1. Experimental Aerodynamics and Concepts Group Narayanan Komerath, Brendan Dessanti, Shaan Shan Daniel Guggenheim School of Aerospace Engineering Georgia Institute of Technology Millimeter Wave Space Power Grid Architecture Development 2012 Acknowledgments This work was partially supported as an advanced concept development study under the NASA EXTROVERT cross-disciplinary innovation initiative. Mr. Anthony Springer is the technical monitor.

2. Experimental Aerodynamics and Concepts Group Space Power Grid Architecture for Space Solar Power Features of the approach Economics Status of systems Assumptions and technical status Starting the roadmap: proposed ISS experiment

3. Experimental Aerodynamics and Concepts Group Space Power Grid Approach to SSP Sun-synchronous or other 2000 km orbits with dynamic beaming, instead of 36000 km GEO. Continuous beaming achieved with small constellation, with 45 degree half-cone of visibility 220 GHz transmission. Aerostat-tether- waveguide solution for low atmosphere transit Phase 1 generates revenue by using space as means of power exchange Standard 1 GWe collector-converter system for expansion to TeraWatts: Mirasol 2 GW collector; Girasol 1GWe converter

4. Experimental Aerodynamics and Concepts Group Economically viable growth path to over 5.6 Terawatts of Space Solar Power in 50 years from the start of the project. First 1GW satellite launched in Year 12. Brayton cycle conversion allows economies of scale compared to the linear mass-power relationship of PV conversion. 3600 K primary helium cycle offers > 80 percent cycle efficiency. Intensifed Conversion Archtecture (Brayton cycle ) achieves breakeven by Year 31 with NPV trough less then 3 Trillion dollars, at a selling price of 11 cents per kWh, or can breakeven by Year 48 with an NPV trough of 9 Trillion dollars, at a selling price of 6 cents per kWh. Exceeds 5.6TW by Year 50 -Improved design of collector/converter link: Mirasols also in 2000 Km orbits -Reduced uncertainty of primary heater -Graphene-based radiator conservatively meets mass budget for 190K inlet. -Waveguide-tethered aerostats solve moist-atmosphere transit problem Prior Work Update

5. Experimental Aerodynamics and Concepts Group Economic Viability Viability Parameter Specific Power s GEO/PV proposals: ~ 0.2 to 0.3 Baseline SPG PV-based: ~ 0.9 Brayton cycle SPG: 1.5 Launch Cost: c: We assume $2500/kg to LEO in Phase 1 (Years 1-12), down to $1300/kg in Phase 2 using runway-based reusable launch Efficiency η Present value ~ 0.48 Has to be ~ 1 for Space Solar Power to be economically viable. GEO-based PV proposals are not above 0.3.

6. Experimental Aerodynamics and Concepts Group

9. Technical and Economic Results: Breakeven vs. Selling Price Baseline: SPG Architecture presented at March 2011 IEEE Aero Conf IηCA: Current architecture including Brayton cycle conversion For Given Price of Power, Significant Improvement in Viability

10. Experimental Aerodynamics and Concepts Group ARCHITECTURE UPDATE 1.Improved orbital calculations for the Mirasol-Girasol link. The Mirasol is now brought down to 2000 km, to overcome the solar beam divergence and spot size problem without major optical system complexity. 2. The heater design for the primary solar heat absorption is improved. 3. The design of the radiator for the closed helium Brayton cycle is improved. Coated graphene sheets project a large reduction in TCS mass. 4. Waveguide transmission between the ground and antenna inside aerostat at 4000m 5. The component mass estimates for the SPG architecture are compared with those developed for other SSP concepts by other teams, and confirms the large improvement in specific power with SPG.

11. Experimental Aerodynamics and Concepts Group

13. Comparison with Other Architectures

14. Experimental Aerodynamics and Concepts Group 1.Antenna mass scales approximately in inverse proportion to frequency, and direct proportion to distance. 220 GHz vs 2.4/5.8 GHz; 2000km vs. 36000 km. Millimeter wave antenna may have higher mass per unit area than microwave antennae. Net result is that antenna mass becomes a minor component of total mass. 2. PV array mass is much greater than that of Brayton cycle converter, at high power level. 3. High thermodynamic efficiency compared to PV: Collector area for PV >> that for a Brayton cycle system, for the same converted power. 4. Extremely high conversion efficiency from mechanical to AC, compared to DC- AC conversion of PV. 5. ACTS must be 3 to 4 times bigger in the PV case. 6. Propulsion mass scales proportionally to the baseline system mass above, Architecture Sanity Check Thus we see that the mass total is easily an order of magnitude less in the SPG architecture than in photovoltaic GEO-based microwave architectures.

15. Experimental Aerodynamics and Concepts Group 1. Reciprocal 90 % conversion to and from DC/AC or 220 GHz. 2. Aereal density of 3 to 7 grams per square meter cited for solar sail craft. We project 60 grams per square meter for Mirasol including mirror array actuators. 3. Ground antenna efficiency is taken as 0.9 based on claims of people in the microwave beaming community. 4. Limiting temperature achieved by concentrated sunlight yet TBD. 5. 220 GHz conversion at 0.087 kg/kWe from mechanical work. Requires some form of mechanically pumped resonant waveguide for amplification. 6. Launch cost of $2500/kg to LEO in Phase 1 (12 years). Launch cost of $1300/kg to LEO in Phase 2/3 (years 12-50). Present Critical Assumptions

16. Experimental Aerodynamics and Concepts Group Reciprocal 90 % conversion to and from DC/AC or 220 GHz Efficiency vs. frequency with gyratrons Mass needed for gyratron conversion to millimeter wave: ~ 1.36 kg per KW at 1 MW level. Very far from 0.02 – 0.08 kg/kW!! ? ?

17. Experimental Aerodynamics and Concepts Group At specific power of 1 kW/kg: 5600 GWe in 40 years: 140 GWe installed per year 140,000,000 kWe 140 million kg per year. Assume 100,000 kg delivered to LEO per launch: 1,400 launches per year. Assume 7 launch sites: 200 launches (and landings) per year. 5 launch-land operations per site per week. Assume each vehicle undergoes 1 week refurbishment between launches: Need 5 vehicles per site, or 35 total. With spares, fleet of 50 vehicles needed. At 8 km/s delta V, mean Isp of 800 seconds using airbreathing stages, Mass ratio = exp (8000/(9.8*800)) ~ exp(1) = 2.7 Propellant mass ~ 170,000 kg per flight Conservative estimate: Need 300 tons per day of LH2/LOX propellant per site. Need airline-type efficiency and scale of operations. Fully reusable vehicles. Strong case for LH2-LOX to avoid pollutants and CO2 Strong case for LACE; air-breathing acceleration in upper atmosphere. Payloads not flat-panel (not PV), but more like turbomachines and round ducts Runway-Based Reusable Vehicle Requirements for Full-Scale SSP Deployment

18. Experimental Aerodynamics and Concepts Group 1.Millimeter wave conversion to and from alternating current, continues to be a first-order uncertainty in its efficiency and specific power when applied on a multi-megawatt scale. 2. Runway-based space launch vehicles, with airbreathing liquid air cycle engines. 3. Limiting temperatures reachable by intensifying sunlight. 4. High-temperature materials such as Hafnium Carbide, and their performance for applications such as that in the blades of the first turbine stage. SUMMARY OF NEEDED RESEARCH AND DEVELOPMENT

19. Experimental Aerodynamics and Concepts Group Starting Along the Road Map We are proposing two international experiments: 1.Dynamic beaming from the ISS at 220 GHz, to ground antennae located in 5 nations. Average of 100 seconds of transmission time per pass. The experiment aims to solve collaboration issues, pointing and reception issues, and obtain long-duration data on atmospheric propagation of millimeter waves. 2. Multinational satellites (4 to 6) to exchange power between terrestrial power plants located around the world. 3. Success in these two endeavors will allow development and launch of the Phase 1 Space Power Grid constellation.

20. Experimental Aerodynamics and Concepts Group 1.The specific power for the space-based component of the Space Power Grid architecture for SSP now promises to be above 1.5 kW/kg. 2. Millimeter wave dynamic beaming from 2000 km orbits provides the expected reduction in system mass over architectures with GEO-based microwave beaming. 3. The use of a solar Brayton cycle provides the remaining improvement in specific power, due to the favorable scaling of turbomachine system mass with increasing power compared to the linear scaling of photovoltaic architectures. 4. Bringing down the Mirasol solar collectors to 2000 km orbits enables better capture of the solar power using the Girasol concentrators. 5. The specific power can be further improved using graphene-based radiators. 6. The mass budgets specified in the 2012 version of the SPG architecture are shown to be reachable. 7. Economic viability estimate remains valid. CONCLUSIONS

21. Experimental Aerodynamics and Concepts Group 220 GHz transmission instead of 2.4 or 5.8 GHz Compact transmitter and receiver 94% capture instead of 84% Tethered waveguide transit of lower atmosphere R&D lead time to refine conversion efficiency and mass-specific power

22. Experimental Aerodynamics and Concepts Group Girasol Turbomachinery 1)300m Collector 2)Intensified Feed 3)Heater 4)Compressor 5)Turbine and Generator 6)Radiator 7)Phase Array Antenna Components: 1 GWe satellite conceived as 8 turbomachine modules sharing common heater core, with 16 transmitter antennae, evolved from Phase 1 Space Power Grid satellites.

23. Experimental Aerodynamics and Concepts Group 6. Standard 1 GWe collector-converter system for expansion to TeraWatts Change the focus from “cost to first power” and massive GEO stations, to a constellation expanding to replace terrestrial fossil power. 2GW solar collector, separated from converter: “Mirasol” 1 GWe converter “Girasol” 1 GWe size compatible with terrestrial power grid based on 1 GW nuclear reactors. Girasol 1 GWe converter Mirasol: 2 GW unltralight solar collector array

24. Experimental Aerodynamics and Concepts Group Space as a Dynamic Power Grid Use Space for synergy with terrestrial power sources Phase 1 generates revenue by using space as means of power exchange Makes terrestrial solar and wind more viable (and more green, by eliminating need for fossil fuel based auxiliary generators) Creates an evolutionary path to SSP Early Revenue Generation Modest Initial Investment

25. Experimental Aerodynamics and Concepts Group Brayton cycle converter specific power increases as power level rises. Conversion efficiency > 80 % minimizes radiator mass. 3650K He Gas Turbine Cycle

26. Experimental Aerodynamics and Concepts Group

30. Graphene and Carbon Nanotubes: High thermal conductivity, low mass densities. 3000 W/mK to 5300 W/mK Graphene: single plane of sp2 bonded carbon-carbon atoms and as a result. Heat pipe technology offers strong potential. ATCS with < 1 kg/m 2 is a very conservative choice. Girasol Thermal Control System Design

31. Experimental Aerodynamics and Concepts Group Thermal Control System