Phenolic Impregnated Carbon Ablator Research Articles

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.

Multidimensional Finite Volume Fully Implicit Ablation and Thermal Response Code

A finite volume, fully implicit ablation and thermal response code that simulates the pyrolysis gas flow, thermochemical ablation, and shape change of thermal protection materials and systems in multiple dimensions is developed. Both structured and unstructured grid approaches are implemented. The governing equations (which include energy conservation, pyrolysis gas mass conservation with Darcy’s law, a multicomponent decomposition model, and a surface energy balance) are solved with a moving grid system to simulate the response of charring ablators in aerothermal heating environments. This work demonstrates and validates new capabilities that are added to the code. These expanded capabilities include fully implicit time integration, pyrolysis gas flow due to Darcy’s law, an unstructured grid option, and parallel computing. Three groups of test cases that consider three different charring ablators are presented. In the first group, three-dimensional iso-q-shaped phenolic-impregnated carbon ablator test models with various throughthickness conductivity directions are studied. In the second group, axisymmetric flat-faced dual-layer woven carbon phenolic models are examined. Finally, in the third group, axisymmetric iso-q-shaped three-dimensional multifunctional ablative thermal protection system models are analyzed. For all cases, predictions are compared with available temperature data.

A novel method for analysing the thermal behaviour of charring ablator

Full comprehension of the thermal behavior of the charring ablator is of great importance in designing the thermal protection system (TPS) of a reentry vehicle. To analyze the thermal response of the charring composites under aerodynamic heating, we have improved one-dimensional coupled pyrolysis layer model, in which the transfer of the heat and mass, the moving interfaces and the ablation surface are coupled. A new iterative method is proposed to determine the moving distances of the two moving interfaces and the ablation surface in each time step. The implicit finite difference formulas are derived from the governing equations, and programmed in MATLAB. The iterative method is validated with the arcjet test data in NASA Ames Research Center, and then the thermal response of the phenolic impregnated carbon ablator (PICA) in the Orion vehicle during a lunar return is predicted. This study will be helpful for the optimization of the TPS in reentry vehicles.

In Situ Ablation Recession and Thermal Sensor for Thermal Protection Systems

In this research, a break-wire-like ablation recession and thermal sensor based on commercial, affordable thermocouples was designed, fabricated, and tested. The objective of this paper was to demonstrate the capability of this technique to provide real-time measurement of in-depth temperature and recession rates. The in situ ablation recession and thermal sensors were assembled on ablatives ranging from low to high densities (). The different types of ablatives adapted for this study were phenolic impregnated carbon ablator, AVCOAT, carbon/phenolic, and carbon/carbon composites. These ablation recession and thermal sensors were tested under an oxidative hyperthermal environment at a heat flux in the range of created by an oxyacetylene test bed. The results were very promising, demonstrating that the proposed approach can provide accurate recession rate data of a variety of ablatives. The sensing technology developed is very versatile and can be applied to the thermal protection systems of spacecraft, and can also be exploited on rocket nozzles. The key features of this sensor system are the use of inexpensive, commercially available thermocouples and industry-standard drilling techniques demonstrated on different ablatives.

Multidimensional material response simulations of a full-scale tiled ablative heatshield

The Mars Science Laboratory (MSL) was protected during Mars atmospheric entry by a 4.5 meter diameter heatshield, which was constructed by assembling 113 thermal tiles made of NASA's flagship porous ablative material, Phenolic Impregnated Carbon Ablator (PICA). Analysis and certification of the tiles thickness were based on a one-dimensional model of the PICA response to the entry aerothermal environment. This work provides a detailed three-dimensional heat and mass transfer analysis of the full-scale MSL tiled heatshield. One-dimensional and three-dimensional material response models are compared at different locations of the heatshield. The three-dimensional analysis is made possible by the use of the Porous material Analysis Toolbox based on OpenFOAM (PATO) to simulate the material response. PATO solves the conservation equations of solid mass, gas mass, gas momentum and total energy, using a volume-averaged formulation that includes production of gases from the decomposition of polymeric matrix. Boundary conditions at the heatshield forebody surface were interpolated in time and space from the aerothermal environment computed with the Data Parallel Line Relaxation (DPLR) code at discrete points of the MSL trajectory. A mesh consisting of two million cells was created in Pointwise, and the material response was performed using 840 processors on NASA's Pleiades supercomputer. The present work constitutes the first demonstration of a three-dimensional material response simulation of a full-scale ablative heatshield with tiled interfaces. It is found that three-dimensional effects are pronounced at the heatshield outer flank, where maximum heating and heat loads occur for laminar flows.

Optimization of the curing process of phenolic impregnated carbon ablator

ABSTRACTPhenolic impregnated carbon ablator (PICA), which is composed of the phenolic resin (PR) and carbon fiber, is of particular interest to researchers in the aerospace field. In this work, PICA was prepared by the double‐stage isothermal heating curing method and the curing process of PICA was optimized based on non‐isothermal differential scanning calorimetry, extrapolation method and orthogonal experiment. Orthogonal array chose the standard L9 (34). The results show that the optimum curing process of PICA is 120 °C 1 h + 170 °C 1 h, based on considering the better compressive property as the standard. And the results of the Fourier transform infrared spectra indicate that the more dimethylene ether groups result in the higher compressive strength of PICA cured at 120 °C 1 h + 170 °C 1 h. The existence of the more dimethylene ether groups can increase the amount of gases generated and bring off more heat during the ablative process of materials. This may provide a new vision for improve the ablative property of materials. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46230.

Curing kinetics and curing process of phenolic impregnated carbon ablator

ABSTRACTPhenolic impregnated carbon ablator (PICA), which is composed of the phenolic resin (PR) and carbon fiber, is of particular interest to researchers in the aerospace field. In this work, PICA was prepared by the double‐stage isothermal heating curing. Then, the curing kinetics of boron‐modified phenolic resin (BPR) was investigated by non‐isothermal differential scanning calorimetry method in order to optimize the curing temperature of BPR. Further, the effect of the heating rate during curing process on the compressive strength of PICA was discussed in detail. The experimental data show that the curing of BPR needs more energy so that the curing temperature of BPR under different condition is higher than that of virgin PR. Notably, with the increasing heating rate during the curing process, the micro‐cracks increase and the compressive strength of PICA decreases. Once the heating rate exceeds a critical value, the micro‐cracks no longer increase and the heating rate has insignificant effect on the compressive strength. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 45434.

Decomposition of Phenolic Impregnated Carbon Ablator (PICA) as a Function of Temperature and Heating Rate.

Material response models for phenolic-based thermal protection systems (TPSs) for atmospheric entry are limited by the lack of knowledge of the nonequilibrium processes that may govern the decomposition pathways of phenolic resin at heating rates up to tens of degrees Celsius per second. We have investigated the pyrolysis of phenolic impregnated carbon ablator (PICA) by measuring the molar yields of the volatile decomposition products as a function of temperature at four nominal heating rates of 3.1, 6.1, 12.7, and 25 °C s-1, over the temperature range of 100-1200 °C. A mass spectrometer was used to probe the 14 significant gaseous products directly as PICA samples were heated resistively in vacuum. Four products, H2, CH4, H2O, and CO, overwhelmingly dominated the molar yields. However, in terms of mass yield, phenol and its methylated derivatives, cresol and dimethyl phenol, were significant. The temperature-dependent molar yields of the observed products exhibited a marked dependence on heating rate. The heating-rate-dependent behavior of the molar yields has been attributed to two main competing decomposition processes that occur as the temperature passes from roughly 300 to 500 °C: (1) cross-linking reactions that produce ether functional groups and carbon-carbon bonds and eliminate H2O and (2) breakdown of the polymer backbone through scission of methylene bridges and liberation of phenol and its methylated derivatives. The latter process competes more effectively with the former as the heating rate increases. The relative rates of these processes appear to have a significant effect on the molar yields of volatile products from subsequent decomposition processes as the temperature is increased further. Thus, the heating rate strongly affects the pathways taken during the pyrolysis of the phenolic resin in PICA. The new data may be used to test nonequilibrium models that are designed to simulate the response of TPS materials during atmospheric entry of spacecraft.

Microstructure and ablation behavior of an affordable and reliable nanostructured Phenolic Impregnated Carbon Ablator (PICA)

Among polymeric Thermal Protection System (TPS) materials, Phenolic Impregnated Carbon Ablators (PICAs) offer substantial specific weight reduction. A traditional NASA developed PICA is made starting from a preform or using Milled Carbon Fibers (MCFs) to produce a porous skeleton impregnated with a liquid phenolic matrix; the production methods, allow to obtain a low density porous material showing high ablation resistance. The aim of this research was to produce a nanostructured PICA (n-PICA) based on the use of MCFs, a liquid phenolic matrix, and selected nanofillers. Two different nanofillers were used: Multi-Walled Carbon Nanotubes (MWNTs) and nanoclays. The synergistic effect of the two nanofillers was evaluated in terms of thermal and dimensional stability, mechanical properties and ablation resistance via oxy-acetylene torch. Scanning electron microscopy was used to investigate the exposed and in depth surface morphologies of the oxy-acetylene torch tested specimens in order to highlight the differences in the thermal erosion mechanisms.

Conceptual Design of a Small Earth Reentry Vehicle for Biological Sample Return

A conceptual design of an Earth return vehicle is presented with the goal of safely returning biological samples from orbit. Key entry, descent, and landing trade studies were completed at the conceptual level for two different mission scenarios: return from the International Space Station and an autonomous, free-flying vehicle returning from low Earth orbit. The analyses that follow for each key subsystem drove design decisions to create the Biopan Deployment in Orbit for Microgravity Exposure vehicle with the versatility to satisfy both of the aforementioned mission scenarios. The final design features a ballistic entry, a 45 deg sphere–cone aeroshell with a diameter of 88 cm, a phenolic impregnated carbon ablator heat shield with a thickness of 7.7 cm, and a passive landing system containing an 8-m-diam ringsail parachute combined with a 7.8-cm-thick crushable carbon foam. Analysis of the vehicle performance verified survivability of biological samples due to heat and deceleration loads from entry. Trajectory dispersion analysis yielded crossrange and downrange limited to and , respectively, whereas landing velocity was confirmed to be for all cases.

Detailed analysis of species production from the pyrolysis of the Phenolic Impregnated Carbon Ablator

Many modern materials that are being developed to protect space vehicles entering planetary atmospheres use phenolic impregnated carbon fiber substrates as the basic material architecture. To mitigate the heat flux into the material, the decomposition of phenolic phase generates protective gases that blow into the boundary layer and help shield the material. The goal of this paper is to measure the decomposition products of cross-linked phenolic as used in the NASA's Phenolic Impregnated Carbon Ablator (PICA). A custom batch reactor was designed to quantitatively determine detailed species production from the pyrolysis of PICA. A step-wise heating procedure using 50K increments from room temperature up to 1250K was followed. An initial PICA mass of 100mg was loaded in the reactor, and the mass loss was measured after each 50K step. Species production after each step was quantified using gas-chromatography techniques. The quantitative molar yields of pyrolysis products as a function of reaction temperature are compared to those from resole phenol-formaldehyde resin pyrolysis reported in the literature. The differences in product distributions between PICA pyrolysis and resole phenol-formaldehyde pyrolysis confirm that the decomposition products are sensitive to the composition of the material and the cross-linking process. These results indicate that characterizations need to be performed for all variations of phenolic-matrix based ablators. Such information is also critical for the development of next generation material response models.

Evaluation of Finite-Rate Gas/Surface Interaction Models for Carbon-Based Ablator

Two sets of finite-rate gas/surface interaction model between air and the carbon surface are studied. The first set is an engineering model with one-way chemical reactions, and the second set is a more detailed model with two-way chemical reactions. Each of these two proposed models intends to cover the carbon surface ablation conditions including the low-temperature rate-controlled oxidation, the midtemperature diffusion-controlled oxidation, and the high-temperature sublimation. The prediction of carbon surface recession is achieved by coupling a material thermal response code and a Navier–Stokes 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 on hypersonic space vehicles. The flow code solves the reacting full Navier–Stokes equations using the data parallel line relaxation method. Recession analyses of stagnation tests conducted in NASA Ames Research Center arc-jet facilities with heat fluxes ranging from 45 to are performed and compared with data for model validation. The ablating material used in these arc-jet tests is phenolic impregnated carbon ablator. Computational predictions of surface recession and shape change are in good agreement with measurement for arc-jet conditions.

Inverse Estimation of the Mars Science Laboratory Entry Aeroheating and Heatshield Response

The Mars Science Laboratory entry vehicle successfully landed the Curiosity rover on the Martian surface on 5 August 2012. A phenolic impregnated carbon ablator heatshield was used to protect the spacecraft against the severe aeroheating environments of atmospheric entry. This heatshield was instrumented with a comprehensive set of pressure and temperature sensors. The objective of this paper is to perform an inverse estimation of the entry vehicle’s surface heating and heatshield material properties. The surface heating is estimated using the flight temperature data from the shallowest thermocouple. The sensitivity of the estimated surface heating profile to estimation tuning parameters, measurement errors, recession uncertainty, and material property uncertainty is investigated. A Monte Carlo analysis is conducted to quantify the uncertainty bounds associated with the nominal estimated surface heating. Additionally, a thermocouple driver approach is employed to estimate heatshield material properties using the flight data from the deeper thermocouples while applying the shallowest thermocouple temperature as the surface boundary condition.

Conformal Phenolic Impregnated Carbon Ablator Arcjet Testing, Ablation, and Thermal Response

A new conformal type of phenolic impregnated carbon ablator was manufactured by resin impregnation of carbon felt. Based on property measurements of the conformal material and the existing model for a standard, rigid phenolic impregnated carbon ablator, a midfidelity material response model for the conformal material was developed. The rigid and conformal materials were arcjet tested simultaneously on a sphere-cone geometry in several environments with frustum heat flux up to . Good agreement between the predictions and data was obtained for recession, surface temperature, and in-depth temperatures. The conformal material has a slightly greater ablation rate but significantly lower thermal diffusivity than the rigid material.

Pyrolysis of phenolic impregnated carbon ablator (PICA).

Molar yields of the pyrolysis products of thermal protection systems (TPSs) are needed in order to improve high fidelity material response models. The volatile chemical species evolved during the pyrolysis of a TPS composite, phenolic impregnated carbon ablator (PICA), have been probed in situ by mass spectrometry in the temperature range 100 to 935 °C. The relative molar yields of the desorbing species as a function of temperature were derived by fitting the mass spectra, and the observed trends are interpreted in light of the results of earlier mechanistic studies on the pyrolysis of phenolic resins. The temperature-dependent product evolution was consistent with earlier descriptions of three stages of pyrolysis, with each stage corresponding to a temperature range. The two main products observed were H2O and CO, with their maximum yields occurring at ∼350 °C and ∼450 °C, respectively. Other significant products were CH4, CO2, and phenol and its methylated derivatives; these products tended to desorb concurrently with H2O and CO, over the range from about 200 to 600 °C. H2 is presumed to be the main product, especially at the highest pyrolysis temperatures used, but the relative molar yield of H2 was not quantified. The observation of a much higher yield of CO than CH4 suggests the presence of significant hydroxyl group substitution on phenol prior to the synthesis of the phenolic resin used in PICA. The detection of CH4 in combination with the methylated derivatives of phenol suggests that the phenol also has some degree of methyl substitution. The methodology developed is suitable for real-time measurements of PICA pyrolysis and should lend itself well to the validation of nonequilibrium models whose aim is to simulate the response of TPS materials during atmospheric entry of spacecraft.

Mars Science Laboratory Heatshield Development, Implementation, and Lessons Learned

The Mars Science Laboratory is a scientific mission that placed a 900 kg rover with 10 science instruments on the surface of Mars. A key component for landing on the surface of Mars is the aeroshell, which decelerates the entry vehicle to just 1% of its initial kinetic energy at atmospheric entry. The forward portion of the aeroshell is the heatshield, which must be designed to withstand the pressure, heating, and shear stresses it experiences during the atmospheric deceleration process. The original thermal protection system chosen for the heatshield, SLA-561V, was the material used on all previous Mars aeroshells but required new qualification testing. Unfortunately, the material failed testing in the more extreme environment three months after the aeroshell and project critical design reviews. A rapid decision was required to switch to another material under test by the Orion program called phenolic impregnated carbon ablator. With 18 months left to design, fabricate, test, and deliver this new heatshield for the 2009 launch opportunity, the heatshield team had to rapidly respond to the challenge and work in a manner that ensured success. Lessons learned can be had from both the failure of the original SLA-561V design and the success of the phenolic impregnated carbon ablator implementation.

Sizing and Margins Assessment of Mars Science Laboratory Aeroshell Thermal Protection System

The methodology employed for the thermal design and margins assessment of the Mars Science Laboratory aeroshell thermal protection system is reviewed. A new thermal margins policy was developed in the course of this work that provides additional rigor over previous methods. Because of a late change of thermal protection materials from the heritage super lightweight ablator 561V to phenolic impregnated carbon ablator, the design of the heat shield followed a nontraditional path in which the flight thickness was selected based on a mass (rather than thermal) limit. The material switch was followed by detailed thermal analyses that demonstrated that the baselined thickness was sufficient to provide adequate thermal protection to the vehicle without violating design requirements during a 3-sigma worst-case entry condition. The backshell material thickness was also finalized before the thermal sizing was completed, and the resulting analysis showed that there was more than sufficient material on the backshell. The parachute cone cover and backshell interface plate were the only major thermal protection system elements that followed a standard design process. Thermal sizing was performed for acreage and special features on the cone cover and interface plate, and the hardware was manufactured according to those analyses.

Two-Dimensional Ablation and Thermal Response Analyses for Mars Science Laboratory Heat Shield

This paper examines transient simulations performed to predict in-depth thermal response and surface recession of the proposed heat shield material for the Mars Science Laboratory entry capsule, that is, phenolic impregnated carbon ablator. The finite volume material response code used in this paper solves the time-dependent governing equations, including energy conservation and a three-component decomposition model, with a surface energy-balance condition and a moving grid system to predict shape change due to surface recession. The predicted in-depth thermal response of heat shield material generally agrees well with the thermocouple data under various arcjet conditions. Also, two-dimensional computations using aerothermal environment for Mars entry (derived from a proposed three-sigma trajectory) are performed around the heat shield shoulder region, where high heating occurs as the result of angle of attack. Parametric studies are conducted to examine the effects of carbon-fiber orientation, material properties, and surface recession on heat shield bondline temperature history. It is proved that the fiber orientation configuration of the baseline heat shield has the lowest maximum bondline temperature.

Arcjet Testing in Shear Environment for Mars Science Laboratory Thermal Protection System

Thermal protection system materials for the Mars Science Laboratory mission were tested in the NASA Ames 60 MW arcjet in a shear environment. Shear tests were performed on candidate ablative heat shield materials in wedge and swept cylinder test fixtures. In portions of the expected flight environment, the proposed main heat shield material, known as SLA-561V, recessed orders of magnitude faster than predicted. An alternate main heat shield material, known as phenolic impregnated carbon ablator, behaved reasonably well in all regions of the flight envelope investigated here. However, the measured recession rate of the phenolic impregnated carbon ablator was on average 50% greater than the predicted recession by the fully implicit ablation and thermal response code, and in some cases the measured recession was as much as 150% greater (uncertainties included). Phenolic impregnated carbon ablator’s higher recession rate than predicted (in shear) resulted in adding thickness margin to the flight heat shield design. Other tests of the phenolic impregnated carbon ablator heat shield system, including tests of various gap fillers, damage scenarios, and repair scenarios, demonstrated the robustness of this heat shield system for use in the predicted Mars Science Laboratory flight environment.

Development of the Mars Science Laboratory Heatshield Thermal Protection System

Early in the development of the Mars Science Laboratory thermal protection system on the heatshield, project management planned to use Lockheed Martin’s Super Light Ablator in honeycomb as the ablative material based on successful use on previous Mars entry heatshields and on stagnation arcjet tests at heating rates beyond the design levels. Because this heatshield would be the first to experience combined turbulent flow and high shear environments as it entered the Mars atmosphere, tests were performed in various arcjet facilities on flat-plate, wedge, and swept-cylinder specimen configurations in order to ascertain the effects of shear on the material. During the course of these tests, a set of conditions within the flight envelope was identified that resulted in catastrophic failure in the SLA-561V. Consequently, project management decided to replace the SLA-561V with the phenolic-impregnated carbon ablator, the material that had flown successfully on the Stardust mission and was undergoing intense testing and characterization for the Crew Exploration Vehicle Thermal Protection System Advanced Development Program. With only two years remaining before the expected launch date, and less than 18 months before the heatshield delivery date, the Mars Science Laboratory team developed and built NASA’s first tiled ablator flight heatshield that contributed to the outstanding success of the spacecraft’s entry, descent, and landing on 5 August 2012.