Numerical assessment of camphor ablation flight relevance in hypersonic wind-tunnel testing

Designing thermal protection systems (TPS) for ballistic reentry capsules requires both reliable numerical tools and experimental facilities to evaluate surface ablation and its shape change. Low-temperature ablators, such as camphor, can be employed in low-enthalpy wind tunnels to overcome limitations of standard hypersonic experimental facilities, which cannot induce the actual TPS ablation due to limits in inflow total enthalpy, velocity, or test time duration. However, the flight relevance of such an experiment needs to be accurately assessed, as both the capsule dimension and the ablative material during the test differ from the flying ones. The main goal of this work is to perform numerical simulations in order to assess the flight relevance of the ground tests. Computational fluid dynamics (CFD) simulations have been performed for both low-enthalpy wind-tunnel conditions for a camphor scaled-down capsule and actual reentry flight conditions, assuming a carbon-based heat shield. Moreover, numerical analyses have been improved by coupling the CFD solver with a material response code named PATO, which models the heating transient process. The obtained results show how the CFD/material response coupling has a more important effect for the on-ground testing conditions considered, resulting in a more favorable agreement with the available experimental data. Finally, the shape change relevance of the analyzed low-temperature ablator has been verified by providing a similitude criterion between the on-ground and in-flight capsule recession. In particular, a few seconds of exposure are sufficient to generate the same shape change that is expected in flight.