Ablative Thermal Protection Materials Research Articles

The effects of ZrSi2 on the ablation and insulation performances of low-density carbon-phenolic composites

Abstract Carbon-phenolic (C-Ph) composites are typical ablative thermal protection materials. Excellent ablation and insulation performance indicate a decreased thickness of the thermal shield. Thus, ZrSi2 particles were introduced to improve the performance of the low-density C-Ph composite. An oxyacetylene flame torch was used to examine the ablation and insulating characteristics. The curing process of the matrix resin was not affected by ZrSi2. The thermal conductivities of the composites with different ZrSi2 contents ranged from 0.219 ∼ 0.254 W K−1·m−1. A continuous but not very compact cover was formed on the ablating surface of the C-Ph composite with 10% ZrSi2, which limited the escape of the charred matrix. The passageways for the gaseous products of the charring phenolic compounds were also not impeded by this cover. The C-Ph composite with 10% ZrSi2 exhibited the lowest linear loss rate (0.0081 mm s−1) and the best heat insulating performance.

Improvement of carbon fiber/phenolic composite properties by low-loading graphene oxide and SiO2 nanoparticles

As an ablative thermal protection material, the mechanical properties and ablative resistance of carbon fiber/phenolic (C-Ph) composite should be improved to meet the high requirements in deep space exploration. Herein, the properties of C-Ph composites were improved by the synergistic effect of low-concentration graphene oxide (GO) solution and low mass fraction of SiO2 nanoparticles. Compared to the unmodified C-Ph composite, the compressive strength of the modified composites was increased by 40.75 % and the linear and mass ablation rates were reduced by 17.78 % and 43.95 %, respectively. The introduction of GO effectively improved the interfacial bonding between the fiber and phenolic resin. Meanwhile, the uniformly dispersed SiO2 nanoparticles resisted the applied loads, thus synergistically enhancing the mechanical properties of the composites. In terms of improving ablation resistance, GO promoted the graphitization of fibers and enhanced the stability of the char layer. Concurrently, SiO2 nanoparticles reacted with the carbon matrix under high temperatures to form ablation-resistant SiC, thus synergistically enhancing the ablative properties of the composite.

Detailed insights of polydimethylsiloxane (PDMS) degradation mechanism via ReaxFF MD and experiments

Polydimethylsiloxane (PDMS) serves as a widely used silicone polymer in ablative thermal protection material. Nevertheless, the intricate degradation mechanism governing PDMS at ultra-high temperatures remains unclear. Herein, PDMS pyrolysis was in-depth investigated by employing a combination of experimental methodologies and ReaxFF molecular dynamics (ReaxFF-MD) simulations. PDMS model was firstly validated by product distribution analysis and kinetics analysis. Molecular reaction tracking unveils that the pyrolysis pathway primarily is triggered by the cleavage of the Si-C bond, leading to cyclization reactions and fractures within the Si-O framework at elevated temperatures. The pathways leading to the formation of gaseous, liquid, and solid products during thermal cracking are systematically unveiled. Drawing from these insights, DFT calculations reveal Si-C bond dissociation energies (BDEs) of 98.8, 102.2, and 108.2 kcal/mol for Si-CH3, Si-C2H3, and Si-C6H5, respectively. MD simulations further indicate that the heightened Si-C BDEs across various silicone rubbers impede their decomposition, thereby augmenting the thermal stability of the material. This study augments our comprehension of the PDMS pyrolysis mechanism and offers theoretical insights for enhancing its thermal stability.

Enhancing ablation and oxidation resistance of phenolic resin with modified pickling asbestos

AbstractGiven the increasing requirements for ablative thermal protection materials, there is an urgent need for boron phenolic resin (BPR) composites with excellent high‐temperature strength properties as well as ablation performance to address the structural failure during the ablation process of these devices. Herein, modified pickling asbestos (MPA) was successfully incorporated into BPR to prepare MPA‐modified BPR (MPABPR). In addition, one‐dimensional MPA with improved dispersion and compatibility was used to fabricate a fiber‐reinforced network inside of BPR for the first time, which effectively increased the high‐temperature strength and ablation resistance of BPR. The results demonstrated that the high‐temperature strength of MPABPR was optimally improved by 25% compared to pure BPR. Meanwhile, when the content of MPA was only 5 wt%, the linear and mass ablation rates of MPABPR could reduce to 0.046 mm/s and 0.043 g/s, which were 34.3% and 23.2% lower than that of pure BPR, respectively. This study has the enormous potential to provide a new strategy for preparing high‐performance BPR matrix materials.Highlights The dispersion and compatibility of PA are improved by modification. MPA constructs a fiber‐reinforced network that can strengthen the carbon layer. The ablative properties of MPABPR have great advantages over the BPR. The compressive strength of carbonized MPABPR5 is 25% higher than that of BPR.

The correlation between multilevel micro-nano structures and thermal conductivity of nanoporous phenolic composites reinforced by needled fiber preforms

Nanoporous phenolic composites (NPC) are the most promising ablative thermal protection materials, but accurately modeling and effective thermal conductivity (λeff) prediction remains a significant challenge due to their intricate micro-nano structures. Herein, a modified generation method, which incorporates structural parameters from micro-CT, is proposed to develop precise NPC structural models for assessing the correlation between micro-nano structures and λeff. The Lattice Boltzmann method and Fourier's law are employed to predict their λeff, with only 8% discrepancy from experiment. Results from phenolic matrix reveals that once the porosity of matrix exceeds 60%, any further increases have a limited effect on λeff due to the Knudsen effect at nanoscale. Additionally, fiber structure analysis indicates that an increase in yarn height or width results in a decrease in λeff, yet if product of the two is maintained, λeff remains unchanged as the yarn cross-sectional area remains constant. Furthermore, investigation into stacked preforms demonstrates that λeff increases linearly with the number of fabric layers at a constant felt density, but decreases at a constant preform density owing to densely packed yarns. These results reveal the correlation between multilevel micro-nano structures and thermal conductivity of NPC, providing important guidance for optimizing the NPC thermal insulation performance.

Thermal conductivity of nanoporous phenolic matrices: Measurements and predictions

Nanoporous phenolic-based composites are widely used as lightweight ablative thermal protection materials, but an accurate prediction of the effective thermal conductivity (λeff) associated with the nanoporous phenolic matrix (NPM) remains a significant challenge. Herein, a series of NPMs with different porosities are prepared, and their thermal conductivities measured experimentally. Precise nanostructure models for NPM are established by measuring the porous structural characteristics. These models have facilitated an accurate predication of λeff by combining the non-equilibrium molecular dynamics with the Lattice Boltzmann method, with an associated maximum deviation less than 5 %. Furthermore, the effects on λeff of intrinsic structure such as porosity, particle overlapping, pore and particle size, in addition to environmental factors are investigated. The results demonstrate that changes in λeff due to porosity are primarily confined to porosity values lower than 50 %. When the degree of particle overlapping is below 0.6, any increase in particle overlapping results in a marked increase in λeff. In addition, λeff exhibits a linear relationship with respect to temperature (below 673 K), and a response to changes in pressure is primarily observed within the range 104–107 Pa. This research provides significant insight into the thermal insulation characteristics and mechanisms of NPM, and can serve as a useful guide for the structural optimization of lightweight thermal protection materials.

Mechanical, thermal insulation, and ablation behaviors of needle-punched fabric reinforced nanoporous phenolic composites: The role of anisotropic microstructure

Needle-punched fabric reinforced nanoporous phenolic composite (NPC) is a kind of promising ablative thermal protection material for spaceflight. However, in practical applications, typically anisotropic microstructure of NPC may lead to different performances and damage mechanisms under various directional mechanical or thermal loads. Herein, NPC is prepared and cut into specimens along three typical plane directions including XY-plane (0°), Z-plane (90°), and transitional-plane (45°), and their mechanical, thermal insulation, and ablation behaviors are systematically investigated. Benefiting from the woven fabric in XY-plane, NPC in 0°-plane direction exhibits highest tensile strength (169.2 ± 12.6 MPa), and CT image-based simulation further verifies that woven fabrics are primary load-bearing structure. Meanwhile, NPC in 45°-plane direction shows highest compressive strength (443.1 ± 18.2 MPa) but low compressive stress at low strain, demonstrating a weak bonding between two layers of woven fabrics. Moreover, the heat transfer simulation indicates that horizontally stacked woven fabrics effectively protect the internal material from thermal erosion, thus NPC in 0°-plane direction exhibits optimal thermal insulation. The ablation testing and micro-CT observations further demonstrate that the angle between woven fabric and thermal load significantly influences the ablation mechanism. The present work will further promote the structural reliability and optimization of needle-punched composites.

3D infrared temperature maps measurements of ablative materials during plasma wind tunnel experiments

Accurate surface temperature and recession measurements are crucial experimental data for plasma wind tunnel testing of ablative thermal protection materials. In this work we propose a novel methodology to reconstruct infrared temperature maps on 3D geometries undergoing surface recession. An optical calibration technique is used to extract 3D metric information from the 2D thermal images, thanks to a specific calibration target with control points. The evolution of the sample shape during the experiment is tracked from a side view, using a visual-range camera with dedicated image processing, and reconstructed in 3D under the assumption of axisymmetry. The surface temperature is projected on the time-varying 3D geometry from the calibrated thermograms. The technique is demonstrated on a graphite ablation experiment in air plasma. The reconstructed maps provide detailed multidimensional information about the transient temperature evolution, overcoming traditional approaches that are limited around the stagnation point. The technique allows to study ablation mechanisms with further detail, improving experimental data for material qualification and validation of simulation codes.

Experimental research on online monitoring of high temperature and high speed airflow ablation of quartz ceramic thermal protection structure using guided wave

Thermal protection structure (TPS) plays a key role in protecting hypersonic flight vehicles from harsh service environment and has been widely used. As a common ablative thermal protection material, quartz ceramic TPS realizes thermal insulation by generating ablation during the reentry phases. The structural state of quartz ceramic TPS therefore is of great importance to the service performance and safety of hypersonic flight vehicles, and shows an urgent need for structural health monitoring (SHM). However, the monitoring of real ablation of quartz ceramic structure is barely considered in the current study, let alone online monitoring under harsh service environment, which brings a great challenge to the implementation of reliable ablation monitoring. Aiming at online ablation monitoring of quartz ceramic structure, this paper performs experimental research by taking advantage of guided wave (GW)-based monitoring method. An ablation experiment and monitoring system is built to generate real ablation under high temperature and high speed airflow and perform GW monitoring. The maximal temperature of the system can reach about 2100 °C. The feasibility of ablation monitoring is verified by analyzing ablation-induced influence on GW signals. Gaussian mixture model (GMM) is adopted to combine with GW method to suppress the high temperature variation caused by uncertainty influence on GW signals during the ablation process, so that reliable monitoring of ablation can be realized. Experimental results show that online monitoring of ablation propagation of quartz ceramic structure under high temperature variation is achieved, which can be further used for ablation quantification evaluation.

BAYESIAN IDENTIFICATION OF PYROLYSIS MODEL PARAMETERS FOR THERMAL PROTECTION MATERIALS USING AN ADAPTIVE GRADIENT-INFORMED SAMPLING ALGORITHM WITH APPLICATION TO A MARS ATMOSPHERIC ENTRY

For space missions involving atmospheric entry, a thermal protection system is essential to shield the spacecraft and its payload from the severe aerothermal loads. Carbon/phenolic composite materials have gained renewed interest to serve as ablative thermal protection materials (TPMs). New experimental data relevant to the pyrolytic decomposition of the phenolic resin used in such carbon/phenolic composite TPMs have recently been published in the literature. In this paper, we infer from these new experimental data an uncertainty-quantified pyrolysis model. We adopt a Bayesian probabilistic approach to account for uncertainties in the model identification. We use an approximate likelihood function involving a weighted distance between the model predictions and the time-dependent experimental data. To sample from the posterior, we use a gradient-informed Markov chain Monte Carlo method, namely, a method based on an Ito stochastic differential equation, with an adaptive selection of the numerical parameters. To select the decomposition mechanisms to be represented in the pyrolysis model, we proceed by progressively increasing the complexity of the pyrolysis model until a satisfactory fit to the data is ultimately obtained. The pyrolysis model thus obtained involves six reactions and has 48 parameters. We demonstrate the use of the identified pyrolysis model in a numerical simulation of heat-shield surface recession in a Martian entry.

2.5D quartz fabric reinforced nanoporous phenolic composites with weakened heat transfer and optimized mechanical properties

Phenolic-based composites are the most promising ablative thermal protection materials for space applications, but optimizing their thermal insulation performance while maintaining high strength remains extremely challenging. Herein, phenolic-based composites with co-optimized thermal insulation and mechanical properties are prepared via 2.5D quartz fabric reinforcing nanoporous phenolic. The pore size of phenolic matrix is efficiently refined to ∼35 nm by adjusting resin concentration, enabling composites to exhibit high density but low thermal conductivity by enhancing Knudsen diffusion. Finite element analysis shows that low braiding angle of 2.5D fabric can further weakening heat transfer of composites due to anisotropic thermal conductivity of fiber yarns. In-situ X-ray micro-CT results indicate that 2.5D fabrics can reinforce composites through interwoven yarns, and straighter weft yarns result in higher strength along weft direction than that along warp direction. Compared with dense quartz/phenolic composite, the resulting composites exhibit 16.5% lower density (1.32 g/cm3), 58.8% lower thermal conductivity (0.21 W/(m·K), ≥372.7% higher tensile strength (≥182.0 ± 9.6 MPa) and comparable ablation resistance. The present results will further advance the application of phenolic-based composites in more extreme re-entry environments.

Arc-jet measurements of low-density ablator spallation

The origin and behavior of particles resulting from the spallation of ablative thermal protection materials was investigated using an arc-jet wind tunnel. Particle tracking velocimetry was used to track particles produced through spallation by using high-speed images that determine their trajectories once entering the surrounding flow, which is the post-shock region of a Mach 5 free-stream. Analysis of particle trajectories revealed dependence on flow composition and sample geometry while indicating that the presence of impregnated phenolic resin did not affect spallation. Geometric effects could be attributed to changes in the aerodynamic shear stress at the sample surface, and oxidation was also found to play an important role in increasing the production of spalled particles. Spallation rates were found to be related to the mass loss rate due to ablation, although some geometric effects were also observed. To estimate the contribution of spallation towards overall mass loss, the acceleration of individual particles along their trajectories was analyzed to produce an estimate of spallation particle size distribution. Diameters in the 1 to 100μm range were found, suggesting that these particles result from the breaking of individual carbon fibers of the ablative material, and the overall contribution to mass loss was found to be approximately 6%.

Detailed Modeling of Cork-Phenolic Ablators in Preparation for the Post-flight Analysis of the QARMAN Re-entry CubeSat

This work deals with the analysis of the cork P50, an ablative thermal protection material (TPM) used for the heat shield of the qarman Re-entry CubeSat. Developed for the European Space Agency (ESA) at the von Karman Institute (VKI) for Fluid Dynamics, qarman is a scientific demonstrator for Aerothermodynamic Research. The ability to model and predict the atypical behavior of the new cork-based materials is considered a critical research topic. Therefore, this work is motivated by the need to develop a numerical model able to respond to this demand, in preparation to the post-flight analysis of qarman. This study is focused on the main thermal response phenomena of the cork P50: pyrolysis and swelling. Pyrolysis was analyzed by means of the multi-physics Computational Fluid Dynamics (CFD) code argo, developed at Cenaero. Based on a unified flow-material solver, the Volume Averaged Navier–Stokes (VANS) equations were numerically solved to describe the interaction between a multi-species high enthalpy flow and a reactive porous medium, by means of a high-order Discontinuous Galerkin Method (DGM). Specifically, an accurate method to compute the pyrolysis production rate was implemented. The modeling of swelling was the most ambitious task, requiring the development of a physical model accounting for this phenomenon, for the purpose of a future implementation within argo. A 1D model was proposed, mainly based on an a priori assumption on the swelling velocity and the resolution of a nonlinear advection equation, by means of a Finite Difference Method (FDM). Once developed, the model was successfully tested through a matlab code, showing that the approach is promising and thus opening the way to further developments.

Development of the Low-Density Phenolic-Based Fibrous Ablator ZURAM-K

ZURAM is an ablative carbon–phenolic thermal protection material, developed by the DLR, German Aerospace Center (DLR) in cooperation with the Institute of Space Systems, University of Stuttgart. In addition to building up a basic competence in the sector of ablative thermal protection materials, an objective of the development was to provide a nonclassified ablative material for university research. ZURAM is comparable to similar low-density ablative thermal protection materials with respect to the material density and general ablative performance. It is based on a rigid, highly porous carbon fiber preform, which is filled with a nano-porous matrix derived from a phenolic resin. In the context of the ongoing material development, possibilities to improve the material were investigated with the aim to lower its thermal conductivity and to enhance the adaptability to varying mission profiles. Preliminary results indicate that by substituting the carbon preform with a phenolic fiber preform, a significant reduction of the thermal conductivity can be achieved while recession rates are comparable to the ones of standard ZURAM. The scope of the present work is the development of ZURAM-K based on phenolic Kynol fibers. Furthermore, results from tests in the arc-heated DLR facilities L2K and L3K, as well as data on the thermal conductivity and heat capacity, will be presented.

Numerical Simulation of Chemical Erosion in Vega Solid-Rocket-Motor Nozzles

The erosion of rocket-nozzle ablative thermal protection materials during the solid-rocket-motor burning time needs to be accounted for to get reliable performance predictions, especially for long-duration firings and/or small nozzles (that is, upper stages). This work numerically investigates the erosion behavior as a function of chamber conditions for the nozzles of the Vega launch vehicle solid rocket motors: Zefiro 9, Zefiro 23, and P80. This study has been performed using a Reynolds-averaged Navier–Stokes equation solver with specific application to ablative nozzles. The throat region of the nozzle, and specifically the carbon/carbon nozzle insert, has received special attention because its evolution has a significant impact on the motor performance. The analysis of the throat insert has been performed for the three full-scale nozzles by comparing the predictions to measured data from static firing tests. A sensitivity analysis to the most important model parameters that can influence the throat erosion prediction has been performed to highlight modeling capabilities and limits, as well as numerical uncertainties. Finally, a characterization in terms of nozzle throat ablation mass flux dependency upon the motor chamber pressure has been derived for the three nozzles via a power fitting of the numerical data performed at discrete chamber pressure levels.

Low thermal conductivity carbon fibrous composite nanomaterial enabled by multi-scale porous structure

Low thermal conductivity carbon is a type of material for special uses such as thermal insulation/protection and particularly for ablative thermal protection material of reentry vehicles and rocket engine components. In this research, a low thermal conductivity carbon nanofibrous material was prepared by electrospinning polyacrylonitrile (PAN) with poly (methyl methacrylate) (PMMA) as well as silica nanoparticles (SNPs) followed by stabilization and carbonization. Morphology and structure of this carbon nanofibrous material were characterized by electron microscope, X-ray diffraction, Raman spectroscopy, and BET surface area analysis and correlated with its thermal conductivity. Introduction of PMMA and SNPs to PAN precursor nanofibers through multi-component electrospinning enabled a unique concurrent multi-scale (micro-, submicro- and nano-) porous structure in the resultant carbon nanofibrous mat and synergistically reduced the thermal conductivity by up to 98% with respect to the non-porous carbon film counterpart. This research demonstrated a novel and effective way to design and manufacture low thermal conductivity carbon materials.

High temperature permeability of fibrous materials using direct simulation Monte Carlo

Porous carbon fiber materials are used as effective insulators in many applications where high temperatures are involved. In particular, they are used as the substrate of ablative thermal protection materials for atmospheric entry systems. In this application and in many other industrial uses, quantifying the permeability of porous materials is needed to compute the flow rate of gases through them, under certain environmental conditions. In this work, direct simulation Monte Carlo (DSMC) simulations are used to compute permeability of several fibrous substrates to high temperature gases. The actual porous geometry of the materials is digitized using X-ray microtomography. Numerical results at various pressures and Knudsen numbers are compared with experimental data published in the literature. The method confirms that the pressure dependence of effective gas permeability is well represented by the Klinkenberg formulation. The method is validated by showing close agreement between measurements of permeability from simulations and experimental investigations. Four carbon fiber materials with different microstructures are investigated. We show that the permeability strongly depends on the pore size distribution, as well as on the porosity of the material.

Arcjet Screening of Candidate Ablative Thermal Protection Materials for Mars Science Laboratory

Arc plasma tests of a range of ablative materials considered candidates for the thermal protection system for the Mars Science Laboratory are described. The objective of these tests was to identify ablative thermal protection materials that offer weight, performance, cost, or other advantages relative to SLA-561V. Several low-density silicones with a mixture of low-density fillers and polymer-impregnated ceramics were tested in the Ames Interaction Heating Facility at conditions representative of nominal stagnation point and worst-case conical heating on the Mars Smart Lander forebody. A wide variety of low-density ablatives and impregnated flexible materials were tested in the Ames Panel Test Facility at nominal and worst-case heating for the Mars Smart Lander backshell. The results of these tests have identified several attractive materials worthy of further investigation.