Dynamic Radioisotope Power System Development for Space Explorations Jeffrey Rusick A. Lou Qualls Paul Schmitz NASA Glenn Research Center Oak Ridge National Laboratory NASA Glenn Research Center 21000 Brookpark Road 21000 Brookpark Road P. O. Box 2008 Cleveland, OH 44135 Oak Ridge, TN 37831-6165 Cleveland, OH 44135 216-522-1260 865-574-0259 216-433-6174 [email protected] [email protected] [email protected]

Dirk Cairns-Gallimore June F. Zakrajsek Dave F. Woerner U. S. Department of Energy NASA Glenn Research Center NASA Jet Propulsion Laboratory 19901 Germantown Road 21000 Brookpark Road 4800 Oak Grove Drive Germantown, MD 20874 Cleveland, OH 44135 Pasadena, CA 91011 301-903-3332 216-977-7470 818-393-2000 dirk.cairns- [email protected] [email protected] [email protected]

Abstract—Dynamic power conversion offers the TABLE OF CONTENTS potential to produce Radioisotope Power Systems (RPS) that generate higher power outputs and utilize the Pu- 1. INTRODUCTION ...... 1 238 radioisotope more efficiently than Radioisotope 2. DESIGN CONSIDERATIONS...... 2 Thermoelectric Generators. Additionally, dynamic 3. RELIABILITY ...... 4 systems offer the potential of producing generators with 4. SPACECRAFT AND MISSION INTEGRATION ...... 5 significantly reduced mechanical degradation over the ESIRABLE CONVERTOR FEATURES course of deep space missions so that more power will be 5. D ...... 5 available at the end of the mission when it is needed for 6. NEW OPPORTUNITY AND APPROACH TO FLIGHT .. 5 both powering the science and transmitting the results. 7. CONCLUSION ...... 6 The development of dynamic generators involves ACKNOWLEDGEMENT ...... 6 addressing technical issues not typically associated with REFERENCES ...... 6 traditional thermoelectric generators. Developing long- BIOGRAPHY ...... 6 life, robust and reliable dynamic conversion technology is challenging yet essential to building a suitable generator. Considerations include working within 1. INTRODUCTION existing handling infrastructure, where possible, so that Radioisotope power systems (RPSs) for NASA deep space development costs can be kept low and integrating science missions have historically used static dynamic generators into spacecraft, which may be more thermoelectric-based designs, which have proven to be complex than integration of static systems. Methods of highly reliable. Within thermoelectric generators, interfacing to and controlling a dynamic generator must radioisotope heat sources are passively cooled as long as also be considered and new potential failure modes must waste heat is properly removed from the generator housing. be taken into account. This paper will address some of Generators based on dynamic power conversion systems the key issues of dynamic RPS design, development and offer energy conversion efficiencies nominally four times adaption. higher than heritage thermoelectric designs. However, dynamic convertors become significant active coolers of the Index Terms—Dynamic Power Conversion, General radioactive heat sources and the failure of a convertor can Purpose Heat Source (GPHS), isotope, radioisotope, potentially lead to large and sudden power decreases and radioisotope power system (RPS), Radioisotope increased internal generator temperatures. Increased Thermoelectric Generator (RTG) efficiency proportionately reduces the amount of radioisotope fuel needed for the same electrical power output.

U.S. Government work not protected by U.S. copyright.

This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication,1 acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

Recently, a significant effort to develop a dynamic RPS, the Dynamic power systems have the potential to 1) produce Advanced Stirling Radioisotope Generator (ASRG) [1], higher electrical power outputs within the same volume shown in Fig 1., was conducted by NASA and the while using less Pu 238, and 2) experience very little Department of Energy (DOE). Under this effort, many mechanical conversion efficiency degradation over the dynamic system development issues were addressed and course of long missions because of the virtual elimination of successfully demonstrated. Key differences of dynamic wear mechanisms within the convertor itself. These systems include 1) vibration from moving parts, 2) an advantages come with the need to 1) develop the alternating current output from potentially multiple technology, 2) demonstrate sufficient performance, convertors, 3) the necessity of a generator controller, and 4) degradation rates, and reliability, and 3) accommodate the coping with potential convertor degradation and potential new operating characteristics on a spacecraft within the failure. The development becomes a trade among context of a mission. performance, complexity, cost, and risk. These trades are under investigation as part of the Dynamic Radioisotope NASA working with the DOE has initiated a technology Power System (DRPS) project, which is a joint NASA/DOE maturation effort that has two major efforts: one that project funded by the NASA Radioisotope Power System matures the dynamic technology of the convertors called Program. Dynamic Convertor Technology (DCT) and an effort that grounds the technology maturation from a mission and system perspective call Dynamic RPS (DRPS) engineering. The DCT and DRPS engineering efforts have collaborated and have developed a path that will lead to the successful development of a dynamic RPS. Key efforts by this team include 1) a procurement activity to obtain from industry mature convertor designs and evaluate the designs and hardware to meet NASA requirements, 2) the establishment of a Surrogate Mission Team to develop the generator system and interface requirements, 3) engagement with industry to assess technologies to understand the technology Fig. 1. Diagram of ASRG components and systems. readiness and risks to consider for a near-term flight development activity, and 4) generator concept evaluation Radioisotope Thermoelectric Generators (RTGs) are proven activity that assesses the potential characteristics of a space system power sources. The currently available system, generators that would use a particular technology. the Multi-Mission Radioisotope Thermoelectric Generator Generator requirements include exsiting hardware and (MMRTG)[2], shown in Fig. 2, has a beginning of mission ground processing constraints and mission considerations power of approximately 110 We. It has an annual such as requirements that have the generator use existing degradation rate on the order of 4%/year, due to the natural and available infrastructure and requirements that are decay of the Pu 238 heat source and performance derived from a large possible mission set that identify the degradation of the thermoelectric materials. This results in most likely needs of the largest number of missions that are reduced electrical power output when the spacecraft arrives of interest to NASA. Second level requirements include as destinations far from Earth. Thus, the power available for consideration of detailed performance metrics such as mass, science and communication of data from that science is volume, vibration, thermal, radiation and EMI signatures. limited when it is most needed for outer planet exploration. The generator concept evaluation activity is looking at the characteristics of convertor technology and formulating conceptual level generators based on them. This provides an understanding of the inherent trades and limitations in dynamic generator design. These initial conceptual designs will be modified and improved as information from industry becomes available for consideration.

2. DESIGN CONSIDERATIONS Certain aspects of RPS development are consciously limited due to the investment and time required to modify them. The heat source to be used is the Step 2 General Purpose Heat Source (GPHS). It is available, produced today, and has been recently launched. GPHSs power NASA’s Curiosity Rover that is currently operating on Mars as part of its MMRTG power system. It is prohibitively expensive to modify because of the investment required to qualify a Fig. 2. Diagram of MMRTG components and systems. new or modified heat source. Thus, the GPHS will be the 2

heat source for the dynamic generator. The infrastructure reduces the potential for sequential convertor loss due to used to fuel, test, and ship a RPS can also be expensive to internal thermal issues. For a dynamic system, the loss of a modify. Gloveboxes at the Idaho National Laboratory (INL) convertor may result in a significant increase in the amount have a limited opening that equipment must pass through, of heat the system must reject, which can potentially lead to and the final generator must fit within the available shipping damaging temperatures. Previous work on Stirling container used to transport a completed RPS from INL to Radioisotope Generators (SRG) estimated that for some the launch complex. New shipping containers can take many convertor failures the majority of the temperature rise will years and millions of dollars to develop and qualify. Also, occur within the first few hours of the failure. Figs. 4 and 5 the available transportation vehicles are compatible with the show a 4-GPHS, 4-Stirling convertor generator concept. currently available shipping cask design and would require Simulations of the dynamic system operation in Matlab modification or replacement if another cask were used. Simscape [3] show that after a single convertor failure, Thus, the dynamic generator will use GPHS, shown in Fig without adjusting the stroke of the remaining convertors, the 3., as the basic heat input block and fit within an existing temperature rise in an adjacent convertor occurs over shipping cask. approximately 5 hours (failure occurs at hour 69 and completes by hour 74) and is over 300K, as seen in Fig. 6.

Fig. 3. Exploded diagram of GPHS and modular combination example.

The major components of generator are heat sources, Fig. 4. Plan view: 4 GPHS, 4 Stirling convertor generator conversion components, insulation, and a housing. In the concept. MMRTG, Eight GPHSs are used to produce ~110 We at the beginning of a mission. The GPHSs are stacked in the center of the housing, surrounded by thermoelectric convertor modules and insulation. Insulation within a generator serves as both thermal insulation to reduce parasitic heat loss and as structural support for the internal components. There are a limited number of materials that retain both the needed structural integrity and the insulating performance at the temperatures of interest. MinK-1400 insulation is commonly considered and its operation is generally limited to approximately 800°C. Insulation materials that operate above that of MinK generally do not retain the same level of structural integrity.

For a dynamic RPS, the failure of a convertor can result in the loss of a primary heat sink for the GPHS, which can lead to increased temperatures. If the insulation exceeds allowable operating ranges, then the insulation may not have Fig. 5. Elevation view: GPHS, 4 Stirling convertor generator the necessary strength to absorb mechanical energy during concept. entry into a science environment. Additionally, failed insulation could interfere with the thermal balance or operation of other convertors within a generator. Thus a single convertor failure could propagate to another, eventually leading to the failure of the entire generator. The ASRG used two GPHSs and Stirling convertors to produce approximately the same power as an MMRTG. Placing the heat sources at opposite ends of the ASRG generator 3

5) require a controller.

3. RELIABILITY Power systems for deep space exploration require high reliability to ensure operation for the many years required to reach its science destination. Thus, the components in the system must have high reliability, low wear features, and must be engineered into a robust generator. Dynamic components can be made highly reliable with no maintenance. Think for example of the compressors in home or automobile air conditioners or turbochargers for engines.

Within an MMRTG there are 16 thermoelectric modules Fig. 6. Simulation results of temperature increase for connected electrically in series to produce from 28 to 32 individual convertor with failure of adjacent convertor. volts. Each thermoelectric module contains 48 thermoelectric couples arranged in a series/parallel ladder Additionally, the temperature of the adjacent convertor arrangement for additional redundancy for a total of 768 exceeds the insulation and the Stirling convertor thermoelectric elements. The degradation, or even failure, of temperature limits. Given the assumptions in this generator a few elements has a small impact on overall operation. model, the controller would need to autonomously detect the However, with dynamic power systems each convertor will failed convertor and adjust the operation of the remaining output a large fraction of system power. If a generator has convertors within the generator to prevent generator failure. only a single convertor, the loss of that single convertor The response to failures is different depending on the would represent the total loss of a generator. If a system redundancy strategy. Also, spared convertors or generators with two convertors were to lose a convertor, then may take time to begin producing power compared to those nominally 50% of output power would be lost. (However, continually running below peak output. the loss may be slightly higher because the controller losses remain relatively constant.) A dynamic generator could use any viable conversion technology, but the most common considered are Stirling, Thus, a mission needing high reliability must use reliability Thermoacoustic, Brayton, and Rankine. Stirling and and redundancy strategies to achieve an acceptable thermoacoustic convertors have oscillating forces along probability of success. With dynamic power systems the their axes that can induce vibration unless they are internally choices include one or a combination of the following: 1) self-balancing. Rotating equipment can impose have a single highly reliable convertor and generator, 2) uncompensated torque unless self-balanced. Generally accommodate multiple convertors within a single generator, impulse forces from unbalanced convertors can be or 3) carry multiple generators on the mission. A single accommodated using one or a combination of three convertor generator, although possible, may require some strategies: 1) balance the force with an identical convertor components to counter impulse forces. If the mission operating out of phase or in the opposite direction, 2) use an requires full power under all failure scenarios, strategies to active balancer, or 3) isolate the generator from the produce nominal full power within a generator in the event spacecraft. Each strategy has its benefits and detractions. of the loss of a single convertor (a presumed credible but For example, if a convertor is balanced using a second worst-case operational scenario) include operating all convertor and one of them fails, both convertors may be convertors below their peak operating capability and required to be shut down to prevent excessive vibration. increasing power to compensate or to carry spares.

The basic characteristics of a dynamic generator therefore Table 1 shows the expected probability of failure (POF) as include: well as the projected overall reliability of a two-convertor dynamic generator. The basic controller unit in this case is 1) use of Step 2 GPHS, one of the components with a high POF. Assuming a 2) fit within the existing, cooled ~2” × ~4” shipping controller is composed of three identical Stirling controllers, cask that has limited capacity for electrical each of which is capable of controlling and synchronizing a feedthroughs and no capacity for direct generator single convertor, and each of these cards had an estimated cooling, POF of 5.9%, when configured with one of the cards in 3) utilize either self-balancing convertors, or an even standby mode, the controller has a decreased POF of ~1.3%, number of balanced convertors or they require as opposed to 5.9% if just two cards are used. If a dual card some form of active balancing, controller with an active balancer is used (one controller 4) designed to manage temperatures in the event one card per convertor) the overall system reliability would have convertor fails, which is presumed to be a credible dropped from an estimated 96.9% to 91.1%. scenario, and

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Table 1. Comparison of a controller component 5. DESIRABLE CONVERTOR FEATURES configurations and probabilities of failures Desirable features of power conversion components within a Case Generator Generator dynamic generator include high conversion efficiency, high Controller has Using two-card Description reliability, and inherently safe operation during a temporary backup card ACU loss of controller function. The trades inherent within a ACU 1.29% 5.91% generator design are the power level of the generator, power ASC 1.76% 0.91% level of individual convertors within a generator, number of Housing 0.07% 0.07% convertors, the number and location of GPHSs, methods to Balancer 0.00% 2.00% move and reject waste heat, balancing strategies, and Extra Alternator 0.00% 0.00% response strategies when a convertor fails. System POF 3.10% 8.90% Stirling Reliability 96.9% 91.1% The power level target for dynamic systems within the DRPS project is bounded on the lower end by the power Future RPSs may mimic the parallel string architecture of levels of the existing MMRTG, which is a 100 W-Class RTG systems to gain additional system level reliability. For system. Science missions generally do not use power levels example, using a single string (convertor, housing, and above 500 We. Thus, the target power range for dynamic controller) with reliability of 91% and allowing ¼ of those systems is approximately 200 to 500 We. The power levels strings to fail can increase overall generator reliability to pursue are dependent on programmatic issues and depending on the number of strings used. Table 2 shows the technology capability. It is essential to develop a generator generator level reliability given ¼ of the convertors fail if around a reliable convertor. The reliability that can be each is treated as an independent single string. Using 4 achieved with available technology will to some degree convertors/housing/controller strings and allowing a single dictate redundancy strategies. string to fail, an estimated 95.8% reliability can be achieved. However, if the total number of convertors is increased to 6. NEW OPPORTUNITY AND APPROACH TO FLIGHT 16 and 4 are allowed to fail, the power system reliability is an estimated 99%. Because RPS missions typically are A dynamic power system will be a new and unique design. flown on high cost NASA missions, high reliabilities are While some constraints are fixed, others areas are open. A required for each of the spacecraft subsystems. This may new design provides an opportunity to re-think generator require multiple convertors strings in a single generator or design. It is difficult and expensive to develop and qualify multiple generators on a mission, or both. flight hardware. It is also difficult to meet the needs of all missions with a single generator design. Thus, the concept Table 2. Generator level reliability considerations with 25% of modularity may be useful for facilitating a dynamic failure rate. Assuming each convertor is a single string. power system into service. The principle would be to Total Number of develop components once but to combine and use them in 4 8 12 16 Convertors (N) different ways. This approach could be used with a series of # Convertors increasingly challenging missions that serve as qualification 3 6 9 12 Required (R) activities for following missions. Chance of Providing Full 95.8% 97.2% 98.3% 99.0% The heart of a dynamic power system is the convertor. The Power at EOM highest priority is to have a robust and reliable convertor. A single convertor with a nominal efficiency of 20% could potentially produce 50 We from a single GPHS. This might 4. SPACECRAFT AND MISSION INTEGRATION represent the smallest dynamic generator of interest or a An attractive feature of the thermoelectric system is its portion of the GPHS heat could be used to warm the inherent direct current output and the lack of a controller. spacecraft and less electrical power could be produced. A generator with a single unbalanced convertor would Dynamic systems will require a controller. The controller probably require an active balancer. A pair of 25 We will need to function in an active way when interfacing with convertors could produce the same power and would not the spacecraft as opposed to passively operating if the spacecraft is experiencing electrical issues. Thus, the need the active balancer, but one could be added to allow interaction between the dynamic generator and the for continued operation in the event of a convertor failure. spacecraft electrical system is expected to be more complex In this case, the GHPS would increase in temperature if only actively cooled by one of the two convertors, so it follows compared to a static DC bus of an RTG. Also, a controller that convertors with higher peak power levels could be used may need to take action based on environmental changes or to produce full power after the loss of one convertor. The convertor performance degradation (or failure) and this action may have to occur before a human could intervene. upper limit is to use two 50 We generators and to normally (The Pluto New Horizons spacecraft takes about 4 ½ hours operate them at half power. Using this approach, it is to receive a signal from Earth.) important that the convertor have the capability to operator off peak power with limited loss of performance. Lower 5 peak power level can also be considered. power ranges of interest. A set of criteria is being assembled to assess available technologies. A part of this assessment is Basic modular technologies can be developed, tested and an evaluation of how specific conversion technologies even flown and the initial investment in those critical integrate into a generator and how that generator would elements would occur only once. Components would be integrate with a spacecraft to accomplish missions of configured or potentially upgraded to make higher power interest to NASA. As information about existing systems. For a dynamic system, a key identifier is the conversion technology becomes available generator number of GPHS used. This determines heat input and performance expectations will be studied in more detail and (depending on convertor efficiency) electrical power output. these studies are anticipated to result in a set of realistic A four GPHS system would have a nominal heat input of system requirements that can be used to develop a dynamic 1000 Wt and could have an electrical power output ranging power system for flight. from 200-300 We (depending on the efficiency of the convertors and the design of the generator). The number of ACKNOWLEDGEMENT convertors will, to some degree, be determined by the optimal size of the convertors. A reliable generator will This work is sponsored under the NASA Radioisotope have a convertor that resides within a comfortable design Power System Program Office within NASA’s Science space. Mission Directorate’s Planetary Science Division.

A similar, spiral-development approach can be considered This material is based upon work supported by the U.S. for generator lifetime requirements. Outer planet missions Department of Energy Office of Nuclear Energy under can require a power system lifetime of ~17 years. It is contract number DE-AC05-00OR22725. difficult to gather enough data on first generation equipment to confidently predict such long life operation. Thus a new REFERENCES dynamic power system could be designed for a 17-year life but used initially for shorter duration missions and their [1] Chan, J., Connolly, D., LeRoy,R,, Quinn, R., Shultze, E., performance beyond the original mission need could be “Concept for an ASRG Hosted Payload Mission,” monitored to build confidence for its use on longer Proceedings of 2014 IEEE Aerospace Conference, Big missions. Sky, MT, March 1-8, 2014.

Critical design elements for a dynamic generator include the [2] Woerner, D. F., Moreno, V., Jones, L., Zimmerman, R., convertor itself and the controller that goes with it. The Wood, E., “The Mars Science Laboratory (MSL) controller must consider the ability to operate multiple MMRTG In-Flight: A Power Update,” Proceedings of convertors together and communicate and integrate with Nuclear and Emerging Technologies for Space 2013, spacecraft operation. Ways of integrating multiple Albuquerque, NM, February 25-28, 2013, Paper 6748. convertors to take heat from GPHS and to fit within [3] “MATLAB and Statistics Toolbox, ” 2014b ed. Natick, available shipping infrastructure are key elements of design. Massachusetts, United States: The MathWorks, Inc., Methods to accommodate possible failure scenarios must be 2014. considered. This would include strategies to manage temperature excursions and external forces in the event of a convertor failure. A robust generator would be designed to BIOGRAPHY sustain operation after the loss of a convertor as well with Lou Qualls graduated from the the expectation that most, if not all, of the science could still University of Tennessee with a Ph.D. in be acquired. Higher power generators are possible if more nuclear engineering in 1991 and is GPHS are used. The target level of a generator is a currently a Distinguished Researcher at programmatic issue as well as a technical issue. If most Oak Ridge National Laboratory. He missions require 300 We of power and no mission requires supports the Department of Energy in more than 500 We, then two 300 We generators can collaborations with NASA. accommodate all mission needs while most would require only one. Reliability issues may however dictate the use of multiple generators. Paul Schmitz was born in Chicago, Ill,

in 1963 and currently resides in Avon 7. CONCLUSION Lake, OH. He has B.S. in Physics from It is expected that development of a dynamic radioisotope Sam Houston State University, a M.S. system will follow from the selection of the most promising Degree in Physics from Case Western conversion technologies that lend themselves to generator Reserve University and a M.S. in integration and spacecraft operation. Within the DCT and Nuclear Engineering from Texas A&M DPRS engineering effort, NASA and DOE are looking to industry for promising convertor technology within the He is nuclear systems engineer at the NASA Glenn Research Center (GRC) currently focused on 6 radioisotope power systems. He began work at GRC in Radioisotope Power System Program. Woerner has 1989 and has worked on both the SP-100 program and worked at JPL on such missions as Galileo, Cassini, Jupiter Icy Moons Orbiter. Beyond the early years Magellan, Mars Pathfinder, and MSL. He was the Chief focused on nuclear reactors he has worked on a wide Engineer of the avionics for the Mars Pathfinder mission range of projects as diverse as radioisotope power that successfully landed on Mars on July 4, 1996. He is systems, high altitude power IC engines for atmospheric the Chair of the Board of Directors for the IEEE science and fuel cells for uninterruptible power supplies Aerospace Conferences. He has won numerous NASA and long endurance aircraft. He is currently focused on awards including earning NASA’s Exceptional Service analysis of Dynamic Radioisotope Generators. and Exceptional Achievement Medals.

Jeffrey Rusick received a B.S. in Physics from Miami University of Ohio in 1974, and a Masters in Nuclear Engineering in 1976 from Ohio State University. His career started at General Electric and then Babcock and Wilcox working on advanced breeder reactor research for the Department of Energy (DOE), until he was hired by NASA to work on the International Space Station electrical power system in 1987. He has been a Chief Safety and Mission Assurance Officer (CSO) at NASA Glenn for the last 10 years, responsible for promoting mission success for a variety of highly visible NASA programs and projects, including the Radioisotope Power System Program.

June F. Zakrajsek has over 20 years of aerospace systems development, research and project management experience. She has led internal discipline teams for space systems health management, ISS power systems analysis, and Biotechnology. She has worked as a project manager in the areas of health management, systems engineering and analysis, propulsion system development, Orion Crew Module and Test & Verification, and Radioisotope Power Systems. Currently June serves as the Program Planning and Assessment Manager for NASA's Radioisotope Power Systems Program. This area is responsible to develop and maintain the implementation strategy for the Program by managing mission and systems analysis functions, integration of new technology into generators, and interfaces with potential missions considering utilizing Radioisotope Power Systems. She holds a Masters in Biomedical Engineering from Case Western Reserve University and Masters and Bachelors in Mechanical Engineering.

David Woerner has more than 30 years’ experience as a systems engineer and manager at JPL including as the MMRTG Office Manager for the Mars Science Laboratory mission. He is presently leading the engineering of an enhanced MMRTG and is the RTG Integration Manager and Deputy Program and Planning Manager for NASA’s

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