Reusable Thermal Protection System Research Articles

Prediction of thermal contact resistance for reusable heat-pipe cooled thermal protection system based on an inverse thermo-mechanical coupling method

Thermal contact resistance (TCR) is a critical characteristic on increasing or decreasing thermal energy transmission efficiency between two bodies in thermal management systems. However, it is difficult to accurately determine the thermal contact resistance in actual engineering structures, due to the complicated influence factors, such as pressure, temperature, and physical properties. In the work, a new method is presented to estimate the thermal contact resistance for a three-dimensional (3D) reusable heat-pipe cooled thermal protection system based on boundary measurements, by solving transient inverse thermo-mechanical coupling problems. Moreover, the thermal contact resistance varies with interface pressure, temperature, and spatial position, which is more practical and challenging. The thermo-mechanical coupling analysis is conducted by the finite element method, and the inversion is carried out by the gradient-based algorithm. First, the present method is validated by identifying the constant TCR, based on the available experimental data. Then, the thermal contact resistance with the functional form in the 3D reusable thermal protection system is accurately estimated. Finally, the convergence stability and robustness of the proposed method are evaluated, by considering the effects of initial guess value and measurement error, respectively. The accurate determination of thermal contact resistance of the reusable thermal protection system effectively avoids the overweight of aircraft and improves its utilization. The present work provides a novel approach for the determination of thermal contact resistance in actual engineering applications.

Irradiation resistance of thermo-optical properties of zirconium diboride by 3 MeV electrons

Due to good thermal conductivity and thermal shock resistance, ultra-high temperature ceramics such as zirconium diboride (ZrB2) have been investigated as promising materials to be used in reusable thermal protection systems TPSs are vital to the heat balance of a spacecraft during atmospheric reentry and subsequent operation in space. Hence, the thermal and optical properties are especially critical for such applications. Meanwhile, radiation exposure in space can pose risks of degrading such material properties, especially over a prolonged mission duration. The interaction of electron radiation-which can be found in the outer Van Allen belt, with ZrB2 has not been studied previously and was chosen as the main scope of this study. An electron source of 3 MeV with different radiation exposure time was used. The response of thermo-optical properties of ZrB2 to increasing electron radiation fluences was investigated. ZrB2 samples were made through spark plasma sintering into sintered pellets and then exposed to 3 MeV electron irradiation. These ZrB2 samples were characterized by their microstructure, thermal conductivity, coefficient of thermal expansion (CTE), emittance, absorptivity, and surface roughness before and after irradiation. It was found that ZrB2’s thermo-optical properties showed high radiation resistance at these fluences, and no apparent microstructural change was observed after irradiation. However, the irradiated samples had, on average, a 29% lower surface roughness than the unirradiated samples, possibly originating from electron sputtering.

Study on Sintering Behavior of Reaction-Cured Glass Coating

High-emissivity coatings constitute an essential component of reusable thermal protection systems, determining the success or failure of hypersonic spacecraft. Reaction-cured glass coating is the basis for all current high-emissivity coatings, and the study of its sintering behavior is of great scientific significance for the development and performance enhancement of the coating. Microstructures and phase compositions of the samples before and after the sintering process were determined using SEM, XRD, and EDS. The sintering temperature, inserting temperature, and heating rate were systematically investigated. The results show that the effects of the sintering temperature, inserting temperature, and heating rate on the coating occur in decreasing order. The optimum condition for coating sintering in this study is an insertion temperature of 1100 °C, a heating rate of 10 °C/min, and a sintering temperature of 1200 °C, and a crack-free and containing SiB4 borosilicate glass coating was successfully prepared.

Fabrication of Light-weight Ceramic Insulation Materials by Using Oxide Ceramic Fibers for Reusable Thermal Protection Systems

Fabrication of Light-weight Ceramic Insulation Materials by Using Oxide Ceramic Fibers for Reusable Thermal Protection Systems

Catalytic characterization in plasma wind tunnels under the influence of gaseous recombination

The catalytic properties of materials used in re-usable thermal protection systems (TPSs) of re-entry vehicles are mainly characterized in plasma wind tunnels. These facilities are adequate to reproduce the thermo-chemical environment found in hypersonic flights. Catalytic recombination coefficients (W) determined empirically for the TPS design process are used to validate catalytic models. Recent experiments in plasma wind tunnels present some contradictions with current catalytic models used in computational fluid dynamics simulations. This work explores the coupling between homogeneous and heterogeneous reactions and its influence on the heat flux measurements, in order to explain such contradictions. For that, three copper calorimeters (at 350 K) are inserted in probes of different sizes, and they are tested in the Plasmatron facility at the von Karman Institute under different chemical regimes, which are modified by the probe diameter. A dimensional analysis of the acquired data based on previous non-equilibrium boundary formulations shows that an empirical recombination coefficient can be falsified by the presence of homogeneous reactions. Then, in the framework for the post-flight analysis of the European Space Agency's intermediate experimental vehicle, the local heat transfer simulations methodology is applied under peak heating conditions. A new probe is designed, manufactured, and tested in the Plasmatron to measure the recombination coefficient on C/SiC under the closest flow conditions with respect to the flight.

Experimental and numerical investigation of progressive damage and failure behavior for 2.5D woven alumina fiber/silica matrix composites under a complex in-plane stress state

The focus of this work is to characterize the progressive damage and failure behavior of 2.5D woven alumina fiber reinforced silica ceramic matrix thin composite specimen under a planar tension-shear stress state. The tensile and shear stress–strain diagrams in the principal material coordinate system were obtained using the off-axis tension test with the digital image correlation technique. A mesoscale finite element model of the composite material was developed within the framework of continuum damage mechanics. The thickness effect on the macroscopic mechanical response was considered by removing the periodic boundary conditions along the thickness direction. The macroscopic stress–strain response given by the numerical model was validated by the experimental results. The interaction of the planar biaxial tension and shear stresses was taken into account in the model, quantified using a shear contribution coefficient in the modified 3D Hashin failure criterion. The results from the damage evolution analysis indicate that the linear tensile stress–strain response with a brittle fracture characteristic for the on-axis tension specimen is governed by the axial fiber bundle. The nonlinear tensile stress–strain response with the significantly reduced ultimate stress under biaxial tension and shear stress state results from multiple damages with evolution in both axial and transverse fiber bundles. Both the effective tensile modulus and the ultimate tensile strength increase with increasing the thickness of the specimen due to the increase of the fiber volume fraction. The strengthening induced by the size effect is less significant for the specimen under biaxial tension and shear stress state than that for the on-axis tension specimen owing to the involvement of shear response dominated by the matrix of the material. The results of this study provide new insight into the failure of 2.5D woven ceramic-based composite material, which contributes to the optimal design of the reusable thermal protection system structure.

Preparation of a tantalum-based MoSi2–Mo coating resistant to ultra-high-temperature thermal shock by a new two-step process

To develop an ultra-high-temperature resistant coating for a reusable thermal protection system, the preparation of a tantalum-based MoSi2-Mo coating by a new two-step process of multi-arc ion plating and halide activated pack cementation is presented. The coating has a dense structure and is well compatible with the tantalum substrate, which can be thermally shocked from room temperature to 1750 °C for 360 cycles without failure. The mechanism of the coating’s excellent resistance to high-temperature thermal shocks is that a strong-binding gradient interface and a dense SiO2 oxide scale with good oxygen resistance are formed by the high-temperature self-diffusion of Si.

Studies on Fibre Orientation of Carbon Fibre Preforms for Fabrication of C/SiC Fasteners

Continuous carbon fibre-reinforced silicon carbide (C/SiC) ceramic matrix composites (CMCs) are one of the most promising candidate materials for many high-temperature applications, particularly as thermo-structural hardware for aerospace and aircraft applications. However, the major challenge lies in joining these composites to the airframe for their application as re-usable thermal protection systems for re-entry vehicles. This necessitates development of suitable fastening mechanisms which can withstand the aero-thermal loads of structures during re-entry conditions. Conventionally used high-temperature fasteners are molybdenum based and they suffer from problems related to oxidation at temperatures above 800 °C in air. Thus for such applications, C/SiC fasteners are considered more reliable due to the oxidation resistance offered up to 1650 °C. This study highlights the importance of fibre orientation in continuous C/SiC composite used for fabricating C/SiC fasteners for futuristic re-entry launch vehicles. The study describes fabrication of four different types of continuous fibre-oriented C/SiC composites viz., 2D stitched, 3D woven, 4D NOOBED and 5D NOOBED configurations via chemical vapour infiltration (CVI) route, and compares their tensile property at room temperature so as to find out a suitable preform for fabrication of C/SiC fasteners. It was found that 2D stitched preform-based C/SiC specimen gave the highest tensile strength (157 ± 14 MPa), followed by 3D woven preform-based specimen (130 ± 25 MPa), while the tensile strength of 4D- and 5D-based specimens was very low. Through this study, two carbon fibre preforms viz., 2D stitched and 3D woven are selected for realizing C/SiC fasteners. The role of pyrocarbon interphase coating on carbon fibres, in obtaining high mechanical properties is also understood.

Effects of residual Si on the ablation properties of biomorphic C/SiC composites for reusable thermal protection systems

Biomorphic C/SiC composites were prepared by infiltrating liquid Si into a carbon preform made from medium-density fiberboard. The effects of residual Si on the ablation properties of the composites were investigated using an oxy-kerosene hypersonic torch. The results revealed that reducing the amount of residual Si plays an important role in increasing the ablation and thermal shock resistances. High residual Si resulted in thermal-shock cracking due to the high thermal expansion coefficient and volume change during melting. The composite infiltrated with 30 wt.% Si showed 0.9 × 10–4 g/cm2∙s of the ablation rate and no thermal-shock cracking after three successive ablation tests due to the formation of a thin SiO2 layer. Our findings suggest that biomorphic C/SiC composites with minimal residual Si may be suitable as reusable thermal protectors for reentry vehicles.

Energy Accommodation Coefficient Calculation Methodology Using State-to-State Catalysis Applied to Hypersonic Flows

The interplay of a gas–surface interaction and thermal nonequilibrium is still an open problem in aerothermodynamics. In the case of reusable thermal protection systems, it is unclear how much of the recombination energy is stored internally in the molecules produced by surface catalytic reactions, potentially leading to nonequilibrium between their translational and internal energy modes. A methodology is developed to calculate the energy accommodation coefficient using a rovibrational state-to-state chemical mechanism for a nitrogen mixture coupled with a generalized form of the catalytic recombination coefficient. The flow around a spherical body is simulated in hypersonic conditions, allowing study of the amount of energy deposited on the surface and stored in the recombining molecules. Internal energy quenching into translational energy is found, which is a phenomenon also observed experimentally, keeping the total energy transferred to the surface overall constant. The methodology developed for the application of a state-to-state model in the computational fluid dynamics framework coupled with catalysis is generic and applicable to a variety of other similar mechanisms.

Resilient carbon fiber network materials under cyclic compression

The thermal protection system (TPS) for the reusable reentry spacecraft is designed to protect the structure from extremely high temperatures for many times launch and reentry. To meet the requirements, the TPS requires excellent thermal insulation performance, mechanical properties and strong ability to recover deformation with a minimum lifetime of dozens of missions. In this paper, we fabricated carbon-bonded carbon fiber composites (CBCFs) with a 3D network structure, and the deformation mechanisms under cyclic compression were systematically analyzed. This work demonstrated the excellent resilience behavior of CBCFs with cyclic loading. Quantitative numerical modeling and experimental tests revealed that the CBCFs could completely recover the deformation with strain up to 40% and maintained structural integrity with low energy dissipation (<∼0.3). The resilient behavior in the out-of-plane direction was mainly attributed to elastic deformation of fibers and contact of neighboring fibers, and most of the energy was dissipated by the sliding between fibers. The in-plane deformation of CBCFs was mainly attributed to network collapse and buckling. This paper provided a fundamental insight into the mechanical properties and deformation mechanisms of the CBCFs under cyclic compression, which may contribute to the development for the new generation of reusable TPS.

Arc-Jet Testing of a Variable-Transpiration-Cooled and Uncoated Carbon–Carbon Nose Cone

The last decade has witnessed a considerable effort in the development of materials and structures for reusable thermal protection systems with the final goal of enabling long-duration hypersonic flights and more affordable access to space by multiple reuse of recoverable rocket stages. While the development of materials such as carbon-based and silicon-based ablators has extended the typical lifespan of heat shields for extremely high heating-rate environments, the limited reusability yet prevents their use for the aforementioned applications and, therefore, active cooling techniques have to be considered to preserve the thermostructural integrity of thermal protection systems. For all of these reasons, active cooling of thermal protection systems is required and transpiration cooling has been demonstrated to be one of the most promising cooling strategies in terms of coolant mass saving and minimum disturbances of the external flow. The prototype heat shield used for the experiment on variable transpiration cooling is an axisymmetric uncoated carbon–carbon cone having tailored porosity and variable thickness prescribed at the manufacturing level in order to reproduce a variable blowing profile. The high-enthalpy test demonstrated the capability of transpiration cooling to preserve the cone, characterized by a 0.25-in. radius of curvature, for approximately 92 s while being exposed to a peak heating rate of 637 (stagnation region). Heat shield failure was detected after 92 s due to a high-temperature flow leak inside the sheeting sleeve of the coolant line upstream the sample.

Microstructure and oxidation resistance of a YSZ modified silicide coating for Ta-W alloy at 1800 °C

To develop an ultra-high temperature resistant coating for a reusable thermal protection system, a YSZ-modified Si-Mo coatings were prepared on Ta-10 W alloy by slurry reaction sintering method. The coating had a MoSi2-ZrSi2-SiO2 outer layer and a TaSi2-WSi2 inner layer. The coated sample remained intact after oxidation at 1800 °C for 10 h, while the untreated alloy was pulverized after 6 min. The coating withstood more than 200 shock heating cycles from room temperature to 1800 °C. Its outstanding oxidation resistance is attributed to its thermal matching with the alloy and formation of protective SiO2 scale containing high-melting ZrSiO4 and ZrO2.

Robust reconstruction of the catalytic properties of thermal protection materials from sparse high-enthalpy facility experimental data

Quantifying the catalytic properties of reusable thermal protection system materials is essential for the design of atmospheric entry vehicles. Their properties quantify the recombination of oxygen and nitrogen atoms into molecules, and allow for accurate computation of the heat flux to the spacecraft. Their rebuilding from ground test data, however, is not straightforward and subject to uncertainties. We propose a fully Bayesian approach to reconstruct the catalytic properties of ceramic matrix composites from sparse high-enthalpy facility experimental data with uncertainty estimates. The results are compared to those obtained by means of an alternative reconstruction procedure, where the experimental measurements are also treated as random variables but propagated through a deterministic solver. For the testing conditions presented in this work, the contribution to the measured heat flux of the molecular recombination is negligible. Therefore, the material catalytic property cannot be estimated precisely. Moreover, epistemic uncertainties are rigorously included, such as the unknown reference calorimeter catalytic property.

Design, formation, and property of high emissivity WSi2-Si-glass hybrid coating on fibrous ZrO2 ceramic for reusable thermal protection system

The objective of this research was to develop a high emissivity coating on the low thermal conductive insulation for reusable thermal protection system. The design principles including high emissivity, match of the coefficients of thermal expansion and excellent high temperature stability were proposed for the first time. A novel WSi2-Si-glass hybrid coating with optimized composition was successfully prepared on fibrous ZrO2 ceramic insulation by slurry dipping and subsequent high temperature rapid sintering method. The total emissivity of the as-prepared coating is up to 0.92 in wavelength of 0.3–2.5µm. After oxidation at 1673K for 100h and 1773K for 20h, the total emissivity values of the coatings are still as high as 0.89 and 0.87. A competitive mechanism between the “defects formation” caused by aging stress, the CTE mismatch stress and the volatilization of low melting point oxides and the “defects healing” resulted from self–healing phases during the different oxidizing conditions was discussed.

Evolution of microstructure and radiative property of metal silicide–glass hybrid coating for fibrous ZrO2 ceramic during high temperature oxidizing atmosphere

To develop a high emissivity, high-temperature-resistant coating for a reusable thermal protection system, a metal silicide–glass hybrid coating was prepared on fibrous ZrO2 ceramic by rapid sintering method, which comprises a TaSiO glass layer and a dense MoSi2–TaSi2–glass layer and exhibits improved oxidation behavior at 1773K in air for 50h. During oxidation, the total emissivity in 0.3–2.5μm slightly fluctuates around 0.9 without obvious degradation. The emissive mechanism was identified as electron transition in interband of metal silicides and in impurity states of amorphous SiO2 and lattice vibration of crystalline SiO2.

Experimental and computational analysis of thermo‐oxidative‐structural stability of ZrB2–SiC–Ti during arc‐jet testing

AbstractThe development of new ultra‐high temperature ceramics for thermal protection system (TPS) of hypersonic cruise and re‐entry vehicles requires performance‐qualification testing under simulated flight conditions. The present work, encompassing experiments and computational analysis, critically analyzes the thermo‐oxidative‐structural stability of flat surface disks of spark plasma sintered ZrB2–18SiC–xTi composites (x=0, 10, 20; composition in wt%) under arc jet flow with heat flux of 2.5 MW/m2 for 30 seconds. Such testing conditions effectively simulate the aero‐thermal environment in ground facility, as experienced by hypersonic vehicles. Based on the extensive XRD, SEM‐EDS and electron probe microanalyzer based analysis of the surface/sub‐surface of arc jet exposed ceramics, the oxidation mechanisms are qualitatively discussed. Importantly, thick oxide layers (~400‐950 μm) were found to be adherent, thereby providing good structural stability of such ceramics for reusable TPS. The careful finite element (FE) analysis with high quality structural elements, being generated using HyperMesh, was conducted to understand the underlying reasons for observed oxidation. Such analysis allows us to determine the temporal evolution of through‐thickness temperature distribution. FE‐based calculations were subsequently validated using experimentally measured backwall temperatures. The thermodynamic feasibility of competing oxidation reactions at the analytically computed front wall temperatures was thereafter realistically assessed to support the oxidation mechanisms. Taken together, the present work provides guidelines for better understanding of the thermo‐oxidative‐structural stability of ceramics under arc jet testing and also establishes the good stability of ZrB2–18SiC–20Ti composites for potential application in TPS of hypersonic space vehicles.

Diffusion effects on the determination of surface catalysis in Inductively Coupled Plasma facility

Atomic recombination is an important process to consider when computing the heat flux transferred to the wall of a re-entry vehicle. Two chemical processes are influencing the species diffusion in the boundary layer surrounding a re-usable Thermal Protection System: gas phase reactions and catalytic recombination at the surface. The coupling between them is not normally taken into account when determining the catalytic recombination coefficient (γ) in plasma facilities. This work aims to provide evidence of such coupling based on both a theoretical analysis and an experimental campaign in the VKI-Plasmatron facility. Recombination coefficient measurements at off-stagnation point configuration on a linear copper calorimeter are provided. An evolution from a high-catalytic to a low-catalytic condition due to the boundary layer growth along the probe is observed. This result is consistent with a parametric analysis carried out using the in-house non-equilibrium boundary layer solver, which shows how the experimentally determined catalysis could be influenced by the amount of gas-phase recombination inside the boundary layer.

Characterization of Complex Porous Structures for Reusable Thermal Protection Systems: Porosity Measurements

This work is focused on the nonintrusive characterization of the local and average porosity of a prototype carbon–carbon nose, representative of a reusable thermal protection system based on transpiration cooling. A study based on the x-ray computed tomography scan of the specimen has been carried out with the purpose of defining the most important guidelines for the permeability tests, which are the minimum area to be probed with a hot-film anemometer and the correct distance of the mass flux sensor from the wall. The former has been calculated from the average porosity calculation, whereas the latter has been retrieved from the statistical analysis of the dimensions, and the distribution of the void structures inside the porous network coupled to the theory of fluid flow through perforated plates. Several longitudinal and transversal sectioning planes with respect to the symmetry axis of the carbon mask have been analyzed to calculate the internal porosity from the two-dimensional images, whereas the three-dimensional reconstruction of the sample has been used to retrieve the average volumetric porosity. Both the nominal values of the two-dimensional porosity and volumetric porosity have provided the same dimension of the characteristic area to be probed with a hot-film sensor for the permeability measurements. Preliminary permeability tests, performed within the predicted dimension of the control surface, have confirmed the uniformity of the mean velocity field and allowed verifying the range of variation of the correct distance of a hot-film sensor from the wall obtained from the statistical analysis of the computed tomography images.

Variable Transpiration Cooling Effectiveness in Laminar and Turbulent Flows for Hypersonic Vehicles

Reusable thermal-protection systems with active cooling, such as transpiration, are among the promising technologies for thermal management of hypersonic vehicles designed as practical, long-range transportation systems. This paper numerically investigates the effectiveness and efficiency of a variable-velocity transpiration technology for fully laminar and fully turbulent hypersonic flows over a two-dimensional blunt leading-edge geometry. For both flow types, variable transpiration based on a sawtooth velocity distribution is compared to a uniform-velocity transpiration approach. An equal amount of coolant has been imposed to compare the cooling effectiveness between two strategies. The results numerically demonstrate the significant reduction in stagnation-point heating and coolant mass savings achievable with the variable-transpiration strategy, which is observed both in laminar and turbulent flows. The transpiration cooling efficiency is shown to be higher in laminar flow compared to turbulent flow downstream of the leading edge (ramp region). In such regions, for turbulent flows, the amount of total coolant must be increased by a factor of 2 to match the cooling efficiency in laminar flows. The thermal response of a porous thermal-protection-system material is investigated in laminar and turbulent flows with variable transpiration to gain important insight about the matrix and coolant behavior in response to external flow.