Mechanical erosion of charring ablators in ground-test and re-entry environments.

Abstract

The use of charring ablators as thermal-protection materials for advanced ballistic re-entry is partially limited by the problem of char-layer structural integrity. The dynamic thermal/structural response of carbon/phenolic is analyzed for a high-performance ballistic-reentry environment and for hyperthermal ground-test simulation of this environment. Normal pyrolysis-gas pressure stress along with radial and tangential thermal stresses are predicted based on thermal response of the char layer, pyrolysis zone, virgin sublayer, and a metallic substructure. An in-depth char failure mode is postulated and repetitive char spallation calculated for exposure to a high-pressure ground-test environment. This mechanical recession, added to surface chemical consumption, substantially agrees with measured surface erosion of carbon/phenolic cylinders in the ground tests. Analysis of elastic/plastic thermal stresses in a carbon/phenolic heatshield during atmospheric re-entry shows high tangential char stresses and accompanying high-frequency char spallation before, at, and after peak heating. In this example, mechanical erosion accounted for two-thirds of the total re-entry surface recession.