Research on the effects of damage on the thermomechanical performance and structural integrity of thermal protection systems (TPS) has been limited. The objective of this research is to address this need by conducting experiments and finite element (FE) analysis on damaged TPS. The TPS selected for study is the High-Temperature Reusable Insulation (HRSI) tiles used on NASA's Space Shuttle Orbiter. The TPS consists of a LI-900 tile, the strain isolator pad and the underlying structure, is subjected to the thermal loading and re-entry static pressure of the Access to Space reference vehicle. The damage to the TPS emulates hypervelocity-impact-type damage.
Preliminary FE analysis using several simplifying assumptions, was conducted to determine the accuracy of using an approximate axisymmetric model compared to a complete three-dimensional model. Temperature results were found to be reasonable close; however, thermal stress results displayed significant differences. The sensitivity of the FE results to the various simplifying assumptions was also examined and it was concluded that for reliable results, the simplifying assumptions were not acceptable. Subsequently, an exact three-dimensional model was developed and validated by comparison with experimental data.
Re-entry static pressures and temperatures were simulated using a high-temperature experimental facility that consists of a quartz radiant heater and a vacuum chamber with appropriate instrumentation. This facility was developed during the course of this dissertation. Temperatures on the top and bottom surfaces of the TPS specimen as well as strains in the underlying structure were recorded for FE model validation.
The validated FE model was then combined with improved thermal loads based on the interactions of hypersonic flow past a cavity representing the damage. Damage increases the thermal loads on the TPS and significantly reduces the heat rejection capability of 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. For the damage sizes considered, the elevated thermal stresses alone are not likely to cause material failure. However, a modest damage size of 0.5" is capable of raising temperatures in the tile to exceed its melting point.Ph.D.Aerospace EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/57597/2/whn_1.pd
