Thermomechanical Analysis of a Thermal Protection System with Defects and Heat Shorts

The thermomechanical behavior of damaged space shuttle tile thermal protection system is determined using the finite element method. The relative effects of damage on the thermal protection capability and the induced thermal stresses in the TPS are determined by comparing the thermal and structural response of the damaged configurations with the undamaged configurations. The TPS, consisting of 3 components (the LI-900 tile, the strain isolation pad and the underlying structure), is modeled as a discrete three-layer structure. The TPS is subjected to the re-entry heating and pressure profile of the Access to Space vehicle, and the transient temperature distribution and the resultant thermal stresses in the system are computed. Three different sizes of damage having diameters 0.5”, 1” and 1.5”, based on hypervelocity impact, are considered. The validity of the simplifying assumptions used in a recent study is systematically examined. Some of these assumptions are relaxed and their effects on the system response are studied in the present work. The effect of damage location on the overall behavior of the TPS is also examined. Damage changes the surface properties of the tile and increases the surface area exposed to heating. It significantly reduces the radiation heat loss from the surface of the tile, resulting in elevated temperatures in the TPS. The elevated temperatures, with the stress concentrations introduced by the damage increases the thermal stresses significantly. Results suggest that the damage considered is capable of raising the maximum temperature in the tile to beyond its melting point and may cause structural failure.