Editor's Note: This is an excerpt from the summary of 'Expanding Frontiers with Radioisotope Power Systems,' a report prepared by the Jet Propulsion Laboratory and California Institute of Technology through an agreement with NASA. Click on the image to the right to get a copy of the full 112-page report.
The purpose of this report is to identify the range of mission concepts and applications that could be enabled by the newest generation of standard multi-mission radioisotope power systems, the MMRTG and SRG. It describes the results of the most recent set of JPL mission studies using realistic estimates of RPS performance, and provides information for review by potential users that may benefit from these types of power systems. The report also identifies the possible advantages of each type of standard RPS unit as a function of mission category and application. Also identified are the potential operating environments (pressure, temperature, atmospheric composition and g-load) that future spacecraft, and thus the standard RPSs, may encounter. This data is meant to benefit the RPS technology community in assessing the environmental operating requirements of the MMRTG and SRG units. This report also provides a current set of top-level performance requirements for each standard RPS type to assist the mission studies community in performing realistic system trades using radioisotope power systems.
This report is divided into three sections. Section 1 summarizes the results of the activities to date and lists the space science and human precursor missions that could potentially be enabled by standard RPSs. Section 2 presents the detailed results of four mission concept studies that demonstrate the overall feasibility of standard RPS-powered missions and summarizes the toplevel goals and objectives of the remaining missions identified in this study. Section 3 summarizes the current technical performance characteristics of the MMRTG and SRG units for mission planning purposes.
Twenty-seven potential missions and applications were identified in this study (Table 1-2) that could potentially be enabled by standard RPS technology. These concepts were, in many cases, based on the priorities defined in the Decadal Surveys of the National Academies , or support the goals and objectives outlined in the Vision for Space Exploration . Nine concepts are space missions, eight are mobility missions (e.g., aerobots and surface rovers), five are lander missions, and five are human base infrastructure support applications. Two flight projects are also included in the table (MSL and Solar Probe) as they are currently baselining the standard RPS power source.
Detailed studies were performed for four mission concepts, including a Triton lander, a DualMode Lunar Rover Vehicle (DMLRV), a Titan Aerobot, and a Saturn Ring Observer. The results of the mission studies indicate that the MMRTG and SRG each have distinct benefits with regards to their use on deep space missions. The following paragraphs identify the potential benefits of each RPS system for a given set of mission parameters, and suggest the favored RPS unit for a given mission configuration.
The MMRTG would be well suited for missions able to utilize the excess heat (~1900 Wt at BOM) generated by its eight GPHS modules, compared with the SRG?s two GPHS modules (~400 Wt of excess heat at BOM). Missions that could potentially benefit from the excess RPS heat are those that would operate in extremely cold environments such as the surface of Europa, Titan, and permanently shadowed areas of the Moon. These concepts could potentially use the excess RPS heat to maintain spacecraft operating temperatures (i.e., via heat pipe systems, etc.) in place of electric heaters, potentially freeing up electrical power for instruments and other subsystems.
The MMRTG utilizes thermoelectric conversion, which is a vibration-free process. This would potentially make the MMRTG better suited for missions using vibration-sensitive instruments (e.g., seismometers) that measure low-amplitude motions (such as seismic activity from tectonic motions or volcanic events). Though the SRG uses synchronous opposed Stirling converters  to minimize vibration, it remains a dynamic conversion process and could have residual motion that might impact sensitive seismic measurements.
The MMRTG would be favored from the perspective of proven reliability and lower technical risk. MMRTG technology is very mature, sharing significant design heritage with the SNAP-19 RTGs used on the Viking surface missions, with the MHW-RTGs used on the Voyager deepspace missions, and with the GPHS-RTGs used on the Galileo, Ulysses and Cassini deep space missions. The failure modes of the MMRTG are well understood, and are more likely to provide graceful degradation than the SRG.
Initially, the MMRTG could have an advantage from a mass perspective, as current NASA/DOE guidelines recommend that early missions using SRGs carry at least one redundant SRG unit until its reliability has been verified . This means that early missions using SRGs would need to carry a minimum of two SRG units. Thus, for early missions (where a redundant SRG would be required), the MMRTG (at <45 kg ) would be the lighter option for spacecraft requiring one or two RPS units. At three RPSs, the mass difference between using MMRTGs and SRGs (including a mandatory fourth spare unit) becomes minimal, representing a breakpoint from a mass perspective (i.e., the mass of three MMRTGs is nearly the same as four SRG units). Missions requiring more than three RPSs would benefit overall from the SRG?s lighter mass (~34 kg ), even with the addition of one redundant unit. However, the redundant SRG would not simply be a ?dead weight?, and could be used to enhance mission performance, if the spacecraft was able to use the additional power to exceed its baseline performance values. Rover missions, for example, would be well suited to use the added power of the redundant SRG to increase the rover?s speed and range beyond the ?nominal? mission goal. In the event that an SRG were to fail during the mission, the rover would simply return to its ?nominal? power level, having already capitalized on the excess power to achieve enhanced mobility. Once the early SRG-powered missions have flown and the SRG?s reliability successfully demonstrated, the redundant-SRG policy would likely be relaxed making this RPS the lightest option overall.
Both the MMRTG and SRG are specified to have an electrical power output of >110 We at BOM. However, the MMRTG is currently predicted to generate ~125 We in deep space (BOM), and 123 We on the surface of Mars (BOM) . The SRG, on the other hand, is currently predicted to generate 116 We in deep space (BOM), and 103 We on Mars (BOM) . The higher BOM power output of the MMRTG (particularly on Mars) would be preferred from a total power perspective. Note, however, that both standard RPS units are in development, and thus their power outputs continue to evolve as their designs mature.
The SRG could be favored for missions where there would be difficulty in rejecting excess heat to the environment. The SRG generates 25% of the thermal power of the MMRTG, which could be a significant benefit for missions that require the RPS to be housed within an aeroshell (e.g., for atmospheric entry, performing an aerocapture maneuver, etc.) or integrated within a spacecraft fuselage where the heat could not be directly radiated to space. Both the MMRTG and SRG have fluid lines that could be used to cool the RPS in addition to their radiator fins; however, an external pump would be required to operate the fluid loop, and the greater quantity of waste heat from the MMRTG could result in a larger, more complicated pumping system being required relative to that for the SRG.
The radiation levels of the MMRTG and SRG are both relatively low, and not expected to pose any significant issues for most mission concepts. However, for missions that require minimal radiation dose, the SRG has the advantage of generating only 25% of the radiation of the MMRTG due to the SRG's higher conversion efficiency. The radiation dose from the MMRTG and SRG can be further reduced by using additional shielding (with an additional mass penalty) or by physically separating the RPS from the payload or crew.
The higher efficiency of the SRG would also make this RPS favored from the standpoint of fuel conservation. Each SRG contains two GPHS modules corresponding to about 1 kg of Pu-238, while each MMRTG contains eight GPHS modules, corresponding to about 4 kg of Pu-238. Plutonium is an expensive component of the RPS in terms of cost and the time it takes to acquire and manufacture the fuel. The fact that the United States currently does not have the capability to produce its own Pu-238, and must purchase it from foreign sources, makes the more efficient SRG an attractive option from the perspective of making future missions less susceptible to potential fuel shortages.
All missions identified in this study had a maximum g-load requirement expected to be achievable with the existing MMRTG and SRG designs. The greatest accelerations would be expected to occur during launch, atmospheric entry, and landing, and would require an appropriate method (e.g., parachutes, airbags, or Sky Cranes in an atmosphere environment; soft landers in vacuum environment) to reduce the deceleration load below the 30g design requirement. Were the MMRTG and SRG capable of withstanding larger acceleration loads (hundreds of g?s), then airbag landings on the Moon, Europa, Triton, and other bodies with minimal or no atmosphere might be possible. System-level trades would need to be performed to assess the relative mass and cost penalties of a reinforced RPS and whether they were offset by the simpler airbag landing system relative to a soft lander approach.
The minimum lifetime requirement of both RPSs is specified as 14 years from BOM . However, the MMRTG and SRG are expected to be robust units, and there is nothing intrinsic in their design that would prevent them from running longer, albeit at decreasing power levels. The fact that the RTGs on both Voyager spacecraft are still operating nearly 30 years after launch demonstrates this robustness. The thermal and electrical power output from the standard RPSs is expected to gradually and predictably decrease due primarily to 1) Pu-238 decay (MMRTG and SRG), 2) sublimation of the thermoelectrics within the MMRTG, and 3) degradation of the thermal insulation within the SRG.
Both standard RPS designs include integrated radiator fins to reject their excess heat to the ambient environment. These fins make up a sizable fraction of the total physical envelope of the MMRTG, and to a lesser degree with the SRG, and must be properly oriented to the environment to be effective. Both RPS designs also include cooling tubes that could be charged with a working fluid and externally pumped by the spacecraft to reject the excess heat via alternate pathways. For spacecraft concepts having significant size and configuration constraints (e.g., the DMLRV concept of Section 2.4), this study suggests that a variant of the MMRTG or SRG without integrated radiator fins could be beneficial. Heat removal would be performed by an external pumping system using the RPSs cooling tubes and a separate radiator optimized for the overall spacecraft design. The removal of the fins could permit the RPS unit to be closely spaced to other spacecraft subsystems or tightly packed against other RPSs. This would permit greater flexibility in designing the spacecraft, in optimizing the heat rejection system, and could result in a vehicle that is smaller overall. In addition, missions to extremely cold bodies with convective atmospheres (e.g., Titan) may require the fins be significantly shortened or removed completely.
The MMRTG and SRG have each been designed to operate in a range of environments that includes Earth (for assembly, storage, and launch), Mars (surface operations) and deep space. This requires that the MMRTG and SRG operate over a pressure range of at least one atmosphere down to vacuum, over a sink temperature range of -269 to 31 oC (4 to 304K), and within atmospheres rich in oxygen (O2), carbon dioxide (CO2) and nitrogen (N2). These RPS requirements represent a broad range of environments that cover the majority of the mission destinations identified within this study. Venus surface missions, however, are beyond the capabilities of either standard RPS unit due to the extreme temperatures (464 oC) and pressures (90 bar) existing there [17-19], and thus are not considered in this report.
Missions to the surface of the Moon, Mercury and Titan need additional thermal analysis to assess their overall feasibility and any additional spacecraft/RPS requirements. A Moon surface mission in direct view of the sun could expose the RPS to temperatures as high as 110 oC. Though both RPS units could operate at this temperature, their conversion efficiency could be decreased due to the higher heat rejection temperature. Using a sunshade to shield the RPS from direct solar exposure could be one mitigating option for missions where the reduction in efficiency was deemed unacceptable. A mission to a permanently shadowed crater at the pole of Mercury, or to Mercury?s dark side, could expose the spacecraft to temperatures as low as -183 oC in the shade. This low temperature is not an issue for either RPS design; however, the RPS would need to be shielded to prevent direct exposure to the Sun during approach and landing, where the temperature could exceed 400 oC. Judicious spacecraft design (e.g., using thermal shields) and orientation relative to the Sun could potentially mitigate this issue and allow the RPS to maintain its nominal operating temperatures. The extremely cold environment of Titan also may pose a challenge for the baseline RPS designs, and NASA and DOE are planning studies to assess any potential RPS impacts and approaches to withstanding Titan?s atmosphere .
Mission concepts that would likely require a modified MMRTG or SRG design include a Europa orbiter or lander. A mission to Europa would expose the spacecraft to intense levels of radiation (potentially several Mrads (Si) behind 100 mils of aluminum), which could damage key components of the MMRTG and SRG. Future analyses will be performed by NASA and DOE to assess the radiation impacts to the RPSs and possible mitigation strategies for tolerating a total dose up to 4 Mrads .
The MMRTG and SRG are the next generation of multi-mission radioisotope power systems expected to be available by 2009. Both units represent a significant new capability for the mission design community in their ability to operate both in deep space (vacuum) and within an atmosphere. Twenty-seven mission concepts and applications have been identified within this study that could potentially be enabled by the MMRTG and SRG, and two flight missions (MSL and Solar Probe) are currently baselining the standard RPS as their power source. The mission concepts and applications identified in this study are, in many cases, based on the priority missions outlined in the Decadal Surveys and the Vision for Space Exploration. Four mission concepts, covering a broad range of goals and objectives, were analyzed in detail to demonstrate the overall feasibility of MMRTG and SRG-powered missions, to identify the mission benefits of each type of RPS system, and to assess their preliminary power requirements. The concepts include a Triton Lander, Dual-Mode Lunar Rover Vehicle, Titan Aerobot, and Saturn Ring Observer. Key benefits of the MMRTG include its higher predicted electrical power output at BOM, significant flight heritage (e.g., SNAP-19, MHW-RTG, and GPHS-RTG), and vibration-free operation (important for experiments involving seismometers, microphones, etc.) The potential benefits of the SRG are its significantly higher conversion efficiency (>20% at BOM) requiring only a fraction (25%) of the Pu-238 fuel used by the MMRTG, the associated lower radiation dose (potentially important for manned missions to the Moon and Mars), and the lower unit mass. Both RPS units were identified as potentially able to support all missions identified in this study, with the exception of two concepts with extreme radiation environments (Europa Lander and Europa Orbiter concepts) that would require modifications to the current RPS designs. In summary, the MMRTG and SRG promise to extend the boundaries of exploration by enabling missions that would otherwise not be possible.
Last Updated: 2 March 2011