Rapid Access to the Interstellar Medium via Solar Thermal Propulsion

To access the interstellar medium with current approaches requires 30 to 40 years, significantly longer than most mission lifetimes. The goal of this study was to explore mission concepts that will reach the interstellar medium in a primary mission’s lifetime (15 years or less). Faster access to the interstellar medium would allow high-capability science probes, with many relevant instruments, to explore the galaxy beyond our solar system in-situ. Science targets include the hydrogen wall structure, bow wave/shock, gravitational lens, foreground emissions and interstellar dust, just to name a few. Further, such a capability would enable rapid exploration of Kuiper belt objects in a much shorter time frame than current methods. Finally, distant targets include the solar gravitational lens, which may enable direct imaging of exoplanets.

Figure 1: The Interstellar Medium Science Targets

We examine a solar thermal propulsion (STP) system to rapidly access the local interstellar medium via a solar perihelion burn. This approach uses several Venus and Earth gravity assists to fly out to Jupiter and then would dive towards the Sun. Approaching within 3 solar radii a perihelion burn would be performed, maximizing the spacecraft’s ΔV to achieve high Solar System escape velocities. A unique aspect of the STP mission concept is that the Sun is not only used as a gravity well for an Oberth maneuver, but also to heat the fuel to ultra-high temperatures (>3000 K), enabling a monopropellant burn with high specific impulse (I_sp). An in-depth modeling exercise found this approach to be preliminarily feasible, with escape velocities of around 9 AU/yr achievable with current technology, and up to 16 AU/yr with significant future technological advances.

While the baseline STP design is capable of providing just under 9±1 AU/yr, Figure 2 highlights areas of key technological improvements that could be explored. Ultimately, if all technological paths could be implemented, the overall performance as a best-case scenario could reach approximately 16 AU/yr.  Figure 2 also qualitatively ranks these improvements from most likely to least likely when reading the graph bottom up. For example, implementing turbopumps in the system is likely more readily feasible than reducing the liner thickness in the near future. It is assumed that these upgrades can be implemented in the future without incurring any additional mass penalty over the baseline design. Thus, it predicts best case performance, and actual values would likely be lower. The improvements could be the result of a single point improvement, or a propagation of several developments.

Figure 2: Overview of Solar Thermal Propulsion Performance

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 12 Au/yr. 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, which would provide one of the fastest ways to exit the solar system. Of particular importance is heating the hydrogen to 3,500K. Using a perihelion burn as a kick stage for a nuclear electric propulsion system was found to be particularly effective for achieving even higher escape velocities, up to 19.5 Au/yr.