Mission analysis for next-generation RTG study

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.

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.

Chapter Seventeen - Space nuclear power: Radioisotopes, technologies, and the future

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.

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.

Saturn remains one of the most fascinating planets within the solar system. To better understand the complex ring structure of this planet, a conceptual Saturn Ring Observer (SRO) mission is presented that would spend one year in close proximity to Saturn’s A and B rings, and perform detailed observations and measurements of the ring particles and electric and magnetic fields. The primary objective of the mission would be to understand ring dynamics, including the microphysics of individual particles and small scale (meters to a few kilometers) phenomena such as particle agglomeration behavior. This would be accomplished by multispectral imaging of the rings at multiple key locations within the A and B rings, and by ring‐particle imaging at an unprecedented resolution of 0.5 cm/pixel. The SRO spacecraft would use a Venus‐Earth‐Earth‐Jupiter Gravity Assist (VEEJGA) and be aerocaptured into Saturn orbit using an advanced aeroshell design to minimize propellant mass. Once in orbit, the SRO would stand off from the ring plane 1 to 1.4 km using chemical thrusters to provide short propulsive maneuvers four times per revolution, effectively causing the SRO vehicle to “hop” above the ring plane. The conceptual SRO spacecraft would be enabled by the use of a new generation of multi‐mission Radioisotope Power Systems (RPSs) currently being developed by NASA and DOE. These RPSs include the Multi‐Mission Radioisotope Thermoelectric Generator (MMRTG) and Stirling Radioisotope Generator (SRG). The RPSs would generate all necessary electrical power (⩾330 We at beginning of life) during the 10‐year cruise and 1‐year science mission (∼11 years total). The RPS heat would be used to maintain the vehicle’s operating and survival temperatures, minimizing the need for electrical heaters. Such a mission could potentially launch in the 2015–2020 timeframe, with operations at Saturn commencing in approximately 2030.

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.

In the past decade, CubeSats have emerged as a cost-effective solution for scientific missions beyond Earth’s orbit, though they have not yet gone further than orbiting Mars, as with NASA’s MarCO CubeSats. This paper discusses the technical challenges and solutions for designing CubeSats for Outer Solar System missions, specifically in the context of the proposed Astraeus Mission to Titan. These CubeSats, called the Mites, aim to measure heavy anions in Titan’s upper atmosphere and have undergone analysis to determine their optimal size and drag coefficient. A significant challenge addressed is the CubeSats’ power system, with solar panels being less effective at greater distances from the Sun. The paper proposes investigating Radioisotope Power Systems (RPS) as an alternative. However, there is the additional challenge of packaging the RPS in a sufficiently small form factor so that the upper atmospheric experiment can be completed. This must cover the greatest range of longitudes, latitudes, and ranges hence the orbital decay rate must be controlled to achieve this. The paper also explores the Mites’ long-duration exposure to space during transit and strategies to minimise cosmic radiation exposure. Whilst this is completed in the context of the Astraeus Mission, the data obtained can guide similar missions and aid others in overcoming the limitations of CubeSats so that they can be used more frequently for Outer Solar System science missions. Keywords: Titan, CubeSat, Radioisotope Thermoelectric Generator, Orbital Decay, Outer Solar System

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.

The National Research Council's solar system exploration decadal survey identified Jupiter's moon Europa as its highest-priority destination for near-term exploration. Voyager and Galileo missions to the Jupiter system provided evidence consistent with a subsurface ocean on Europa, which is of great interest as a potential abode for extraterrestrial life. This paper describes a conceptual flagship-class Europa orbiter concept that was assumed to launch as early as 2012, arriving at Europa approximately 8 years later using inner solar system gravity assists to reach Jupiter. Jupiter's intense radiation environment limits the mission duration at Europa to 30 days for this study, though the duration is a result of multiple trades and is by no means fixed. The Europa subgroup of the outer planets assessment group identified six primary science objectives for this concept. An /spl sim/150-kg instrument suite selected for the study addresses those objectives. Large heliocentric distances, high power levels required, and especially the harsh Jovian radiation environment drove the selection of radioisotope thermoelectric generators (RTGs) for all onboard electrical power, with the excess heat aiding spacecraft thermal control. Mass and architecture trades were performed using different spacecraft trajectories, launch vehicle types, radioisotope power systems, and mission durations. The study shows that new mission constraints allow a scientifically compelling Europa orbiter mission that might also deliver a Europa lander.

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.