Phase I Final Report NASA Institute for Advanced Concepts Combined Heat Shield and Solar Thermal Propulsion System for an Oberth Maneuver

As humanity continues its exploration of space, many space missions are enabled by increases in speed. Examples include outer planet and dwarf planet exploration missions and missions that travel through our solar system into interstellar space. For many of these applications speeds of >10 astronomical units per year (AU/yr) are desired. A powered gravity assist around the Sun may offer the best option for reaching this goal; however, current heat shields and kick stages are too heavy or generate too little thrust. Solar thermal propulsion overcomes this tradeoff by converting the heat of the Sun into thrust. By tripling the specific impulse relative to chemical propulsion and by enabling a smaller perihelion through active cooling, this approach nearly doubles the escape velocity.
Our team has designed and built working solar thermal propulsion prototypes out of materials that can survive 2700 K at a 30 x 30 cm scale. These benchtop-scale demonstrations have thus far validated the simplifying assumptions that underlie our thermal and propulsion models. Despite growing confidence that a full-scale heat shield/heat exchanger can survive an Oberth maneuver, many questions remain regarding the feasibility of long-term cryogenic storage of hydrogen propellant.
We therefore performed a full trade study of alternate propellants to determine the maximum escape velocity for a given total system mass, including spacecraft, heat shield, propellant storage, and attitude control system. The main propellants of interest include H2, LiH, Li, CH4, and NH3. The key question is whether alternatives to H2 enable higher escape velocities by offsetting the loss of specific impulse through reductions in dry mass and system complexity. Our calculations showed, in fact, that lithium, lithium hydride, and ammonia enable a greater escape velocity than hydrogen when the full system trade is performed. Lithium hydride had the highest escape velocity, at over 12 AU/yr. However, ammonia is more attractive because it removes most of the risk associated with propellant storage and handling. It would allow future TRL advancement to focus on the development of a heat shield that doubles as a heat exchanger for a solar thermal propulsion system. Escape velocities using ammonia are still predicted to exceed 10 AU/yr even if the novel heat shield is unaccompanied by any other technological advances.