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F TA03 Space Power and Energy Storage INTRODUCTION The draft roadmap for technology area (TA) 03, Space Power and Energy Storage, is divided into four level 2 technology subareas:1 • 3.1 Power Generation • 3.2 Energy Storage • 3.3 Power Management and Distribution • 3.4 Crosscutting Technology NASA has many unique needs for space power and energy storage technologies that require special technol - ogy solutions due to extreme environmental conditions. For example, • Venus surface operations require very high sustained temperatures (~460 °C), • Surface penetrators must survive high-impact decelerations (hundreds of g’s or more), • Deep space planetary surface missions operate at very cold temperatures, • Missions that travel very far from the Sun cannot rely on solar energy, and • Missions to Jupiter and many other destinations must survive high-radiation environments. These missions would all benefit from advanced technologies that provide more robust power systems with lower mass. In particular, this technology area encompasses pacing technology challenges for the volume, mass, and reliability critical space exploration systems. Even in the reduced gravity of the Moon or Mars, the large mass of EVA suits degrades crew operations. Advanced power and energy storage systems would directly improve the performance of EVA suits, rovers, surface habitats, and spacecraft. The ability of space power and energy storage technologies to enable and enhance NASA’s ability to learn about Earth and the solar system is illustrated by the following quotes from a recently completed decadal survey on planetary science (NRC, 2011): 1 The draft space technology roadmaps are available at http://www.nasa.gov/offices/oct/strategic_integration/technology_roadmap.html. 131
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132 NASA SPACE TECHNOLOGY ROADMAPS AND PRIORITIES As future mission objectives evolve, meeting these challenges will require continued advances in several technology categories, including . . . more efficient power and propulsion for all phases of the missions. Of all the multi-mission technologies that support future missions, none is more critical than high-efficiency power systems for use throughout the solar system. Since more efficient use of the limited plutonium supply will help to ensure a robust and ongoing planetary program, the committee’s highest priority for near-term multi-mission technology investment is for the completion and valida - tion of the Advanced Stirling Radioisotope Generator. The committee recommends that NASA consider making equivalent systems investments in the advanced Ultraflex solar array technology that will provide higher power at greater efficiency. . . Investing in these system capabilities will yield a quantum leap in our ability to explore the planets and especially the outer solar system and small bodies. Prior to prioritizing the level 3 technologies included in TA03, several technologies were renamed, deleted, or moved. The changes are explained below and illustrated in Table F.1. The complete, revised technology area breakdown structure (TABS) for all 14 TAs is shown in Appendix B. Energy storage can be accomplished using many fundamentally different approaches. The current roadmap includes three: batteries, flywheels, and regenerative fuel cells. Two other approaches may also prove feasible for space applications: (1) electric and magnetic field storage and (2) thermal storage (especially for surface power applications). Accordingly, the structure for this roadmap has been modified by adding two new level 3 technologies: • 3.2.4. Electric and Magnetic Field Storage • 3.2.5. Thermal Storage TABLE F.1 Technology Area Breakdown Structure for TA03, Space Power and Energy Storage NASA Draft Roadmap (Revision 10) Steering Committee-Recommended Changes Two technologies have been added. TA03 Space Power & Energy Storage 3.1. Power Generation 3.1.1. Energy Harvesting 3.1.2. Chemical (Fuel Cells, Heat Engines) 3.1.3. Solar (Photovoltaic & Thermal) 3.1.4. Radioisotope 3.1.5. Fission 3.1.6. Fusion 3.2. Energy Storage 3.2.1. Batteries 3.2.2. Flywheels 3.2.3. Regenerative Fuel Cells Add: 3.2.4. Electric and Magnetic Field Storage Add: 3.2.5. Thermal Storage 3.3. Power Management & Distribution 3.3.1. Fault Detection, Isolation, and Recovery (FDIR) 3.3.2. Management & Control 3.3.3. Distribution & Transmission 3.3.4. Wireless Power Transmission 3.3.5. [Power] Conversion & Regulation 3.4. Crosscutting Technology 3.4.1. Analytical Tools 3.4.2. Green Energy Impact 3.4.3. Multi-functional Structures 3.4.4. Alternative Fuels
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133 APPENDIX F TOP TECHNICAL CHALLENGES The panel identified four top technical challenges for TA03, all of which are related to the provision of safe, reliable, and affordable in-space power systems consistent with NASA’s current and potential mission needs. They are listed below in priority order. 1. Power Availability: Eliminate the constraint of power availability in planning and executing NASA missions. Power is a critical limitation for space science and exploration. The availability of more power opens up new paradigms for how NASA operates and even what individual missions can accomplished. Increased power avail - ability for human exploration missions translates into capabilities to support more astronauts at larger outposts with higher-capacity in situ resource utilization (ISRU) systems, higher data transmission rates, more capable mobility systems with shorter turnaround times, and higher capability science instruments. For robotic science missions, power availability has become critical in determining the scope of a mission that can be planned and how long it takes to reach mission destinations. This is due to the emergence of electric propulsion systems that enhance robotic mission design, so that the more power that is available, the shorter the trip time to any destination. Once at the destination, high power levels enable scientists to develop new approaches to scientific discovery and to communicate larger volumes of information more quickly back to Earth. 2. High-Power for Electric Propulsion: Provide enabling power system technologies for high-power electric propulsion for large payloads and planetary surface operations. Advances in solar and nuclear technologies in the United States and elsewhere during the past decade offer the potential of developing power generation systems that can deliver tens to hundreds of kilowatts. For example, inverted metamorphic (IMM) solar cells are being developed to deliver 40 percent efficiency with very little mass due to removing the thick substrate used to grow the multi-layer photovoltaic semiconductor materials. New light - weight structures also greatly reduce the mass of solar arrays and enable higher power outputs. As solar arrays grow to large sizes (such as hundreds of kW for electric propulsion planetary missions), new technology will be needed for the control and pointing of the large arrays. Nuclear fission system concepts have been developed for lunar and Mars missions that provide pathways to reasonable mass reactors that can be placed and operated on a planetary surface to deliver 10 kW to 100 kW. These designs use proven fuels, power conversion technologies, and reactor materials to reduce the development and operations risk to acceptable levels. Other aspects of fission systems require technology development including heat exchangers, fluid management, scaling of power conver- sion devices, heat rejection components, radiation shielding, and aspects of system integration and testing. 3. Reduced Mass: Reduce the mass and stowed launch volume of space power systems. Power systems typically comprise one third of the mass of a spacecraft at launch, and the volume available in the launch vehicle fairing can limit the size of solar arrays that can be packaged on the vehicle. New power generation, energy storage, and power delivery technologies have the potential to cut the mass and volume of these systems by a factor of two to three. Successfully developing these technologies would enable missions to include more science instruments, use smaller and less expensive launch vehicles, and/or provide higher power levels. 4. Power System Options: Provide reliable power system options to survive the wide range of environments unique to NASA missions. NASA missions require power systems and components to survive many different types of extreme environ - ments. This can include high radiation levels, very high or very low temperatures, very high impact forces (for planetary surface penetrators), highly corrosive environments, dusty atmospheres, and other unique extremes. In all of these challenging environments, the power system must operate predictably and reliably or the mission is
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134 NASA SPACE TECHNOLOGY ROADMAPS AND PRIORITIES lost. Continued advances in space power technology that improve NASA’s ability to overcome these challenges will enable NASA to plan and execute a wider array of missions. QFD MATRIX AND NUMERICAL RESULTS FOR TA03 The panel evaluated 20 level 3 technologies in TA03, Space Power and Energy Storage. Eighteen of these technologies appear in the draft roadmap for TA03; the other two were added by the panel, as explained above. The results of the QFD scoring are shown in Figure F.1. Figure F.2 shows the breakdown of the technologies into high, medium, and low priority categories. Six technologies were assessed to be high-priority technologies: • Solar (photovoltaic and thermal), • Fission, • Distribution and transmission, • Conversion and regulation, • Batteries, and • Radioisotope power systems. The first five technologies were designated as high-priority technologies because they received the highest QFD scores based on the panel’s initial assessment. The panel subsequently decided to override the QFD scoring results ls oa ds lG ee na N ch io s at es Te N en o ce er bl pa A ds na os SA ee so er N g ea A in d) -N -A SA R m te on on d A Ti gh an N N N d ei rt ith ith ith an k (W fo is y tw tw tw rit Ef ng R e rio l or en en en ci d ca an lP en Sc nm it nm nm ni ef qu ne e ch FD en lig lig lig m Se Pa Te Ti Q B A A A Multiplier 27 5 2 2 10 4 4 0/1/3/9 0/1/3/9 0/1/3/9 0/1/3/9 1/3/9 -9/-3/-1/1 -9/-3/-1/0 Alignment Risk/Difficulty Technology Name Benefit 60 L 3.1.1. Energy Harvesting 1 3 1 1 3 -3 -1 130 M 3.1.2. Chemical (Fuel Cells, Heat Engines) 3 3 3 3 3 -1 -1 406 H 3.1.3. Solar (Photovoltaic and Thermal Power) 9 9 9 9 9 1 -3 122 H* 3.1.4. Radioisotope (Power) 3 3 1 1 3 1 -3 374 H 9 9 1 1 9 1 -3 3.1.5. Fission (Power) ‐39 L 3.1.6. Fusion 0 3 1 3 1 -9 -9 192 H 3.2.1. Batteries 3 9 9 9 3 1 -1 76 L 3.2.2. Flywheels 1 3 3 3 3 -1 -1 50 L 3.2.3. Regenerative Fuel Cells 1 1 1 1 3 -1 -3 64 L 3.2.4. Electric and Magnetic Field Storage 1 3 3 1 3 -3 -1 118 M 3.2.5. Thermal Storage 3 3 1 3 3 -3 -1 118 M 1 9 9 3 3 -1 -1 3.3.1. (Power) Fault Detection Isolation and Recovery 118 M 3.3.2. Management and Control 1 9 9 3 3 -1 -1 216 H 3.3.3. Distribution and Transmission 3 9 9 3 9 -3 -3 60 L 3.3.4. Wireless Power Transmission 1 3 3 3 3 -3 -3 216 H 3 9 9 3 9 -3 -3 3.3.5. (Power) Conversion and Regulation 106 M 3.4.1. Analytical Tools 1 9 3 1 3 -1 0 54 L 3.4.2. Green Energy Impact 1 1 1 3 3 -3 -1 102 M 3.4.3. Multi-functional Structures 1 9 3 1 3 -1 -1 52 L 3.4.4. Alternative F l 3 4 4 Alt ti Fuels 1 3 1 1 3 -3 3 -3 3 FIGURE F.1 Quality function deployment (QFD) summary matrix for TA03 Space Power and Energy Storage. The justification for the high-priority designation of all high-priority technologies appears in the section “High-Priority Level 3 Technologies.” H = High Priority; H* = High Priority, QFD score override; M = Medium Priority; L = Low Priority.
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135 APPENDIX F 0 50 100 150 200 250 300 350 400 3.1.3. Solar (Photovoltaic and Thermal Power) 3.1.5. Fission (Power) 3.3.3. Distribution and Transmission High Priority 3.3.5. (Power) Conversion and Regulation 3.2.1. Batteries 3.1.2. Chemical (Fuel Cells, Heat Engines) 3.1.4. Radioisotope (Power) 3.2.5. Thermal Storage Medium Priority 3.3.1. (Power) Fault Detection Isolation and Recovery 3.3.2. Management and Control 3.4.1. Analytical Tools 3.4.3. Multi‐functional Structures 3.2.2. Flywheels 3.2.4. Electric and Magnetic Field Storage Low Priority 3.1.1. Energy Harvesting 3.3.4. Wireless Power Transmission 3.4.2. Green Energy Impact 3.4.4. Alternative Fuels High Priority (QFD Score Override) 3.2.3. Regenerative Fuel Cells 3.1.6. Fusion FIGURE F.2 Quality function deployment rankings for TA03 Space Power and Energy Storage. to designate radioisotope power systems as a high-priority technology to highlight the critical importance of cur - rently funded and planned programs for Pu-238 production and Stirling engine development and qualification. Figure F.3 correlates the applicability of the top technical challenges, as described above, to each of the Space Power and Energy Storage level 3 technologies. This shows that the high-priority technologies, discussed in the next section, provide the potential solutions that will meet these challenges. The medium- and low-ranked technologies are judged to have a weak linkage because of the limited benefit of investing in these technologies regardless of how closely they may overlap with various challenges in terms of subject matter. HIGH-PRIORITY LEVEL 3 TECHNOLOGIES Panel 1 identified six high-priority technologies in TA03. The justification for ranking each of these technolo - gies as a high priority is discussed below. Technology 3.1.3, Solar (Photovoltaic and Thermal) Photovoltaic space power systems have been the workhorse of NASA science missions as well as the foundation for commercial and military space systems. Solar cells directly convert sunlight into electricity. Today’s solar cells are made from III-V materials2 and are composed of multiple junctions of various band gaps to achieve solar conversion efficiency of 30 percent. Current emphasis is on the development of high-efficiency cells. NASA also needs • Cells that can effectively operate in low-intensity/low-temperature (LILT) conditions (which is typical when spacecraft are more than three astronomical units from the Sun), • Cells and arrays that can operate for long periods at high temperatures (>200°C), • High specific power arrays (500 to 1000 W/kg), and • Electrostatically clean, radiation tolerant, dust tolerant, and durable, re-stowable, and/or deployable arrays. 2 III-V materials are semiconductor materials made of materials from Groups III and V of the periodic table, such as gallium-arsenide.
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136 NASA SPACE TECHNOLOGY ROADMAPS AND PRIORITIES Top Technology Challenges 2. High-Power for Electric Propulsion: Provide enabling power 4. Power System system technologies for Options: Provide reliable 1. Power Availability: 3. Reduced Mass: Eliminate the constraint high power electric Reduce the mass and power system options to of power availability in propulsion for large stowed launch volume survive the wide range planning and executing payloads and planetary of space power of environments unique Priority TA 03 Technologies, Listed by Priority NASA missions. surface operations. systems. to NASA missions. ● ● ● ● H 3.1.3. Solar (Photovoltaic and Thermal Power) ● ● ● ● H 3.1.5. Fission (Power) ○ ● ○ H 3.3.3. (Power) Distribution and Transmission ○ ● ○ ○ H 3.3.5. (Power) Conversion and Regulation ○ ● ● H 3.2.1. Batteries ○ ● H 3.1.4. Radioisotope (Power) ○ M 3.1.2. Chemical (Fuel Cells, Heat Engines) M 3.2.5. Thermal Storage M 3.3.1. (Power) Fault Detection Isolation and Recovery M 3.3.2. Management and Control M 3.4.1. Analytical Tools M 3.4.3. Multi-functional Structures L 3.2.2. Flywheels L 3.2.4. Electric and Magnetic Field Storage L 3.1.1. Energy Harvesting L 3.3.4. Wireless Power Transmission L 3.4.2. Green Energy Impact L 3.4.4. Alternative Fuels L 3.2.3. Regenerative Fuel Cells L 3.1.6. Fusion Strong Linkage: Investments by NASA in this technology would likely have a major ● impact in addressing this challenge. Moderate Linkage: Investments by NASA in this technology would likely have a ○ moderate impact in addressing this challenge. Weak/No Linkage: Investments by NASA in this technology would likely have little or [blank] no impact in addressing the challenge. FIGURE F.3 Level of support that the technologies provide to the top technical challenges for TA03 Space Power and Energy Storage. Because of the critical importance of photovoltaic power systems, the Department of Defense (DOD) is funding technology development at the cell level to raise that efficiency first to 33 percent and then to 39 percent. The most common usage is in planar solar arrays but certain concentrator arrays have flown successfully and two types are currently under development. Concentrator arrays offer cost reductions due to reduced solar cell material and efficiency gains. Nearly all spacecraft flown to date have been powered by solar arrays. Photovoltaic power systems provide the energy for NASA science missions in low Earth orbit (LEO), including the International Space Station (ISS) and higher altitude communication systems such as the Tracking and Data Relay Satellite Systems (TDRSS). They have powered science missions to Mars (both in orbit and on the surface), Venus, and Mercury. Advanced photovoltaic power systems will be used on the Juno mission to Jupiter and the Solar Probe Plus near the Sun. Commercial communications satellites in geosynchronous Earth orbit (GEO) rely exclusively on photovoltaic power systems. NOAA polar orbiting weather satellites also rely on photovoltaic power systems. The DOD uses photovoltaic power systems for satellites in LEO, GEO and mid-Earth orbits (MEO) for observation, event detec - tion, navigation (the Global Positioning System), and others applications. The high priority assigned to this level 3 technology is based on the benefit provide by photovoltaic research. The draft roadmap for TA03 also includes
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137 APPENDIX F solar thermal systems in technology 3.1.3, but the panel does not perceive solar thermal power technology as a high priority because it has not been used in space and it has not proven to be cost competitive. Many photovoltaic power systems have achieved a TRL of 9, including the III-V triple junction solar cells. Advances in new photovoltaic technology, such as IMM photovoltaic cells (which remove the substrate to leave a cell that is only a few microns thick and has a power conversion efficiency of more than 30 percent with exceedingly low mass and inherent radiation tolerance. IMM arrays will be storable and deployable. The increased efficiency of IMM cells will enable smaller arrays, and the lightweight cells may enable alternate array structures to further reduce mass. Lightweight solar array structures with specific mass beyond the 100 W/kg offered by the current state of the art would also be beneficial. These technologies are currently at TRL 3 or below. As discussed in Appendix E (TA02, In-Space Propulsion Technologies), NASA has a vital interest in photo - voltaic power system developments for high-power electric propulsion (EP) missions. Because power system mass reduction is critical to EP missions, advanced array technologies that offer high specific mass (>500 W/kg) and high power density (>300 W/m2) are critical technology development areas. Increases in solar cell efficiencies offered by IMM photovoltaic cell technology and lightweight array technology, including both planar and concentrator approaches (now at TRL 2), is expected to achieve a specific mass of more than 600 W/kg and a power density of more than 400 W/m2. The NASA Glenn Research Center has been the primary NASA center developing photovoltaic technolo - gies, with the Jet Propulsion Laboratory (JPL) and NASA Goddard Space Flight Center (GSFC) also providing significant capabilities and facilities to advance the state of the art. NASA is well qualified to lead development of high-power solar array technology because of its expertise and capabilities plus the diversity of its mission needs, while collaborating with DOD, DOE, commercial industry, and academia. While the DOD has a modest investment in IMM solar cell technology, NASA is highly motivated to invest in IMM technology to ensure timely development of next generation solar cells to meet its own mission needs. Continued interactions between NASA and other countries investing in space photovoltaic technology, including multiple European countries and Japan, would be beneficial. The ISS has been used to test and qualify new photovoltaic cell technologies in the past, and it remains available for this function. Photovoltaic power technology is a high priority because of the game-changing impact that higher power, lighter weight solar arrays would have on future NASA missions. Solar power generation applies to virtually all NASA mission areas plus DOD, commercial, and civil space enterprises. Development efforts will therefore lead to widespread benefits across the user spectrum. The development risks for high-power solar arrays are moderate to high, which is appropriate for NASA technology investments. Space demonstration tests of lightweight arrays with power output of 30 to 50 kW level are warranted, and the risks at that size are relatively low. Development of larger arrays with power output as high as 1 MW will be required to support large-scale exploration missions. Solar arrays with such a high power output would be game changing. Structures large enough for such a large array have not been developed, however, and a focused development program for large arrays, that addresses control and pointing issues and the higher risk associated with higher power arrays, is warranted to support NASA exploration missions. Technology 3.1.5, Fission Space fission power systems use heat generated by fission of a nuclear fuel to power a thermal to electric conversion device to generate electric power. Key subsystems include the reactor, heat exchanger (to move the heat out of the reactor and into the power converter), power converter, heat rejection radiator, and radiation shield. As noted in an earlier study that focused on space applications of fission reactor systems (NRC, 2006, pp. 10-11): Nuclear reactor systems, which can provide relatively high power over long periods, make it possible to design mis - sions with more numerous and more capable science instruments, high-bandwidth communications systems, shorter transit times, and greater flexibility to change the course and speed of spacecraft enough to conduct extended investi - gations (rather than brief flybys) of bodies of interest; visit multiple bodies much more easily; and significantly alter a spacecraft’s trajectory in response to information collected during a particular mission. Nuclear reactors have the potential to overcome limitations associated with low energy and power. They do this by providing electricity and
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138 NASA SPACE TECHNOLOGY ROADMAPS AND PRIORITIES propulsion over a wide range of power levels for extended periods (years to decades), including during both transit and surface operations, without regard to the availability of either solar energy or large quantities of chemical fuel. Nuclear reactor systems, however, are expensive to develop, and their potential will be realized only if key technol - ogy issues can be overcome. Space fission technology is currently assessed to be at TRL 3. While some components are demonstrated at higher TRLs, many of the required elements require technology development to advance beyond TRL 3. Other components have reached higher TRLs in past programs such as the SP-100 and Prometheus programs, but tech - nology capability has been lost and must be redeveloped. Key subsystems that must be addressed include the reac - tor (including instrumentation and control/safety), energy conversion, heat transfer, heat rejection, and radiation shields. NASA has some of the expertise needed to develop space fission power system technologies, but it will need to work in collaboration with the DOE and private industry in order to advance the technology to TRL 6 and beyond. Because of their unique capabilities and statutory requirements, DOE must take the lead on the development reac - tor components and technologies, including the fuel. NASA centers including the Glenn Research Center and the Marshall Space Flight Center have the expertise and facilities to lead development of the energy conversion and heat rejection subsystems. NASA is also qualified to lead overall systems engineering efforts, with DOE assistance for nuclear subsystems. This is a technology that is primarily suited to space exploration needs, and therefore NASA is best positioned to take the lead in maturing the technology to TRL 6 or beyond. There may be opportunities to collaborate with some international partners that have capabilities and facilities in fast reactor technologies, such as Russia, Japan, and France. Use of the space station is not appropriate for developing this technology. Space fission power systems would be game-changing due to the potential (1) to provide a power rich envi - ronment to planetary surface exploration missions, especially for crewed missions, and (2) to enable high-power electric propulsion systems for deep space exploration and science missions. The alignment of this technology with NASA’s needs is high because of the game-changing impact it would make on both robotic science and human exploration capabilities. Alignment with other aerospace and national needs is considered to be low because space power reactors will be designed as fast neutron reactors, and there are no significant terrestrial applications for fast neutron reactor technology. The risk is assessed to be moderate to high, which is appropriate for NASA. (This risk level assumes that NASA will set realistic goals for the fission system to be developed; otherwise the risk could increase to the “very high” level.) The next space fission development program would hopefully adopt performance and life goals that are not as ambitious as the fission power system goals incorporated in the cancelled Jupiter Icy Moons Orbiter (JIMO) mission, so that entirely new materials or fuels will not need to be developed. A space reactor concept based on liquid metal cooling, stainless steel structures, and UO 2 fuel would have the strongest technology base, drawing on decades of development. The pursuit of higher temperature systems has been the primary source of technical problems and associated cost and schedule overruns in prior space fission power system development projects. The cost of the power system development is expected to be high, in the range of $1 billion to $2 billion over 10 to 12 years. This cost is easily justified, however, by the potential benefits that can be realized in both the human and robotic exploration of the outer solar system and beyond. Thus, fission is judged to be a high-priority technology. Technology 3.3.3, Distribution and Transmission Interest in the components of a spacecraft electrical power system often centers on the power generation and storage functions, and many resources have been devoted to exploring options, developing alternatives, and improv- ing the specific power and energy of the systems that serve these two functions. As game-changing science and human exploration missions of the future are examined, the need for significant increases in electrical power on spacecraft becomes a clearer and higher priority. With these higher power levels, an extrapolation of the current technologies for the distribution and transmission (D&T) of power would result in unacceptably high mass and complexity. Thus, D&T is judged to be a high-priority technology area. Distribution and transmission on a space system is comprised primarily of cables (copper conductors and insulation) and connectors. Currently, electrical power on spacecraft is distributed by direct current (DC) at fixed
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139 APPENDIX F voltages ranging from 28V DC to 100V DC. The ISS is the exception to this rule, with a 120V DC distribution architecture. As higher power systems are developed, higher voltage distribution will be required to avoid very heavy wire bundles. This can be done with either DC or alternating current (AC) architectures. Paschen’s law limits high-voltage DC systems to a maximum of about 270 V DC. Therefore, high power levels will require more attention to AC systems, probably at relatively high frequencies. The demand for lower mass will challenge the traditional copper materials for the bus, and the need to operate at higher temperatures (to increase the efficiency of the thermal management systems as well as electronics efficiencies) may appear counter to the need to reduce ohmic losses in transmission lines. Proposed research under technology 3.3.3 would increase the voltage of D&T subsystems, develop high- frequency AC distribution options for space systems, and identify alternate materials to replace copper conductors. Copper wire has long been a conductor of choice for spacecraft, but as power levels increase, so too will current and voltage and with them the conductor mass will grow. With DC currents, to reduce the mass penalty of larger cables, alternate materials such as superconductors or nano-material conductors may need to be developed, along with lighter space-qualified insulating materials capable of protecting systems at high voltage. With AC power systems, advancing beyond the 116 V AC system in the space shuttle may require very high operating frequen - cies; for example, NASA funded development of a 440 volt, 20 kHz AC power system for Space Station Freedom until it was reconfigured to use a DC power system (Patel, 2005). Technical needs include keeping transmission losses to a minimum, reducing transformer masses, incorporating fault protection and smart telemetry into power distribution architectures, and developing new connectors. Ultimately, the nature of future missions will dictate the architecture and technologies used for vehicle power systems, and that, in turn, will define the requirements for electrical power D&T. For example, the electrical power from a nuclear reactor-turboalternator system will likely be high-voltage AC, while power from photovol - taics is always generated as a DC voltage. If electric thrusters are needed for the mission, very-high-voltage DC power (kilovolts) will be required, perhaps from a nuclear prime-power source. Each of these options will impose D&T technology requirements that are, in most cases, not yet at TRL 3. The risks associated with developing the needed D&T technologies are high over the next two decades, but there may be no alternative to addressing those needs if the needed power is to be delivered to the load with acceptable mass, volume, and efficiency constraints. NASA is well suited to lead development of advanced D&T technologies. Advances in this technology will likely require significant participation from both industry and academia for voltage selection, architecture technol - ogy option development, and advanced transmission and insulation materials. The ISS provides limited benefit to the development this technology, as testing can be accomplished on the ground using vacuum chambers and test fixtures. In addition, the introduction of high voltages on the ISS could pose safety risks, which would increase the cost and schedule of any ISS test program. However, in-space testing is ultimately required (on the ISS or some other platform) to validate new D&T technology in the space environment. For example, in-space testing is needed to address plasma interactions and micro-meteoroid impacts. The panel determined that advanced D&T technology could provide significant benefits in terms of system mass reduction due to the major reductions in cable harness mass that could be achieved with high-voltage sys - tems. Advanced technologies in this area would be broadly applicable to many classes of NASA and non-NASA space systems, and potentially to a broad range of terrestrial systems as well. The risk was judged to be medium to high, within the typical range of NASA technology programs. Technology 3.3.5, Conversion and Regulation The voltage and current of electrical power available on any particular spacecraft will be dictated by the power source and the power management and distribution architecture. Various payloads will then most likely require the power in a different form, such as higher voltage for electric propulsion. The purpose of electrical power conver - sion and regulation is therefore to provide the necessary bridge between the power source and payloads, and to regulate this power to within the tolerances required by the payloads. Currently unresolved issues include the need to (1) space qualify existing terrestrial high-voltage components and (2) replace space qualified components that currently lag significantly behind the commercial state of the
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140 NASA SPACE TECHNOLOGY ROADMAPS AND PRIORITIES art. Important parameters for improving power conversion and regulation devices include increasing conversion efficiency, operating temperature range, and radiation tolerance. Advanced conversion and regulation technology with relatively near-term application is at TRL 4. Future space power generation and distribution systems are likely to operate at high voltages than current systems to increase efficiency. Higher current ratings, lower switching and conduction losses, and higher junction temperature tolerances would improve the functionality of future system components. The need for high-voltage regulation is also associated with some electric propulsion technologies that require high voltages (kilovolts) to function. Development of high-voltage regulation capabilities would require a major project that includes the development of many new technologies and facilities. The next generation technology for power conversion and regulation is at TRL 2 to 3. An example of advanced conversion and regulation technology is a higher band gap material such as silicon- carbide or gallium-nitride that would replace the traditional silicon materials in switching components, thereby increasing device operating temperature and efficiency while decreasing mass and volume. Another example is advanced magnetics for improved conversion and regulation devices. NASA has the extensive capabilities, equipment, and facilities needed to lead research and technology develop - ment of advanced conversion and regulation technology. NASA will need to collaborate with industry and academia to leverage the power electronics advances being made for other applications, including commercial uses. Advance - ments that NASA makes in this area will have application to future electric vehicles and commercial smart-grid systems where power is generated and regulated close to its point of use. Some aspects of technology development can potentially be accomplished jointly with offices of the DOD and DOE. Qualification of the hardware in the space environment could be facilitated by attaching an experimental payload externally to the ISS, providing a low-risk, in-space demonstration. Conversion and regulation of power was highly ranked because it was identified as potentially providing a major improvement in mission performance, as well as having broad applicability across several NASA mis - sion areas. The main benefits of advanced power conversion and regulation on system performance come both directly in the form of lower subsystem mass, as well as indirectly in the form of better conversion efficiencies that provide higher power margin. These benefits are especially important at the higher power levels needed for electric propulsion systems or high-bandwidth communications. Increasing the efficiency of power conversion could potentially reduce the size of solar arrays, batteries, and thermal control systems by more than 10 percent on lower power systems, with a bigger impact for higher power systems. Conversion and regulation was assessed to have broad application across the general aerospace community, as all spacecraft use power conversion and regulation components. The technical risk associated with future advancement of this technology is moderate to high, which is a good fit to NASA’s level of risk tolerance for technology development, and the likely cost to NASA and the timeframe to complete technology development is not expected to substantially exceed that of past efforts to develop comparable technologies. Technology 3.2.1, Batteries Batteries are electrochemical energy storage devices that have been flown in space since the beginning of the space age. Battery technology has advanced continuously, and further high-payoff improvements are possible through recent scientific discoveries. In space, batteries must survive a variety of harsh, sometimes unique envi - ronments and load profiles are more demanding than for most terrestrial applications. Many batteries are already proven in space, at TRL 9, but a variety of advanced chemistry alternatives have yet to be developed and qualified for spaceflight. After relying on nickel-cadmium and nickel-hydrogen rechargeable batteries for decades, the aerospace indus - try is now moving to lithium-ion (Li-ion) batteries as the standard energy storage component for space systems. Li-ion technology provides a substantial improvement in specific energy, charge and discharge rates, and cycle life at high depths-of-discharge. Li-ion batteries are also being used as a primary battery (where the battery is used once and not recharged) in applications such as launch vehicle electric power systems.
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141 APPENDIX F NASA missions would benefit from new electrochemical power technologies that offer higher specific energy and/or higher specific power. NASA has world-class expertise in space battery technology, with capabilities at multiple centers including JPL, Glenn Research Center, Goddard Space Flight Center, and Ames Research Center. NASA is best qualified to lead development of advanced battery technology for their unique mission needs. Ide - ally, research in this technology would leverage commercial technology developments, as NASA did with the development of Li-ion batteries for space applications. Rechargeable battery technology is advancing rapidly to meet the commercial needs of electric vehicles, cell phones, laptop computers/tablets, and renewable energy systems. NASA would therefore benefit from collabora - tions with DOE and commercial industry in developing advanced battery technology. DOE’s Advanced Research Projects Agency-Energy (ARPA-E) is pursuing a wide range of advanced battery technologies, and the commercial automotive industry is investing in efficient batteries for plug-in hybrid/electric vehicles (where longevity, energy density, and reliability are quite important, as they are in space applications. The ISS can be an asset in testing some advanced batteries (such as those with liquid electrodes or electrolytes) in a relevant thermal, electrical, and microgravity environment. The steering committee assessed the benefit of battery technology to be significant due to the potential to reduce mass for many space systems and to enable missions in extreme environment missions. The alignment of this technology with NASA’s needs is high due to its potential impact on both robotic science and human explora - tion capabilities; its alignment with other aerospace and national needs is also high because batteries are so widely used. NASA can capitalize on the investments by other government and commercial organizations that are making substantial investments in advanced battery technologies. However, the unique requirements posed by NASA mis - sions in extreme environments do require NASA-specific research and development with moderate risk. Technology 3.1.4, Radioisotope Radioisotope power systems (RPSs) provide power to scientific and human exploration missions over long periods almost anywhere in the solar system and beyond. RPSs have enabled many unique deep space and plan - etary exploration missions, making important scientific discoveries possible. RPSs used plutonium-238 (Pu-238) as a heat source, and they have used thermoelectric converters since 1961 to provide reliable electrical power for many missions throughout the solar system, including Pioneer, Viking, Viking landers, Galileo, Ulysses, Apollo 12-17, Cassini, and New Horizons. Demonstrated mission operating lifetimes have exceeded 30 years. Future RPSs could be developed to deliver both lower power levels (watts or fractions of a watt) and higher power levels (hundreds of watts up to 1 kW). The higher power systems would enable radioisotope electric propulsion for deep space missions, making several new classes of missions possible. While RPSs have a well-established foundation, there are significant technology issues that must be overcome to maximize the effectiveness of the United States’ dwindling supply of available Pu-238. DOE no longer has the ability to produce Pu-238 (except for very small amounts for research), and the United States has purchased all available Pu-238 from Russia. No other country, including Russia, is currently producing Pu-238, and multiple studies have shown that there is no other available radioisotope material that can meet even a significant fraction of NASA’s RPS needs. Supported by recent NRC studies (NRC, 2010, 2011), NASA and DOE have been attempt - ing to restart production of a limited annual quantity of Pu-238 for the past few years, but Congress has not yet provided funding to the DOE and/or NASA for this purpose.3 NASA and DOE have been developing advanced RPSs that would use Stirling engines to replace thermoelectric converters. Because the energy conversion efficiency of the Stirling engine under development is about 5 times that of thermoelectric converters, Stirling engines require significantly smaller quantities of Pu-238 to achieve similar power levels. Given the scarcity of Pu-238 (which will persist for years even after Pu-238 production is approved and funded), the much higher efficiency of Stirling engines is necessary if RPSs are to be available for NASA’s 3 By statute, the U.S. DOE is the only federal agency authorized to produce nuclear material. Congressional action through October 2011 indicates that the 2012 budget for DOE will include no funds to restart production of Pu-238. The NASA budget for 2012 may include up to $10 million for this purpose, but it remains to be seen when the DOE will be able to restart production of Pu-238.
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142 NASA SPACE TECHNOLOGY ROADMAPS AND PRIORITIES planned science and exploration missions. As discussed in Chapter 3, establishing a reliable, recurring source of Pu-238 and maturing Stirling engine technology are both critically important to provide power for NASA’s future science and exploration missions that cannot rely on solar power. Radioisotope technology using Stirling engines is currently assessed to be at TRL 6. Although some compo - nents have been demonstrated at higher TRLs, a flight test is needed to advance beyond TRL 6. Using the ISS to demonstrate this technology is not an option due to restrictions regarding operation of nuclear power systems in LEO. RPS technology is somewhat unique to NASA, as interplanetary space missions are the only driving need that has been identified to justify restarting Pu-238 production. NASA and DOE have the unique capabilities and facilities necessary to develop RPSs. By statute, DOE must be responsible for the nuclear aspects of RPS technol - ogy development. NASA Glenn Research Center has led the development of Stirling engines and the Jet Propulsion Laboratory leads NASA efforts in RPS development and spacecraft integration. In assigning the QFD scores for this technology, the panel assumed that Pu-238 production and Stirling technology development would continue as currently planned by NASA. As noted above, RPS technology was selected as a high-priority technology despite its relatively modest QFD score because this technology is criti - cally important to the future of NASA’s deep space missions. The committee assessed the benefit of additional investments in RPS technology to be low because there are few good options with the potential to improve on the performance of RPSs that couple Stirling engines with Pu-238 heat sources. However, as noted above, this rating would be much higher if those technologies were not already being developed. The alignment of this technology to NASA’s needs is high due to the high impact on both robotic science and human exploration capabilities, while alignment with other aerospace and national needs is considered to be low. The risk is assessed to be moderate to high, within the bounds of NASA’s acceptable risk levels for technology development. The cost of the power system development is expected to be moderate. Thus, RPS technology would be assessed as a medium-priority technology based on its QFD score, which is based on two assumptions: (1) the current program for Stirling engine development is continued and (2) domestic production of Pu-238 is restored in a timely fashion. Given that the second assumption remains in doubt, the panel overrode the QFD score to assign this technology a high priority. MEDIUM- AND LOW-PRIORITY TECHNOLOGIES TA03 includes 14 level 3 technologies that ranked low or medium priority. This includes two technologies that were added to the TABS for completeness. Thermal storage was added due to its potential to improve energy storage mass, as compared to advanced batteries, for many special purpose applications. For example, if a heat- engine-based power system is used, energy can be efficiently stored thermally instead of using an electrochemical system. In situ resource utilization (ISRU) is another application where thermal energy storage might be preferable, especially if thermally based processes are used. Electric and magnetic field storage was added as a technology to cover many advancing technologies such as super-capacitors, ultra-capacitors, and superconducting magnetic energy storage. In general, these technologies are useful options when storage times are very short such as peak load management for a high-power-radar instrument. Seven of the eight technologies that were assessed to be low priority were judged to have marginal benefits to NASA missions within the next 20 to 30 years. These technologies included energy harvesting, flywheels, regenerative fuel cells, electric and magnetic field storage, green energy impact, alternative fuel storage, and wireless power transmissions. The marginal benefit (less than 10 percent improvement) evaluation was based on an assessment of the expected improvement, at the system level, in the primary parameter of interest for each technology. In most cases, this was improvement in spacecraft mass or reliability that the panel believed could be achieved given reasonable investments in that technology. While higher claims have been made for some of these technologies, such as flywheels or electric and magnetic field storage, the panel’s review of available information did not produce any credible technology development paths that would achieve the ambitious performance levels specified in the draft roadmap with reasonable investments. Also, currently available approaches for advancing these technologies tended to have a lower risk level than is usually appropriate for NASA technology investments. The remaining low-priority technology, fusion, was judged to provide no likely value to NASA in the next 20 to 30 years due to a very low probability of success during that timeframe.
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143 APPENDIX F The six power and energy storage technologies that were ranked as medium priority included four that had marginal benefit scores and two that had more substantial benefits, but application of those benefits was limited to a small set of mission areas. The four with marginal benefits included fault detection, isolation, and recovery (FDIR), management and control, analytical tools, and multi-functional structures. While the benefit of these technologies was assessed to be similar to the low-priority technologies discussed previously, all four ranked higher because any advancement would apply to all or nearly all NASA and non-NASA space missions in all or most mission classes. In other words, success in these technologies helps everyone. The remaining two medium- priority technologies, thermal storage and chemical power generation, were judged to provide substantial benefit to a smaller set of mission opportunities. Thermal storage applications include heat engine power systems and ISRU, as described above. Chemical power generation, including fuel cells and heat engines, may be valuable in human exploration missions where large quantities of hydrogen and oxygen are being used for propulsion. In these cases, power can be generated essentially for free using the boil-off from the propellant tanks. DEVELOPMENT AND SCHEDULE CHANGES FOR THE TECHNOLOGIES COVERED BY THE ROADMAP Schedules for Space Power and Energy Storage technologies are highly dependent on the level of funding applied to the development programs. The schedules depicted in the roadmap are generally feasible if sufficient resources are applied to each item in the roadmap. OTHER GENERAL COMMENTS ON THE ROADMAP Space Power and Energy Storage is related to several other technical areas. Many challenging requirements arise from high-power electric propulsion applications discussed in TA02. Heat rejection from power and energy storage components relies on technologies from the thermal control systems covered by TA14. Advances in many power technologies are possible due to advancing materials technologies discussed under the materials and struc - tures technologies in TA12 and nanomaterials covered by TA10. PUBLIC WORKSHOP SUMMARY The workshop for the Space Power and Energy Storage technology area was conducted by the Propulsion and Power Panel on March 21, 2011, on the campus of the California Institute of Technology in Pasadena, California. The discussion was led by panel member Douglas Allen, who began with a general overview of the draft roadmap and the NRC’s task for this study. He also provided some direction for what topics the invited speakers should cover in their presentations. Experts from industry, academia, and government were invited to lead a 25 minute presentation and discussion of their perspective on the draft NASA roadmap for TA03. At the end of the session, there was a short open discussion by the workshop attendees that focused on the recent session. At the end of the day, a concluding discussion led by Allen summarized the key points observed during the day’s discussion. Session 1: Solar Arrays Ed Gaddy (Applied Physics Laboratory) started the solar arrays session with a discussion on properly mea - suring the impact of technology improvements. His argument was that often the cost impact of weight saving technology improvement is significantly underestimated. This can lead to under-investment in advanced technolo - gies. He also observed that solar array history has shown that slow and steady investments, which lead to slow and steady technology improvements can be highly successful and, over time, this approach can yield revolutionary improvements in capabilities. He advocated support for Inverted Metamorphic Multijunction (IMM) photovoltaic cells as the next evolutionary step forward in this technology. Finally, he stated operational missions need better insight into the benefits of adopting specific new technologies and/or incentives should be established to encourage mission managers to adopt advanced technologies that have achieved an appropriately high TRL.
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144 NASA SPACE TECHNOLOGY ROADMAPS AND PRIORITIES Alan Jones (ATK) began his review of the TA03 roadmap by saying that its near-term goals were generally achievable, but the long-term goals would be much more challenging. He then reviewed the solar power array chal - lenges that are specific to NASA missions, such as very low or very high environmental temperatures. He endorsed NASA investment in IMM photovoltaic cell technology, but added that support for advanced array technologies is also needed. In particular, he suggested that investing in wing-level structural platforms is needed to fully realize the benefits of the new cells. Ted Stern (Vanguard Space Technologies) discussed integrated array manufacturing methods that could reduce array cost and weight and increase array reliability. He also discussed a concept of modular solar arrays that would simplify spacecraft design by making all arrays using the same method, which would allow for easier qualification. He also discussed the advantages and disadvantages of near-term technology for solar concentrators to improve solar array performance. For the long-term, he supported investments in spectrum converting and multi-photon enabled photovoltaics that may provide game-changing performance. Finally, he noted the value in showing the relevance of improved solar arrays to terrestrial spinoffs. Paul Sharps (Emcore Photovoltaics) began by providing some information on Emcore and noting that it is one of two U.S. developers and manufacturers of high-efficiency multi-function solar cells and arrays for space applications. He noted that while solar cells have received continuous DOD funding for 25 years, there has been little improvements in cell integration and array technologies. He provided his perspective on IMM technology, stating that Emcore had achieved 34 percent energy conversion efficiency with this technology in the laboratory, and work is progressing with concepts that may achieve 37 percent efficiency. (State-of-the-art cells have less than 30 percent conversion efficiency.) Sharps believes that a 34 percent efficient, radiation hard IMM cell is 2 to 3 years away from being inserted into flight hardware. He suggested that NASA focus its photovoltaic cell invest - ments on this technology, adapting it for NASA specific requirements such as very-high- and very-low-temperature environments and the very low light intensity experienced in deep space. In the group discussion the speakers were asked to forecast where state-of-the-art array technology might be in the next 20 years. The speakers seemed to generally agree that a power density of 400 W/kg will be available in the near term, but that long-term forecasting is very difficult given that the next generation of technology that will be successful beyond IMM remains to be determined. The group discussion also covered the benefits that NASA is likely to receive from investing its own resources in solar array technology. The responses included arrays built for NASA specific environments (particularly missions to outer planets, the inner planets, and the Sun), quicker deployment of advanced technologies, and the ability to act as a smart buyer of array technology provided by industry. Session 2: Power Storage Joe Troutman (ABSL Space Products) started the session on power storage by noting that Li-ion technology has driven recent advances in battery performance. He said that basic Li-ion battery technology has been proven, but more work is needed to improving cell performance and safety, primarily through the use of advanced cathode materials and anode coatings. He also suggested that increasing cell voltage would lead to higher energy density and cycle life. Michael Tomcsi (Quallion) said that Li-ion batteries have shown a steady improvement in energy density (W-hr/kg) of about 6 to 8 percent per year for the past 15 years. He then discussed the different materials that can be used to improve batteries in the future. He suggested that small improvements of a few percent per year are achievable. Tomcsi asserted that metallic lithium batteries may provide a leap in performance, although significant safety issues must be overcome. In the group discussion both speakers agreed that cathode advancements will be the biggest challenge in future battery improvements. For advancements needed to support NASA specific needs, they suggested that low- temperature performance could be improved with some loss in performance, but the high-temperature requirements stated in the draft roadmap for TA03 are very challenging and may be achievable only with a completely new type of battery. There was also some discussion on better modeling for battery development and the challenge of developing good physics-based battery models to replace current empirical models.
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145 APPENDIX F Session 3: Power System Engineering The purpose of this session was to obtain insights into how solar-based power systems work as a whole and the performance improvements that could arise from advanced technology. Robert Francis (The Aerospace Corporation) began the session by discussing some of the general issues related to improving solar-based power systems. He noted that the DOD has a technology roadmap with specific performance numbers in terms of energy density and volumetric efficiency for solar arrays. This DOD roadmap projects improvements that are about half the level of improvement proposed in the draft NASA roadmap for TA03. Francis thinks that most of the mass savings in future solar power systems will come from improvements to solar array supporting structures—not from improvements in the solar cells mounted on the arrays. He noted some of the challenges in moving to very-high-power arrays include the need for higher voltages for the spacecraft electrical bus and difficulties in ground testing. Azam Arastu (Boeing) focused most of his talk on the two advanced, lightweight, and compact, solar arrays currently under development at Boeing with the support of the Defense Advanced Research Projects Agency (DARPA) and the U.S. Air Force Research Laboratory. Both the Fast Access Testbed Spacecraft (FAST) and the Integrated Blanket/Interconnect System (IBIS) arrays are capable of generating very high power and employ highly efficient IMM solar cell technology and advanced deployment structural concepts to provide significant improvements in the specific power over the current state of the art. The FAST array uses linear solar concentra - tors and is less expensive as it uses fewer solar cells. It also is more tolerant to radiation and more effective in the low-light environments of deep space, but it has more precise pointing requirements. The IBIS array, which is constructed using thin, flexible, and planar solar modules, packs more compactly than FAST and has less precise pointing requirements. Both array systems are designed to be highly modular and scalable so they can eventually be assembled into systems providing solar arrays capable of generating upwards of 300 kW of power. The group discussion spent some time on the issue of modularity with both speakers saying that the slight increase in weight caused by modular systems is outweighed by the savings due to simplicity and the ability to mass produce components. When asked why NASA needs to invest in arrays when there is already a robust technology development, the speakers suggested that NASA can focus its investment on spacecraft integration issues and the unique environmental requirements for operation in space. One of the speakers also said that even non-financial support from NASA could help ensure technological progress continues. Finally there was some discussion on bus voltage: 200 V DC was identified as a near term possibility; 300 V DC possible over the long term; and improving performance beyond that achievable with 300 V DC may require shifting from a DC power system to an AC power system. Session 4: Nuclear Systems Joseph Nainiger (Alphaport) began the session on nuclear systems by discussing the history of RPSs and fis - sion power systems. He noted that while the United States has extensive flight experience with RPSs, it has only flown one fission power system (in 1965) and has not manufactured and ground tested a fission nuclear thermal rocket since 1972. Over the near term, RPSs will continue to fill the need for a reliable source of electrical power in environments that cannot use solar power (even with advances in solar power technology). Fission systems are enabling for missions with high power demands independent of sun proximity or illumination. Nainiger praised the current effort at NASA to develop and test the non-nuclear components of a fission power system, while adding that a system capable of producing at least 1 MW of electrical power will only be possible with a sustained and aggressive technology development effort. He noted that fission power systems will require a significant invest - ment in infrastructure, and he suggested that efforts should be taken to capture technology developed in the past. Naininger also asserted that restarting the production of Pu-238 is a critical national need. Gary Bennett (formerly with NASA) concurred that NASA has a critical need for Pu-238. He postulated that more than half of the missions proposed in the recent planetary decadal survey (NRC, 2011) would benefit from an RPS, if enough Pu-238 were available. He discussed the new RPSs under development and urged NASA to focus on the Stirling engine power converter that is at the heart of the Advanced Stirling Radioisotope Generator
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146 NASA SPACE TECHNOLOGY ROADMAPS AND PRIORITIES (ASRG). To advance fission power technology, he recommended starting small, using ideas that are known to work and evolving over time with an emphasis on safety and reliability. He said that NASA should acknowledge the high cost and complexity of fission space power systems, and he cautioned against chasing new fission power concepts that are unrealistic and unproven, with no technological foundation. H. Sterling Bailey (formerly with General Electric) focused his remarks on fission power systems, which he supports as a game-changing technology that could dramatically expand NASA’s science and exploration mission capabilities. He noted the programmatic challenges of fission power and the legal requirement that NASA partner with DOE in developing nuclear systems. He also said that the history of fissions space power systems has been characterized by bold efforts that are cancelled before development of operational systems is completed. As a result, confidence in this technology is low and the pool of personnel experienced with this technology is small and shrinking. He advocated the development of a fission space power system that could produce 1 kW of electricity, and he strongly supported ongoing work by NASA’s Fission Surface Power System Technology Project. Bailey cautioned against adopting a megawatt-class system as a goal for a new fission space power program; it would be better to start small. The group discussion period spent some time on what size fission system should be built. Most of the speakers supported development of the 40 kW system that NASA is currently funding. (The current effort, which is funded entirely by NASA, does not include the development of nuclear technology.) In the end, system performance requirements should be based on the requirements of whatever missions are expected to use the system. Several speakers noted that efforts to develop new nuclear power systems typically face both technological and political challenges. A member of the NRC panel noted that the latter is outside the scope of this study, which is focused on technology. REFERENCES NRC (National Research Council). 2006. Priorities in Space Science Enabled by Nuclear Power and Propulsion. The National Academies Press, Washington, D.C. NRC. 2010. Radioisotope Power Systems: An Imperative for Maintaining U.S. Leadership in Space Exploration. The National Academies Press, Washington, D.C. NRC. 2011. Vision and Voyages for Planetary Science in the Decade 2013-2022. The National Academies Press, Washington, D.C. Patel, M.R. 2005. Spacecraft Power Systems. CRC Press, Boca Raton, Fla.