Array 157 92 Program Phasing 183
LIST OF TABLES Table No. Page 1 Flight Control Requirements 8 2 Microwave Power Transmission Efficiencies 14 3 SSPS Structural Model Weights 27 4 Dynamic Model — Elastic Body Characteristics 28 5 Dynamic Model — Elastic Body Characteristics — Anti-Symmetrical 29 6 Pitch Axis Mass Properties 40 7 Pitch Axis Bending Mode Data 41 8 Pitch Axis Disturbance Torques 42 9 Roll Axis Mass Properties 52 10 Roll Axis Bending Mode Data 54 11 Roll Axis Disturbance Torques 55 12 Yaw Axis Mass Properties 63 13 Yaw Axis Bending Mode Data 65 14 Yaw Axis Disturbance Torques 65 15 CFA Intra-Spectrum Noise Measurements $3 16 Electrical Characteristics of the Transmitting Antenna 96 17 Summary of Power Densities Distributions and Radii 99 18 Service Frequencies — Cases (a) and (b) Hl 19 Design Resources to Meet Requirements Placed on Microwave Power Transmission in SSPS 113 20 Microwave Power Generation, Transmission, and Rectification Issues 114 21 Power Conversion Transmission, Reception, and Control 116 22 Key Study Areas in Solar Energy Conversion 122 23 SSPS Critical Materials Supply 133 24 Rainfall Rates 136 25 Solar Energy 141 26 Representative Elements of the Radiation Environment Determining Environmental Temperature 143 27 Radiation Regimes for Hypothetical Environments 144 28 Range in Components of the Water Balance for Desert and Dry Grassland Communities in Western United States 147 29 Energy Consumption (EC) Density in Selected Industrial and Urban Areas 148 30 Shuttle Vehicle Water Vapor Injection into the Stratosphere 151 31 Natural and Artificial Flux of NO into the Stratosphere 153 32 Cost Projection Comparisons for Two SSPS Solar Collector Array Configurations 156 33 Cost Estimates on Transmitting Antenna 161 34 Receiving Antenna Statistics 162 35 Cost Elements of Receiving Antenna 162 36 Estimated Capital Costs 163 37 Annual Expenses Associated with $100 of Plant Equipment 17 2 38 I/O Model of Economy 174
SUMMARY This is the Final Report of a feasibility study of a satellite solar power station (SSPS) carried out by Arthur D. Little, Inc., Grumman Aerospace Corporation, Raytheon Company, Spectrolab, a division of Textron Inc., for the National Aeronautics and Space Administration under Contract NAS 3-16804. The primary objectives of this study were (1) to explore how an SSPS could be “flown” and controlled in orbit, (2) to determine the techniques which would be required to avoid radio frequency interference with other users of the electromagnetic spectrum, and (3) to determine the key environmental and economic issues which would have to be assessed. Structure and Control Techniques Structural and dynamic analyses of the SSPS structure were performed to provide elastic characteristics (natural frequencies, generalized masses, and mode shapes) of the structure for use in an analytical investigation of the elastic coupling between the SSPS attitude control system and the spacecraft's structural modes. Deflections and internal member loads resulting from the various flight loading conditions were determined to verify structural integrity. Desirable material characteristics were assessed and technology developments identified to provide inputs leading to the design of stnicture and attitude control systems for the very large-area, light-weight space structures represented by the SSPS. The flight control performance of the SSPS baseline design was evaluated and parametric sizing studies performed to determine the influence of structural flexibility upon attitude control system performance. RFI Avoidance Techniques The study of RFI avoidance techniques included three principal areas: (1) optimization of the microwave transmission system; (2) device design and expected RFI; and (3) effects of SSPS RFI on other users. System Optimization. — To optimize the microwave transmission system, a model and a set of assumptions were first defined. The model included data on orbital and ground location, ground power transmission, device characteristics, phase-front control, efficiencies, RF environment, attenuation, frequencies, users, and equipment. Device Design. — The Amplitron, a very efficient microwave generator, was evaluated from the viewpoint of its design versus its operating frequency for the SSPS concept. The choice of 3.3 GHz as the fundamental frequency for the SSPS was based on a set of assumptions for filter design and recognition of existing allocated radio astronomy and fixed satellite space-to-Earth bands. Effects of SSPS RFI on Other Users. - This phase of the study concentrated on (1) the transmitting antenna and nature of the transmitted beam; (2) the receiving antenna; and (3) noise
emanating from the SSPS. The analysis showed that the approach required for Amplitron design and filtering techniques would minimize RFI with other users, and hence national and international agreement on frequency allocation for the SSPS would be achievable. Identification of Key Issues Technological Issues. — a. Microwave Generation, Transmission and Rectification The microwave portion of the electromagnetic spectrum has been selected as the most useful for SSPS power generation, transmission, and rectification. From a device point of view, the Amplitron appears most promising because of its unique performance characteristics. b. Solar Energy Conversion Design approaches for the solar collector, solar cell blankets, and power collection and distribution methods were evolved to meet the requirements of the structure and control technique analyses. Environmental Issues. — a. Environmental/Ecological Impact The environmental and ecological impacts of the SSPS were explored, with attention focussed on the environmental and ecological impact at the receiving antenna, and the possible contamination of the stratosphere by the space transportation system. Waste heat released at the receiving antenna does not constitute a significant thermal effect on the atmosphere, and, with RF shielding incorporated below the rectifying elements, the receiving antenna operation can be compatible with other land uses. b. Biological Effects There exist conflicting interpretations of the effects of microwave exposure throughout the scientific community. Because of the lack of internationally accepted standards, based on experimental data, to place a specific and allowable level on microwave exposure, the SSPS will have to be designed to accommodate a wide range of microwave power flux densities. Economic Issues. — Three of the key issues that will have to be addressed when making an economic comparison of the SSPS with other means of generating power and the methodology to deal with these issues include: 1. The costs and benefits associated with the SSPS which have to be evaluated to determine the economic feasibility of an investment of this type.
2. The macro-economic interindustry effects produced by the SSPS which have to be examined to analyze the effects such an investment might have on the structure of the economy as a whole. 3. The consumption effects created by the SSPS which will be reflected in both the cost/benefit analysis and the analysis of macro-economic interindustry effects. GLOSSARY OF SYMBOLS Attitude control force along the x,y,z axes, respectively kg slugs Principal moment of inertia about the x,y,z axes respectively kg-m2 slug-ft2 Attitude control system gain Portion of external disturbance torque that is proportional kg-ni/rad ft-lb/rad to attitude angle errors acting about the x,y,z axes, respectively Rate sensor gain rad/(rad/sec) Spacecraft length along the x and y axes, respectively m ft Generalized displacement of the i^1 mode m ft Portion of external disturbance torque that is constant kg-m ft-lb and acting about the x,y,z axes, respectively Control system response time for the x,y,z axes, sec respectively Rigid body damping ratio Spacecraft rigid body rotational attitude about the x,y,z rad axes, respectively Rotational attitude commands about the x,y,z axes, rad respectively Rotational attitude errors about the x,y,z axes, rad respectively
Normalized slope of the mode at the right end rad/m rad/ft Normalized modal de fleetion of the mode at the m/m ft/ft right end Undamped natural frequency rad/sec Natural frequency of Is* anti-symmetric mode rad/sec
INTRODUCTION Solar energy is being seriously considered as an alternative energy source for a wide range of applications not only as a result of technological advances, but in response to a variety of economic, environmental, and social forces. As limitations on conventional energy sources and the environmental consequences of energy production become more apparent, solar energy stands out as an inexhaustible alternate energy source if it can be harnessed within economic, environmental, and social constraints. Recently the potential of solar energy to meet future needs has been re-examined (1). Today opportunities for harnessing solar energy, both over the long and short term, are being investigated by government and industry. The magnitude of solar energy theoretically available is far in excess of future needs. Although the sun radiates vast quantities of energy, they reach the Earth in a very dilute form. Thus, any attempts to harness solar energy on a significant scale will require devices which occupy a large area as well as locations that receive a copious supply of sunlight. These requirements restrict Earthbased solar energy conversion devices for producing power to a few favorable geographical locations. Even for these locations energy storage must be provided to compensate for the day-night cycle and cloudy weather. One way to harness solar energy effectively would be to move the solar-energy conversion devices off the surface of the Earth and place them in orbit away from the Earth's active environment and influence and resulting erosive forces (2). The most favorable orbit from the power density point of view would be one around the sun, but a synchronized orbit around the Earth could be used where solar energy is available nearly 24 hours of every day. In the five years since the concept of a satellite solar power station (SSPS) was first presented as an alternative energy production method (3), the energy crisis experienced in the technologically advanced countries has intensified because of increasing energy use and demands for a clean environment. An assessment of the feasibility of the SSPS concept has shown that it is worthy of Consideration as an alternative energy production method (4-9). Its development can be realized by building on scientific realities, on an existing industrial capacity for mass production, and on demonstrated technological achievement (10).
BASELINE DESIGN Principles of a Satellite Solar Power Station Figure 1 shows the design concept for an SSPS. Two symmetrically arranged solar collectors convert solar energy directly to electricity by the photovoltaic process while the satellite is maintained in synchronous orbit around the Earth. The electricity is fed to microwave generators incorporated in a transmitting antenna located between the two solar collectors. The antenna directs the microwave beam to a receiving antenna on Earth where the microwave energy is efficiently and safely converted back to electricity. An SSPS can be designed to generate electrical power on Earth at any specific level. However, for a power output ranging from about 3,000 to 15,000 MW, the orbiting portion of the SSPS exhibits the best power-to-weight characteristics. Additional solar collectors and antennas could be added to establish an SSPS system at a desired orbital location. Power can be delivered to most desired geographic locations with the receiving antenna placed either on land or on platforms over water near major load centers, and tied into a power transmission grid. The status of technology and the advances which will be required to achieve effective operation for an SSPS are described below. Location of Orbit. — The preferred locations for the SSPS are the Earth's equatorial synchronous orbit stable nodes which occur near the minor axes, at a longitude of about 123° West FIGURE 1 DESIGN CONCEPT FOR A SATELLITE SOLAR POWER STATION
and 57° East. The minor axes are stable node points and the major axes unstable. The SSPS would be positioned so its solar collectors always face the sun, while the antenna directs a microwave beam to a receiving antenna on Earth. The microwave beam would permit all-weather transmission so that full use could be made of the nearly 24 hours of available solar energy. In an equatorial, synchronous orbit, the satellite can be maintained stationary with respect to any desired location on Earth. There are three major influences on the SSPS which would cause it to drift from its nominal orbital location (Figure 2): FIGURE 2. - NOMINAL ORBIT PERTURBATIONS 1. The ellipticity of the Earth causes the SSPS to seek out the Earth's minor axes; 2. The interaction of the gravitational effects of the sun and the moon would cause the orbit to regress so that its inclination would change with respect to the equator; and 3. Solar pressure would distort the orbit from circular to elliptical and back again over a one-year period. In addition, there would be an effective altitude change which
would increase the orbital period and then restore it to nominal over the same elapsed time. A summary of the flight control requirements is presented in Table 1. TABLE 1 FLIGHT CONTROL REQUIREMENTS Gravity Gradient 1.67x10 9 newtons m sec Solar Pressure Ty = 5500 newton-m Tx = 136 newton-m Electromagnetic Field Interactions < 10"5 newton-m Rotary Joint Friction 216 newton-m Microwave Transmission Recoil Pressure 11 newtons Aerodynamic 22x10~6 newton RCS (Ion Thruster, ISP 8000 sec) Propellant Weight to Control SSPS to within 1 Deg 43.6 kg/day 15,500 kg/year An SSPS in synchronous equatorial orbit would pass through the Earth's shadow around the time of equinoxes, at which time it would be eclipsed for a maximum of 72 minutes a day (near midnight at the SSPS longitude). This orbit provides a 6- to 15-fold conversion advantage over solar-energy conversion on Earth. A comparison of the maximum allowable costs of photovoltaic energy-conversion devices indicates that for a terrestrial solar-power application these devices are competitive with other energy-conversion methods if they cost about $2.30 per square meter. Because of the favorable conditions for energy conversion that exist in space, these devices are competitive if they cost about $45 per square meter in an Earth-orbit application (11). Solar Energy Conversion. — The photovoltaic conversion of solar energy into electricity is ideally suited to the purposes of an SSPS. In contrast to any process based on thermodynamic energy conversion, there are no moving parts, fluid does not circulate, no material is consumed, and a photovoltaic solar cell can operate for long periods without maintenance. There has been a substantial development in photovoltaic energy conversion since the first laboratory demonstration of the silicon solar cell in 1953. Today, such cells are a necessary part of the power supply system of nearly every unmannea spacecraft, and considerable experience has been accumulated to achieve long-term and reliable operations under the conditions existing in space. Thus, the “Skylab” spacecraft relies on silicon solar cells to provide about 25 kW of power. As a result of many years of operational experience, a substantial technological base exists on which further developments can be based (12). These developments are directed towards increasing the efficiency of solar cells, reducing their weight and cost, and maintaining their operation over extended periods.
a. Efficiency Increase The maximum theoretical efficiency of a silicon solar cell is about 22%. The most widely used single-crystal silicon solar cells can routinely reach an efficiency of 11% and efficiencies of up to 16% have been reported (13). Development-programs to increase the efficiency of silicon solar cells up to 20% have been outlined (14). The silicon solar cell which is produced from single-crystal silicon is typically arranged with the P-N junctions positioned horizontally. More recently, vertically illuminated multijunction silicon solar cells have been investigated (15). These have the potential to reach higher efficiencies and to be more resistant to solar radiation damage. Solar cells made from single-crystal gallium arsenide exhibit an efficiency of about 14% with a theoretical limit of about 26%. Recently, a modified gallium arsenide solar cell was reported to have reached an efficiency of 18% (16). This substantial increase in efficiency is particularly significant, because these cells can operate at higher temperatures than silicon solar cells, are more radiationresistant and can be prepared in thicknesses about one-tenth that of a silicon solar cell. Several other materials may be suitable for photovoltaic solar energy conversion. Among these are various combinations of inorganic semiconductors which have only partially been investigated. Organic semiconductors which exhibit the photovoltaic effect and which do not have known boundaries to the theoretical efficiency also remain to be explored, so that their potential for photovoltaic solar-energy conversion can be established (17). b. Weight Single-crystal silicon solar cells are presently 500 to 1000 microns thick, although their thickness could be reduced to about 50 microns without compromising efficiency. However, gallium arsenide cells need be only a few microns thick. The individual solar cells have to be assembled to form the solar collector. The weight of a solar cell array can be reduced by assembling the solar cells in a blanket between thin plastic films, with electrical interconnections between individual cells obtained by vacuum-depositing metal alloy contact materials. The collector weight can be further reduced when solar energy concentrating minors arranged to form flatplate channels are used so that a smaller area of solar cells is required for the same electrical power output (see Figure 3). The weight and cost of a given area of a reflecting mirror used to concentrate solar energy are considerably less than those for the same area of solar cells. Suitable coatings on mirrors to reflect only the component of the solar spectrum most useful for photovoltaic conversion can reduce heating of the solar cells and thus increase efficiency. There is a balance between concentrating the solar radiation per unit area of the cell, which may lead to a rise in temperature and a consequent decrease in solar cell efficiency, and the desire to maximize the collection of solar energy. An array configuration that includes mirrors with a concentration factor of about 2 has been chosen for the solar collector.
FIGURE 3. - SSPS DIMENSIONS Since 1965, the solar cell weight has decreased substantially. With the development of blanket-type construction for solar cell arrays, this weight is projected to drop to 4 kg/kW by 1975. The use of solar energy-concentrating mirrors can reduce the solar collector weight to about 1 kg/kW if 100-micron thick silicon solar cells are used, and to even less if gallium arsenide solar cells are used. c. Electrical Interconnections Because solar cell arrays with large areas will be required (e.g., one of the two solar arrays for the SSPS is about 5 km by 5 km), the satellite structures must be designed to combine mechanical and power distribution network functions to achieve high-voltage DC output. In the very large arrays, solar cells can be connected in series to produce any voltage desired. With existing solar cells, a series string can be assembled to build up the voltage to 50 kV or more. Development of the vertically illuminated, multijunction solar cell could product solar cells of high-voltage output. In such a solar cell, there may be a thousand junctions in series for each 1-cm wide cell. Thus, each cell may put out several hundred volts instead of the 0.5 volt from present solar cells. This type of cell will make it easier to build up a high voltage with a small number of cells, and thus allow most of the circuit to be in parallel and to be less susceptible to losses from open circuits. This arrangement also would preclude the loss of a total string of solar cells because of the loss of any one link.
The power bus interconnecting the major segments of the solar cell arrays, which will have to carry several hundred thousand amperes, must be designed to minimize magnetic field interactions. This can be done by suitable arrangement of the power distribution circuits. High-voltage switching circuits will have to be developed to control sections of the solar cell array for maintenance and operational purposes, and to protect the solar cell arrays when they enter and leave the Earth's shadow. The system also must provide the capability of switching off all power by open-circuiting solar cells instantaneously in the event of system malfunction. Because the SSPS system provides no energy storage, it will be safer than conversion systems that rely on thermodynamic power. d. Effects of the Space Environment The state of the art of solar cells is now at a level where lifetimes of 10 years are achievable. For example, the effective life expectancy of the Intelsat IV satellite is eight years. But the operations of the solar cell arrays will be influenced by the space environment. One influencing factor in space will be solar radiation. Solar radiation damage will cause a logarithmic decay of sok.r-cell effectiveness. However, improvements in radiation-resistant solar cells are expected to result in a 30-year minimum operational lifetime for the SSPS, after which normal SSPS effectiveness can be restored by adding a small area of new solar cells. Thus, there will be no absolute time limit on elfective SSPS operation. Another aspect of the space environment that will influence SSPS operations is the impact of micrometeoroids. In synchronous orbit, the SSPS is expected to suffer a 1% loss of solar cells, based on the probability of damage-causing impacts by micrometeoroids during a 30-year period. The benign nature of the space environment and the absence of significant gravitational forces, however, permit the design of solar collector arrays which have a minimum material mass. In addition, their performance would be much more predictable than that of an Earth-based solar energy conversion device because of th. absence of the vagaries of the Earth environment. Microwave Power Generation, Transmission, and Rectification. — The power generated by the SSPS in synchronous orbit must be transmitted to a receiving antenna on the surface of the Earth and then rectified. The power must be in a form suitable for efficient transmission in large amounts across long distances with minimum losses and without affecting the ionosphere and atmosphere. The power flux densities received on Earth must also be at levels which will not produce undesirable environmental or biological effects. Finally, the power must be in a form that can be converted, transmitted, and rectified with very high efficiency by known devices. All these conditions can best be met by a beam link in the microwave part of the spectrum. In this part of the spectrum a desirable frequency can be selected, e.g., about 3.3 GHz, and induced radio frequency interference limited so that an appropriate internationally agreed upon frequency could be assigned to an SSPS.
Fortunately, man has considerable experience in high-power microwave generation, transmission, and rectification. As early as 1963, Brown (18) demonstrated that large amounts of power could be transmitted by microwaves. The efficiency of microwave power transmission will be high when the transmitting antenna in the SSPS and the receiving antenna on Earth are large. The dimensions of the transmitting antenna and the receiving antenna on Earth are governed by the distance between them and the choice of wavelength (19). The size of the transmitting antenna is also influenced by the inefficiency of the microwave generators due to the area required for passive radiators to reject waste heat to space and the structural considerations as determined by the arrangement of the individual microwave generators. The size and weight of the transmitting antenna will be reduced as the average microwave power flux density on the ground is reduced by increasing the size of the receiving antenna and as higher-frequency microwave transmission is used. The size of the receiving antenna will be influenced by the choice of the acceptable microwave power flux density, the illumination pattern across the antenna face, and the minimum microwave power flux density required for efficient microwave rectification. a. Microwave Attenuation (20) Ionospheric attenuation of microwaves is low (less than 0.1%) for wavelengths between 3 and 30 cm and for the microwave power flux densities occurring within the beam. Tropospheric attenuation is low for wavelengths near and above 10 cm, but attenuation will increase as wavelengths are reduced. Moderate rainfall attenuates microwaves approximately 10% at a wavelength of 3 cm and 3% at a wavelength of 10 cm at a nadir angle of 60 deg. The efficiency of transmission through the atmosphere in temperate latitudes, including some rain (2 mm/hr), is approximately 98% and decreases to 94% for moderate (33 mm/hr) rainfall, depending on location (see Figure 4). b. Microwave Transmission System Efficiency The efficiency of the microwave power transmission system is a product of the efficiency of dc-to-microwave power conversion, the efficiency with which the microwave power is transmitted to the receiving antenna by the microwave beam link, and the efficiency with which the microwave beam is controlled, pointed toward, and intercepted at the receiving antenna, and there rectified to de. This overall transmission efficiency can also be measured experimentally as a ratio of the de power output at the receiving antenna to the de power input at the transmitting antenna. Table 2 indicates the efficiencies that have been demonstrated in the three major functional categories of the microwave transmission system and the projected efficiencies which should be achievable with further development (21). Including microwave attenuation, the overall efficiency of microwave transmission from de in the SSPS to de on the ground is projected to be about 70%.
FIGURE 4. - ATMOSPHERIC ATTENUATION OF MICROWAVES IN TWO UNITED STATES LOCATIONS c. Microwave Generation The microwave generator design is based on the principle of a crossed-field device which has the potential to achieve a high reliability and a very long life (22). A pure-metal, self-starting, secondary emitting, cold cathode is employed in a non-reentrant circuit, matched to an input and output circuit so as to provide a broadband gain device. The device is designed to be capable of automatically self-regulating its power output. The use of samarium-cobalt permanent magnet material leads to substantial weight reduction compared to previously available magnet material. The vacuum in space obviates the glass envelope required on Earth. The cathode and anode of the microwave generator are designed to reject waste heat with passive extended-surface radiators which radiate to space. The output of an individual microwave generator weighing a fraction of a kilogram per kilowatt can range from 2 to 5 kW.
TABLE 2 MICROWAVE POWER TRANSMISSION EFFICIENCIES Efficiency Presently Demonstrated3 Efficiency Expected with Present Technology3 Efficiency Expected with Additional Development3 Microwave Power Generation Efficiency 76.7b 85.0 90.0 Transmission Efficiency from Output of Generator-to-Col lector Aperture 94.0 94.0 95.0 Collection and Rectification Efficiency (Rectenna) 64.0 75.0 90.0 Transmission, Collection, and Rectification Efficiency 60.2 70.5 85.0 Overall Effici ncy 26.5C 60.0 77.0 The quantity of 1 to 2 million tubes that would be needed for each SSPS is large enough to warrant large-scale, highly efficient mass production. There is substantial production experience on magnetrons, similar in many respects to the Amplitron device projected for use in the SSPS. d. Microwave Transmission A series of microwave generators will be combined in a subarray (e.g., about 5 m by 5 m) which forms part of the antenna. Each subarray must be provided with an automatic phasing system so the individual antenna radiating elements will be in phase. These subarrays will radiate through a slotted waveguide and form a phased-array transmitting antenna about 1 km in diameter to obtain a microwave beam of a desired distribution. The distribution can be designed to range from uniform to near gaussian. For this 1-km diameter antenna, the diameter of the receiving antenna on Earth would have to be about 7 km for gaussian distribution in the beam within which 90% of the transmitted energy is intercepted. The use of such a large receiving antenna area would reduce the microwave power flux density on the Earth to a value low enough so that the flux density of the edges of the receiving
antenna would be substantially less than the continuous microwave exposure standard presently accepted in the United States (i.e., 10mW/cm2). Within several kilometers beyond the receiving antenna, the microwave density levels drop to less than 1 pW/cm2(23). (See Figure 5.) The data in Figure 5 are for a total power of 1.17 x 107 kW for a single SSPS whereas the SSPS baseline design is for about half that value. The curves, however, indicate the significant decay rate with increased radius. FIGURE 5. - HYPOTHETICAL DISTRIBUTIONS OF MICROWAVE POWER DENSITY FROM THE BEAM CENTER (Ideal Gaussian) To achieve the desired high efficiency for microwave transmission, the phased-array antenna will be pointed by electronic phase shifters (24). Proper phase setting for each subarray must be established to form and maintain the desired phase front. Deviations can be detected and appropriate phase shifts made to minimize microwave beam scattering. A master phase control in the antenna will have to be developed if the microwaves are to be transmitted efficiently and the microwave beam always directed toward the receiving antenna (25). The master phase front control system can be designed to compensate for the tolerance and position differences between the subarrays by sensing the phase of a pilot signal beamed from the center of the area occupied by the receiving antenna to control the phase of the microwaves transmitted by each subarray. The pilot signal will be of a substantially different frequency than that of the microwave power beam, so wave filters could be used to separate them.
Precision pointing of the receiving antenna is not necessary to the operation of the SSPS and inhomogeneities in the propagation path are not significant. Any deviation of the microwave beam beyond allowable limits would preclude acquisition of the pilot signal. Without the signal, the coherence of the microwave beam would be lost, the energy dissipated, and the beam spread out so the microwave power density would approach communication signal levels. This phase-control approach would assure that the beam could not be directed either accidentally or deliberately towards any other location but the receiving antenna. This inherent fail-safe feature of the microwave transmission system is backed up by the operation of the switching devices, which would open-circuit the solar cell arrays to interrupt the power supply to the microwave generators. e. Microwave Rectification The receiving antenna is designed tQ intercept, collect, and rectify the microwave beam into de which can then be fed into a high-voltage de transmission network or converted into 60-Hz ac. Half-wave dipoles distributed throughout the receiving antenna capture the microwave power and deliver it to solid-state microwave rectifiers (26). Schottky barrier diodes have already been demonstrated to have a 80% microwave rectification efficiency at 5W of power output. With improved circuits and diodes, a recreation efficiency of about 90% will be achievable. The diodes combined with circuit elements which act as half-wave dipoles are uniformly distributed throughout the receiving antenna, so the microwave beam intercepted in a local region of the receiving antenna is immediately converted into de. The collection and rectification of microwaves with a receiving antenna based on this principle has the advantages that the receiving antenna is fixed and does not have to be pointed precisely at the transmitting antenna. Thus, the mechanical tolerance in the construction of the receiving antenna can be relaxed. Furthermore, the illumination distribution of the incoming microwave radiation need not be matched to the radiation pattern of the receiving antenna; therefore, a distorted incoming wavefront caused by non-uniform atmospheric conditions across the antenna will not reduce efficiency. The amount of microwave power that is received in local regions of the receiving antenna can be matched to the power-handling capability of the solid-state diode microwave rectifiers. Any heat resulting from inefficient rectification in the diode circuits can be convected by the ambient air in the local region of the receiving antenna with atmospheric heating similar to that over urban areas. Only about 10% of the incoming microwave beam would be lost as waste heat. The low thermal pollution achievable by this process of rectification cannot be equaled in any known thermodynamic conversion process. The rectifying elements in the receiving antenna can be exposed to local weather conditions. The antenna can be designed so that sunlight would still reach the land beneath it, with only a fraction lost due to shadowing. Thus the land could be put to productive use.
f. Power Output Levels Bounds can be placed on the range of potential SSPS power levels as shown in Figure 6. The SSPS design can be adjusted to provide from 4 to 40 mW/cm2 of rectified power at the receiving antenna. This range of power level postulates a receiving antenna diameter of 10 to 20 km and a transmitting antenna diameter of 1 to 2 km. An idealized Gaussian distribution was chosen to establish the transmitting and receiving antenna diameters. There is an additional cutoff established by the inability of the transmitting antenna's passive thermal control system to reject the waste heat of the microwave generator when the microwave power density of the transmitting antenna rises above 4.13 W/cm2. Thus, in principle, an SSPS could be designed to generate electrical power on Earth at power outputs ranging from about 2,000 to 20,000 MW. It is likely that a narrower range of power output, ranging from 3,000 to 15,000 MW will be more effective. A nominal power output level of 5000 MW at the receiving antenna falls about in the middle of the range of interest represented by the shadowed area in Figure 6, and therefore it was chosen as representative for the SSPS baseline design. The overall capacity of the future transmission grid system will place an upper boundry on the SSPS power output to allow for the possibility that one SSPS has to be taken out of service. SSPS Flight Control Although the SSPS is orders-of-magnitude larger than any spacecraft yet designed, its overall design is based on present principles of technology. Thus, its construction and the attainment of a 30-year operating life require not new technology, but substantial advances in the state of the art. The SSPS structure is composed of high-current-carrying structural elements whose electromagnetic interactions will induce loads or forces into the structure. Current stabilization and control techniques are capable of meeting the requirements of spacecraft now under development. Most of these spacecraft have comparatively rigid structures and the amenable to control as a single entity by reaction jets or momentum storage devices. But the large size of an SSPS suggests that new structural and control system design approaches may be needed to satisfy orientation requirements. This study, however, indicates that present analytical techniques/tools are adequate and that an SSPS can be controlled to better than ± 1 degree. Low-thrust, ion propulsion systems appear promising for SSPS control, because their performance characteristics are compatible with the potential lifetime required of the SSPS. Earth-to-Orbit Transportation A high-volume, two-stage transportation system will be required for an SSPS: (1) a low-cost stage capable of carrying high-volume pay loads to low-Earth orbit (LEO); and (2) a high-performance stage capable of delivering partially assembled elements to synchronous or some intermediate orbit altitude for final assembly and deployment. The factors affecting flight mode selection include payload element size, payload assembly techniques, desirable orbit locations for assembly, time constraints, and requirements for man's participation in the assembly. The choice of transportation
system elements includes currently planned propulsion stages and advanced concepts optimized for an operational SSPS system. Minimum-cost transportation combinations will have to be identified which can fulfill the requirements for SSPS delivery, assembly, and maintenance for an operational system. The challenge of an SSPS capable of generating 5,000 MW of power on Earth is to place into orbit a payload of about 25 million pounds and propellant supplies for station-keeping purposes of about 30,000 pounds per year. FIGURE 6. ~ POTENTIAL SSPS POWER LEVELS
STRUCTURE AND CONTROL TECHNIQUES Structural and Dynamic Analysis of the SSPS Summary. — The purpose of this task was to perform structural and dynamic analyses of the SSPS structure for the purposes of: • Providing elastic characteristics (natural frequencies, generalized masses and mode shapes) of the structure for use in an analytical investigation of the elastic coupling between the SSPS attitude control system and the spacecraft's structural modes; • Determining deflections and internal member loads resulting from the various flight loading conditions to verify structural integrity; • Assessing desirable material characteristics; and • Identifying desirable technology developments. Results of this task will be applied to the design of structure and attitude control systems for very large-area, light-weight space structures represented by the SSPS. The SSPS will require the design of a structure that can not only support the solar cell blankets, concentrator mirrors, and transmission bus/structure for the various flight loadings, but also one that can be both assembled and controlled in space. To investigate the structural and dynamic aspects connected with controlling such a structure in space required: * The establishment of a baseline configuration, a structural math model, a dynamic math model, and flight loading conditions; • The input of flight and control system loadings into the structural model to determine internal loads and structural deflections; ® Assessment of the baseline configuration for internal loads and deflections, as well as candidate material characteristics; and • Identification of areas requiring future analysis and the analytical tools needed for analysis. Baseline Configuration.- The SSPS characteristics and the baseline configuration used in this task are shown in Figures 7, 8, and 9. The main structural framework for each solar array consists of a large-diameter coaxial mast transmission bus, four transverse DC power buses, and non-conductive struts. Shear loads are transmitted by tension-only wires. Structural continuity between the two solar arrays is supplied by the mast and by non-conductive structure running