Dual-mode Propulsion System Enabling CubeSat Exploration of the Solar System

The primary goal of this study was to flesh out the initial concept of a radioisotope-based propulsion system for small payloads. The design of which was performed in the context of delivering a 6U payload to the orbit of Enceladus. Building on the work completed here the system concept will need to be further matured as its design is further optimized. Modeling of the thermal capacitor core, using programs such as COMSOL Multiphysics, will need to become more integrated to fully determine the thermal interactions of the various central components of the core including the thermal capacitor, insulation layers, support structures, etc and their interactions with one another. Additionally, as design changes occur through the evolution of this concept, modeling will need to be used to address those changes as well. Optimization of the core design will need to be continued, progressing to the inclusion of an appropriate containment method for a PCM thermal capacitor. Research on silicon will need to be continued and experiments will need to be conducted, cycling silicon through its liquid-to-solid phase change in order to determine potential stresses on the system. Additionally, methods to alleviate potential stresses and ensuring thermal conductive pathways between a PCM thermal capacitor and flow channels are maintained will need to be further evaluated. Proper understanding and functionality of the thermal capacitor is key to the development of this concept and Figure 34 shows details of the CSNR's remote laser heating apparatus that can be used to better understand silicon's phase change process. Follow on work will need to include the use of a CFD code, such as Starr CCM+, to model the thermal hydraulics of the system and to model the exchange of energy between both the thermal capacitor and the absorber heat rejection subsystem with a flowing gas. Ultimately, experimentation will need to be conducted demonstrating the heat transfer from a PCM to a flowing gas to demonstrate this key technology of the concept. This can be performed using existing equipment and facilities available to the CSNR. A laboratory-scale test rig used to perform flow experiments through the CSNR's Mars Hopper concept is seen in Figure 14 (a). A similar test rig will be constructed for possible thermal hydraulic experiments, designed specifically for a PCM thermal capacitor. Figure 14 (b) shows the high temperature blowdown facility that is capable of safely housing/conducting the potential thermal hydraulic experiments. These same facilities can be used to house a sub-scale test article for thermal hydraulic experiments to demonstrate the absorption of energy from a flowing gas within the heat rejection subsystem, which is a key technology to a low mass, low footprint, integrated conversion subsystem. Experimental data will aid in validating CFD modeling, which in turn will lead to better refinement of the various flow loops of the system and a better understanding of a fully integrated system. Utilizing existing CSNR facilities to conduct experimental work and to demonstrate the thermal hydraulics of the system allows for more results to be achieved in future studies. power requirement of the system is continually met; resulting in the design of a fully integrated conversion system. Additionally, a better assessment of the Brayton engine's turbo-machinery components will need to be conducted. Ultimately, data gathered through thermal hydraulic experiments and CFD modeling will allow for a better prediction of the expected turbine inlet and outlet temperatures; aiding in the turbo-machinery assessment. The overall mission architecture of an Enceladus orbiter mission will need to be further refined. In future studies the instrumentation system will continue to evolve and the integration of each instrument in to the propulsion system will need to be completed. An assessment of a larger payload can prove to be beneficial, given the added mass is acceptable. The trajectory phases of the mission will need to be further developed and optimized. Specifically, the periapsis pumping phasing maneuvers need to be further refined, and an assessment of non-gravity assisted trajectories for the cruise phase may also be beneficial to alleviate tight launch windows. Finally, a design of a fully integrated propulsion system along with the design of the various subsystems will be beneficial, aiding in better assessing the size, footprint and masses of the system and subsystems. For the mission studied here it is apparent the power requirement of the communication subsystem is significant. Employing an RF-based system, such as the one designed here, may prove to be problematic. Follow on work will need to be done to further evaluate a proper communication subsystem and for RF technology an assessment of their limitation will be needed. Additionally, to ensure the communication subsystem is maintained on the curve for communication technology an assessment of alternative communication technologies must be performed, specifically a comparable laser-based system must be assessed. As more work is performed in maturing this concept, its motivation will continue to target lower mission costs. This will be continued through using the micro - satellite platforms and continuing with the design constraint of an IMLEO of < 1,000 kg; taking advantage of smaller launch vehicles.