Isotope Selection:

18. Pu-238 (PuO2) modeled for baseline and accuracy (compared to existing papers) Neutron - 0.361±0.002 mrem/hr Photon - .65±0.03 µrem/hr Total: .36368 mrem/hr Cu-244 (Cu2O3) Neutron – 2.01±0.03 rem/hr Photon - 56.5±0.2 mrem/hr Total: 2.0967 rem/hr

21. Questions?

22. Evaluation of General Purpose Heat Source Powered Stirling Technologies for a2.5 kWe Lunar Surface Power SourcePresented By Chris Miller and Troy ReissContributions by Chris Miller, Troy Reiss, Jeff Katalenich, Logan Sailer, Caleb Robison August 10, 2007

23. Contents Overview of existing space power conversion technology Justification for selection of Stirling power conversion system Review of current Stirling systems for space applications Concepts offering potential improvement to existing technology

24. Radioisotope Thermoelectric Generator (RTG) Flown on Galileo, Ulysses, Cassini, and New Horizons missions Powered by Pu-238 General Purpose Heat Source (GPHS) Produced approximately 300 We using 572 silicon germanium thermoelectric elements Thermal power 4400-4500 W from 18 GPHS modules, mass 55.9 kg, specific power 5.5 We/kg, efficiency 6.7%-6.8% Reliable, but electrical power output well below lunar surface mission requirement

25. Stirling Power Conversion System Other power conversion systems explored to provide greater power output than RTG Stirling and Brayton systems found to be capable of supplying desired power Stirling determined to be best power conversion system due to better scaling at desired power level Better scaling results in lower mass and higher efficiency than Brayton cycle based power conversion system

26. Stirling vs Brayton1 kW to 10 MW

27. Hot end temp 923K Cold end temp 333K 2 55 We Stirling Led to development of the SRG110 Achieved 26,000 hours of operation Specific power of 3.5 We/kg Existing TechnologyDual-Opposed Technology Demonstration Converters

28. Sunpower Advanced Stirling Converter (ASC) Resulted from the SRG110 Free piston design Weight and size reduction 88 We at 38% Efficiency Hot end 1123K

29. Advanced Stirling Radioisotope Generator(ASRG) Lifetime of 14 years plus 3 years of storage BOM Power Output: 140 We EOM: 126 We (14 yrs) Projected Mass: 20.24 kg Projected Specific Power: 7.0 We/kg (Using Pu) 72.5 cm L x 41 cm H x 29.3 cm W Beryllium housing Future model projects 8.5 We/kg

30. Stirling Lunar Power System (LPS) Stacking GPHS modules has limits as distance increases from the Stirling converter Radial configuration allows all GPHS modules to be in contact with the hot shoe Can accommodate many GPHS modules Ni-200 hot shoe for high thermal conductivity

31. Potential Improvements Problems Current technology fails to meet the 2.5 kWe goal Multiple units of the ASRG to meet power requirements would be too heavy Current heat rejection systems subjected to lunar day/night cycles Changing environmental conditions changes cold end temperature, affects Stirling performance Solution Development of Stirling converters with a higher electrical power output Designs to incorporate multiple Stirling converters were developed Concept to reduce cold end variation during changing environmental conditions proposed

32. Development of New Stirling Converters Nasa has recently funded development of a 5 kWe free-piston stirling converter for lunar application Sunpower’s EG1000 is a 1.2 kWe free piston stirling converter has been in use for DOD applications for several years Infinia Corp. is working to develop a 3 kW free-piston stirling convertor for solar applications

33. Based on LPS configuration Used to determine capability of other configurations Results would be baseline for other designs Tri-core with Two Stirling Converters

34. Quad-core and Octagon-core Based on LPS design Ability to attach multiple Stirling converters Stirling converters with higher power output could be attached

35. Heat Pipe Concept 4 converters ~ .75 kWe each 5 GPHS blocks (Cm 244) Heat pipes couple GPHS modules to Stirling hot ends Working fluid transport via capillary forces in a wicking structure Heat rejected to surroundings via cold end radiators Possible power output of 2.5 to 3 kWe Designed to be easily assembled prior to or after launch

36. Stirling Cooling All designs discussed thus far cooled from a cold flange attached to the cold end of the engines The cold flanges are coupled to the outer shell which acts as the radiator for the unit Issues arise from changing lunar day/night temperatures and lunar dust collection on radiators Method of heat rejection from cold end identified as major potential area for improvement of existing concepts

37. Sub-lunar Surface Heat Sink Concept Current lunar Stirling concepts exposed to changing environmental conditions during lunar day/night cycle Potential to eliminate this complication through use of constant -30 °C sub-lunar surface temperature as heat sink Liquid metal or sulfur injected into bedrock or regolith during drilling operation Liquid diffuses into lunar material, providing higher thermal conductivity sink than lunar material alone Stirling cold end coupled to sink with high thermal conductivity material prior to freezing Heat rejected via conductive path to sink instead of radiator

38. Sub-lunar Surface Heat Sink Concept Advantages Elimination of fluctuating cold end temperature and power output No exposure of radiators to lunar dust Reduced shielding and insulation mass if entire assembly placed below lunar surface Potential mass savings from removal of radiators Remaining issues Thermal analysis must determine necessary size of heat sink Cold end temperature must be determined and compared with cold end temperatures of current concepts Tradeoffs between sulfur and metal sink must be determined and evaluated Physical location of sink, converter, and GPHS units must be determined

39. Conclusions and Future Work Stirling converters best power conversion option for 2.5 kWe lunar surface radioisotope power system Free piston Stirling should be basis for such systems Great potential for improvement of existing Stirling systems through utilization of sub-lunar surface heat sink Extensive modeling and thermal analysis must be performed on all proposed concepts to determine if they offer improvements over existing Stirling systems

40. Questions?

41. RPS Cooling Options on the Moon Holly Szumila, Mookesh Dhanasar Benjamin Schreib Center for Space Nuclear Research August 10, 2007

42. Outline Assembly / In-transit cooling (active) Cooling options on the moon Surface Sub-surface Analysis Thermal models (1-D analytical, 2-D numerical) Compare different sources Conclusions Questions

43. RPS Cooling (Assembly and In-Transit) Already have assembly active cooling. In transit cooling systems already exist. Include fins.

44. Cooling options on the moon

45. Lunar Surface

46. Radiation considerations Solar wind, peak solar flares, galactic cosmic radiation. Primary concern solar cosmic radiation, or solar flares. Heavy ion fluxes not accounted for using MCNPX, but can travel through layers of shielding and spallation effects (high energy neutron fluxes). Proton and neutron fluxes can only cause heat deposition to RTG on nano-Watt magnitude. Lunar Dust Micrometeorites Caused thin films on Apollo structures, thought to pile a great deal and cause wear to metal over extended periods of time.

47. Lunar Surface - Conditions

48. Lunar Sub-Surface (Regolith) Lunar sub-surface (Regolith) Thermal shield. Constant temperature. Radiation shield. Shielding against micro-meteorites.

49. Lunar Bedrock Difficulty in drilling Less known on bedrock Constant temperature sink

50. Lunar Sub-Surface Thermal analysis (Regolith)

51. RPS Thermal Analysis-Model

52. RPS Thermal Analysis-Assumptions The analysis is carried out in 1-D only. Steady-State conditions apply. Heat generation is constant and uniform. The thermal conductivity for the material is constant. The RTG is symmetric about its centerline. There is a conduction-convection interface with the outer surface of the RTG and the medium.

53. RPS Thermal Analysis – Theory (Analytical)

54. RPS 1-D Temperature Profile

55. Surroundings Analysis - Model

56. Surroundings Analysis - Assumptions 1-D heat transfer. Steady State conditions apply. There is no heat generation in the region of interest. There is no bulk fluid motion, so heat transfer is a special case of conduction. The temperature at the surface of the RTG is known. The ambient temperature is known.

57. Surroundings Analysis – Theory (Analytical)

58. RPS Surrounding Area Temperature Profile

59. Surroundings 2-D Analysis (Numerical)

60. Surroundings 2-D Analysis (Point Source) Results

61. Surroundings 2-D Analysis (Numerical) Results

62. Compare Various RPS Sources

63. Compare Various RPS Sources

64. Compare Various RPS Sources

65. Compare Various RPS Sources

66. Conclusion From our research it is desirable to have the RPS buried in the regolith. From the thermal analysis, a simple heat transfer tool was created. It was used to determine the thermal profile for a variety of sources. From this analysis it is observed that for the commonly used isotope source, approximately 0.8 m of sulfur is required before phase change occurs.

67. Future Work Refine model. Detail 3-D Investigate various convective mediums.

68. Questions

69. Acknowledgments Dr. Steve Howe John Bess, Jon Webb Ms. Kristi Bailey INL 2007 CSNR Summer Fellows

95. 16.5 kg Cm2O3 required for 40 kW (thermal) Unshielded dose: 8.6 rem/hr 300 kg LiH: ~50 mrem/hr 90 kg LiH and 190 kg of a Gd/U mixture: ~60 mrem/hr Pure LiH may be more attractive than including Gd/U if volume is not important; 300 kg of LiH should fit inside the launch vehicle

98. Questions