Experimental and Analytical Evaluation on an Aerospace Thermal Protection System: Carbon Monolith Ablator

Re-entry vehicles are exposed to high temperatures during re-entry owing to severe aerodynamic heating. Therefore, thermal protection systems (TPS) are important to protect the aircraft from heating. Ablator is a typical TPS used as high-speed re-entry vehicles. Generally, Ablator is a carbon-based material impregnated with resin that protects the vehicle from heating through thermal decomposition and ablation because the decomposition of resin is an endothermic reaction and gas evolved by the decomposition of resin prevents aerodynamic heating of the surface. Phenolic Impregnated Carbon Ablator (PICA) is a thermal protection system with used successfully on missions to the Moon and Mars. On the other hand, compressive strength of PICA is low and mechanical erosion occurs because substrate of PICA is a carbon felt. In addition, it is not possible to use resin with a low residual carbon content because a carbon felt cannot bear the load during operation. The use of a resin with a low residual carbon content is expected to increase the prevention of recession by the decomposition of resin. In the previous study, Three-dimensional Networked Porous Carbon (TNPC), which has approximately 25 times higher compressive strength than PICA and was used as a substrate for an ablator. Since TNPC has a continuous structure, we developed and evaluated Porous Carbon Ablator (PCA) by impregnating TNPC with acrylic resin that has a residual carbon content of ~0%. As a result, the recession characteristics of PCA were comparable to those of PICA due to oxidation and sublimation of the struts of TNPC.<br/>In this study, we have developed a carbon monolith with a three-dimensional network structure by the carbonization of phenol monolith with a three-dimensional network structure. Although it is denser than TNPC, it can be easily fabricated by phase separation of phenolic resin and carbonization. Furthermore, the pore size can be controlled by the condition of phase separation because three-dimensional network structures of phenol monolith are maintained after carbonization. Since the recession of carbon monolith is smaller than that for TNPC, it is expected that ablator with carbon monolith substrate realizes the improvement of ablation behavior compared to PCA. Therefore, Carbon Monolith Ablator (CMA) was prepared by impregnating carbon monolith with acrylic resin, and its recession behavior and thermal properties were evaluated in an arc-wind tunnel test. The amount of recession was calculated based on the test conditions. To evaluate thermal conduction and temperature distribution of ablator during test, an analytical model simulating an arc-wind tunnel was created by conducting a heat conduction analysis using the finite element method. The thermal stresses were also evaluated using the temperature distribution obtained from the model. In this presentation, design guidelines and optimal analytical models for ablators using porous materials as substrate will be discussed based on the experimental and analytical results.