Impact of spallation and internal radiation on fibrous ablative materials

Space vehicles are equipped with Thermal Protection Systems (TPS) that encounter high heat rates and protect the payload while entering a planetary atmosphere. For most missions that interest NASA, ablative materials are used as TPS. These materials undergo several mass and energy transfer mechanisms to absorb intense heat. The size and construction of the TPS are based on the composition of the planetary atmosphere and the impact of various ablative mechanisms on the flow field and the material. Therefore, it is essential to quantify the rates of different ablative phenomena to model TPS accurately. In this work, the impact of two ablative mechanisms is studied. The first ablative mechanism studied is spallation, a phenomenon in which the TPS material ejects particles when exposed to atmospheric entry conditions. It is typically modeled as an added percentage of safety based on the overall ablation rate. A data-driven adaptive technique was performed to numerically reconstruct particle trajectories from spallation experiments at the NASA HyMETS facility to evaluate the effects of spallation on ablative materials. Several numerical models were developed and integrated into a Lagrangian particle trajectory code to ensure accurate results of this reconstruction. More specifically, a blended drag coefficient model to compute accurate particle dynamics, a non-sphericity model to account for irregular shapes of the particles, and a backtracking model to simulate the trajectories reversed in time from the first experimental point to the ejection location on the sample were developed. The reconstructed results were analyzed statistically to provide more information on these spalled particles\u27 size and ejection parameters. The results would estimate the mass loss due to spallation and probable causes for the ejection of particles. In addition, coupling was performed between the trajectory code and a hypersonic aerothermodynamic code to evaluate the effect of these hot, chemically reactive spalled particles on the flow field. This comprehensive study in spallation provides more insights into the phenomenon and tools to quantify its impact. The second mechanism studied in this work is internal radiation. Recent laser heating experiments have concluded that spectral radiative heat fluxes penetrate the ablative materials. The penetration distance is inversely proportional to the absorption coefficient of the material at the corresponding wavelength. Since the shock layer produced at the atmospheric entry conditions around the material can be expressed as a group of lasers of different wavelengths, the radiation penetration might be significant, especially in radiation-dominated entries. A radiation transfer equation is fully coupled to the in-house material response code to evaluate the impact of radiation penetration. In addition, a band model of unequal widths for the material was developed to investigate the effect of shock layer radiation within the material. The results showed high internal temperatures and internal decomposition. The tools developed in this work can be useful in accurately modeling the heat transfer through the material

Modeling of spallation phenomenon in an arc-jet environment

Space vehicles, while entering the planetary atmosphere, experience high loads of heat. Ablative materials are commonly used for a thermal protection system, which undergo mass removal mechanisms to counter the heat rates. Spallation is one of the ablative processes, which is characterized by the ejection of solid particles from the material into the flow. Numerical codes that are used in designing the heat shields ignore this phenomenon. Hence, to evaluate the effectiveness of spallation phenomenon, a numerical model is developed to compute the dynamics and chemistry of the particles. The code is one-way coupled to a CFD code that models high enthalpy flow field around a lightweight ablative material. A parametric study is carried out to examine the variations in trajectories with respect to ejection parameters. Numerical results are presented for argon and air flow fields, and their effect on the particle behavior is studied. The spallation code is loosely coupled with the CFD code to evaluate the impact of a particle on the flow field, and a numerical study is conducted