Mechanical Shock Analysis and Testing of an Air-Dropped Antarctic Ice Penetrator

Abstract

The Seismo-Geodetic Ice Penetrator (SGIP) is an air-dropped kinetic penetrator that will deposit a geophysics-grade seismometer and GPS receiver into Antarctic ice shelves. These instruments will measure vibrations in the infragravity (< 0.1 Hz) range to improve understanding of forces that cause ice shelf calving. The penetrator will impact at its terminal velocity---roughly 40 m/s---to deploy the seismometer at least 2 meters below the ice shelf surface. SGIP will separate into two components, the body and flare, on impact to embed the seismometer into the ice shelf and transmit data from the ice shelf surface, respectively. However, the penetrator's impact can accelerate the primary payload up to 129 g along its central axis, which can damage the delicate seismometer. SGIP uses shock isolation to reduce the seismometer's peak acceleration. A structural response model is used to predict the seismometer's dynamic response to ice shelf impacts with hundreds of potential shock isolation designs. This structural response model is used to select a candidate shock isolation design based on the SGIP prototype's volume constraints. A 22 cm long, 4.8 cm diameter cylinder made of IMPAXX 300 foam can be used to limit the seismometer's peak axial accelerations to 55 g, which represents a 57\% reduction from the maximum expected peak axial acceleration. A shear pin assembly is designed to rigidly connect the body and flare during descent yet deliberately shear on impact to separate the body and flare. Risk reduction tests are conducted to lower the probabilities of the shear pins' four primary failure modes.