Design and analysis of penetrator probes for planetary science applications

Abstract

Penetrator probes emplace by impact into a solid medium, carrying scientific instrumentation to fulfil specified post-impact mission objectives. They have been used successfully for multiple terrestrial applications, but only flown once unsuccessfully as a planetary exploration vehicle, with many proposed penetrator missions over several decades being cancelled at various stages of development. We examine past applications of penetrator probes alongside current Antarctic research instrumentation, setting out the context for design and analysis work carried out on the Geodetic Seismic Ice Penetrator (GSIP). GSIP is a separable two-body penetrator designed for air-deployment from LC-130 aircraft into snow pack on the Ross Ice Shelf, using existing polar research aircraft as the deployment method. The mission objective is to measure seismic readings and ice shelf displacement for a full year in order to improve understanding of the dynamic effect of ocean forcing on ice shelf stability. GSIP also aims to improve upon current Antarctic research instrumentation by reducing the on-ice footprint and therefore reducing the risk to team members, as well as reducing the deployment time and cost when compared to deployment by hand on the ground. Using an air-deployed sensor vehicle improves ease of instrumenting remote and crevassed areas to establish a wide network of seismic sensors, in order to build up a large-scale overview of dynamic response across the ice shelf. The current design of the GSIP system is presented, including the need for the penetrator to be aerodynamically stable with a low centre of gravity in order to rapidly damp oscillations during the falling phase and maximise the probability of emplacing at a vertical or near-vertical angle. This is required both to achieve stable seismic coupling with the snow pack and to fulfil the microseismometer positioning requirements. A two-body design was selected due to the ability to separately optimise the forebody design to emplace at least 100% of its length to achieve secure seismic coupling and the afterbody design to decelerate rapidly upon impact to ensure the antennas are placed sufficiently high above the surface to remain uncovered after a full year of snow accumulation. This separable design improves GSIP's robustness to the wide possible range of impact medium properties which may be encountered. Due to a strong reliance of success on the properties of the impact medium, a MATLAB simulation of snow penetration was developed and used to compare a range of design variables. Although snow mechanics is a complex field requiring large amounts of simplification to succeed in solving specific practical problems, this simulation was partially validated using data from drop testing into snow and shown to have low error values. The model was used to help refine the design for GSIP but also to aid in the development of a miniaturised earthquake-monitoring soil penetrator, broadening the range of impact medium variables which were considered by the penetration simulation.