Equilibrium model for the ablation response of silicone-coated PICA

Abstract

A novel equilibrium ablation and thermal response model for NASA’s Phenolic Impregnated Carbon Ablator (PICA) is presented. For the first time, the model accounts for the effect of NuSil, a silicone overcoat applied as contamination control during assembly, test, and launch operations for both the Mars Science Laboratory and the Mars 2020 heatshields. The protective effect of NuSil against atomic oxygen is discussed based on experimental results collected under arc jet environment. A detailed mass and heat transfer analysis for PICA-NuSil was implemented in the Porous material Analysis Toolbox based on OpenFOAM, PATO. The model includes specific boundary conditions for the coating removal and surface equilibrium processes using representative elements of the NuSil-environment system. The charred NuSil surface was modeled as SiO2 based on the observation that a silicon oxycarbide layer, formed during the decomposition of NuSil, transforms into domains of pure silica and charred PICA. Two-dimensional material response simulations were performed to compare PICA and PICA-NuSil using boundary conditions calibrated with arc jet data. The present work constitutes the first demonstration of a multi-dimensional ablation response of a silicone-coated carbon-phenolic ablator. It is shown that simulations agree with experiments, and the model reproduces measured temperature and recession. Differences between the PICA-NuSil response under Earth and Mars atmospheres are discussed.