Ablation is a term denoting the physical-chemical phenomena undergone by a material exposed to severe aerothermal stress such as the ejection of gas from a nozzle.
With the rise in temperature, the material generally undergoes internal transformations (pyrolysis) by injecting volatile products into the fluid, followed by various chemical reactions taking place at the wall between the material and the fluid.
Ablation results in a transfer of mass from the material to the fluid. The phenomenon is also called thermochemical erosion.
The energy dissipation due to these reactions delays the penetration of heat into the material. We generally refer to thermal protection by ablation.
The applications of ablative materials are:
• thermal shields for hypersonic craft re-entering the atmosphere;
• the propulsion nozzles of rocket engines;
• the internal protection of the body of thrusters.
There are materials which are:
• pyrolysable, combining refractory reinforcement (asbestos, glass, silica, carbon, etc.) and a resin of which the thermal decomposition is accompanied by a large coke residue;
• refractory, the most widely used being carbon-carbon materials.