Thermomechanical Analysis of a Damaged Thermal Protection System

Thermomechanical analysis of the space shuttle tile thermal protection system (TPS) subjected to damage is conducted using the finite element method. The thermal and structural response of the damaged configurations are compared with the undamaged configuration to determine the relative effects of damage on the thermal protection capability and the induced thermal stresses in the TPS. The TPS is modeled as a discrete three-layered structure, consisting of the LI-900 tile, strain isolation pad (SIP) and the underlying structure. The atmospheric re-entry heating profile of the Access to Space (ATS) vehicle is used for the analysis. The transient temperature distribution in the system is calculated and these results are then used to determine the thermal stresses in the system. The temperature and thermal stress solutions are assumed to be uncoupled. Three different sizes of damage having diameters 0.5”, 1” and 1.5”, based on hypervelocity impact, are considered. Material nonlinearity as well as cavity radiation is included in the models. The axisymmetric case is considered first and subsequently, square configurations that resemble the actual system are analyzed. The damage increases the surface area exposed to heating and affects radiation heat loss from the surface, allowing more heat to penetrate into the system. The resulting higher temperatures, coupled with stress concentrations introduced by the damage cause significant increases in the thermal stresses.