NASA CR-134886 ER75-4368 MICROWAVE POWER TRANSMISSION SYSTEM STUDIES VOLUME I EXECUTIVE SUMMARY RAYTHEON COMPANY EQUIPMENT DIVISION ADVANCED DEVELOPMENT LABORATORY SUDBURY, MASS. 01776 prepared for NATIONAL AERONAUTICS AND SPACE ADMINISTRATION NASA Lewis Research Center Contract NAS 3-17835 This copy may not be removed from file room
1. Report No. NASA CR-134886 2. Government Accession No. 3. Recipient's Catalog No. 4. Title and Subtitle MICROWAVE POWER TRANSMISSION SYSTEM STUDIES Volume I - Executive Summary 5. Report Date December 1975 6. Performing Organization Code 7. Authors} -- O. E. Maynard, W.C. Brown, A. Edwards, G. Meltz, J. T. Haley, J. M. Howell - Raytheon Co. ; A. Nathan - Grumman Aerospace Corp. 8. Performing Organization Report No. ER75-4368 10. Work Unit No. 9. Performing Organization Name and Address Raytheon Company Equipment Division 528 Boston Post Road Sudbury, Massachusetts 01776 11. Contract or Grant No. NAS 3-17835 13. Type of Report and Period Covered Contractor Report 12. Sponsoring Agency Name and Address National Aeronautics and Space Administration Washington, D. C. 20546 14. Sponsoring Agency Code 15. Supplementary Notes Mr. Richard M. Schuh NASA Lewis Research Center Cleveland, Ohio 44135 16. Abstract A study of microwave power generation, transmission, reception and control was conducted as a part of the NASA Office of Applications* joint Lewis Research Center/Jet Propulsion Laboratory five-year program to demonstrate the feasibility of power transmission from geosynchronous orbit. This volume (1 of 4) serves as an executive summary of results concerning design approaches, estimated costs (ROM), critical technology, associated ground and orbital test programs with emphasis on de to rf conversion, transmitting antenna, phase control, mechanical systems, flight operations, ground power receiving-rectifying antenna with systems analysis and evaluation. Recommendations for early further in-depth studies complementing the technology program complete the volume. 17. Key Words (Suggested by Authors}) Microwave power transmission; power from space; satellite power transmission; phased array power transmission; rectifying antenna (rectenna). 18. Distribution Statement Unclassified - Unlimited 19. Security Qassif. (of this report} Unclassified 20. Security Ciassif. (of this page) Unclassified 21. No. of Pages
TABLE OF CONTENTS VOLUME I - EXECUTIVE SUMMARY Section Page 1. Introduction 2 2. DC to RF Conversion 4 3. Transmitting Antenna and Phase Front Control 9 4. Mechanical Systems 16 5. Flight Operations 22 6. Receiving Antenna 25 7. Systems Analysis and Evaluation 28 8. Critical Technology 34 9. Critical Technology and Test Program 37 10. Recommendations for Additional Studies 42 VOLUME II - Sections 1 through 7 with Appendices A through G Page 1. INTRODUCTION, CONCLUSIONS AND RECOMMENDATIONS 1. 1 Introduction 1"1 1.2 Conclusions and Recommendations 1-3 1. 2. 1 General 1~3 1.2.2 Subsystems and Technology 1“5 1.2.2. 1 Environmental Effects - Propagation 1-5 1.2. 2.2 DC-RF Conversion 1-5 1.2. 2. 3 Power Interface and Distribution (Orbital) 1-7 1.2.2.4 Transmitting Antenna 1-7 1.2. 2. 5 Phase Front Control 1-8 1.2. 2. 6 Mechanical Systems and Flight Operations 1-9 1.2.2. 7 Receiving Antenna 1-10 1.2. 2. 8 Radio Frequency Interference and Allocation 1-11 1.2. 2.9 Risk Assessment 1-12 1.2.3 System Analysis and Evaluation 1-15
1.2.4 Technology Development and Test Programs 1-16 1. 2. 4. 1 Technology Development and Ground Test Program 1-16 1. 2. 4. 2 Technology Development and Orbital Test Program 1-16 1.2.5 Additional Studies 1-17 1. 3 Report Approach and Organization 1-17 2. ORGANIZATION AND APPROACH 2.1 Organization 2-1 2.2 Approach 2-3 3. ENVIRONMENTAL EFFECTS - PROPAGATION 3. 1 Introduction 3- 1 3. 2 Atmospheric Attenuation and Scattering 3-2 3. 2. 1 Molecular Absorption 3-2 3. 2. 2 Scattering and Absorption by Hydrometeors 3-4 3.3 Ionosphere Propagation 3-12 3.3.1 Ambient Refraction 3-12 3. 3. 2 Scintillations Due to Ambient Fluctuations and Self-Focusing Instabilities 3-13 3.4 Ionospheric Modification By High Power Irradiation 3-19 3. 5 Faraday Rotation Effects 3-21 3.5.1 Introduction 3-21 3. 5. 2 Diurnal and Seasonal Changes 3-21 3. 5. 3 Midlatitude Geomagnetic Storms 3-22 3. 6 Conclusions and Recommendations 3-24 4. DC-RF CONVERSION 4.1 Amplitron 4-1 4. 1. 1 RF Circuit 4-2 4. 1.2 Pyrolytic Graphite Radiator 4-4 4. 1. 3 Magnetic Circuit 4-4 4. 1.4 Controlling the Output of Amplitrons 4-5 4.1.5 Weight 4-5
6. TRANSMITTING ANTENNA 6. 1 Aperture Illumination and Size 6-1 6.2 Array Types 6-10 6. 3 Subarray Types 6-16 6.4 Subarray Dimensions 6-16 6. 5 Subarray Layout 6-20 6. 6 Tolerance and Attenuation 6-26 6. 6. 1 Frequency Tolerance 6-26 6.6.2 Waveguide Dimensional Tolerances 6-27 6.6*3 Waveguide Attenuation 6-28 6. 7 Mechanical Design and Analysis 6-29 6. 7. 1 Thermal Analysis and Configuration 6-29 6. 7C 2 Materials 6-38 6. 7. 3 Transportation, Assembly and Packaging 6-43 6. 8 Attitude Control and Alignment 6-47 6.9 Conclusions and Recommendations 6-49 7. PHASE FRONT CONTROL 7. 1 Adaptive Phase Front Control 7-4 7.2 Command Phase Front Control 7-10 7. 2. 1 Phase Estimation 7- 10 7.2.2 Bit Wiggle 7-14 7.3 Conclusions and Recommendations 7-14 APPENDIX A - RADIO WAVE DIFFRACTION BY RANDOM IONOSPHERIC IRREGULARITIES A. 1 Introduction A-1 A. 2 Model for Electron Density Irregularities A-2 A. 3 Phase Fluctuations and Their Spatial Correlation at the Diffracting Screen A-3 A, 4 Phase and Amplitude Fluctuations and Their Spatial and Temporal Correlation Functions on an Observational Plane A-4
APPENDIX B - SELF-FOCUSING PLASMA INSTABILITIES B-l APPENDIX C - OHMIC HEATING OF THE D-REGION C-l APPENDIX D - CAVITY CIRCUIT CALCULATIONS D. 1 Input Impedance D-1 D. 2 Input Power and Gain at Saturation D-2 D. 3 Intermediate- and Output-Gap Voltages D-3 D. 4 Cavity Tunings D-3 D. 5 Output Cavity and Circuit Efficiency D-4 APPENDIX E - OUTPUT POWER OF THE SOLENOID-FOCUSED KLYSTRON E-l APPENDIX F - KLYSTRON THERMAL CONTROL SYSTEM F. 1 Heat Conduction F-1 F. 2 Temperature, Area and Weight of Radiators F-2 F. 2. 1 Collector F-3 F. 2. 2 Collector Reflector and Heat Shield F-4 F. 2. 3 Body Radiator F-4 F. 2. 4 Body Reflector and Heat Shield F-5 F. 3 Weight of Heat Pipes F-5 APPENDIX G - CONFINED-FLOW FOCUSING OF A RELATIVISTIC BEAM G-1 VOLUME III - MECHANICAL SYSTEMS AND FLIGHT OPERATIONS (Section 8) 1. INTRODUCTION 1-1 2. SUMMARY 2. 1 Task 1 - Preliminary Design 2-1 2. 1. 1 Control Analysis 2-1 2. 1. 2 Thermal/Structural Analysis 2- 1 2. 1. 3 Design Options and Groundrules for Task 2 Concept Definition 2-5
2. 2 Task 2 - Concept Definition 2-9 2. 2. 1 Mission Analysis 2-9 2.2.2 Antenna Structural Definition 2-9 2.2.3 Configuration Analysis 2-14 2.2.4 Assembly 2-21 2.2.5 Cost 2-31 2.3 Recommendations 2-33 3. TECHNICAL DISCUSSION 3. 1 Mission Analysis 3. 1-1 3. 1. 1 SSPS Configuration and Flight Mode Descriptions 3. 1-1 3. 1.2 Transportation System Performance 3. 1-1 3. 1. 3 Altitude Selection 3. 1-8 3.1.4 SEPS (Ion Engine) Sizing 3.1-14 3. 2 Antenna Structural Concept 3. 2- 1 3. 2. 1 General Arrangement 3. 2- 1 3.2.2 Rotary Joint 3.2-1 3.2.3 Primary/Secondary Antenna Structure 3.2-15 3.2.4 Structure/Waveguide Interface 3.2-15 3. 2. 5 Antenna Weight and Mass Properties 3. 2-18 3.3 Configuration Analysis 3.3-1 3. 3. 1 Control Analysis 3. 3-1 3. 3. 2 Thermal Evaluation 3. 3-9 3. 3. 3 Structural Analysis 3. 3-41 3.4 Assembly and Packaging 3.4-1 3.4. 1 Detail Parts 3.4-1 3.4.2 Structural Assembly 3.4-9 3. 5 Cost 3. 5. 1 3. 5. 1 Task 1 - Preliminary Design Results 3. 5- 1 3. 5. 2 Task 2 - Concept Definition Results 3. 5-5 3. 5. 3 MPTS Structural Costs 3. 5-19
4. TECHNOLOGY ISSUES 4. 1 Control System 4-1 4. 1. 1 Evaluation of Alternate Power Transfer and Drive Devices 4-1 4. 1. 2 Detailed Control System Analysis 4-2 4.2 Structural System 4-3 4. 2. 1 Composite Structures and Assembly Techniques 4-3 4. 2. 2 Tension Brace Antenna Feasibility Assessment 4-4 4. 2. 3 Local Crippling Stress Evaluation 4-4 4.2.4 Design Environments 4-5 4. 2. 5 Optimum Antenna Structures 4-5 4.2.6 Finite Element Model Development 4-6 4. 2. 7 Composite Waveguide 4-6 4.3 Thermal System 4-7 4. 3. 1 Maximum Temperature 4-7 4.3.2 Transient Analysis 4-8 4.4 Assembly 4-9 4.4. 1 Assembly Cost 4-9 4.4.2 Man’s Role in Assembly and Maintenance 4-10 5. REFERENCES 5-1 VOLUME IV - Sections 9 through 14 with Appendices H through K 9. RECEIVING ANTENNA 9. 1 Microwave Rectifier Technology 9*1 9.2 Antenna Approaches 9-9 9.3 Topology of Rectenna Circuits 9*14 9.4 Assembly and Construction 9*21 9. 5 ROM Cost Estimates 9*21 9. 6 Power Interface Estimates 9*25 9. 6. 1 Inverter System 9*30 9.6.2 Power Distribution Costs 9*30 9. 6. 3 System Cost 9*31 9.7 Conclusions and Recommendations 9-31
10. FREQUENCY INTERFERENCE AND ALLOCATION 10-1 10. 1 Noise Considerations 10-3 10. 1. 1 Amplitron 10-3 10.1.2 Klystron 10-4 10. 1. 3 Interference Limits and Evaluation 10-6 10.2 Harmonic Considerations 10-6 10. 3 Conclusions and Recommendations 10-12 11. RISK ASSESSMENT 11. 1 Technology Risk Rating and Ranking 11-1 11.2 Technology Assessment Conclusions and Recommendations 11-16 12. SYSTEM ANALYSIS AND EVALUATION 12- 1 12. 1 System Geometry 12-1 12.2 Parametric Studies 12-3 12,2.1 System Relationships 12-3 12.2.2 Efficiency, Weight and Cost 12-8 12.2.3 Converter Packing 12-12 12.2.4 Capital Cost Vs Power and Frequency Results 12-13 12.2.5 Ground Power Density and Power Level Selection 12-19 12.2.6 Frequency Selection 12-22 12. 2. 7 Characteristics of 5 GW and 10 GW Systems 12-22 12.2.8 Energy Cost 12-36 12.3 Final System Estimates 12-41 12. 3. 1 Cost and Weight 12-41 12.3.2 Efficiency Budget 12-43 12. 3. 3 Capital Cost and Sizing Analyses 12-45 12.4 Conclusions and Recommendations 12-45 13. CRITICAL TECHNOLOGY AND GROUND TEST PROGRAM 13. 1 General Objectives 13-1 13.2 Detailed Ground Test Objectives 13-2 13.3 Implementation - Ground Test 13-3 13.3.1 Summary 13-3
13.3.2 Phase I 13-5 13.3.3 Phase II 13-5 13.3.4 Phase III 13-9 13.3. 5 Alternate Phase I Converter Implementation 13-11 13.4 Critical Technology Development 13-14 13.4.1 Amplitron 13-14 13.4.2 Klystron 13-14 13.4,3 Phase Control 13-14 13.5 Schedule and Cost 13-15 13. 6 Conclusions and Recommendations 13-17 14. CRITICAL TECHNOLOGY AND ORBITAL TEST PROGRAM 14. 1 Orbital Test Objectives 14-1 14.2 Implementation 14-3 14.2. 1 Geosatellite (Mission 1) 14-4 14.2.2 Shuttle Sorties (Missions 2 through 11) 14-4 14.2.3 Orbital Test Facility 14-23 14. 3 Cost and Schedule 14-25 14.4 Conclusions and Recommendations 14-30 APPENDIX H - ESTIMATED ANNUAL OPERATIONS AND MAINTENANCE COST (5 GW System) H-l APPENDIX I - ANNUAL OPERATIONS AND MAINTENANCE COST (10 GW System) I-1 APPENDIX J - SYSTEM ANALYSIS EXAMPLES J. 1 Introductory Analysis of Initial Operational System With Minimum Size Transmitting Antenna J-1 J. 2 Analysis of the Final Operational System and Their Goals J-10 J. 3 Analysis of the Initial Operational System Based On the Final System Configuration J-21 J. 4 Weight and Cost Analysis for the Initial and Final Operational Systems J-25 J. 5 Energy Cost J-27
APPENDIX K - DETAILS OF GROUND AND ORBITAL TEST PROGRAM K. 1 Introduction K-1 K. 2 Objectives Implementation Equipment and Characteristics K-1 K. 3 Implementation of Objectives Hl, H2, DI and D2 Using Low Earth Orbit Sortie Missions K-3 K. 4 Defining an MPTS Orbital Test Facility Program K-13 K. 4. 1 Assumptions K-13 K. 4. 2 Sizing the Phased Array Antennas K- 14
1 MPTS Concept 3 2 MPTS Functional Diagram 3 3 Comparison of Gaseous Absorption and Rain Attenuation 5 4 Transmission Efficiency - Molecular Absorption and Rain 5 5 Amplitron Assembly 6 6 Amplitron Weight/Cost/Efficiency Vs Frequency 6 7 Amplitron Weight and Cost Vs Power 6 8 MPTS 5 kW Amplitron Parameters 7 9 MPTS 5 kW Amplitron Power Budget 7 10 Efficiency Vs Output Power for Solenoid-Focused Klystron 8 11 Outline of 48 kW Klystron with Solenoid Focusing 8 12 MPTS 48 kW Klystron Parameters 9 13 MPTS 48 kW Klystron Power Budget 9 14 Microwave Power Beam - Idealized 10 15 Receiving Antenna Sizes for Truncated Gaussian Beam Tapers 10 16 Taper Effect on Pattern and Efficiency 11 17 Array-Subarray Organization 11 18 Subarray Size Considerations 12 19 Command and Adaptive Phase Front Control Concepts 13 20 Subarray Types 14 21 Alternative Array Types 14 22 Amplitron Thermal Model 15 23 Subarray Deflection Vs Size 15 24 SPS Incremental Cost Vs Subarray Size 17 25 Subarray L^ayout 17 26 Transmitting Antenna Pointing System 18 27 Rotary Joints 18 28 Antenna Structural Arrangement 20 29 Structure/Waveguide Interface 20 30 Comparison of Max Temp and Thermal Gradients 21 31 Structural Joints 21 LIST OF ILLUSTRATIONS
32 Typical Antenna Deflections Due to Thermal Gradients 22 33 Typical Slopes of Structure Due to Thermal Gradients 22 34 Assembly Functional Flow 23 35 Mission Options 23 36 Waveguide Weight and Packaging Density 23 37 Detail Part Assembly Summary 24 38 Traffic and Fleet Size Summary 24 39 Transportation and Assembly Cost - Plan 1 26 40 Comparison of Antenna Approaches 26 41 Rectenna Element Efficiency Vs Frequency 27 42 Rectenna Elements 27 43 Diode Production Experience 29 44 Orbital Transportation/Assembly and Power Source Parameters 29 45 SPS Capital Cost Vs Frequency - 100 $/kg 29 46 SPS Capital Cost Vs Frequency - 300 $/kg 30 47 Peak Ground Power Density Vs Frequency 30 48 Amplitron-Aluminum MPTS Comparison 32 49 Comparison of 5 GW Systems 32 50 MPTS Efficiency Budget 33 51 SPS Capital Cost for Various Power Source Characteristics 33 52 SPS Energy Cost for Various Power Source Characteristics 33 53 SPS Energy Cost for Various Rates of Return 33 54 SPS Energy Cost for Various Construction Cycles 33 55 Suggested Nominal Values for SPS and MPTS 34 56 Technology and Hardware Development Risk Rating Definition 35 57 Satellite Power System Technology Risk Assessment 36 58 MPTS Ground Test Functional Block Diagram 38 59 MPTS - Critical Technology Development and Ground Test Program 38 60 1975 Dollar ROM Costs, $K, for Critical Technology and Ground Test Program 39 61 Microwave Orbital Test Program 40
62 Mission Schedule ±U 63 Critical Technology Schedule 41 64 MPTS Orbital Test Program ROM Cost Summary 41
AFCRL Air Force Cambridge Research Laboratory ATC Air Traffic Control ATS Applications Technology Satellite CFA Crossed Field Amplifier CPU Central Processor Unit GaAs Gallium Arsenide HLLV Heavy Lift Launch Vehicle Met Meteorological MPTS Microwave Power Transmission System MW Microwave N. F. noise factor PPM periodic permanent magnet ROM Rough Order of Magnitude SCR Silicon Controlled Rectifier SEPS Solar Electric Propulsion Stage Sm-Co(SMCO) Samarium Cobalt SPS Satellite Power System SSPS Satellite Solar Power Station TDRS Tracking and Data Relay Satellite TEC Total Electron Content LIST OF NON-STANDARD TERMS
EXECUTIVE SUMMARY This volume summarizes results obtained in the study concerning, design approaches, estimated costs and technology requirements for systems that transmit power from space to earth, a concept leading to a potential source of comparatively pollution-free power. Basic elements of such systems are an extraterrestrial power source, e.g., a solar powered device or a nuclear reactor, and a transmission system to condition the power, beam it to earth and again condition it for distribution. The transmission system uses microwave technology which has the potential for high efficiency, large power handling capability and controllability. The transmitting antenna would be in geosynchronous orbit on a fixed line of sight to the ground antenna. The work was conducted in 1974-1975 by the Raytheon Company. Raytheon was supported by the Grumman Aerospace Corporation on mechanical systems and flight operations, and by Shared Applications, Inc. on klystrons for microwave conversion. The transmitting antenna is a planar phased array about 1 km in diameter constructed of aluminum or composites and weighing about 6 x 106 kg. It consists ofj 18M x 18M slotted waveguide subarrays which are electronically controlled to direct the power beam at the ground receiving antenna with an rms error of only 10M. The subarrays use groups either of 5 kW amplitrons in series or 50 kW klystrons in parallel to convert input de power to microwave power. The receiving antenna is an array about 10 km in diameter consisting of dipole elements each connected to a solid state diode which converts microwave power back to de power. An operating frequency of 2.45 GHz in the USA industrial band re- bulls in near optimum efficiency, avoids brownouts in rain and should have minimal problems in radio frequency interference and allocation. A 5 GW ground power output provides economy of scale while keeping the peak microwave power density in the center of the beam at earth about 20 mW/cm^. Microwave system transmission efficiency is about 60% and cost is about 500 $/kW including assembly and transport of the transmitting antenna to geosynchronous orbit at 200 $/kg. The orbital transportation and assembly cost should not exceed about 200 $/kg if a satellite power system is to have energy costs comparable with projections for ground based fossil and nuclear plants. The recommended flight plan is transport to low earth orbit using a reusable heavy lift launch vehicle, assembly in low earth orbit and then transport to synchronous orbit usirg a solar electric propulsion stage. Emphasis is placed on orbital manufacture and assembly to achieve faborable launch vehicle packaging densities. The critical technology items needing early development are the de to microwave converters, materials, electronic phase control subsystems and the transmitting antenna waveguide and structure including their interfaces with the microwave converters. A
critical technology development and test program is presented. A ground test involving transmitting and receiving antennas is recommended to obtain data on beam controllability and radio frequency interference, which will provide design confidence for orbital tests. The planned orbital test program demonstrates operation of the open cathode dc to rf converter, satisfactory vacuum high voltage plasma interaction, orbital assembly techniques, and operations and maintenance development. The orbital test program for the microwave power transmission technologies assumes availability of the Shuttle transportation system and a power source presumed to be a part of its own orbital test program. 1. INTRODUCTION Microwaves can traverse the atmosphere with low attenuation, and advances in microwave power technology have been considerable since the first demonstration of appreciable power transfer by Brown (1963). The combination of a solar photovoltaic power source in geosynchronous orbit with microwave transmission to earth was first proposed by Glaser (1968). This Satellite Solar Power Station (SSPS) concept received increasing attention (J. Microwave Power, 1970; Brown, 1973) and led to a feasibility study conducted by a team consisting of Arthur D. Little, Inc., Grumman Aerospace Corp., Raytheon Co., and Textron Inc. under NASA sponsorship (Glaser, 1974). Results were sufficiently promising to warrant support of more detailed studies in the technologies involved. The study concentrated on the microwave power transmission system (MPTS) for transmitting energy from space to earth, and as such the results are independent of the power source selected. For examples, a solar thermal converter or a nuclear reactor in orbit could be considered in place of a solar photovolataic source. Nevertheless, the solar photovoltaic source remains the best known and studied of alternatives, and so was used for purposes of illustration where required. The study involved preliminary analysis, conceptual design, technical and economic evaluation, and planning for a technology development, a ground demonstration and an orbital test program. The concept of space to earth microwave power transmission is illustrated in Figure 1. A transmitting antenna in geosynchronous orbit beams microwave power to a ground antenna where it is rectified to de power. Functional blocks of such a power transmission system are shown in Figure 2. Efficiency is a prime consideration in any transmission system, and it is evident that elements must average over 90% if overall efficiency is to be a modest 60%. These efficiency considerations dictate that the antennas be extremely large scale, e.g., the transmitting antenna is on the order of 1 km in diameter and the receiving antenna is on the order of 10 km, because of the long transmission distance of 37000 km. This scale implies that large units of power, on the order of 5 GW-10 GW, must be transferred and that the power source in turn must be very large scale.
Figure 1 MPTS Concept Figure 2 MPTS Functional Diagram
High efficiency requirements also dictate the band of microwave frequencies that can be considered. The effect of molecular absorption shown in Figure 3 limits frequencies to below 10 GHz to 16 GHz. The upper limit reduces further if brownouts in light rain (5 MM/HR) are to be excluded, and the avoidance of brownouts in heavy rain and severe thunderstorms, for which attenuations are shown in Figure 4, would place an upper limit not far above 3 GHz. The severe rain conditions are experienced even in the desert locations that are prime candidates for the ground receiving antenna location. Having introduced scale and frequency considerations, we proceed to examine the technical and cost aspects of the major systems building blocks, then discuss the transportation and assembly of the orbital elements, present a selection of recommended system parameters based on overall economic and technical considerations, and finally summarize the areas of critical technology requiring priority attention in the future. A 30 year useful life is taken as a design goal for candidate configurations. 2. DC TO RF CONVERSION The study examined two generic types of devices for converting dc power to rf power at microwave frequencies: the amplitron, or crossed field amplifier (CFA), and the klystron, a linear beam device. In current usage the amplitron is characterized by high efficiency and low gain; the klystron is known for moderately high efficiency, high gain and low noise. A cross section view of an amplitron designed for the MPTS application is shown in Figure 5. Special features of the design include open construction for low weight and reliability, and a platinum metal cathode operating on the principle of secondary emission to achieve an essentially infinite cathode life. Tube de voltage input is 20 kV. Samarium Cobalt magnets provide a very low specific weight, and pyrolytic graphite with low density and high emmissivity radiates waste heat to space. The only element with a potential wearout mechanism is the movable magnet shunt designed for regulating the output as input voltage fluctuates. A regulating concept eliminating moving parts by using an impulse magnetic control is proposed as an a1tern a tive approach. Specific weight, specific cost and efficiency trends with frequency as a variable are shown in Figure 6. These favor a selection near 2.45 GHz which is in the center of the industrial microwave band of 2.40 GHz - 2.50 GHz. An output power level selection at 2.45 GHz should be near 5 kW as indicated in the weight and cost trends of Figure 7. The dominance of the thermal radiator in overall weight is indicated in the breakdown of Figure 8 for a 5 kW, 2.45 GHz amplitron. The pyrolytic graphite radiator is sized by an 85% tube efficiency and by the maximum temperature (350°C) allowed for the Samarium Cobalt magnet. Power budget for the MPTS amplitron is given in Figure 9. Improvement to 90% efficiency is believed a reasonable development goal since amplitrons already have reached 85% [Brown, 1974].
Figure 3 Comparison of Gaseous Absorption and Rain Attenuation Figure 4 Transmission Efficiency - Molecular Absorption and Rain
Figure 5 Amplitron Assembly Figure 6 Amplitron Weight/ Cost/Efficiency Vs. Frequency Figure 7 Amplitron Weight and Cost Vs. Power
Figure 8 MPTS 5 kW Amplitron Parameters Figure 9 MPTS 5 kW Amplitron Power Budget Preliminary studies of the klystron indicated the 1.5 GHz to 3 GHz region would yield relatively low specific weight and cost designs as was the case for the amplitron. They also showed a possibility of purely passive cooling at power levels below 10 kW, but further study indicated all klystrons would require at least a heat pipe cooling scheme. Attention then shifted from low power, permanent magnet-focused tubes to higher power tubes of the solenoid-focused type where focusing power becomes less significant in the power budget. This trend is shown in Figure 10. A klystron design at 2.45 GHz representative of the higher power versions is shown in cross section for a 48 kW tube in Figure 11. Like the amplitron the tube would be open construction. Most power loss occurs at the collector where it is radiated at high temperature. However, removing beat from the body is a difficult challenge, solved in this example by using heat pipes which remain to be detailed in future studies. The tube incorporates a hot cathode which must be designed to achieve a life commensurate with a 30 year system life. Cold cathodes have not been demonstrated in klystrons as they have in amplitrons, although they might be realized by further development. The power budget and tube parameter summary in Figures 12 and 13 indicate that relative to the amplitron, efficiency is about four percent lower, specific weight is three hundred percent higher and specific cost is slightly higher. However, the klystron may have two potential advantages over the amplitron: (1) fewer, higher power tubes may simplify the orbital assembly task and (2) low noise and narrow bandwidth reduce radio frequency interference .
Figure 10 Efficiency Vs Output Power for Solenoid-Focused Klystron Figure 11 Outline of 48 kW Klystron with Solenoid Focusing
Figure 12 MPTS 48 kW Klystron Parameters Figure 13 MPTS 48 kW Klystron Power Budget 3. TRANSMITTING ANTENNA AND PHASE FRONT CONTROL Goubau and Schwering [1961] showed theoretically that microwave power can be transferred at high efficiency when the transmitting antenna is illuminated with an amplitude distribution that is near Gaussian, as illustrated for the MPTS in Figure 14, and when the phase front of the beam is focused on the receiving antenna. For the extreme transmission distance from geosynchronous orbit, the curvature of the phase front is very slight, but nevertheless the front must be controlled with high precision to maintain high efficiency. Figure 15 shows the effect of the transmitting antenna amplitude taper (from antenna center to its edge) on receiving antenna dimension for several high beam interception efficiencies. We can expect that attractive combinations will be found in the 5 dB to 10 dB range to limit the size of the receiving antenna while achieving high efficiencies. It is interesting to note that other microwave applications generally use a uniform illumination, or 0 dB taper, which achieves maximum intensity in the center of the beam but also places a higher proportion of power in sidelobes. This trend is illustrated in Figure 16. The recommended approach to control of the phase front to the required precision requires that the antenna be sectored into numerous subarrays. A typical quadrant for an antenna on the order of 1 km is shown in Figure 17, which also gives an example of how the array could be organized to provide the necessary center to edge amplitude taper. Figure 18 illustrates the factors entering into the choice of subarray size. Large subarrays individually have narrow, high gain radiation patterns that will result in large power loss if the overall array is mechanically offset to a substantial degree from pointing to the ground, due for example to attitude control limit cycling. This power loss cannot be offset electronically, so smaller more numerous subarrays are selected, as shown. (Total radiated power remains the same.) Phase control electronics must be present in each subarray (definition of a subarray) so that
Figure 14 Microwave Power Beam - Idealized Figure 15 Receiving Antenna Sizes for Truncated Gaussian Beam Tapers
Figure 16 Taper Effect on Pattern and Efficiency Figure 17 Array-Subarray Organization
Figure 18 Subarray Size Considerations there will be a tradeoff of power loss vs controls cost. Figure 18 also shows other factors producing power loss: subarray tilt relative to the overall array nominal position, and subarray distortion, both of which will be strongly influenced by thermal effects. Efficiency and safety needs dictate that a closed Loop form of control be implemented for phase front or beam formation. Two approaches, adaptive and command, have been formulated and are illustrated in Figure 19. The command system uses a matrix of sensors at the ground antenna to determine the received power beam center and shape. A processor then develops commands which are routed to the subarrays over the telecommunications link. This approach has limited resolution, but nevertheless it is anticipated that antenna thermal distortions, a major source of error, can be accurately modeled and suitable command algorithms developed. In any event it will serve as a system monitor and as a safety override function. A potentially more accurate scheme calls for a reference beam to be launched from the center of the ground antenna. This is sensed at each subarray and at a reference subarray in the antenna center. The latter transmits the reference to the sub- array over a calibrated coaxial cable at which point it is compared with the incoming beam. A difference in phase between these signals is interpreted as a displacement of the subarrays from the nominal reference plane, due for example to thermal distortion of the structure, and a correction is applied to the phase of the transmitted beam at the subarray so that the required beam front is launched toward the ground antenna. A
Figure 19 Command and Adaptive Phase Front Control Concepts similar retrodirective technique is used in other applications [IEEE Trans., 1964 j, although not with the primary purpose of compensating for structural deflections, which requires that a reference signal be distributed to each subarray. A number of possible approaches to subarray designs are shown in Figure 20. The slotted waveguide approach is selected because it has very high antenna efficiency while also serving as an efficient means to distribute the microwave power from the converters to the radiating elements. The spacefed array shown in Figure 21 represents a radically different approach to the overall antenna mechanization that was devised to simplify converter repair or replacement by centrally locating them. However mechanical complexity, lower efficiency and need for active heat transfer to a radiator (not detailed) are important disadvantages. A second alternative also shown is a cylindrical array using electrical switching to eliminate the rotary joint to a solar oriented power source. It is too heavy, costly and complex to compete with the recommended planar approach. Having selected the waveguide approach to antenna design, we proceeded to select material wall thickness and subarray dimensions. The thermal interface between converter and waveguide, shown in Figure 22 for the amplitron, was analyzed to obtain the deflection data shown in Figure 23. This is for a wall thickness of 0.5 mm which is believed to be the minimum produced to date. The aluminum deflection over 5 meters is sufficient to produce a 1% beam power loss at the subarray, while graphite composites can extend out to about 18 meters for a 1% loss. The graphite polyimide is a potentially attractive candidate, being 0.6 the density of aluminum and having a higher maximum temperature (290°C) than either aluminum (175cC) or graphite exoxy
Figure 20 Subarray Types Figure 21 Alternative Array Types
Figure 22 Amplitron Thermal Model Figure 23 Subarray Deflection vs Size
(175°C). However, there are questions of the extent of outgassing and its potential effect on the open tube converters as well as questions of stability over 30 years in the space environment. An all aluminum solution must be carried forward until these questions are resolved, and a concept of sectoring the subarray into independent 5-meter segments was devised to alleviate the aluminum deflection problem. Figure 24 illustrates the tradeoff between capital cost of power lost by increasing subarray size versus capital savings for reduced electronic investment. Subarray dimensions of 18M x 18M were selected because 18M is within the acceptable range and corresponds to the maximum dimensions that can be carried in the Space Shuttle Orbiter, which could be expected to play a key role in early development flights. The detailed implementation of a subarray with amplitrons is shown in Figure 25. Phase reference electronics are shown, as are the mounting of circuits for power control (crowbars). Screwjacks at the corners provide for mechanical attitude adjustment of the subarray to compensate for installation errors and for deflections that may arise over years of operation. The fully packed tubes with thermal radiators touching represent the highest power density that can be implemented, and so this is a centrally located subarray. Power taper toward the edge of the antenna is achieved by spacing the tubes farther apart. Maximum radiated power for this unit is 7 megawatts, an impressive figure by any earth based standards. 4. MECHANICAL SYSTEMS Fine pointing by electronic phase control directs the power beam to an accuracy of about 0.04 arc sec (about 10M at Earth), but as noted earlier there is reduced efficiency if mechanical pointing is not reasonably accurate. An error of 1 arc minute corresponding to a power loss under 1% was selected for the design goal and is accomplished with control in elevation and azimuth as shown in Figure 26. The azimuth rotary joint is located at the mast interface with a solar oriented power source for which relative rotation is 360° per day. Additional antenna motion in azimuth and elevation is required to compensate for spacecraft (power source) limit cycling which would nominally be on the order of 1 degree [Glaser, 19741. The principal disturbance torque on the antenna by far is the frictional torque in azimuth due to contact pressure between the brushes, carrying electric power, and the rotary joint ring. It is estimated at about 10$ Nm (8 x 10$ Ft-Lb). Details of the rotary joints are given in Figure 27. Power is carried across the azimuth interface by silver alloy brushes and slip rings, and across the elevation drive by flexible cable where motion is limited to ± 8 degrees. Orientation drive is nominally by DC torque motor
Figure 24 SPS Incremental Cost vs Subarray Size Figure 25 Subarray Layout
Figure 26 Transmitting Antenna Pointing System Figure 27 Rotary Joints
with spur geax* drive, but a linear induction drive may prove superior for longer life. Sizing requirements exhibit high torques but surprisingly low power demands: The overall structural concept for the transmitting antenna is shown in Figure 28. Nominal design is 40 meters deep and, depending upon system considerations to be covered later, is about 1 km in diameter. It is assembled in two rectangular grid structural layers. The rectangular grid was found to be less massive than a competing radial spoke design. Primary structure is built up in 108 x 108 x 35 meter bays using triangular grid compression members 18 meters long and 3 meters deep, The secondary structure is used as support points for the waveguide subarrays and is built up as 18 x 18 x 5 meter bays. The structure to waveguide^ interface shown in Figure 29 uses three gimballed screwjack assemblies to correct up to a 4 arc min subarray misalignment and a 40.5 cm linear displacement. Temperature considerations resulted in a choice of a triangular hat construction technique as noted in Figure 30, and a simple locking mechanism to expedite assembly of structural joints is suggested as shown in Figure 31. Thermal analysis of the overall structure was a key aspect of the study since distortion and bending impact the error budget for beam phase control. Figures 32 and 33 show the displacements and slopes over the antenna for a full range of sun angle conditions and for aluminum and composite materials. The adaptive phase front control will compensate for the deflection effects, and the screwjacks previously noted can be set to compensate for the average slope error; however, the subarray size must be sufficiently small to keep efficiency (gain) loss tolerable for the deviations about the mean slope that will occur on a daily basis. A further result of the thermal analysis was a determination of maximum heat flux density that could be tolerated at the center of the antenna to stay within structural material temperature limits. It was found that microwave converters could be fully packed with their individual radiators touching for any of the materials investigated, with graphite polyimide showing the greatest temperature margin.
Figure 28 Antenna Structural Arrangement Figure 29 Structure/Waveguide Interface
Figure 30 Comparison of Max Temp and Thermal Gradients Figure 31 Structural Joints
Figure 32 Typical Antenna Deflections Due to Thermal Gradients Figure 33 Typical Slopes of Structure Due to Thermal Gradients 5. FLIGHT OPERATIONS The transportation of a satellite power station with the MPTS to synchronous orbit and the assembly techniques in space are key factors in system cost and technical feasibility. In fact we may expect that these aspects will influence design and tradeoffs to a significant degree, and conversely, expect that the satellite requirements would dictate transportation design to the point of justifying a dedicated system for operational deployment. This study for the most part examined the constraints and cost associated with a system based on the present Space Shuttle Orbiter which would play a key role in satellite development, and a support role in operational deployment. Two flight plans for assembly and transport to geosynchronous orbit were developed: (1) low altitude assembly and transport to geosynchronous using solar electric propulsion (SEP), and (2) assembly just above the Van Allen belts and transport to geosynchronous using SEP. A complete SSPS using photovoltaic cells was assumed to establish overall time and support requirements, with assembly flow as shown in Figure 34. The mission options with associated transportation performance capabilities are given in Figure 35. The challenge in packaging concepts is shown in Figure 36, where a prefabricated waveguide is efficiently stowed, but still is short of the ideal packaging density for the shuttle at the waveguide design wall thickness of 0.5 mm. A space manufacturing and assembly approach would reduce the number of flights here, and similarly so for the structure and for the larger and more massive power source as well. A concept suggested for on-orbit manufacturing of structural members to meet this need is shown in Figure 37. The traffic and fleet requirements for three plans, two at low altitude with different build times and one at high altitude, are given in Figure 38 fox an SSPS providing 5 GW of ground power. The overall estimated transportation and assembly costs for these
Figure 34 Assembly Functional Flow Figure 35 Mission Options Figure 36 Waveguide Weight and Packaging Density
Figure 37 Detail Part Assembly Summary Figure 38 Traffic and Fleet Size Summary options are: Plan 1 - 594 $/kg, Plan 2 - 1554 $/kg, Plan 3 - 571 $/kg. The time value of capital, as described later, would favor the Plan 1 with the short build cycle. The high altitude assembly appears out of the running on cost grounds. However, the low altitude options must make allowance for SEPS solar cell degradation and must protect the SSPS cells while traversing the Van Allen belts over relatively long time periods.
The cost breakdown for Plan 1 in Figure 39 shows that only about 10% is applicable to the orbital assembly task, so that major cost reduction is possible if a more economical basic transportation mode to low earth orbit were developed. If, as forecasted, an unmanned fully reusable, fly back heavy lift launch vehicle (HLLV) with a 180,000 kg payload can be launched at a unit cost comparable to the Shuttle Orbiter, which has a 30,000 kg payload, then the transportation and total costs would approach 125 $/kg and 150 $/kg respectively. Assembly operations of course would still require a manned Shuttle. 6. RBCSIVING ANTENNA A review of options for the antenna design at the ground receiving site quickly confirmed that an array of solid state diode rectifier elements each combined with individual dipole antenna and suitable filter was the only choice combining both high efficiency and low cost. These and other significant factors are noted in a comparison of approaches in Figure 40. This integrated reception-collection-rectification antenna concept is termed a rectenna, and its technology is the farthest advanced of those related to efficient transmission of microwave power. The most recent evidence of this was the achievement of an 82% efficiency at an output power level of 32 kW in a demonstration at the Goldstone, California facility of the Jet Propulsion Laboratory [Raytheon Co., 1975]. This subsystem efficiency is very close to the individual element efficiency plotted as a function of frequency in Figure 41. Overall construction of the rectenna which covers an area of about 100 km was shown in Figure 1. The panels aro trited to normality with the incoming phase front, but the accuracy need not be great since the individual antenna elements have broad dipole gain patterns; and for the same reason the phase front can be distorted by the atmosphere or ionosphere without appreciably affecting efficiency. Figure 42 shows the detailed organization of the rectenna where the DC power is collected at each element in parallel. At the next level it is summed in a series connection to reach voltage levels at which efficient conversion or distribution can be made. The ground plane is open metal construction for low cost and low wind resistance. Sealing of the rectenna elements within a plastic tube is suggested as a means to achieve economical environmental protection. Principal concern as regards weather phenomena would be damage due to large hailstones and this must be considered in site selection. The rectenna’s large scale demands a very low cost, mass production approach to manufacture of the several billion elements and the supporting structure. Cost estimates for a 2.45 GHz operating frequency are:
Figure 39 Transportation and Assembly Cost - Plan 1 Figure 40 Comparison of Antenna Approaches
Figure 41 Rectenna Element Efficiency vs Frequency Figure 42 Rectenna Elements
Extension of learning curves for diode production shown in Figure 43 indicate that one cent per diode cost at high quantity can be projected. Power distribution at about 47 $/kW (2.50 at 5 GW) also is important. The cost factors for real estate and site preparation assuming a generally suitable location has beer- selected are relatively small. 7. SYSTEMS ANALYSIS AND EVALUATION Frequency and power output level are the two prime parameters affecting MPTS performance, given the subsystem and component characteristics discussed above. MPTS cost and efficiency together determine its performance, and these can be combined into a single index of capital cost per kilowatt of ground output power if the power source characteristics are included, so that the cost impact of MPTS inefficiency is accounted for. We must also include the orbital transportation and assembly costs which can be more significant than the equipment factory manufacturing costs. The range of power source and transportation-assembly factors included in the study is shown in Figure 44. The high transportation and assembly cost of 600 $/kg derives from the Shuttle based estimate, and the low figure of 100 $/kg, which is below the transportation factor for the HLLV noted earlier, represents a probable lower extreme for a combined transportation and assembly cost for operational systems with deployment extending into the next century. The power source estimates represent a composite assessment of the range of values appropriate to the ISoO’s and beyond for solar photovoltaic [Glaser, 1974], solar thermal [Woodcock, 1974J and nuclear [williams, 1973] technologies, which have been studied elsewhere in decreasing detail in the order given. Points of reference are the cost goals for ground based solar arrays recommended as realistic for the U.S.A, by a National Science Foundation Study [1974], which are 500 $/kW by 1985 and 200 $/kW in a subsequent development phase. Other reference points are an estimate of 313 $/kW for an orbital silicon photovoltaic array with 2:1 concentration ratio and 18% cell efficiency made in the prior feasibility study [Glaser, 1974] and an updated weight estimate of 1.46 kg/kW made in this MPTS study for a 2:1 concentration ratio and 14% cell efficiency. The high values in Figure 44 represent what are thought to be achievable with high confidence in the required time frame. SPS capital cost as a function of frequency for low and medium level transportation assembly and power source parameters are given in Figures 45 and 46. Plotted are the lowest cost solutions representing tradeoffs between costs of orbital equipment, including transportation and assembly, and the rectenna costs. The latter pertain to an elevation angle of 50 degrees, which would be the case in the Southwest USA. Frequency range has been limited to below 5 GHz because of the increasing susceptibility to rain brownouts above 3 GHz.
Figure 43 Diode Production Experience Figure 44 Orbital Transportation/Assembly and Power Source Parameters Figure 45 SPS Capital Cost vs Frequency - 100 $/kg
Figure 46 SPS Capital Cost vs Frequency - 300 $/kg Figure 47 Peak Ground Power Density vs Frequency The significant trend is to reduced cost at higher power levels which is due to the fact that the rectenna is better utilized as power densities on the ground increase. The relative improvement slows above 5 GW as the basic power source costs dominate. Cost at low power levels trends lower with increasing freouencv because the transmitting antenna gain increases with reduced microwave wavelength. At the relatively economical pow’er levels of 5 GW and above, however, there are broad minima around 2 GHz, caused by the dc-rf converter packing limit resulting in a larger transmitting antenna than would otherwise be optimum at higher frequencies. A critical factor in addition to direct cost to be considered in selecting frequency and power is ground power density, which has implications for biological and environmental effects. Figure 47 shows peak ground power density assuming dc-rf converters are fully packed at the center of the transmitting antenna, an approach which minimizes antenna diameter and thus minimizes ground power density. Reference values are 100 mW/cm2 for sunlight at ground level and 10 mW/cm2 for the USA standard for continuous exposure to microwaves. An estimate for onset of ionosphere modification effects, based on scaling from experiments at much lower frequencies [Meltz, 1974j, is also shown. Levels above the biological standard could be accommodated (restricting rectenna area and its air space to fly throughs would limit exposure to short periods), and ionosphere modifications probably will be localized and have negligible effect on other users; nevertheless it would be prudent to limit power to levels as low as can be economically useful, such as 5 GW or 10 GW at most, for planning purposes.
Still another key factor to be considered is radio frequency allocation and radio frequency interference. As a general rule, the lower the frequency the more impact there would be on established users of the radio spectrum and the less economically attractive would be an SPS on a cost benefit basis. Specific ’’natural" frequencies to be avoided if possible are the space hydrogen and hydroxyl emission lines at 1.4 GHz and 1.7 GHz respectively, which are under continuous observation by the radio astronomy community. Although current and planned usage of the spectrum for space activities, including the NASA unified S-band from 2.1 GHz to 2.3 GHz and the communications satellites beginning at 3.7 GHz, could be shifted to other frequencies in the extended time frame needed for an SPS development, it would also be prudent to expedite MPTS development by avoiding these frequencies as well. The recommended choice is the industrial microwave band of 2.45 GHz ± 0.05 GHz. This is cost effective on both a system and DC-RF converter level, as previously shown, provides continuity with prior rectenna development, avoids potential frequency allocation problems, and minimizes impact on other spectrum users. Users of the spectrum from 2.4 GHz to 10 GHz with sensitive receivers would have to protect themselves with notch filters against the fundamental and up to third or fourth harmonic emissions. High gain antenna (60 dB) radio astronomy observers in view of an SPS would be denied only the basic" 2.4-2.5 GHz band if klystrons were utilized as the dc-rf converters. The higher projected noise output of amplitrons, even with filters, would exclude an additional range up to 2.7 GHz; and where the SPS were in the main lobe of a 60 dB antenna, would exclude observations above 1.9 GHz. The amplitron noise estimates are based on measurements of pulsed tubes and as such may be too high, since there is some evidence that continuous wave operation as proposed for MPTS may reduce noise considerably. The trend of MPTS characteristics at a design center of 2.45 GHz and 5 GW-10 GW power level is shown in Figure 48 for ampli- tron-aluminum configurations. SPS cost changes little at a given power level so that the taper-beam interception efficiency choice can be made on the basis of such factors as minimum ground power density, reduced land use, minimum antenna weight, etc. We can expect in fact that the modular designs described for the subsystems would lend themselves to configurations tailored for a particular site, with variations in the weight given to the key factors. The 5 GW, 5 dB taper, 90% beam interception case is chosen as the single baseline for further evaluation. Figure 49 provides a comparison of dc-rf converters and material choices for a 5 GW baseline system. We see that the klystron designs are substantially heavier and more costly as could be expected from the component characteristics provided earlier. For waveguide and structure, the graphite material choice reduces weight but has negligible impact on cost because of an offsetting increase in processing cost relative to aluminum; however, development of manufacturing and assembly concepts, especially for orbital use, could very well shift preference strongly to one or the other material.
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