Evaluation of proof testing as a means of assuring mission success for the Space Shuttle thermal protection system

The reliability analysis of a thermal protection system (TPS) incorporating low density, fibrous ceramic materials must take into account the time-dependency and variability of the system's strength. Fracture mechanics concepts can be used to estimate the allowable strength and expected lifetime, as well as to define a proof test scheme for assuring mission success of TPS. The aim of this study was to evaluate the proof testing scheme used on the TPS of the Space Shuttle with these fracture mechanics concepts. The analysis confirmed that proof testing was necessary for the undensified system and that proof testing should lead to adequate reliability with respect to the design stresses. For the undensified system, fracture mechanics predictions were confirmed by measuring the strength of samples that survived proof testing. It was also found that the time-dependent nature of the system's strength was controlled mainly by the ceramic and that this time-strength dependency is similar to that observed for bulk glasses that have compositions similar to the glass fibers present in the ceramic tiles.