Project title: Combined Effects of Roughness and Blowing in Ablatives

High speed spacecrafts, when entering planetary atmospheres, produce incredibly high gas temperatures due to the conversion of kinetic energy into internal energy within the gas. These high temperatures create the need for thermal protection systems (TPS) put in place to reduce the large heat fluxes incident on the spacecraft, ensuring its structural integrity during descent.

The most common type of TPS are ablatives heat shields, a layer of material that, when subject to high heat fluxes, undergoes several processes in response. These processes include the production of a porous layer of charred fibres and the generation of relatively cool pyrolysis gases (pyrolysis is the decomposition of a substance as a consequence of high temperatures).

The charred layer results in a very rough external surface, with a roughness height that greatly exceeds the boundary layer thickness - the thin layer of fluid in the immediate vicinity of the spacecraft walls. This results in material protruding into the high-speed flow ultimately serving to increase the convective heat flux incident on the spacecraft (a convective heat flux is energy transferred in the form of heat in a fluid due to the presence of a temperature gradient in the medium).

Counteracting this increase, the injection of the pyrolysis gases onto the external surface reduces the incident heat flux through the generation of a cold buffer layer between the surrounding flow and the material.

These phenomena and their overall augmentation to the heat flux that would be incident on the spacecraft walls have been understood through a plethora of experiments in isolation. The same cannot be said for the phenomena as they occur during re-entry. There as been little exploration into the augmentation of the incident heat flux due to the combined presence of roughness on large scales and transpiration (the injection of gas into the boundary layer as a means of cooling). This lack of understanding of these competing phenomena in conjunction results in the development of models that incorrectly categorise the resultant heat flux.

My research, which falls within the EPSRC engineering research area, will contribute to the lessening of this knowledge gap through experimentation to aid the improvement of ablative models in simulation and thus how they are designed in practise. The key goal is to develop a model that, in essence, allows the evaluation of the total augmentation to the heat-flux caused by the ablative heat shields. As a result, simulations will more accurately encapsulate the behaviour of ablative TPS in practise leading to better more efficient use and design of them.