Phenolic Impregnated Carbon Ablator Research Articles

Microscale Thermal–Structural Modeling for Carbon Fibers Subjected to a Hypersonic Boundary Layer

With regard to the thermal protection system, the thermal–structural responses of modern ablative materials are of primary importance in many aspects, including material selection and sizing. Due to the intricate nature of their porous structure and complicated thermal conditions they are subjected to, ablative materials may exhibit unexpected behavior, which can potentially lead to material failure. In this study, two solvers—a direct simulation Monte Carlo solver and a finite-volume-based material response solver—are coupled together to predict the microscale thermal–structural performance of a thermal protection system material like phenolic impregnated carbon ablator. In this approach, individual fibers are modeled at the microscale, which provides valuable knowledge of porous media behavior. Nonuniform boundary conditions, including the heat flux and external force, are captured by the direct simulation Monte Carlo solver, and the detailed thermal and structural performance of the fiber is captured by the material response solver. The results show that individual fibers do not fail based on the temperature gradient and applied aerodynamic forces. However, it is shown that the attachment points of the fibers are the most vulnerable. This vulnerability can potentially lead to a breakdown of the binders, which would separate fibers and cause material failure.

Establishment of strain rate‐dependent intrinsic models for short‐cut carbon fiber/nitrile composites in different pyrolysis states

AbstractFlexible nozzle extension technology has been extensively researched throughout the world, but traditional materials cannot meet the growing demands in harsh environments. Short‐cut carbon fiber‐reinforced nitrile rubber (CF/NBR) composites, which are similar to the phenolic impregnated carbon ablator, are currently a hot research topic in this field. The mechanical properties of CF/NBR composites were experimentally tested to study the change in the mechanical behavior of a rocket engine's flexible nozzle extension section under thermal coupling. As indicated by the results, existing models cannot accurately predict changes in materials' mechanical behavior. The theoretical foundation of the nonlinear viscoelastic intrinsic model of rubber is utilized. The uniaxial tensile intrinsic model of CF/NBR composites has been improved by introducing physical parameters, such as fiber reinforcement, strain rate‐dependent correction and temperature‐dependent correction coefficients, and fiber volume fraction. Results show that the intrinsic model developed in this article provides a guide to the uniaxial tensile mechanical behavior of randomly distributed short‐cut CF/NBR composites with known mass‐part formulations and fiber volume fractions at room temperature and under different pyrolysis conditions.

Phase-field model of char oxidation in ablative thermal protection system materials

We develop a phase field model of char oxidation during atmospheric entry in the ablation zone of the Phenolic Impregnated Carbon Ablator (PICA). The phase field model is coupled to a model of heat transport, to capture the interaction between the oxidation and temperature, and it is based on our previous model of carbon fiber oxidation. We explore the char oxidation behavior using three 2D examples. First, we simulate oxidation with an imposed temperature gradient for char structures with increasing amounts of initial porosity. The oxidation time decreases with increasing porosity and the oxidation rate fluctuates as pores are opened to oxygen transport. Next, we predict the decrease in temperature due to the endothermic oxidation reactions in thermally-insulated systems with and without transport of oxygen through the top boundary. In both cases, the temperature decreases until oxidation stops; it stops due to the decrease in temperature in the case open to oxygen transport and due to lack of oxygen in the case closed to oxygen. Finally, we explore the oxidation behavior under different heat fluxes. The oxidation reactions compensate for the heat flux until the char is fully consumed and then the temperature increases. Our results demonstrate the importance of considering the interactions between the oxidation reactions and the temperature when modeling char oxidation.

Equilibrium model for the ablation response of silicone-coated PICA

A novel equilibrium ablation and thermal response model for NASA’s Phenolic Impregnated Carbon Ablator (PICA) is presented. For the first time, the model accounts for the effect of NuSil, a silicone overcoat applied as contamination control during assembly, test, and launch operations for both the Mars Science Laboratory and the Mars 2020 heatshields. The protective effect of NuSil against atomic oxygen is discussed based on experimental results collected under arc jet environment. A detailed mass and heat transfer analysis for PICA-NuSil was implemented in the Porous material Analysis Toolbox based on OpenFOAM, PATO. The model includes specific boundary conditions for the coating removal and surface equilibrium processes using representative elements of the NuSil-environment system. The charred NuSil surface was modeled as SiO2 based on the observation that a silicon oxycarbide layer, formed during the decomposition of NuSil, transforms into domains of pure silica and charred PICA. Two-dimensional material response simulations were performed to compare PICA and PICA-NuSil using boundary conditions calibrated with arc jet data. The present work constitutes the first demonstration of a multi-dimensional ablation response of a silicone-coated carbon-phenolic ablator. It is shown that simulations agree with experiments, and the model reproduces measured temperature and recession. Differences between the PICA-NuSil response under Earth and Mars atmospheres are discussed.

Evaluation of Sublimation Kinetics for Phenolic Impregnated Carbon Ablator

Recession analyses of stagnation tests conducted in NASA Ames Research Center arcjet facilities with surface heat fluxes high enough for carbon char to sublimate are performed and compared with data. The axisymmetric recession computations are based on both the finite-rate gas–surface interaction model and the standard chemical equilibrium surface ablation model. The finite-rate gas–surface interaction model is studied using three different sets of sublimation coefficients, which are obtained through direct measurements. The standard chemical equilibrium ablation model is established through the tables. The ablating material used in these arcjet tests is phenolic impregnated carbon ablator. The prediction of axisymmetric surface recession is achieved by implicitly coupling a material thermal response code and a flow code. The material thermal response code used in this study is the Two-Dimensional Implicit Thermal-Response and Ablation program, which predicts charring material thermal response and shape change. The flow code solves the reacting Navier–Stokes equations using the data parallel line relaxation method. The recession predictions for three selected arcjet conditions with various base radii are presented. The effect of sticking coefficient for carbon sublimation on recession prediction is investigated, and the prediction discrepancy between finite-rate and chemical equilibrium models is examined and discussed in detail.

Facile fabrication of lightweight carbon fiber/phenolic ablator with improved flexibility via natural rubber modification

The rigidity and low strain-to-failure of the phenolic impregnated carbon ablator seriously restrict the application of this state-of-the-art thermal protection material. In this study, to improve the flexibility of carbon fiber/phenolic ablator (CFPA), natural rubber (NR) was deposited homogeneously onto the surface of the carbon fibers through impregnation. Thus, a “bridge” was built between the carbon fiber reinforcement and the phenolic resin aerogel matrix, allowing the interface better connection and better deformation ability. Experimental results indicate that, compared with the unmodified, modified CFPA has lower flexural modulus and higher fracture strain, 6.3 MPa and 39.6% at the optimal NR latex mass fraction respectively, while maintains low density, good thermal insulation properties, and excellent ablation properties. The facile fabrication via natural rubber modification of CFPA improved its flexibility and could broaden the application scenarios of lightweight carbon/phenolic composite.

Phase-field modeling of carbon fiber oxidation coupled with heat conduction

In this paper, we developed a multiphysics mesoscale model of carbon fiber oxidation in the upper most layer of a phenolic impregnated carbon ablator thermal protection system. The presented model uses the phase-field method to capture the reduction of carbon fibers due to an oxidation reaction between carbon and oxygen to form carbon monoxide. The model is fully coupled with heat transport in the system, including the heat absorbed by the endothermic oxidation reaction. We implemented the model in the new Macaw application, created using the Multiphysics Object-Oriented Simulation Environment (MOOSE). The model was verified against an analytical solution of surface reactions. A sensitivity analysis revealed that the parameters that most impact the oxidation time are those related to the reaction rate. The model was demonstrated using 2D and 3D simulations of the oxidation of multiple fibers, illustrating the impact of fiber diameter, temperature, and gas flow on the fiber oxidation.

Thermal Analysis for Phenolic Impregnated Carbon Ablator/CV-1144-0 in Hypersonic Test Environments

Analysis of multidimensional in-depth thermal response for the phenolic impregnated carbon ablator (PICA)/CV-1144-0 material system in NASA Langley Research Center Hypersonic Materials Environmental Test System environments was performed using the 3-D Fully Implicit Ablation and Thermal (3dFIAT) code, which was enhanced to enable the simulation of multidimensional thin-coating burn-through and multistreams of pyrolysis gas flow. CV-1144-0 is a controlled volatility room-temperature vulcanized silicone oxygen protective coating. Three proposed in-depth thermal response models for CV-1144-0 were examined to assess their accuracy and understand how the coating affected PICA thermal performance. These proposed in-depth thermal response models were used to predict the in-depth temperature history for three groups of arcjet test models. Pyrometer surface temperature measurements and recessed surface data from laser scans were used as the boundary conditions in 3dFIAT in-depth response simulations. Data-parallel line relaxation was used to compute the surface heat flux and pressure distributions based on the surface geometries of pretest and posttest models. Comparisons between computations and thermocouple data are presented and discussed for each test model. The uncertainty of surface temperature and recession measurements on in-depth temperature prediction is investigated. The effect of CV-1144-0 coating thickness on the modeling of PICA in-depth thermal response is also studied.

Influence of ambient pressure on thermal response of polymer matrix composites

AbstractAmbient pressure is crucial to the thermal performance of polymer matrix composites, which are usually used as the thermal protection system (TPS) materials of hypersonic vehicles. Aiming at studying the influence of ambient pressure on thermal response of polymer matrix composites, a novel pyrolysis layer model with pressure is presented, and its mathematical model is deduced and programmed in MATLAB. Comparison between numerical results and experimental results of theoretical ablative composite for open testing is given to verify the accuracy of the presented model. After that, thermal responses of phenolic impregnated carbon ablator under ambient pressures range from 1 to 500 kPa are simulated. It is found that accumulation and backstreaming of generated pyrolysis gasses begin to appear with the increase of ambient pressure. And there is an obvious linear correlation between the reciprocal of the material internal pressure and the ambient pressure. In addition, bondline temperature of polymer matrix composites declines as the ambient pressure decreases. Effect of ambient pressure on thermal response of polymer matrix composites cannot be neglected, which should be taken into account in the TPS design. This study contributes to the accurate prediction of thermal response of polymer matrix composites.

Lightweight, flexible, and heat‐insulated phenolic impregnated carbon ablator (PICA) with adjustable flexibility and high compressive resilience property

AbstractPhenolic impregnated carbon ablator (PICA) as a typical lightweight ceramic ablative material is widely used in thermal protection systems (TPS). However, the rigidity and poor mechanical property of the existing PICA limit its application in flexible TPS. For the purpose of solving these problems, a novel lightweight, flexible, and heat‐insulated PICA material is first successfully fabricated using resorcinol‐formaldehyde (RF) aerogel as matrix and flexible carbon fiber felt as reinforcement through vacuum impregnating and ambient pressure drying. The prepared composites have relatively low densities (0.20 ~ 0.27 g/cm3) and low thermal conductivities (~0.11 W/[m·K]) at room temperature. The flexible PICA has adjustable flexibility through changing the RF concentration of impregnated sols and exhibits excellent folding endurance, which can be folded more than 500 times without fracture. The material can recover to its original shape without fracture when the compressive strain is up to 60%, which shows high compressive resilience properties. The result of the back‐face temperature test indicates the prepared flexible PICA composite has improved thermal insulation property at high temperature by impregnating with RF aerogels. These results demonstrate the flexible PICA prepared in this work is a promising material that can be used in the flexible thermal protection field of aerospace.

Pyrolysis layer model of polymer matrix composites with heating rate and pressure

Heating rate and ambient pressure affect the resin matrix pyrolysis process, the pyrolysis gases flow state and the temperature distribution of polymer matrix composites as heatshield of spacecraft. Therefore, a pyrolysis layer model with heating rate and pressure is presented. Accuracy of prediction of heating rate distribution calculated by the presented model is validated by comparing Arcjet wind tunnel test results of phenolic impregnated carbon ablator (PICA). After that, the thermal behavior of PICA under different heating rates is predicted by this presented model. It is indicated that the mass injection rate of pyrolysis gases increases, the bondline temperature declines and the char layer's density decreases as the heating rate rises. Besides, from material bondline to material surface, the heating rate maximum increases exponentially along material thickness. This study contributes to the optimal design of the thermal protection system of spacecraft.

Theoretical prediction for effective thermal conductivity of composite materials with random structure

Based on the random generate-growth method, a theoretical method employing linked lists was implemented to predict the effective thermal conductivity of fiber-reinforced composite materials with random structure, which has the feature of physical intuition and independent of mesh. The fiber perform of like-PICA (Phenolic impregnated carbon ablator) was studied to reveal the relevant factors that can influence the effective thermal conductivity. The results show that the effective thermal conductivity is not an intrinsic property and it may be related with the specimen size when it is close to fiber length. There is a positive nonlinear correlation between the fiber quantity per volume and the effective thermal conductivity. However, the effective thermal conductivity is not a uni-variate function of volume fraction and the concept of the effective length is present to describe the connectivity of fiber perform, which is negative correlation with the effective thermal conductivity.

Analytical solution for multi-component pyrolysis simulations of thermal protection materials

This paper provides the analytical solution to a one-parameter Šesták–Berggren kinetic model for the thermoanalytical study of pyrolysis reactions involving multiple independent parallel reactions. Such multiple independent parallel reactions are widely used, for instance, in the modeling of the pyrolysis of thermal protection materials used in heatshields for spacecraft during the atmospheric entry phase. Solving inverse problems to infer parameters of the kinetic model through optimization techniques or Bayesian inference methods for uncertainty quantification may require a large number of evaluations of the response and its sensitivities (derivatives with respect to parameters). Moreover, in the case of kinetic equations, the Arrhenius parameters can exhibit strong dependence that can require further model evaluations for an accurate parameter calibration. The interest of this analytical solution is to reduce computation cost while having high accuracy to perform parameter calibration from experiments and sensitivity analysis. We propose to use exponential–integral functions to express the solution of the temperature integral, and we derive the analytical solution and its sensitivities for the parallel reaction model both for constant temperature (isothermal) and for constant heating rate conditions. The solution is validated on a six-equation model using parameters inferred in a previous work from the experimental data of the pyrolysis of a phenolic-impregnated carbon ablator material, and we compare the computational cost and accuracy of the implemented analytical solution with a numerical solution. Our results show that the use of such analytical solution with an accurate computation of the exponential–integral function significantly reduces the computational cost compared to the numerical solution.

A multiscale cohesive law for carbon fiber networks

Better predictive models of mechanical failure in low-weight heat shield composites would aid material certification for missions with aggressive atmospheric entry conditions. Here, we develop such a model for the rapid engineering analysis of the failure limits of phenolic impregnated carbon ablator (PICA) - a leading heat shield material whose structural component is a carbon fiber network. We hypothesize inelastic deformation failure mechanisms and model their behavior using molecular dynamics simulations to calculate the binding energy. We then upscale this binding energy to the macroscale using a renormalization argument. The approach delivers insightful and reasonably accurate macroscale predictions that compare favorably to experiments. In application, the model is validated for a particular variety of PICA by comparison to experiment and would then be used to study design scenarios in different entry conditions.

Competitive kinetic model for the pyrolysis of the Phenolic Impregnated Carbon Ablator

Carbon/phenolic ablators are successfully used as thermal protection material for spacecraft. Nevertheless, their complex thermal degradation is not yet fully understood, and current pyrolysis models do not reproduce important features of available experimental results. Accurate and robust thermal degradation models are required to optimize design margin policy. We investigate whether the competitive kinetic schemes commonly used to model biomass pyrolysis are appropriate to describe the thermal degradation of carbon/phenolic composites. In this paper, we apply competitive pyrolysis mechanisms for the thermal degradation of the carbon/phenolic ablator PICA. Model parameters are then calibrated using a robust two-step methodology: first deterministic optimization is used to obtain the best estimation of the calibration parameters based on the experimental data, then a stochastic Bayesian inference is performed to explore plausible set of solutions taking into account the experimental uncertainties. The proposed calibrated model provides an accurate description of the pyrolysis process at different heating rates. The model shows great flexibility and robustness at a similar computational cost as the traditional devolatilization models. This opens the possibility for more complex mechanisms when more experimental data becomes available.

Constitutive Model of Fiber Substrate Ablators for Conceptual Design

Fiber substrate-based ablators such as Phenolic-Impregnated Carbon Ablator are composite materials comprising a fibrous substrate and a polymer matrix. This paper presents an approach for estimating selected bulk material properties for the purposes of conceptual design and manufacturing evaluations. A filamentary analog model of the substrate, using a volumetric approximation of the individual fibers, is combined with known properties of the cured resin to approximate the composite material, resulting in closed-form expressions for the virgin density, resin mass fraction, and char yield. Two variants of a low-density carbon felt/phenolic resin ablator were fabricated for comparison to the analytical approximations. Bounds on the material properties were generated from the closed-form expressions and then compared to Monte Carlo results. A linear regression analysis was then used to correlate output uncertainties to those of the input parameters to understand the drivers of material variability.

A high heating rate pyrolysis model for the Phenolic Impregnated Carbon Ablator (PICA) based on mass spectroscopy experiments

A novel model for the pyrolysis of the Phenolic Impregnated Carbon Ablator (PICA) at high heating rate is developed and calibrated based on high fidelity thermal decomposition experiments. The calibration relies on accurate quantification of pyrolysis gases obtained from mass spectroscopy analysis during thermal decomposition at fast heating rates simulating flight conditions. Model calibration is achieved by coupling the Porous material Analysis Toolbox based on OpenFOAM (PATO) with an optimization software (Dakota). A multi-objective genetic algorithm is used to fit the experimental data by optimizing the model parameters for an element and a species-based formulation. The new model captures both the material mass loss and the gaseous species produced during pyrolysis.

Assessment of a one-dimensional finite element charring ablation material response model for phenolic-impregnated carbon ablator

In this study, mathematical formulations to model charring ablation problems were numerically implemented using finite element analysis (FEA) with ABAQUS, which account for the material decomposition and progressive surface removal in the heat conduction and the surface energy balance equations. FEA was performed for a one-dimensional model to predict the temperature and ablation histories of a phenolic-impregnated carbon ablator sample (i.e., a common heat shield material for hypersonic vehicles and spacecraft) subjected to oxy-acetylene torch flame (i.e., 0.8 SLPM acetylene gas to 2.7 SLPM oxygen gas). The recovery enthalpy and convective heat transfer coefficient for the ablation model were calculated based on gas compositions and two assumed surface conditions (i.e., equilibrium and frozen). Simulations using the calculated recovery enthalpy and convective heat transfer coefficient resulted in a recession rate of 6.38 times (equilibrium) and 14.08 times (frozen) higher than the experimental data, despite fair agreement of the surface temperature. In addition, the effect of the heat transfer coefficient was investigated through a steady-state ablation analysis. The results of the analysis indicate that there is not one single value for the heat transfer coefficient that would allow the prediction to match both measured recession rate and surface temperature. Possible reasons for such an inconsistency are provided and discussed.

Ablative thermal protection system under uncertainties including pyrolysis gas composition

Spacecrafts such as Stardust (NASA, 2006) are protected by an ablative Thermal Protection System (TPS) for their hypersonic atmospheric entry. A new generation of TPS material, called Phenolic Impregnated Carbon Ablator (PICA), has been introduced with the Stardust mission. This new generation of low density carbon-phenolic composites is now widely used in the aerospace industry. Complex heat and mass transfer phenomena coupled to phenolic pyrolysis and pyrolysis gas chemistry occur in the material during atmospheric entry. Computer programs, as the Porous material Analysis Toolbox based on OpenFoam (PATO) released open source by NASA, allow to study the material response. In this study, a non-intrusive Anchored Analysis of Variance (Anchored-ANOVA) method has been interfaced with PATO to perform low-cost sensitivity analysis on this problem featuring a large number of uncertain parameters. Then, a Polynomial-Chaos method has been employed in order to compute the statistics of some quantities of interest for the atmospheric entry of the Stardust capsule, by taking into account uncertainties on effective material properties and pyrolysis gas composition. This first study including pyrolysis gas composition uncertainties shows their key contribution to the variability of the quantities of interest.

Thermal behavior of gradient charring materials with improved iterative method

ABSTRACTThe lightweight material of the thermal protection system for a reentry vehicle has become a trend in order to maximize the payload. To explore the effect of gradient density on the thermal behavior of charring materials, a pyrolysis layer model considering of variable density is developed and discretized with the central difference method. An iterative method is improved to look for the locations of the moving interfaces and boundary in each time step. Taken different gradient phenolic impregnated carbon ablator under a return trajectory heat flux for the Orion vehicle as example, their thermal behavior and effective heat capacities are simulated using the code written. Numerical results indicate that the gradient designs can improve the thermal protection performance of charring materials effectively. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47076.