Ablative Thermal Protection System Research Articles

Flow Mechanics in Ablative Thermal Protection Systems

Ablative thermal protection systems have experienced renewed interest in the past decade owing to the retirement of NASA's Space Shuttle fleet and the US presidential mandate to develop technologies that enable humans to explore space beyond low Earth orbit. Blunt body architecture for spacecraft and the use of ablators for thermal protection systems returned as the primary choice in mission planning. This review addresses current progress in modernizing predictive tools for ablative material response. Current theory development leverages progress made in the theory of flows in porous media. This development, combined with progress in experimental techniques and high-end computing, is enabling the development of 3D macroscale models with realistic closure coefficients derived from direct numerical simulations of 3D microscale geometries of actual materials. While flight data quantifying ablative material response remain sparse, the next decade will be one of exploration in which heatshield instrumented spacecraft will provide crucial flight data for refining and validating closure models.

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.

Validation of Ablation and Thermal Response Model for Three-Dimensional Multifunctional Ablator

A semi-empirical physics-based ablation and thermal response model was developed for the 3-Dimensional Multifunctional Ablative Thermal Protection System (3D-MAT) material. Model validation was achieved through comparison between computation and available data obtained in the arc-jet test series, which were conducted at NASA Ames Research Center from 2014 to 2020. The charring ablator simulations for arc-jet test models presented in this work were computed by the Two-Dimensional Implicit Thermal Response and Ablation (TITAN) code, and the Data-Parallel Line Relaxation Method (DPLR) code was used for arc-jet flow simulations to estimate the arc-jet stream total enthalpy and define the aerothermal boundary conditions over the test model surface for TITAN simulations. Because of low surface catalytic efficiency of 3D-MAT char, the exact surface heating could not be determined. Thus, three different types of boundary conditions, including 1) fully catalytic surface heating, 2) noncatalytic surface heating, and 3) surface temperature and recession, were used in the TITAN simulation for model validation. The predicted surface and in-depth temperature history for arc-jet test models were compared with pyrometer and thermocouple data, and the predicted test model surface recession and char depth were compared against the posttest measurements.

Implementation of Practical Algorithms in the Inverse Problem with Unknown Boundary Conditions

This work attempts to propose a practical methodology for the one-dimensional heat conduction problems with unknown boundary conditions of the first kind at both ends. The methodology can be effectively applied when the remote boundary condition is inaccessible or internal temperature is required, such as a cutting process with cooling lubricant, and ablative thermal protection system. The proposed methodology utilizes the shifting function method in conjunction with the least squares error method to transform the inverse heat conduction problem into an approximate “well-posed” problem. Consequently, the temperature and the heat flux distributions over the entire time and space domains are determined by using half-range Fourier cosine series solutions. Experimental examples of spray cooling problems are provided to illustrate the advantages of the proposed methodology, including fast convergence of the temperature function, fewer discrete measured times in numerical analysis, and regardless of two interior temperature probes positions.

Investigations on Phthalonitrile – Cyanate ester blends and their light weight composites; Synthesis, thermal, mechanical and ablative characteristics

Newer high performing polymer matrix resins are highly demanded given the rapid development trend in aero-space exploration sector. The present work focuses on synthesis and performance evaluation of Phthalonitrile- Cyanate ester blend as matrix resin for ablative composite applications. Incorporation of Cyanate ester facilitated early cure initiation (by about 35 °C) of Phthalonitriles without comprising thermal stability, a novel strategy that improves the sluggish cure profile of phthalonitriles. Low density (0.5g/cc) syntactic foam composites prepared from the blend exhibited low Coefficient of Thermal Expansion (CTE) (1.3 × 10−5 – 1.7 × 10−5/°C), high specific heat (0.32–0.38 J/g/°C at 80 °C) and low thermal conductivity (0.11–0.13 W/mK at 80 °C). Exposure of the composites to moderate re-entry heat flux conditions using radiant heating facility (∼55W/cm2, 1200s) and Plasma Arc jet (70&125 W/cm2, 50s) resulted in excellent surface uniformity with minimal of mass loss (4.77–5.77%) and excellent heat of ablation (8000–12000 cal/g) with reduced back-wall temperature (230–260 °C). The composites possessed very good mechanical properties such as Flexural strength of 5–6.5 MPa and compressive strength of 5–9 MPa. 1:1 combination of pyrolyzed system possessed high degree of graphitization. The promising results confirm that the new matrix blend can be a potential replacement for conventionally used phenolics as ablative Thermal Protection System (TPS) matrix.

Enhanced ablation resistance of phenolic composites through the construction of a sea-island nanostructured char layer via in-situ catalytic graphitization of incorporated Salen-Ni complex

Phenolic resin (PR) are widely used as ablative thermal protection systems for aerospace applications. However, it remains a challenge to improve the ablation resistance of phenolic composites by designing the structures of char layer via a facile approach. In this paper, a Salen-Ni complex (CL) modified boron phenolic composite (CLBPR) was designed with low concentrations of Ni (0.05–0.45 wt%). The chemically incorporated Salen-Ni structures enabled the uniform distribution of Ni element, and the enhancement of graphitization degree of char layer through the significant catalytic effect. XPS, XRD, RAMAN and TEM analyses showed that the metallic Ni nanoparticles arising from Salen-Ni structures could effectively catalyze the ablated char layer to generate various type of graphitic nanostructures, and construct a sea-island nanostructured char layer during ablation. Furthermore, we found that the inlaid graphitic phases formed through Solid-Liquid-Solid mechanism in amorphous carbon (AC) can hinder the growth of defects, while improve the graphitization degree of char layer. The results demonstrated that the linear and mass ablation rates of CLBPR were reduced by 128.9% and 32.6% compared to the counterpart at 4 MW/m2. This research will provide some inspirations more or less for the development of more advanced polymer-matrix ablative composites in the future.

Thermal-insulation and ablation-resistance of Ti-Si binary modified carbon/phenolic nanocomposites for high-temperature thermal protection

Ablative thermal protection systems are considered to be one of the most promising means of protection for spacecraft re-entry, but challenges remain. Herein, efficient Ti-Si binary modified carbon fiber reinforced phenolic aerogel nanocomposites (Cf/TS-PR) were originally designed based on a multi-stage construction strategy. The Cf/TS-PR exhibited desirable mechanical strength, good thermal insulation with a minimum thermal conductivity of 0.0756 W/(m·K), remarkable thermal stability and outstanding ablation resistance, benefiting from the features of TiO2 and the special structure of TiO2-SiO2 coating. The linear ablation rates are as low as 0.004 and 0.003 mm/s at the heat flow of 1.0 and 1.5 MW/m2, respectively. The mass loss rates are as low as 0.006 and 0.009 g/s at the heat flow of 1.0 and 1.5 MW/m2, respectively. These results indicate that the Cf/TS-PR nanocomposites can be a reliable thermal protection material for the future high-temperature service environment of ultra-high-speed vehicles and neutral rockets.

Heat transfer and pyrolysis gas flow characteristics of light-weight ablative thermal protection system in the blunt body

Thermal protection system (TPS) is the critical system to protect the structure and electronic components of hypersonic vehicles from aerodynamic heating. Since the blunt body suffers the most severe thermal environment in a flight vehicle, the design of TPS for the blunt body is crucial. To investigate the thermal performance of the light-weight ablative material in the blunt body, a multi-dimensional thermal response model is proposed considering heat transfer, pyrolysis gas flow, and gas blowing effect. The characteristics of heat diffusion, pyrolysis process, gas flow, and the effect of pyrolysis gas blowing are investigated. It shows that the developed model can accurately predict temperature and gas flow. The temperature and charred thickness at the stagnation point are the largest, decrease along the surface of the sphere nose, and keep almost the same at the cone. Due to the surface pressure distribution, a large part of the pyrolysis gas in the sphere nose flows through the ablative material and injects into the air at the sphere-cone junction. In contrast, the gas at the cone flows vertically to the cone surface. The gas flow rate and pressure decrease as time elapses. Moreover, gas blowing mainly influences the thermal response near the sphere-cone junction, leading to a minimum blowing factor of 0.965 and a max temperature reduction of 7.4% at 100s. The effect of gas blowing is enhanced with the increase of phenolic resin content. This study deepens the understanding of the thermal performance of light-weight ablative material in the blunt body and guides the ablative thermal protection design.

Healable ablative composites from synergistically crosslinked phenolic resin

Due to the excellent processability and high char yield, phenolic resin plays a pivotal role in ablative thermal protection systems for the aerospace industry. However, internal defects in conventional phenolic resin matrix composites could potentially scrap the entire thermal protection system and cause devastating safety hazards to the spacecraft. Herein, hexamethylenetetramine (HMTA) and phenylboronic acid are applied as the co-curing agents of novolac resin. With the HMTA amount below 2.5 phr, a series of synergistically crosslinked phenolic resin (HPNR), which consist of both traditional methylene linkages and dynamic boronic ester linkages, are successfully synthesized. By tuning the ratio of the traditional/dynamic bonds, the HPNR demonstrates high char yield and controlled plasticity. Compared with its conventional counterparts, the carbon fabric reinforced HPNR matrix composites show healing capability and superior mechanical properties and ablation resistance, with an interlayer shear strength and mass ablation rate of 37.0 MPa and 0.055 g/s, respectively. The synergistic crosslinking strategy provides a promising pathway for designing and preparing healable ablative thermal protection system materials.

Thermomechanical response of out-of-autoclave infused carbon-phenolic laminates for rocket engine applications subjected to surface ablation

Cost-affordable and reliable manufacturing technologies to produce Carbon/Phenolic Composites (CPCs) are essential for competitive access to space. These materials are used as ablative Thermal Protection System (TPS) for space applications, such as rocket nozzles or the heat shields of space vehicles. Out-Of-Autoclave (OOA) infusion processes can reduce the manufacturing cost of composites: in this work, OOA produced CPC laminates were compared to Carbon/Epoxy Composites (CECs). The CPCs produced in this work were mechanically characterized to assess their quality and compared with state-of-the-art counterparts through the data available in the open literature. CPC laminates instrumented with ultra-small thermocouples were subjected to Oxy-Acetylene Torch (OAT) ablation testing and a numerical approach was assessed to predict the temperature reached in the laminate layers. Post-burning mechanical tests performed on specimens subjected to the ablation test showed that when the temperature does not exceed a given threshold, the material retains adequate properties to perform a structural role.

A novel solution for inverse heat conduction problem in one-dimensional medium with moving boundary and temperature-dependent material properties

A novel approach for solving inverse heat conduction problems in one-dimensional domain with moving boundary and temperature dependent material properties is presented. In this study, it is assumed that two thermocouples are used to measure temperature at two interior locations within the medium while the front boundary experiences recession (moving towards the back surface). A filter coefficient-form Tikhonov regularization method is used to develop the solution for this problem. It is hypothesized that interpolation of pre-calculated filter coefficients for a medium with various thickness values can be used to calculate the filter coefficients for an IHCP with moving boundary, knowing the current thickness of the domain. This can be used along with interpolation among the pre-calculated filter coefficients at different temperatures, knowing the temperature value, to find the correct filter coefficients at each time step and accurately estimate the conducted heat flux in a near real-time fashion. The hypothesis is then validated through several numerical test cases developed in COMSOL. It is shown that the proposed solution method can accurately estimate the conducted heat flux in a domain with a moving boundary in a near real-time fashion. The main advantage of the proposed solution is in it's computationally efficient nature as well as possibility of implementation for monitoring and control purposes due to the near real-time operation of the method. The developed method can be used for calculating surface heat flux in applications that involves moving boundary and large temperature variations such as ablative thermal protection systems for space vehicles.

Thermal ablation mechanism of polyimide reinforced with POSS under atomic oxygen bombardment

The thermal stability and reactive atomic oxygen (AO) resistance of polyhedral oligomeric silsesquioxane (POSS)-reinforced polymer nanocomposites have attracted significant interest from both experimental and theoretical researchers. However, understanding the damage-mitigation mechanism of inorganic core constituents in POSS hybrids is still limited due to the lack of empirical insights. In this study, the ablation resistance of polyimide (PI)/POSS nanocomposites was examined using reactive molecular dynamics and first-principles calculations in terms of physicochemical changes in the POSS cage structure. According to the AO collision kinetic energy and frequency level, the origin of ablation resistance in the system is classified. Each damage-mitigation behavior is strongly correlated with the normalized residue mass and byproduct distribution during the erosion of the surface. In particular, the results suggest that the kinematic collapse of POSS prevents the exfoliation of large carbonic molecules and plays a crucial role in the formation of a ceramic passivation layer, which significantly improves the damage-mitigation effect. Therefore, this study provides a new perspective on the design of ablative thermal protection systems by understanding the behavior of the inorganic nanocage constituting POSS hybrids at the molecular level.

Elucidating multiple-scale reaction behaviors of phenolic resin pyrolysis via TG-FTIR and ReaxFF molecular dynamics simulations

Phenolic resin is a matrix material widely used in ablative thermal protection systems (TPS) and its pyrolysis behavior is crucial for the analysis and prediction of TPS thermal response. However, it still remains a huge challenge to elucidate the involved reaction mechanisms due to the process complexity and unpredictability. Herein, a combination of TG-FTIR and ReaxFF MD simulation method was adapted to explore phenolic resin pyrolysis mechanism. Experiment results indicate that the pyrolysis process of phenolic resin can be divided into three stages, and the activation energy of each stage increases gradually. Heating rate affects the initial pyrolysis temperature of phenolic resin, but has no obvious effect on the types of pyrolysis products. Additionally, an accurate and reasonable decomposition kinetic model is established based on 30 kinds of kinetic function models, involving diffusion, random nucleation and nuclei growth, phase interface reaction and chemical reaction. The cook-off simulations results suggest that the major pyrolysis products of phenolic resin at high temperature include H2, CO, C2H2, and H2O, which formation mechanisms are also discussed according to the simulation trajectories. The reaction model and the activation energy of phenolic resin pyrolysis based on simulations are in good agreement with the experimental results, contributing to the deep understanding of phenolic resin pyrolysis behaviors at atom scale. This study may provide theoretical data basis for the establishment of thermal response calculation model of resin-based composite thermal protection materials.

Experimental Studies of Resin Systems for Ablative Thermal Protection System

The present work was initiated to finalize resin for the development of thermal protection system (TPS) for the external surface of a polymeric composite rocket motor case made up of Carbon roving and Epoxy resin. The temperature on the outer surface of the composite case increases due to kinetic heating caused by aerodynamic drag and vehicle velocity. These rocket motor casings are functionally required only in the ascent phase of missile trajectory till motor action time and stage separation. Due to which the experienced heat flux is relatively less, and the temperature on the external composite case is in order of 250 °C - 300 °C depending on missile configuration & trajectory, unlike extreme thermal conditions on ablative nozzle liners exposed to rocket motor exhaust. The maximum allowable temperature in the present study for the Carbon-Epoxy case is 100 °C due to degradation in mechanical properties. The thermal protection system on the external surface will function as a heat-insulating layer based on the working mechanism of ablation. The resin of the thermal protection layer has a substantial impact on the manufacturing process and curing aspects, especially compatibility with the pre-cured carbon epoxy case layer. The generation of test results for thermal stability, cure characteristics and Tg for Epoxy resin has also been included in present studies as an additional objective that provides significant inputs for process development. The test results for Epoxy resin is also used as a basis for the finalization of resin for the thermal protection layer for processing aspects apart from its basic thermal stability characteristics. The ablative thermal protection working mechanism is based on the ablation phenomenon. In the case of ablation, resin plays a vital role due to pyrolysis and other thermal characteristics. In the present experimental studies, the Phenolic resin and Silicone resin are considered as candidate resin materials for ablative thermal protection system based on available literature and in house experience. The main objective of the present studies is to evaluate thermal stability, char yield after final decomposition through DSC and TGA techniques for both resins as these are fundamental characteristics needed for the present specific application. The test results for specific grades (formulation) of phenolic and Silicone resins are generated and compared. In the present work, the experimental studies to evaluate glass transition temperature (Tg), thermal stability, and cure characteristics for Epoxy resin is also carried through DSC. The test results of specific grade Epoxy resin provides a basis to assess thermal margins for resins selected for ablative thermal protection system and inputs for process development and design requirements. The scope of the present studies is aimed to finalize the resin system for external thermal protection of composite rocket motor case based on thermal characteristics test results and other compatibility aspects with the structural layer.

Inverse Identification of Temperature-Dependent Thermal Conductivity for Charring Ablators

Estimation of thermal conductivity of charring composite is a key issue for both design and optimization of an ablative thermal protection system. This paper presents a method for predicting temperature-dependent thermal conductivity of charring ablator using genetic algorithm and cubic spline interpolation, and a special ablative material composed of phenolic resin, glass fibers and quartz fibers is concerned. In this study, by considering the effects of pyrolysis, carbon–silica reaction, and the convective heat transfer between pyrolysis gas and charred layer, the ablative heat transfer model is developed to simulate the ablation process first. Then based on the measured temperature profiles and built model, the temperature-dependent thermal conductivity without prior information on the functional form is predicted which is parameterized by cubic spline. The predicted thermal conductivity curve is continuous and smooth everywhere, and the changes of the curve can reveal the ablation characteristic of the concerned material precisely. Consequently, the calculated temperature profiles using the predicted results show good agreement with experimental data. Also, the reliability of the prediction is discussed through analyzing the iterative process and the number of interpolation nodes. It is concluded that the improvement is limited through adding nodes when the thermal conductivity is already well defined by existing nodes.

Study of Ablative Thermal Protection System with Density Gradient

Study of Ablative Thermal Protection System with Density Gradient

Ion Beam Analysis for recession determination and composition estimate of Aerospace Thermal Protection System materials

The study of Thermal Protection Systems (TPS) is largely performed in Plasma Wind Tunnel (PWT) facilities where sample wear rate is of main interest and the knowledge of its relation with sample surface temperature desirable. The intrusive and indirect methods employed so far reduce the reliability and the accuracy of these tests.A new joined technique based on both the Surface Layer Implantation of radioactive tracers and on the emissivity-free Dual Color Infra-Red thermography allows the online measure of both the TPS wear rate and the surface temperature without any preparation (i.e. modification) of the sample. Other Ion Beam Analysis techniques like the Rutherford Backscattering Spectrometry (RBS) can be employed off-line to perform integrative, quantitative and still not intrusive analysis of TPS composition and of the structure modifications occurred during the PWT tests.In this work we present the results of a discrete wear rate measurement of an ablative TPS implanted with 7Be ions and the feasibility study of the RBS technique to investigate the oxidation of ceramic TPS.

Ablative Heating Technology in Hypersonic Re-entry Vehicles and Cruise Aircrafts

The aim of researches conducted in thermal protection systems in aeronautics and astronautics field of engineering is to generally defend the craft from high heat loads during operation while operating at hypersonic regimes in air and space. The motive of the following composition is to draft a review analysis on ablative heating materials as thermal protective equipment on reusable planetary/atmospheric re-entry vehicles such as a space shuttle, an inter-continental ballistic missile, or a hypersonic cruise missile. The heat liberation can cause much damage to the aircraft/spacecraft whilst operation which is generally beyond repair. It is therefore of utmost importance to research multiple strategy to reduce the effect of shockwaves damage to spacecraft/aircraft materials. We shall initiate the analysis by mentioning some re-usable tile thermal protection system types such as high temperature reusable surface insulation tiles (H.R.S.I), fibrous refectory composite insulation tiles (F.R.C.I), low temperature reusable surface insulation tiles (L.R.S.I) and gradually move on to ablative thermal protection systems with the advent of reinforced carbon-carbon’s application in astronautics and aeronautics respectively.

Transport properties of carbon-phenolic gas mixtures

The development of a new generation of ablative thermal protection systems based on carbon-phenolic composite materials demands the accurate determination of transport properties of pyrolysis gases mixed in various atmospheres. The present work provides recommended collision data for the calculation of the transport properties for Mars and Earth re-entries. A review of available potential data is given for species in the atmospheres of Earth and Mars. Additional data are provided for species formed during the degradation of the theoretical ablative composite for open testing material. Dipole polarizabilities are calculated from ab initio methods and used for the calculation of transport properties of reduced pyrolysis mixtures over a temperature range representative of the boundary layer for atmospheric entry flows.

Evaluation System for Ablative Material in a High-Temperature Torch

Ablative thermal protection system (TPS) has been used to protect a vehicle from the heat load experienced during hypersonic or re-entry flights. Typical materials for ablative TPS are carbon–carbon (C/C) composites and carbon–phenolic composites, which have generally been used at working temperatures that are not very high due to surface oxidation. From a material science perspective, modification with ultra-high-temperature ceramic (UHTC) materials or coating with UHTC materials have been undertaken in an attempt to enhance the performance of ablative TPSs for high-temperature applications. As no single facility can reproduce all the aspects of the harsh environments that a vehicle would experience in hypersonic flight, a facility appropriate for the environment of interest is selectively adopted to assess TPS materials. In this study, an evaluation system for ablative material in a high-temperature torch has been developed. As part of the development, cost-effective experiments were conducted using an oxy-kerosene torch. Performance of the C/C test sample was measured, and the flowfield was diagnosed. Numerical simulation of the oxy-kerosene flowfield and the one-dimensional material response have been performed and compared with the test data.