INITIAL TECHNICAL, ENVIRONMENTAL AND ECONOMIC EVALUATION OF SPACE SOLAR POWER CONCEPTS JSC 11568 VOLUME II - DETAILED REPORT AUGUST 31, 1976 National Aeronautics and Space Administration LYNDON B. JOHNSON SPACE CENTER Houston, Texas
Report Digitized by The Space Studies Institute ssi.org
INITIAL TECHNICAL, ENVIRONMENTAL AND ECONOMIC EVALUATION OF SPACE SOLAR POWER CONCEPTS Lyndon B. Johnson Space Center Houston, Texas VOLUME I - SUMMARY VOLUME II - DETAILED REPORT
INITIAL TECHNICAL, ENVIRONMENTAL AND ECONOMIC EVALUATION OF SOLAR POWER SATELLITES VOLUME II - DETAIL REPORT I. INTRODUCTION II. SUMMARY AND CONCLUSIONS III. PROGRAM REQUIREMENTS A. Projected Energy Demand B. Implementation Scenarios IV. POWER STATION A. System Analysis 1. Efficiencies 2. MPTS/MRCS Analysis 3. Orbit Considerations 4. Configurations 5. Mass Properties B. Solar Energy Collection System 1. Solar Array a. Solar Cell Technology b. Solar Cell Blankets and Concentrators c. Alternate Energy Conversion Concepts 2. Power Distribution 3. Structure 4. Attitude and Orbit Control 5. Instrumentation, Control and Communications 6. Maintenance Station C. Microwave Power Transmission System 1. Antenna Array 2. Microwave Generators a. Microwave Generators b. Radio Frequency Interference
3. Subarrays 4. Phase Control 5. Pointing Control 6. Power Distribution 7. Structure 8. Rotary Joint 9. Thermal Control D. Microwave Reception and Conversion System 1. Rectenna a. Rectenna b. Structural Support and Ground Preparation 2. Grid Interface E. Operations F. Unit Costs APPENDIX - Comparison Study of Thermal Engine (Brayton Cycle) and Photovoltaic SPS Design Concepts V. SPACE CONSTRUCTION AND MAINTENANCE SYSTEM A. System Requirements and Analysis APPENDIX 1 - Construction Studies APPENDIX 2 - Consideration of Joining Processes for Construction APPENDIX 3 - Proposed Construction Experiment for Shuttle OFT Flight B. Construction Base 1. Construction and Manufacturing Facility 2. Orbital Construction and Support Equipment 3. Logistics Facility 4. Integration Management Facility 5. Crew Habitability Facilities 6. Construction Base Configuration Evaluation C. Construction Operations AP PENDIX 1 - SPS Orbital Construction Organization VI. SPACE TRANSPORTATION SYSTEMS A. System Requirements and Analysis B. Heavy Lift Launch Vehicles
1. Summary 2. Modified Single Stage to Orbit Vehicles 3. Winged Launch Vehicle 4. Two Stage Ballistic Vehicle C. Personnel and Priority Cargo Vehicle D. Cargo Orbital Transfer Vehicle E. Personnel Orbital Transfer Vehicles F. Summary of Projected Transportation System Characteristics VII. INTEGRATED OPERATIONS A. System Requirements and Analysis B. Program Model C. Mission Management Concept D. Mission Management Functions E. Key Considerations and Areas for Further Investigation VIII. ENVIRONMENTAL CONSIDERATIONS A. Methodology B. Environmental Questions C. Comparisons with Conventional Systems IX. MANUFACTURING CAPACITY, NATURAL RESOURCES, TRANSPORTATION AND ENERGY CONSIDERATIONS A. Requirements B. Manufacturing Capacity C. Natural Resources D. Surface Transportation E. Energy Payback APPENDIX - Material Summary
X. PROGRAM DEVELOPMENT PLAN A. Program Phasing B. System Development and Exploration C. Technology Advancement Phase D. System Development E. Program Costs XI. PROGRAM COST AND ECONOMIC ANALYSIS A. Methodology B. SPS Costs C. Comparison with Conventional and Other Advanced Systems D. Summary Remarks APPENDIX A - Terrestrial Solar Power APPENDIX B - Cost Sensitivity Analysis
INITIAL TECHNICAL, ENVIRONMENTAL AND ECONOMIC EVALUATION OF SPACE SOLAR POWER CONCEPTS VOLUME II - DETAILED REPORT I. INTRODUCTION The requirements for energy in the U.S. and the world will continue to increase to support a growing population and to improve the quality of life for that population. Projections indicate the U.S. requirements will grow by a factor of two to three between now and the Year 2000. The manner in which we will meet this requirement is not clear. Oil and gas are expected to be depleted within decades. Fuel for the present class of nuclear reactor systems will also be depleted in the same timeframe. The breeder reactor system, when successfully developed, will greatly extend the natural fuel resource but presents continuing safety and environmental concerns, not the least of which is the disposal of nuclear waste as it accumulates from large-scale nuclear energy production. Fusion reactor systems also have potential, but these require significant scientific advances. Coal resources appear sufficient for several hundreds of years. The environmental concerns associated with mining coal, and the subsequent problems or costs in reducing air pollution to an acceptable level during its use, are well known. The logistics of a greatly expanded coal industry is also a significant although not unsolvable consideration. In view of the problems or concerns related to obtaining the required energy from oil, gas, nuclear and coal sources, the nation is actively pursuing alternate sources of energy for the future. Solar energy is an obvious candidate for consideration. Solar energy is inexhaustible and clean and the increasing costs of other sources will make solar energy more attractive in the future. The use of solar energy collected on the earth has several basic limitations, however, which will tend to inhibit its widespread use. At any given location on the earth, a solar collector will be limited by such factors as the day-night cycle, cloud cover and atmospheric attenuation. The day-night cycle particularly requires the use of expensive storage capacity or limits the solar application by requiring additional power sources. A concept has been presented ("Power from the Sun: Its Future," Dr. Peter E. Glaser, Amer.tAssn. Advan. Sci., Vol. 162, 22 November 1968, pp. 857-61) which is intended to alleviate limitations associated with the collection of solar energy on earth. This concept involves placing large solar power satellites in geosynchronous orbit and beaming microwave energy down to collection stations on the earth. Some of the advantages of this concept are that the satellite is in near-continuous
sunlight which is not attenuated by the atmosphere; no electrical storage facilities are required; the land use requirement is reduced by a factor of 5 to 10; and the ground power output can be located near the user rather than in desert-type regions. The space concept, while having advantages, also introduces new requirements. These include the need for transportation of the power station into space and the transmission of power from space to earth by microwave radiation. Several studies conducted in the past few years have been directed toward exploring the feasibility of the concept. The results of these studies have generally been favorable, while reflecting a need for significant technological advancement if the concept is to be economically competitive with ground-based systems. Critical areas were identified during the course of these studies and research and development programs have begun to be formulated to investigate these areas. A particular effort was conducted at the Johnson Space Center during the summer of 1975 to evaluate the need and feasibility of a Space Solar Power Development Laboratory. The study was done in support of the NASA "Outlook for Space" study and was documented in JSC-09991. Possible requirements for a development laboratory or "pilot plant" type solar power satellite were evaluated and the technical feasibility of such a plant was established. In view of past study results, the six-week study, and the conclusions of the "Outlook for Space" study, it was decided to implement at JSC a more detailed study of the Space Power Concept. This document (Volume II) contains the detailed results of that study. Volume I presents a summary of the study results. The study was conducted between September 1975 and June 1976, by JSC personnel. The principle authors of each sub-section are identified by name and JSC organization. The general objectives of Solar Power Satellite (SPS) studies include: 1. Establishment of realistic technical and economic design criteria and requirements for a full scale SPS. 2. Definition of technology development and flight test programs necessary to achieve the optimum SPS design. 3. Comparison of the SPS with other energy generation options to establish the relative economic, environmental, and social advantages/ disadvantages of the SPS concept. These objectives are quite broad and definitive answers will require a number of years of study augmented by technology efforts in a number of areas. Nevertheless, the present study provides further insight into a
number of aspects of the concept and provides a point of departure for further work. The summary (Volume I) presents a number of preliminary conclusions and a synopsis of the more detailed studies which are presented in Volume II. Certain programmatic guidelines were chosen to initiate the study based on deployment of the first operational SPS as early as 1990. a. Program plans and technology projections will be developed based on deployment of the first operational SPS as early as 1990. b. The capability will be provided as early as 1995 to deploy two to four SPS per year. c. Dedicated transportation systems will be developed and optimized specifically for use in deploying and operating the SPS network. d. Materials used in fabricating and operating an SPS will be obtained only from the earth. e. The SPS will be deployed in appropriate geosynchronous orbits only. f. The lifetime of an SPS will nominally be 30 years although liberal refurbishment/replacement of parts may be assumed. g. The SPS will be designed in a manner to optimize participation of man in its fabrication, assembly, and operation. h. Availability of scarce resources will be a major consideration in projecting technologies to be used in fabricating the SPS network. i. Energy as well as economic payback will be assessed in determining the SPS development strategy. j. Aspects of social and environmental impact will be assessed. k. Assembly fabrication strategies for SPS will be developed such as to minimize overall costs. The first two guidelines were modified slightly as the study progressed in that various scenarios were defined and evaluated. Available resources defined the scope and depth of the study. For example, the study was primarily limited to consideration of the photovoltaic concept for solar energy collection and conversion. Similarly, the more detailed design studies were limited to consideration of silicon solar cells. Given these restrictions, a range of power station sizes and weights were determined based on conservative and optimistic estimates of collection, conversion, transmission and receiving efficiencies.
Analyses and/or design studies were conducted for each element of the systems to varying degrees. These studies included several satellite configurations, construction concepts, crew requirements, alternate microwave generator concepts, rotary joint designs, attitude and control concepts, and structural designs. Several program scenarios were developed which defined the number and schedule of space power satellites required to provide varying percentages of the nation's energy needs in the 1995-2025 period. Satellite weights were then coupled with the number and schedules of satellites required to define a range of transportation requirements. These requirements were used to guide the study of various transportation elements and to estimate integrated transportation requirements such as fleet size. Transportation elements for which specific studies were conducted included multi-stage winged and ballistic heavy lift launch vehicles, and of a variety of orbital transfer vehicle thrusters, and personnel launch and transfer vehicle designs. In a similar manner, the satellite and transportation system characteristics, number and schedule were used as a basis to estimate the cost of DDT&E, total program, and mills per kilowatt hour. Preliminary estimates are also provided of natural resource requirements and pollutants emitted from processing and launch operations. Estimates of energy payback are also presented. Figure 1-1 presents the task structure that was used in the study effort. Both Volume I and Volume II are organized according to this task structure.
Figure I-l. - Study task structure.
II. CONCLUSIONS The scope and complexity of the satellite power concept coupled with the limited depth of the present study would make it inappropriate to draw absolute conclusions. However, the SPS concept appears to be technically feasible in that no design or operational problems were encountered that did not appear amenable to solution. The economic viability of the system appears promising but is obviously dependent upon a combination of technology advancement and/or the costs of competitive sources. Within the limitations of the study and based on a variety of assumptions and/or estimates, the following preliminary conclusions are presented. 1. The maximum power output of an individual microwave transmission link to earth is about 5 GW and the transmitting antenna diameter is about 1 km, based on the following assumptions; o An operating frequency of 2.45 GHz o A maximum allowable power density at the ionosphere of 23mW/cm o A maximum allowable antenna waveguide temperature of 485°K resulting in a power density at the antenna of 21 Kw/m o A 10 db Gaussian taper of the microwave beam 2. The estimated mass of a 10 GW SPS (incorporating solar energy converters sufficient for two 5-GW microwave power transmission systems) is between 47 x 10^6 and 124 x 10^6 kg, based on the following assumptions: o Silicon cell arrays with an efficiency of 15 to 17 percent at 30°C and a concentration ratio of 2. o An overall system conversion and transmission efficiency range of 4.2 to 8.0 percent o A weight growth of 50 percent over present estimates The resulting solar array areas ranged from 96 to 183 km^2 . 3. The silicon solar cell arrays make up well over half the weight and cost of the satellite. Consequently, additional effort on solar arrays offers the most potential for overall system improvements particularly with respect to new approaches which could result in significant weight reduction. 4. Considerations of the structure indicated that minimum weight can be achieved if design loads are limited to those encountered on orbit and after construction. If this is done, the structure can be held at a very small percentage (<5%) of the SPS weight. The major factors in design will not be weight but the development of techniques for automated on-orbit construction and for conducting large electrical currents.
5. Development of automated construction techniques is complex and requires a great deal of further effort. A preliminary task evaluation based on a conceptual construction technique suggests that as many as 600 personnel may be required in space to construct an SPS in one year, with minor variations expected in personnel required due to configuration and construction location. Placing and supporting these personnel in orbit is a relatively small factor in the overall transportation requirement. 6. Past studies have indicated an apparent performance advantage of constructing, assembling, or deploying all or a portion of the solar arrays in low earth orbit and then utilizing solar energy with electric thrusters to propel the system or major elements thereof to geosynchronous orbit. The present study concludes that this area needs further study with full consideration given to the following factors: o Degradation of the exposed solar arrays during transit o Protection of unused arrays during transit o Earth shadowing during portions of transit possibly requiring non-solar propulsion o Docking and assembly of large SPS sections at geosynchronous orbit and resulting impact on structural design o Relative simplicity of chemical stages for transfer of "containerized" packages to geosynchronous orbit o Radiation conditions at geosynchronous orbit 7. The SPS in equatorial orbit will be eclipsed both by the earth and by other satellites. These eclipses result in as many as three brief (up to 75min) power outages per day for two six-week periods per year although less than 1% of the available energy is lost- The SPS/ grid system must be designed to accommodate these outages. 8. Conceptual designs and characteristics were developed for two- stage winged and ballistic heavy lift launch vehicles of varying payload capability. While the ballistic systems are much smaller and lighter, recovery and reusability will be key issues in establishing the desired configuration. 9. Heavy lift launch vehicle design considerations established hydrocarbon fuel rather than hydrogen as the choice for first stage propellant because of its greater energy density. 10. Considerations of I and confidence in technical development of candidate electric engines indicate that the MPD arcjet engine appears
to be the best choice for self-powered orbital transfer. These engines are also suitable for subsequent use as thrusters for the SPS attitude control system. 11. The high launch rates required indicate that launch window and related operational considerations may become significant factors. Launch latitudes near the equator greatly expand the launch window and offer performance advantages. 12. Based on varying assumptions as to performance, construction, location, orbital transfer modes and reusability, achievable transportation costs to geosynchronous orbit are estimated to range from $71 to $294 per kilogram. The major contributor to the total transportation costs for a given program was the cost of transporting the necessary material to low earth orbit. 13. The cost of producing electricity from solar power satellites as described herein is estimated to be in the range of 29 to 115 mills/ kWhr. This range of estimates is based on the following assumptions: o An implementation of 112 10-GW satellites over a 30-year period o A range of satellite weights and transportation costs as indicated earlier o A design, development, test and evaluation (DDT&E) cost o A space hardware repair/replacement rate of 1% annually o Plant factor of 92 percent allowing for eclipses and maintenance time o Return on capital investment of 15 percent 14. The cost of producing electricity with conventional (nuclear and fossil) plants is predicted to be in the 30 to 60 mills/kWh range, in the 1995 time period, depending upon the cost, fuel, and type of power plant. The cost of producing electricity with potential ground- based power plant concepts (ground solar, geothermal, wind) are estimated to be from 28 to 121 mills/kWh. 15. The introduction of SPS in lieu of meeting an equivalent portion of the nation's energy needs with new nuclear and coal-burning electrical power plants will result in significant reduction in emissions (particulates, N0v, SO , nuclear waste). X X 16. The microwave power density at the edge of the rectenna (1 mW/cm^2) is about one-tenth of the present U.S. standard for human exposure. The
system is fail-safe in that the beam would be dispersed to harmless intensity levels should be microwave beam pointing control fail. 17. Implementation of SPS on a large scale would create an increased demand for resources such as aluminum and rocket propellant gases (hydrogen and argon). Also, production capacity would have to be substantially increased in the areas of solar cells and reduction of arsenic from oxides (for the manufacture of gallium arsenide diodes). However, there does not appear to be any critical shortages of resources for SPS construction based on world reserves.
III. PROGRAM REQUIREMENTS A. Projected Energy Demand Tony E. Redding and Barry M. Wolfer Urban Systems Project Office Projections of the nation's electrical energy demand have been made by the FPC (Federal Power Commission), the ERDA (Energy Research and Development Administration) and other federal agencies and private organizations. Figure III-l shows the FPC and ERDA projections for electrical energy demand through 1990 and 2000, respectively. The FPC projection through 1990 is given in the 1970 Federal Power Survey Report, Volume 1. The ERDA projection (given in ERDA-48, Volume 1, June 1975) involves six different technological development scenarios and their resulting electrical demands. The highest electricity generation scenario presented by ERDA is based on intensive electrification and it has a 4.4 percent per year growth rate in the year 2000. The lowest electricity generation scenario is based on improved efficiencies in end use and it has a 1.4 percent year growth rate in the year 2000. The FPC projection, which is higher than any of the ERDA projections, has an annual growth rate of about 6.0 percent per year in 1990. Because it represents the most severe requirements in terms of capacity needs and growth rate, the FPC projection was used as a basis for evaluating the potential of the SPS (Solar Power Satellite) system. B. Implementation Scenarios Effective use of solar power implies that SPS must produce a significant portion of the future electrical energy demand. Therefore, scenarios of SPS implementation rates were developed that would provide 25 percent of the new capacity by 2015 (scenario A), 50 percent of the new capacity by 2010 (scenario B), and all of the new capacity by 2005 (scenario C). The new capacity requirements are based on extrapolation of the FPC projection from 1990 to 2025 as shown in figure III-l. Figure III-l shows the cumulative new capacity (GW) addition for each of these scenarios and the resultant electrical energy (GWH) provided by SPS each year until 2025. Scenario B was used as an illustrative example by which to examine the SPS program in terms of its technical requirements and resulting economic analysis. This scenario results in providing a significant quantity of the total electrical energy by 2025 (31 percent based on FPC projection and 50 percent based on the highest ERDA projection). The SPS installed capacity by 2025 would be 1120 GW or about 30 percent of the FPC total. Based on a 10 GW SPS power output, as described in Section IV, a total of 112 satellites will be required to accomplish Scenario B. Figure III-2 shows a year-by-year summary of 10 GW SPS installations for the three scenarios over the 30 year implementation period. Note that in Scenario B, the SPS installation rate reaches 7 per year in 2023.
Reference: (a) 1970 Federal Power Survey, Volume 1 Federal Power Commission (b) ERDA-48, Volume 1, June 1975 Energy Research and Development Administration Figure III-l.- Projections of U.S. electrical energy requirements and possible SPS implementation scenarios.
FIGURE 111-2 POTENTIAL SPS SCENARIOS
IV. PMER STATION A. SYSTEM ANALYSIS IV-A-1. Efficiencies L. E. Livingston Spacecraft Design Div. Three estimates were made of the efficiency of each step in the power collection and transmission process: a minimum, which could be achieved by 1995 with virtual certainty; a maximum, the best that could realistically be expected under the most favorable circumstances; and a nominal or reference, representing the value most likely to be achieved. These are summarized in figure IV-A-1-1, together with the corresponding power levels at each stage for 5 GW DC output from the power interface. Since these estimates were required for subsystem sizing, they were made early in the study. Subsequent work (cf. section IV-A-2) has changed some estimates by several percentage points; these are shown in figure IV-A-1-1 for comparison. However, the "initial reference" data in figure IV-A-1-1 have been used, for the sake of consistency, in all sizing exercises. These changes would reduce the total solar input, and consequently the total array area, about 10 percent from the example configurations discussed in the rest of this report. Insufficient time was available to repeat all the subsystem calculations in detail, and no attempt was made to do so.
Figure IV-A-1-1SPS efficiencies.
IV.A.2 MPTS/MCRS ANALYSIS G. D. Arndt, Avionics Systems Engineering Division INTRODUCTION The initial task in the Satellite Power Station study was to determine an appropriate sizing for the overall satellite system. The station consists of large solar arrays converting solar energy to DC electricity by the photovoltaic process. This electrical energy is transmitted back to the earth using a high power microwave transmission system. The microwave system, consisting of DC-RF amplifiers, a large planar phased array, and a ground antenna/rectifier scheme (rectenna), must be capable of operating at a high efficiency over a 30 year lifetime with a low failure rate. The microwave energy is rectified back to DC electricity in the rectenna and then collected and carried via buss bars to a power distribution interface with commercial landlines. The output DC power from the solar power station will be defined at a collection point near the ground rectenna, prior to any signal conditioning or DC/AC conversion. IV.A.2(a) SYSTEM SIZING The physical size and power capabilities for the Satellite Power Station (SPS) are dependent upon: (1) the amount of DC output power at the ground rectenna, (2) the transmit antenna size, and (3) the system efficiencies. The tradeoffs for defining these system requirements are as fol 1ows: (1) Amount of DC power out of rectenna - Three output power levels, 1GW, 5GW, and 10GW, were studied. Raytheon (ref. 1) and Lewis Research Center (ref. 2) indicate there may be some cost advantages in going to high power systems. The allowable output power level is also dependent upon the size of the microwave transmit array antenna in the SPS as discussed below. (2) Tradeoffs of SPS transmit antenna size - The size of planar phased array in the satellite is primarily dependent upon four factors: (a) Thermal constraints within the antenna - At the center of the phased array, temperature limitations due to heat radiated by the DC to RF converters determine a minimum size for the antenna. The maximum power density (on boresight) at the transmit antenna is given as a function of array diameter in Figure IV.A.2-1. A 10 dB taper aperature illumination for a 5GW system will just meet the thermal limitations for the antenna. (The 10 dB taper illumination rather than a 5 dB taper was.selected because of increased collection efficiency at the rectenna as will be discussed later.) For the model configuration, the maximum power density at boresight will be 20.88 KW/m2 when using a 1 km array 5GW system. This density level gives a temperature of 450° - 485°K at the antenna for a DC to RF amplifier conversion efficiency of 85-90%. This temperature is an upper limit
Figure IV.A.2-l Peak Power Density at Transit Array vs Array Diameter
for aluminum waveguides. The klystrons are mounted behind the waveguides and radiate thermally outward via graphite radiators or heat pipes. However, the area between the waveguides and the thermal radiators for the klystrons will act as an oven and operate in the 450-485° temperature range. Therefore, a 1 km or larger transmit antenna is needed for a 5GW 10 dB Gaussian taper illumination. (b) Microwave power density limitations in the ionosphere - As the antenna array size increases, the maximum power density transmitted through the ionosphere increases. Previous studies (ref. 1, 3) indicate that non-linear interactions between the ionosphere and the power beam begin to occur at some threshold power density level which is dependent upon the operating frequency. This threshold level is 23 mw/cm^ for the model SPS system using an operating frequency of 2.45 GHz. This places a maximum size on the antenna for a given power output at the rectenna. The maximum power density at the rectenna as a function of transmit array size is given in Figure IV.A.2-2. Three output DC power levels, 1GW, 5GW, and 10GW, at the rectenna are shown, together with a 5 dB and 10 dB Gaussian taper for the 5GW system. These curves indicate that a 1 km array, 10 dB taper, 5GW system is at the maximum power density level set by the ionospheric interaction limit of 23 mw/cm^. However, this 23 mw/cm^ density level can only be considered a guideline, not an absolute requirement. There is not sufficient experimental data available to accurately predict the exact threshold level. (c) Maximum output power - If Figures IV.A.2-1 and -2 are superimposed such that the power density limits coincide (Figure IV.A.2 -3), it may readily be determined whether a given combination of output power and transmitting antenna diameter exceed either of the two limits. For example, 6GW output and 0.8 km diameter fall within the ionosphere limit but greatly exceed the transmitting antenna thermal limit. Since the only issue is whether an operating point falls above or below the limit line, the relative sizes of the vertical scales are unimportant for this superposition. For the limits used here, the maximum output power is 5GW and the corresponding antenna diameter is 1.0 km. A high output power has a number of economic and operational advantages discussed elsewhere in this report; consequently, the maximum of 5GW output power, together with 1.0 km transmitting antenna diameter, has been used for sizing purposes throughout this study. These results will change if the maximum power densities are revised. For example, if the transmitter limit were 25 kw/m? and the ionosphere limit 40 mw/crrr, the maximum output power would be 7.1GW and the antenna diameter 1.1 km. (d) Rectenna size - As the transmit array size increases, the beamwidth of the main lobe decreases in proportion to the array area, which causes the rectenna size (and cost) to decrease. The tradeoffs of
Figure IV.A.2-3 - Thermal and Ionospheric Constraints on DC Output Power
rectenna size are shown in Figure IV.A.2-4. The rectenna size is that required for a 90% flux intercept efficiency, or collection efficiency. The ground facility, which is the area fenced off for the rectenna system, was arbitrarily chosen to intercept all power density levels out to .05 mw/ cm2. This density level example is 200 times more stringent than the 10 mw/ cm2 radiation standard set by the United States; however, it is 5 times greater than the .01 mw/cm2 USSR standard. The exact power density standard to use is not known at this time and should be one objective of a microwave radiation study. However, it is felt that the SPS density limit will be somewhere between the present USA and USSR standards. Since the radiation will be continuous, the SPS standard may be closer to the USSR limit. The facility required for a 1 km transmit array covers 80,000 acres - a large land area. In summary, the 1 km transmit array, together with a 10 dB Gaussian taper illumination for a 5GW system, was chosen as the model configuration for three reasons: (1) the system operates just below the 23 mw/cm2 threshold level expected for nonlinear ionosphere interactions, (2) the power density at the transmit array is at the 21 kw/m2 limit due to thermal constraints for the waveguides and klystrons, and (3) a size/cost tradeoff for the rectenna and transmit array has a broad minima at a 1.0 km array diameter. IV.A.2(b) IONOSPHERIC EFFECTS The microwave beam/ionospheric interactions can be divided into two categories: the ionospheric effects on the beam and the beam effects on the ionosphere. Previous work by Raytheon (ref. 1) indicates the ionospheric effects on the beam, such as phase dispersions, beam displacement, and power absorption, will be minimal. However, the beam perturbations to the ionosphere must be considered when sizing the SPS power. As the microwave power density increases above a threshold level, nonlinear interactions between charged particles in the ionoshpere and the power beam begin to occur. This threshold level has been postulated to be approximately 23 mw/cm2 for a 2.45 GHz frequency, which is close to the peak density expected for a 5GW system. When the microwave power approaches the threshold level, the ionosphere will be perturbed as shown in Figure IV.A.2-5. In the "F" layer there will be heating and expansion with a corresponding reduction in the electron/ion density. A "hole" will be punctured in the "F" layer due to the reduction in electron/ion concentration. The neutral particles however will not be appreciably affected by the heating, and their concentration will remain the same. These particles, i.e., the electrons, ions, and neutrons are moving through the ionosphere at some velocity - estimated to be 50 meters/ second (ref. 4). Thus, as these particles sweep pass the "hole" region, they will slowly diffuse back until the normal equilibrium density level is again reached. Rough calculations indicate the "hole" will be closed within five minutes, or about 15 km, after the particles leave the heated region. In the "D" region the microwave heating slows the electron/ion recombination rate and the density may actually increase. There will be no
Figure IV.A.2-5 -Changes in electron/ion density for microwave power densities above the threshold level.
change on neutral particle concentration as was the condition in the "F" region. As the ionosphere drifts out of the heated region, the temperature will almost instantaneously decrease to normal (within a few milliseconds) but the time required for the density to return to normal will be longer (10-to 20-minutes). There should be no change in solar ray absorption by the ionosphere since the neutral particles rather than the electrons and ions, absorb solar radiation. As mentioned previously, the density of the neutral particles remains constant in the hole region. The main problems associated with nonlinear heating and hole creation are possible disruptions in HF, VHF communication systems, and VLF navigation systems due to additional RFI and multipath degradations. The power density levels at which nonlinear effects in the ionosphere begin to occur have not been measured at S-band frequencies and can only be speculated upon. There are a number of possible ground-based tests which can provide information on the nonlinear effects and determine at what power levels these effects actually occur. However, for the model configuration the 23mw/cm2 was taken as the upper bound for the power density level and consequently, was a factor in selecting 5GW as the output DC power. IV.A.2(c) NOMINAL EFFICIENCIES FOR SYSTEM SIZING After the DC output power at the rectenna and the transmit array size are selected, then representative values for efficiencies for each of the subsystems in the SPS are determined. The nominal system efficiency from the RF radiated output of the transmit antenna to the collected DC power at the rectenna is specified to be 76%. Using this efficiency and the 5GW DC output power, the RF power radiated from the transmit antenna is 6.5GW. The microwave system performance curves given in this report are all based upon 6.5GW radiated RF power. The nominal efficiencies for the microwave subsystems and their associated power levels are given in Figure IV.A.2-6. The total microwave system efficiency (nominal) from the DC output of the rotary joint to the collected DC output of the rectenna is 63%. Details on the efficiency tolerances for the total SPS system are given later in the text. IV.A. (d) FREQUENCY SELECTION A frequency of 2.45 GHz was chosen for the JSC system, which is the same as that used in the Raytheon/Lewis studies (ref. 1,2). The 2.45 GHz is at the center of a 100 MHz band reserved for government and non-government indistrial, scientific, and medical use. This band has the advantage in that any radio communication services operating within the 2450 + 50 MHz limits must accept any harmful interference that may be experienced from the operation of industrial, scientific and medical equipment. That is, as long as the microwave energy is confined to this frequency band, there will be no interference problems. Other advantages of this frequency include low atmospheric attenuation even in the presence
Figure IV.A.2-6 - Nominal Efficiencies for the Microwave System
of rain and good operating efficiencies for the microwave components. However, there is one reason for considering a higher frequency such as 3.0 GHz. Since the effective gain of the transmit antenna is proportional to X? (the wavelength squared), a 44% increase in gain is achieved for the same 1 km array by going to 3.0 GHz rather than 2.45 GHz. Such a change in frequency would require approval by the ICCR (International Radio Consultative Committee) which meets again in 1979. IV.A.2(e) ANTENNA APERTURE ILLUMINATION In order to achieve a high transmission efficiency, the transmit antenna must have an aperture distribution across the array surface which maximizes the amount of RF power intercepted by the ground rectenna. The previous work by Raytheon and JPL (ref. 1,5) has shown that a truncated Gaussian taper is a good approximation for an optimum aperture distribution. This illumination function has the form The mainbeam pattern and sidelobe characteristics of the antenna will vary with amount of edge taper as shown in Figure IV.A.2-7. A uniform illumination, that is OdB taper, has a narrow mainbeam and the maximum density at boresight (center of rectenna). Increasing the amount of taper produces a lower boresight density, a wider mainlobe, and lower sidelobes. These power density curves are for a 1 km transmit array with no phase or amplitude errors and no failures. The rectenna collection efficiency, that is, the amount of flux density intercepted by the rectenna, is shown in Figure IV.A.2"8 for the same taper configurations. As would be expected, the collection efficiency for a given rectenna radius increases with the amount of taper. The model configuration has a 10 dB taper which means that the power density at the center of the array is 10 times that at the edge. For the no error/no failure conditions the 10 dB taper system gives a 90% collection efficiency at a radius of 4300 meters. IV.A.2(f) STEP-TAPER ANALYSES The 10 dB Gaussian taper will not be a continuous function across the array surface; rather it will be a physically-realizable quantized approximation. Three configurations, 5 step, 8 step, and 10 step approximations were investigated. Minimizing the number of steps
Figure IV.A.2-7 - Power Density at Rectenna vs Array Taper
figure IV.A.2-8 - Rectenna Collection Efficiency vs Array Taper
simplifies the antenna design at the cost of decreased collection efficiency at the rectenna. The steps are quantized in equal power increments according to the relationship (ref. 1), The collection efficiencies for the 5, 8, and 10 step quantizations are compared to the continuous distribution in Figure IV.A.2-9. There are only small differences between the collection efficiencies of these three step approximations, which is in contrast to previous results (ref. 1). However, since the revenue return over the 30-year lifetime of the SPS is $524 X 10° per 1% collection efficiency (based upon a charge rate of 40 mils per kilowatt-hour), the model configuration will have a 10 step approximation. The 10 step function gives about 1% greater efficiency than the 5 step and the extra $524 X 10° revenue justifies the slightly greater complexity of the antenna. The configuration of the 10 step taper is shown below in Figure IV.A.2-10. The amount of power radiated per subarray is lowered progressively outward from the center of the total array by reducing the number of klystrons per subarray. There is a maximum of 42 klystrons/subarray at the center of the array and a minimum of 6 klystrons/subarray at the edge. A diagram of the normalized power density at the array as a function of radius is shown in Figure IV.A.2-11. There are equal power increments between each step except at the center and end of the array. This is the configuration used in the design of the DC power distribution for the antenna. IV.A.2(g) SUBARRAY SIZE TRADEOFFS The 1 km transmit array is composed of smaller subarrays, each phased together with a feedback reference signal from the ground. Each subarray can be considered an individual antenna, the gain and beamwidth of which is determined by its size. The previous work (ref. 1) used an 18m X 18m subarray. However, the 18m X 18m subarray had such a narrow beamwidth that active positioning devices, i.e., motor-driven screwjacks, were needed in order to mechanically compensate for small misalignments in the antenna. These misalignments were caused by thermal warping of the antenna, tilting of the individual subarrays, etc. Smaller subarrays have the advantage of wider beamwidths, and hence, reduced mechanical alignment requirements. However, the phase control costs increase since each subarray has its own receiver and phasing electronics.
Figure IV.A.2-10 - Ten-Step Quantization of 10 dB Gaussian Taper Figure IV.A.2-11 - Normalized Power Density as a Function of Radius
The effective subarray antenna gains as a function of the tilt angle, or mechanical misalignment, are shown in Figure IV.A.2-12. Three subarray sizes, 4, 10 and 18 meters, were studied. A 2% loss in antenna efficiency was allocated for mechanical misalignments which corresponds to 8.0, 3.2, and 1.9 arc minutes for the 4, 10 and 18 meter square subarrays. The total cost of the transmit antenna as a function of subarray size is shown in Figure IV.A.2-13. The transmit antenna is a 1 km, 5GW system -- the only change is the $64,000 cost for the phasing electronics associated with each subarray. The curve which is calculated using the relationship indicates that small subarrays are impractical due to the large increase in cost. A 4m X 4m subarray configuration cost $2.7 X 10$ more than a 10m X 10m system. Therefore, the model system uses a 100 m2 subarray (approximately 10m X 10m) which is a compromise between the more stringent alignment requirements for larger subarrays and the increased cost of smaller subarrays. The total number of these subarrays within the 1 km total array is 7,850. The mechanical alignment requirement is + 3 arc minutes, giving a 2% loss in antenna efficiency. The corresponding mTsalignment tilt in length is .44 cm for the subarray and .44m for the total array. The stringent mechanical alignment requirement for the subarrays is the tilt angle from boresight to the ground; the vertical displacement of the subarrays with respect to each other is not that critical since the electronic phasing can compensate for different path lengths to the ground. IV.A. 2(h) BASIC SYSTEM PERFORMANCE REQUIREMENTS The preceding analysis has considered only a perfect antenna; the degradations associated with phase and amplitude errors, together with failure rates, will now be determined.
Figure IV.A.2-12 - Effective Subarray Ahtenna Gain Versus Tilt Angle Figure IV.A.2-13 - Transmit Antenna Cast Versus Subarray Size
(1) Random Phase Error - A Gaussian distribution phase error with selected standard deviations wfll be applied to each subarray. These phase errors are due to noise in the phase lock loop reference in the subarray's RF receiver, phase shifts in the klystrons, errors in the calibrated path lengths of the phase reference distribution system, etc. The rectenna collection efficiency for 0°, 7°, 14°, and 20° phase errors are shown in Figure IV.A.2-14. There are no amplitude errors nor failures in these calculations. For a 5000 meter rectenna radius, the efficiency loss as compared to a 0° phase error system varies from 1% for a 7° error to 10% for a 20° error. The model configuration is specified to have a 10° error. (2) Amplitude Tolerance - A Gaussian distribution amplitude illumination error with selected standard deviations will be applied to each subarray. The amplitude errors are due to variations in klystron outputs, losses in the feeds and in the waveguides, etc. The rectenna collection efficiency for 0, + 1 dB, and + 2 dB amplitude errors are shown in Figure IV.A.2-T5. There are no phase errors or failures in the data. For a 5000 meter rectenna radius, the efficiency loss for a + 1 dB amplitude error (the model configuration) is only .5%. (3) Failure Rates - Random failure rates of selected percentages are now applied to the entire transmit array. The failures would be due to the failures in the 130,000 klystrons, the 7,850 RF receivers and phase control circuits, and possibly in DC power distribution system in the antenna. The rectenna collection efficiency for 0, 2%, 5%, and 10% random failure rates are shown in Figure IV.A.2-16 . There are no phase or amplitude errors for this data. Collection efficiency is very sensitive to random failures, with about a 2% drop in efficiency for each 1% failure rate. This corresponds to a loss in revenue over a 30-year life time of over $1 billion for each 1% failure rate. A reasonable goal is 2% failure rate for the model configuration. (4) Combined Phase Errors and Failure Rates - When there are both phase errors and failures in the antenna, the loss in rectenna collection efficiency is compounded. This data are given in Figure IV.A.2“17. for various combined phase error and failure rates. No amplitude errors are present. (5) System Performance with Specified Parameters - The model configuration has an error budget of 10° phase error, + 1 dB amplitude error and a 2% random failure rate. The rectenna collection efficiency for this configuration as shown in Figure IV.A.2-18 is 88% for a 5000 meter rectenna radius. This is the collection efficiency used when calculating the total DC output power. The power density at the rectenna for the baseline configuration is shown in Figure IV.A.2-19. The density varies from approximately 22 mw/cm? at the center of the rectenna to .9 mw/cm? at the edge.
Figure IV.A.2-14 - Rectenna Collection Efficiency vs Phase Error Figure IV.A.2-15 - Rectenna Collection Efficiency vs Amplitude Error
Figure IV.A.2—16 - Rectenna Collection Efficiency Versus Failure Rate Figure IV.A.2-17 - Rectenna Collection Efficiency VS. Phase Errors and Failure Rates
Figure IV.A.2-18 - Rectenna Collection Efficiency for Baseline Configuration
Figure IV.A.2-19 - Power Density at Rectenna for Normal, Partial Failures, and Total Failure Configuration
IV. A. 2 (i) NOMINAL WEIGHT AND COST SUMMARY FOR THE MICROWAVE SYSTEM SIZING The nominal weights and costs for the 5GW microwave system with the 1 km transmit array are summarized below. The transportation cost is assumed to be $164/kg. No construction costs are included.
Ground Systems - (excluding DC bus bars) Rectenna is 10 km in diameter; TOTAL COST The $1.29X109 cost for the rectenna and facility is heavily dependent upon the diode and assembly cost/unit. A large variation in the $.015/unit cost could significantly change the rectenna cost. IV.A.2(j) FAILURE MODE ANALYSIS Studies were made into three types of discrete failures in the transmit antenna (1) a partial breakdown in the phase control system, (2) a total breakdown of the phase control system, and (3) a partial breakdown in the DC power distribution system. If the feedback phase control to the subarrays has large errors induced into the system, the subarrays no longer add in-phase to create a high power, narrow beam signal. Each subarray may act as an individual antenna, radiating its energy to the earth as a wide beamwidth signal. The power density at the rectenna for a large phase error of 135° is shown in Figure IV.A.2i9. The power density into and around the rectenna decreases by a factor of 100 within the main beam when compared to a properly phased-signal. Comparative si delobe levels will increase with distrance from the rectenna.
A total breakdown of the phase control system, as would happen if the pilot beam from the ground was interrupted, is also shown in Figure IV.A.2-19. The power density is reduced to .003 mw/cm2, well below even the stringent USSR microwave limits. There should not be any health hazards if the phase control system fails completely. Two types of failures within the lateral DC power distribution system were investigated. Referring to the diagram below, switching failures in the lateral system near the center of the array (called Type I) and near the edge of the array (Type II)are shown. In each case a 5% loss in the number of radiating subarrays is assumed. The rectenna collection efficiency for these two types of failures are compared with a 5% random failure in Figure IV.A.2-20. As would be expected, the 5% discrete loss at the center of the array has the greatest loss in efficiency while the 5% loss at the edge has the minimum loss. The subarrays at the edge of the array have only 1/10 the radiated power of those in the center, and hence, do no appreciably affect the efficiency. IV.A.2(k) CONTINGENCY ISSUES Some studies have been done into the solutions of possible system problem areas. These include: (1) What can be done if the phase, amplitude, or failure rate requirements are too stringent, or the subarray misalignment tolerances cannot be met? One solution is to increase the size of the transmit array. The rectenna collection efficiency for 1.25 km antenna is compared to the 1 km, model antenna in Figure IV.A.2~21. Both curves have the same error budget, 10° phase error, + 1 dB amplitude error, and 2% failure rate. For the 10 dB taper curves, (1) and (2), the 1.25 km antenna gives 5-7% greater efficiency in the mainbeam and can compensate for greater errors or failure rates. Referring back to Section IV.A.2(i), the increase in transmit array cost for going to a 1.25 km diameter is $700X106, which is the equivalent in revenue cost ($522X106 per 1% collection efficiency) of 1.4% in collection efficiency. Thus, for a cost viewpoint, if the collection efficiency drops because of projected increases in the errors or failure rates, a larger transmit antenna may be justified.
Figure IV.A.2-20 - . Rectenna Collection Efficiency VS Outage Effects Tigure IV?A.2-Z1 - Rectenna Collection Efficiency’Array Drameter and Taper
(2) What should be done if thermal radiation problems at the center of the transmit array cannot be solved without using complex, active cooling systems? One solution is to reduce the taper to 5 dB and increase the antenna size. The collection efficiency for a 1.25 Km, 5 dB taper antenna is shown in Figure IV.A.2 -21. It would be more desirable to increase the diameter to 1.25 Km and keep the 10 dB taper. The data given in Figure IV.A.2-1 indicates the boresight power density would decrease approximately 30% by going to the larger diameter and keeping the 10 dB taper. (3) Klystron versus amplitrons - The model configuration uses 50 Kw klystrons for the power converters in contrast to previous studies which employed 5 Kw amplitrons. It is felt that klystrons would have a higher reliability. From a system viewpoint, there are two critical issues which may determine whether amplitrons or klystrons should be used. There are (1) high efficiency with reduced power - the DC input power to the transmit antenna will decrease by about 30% over the 30-year lifetime. As the current slowly decreases to 70% of its initial value into the power converters, the klystron or amplitron must maintain a high DC-to-RF conversion efficiency. One method to reduce this 30% reduction is to add solar cell arrays periodically to the SPS. (2) High reliability - the rectenna collection efficiency is very dependent upon the failure rate. The % collection efficiency as a function of failure rate for a transmit antenna with 10° phase error and + 1 dB amplitude error is shown in Figure IV.A.2-22. The slope of the curve gives a 2% loss in efficiency for each additional 1% failure rate. As discussed previously, this is one billion dollars loss in revenue over the 30-year life for 1% failure rate. Thus, the power converters must operate efficiently and reliably in a high temperature environment. IV.A.2(1) BASIC MICROWAVE SYSTEM PERFORMANCE SUMMARY The performance characteristics and requirements for the model microwave system may be summarized as follows: (1) Output DC power - 5GW at the rectenna (2) Transmit Array size - 1 km in diameter (3) Array aperture illumination - a 10 step, truncated Gaussian amplitude distribution with a 10 dB edge taper (4) Subarray size - 100 m^ (approximately 10m X 10M) (5) Number of subarrays - 7,850 (6) Error budget - Total RMS phase error for each subarray - 10° (for the phase control system) Amplitude tolerance across subarray - +/- 1 dB Failure rate (total) - 2% over 30 year lifetime
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