System Engineering a Solar Thermal Propulsion Mission Concept for Rapid Interstellar Medium Access

The interstellar medium (ISM) represents the next frontier in space exploration, with many new discoveries to be made. The challenge, being so far away from Earth, the ISM requires many decades to reach. To advance our knowledge of what exists beyond our solar system, new approaches for rapid access are required. One such approach is solar thermal propulsion (STP). The approach uses several Venus and Earth gravity assists to fly to Jupiter and use its gravity well to dive towards the Sun. Approaching within three solar radii a perihelion burn would be performed, maximising the spacecraft’s ΔV to achieve high solar system escape velocities. A unique aspect of the STP mission concept is that the Sun is used not only as a gravity well for an Oberth manoeuvre, but also to heat the fuel to ultra-high temperatures (> 3000 K), enabling a monopropellant burn with high specific impulse (I_sp). Prior preliminary studies indicated escape velocities of over 20 astronomical unit (AU)/year would be possible. An in-depth modelling exercise was undertaken to determine how such a system would perform. The model in this paper showed the current STP design is capable of providing just under 9 ± 1 AU/year, but there are many technology developments that could increase escape velocity. The technologies vary from items that could be implemented in the near term, like turbo-pumps driven by the hydrogen, to items requiring more extensive development programs like thin coatings which do not erode in superheated hydrogen. After reviewing the STP approach, and comparing it to a solid rocket motor (SRM), it was found that with currently available technology, SRM outperforms STP with an escape velocity of approximately 10–12 AU/year. However, future advances in heat exchanger lining materials, turbo pumps, and advanced heat exchanger geometries may enable solar thermal propulsion to provide higher escape velocities, providing one of the fastest ways to exit the solar system. Ultimately, if all technology paths could be implemented with minimal side effects, the performance in a best-case scenario could reach up to 16 AU/year.