The Euratom PULSAR project

The alkali metal thermal to electric converter (AMTEC) used in the Advanced Radioisotope Power System (ARPS) program has been developed around a specific system configuration that was derived from the radioisotope thermoelectric generator (RTG) used for the last 20 years. This configuration, and the restrictions it places on the converter technology, limits the overall generator performance. An advanced AMTEC system design, that is not based on RTG heritage, using AMTEC chimney cells, is better suited for the AMTEC technology and provides substantially better power density and efficiency. It also provides system scale-ability from 10 to 100 Watts. While technology development continues to proceed with the ARPS cell design in order to validate the technology, efforts are also underway to develop the advanced technology to meet the more aggressive mass, efficiency, life and electromagnetic performance requirements of future missions. INTRODUCTION The alkali metal thermal to electric converter (AMTEC) has been considered to be a potential candidate for small radioisotope space power systems since the early 1980's. Recent converter technology advances and system design work by Hendricks et al. and Schock et al.' have shown these systems to be very attractive relative to the state of the art radioisotope thermoelectric generators (RTG's), currently used in all spacecraft radioisotope power system. The AMTEC technology not only reduces the system mass relative to RTG technology, it also requires significantly less radioactive material than the RTG's for any given power requirement. From mid-1997 to mid-1999, the Advanced Radioisotope Power System (ARPS) program, was focused on developing the first AMTEC power system for the Europa and Pluto missions. This power system, which has been described in the literature, is the first generation of AMTEC space systems. It is derived from the RTG system technology and has several features in common with the RTG systems. • The cells are mounted to the inside of a cylindrical housing. This The general purpose heat source (GPHS) is stacked, and the stack is held from each end in much the same way as the RTG. The cells use the same mounting feature used in the RTG unicouple. The cell must support the weight of the multilayer insulation package, system is 14% efficient and 21 kg for approximately 100-120 W, end of mission power. The use of RTG system reduced the system level design effort, however it also increased the AMTEC cell development risk by requiring that the technology be configured for the system as opposed to configuring the system to meet the technology needs. The combination of developing the technology under the constraint of the RTG heritage requirements and a number of changes in the spacecraft power requirements made AMTEC technology development very difficult. During the course of the two years of development, there were two major changes in system power level and one change in configuration. This resulted in 3 different cell revisions that were actually designed and tested. The final E-revision cell is shown in Figure 1. Figure 1. ARPS AMTEC Cell, Revision E. American Institute of Aeronautics and Astronautics (c)2000 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization. With the challenges of developing the materials and fabrication processes for the refractory metal cell (previous designs were stainless steel), these design changes made it very difficult to mature the fabrication process. In mid-1999 DOE and NASA made the decision that AMTEC would not be ready for the first planned ARPS mission in 2003 or 2004. Cell testing had not yet reached the point of demonstrating reliable performance and was not yet ready for life testing at that time. Cell development has continued since mid-1999 and substantial progress has been made, with cells now capable of producing power at or near predicted levels and, in most cases, to operating reliably at nominal and elevated temperatures. These cells are still designed for the RTG heritage system, and while they are not the design that would provide the highest efficiency or power, they are a baseline to fully develop and prove the technology for mission use. Life testing could begin early next fiscal year. ADVANCED SYSTEM DESIGN An AMTEC system design that is significantly lower mass than the current ARPS design has been described in the literature. This design is not constrained by RTG heritage. It places the cells at only two ends of the heat source stack. A new method is used to support only two GPHS. It eliminates the cell mounting features used in the ARPS cell. This system is over 20% efficient and 8.9 kg for approximately 100 W, end of mission. Closure Plate Cell Interconnects

The European Space Agency is funding the research and development of 241Am-bearing oxide-fuelled radioisotope power systems (RPSs) including radioisotope thermoelectric generators (RTGs) and European Large Heat Sources (ELHSs). The RPSs’ requirements include that the fuel’s maximum temperature, Tmax, must remain below its melting temperature. The current prospected fuel is (Am0.80U0.12Np0.06Pu0.02)O1.8. The fuel’s experimental heat capacity, Cp, is determined between 20 K and 1786 K based on direct low temperature heat capacity measurements and high temperature drop calorimetry measurements. The recommended high temperature equation is Cp(T/K) = 55.1189 + 3.46216 × 102 T − 4.58312 × 105 T−2 (valid up to 1786 K). The RTG/ELHS Tmax is estimated as a function of the fuel thermal conductivity, k, and the clad’s inner surface temperature, Ti cl, using a new analytical thermal model. Estimated bounds, based on conduction-only and radiation-only conditions between the fuel and clad, are established. Estimates for k (80–100% T.D.) are made using Cp, and estimates of thermal diffusivity and thermal expansion estimates of americium/uranium oxides. The lowest melting temperature of americium/uranium oxides is assumed. The lowest k estimates are assumed (80% T.D.). The highest estimated Tmax for a ‘standard operating’ RTG is 1120 K. A hypothetical scenario is investigated: an ELHS Ti cl = 1973K-the RPSs’ requirements’ maximum permitted temperature. Fuel melting will not occur.

The Pluto Express Fly-By program requires a radioisotope power system (RPS) to supply spacecraft power for various internal functions and mission instruments and experiments. AMTEC (alkali-metal thermoelectric conversion) power conversion is the DOE-selected technology for an advanced, high-efficiency RPS to power the Pluto Express Fly-By spacecraft. An AMTEC-based RPS using the general purpose heat source (GPHS) has been conceptually designed to satisfy the Pluto Express power requirements. Integrated AMTEC cell and system thermal/electrical design analyses, structural design analyses and mass analyses were performed to define an optimum system design. Using fresh radioisotope fuel at beginning of mission, the RPS produces 102 watts of power, has a mass of 8.35 kg (specific power density=12.2 watts/kg), with a system conversion efficiency of 20.3%. Mass/power scale-up estimates have also been generated, indicating that a 150-watt version of this RPS would weigh approximately 11.3 kg. This paper presents and discusses the key features of this RPS design, the design and analysis methodology, and the numerous system and AMTEC cell tradeoff studies establishing the optimum AMTEC-based RPS.

NASA's Radioisotope Power Systems (RPS) Program began formal implementation in December 2010. The RPS Program's goal is to make available RPS for the exploration of the solar system in environments where conventional solar or chemical power generation is impractical or impossible to meet mission needs. To meet this goal, the RPS Program manages investments in RPS system development and RPS technologies. The current keystone of the RPS Program is the development of the Advanced Stirling Radioisotope Generator (ASRG). This generator will be about four times more efficient than the more traditional thermoelectric generators, while providing a similar amount of power. This paper provides the status of the RPS Program and its related projects. Opportunities for RPS generator development and targeted research into RPS component performance enhancements, as well as constraints dealing with the supply of radioisotope fuel, are also discussed in the context of the next ten years of planetary science mission plans.

The focus of the National Aeronautics and Space Administration's (NASA) Radioisotope Power Systems (RPS) Development program is aimed at developing nuclear power and technologies that would improve the effectiveness of space science missions. The Radioisotope Power Conversion Technology (RPCT) NASA Research Announcement (NRA) is an important mechanism through which research and technology activities are supported in the Advanced Power Conversion Research and Technology project of the Advanced Radioisotope Power Systems Development program. The purpose of the RPCT NRA is to advance the development of radioisotope power conversion technologies to provide higher efficiencies and specific powers than existing systems. These advances would enable a factor of 2 to 4 decrease in the amount of fuel and a reduction of waste heat required to generate electrical power, and thus could result in more cost effective science missions for NASA. The RPCT NRA selected advanced RPS power conversion technology research and development proposals in the following three areas: innovative RPS power conversion research, RPS power conversion technology development in a nominal 100We scale; and, milliwatt/multi‐watt RPS (mWRPS) power conversion research. Ten RPCT NRA contracts were awarded in 2003 in the areas of Brayton, Stirling, thermoelectric (TE), and thermophotovoltaic (TPV) power conversion technologies. This paper will provide an overview of the RPCT NRA, a summary of the power conversion technologies approaches being pursued, and a brief digest of first year accomplishments.

NASA Glenn Research Center (GRC), the Department of Energy (DOE), and Stirling Technology Company (STC) are developing a Stirling convertor for an advanced radioisotope power system to provide spacecraft on-board electric power for NASA deep space missions. NASA GRC is conducting an in-house project to provide convertor, component, and materials testing and evaluation in support of the overall power system development. A first characterization of the DOE/STC 55-We Stirling Technology Demonstration Convertor (TDC) under the expected launch random vibration environment was completed in the NASA GRC Structural Dynamics Laboratory. Two TDCs also completed an initial EMI characterization at NASA GRC while being tested in a synchronized, opposed configuration. Materials testing is underway to support a life assessment of the heater head, and magnet characterization and aging tests have been initiated. Test facilities are now being established for an independent convertor performance verification and technology development. A preliminary FMEA, initial FEA for the linear alternator, ionizing radiation survivability assessment, and radiator parametric study have also been completed. This paper discusses the status, plans, and results to date for these efforts.

Since the launch of the first radioisotope power system (RPS) on Transit 4A in 1961, numerous research and development activities have been performed centered on increasing the overall thermal to electrical conversion efficiency of the selected nuclear fueled power system. The latest U.S. fielded RPS (MMRTG - Multi-Mission Radioisotope Thermoelectric Generator) contains eight GPHS (General Purpose Heat Source) modules which nominally yield ∼2000W Th from the thirty-two 238PU5O 2 ceramic fuel pellets. MMRTG's PbTe/TAGS-85 based thermoelectric couples have a thermal to electrical conversion efficiency of ∼5.5% thus yielding ∼110W e at launch. This paper centers on the discussion of a conceptual idea that entails employing a second set of thermoelectrics on an MMRTG. These would be employed for possibly converting a portion of the excess “waste heat” (∼1750 W th ) into additional electrical mission power. First-order experiments and calculations employing bismuth telluride (Bi 2 Te 3 ) based thermoelectric modules being considered for a European RPS by the University of Leicester indicate that an improvement in the efficiency of an MMRTG could be achieved by integrating them with the PbTe/TAGS-85 thermoelectrics being utilized in the MMRTG in a “dual” or “cascaded” arrangement. This arrangement of two different integral thermoelectric materials suggests the intriguing possibility of the additional harvesting of a portion of the MMRTG's currently unutilized “waste heat”. This appears to be feasible since the cold side temperature of the PbTe/TAGS-85 in an MMRTG is ∼200°C which corresponds to typical hot-side operating temperatures of the Bi 2 Te 3 thermoelectric modules. It is recognized that extensive thermo-mechanical-electrical design, modeling, and analysis are required to fully investigate the cascaded thermoelectric concept. In addition, materials compatibility and assembly aspects will need to be fully addressed in the future. The present work indicates that system-level performance gains could be achieved via a “cascaded” or cMMRTG which could result in electrical power increases at BOM (Beginning of Mission) of up to ∼25%, and perhaps more significantly, gains in EODL (End of Design Life) power of up to ∼40% which could be utilized on a future space mission.

The Office of Space and Defense Power Systems of the Department of Energy (DOE) provides Radioisotope Power Systems (RPS) for applications where conventional power systems are not feasible. For example, radioisotope thermoelectric generators were supplied by the DOE to the National Aeronautics and Space Administration for deep space missions including the Cassini Mission launched in October of 1997 to study the planet Saturn. For the Cassini Mission, ORNL produced carbon-bonded carbon fiber (CBCF) insulator sets, iridium alloy blanks and foil, and clad vent sets (CVS) used in the generators. The Oak Ridge National Laboratory (ORNL) has been involved in developing materials and technology and producing components for the DOE for more than three decades. This report reflects program guidance from the Office of Space and Defense Power Systems for fiscal year (FY) 2004. Production and production maintenance activities for flight quality (FQ) CBCF insulator sets, iridium alloy blanks and foil, and CVS are summarized in this report. In all three cases, production maintenance is assured by the manufacture of limited quantities of FQ components. Technology activities are also reported that were conducted to improve the manufacturing processes, characterize materials, or to develop information for new radioisotope power systems.

The Office of Space and Defense Power Systems of the Department of Energy (DOE) provides Radioisotope Power Systems (RPS) for applications where conventional power systems are not feasible. For example, radioisotope thermoelectric generators were supplied by the DOE to the National Aeronautics and Space Administration for deep space missions including the Cassini Mission launched in October of 1997 to study the planet Saturn. The Oak Ridge National Laboratory (ORNL) has been involved in developing materials and technology and producing components for the DOE for more than three decades. For the Cassini Mission, for example, ORNL was involved in the production of carbon-bonded carbon fiber (CBCF) insulator sets, iridium alloy blanks and foil, and clad vent sets (CVS). This report has been divided into three sections to reflect program guidance from the Office of Space and Defense Power Systems for fiscal year (FY) 2002. The first section deals primarily with maintenance of the capability to produce flight quality (FQ) CBCF insulator sets, iridium alloy blanks and foil, and CVS. In all three cases, production maintenance is assured by the manufacture of limited quantities of FQ components. The second section deals with several technology activities to improve the manufacturing processes, characterize materials, or to develop technologies for new radioisotope power systems. The last section is dedicated to studies related to the production of {sup 238}Pu.

The Office of Space and Defense Power Systems of the Department of Energy (DOE) provides Radioisotope Power Systems (RPS) for applications where conventional power systems are not feasible. For example, radioisotope thermoelectric generators were supplied by the DOE to the National Aeronautics and Space Administration for deep space missions including the Cassini Mission launched in October of 1997 to study the planet Saturn. For the Cassini Mission, ORNL produced carbon-bonded carbon fiber (CBCF) insulator sets, iridium alloy blanks and foil, and clad vent sets (CVS) used in the generators. The Oak Ridge National Laboratory (ORNL) has been involved in developing materials and technology and producing components for the DOE for more than three decades. This report reflects program guidance from the Office of Space and Defense Power Systems for fiscal year (FY) 2005. Production activities for prime quality (prime) CBCF insulator sets, iridium alloy blanks and foil, and CVS are summarized in this report. Technology activities are also reported that were conducted to improve the manufacturing processes, characterize materials, or to develop information for new radioisotope power systems.

The Office of Space and Defense Power Systems of the U. S. Department of Energy (DOE) provides Radioisotope Power Systems (RPS) for applications where conventional power systems are not feasible. For example, radioisotope thermoelectric generators (RTG) were supplied by the DOE to the National Aeronautics and Space Administration (NASA) for deep space missions including the Cassini Mission launched in October of 1997 to study the planet Saturn. For the Cassini Mission, the Oak Ridge National Laboratory (ORNL) produced carbon-bonded carbon fiber (CBCF) insulator sets, iridium alloy blanks and foil, and clad vent sets (CVS) used in the generators. ORNL has been involved in developing materials and technology and producing components for the DOE for more than three decades. This report reflects program guidance from the Office of Space and Defense Power Systems for fiscal year (FY) 2009. Production activities for prime quality (prime) CBCF insulator sets, iridium alloy blanks and foil, and CVS are summarized in this report. Technology activities are also reported that were conducted to improve the manufacturing processes, characterize materials, or to develop information for new RPS.

Radioisotope heater units (RHU) and radioisotope thermoelectric generators (RTG) are currently being developed for the ESA radioisotope power system program. The state-of-the-art for the USA and Russian systems is to use plutonium-238 as the radioisotope fuel; however, for the ESA applications americium-241 has been selected due to its availability and relatively cost-effective production in the European context. The proposed designs implement a multi-layer containment approach for safety reasons, with a platinum-rhodium alloy for the inner containment of the fuel and carbon-based materials for the outer layers. The Am-fueled RHU provides 3 W of thermal power, and makes this design competitive with existing models in relation to specific power. The heat source for the RTG has a 6-side polygonal shape, with a distributed 3-fuel pellet architecture: this configuration allows to maximize the specific power of the RTG, since Am-based fuels have a lower power density than Pu-based fuels. The heat supplied by the fuel is 200 W, with an expected electrical power output of 10 W provided by six Bi-Te thermoelectric modules. Finite element structural and thermal analyses have been performed to assess the theoretical feasibility of the components as initially conceived. Mechanical and electrically-heated prototypes for the systems have already been tested in a representative lab environment at the University of Leicester; these tests have provided initial estimates for the efficiency of the systems. Both the RHU and RTG architectures are currently undergoing a new design iteration process. This paper reports on the overall architecture and design of the Am-fueled RTG and RHU, the modelling results and the experimental data obtained so far.

The Office of Radioisotope Power Systems (RPS) of the Department of Energy (DOE) provides RPS for applications where conventional power systems are not feasible. For example, radioisotope thermoelectric generators were supplied by the DOE to the National Aeronautics and Space Administration (NASA) for deep space missions including the Cassini Mission launched in October of 1997 to study the planet Saturn. For the Cassini Mission, ORNL produced carbon-bonded carbon fiber (CBCF) insulator sets, iridium alloy blanks and foil, and clad vent sets (CVS) used in the generators. The Oak Ridge National Laboratory (ORNL) has been involved in developing materials and technology and producing components for the DOE for more than three decades. This report reflects program guidance from the Office of RPS for fiscal year (FY) 2008. Production activities for prime quality (prime) CBCF insulator sets, iridium alloy blanks and foil, and CVS are summarized in this report. Technology activities are also reported that were conducted to improve the manufacturing processes, characterize materials, or to develop information for new RPS.

The Johns Hopkins University Applied Physics Laboratory (APL) has been tasked by the NASA Heliophysics Division to (re-)study a robotic Interstellar Probe mission. The top-level requirement is to provide input to support the next Solar and Space Physics “Decadal Survey” in the United States, with a nominal time frame of performance from 2023 to 2032.The study approach is to look widely across both relevant scientific and technical communities and assemble a “Menu” of what has been, and what can be, done with respect to Interstellar Probe desires and concepts past. By its nature this assemblage is a “superset” of what might be implemented; “ordering” from the menu will be a charge to a future Science Definition Team – at NASA’s discretion. This approach has been adopted successfully in the past, with the emphasis on informing the Survey participants of valid possibilities, while not dictating a “best” solution.The components of such a menu are not random. They must flow from compelling top-level science goals, through explicit measurement requirements, instrumentation to make those measurements, and an assessment of how those measurements have provided “closure” to the investigation, i.e. have addressed the science goals.For this type of engineering “menu” study, payload instrument possibilities and capabilities, with representative masses and power requirements, are required to help assess the overall spacecraft cost, mass, and achievable speed for a given launch vehicle configuration. Details have and will continue to evolve as the engineering aspects mature and as more people throughout the international scientific community continue to contribute their ideas.Neither critical trade-offs nor enabling technologies are new. For example, with a given launch system, the total energy that can be imparted to the spacecraft is fixed. Thus, with a given set of planetary gravity assists there is a trade between the maximum asymptotic escape speed from the Sun and the total mass of the spacecraft, which, in turn, tends to scale with the payload mass. As a starting point, a range of spacecraft masses from ~300 to 800 kg, corresponding roughly to Pioneer (251.8 kg) through Voyager (825.4 kg), has been considered. Communication has focused on microwave downlink (X-band or Ka-band), which is well developed, known, and robust. While optical laser comm might achieve far higher downlink rates, it requires extreme pointing stability, and the associated lifetime also needs continuing investigation. The need and capability for radioisotope power systems (RPS) for powering deep-space robotic spacecraft operating far from the Sun is well established  and will be required here as well. Also, in this study the use of the Space Launch System (SLS) cargo version is in use along with upper stages. Other “lunar capable” launch vehicles have also been “spot checked,” but more complete performance data is needed from their respective vendors for a definitive assessment.As with any other study of this type, initial engineering requirements must be imposed to begin an inherently iterative process of design. Engineering requirements are needed to frame the engineering study and “bound the box” – but allow for trades. As with other aspects of this study, these are also still evolving:(1)       Enable a mission that can be launched no later than 1 January 2030 (technology driver)(2)       Have the capability to operate from a maximum range of not less than (NLT) 1000 astronomical units (a.u.) from the Sun (communications system driver)(3)       Require no more than 600 Watts of electrical power (We) at the beginning of mission (BOM) and be able to operate at no less than half of the BOM amount at the end of mission (EOM) (power system driver)(4)       Achieve a mission lifetime of NLT 50 years with an “acceptable” probability of success (drives parts program, physics of failure analyses, programmatics, and policy)Given the requirement of near-term flight, we focus entirely upon ballistic solutions with Jupiter Gravity Assists (JGA) in order to maximize the asymptotic solar system escape speed. Three options are under study: The first two use prograde gravity assists at Jupiter, one passive and one active with an upper stage burn in Jupiter’s gravity well; the third uses a retrograde gravity assist and a powered “Oberth maneuver” near the Sun.The current focus is upon a medium-mass, all-heliophysics payload to obtain a first cut of basic architecture approach and initial subsystem master equipment list (MEL) with power requirements and margins and reserves, as appropriate. This is being used to size the communication system, avionics, and guidance and control and data systems. It will also serve as a baseline for looking at both upscale and downscale payload and capability options, and trades with planetary and astrophysics science options, including effects on required spacecraft concept of operations and the payload mass, power, and cost tradespaces.The current effort is well into this initial concept definition via seven focus studies:1) Longevity - Spacecraft lifetimes/failures, long-lasting systems, failure modes, ground operations, staff renewal, and knowledge documentation and retention 2) Instruments - Candidate payload components with parameters + operating requirements 3) Trajectory and launch vehicle trades including Jupiter’s radiation and dust ring keep-out zone(s) 4) Communication and guidance and control trades, achievable pointing accuracy 5) Thermal shield requirements and possibilities for a near-Sun maneuver, including quantifying shield mass, radiation pressure effects, and center-of-mass migration and attitude control requirements during a near-Sun, rocket-motor burn 6) Overall mechanical layout, fairing clearances, payload adaptors, instrument clear fields of view, boom and antenna options, deployments, and associated pointing control 7) Power requirements, including payload and spacecraft systems, parasitic heating of propellant lines, surge capability trades, and shunt requirements for use with a Next Generation (NG)-RPS The study continues to solicit input on all of these topics from the space science and engineering communities via meetings and workshops. We remain on track for delivering the final report to NASA’s Heliophysics Division in late calendar year 2021.

NASA Glenn Research Center developed a nonnuclear representation of a Radioisotope Power System (RPS) consisting of a pair of Advanced Stirling Convertors (ASCs), Dual Convertor Controller (DCC) EMs (engineering models) 2 and 3, and associated support equipment, which were tested in the Radioisotope Power Systems System Integration Laboratory (RSIL). The DCC was designed by the Johns Hopkins University Applied Physics Laboratory (JHU/APL) to actively control a pair of ASCs. The first phase of testing included a Dual Advanced Stirling Convertor Simulator (DASCS), which was developed by JHU/APL and simulates the operation and electrical behavior of a pair of ASCs in real time via a combination of hardware and software. RSIL provides insight into the electrical interactions between a representative radioisotope power generator, its associated control schemes, and realistic electric system loads. The first phase of integration testing included the following spacecraft bus configurations: capacitive, battery, and super-capacitor. A load profile, created based on data from several missions, tested the RPS's and RSIL's ability to maintain operation during load demands above and below the power provided by the RPS. The integration testing also confirmed the DCC's ability to disconnect from the spacecraft when the bus voltage dipped below 22 volts or exceeded 36 volts. Once operation was verified with the DASCS, the tests were repeated with actual operating ASCs. The goal of this integration testing was to verify operation of the DCC when connected to a spacecraft and to verify the functionality of the newly designed RSIL. The results of these tests are presented in this paper.