Thermal-Structural Analysis of PICA Tiles for Solar Tower Test

Thermal protection materials used in spacecraft heatshields are subjected to severe thermal and mechanical loading environments during re-entry into earth atmosphere. The stress analyses for these conditions show the presence of very high in-plane compressive stresses caused by the high temperature gradients that could cause damage to the heatshield and potentially lead to failure. In order to investigate the reliability of PICA tiles in the presence of high thermal gradients and mechanical loads, novel test of a thermal protection system, was designed and conducted at the solar tower facility in Sandia National Laboratory, New Mexico. The tests included several single-tile and multi-tile PICA coupons with PICA on Edge (PoE) and Room Temperature Vulcanized (RTV) gap-fillers. This paper focuses on the design and analysis work for the multi-tile tests series with different gap-filler options. Coupled non-linear thermal-mechanical finite element analyses were conducted, to estimate the stress contours. The tests were successfully conducted at the solar tower facility. No catastrophic failure in the test articles was observed at nominal design loads. However, char cracking was observed due to additional environment loads on test articles. The analysis predictions for this test were very sensitive to material properties input. When average material properties of PICA and gap-filler material were used, the analysis was consistent with tests and able to predict test results. The tests were instrumental in the adoption of policy regarding the use of material properties for heatshield analysis. It was also demonstrated that there is a real failure mode for PICA tiles when subjected to excessive compression, and this limitation needs to be taken into account when designing the heatshield.