Re-entry and hypersonic vehicles, as well as rocket motors, are exposed to extreme heat loads, shear forces and chemically aggressive gasses. To protect the structure of the vehicle, ablative thermal protection system (TPS) materials are commonly used. As these materials protect the vehicle, they sacrifice themselves and, as a result, change in thickness and shape. This adversely affects the insulation and aerodynamic performance of the vehicle. Hence, during testing and certification of ablative TPS materials it is critical to monitor their ablation performance. The main objective of the current research is to use an ultrasonic technique to monitor the material ablation process of silica-phenolic TPS material. Moreover, due to the large temperature gradients within the material, a method for correction of temperature influence on speed of sound was sought after.
Many ablative TPS materials, including silica-phenolic, have anisotropic structure, which causes a diffuse backscatter of ultrasonic pulses propagating through the material. Since these ultrasound reflections come from the internal material micro-structure, they are received earlier than the echo from the ablating surface. Thus, they can be used to correct the decrease in speed of sound as the material temperature rises.
The current study will show how monitoring the temperature dependent behavior of the backscattered echoes was used to correct the speed of sound (SOS) change in the material. Additionally, based on the experimental measurements of the SOS temperature dependence, evaluation of time-varying temperature profiles within the material will be presented. The performance of the developed method is evaluated based on several ablation experiments using an oxy-acetylene torch apparatus. Verification of the developed method is based on post-test micro-CT and ultrasound scans of the ablated samples.
