Characterization of carbon ablators for hypersonic thermal protection systems

Ringel, Benjamin M.

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https://hdl.handle.net/2142/132797 Copy

Description

Title

Characterization of carbon ablators for hypersonic thermal protection systems

Author(s)

Ringel, Benjamin M.

Issue Date

2025-12-04

Director of Research (if dissertation) or Advisor (if thesis)

Panerai, Francesco

Doctoral Committee Chair(s)

Panerai, Francesco

Committee Member(s)

• Elliott, Gregory S
• Villafañe Roca, Laura
• Panesi, Marco
• Lee, Tonghun

Department of Study

Aerospace Engineering

Discipline

Aerospace Engineering

Degree Granting Institution

University of Illinois Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• ablation
• thermal protection systems
• hypersonic
• atmospheric entry
• plasma
• carbon fibers
• oxidation
• spallation

Abstract

Hypersonic entry of a planetary atmosphere is among the most demanding environments in engineering, where a vehicle's kinetic energy is rapidly converted to thermal energy, generating extreme heat and caustic chemical conditions that rapidly destroy even the most advanced materials. Surviving such an environment depends entirely on the thermal protection system (TPS). Ablative materials, which dissipate aerothermal energy through controlled decomposition, are a commonly selected TPS material, with carbon-based variants often preferred for their superior thermochemical performance at high temperatures. Understanding how these materials decompose during hypersonic flight is therefore essential for proper selection and sizing of the TPS, motivating the detailed investigation of carbon ablators presented in this work. Ablation is a complex multiphysics phenomenon, with numerous coupled mechanisms, such as gas-material reactions, sublimation, and mechanical erosion, governing material response. While it is critical to understand their cumulative effect, it is equally important to isolate and characterize the role of individual mechanisms on ablation behavior. Toward this end, the present work details extensive experimental characterization of low-density carbon ablators, including FiberForm and Phenolic Impregnated Carbon Ablator (PICA), first studying their bulk response under entry-like conditions, then focusing on two key ablation mechanisms: oxidation and spallation. Initial experimental bulk characterization of carbon ablatives was conducted in high-enthalpy inductively coupled plasma (ICP) wind tunnels. FiberForm and carbon fiber weaves were exposed to high-temperature plasma in the Plasmatron facility at the von Karman Institute for Fluid Dynamics, where surface temperature, in-depth temperature, and recession were measured in situ under varying heat fluxes and pressures. During testing, spallation - mechanical erosion of surface fibers and bundles - was observed, and post-test analysis revealed oxidative decomposition in-depth during lower-temperature cases. Additional high-temperature tests were performed in the Plasmatron X wind tunnel at the University of Illinois at Urbana-Champaign, assessing the response of various graphite grades under high-temperature flight-relevant conditions. To analyze the transport-reaction competition of atomic oxygen in the boundary layer, a Damköhler number model for plasma wind tunnel environments was developed and implemented, providing an essential tool for ground-to-flight condition matching and finite-rate chemistry modeling. These experiments prompted closer study of oxidation - the primary mass loss mechanism for carbon TPS in air - and the influence of diffusion-reaction competition in porous carbon-fiber ablators. Micron-resolution in situ X-ray microtomography (µ-CT) was conducted on oxidizing FiberForm samples at elevated temperatures across a range of pressures, producing time-resolved 3D (4D) image data of carbon fiber decomposition, enabling measurement of porosity evolution, oxidation depth, and the dimensionless Thiele number. High-temperature cases exhibited surface-limited mass loss, governed by oxygen diffusion, while low-temperature cases showed uniform, reaction-limited oxidation throughout the sample. Despite seemingly benign lower temperatures, reaction-limited oxidation led to in-depth fiber weakening, spallation, and structural collapse, a phenomenon with critical implications for material integrity in-flight. Time-resolved thermal and mass transport properties were extracted using Porous Microstructure Analysis (PuMA) software, revealing surface-localized changes in diffusion-limited regimes and uniform volumetric evolution under reaction-limited conditions. Results were compared with a PuMA oxidation model, which showed strong agreement with experimental data. Building on prior observations, material spallation was investigated in supersonic air and nitrogen plasmas produced by the Plasmatron X ICP wind tunnel at aerothermal conditions representative of atmospheric entry. Spalled particles from FiberForm and PICA wedges were tracked using high-speed imaging, enabling time-resolved analysis of spallation events. Tests in nitrogen revealed high variance in particle production over time, while tests in air exhibited steady particle release. Post-test microscopy and spectroscopy identified a disordered nitrogen-functionalized carbon precipitate that forms exclusively in nitrogen plasma. Under extreme conditions, this deposit decreased surface permeability, enabling subsurface pressure buildup and causing unsteady particle release. Spalled particle size was inferred from velocity data obtained via particle tracking, enabling estimation of spallation mass loss. Spallation was estimated to account for upwards of 45% of total mass loss for tests in nitrogen, underscoring its significance in anaerobic entry conditions. Results suggest that deposit formation, material orientation, and environment conditions collectively govern spallation behavior. Collectively, this work advances the fundamental understanding of carbon ablator performance in extreme environments, offering data, methodologies, and physical insight to guide the design, qualification, and modeling of future thermal protection materials. These advances enable safer, more reliable planetary entry for both national defense applications and next-generation space exploration.

Graduation Semester

2025-12

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/132797

Copyright and License Information

Copyright 2025 Benjamin Ringel

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