Thermomechanical Behavior of a Thermal Protection System with Different Levels of Damage – Experiments and Simulation

Experiments are conducted on damaged and undamaged space shuttle tile thermal protection system (TPS) to determine its thermomechnical behavior. The TPS specimens, which consists of a LI-900 tile, the strain isolator pad and the underlying structure, is subjected to a temperature profile corresponding to the thermal loading of the Access to Space (ATS) reference vehicle. Experiments are conducted in a vacuum chamber to allow the ATS re-entry static pressure to be simulated. Temperatures on the top and bottom surfaces of the TPS, as well as the strains in the underlying structure are recorded. The experimental results were used to validate the finite element model. The validated model was then used to account for the interactions of the high speed external flow past a cavity (that represents damage) with the damaged TPS. The modified thermal loads that account for the effects of flow separation and reattachment are indicative of actual flight conditions. Using the improved model, 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 configuration. Damage increases the thermal loads on the TPS and significantly reduces the radiation heat loss from the surface of the tile, resulting in elevated temperatures. The higher temperatures coupled with the stress concentrations introduced by the damage cause a substantial increase in thermal stresses. Results also indicate that damage can elevate the maximum temperature in the tile to above its melting point.