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Author ORCID Identifier

https://orcid.org/0000-0001-9423-7604

Date Available

5-20-2028

Year of Publication

2026

Document Type

Doctoral Dissertation

Degree Name

PhD in Aerospace Engineering

College

Engineering

Department/School/Program

Mechanical Engineering

Faculty

Savio J. Poovathingal

Faculty

Jonathan Wenk

Abstract

Thermal protection systems (TPS) are critical for enabling atmospheric entry of space vehicles, where materials are subjected to extreme thermal and mechanical loads. The performance of these systems is governed by complex, multi-scale interactions between microstructure, transport processes, and thermochemical degradation. However, conventional modeling approaches often rely on bulk-average material properties, implicitly assuming homogeneity and neglecting the inherent variability present in heterogeneous materials such as FiberForm and resin-based gap fillers.

This work presents a comprehensive framework for the statistical characterization of TPS-relevant materials and the incorporation of microstructural variability into material modeling. High-resolution X-ray computed tomography (XRCT) and focused ion beam scanning electron microscopy (FIB-SEM) are employed to capture the three-dimensional microstructure of fibrous preforms and resin systems across multiple length scales. While XRCT enables mesoscale characterization of fibrous architectures, FIB-SEM provides nano-scale resolution necessary to resolve resin morphology that is otherwise inaccessible due to resolution limitations.

To systematically extract and quantify microstructural features, the Heterogeneous Effective Representative Multi-scale property Extraction Software (HERMES) framework is developed. HERMES integrates stochastic sampling, geometric analysis, and statistical modeling to generate probability distribution functions (PDFs) of key material properties, including porosity, surface area, feature size, and fiber geometry. A key finding of this work is that material properties converge to stable distributions rather than single representative values, even at the representative elementary volume (REV) scale, demonstrating that variability persists across all relevant length scales.

Application of the framework to rayon- and lyocell-based FiberForm reveals distinct microstructural organizations that influence material behavior. Rayon exhibits higher porosity, smaller fiber diameters, and greater spatial heterogeneity, whereas lyocell displays larger fibers and more uniform structural organization. These differences are shown to have direct implications for transport properties and thermochemical response.

In parallel, the thermal degradation of RTV-based gap fillers is investigated under both furnace-controlled and arc-jet environments. Results demonstrate that decomposition leads to the formation of internal cavities, swelling, and eventual surface rupture, generating interconnected pathways that challenge the common assumption of RTV impermeability. Direct simulation Monte Carlo (DSMC) analyses of degraded microstructures confirm that permeability increases significantly with thermal damage, highlighting the importance of accounting for evolving microstructure in TPS modeling.

Overall, this work establishes a unified methodology for linking high-resolution imaging, statistical microstructural characterization, and physics-based modeling. By replacing deterministic material descriptions with distribution-based representations, the proposed framework enables more accurate and predictive simulations of TPS materials under extreme conditions. These findings provide a foundation for advancing multi-scale modeling approaches and improving the reliability of next-generation thermal protection systems.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2026.306

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Archival

Funding Information

This work was supported by a Space Technology Research Institutes grant from NASA’s Space Technology Research Grants Program under grant number 80NSSC21K1117.

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