Integrated Thermal Protection System Research Articles

Experimental Investigations of Failure Behavior for Aerogel-Cored Stitched Sandwich Composites Under Vibration Conditions

Aerogel-cored stitched sandwich composites (ACSSCs), which possess numerous desirable properties, are anticipated to become the preferred choice for next-generation integrated thermal protection systems (ITPSs) in high-speed vehicles. Despite the reinforcement provided by stitching, the ACSSC structures on vehicles’ outer surfaces remain vulnerable to damage resulting from the excessively high deformation and vibration that occur during high-speed flight. Accordingly, experimental tests were conducted to investigate the failure characteristics of ACSSCs under bending vibration conditions. The tests demonstrated that the ACSSC structures were prone to fatigue failure caused by vibration, which resulted in significant shear damage in the aerogel core. Furthermore, the study introduced a new method for indirectly characterizing the fatigue life of shear damage in the core. Finite element analysis was undertaken to elucidate the cause of the cracking near the bottom layer of the core. The results indicated that the crack location of the aerogel core was littered with the high stiffness and deformation of internal stitches.

Thermo-chemical and structural analysis of integrated thermal protection system for a space vehicle

The Thermal Protection System, or TPS, is the layer that shields spacecraft from the extreme aerodynamic heating that occurs during re-entry and launch. We proposed an optimized design of an Integrated Thermal Protection System (ITPS) in this paper. An ITPS provides thermal protection as well as structural load-bearing capability. Thermo-chemical simulations are carried out with the help of the Fully Implicit Ablation and Thermal response program (FIAT) for better understanding of the thermo-chemical behavior of the system under the extreme conditions of a space vehicle. Similarly, a structural analysis is performed to determine the structural integrity of the proposed ITPS design. Both the thermo-chemical and structural analyses show that the proposed corrugated-core sandwich structure is viable in terms of recession, pyrolysis, and structural integrity.

Direct-Homogenization-Based Plate Models of Integrated Thermal Protection System for Reusable Launch Vehicles

Integrated thermal protection systems of reusable launch vehicles (RLVs) can have a corrugated core sandwich structure and experience location-dependent thickness-wise temperature gradient. The sandwich structure can be optimized depending on the location on RLV using finite element method simulations of RLV components. However, such analysis can be computationally challenging due to the disparate length scales between the RLV components and features of the sandwich structure. Two different equivalent plate models based on first-order shear and normal deformation theory, and first-order shear and second-order normal deformation theory (FSSNDT) have been utilized in this work to address this drawback. A direct homogenization technique involving unit cell and cantilever beam analysis has been developed to calibrate the equivalent plate properties for different thickness-wise temperature variations. The accuracy of the plate models has been evaluated by comparing their responses with a full-scale model for different thickness-wise temperature gradients and a uniform pressure. The comparisons clearly indicate that the FSSNDT-based plate model calibrated via direct homogenization captures both the displacements and strains in the in-plane and transverse directions accurately, and can be used to perform RLV component analysis efficiently to obtain location-specific optimal design of corrugated core sandwich structures.

Bilayer lattice structure integrated with phase change material for innovative thermal protection system design

In recent years, ceramic matrix composite (CMC) lattice structure has become a promising candidate for a new generation of integrated thermal protection systems (ITPSs) used in the ultrahigh-temperature environment. However, the thermal short circuit effect caused by lattice cores limits their application. In this work, we propose a bilayer CMC lattice structure filled with phase change material (PCM) and thermal insulation material for integrated thermal protection systems (ITPSs) to mitigate the thermal short circuit effect. The numerical simulation method is used to verify the effectiveness of the bilayer lattice structure in mitigating the thermal short circuit effect compared with one-layer ITPS. By inserting the middle face sheet, the thermal insulation effect enhances about 16%. Furthermore, the effects of thermal insulation materials, thermal conductivity and thickness of paraffin/expanded graphite composite PCM, and the diameter of the lattice core strut on the thermal insulation performance are discussed. The results show that the thickness of PCM has an optimal solution. Optimizing results show that when the aerogel insulation fiber is used as the thermal insulation material, the thermal conductivity k=0.82 W/(m⋅K) and the thickness t3=10 mm of PCM, the diameter of the core strut d=1.5 mm, the bilayer PCM-ITPS has the best thermal insulation performance. When the ultrahigh-temperature load is continuously loaded, the highest temperature of Top Face Sheet is more than 2300°C, but the highest temperature of Bottom Face Sheet is only 30.12°C, which thermal insulation effect is far better than the traditional ITPS. This work gives a novel scheme to solve the thermal short circuit effect, improving the thermal protection performance and accelerate the application of lattice structure in ITPS.

Thermomechanical Behaviors of a Novel Double-Layer Corrugated Core Structure for Thermal Protection System

Corrugated core sandwich structures have great potential in the application to thermal protection system of aerospace vehicles. However, the traditional layout of web plates could inevitably lead to thermal short effects and high risk of buckling failure of the integrated thermal protection system (ITPS). In this paper, a novel double-layer ITPS is proposed by splitting and reorganizing a classical corrugated sandwich structure without additional introducing of weight. Distribution types of parallel, symmetric, and orthogonal of the double layers are designed and studied in detail. Basic theory of the thermomechanical problem as well as finite element simulation is carried out to study the responses of the ITPS. Numerical results show that the orthogonal type has more excellent yield resistance at high temperature and large temperature gradient than the others, while the parallel type has a relatively stronger buckling resistance. In addition, the structural stiffness variation caused by temperature dependencies of material parameters is greater than that caused by thermal stress, which shows the significance of consideration of temperature-dependent material properties in structure vibration analysis.

Comprehensive Comparison of Different Integrated Thermal Protection Systems with Ablative Materials for Load-Bearing Components of Reusable Launch Vehicles

Economic viability of small launch vehicles, i.e., microlaunchers, is impaired by several factors, one of which is a higher dry to wet mass ratio as compared to conventional size launchers. Although reusability may reduce launch cost, it can drive dry and/or wet mass to unfeasibly high levels. In particular, for load-bearing components that are exposed to convective heating during the aerothermodynamic phase of the re-entry, the mass increase due to the presence of a thermal protection system (TPS) must be considered. Examples of such components are aerodynamic drag devices (ADDs), which are extended during the re-entry. These should withstand high mechanical loading, be thermally protected to avoid failure, and be reusable. Ablative materials can offer lightweight thermal protection, but they represent an add-on mass for the structure and they are rarely reusable. Similarly, TPS based on ceramic matrix composite (CMC) tiles represent an additional mass. To tackle this issue, so-called integrated thermal protection systems (ITPS) composed of CMC sandwich structures were introduced in the literature. The aim is to obtain a load-bearing structure that is at the same time the thermally protective layer. However, a comprehensive description of the real lightweight potential of such solutions compared to ablative materials with the corresponding sub-structures is, to the authors’ knowledge, not yet presented. Thus, based on the design of an ADD, this work aims to holistically describe such load bearing components and to compare different TPS solutions. Both thermal and preliminary mechanical designs are discussed. Additionally, a novel concept is proposed, which is based on the use of phase change materials (PCMs) embedded within a metallic sandwich structure with an additively manufactured lattice core. Such a solution can be beneficial due to the combination of both the high specific stiffness of lattice structures and the high mass-specific thermal energy storage potential of PCMs. The study is conducted with reference to the first stage of the microlauncher analysed within the European Horizon-2020 project named Recovery and Return To Base (RRTB).

Topology Optimizations of Integrated Thermal Protection Systems in Multiple Reentry Load Cases

Topology Optimizations of Integrated Thermal Protection Systems in Multiple Reentry Load Cases

Vibration analysis of thick-section sandwich structures in thermal environments

An ultra-light sandwich structure with a thin ceramic-based composite face sheet bonded to a thick thermal insulator is developed as an integrated thermal protection system (ITPS) structure to protect the substructure of a hypersonic flight vehicle. For vibration fatigue assessment and failure prediction, the global dynamic response of the three-dimensional ITPS structure when subjected to a combination of thermal and harmonic excitation is desired. In this study, a novel unified model for vibration analysis of a thick-section sandwich structure is proposed based on the variational asymptotic method. The original 3D sandwich structure problem is reduced to one-dimensional through-the-thickness analysis and two-dimensional (2D) reference plane analysis. The stiffness matrices, including an equivalent transverse shear matrix, are obtained; hence, a shear correction factor is not required. The mechanical response of the 2D reduced-order plate under general boundary conditions in thermal environments is investigated. The static thermal deformation and forced vibration of the sandwich structure with the temperature gradient in the thickness direction are determined using a set of algebraic orthogonal polynomials through the Gram–Schmidt procedure. The thermal deflection is first introduced into the kinematic relationship to modify the total strain energy for the sandwich plate with temperature-independent material properties. To demonstrate the validity and applicability of the model, some numerical results relating to displacements and fundamental frequencies of simply supported sandwich plates are presented and compared to those obtained by other models. The effects of temperature gradients in the thickness direction, boundary conditions, core thickness, and temperature-dependent material properties on the dynamic characteristics and forced vibration of the sandwich structure are discussed by a detailed parametric study. The global dynamic strain response is first obtained using the plate model, which is desirable to determine the 3D local stress and strain field and predict vibration fatigue failure for the ITPS structure.

Integrated Thermal Protection System Design for Hypersonic Vehicle Based on New Thermal–Mechanical Method

With the development of hypersonic vehicles, considerable attention has been paid to thermal protection systems (TPSs). Among the different types of TPS, integrated TPS (ITPS) has attracted extensive interest because of its light weight, cost effectiveness, bearing capacity, and easy maintenance. However, ITPS, with its complex structure and reusable requirement, is facing challenges and it is worth devoting much effort to this. In an effort to overcome these challenges, we develop a novel approximate analytical method based on separation of variables and orthogonal expansion technique, which is presented for the prediction of heat transfer. The approximate analytical method has the ability to consider the effects of temperature-dependent thermal material properties, convection and radiation. Moreover, a C0 higher-order layer-wise finite-element model combined with homogenization techniques and a simplified three-dimensional (3D) finite-element model based on the periodic structure characteristic are proposed to estimate the thermal–mechanical response and stability of ITPS effectively. In addition, an optimization procedure based on the proposed methods for ITPS is developed. Implementation of the optimization is demonstrated by applying it to design the ITPS of space shuttles and hypersonic vehicles. By comparing with conventional thermal protection systems (CTPSs), ITPS exhibits an outstanding advantage in weight and an acceptable disadvantage in size. The objective of this paper is to establish an accurate and efficient design method aiming at determination of the optimal performance of ITPS, and puts forward the foundation for the development of a hypersonic vehicle.

Thermomechanical Homogenization of Corrugated Core Sandwich Structures of Reusable Launch Vehicles

The corrugated core sandwich structure is a potential candidate for the integrated thermal protection system (ITPS) of reusable launch vehicles (RLVs). The sandwich structure can develop a significant thickness-wise thermal gradient during operation, resulting in a temperature-dependent property variation along the cross section as well as thermal stresses. In this work, a homogenization method has been developed that incorporates these effects into the first-order shear deformation plate theory to facilitate thermomechanical structural analysis of RLV components such as wings, fuselage, etc., with the ITPS sandwich panels represented as thick orthotropic plates. The formulation expands the classical plate theory by assuming a polynomial form of thickness-wise temperature variation, as well as a linear dependence of elastic moduli and thermal expansion coefficients on temperature. This expansion introduces unknown constants into the homogenized model that could then be evaluated from unit-cell analyses with periodic boundary conditions and beam analyses of the corrugated core sandwich structure. The accuracy of the homogenized plate has been demonstrated by comparing its response with a full-scale model for a uniform and spatially varying in-plane temperature field. For both cases, a thickness-wise linear temperature variation has been used. The comparisons show that the homogenization approach is reasonably accurate in capturing the midplane deflections as well as stresses on the face sheets. An error of less than 1% could be seen in the maximum midplane transverse deflection for the problem with a spatially varying in-plane temperature field.

Optimization and Re-Design of Integrated Thermal Protection Systems Considering Thermo-Mechanical Performance

Integrated thermal protection system (ITPS) is regarded as one of the most promising thermal protection concepts with both thermal insulation and load-bearing capacities. However, the traditional layout of webs could inevitably lead to thermal short effects and high risk of buckling failure of the ITPS. A topological optimization method for the unit cell of the ITPS was established to minimize the equivalent thermal conductivity and elastic strain energy with the constraint of maintaining structural efficiency. The ITPS was re-designed consulting the optimized cell configuration. In order to control the buckling-mode shape and the associated buckling load of the ITPS, the new design was further optimized, subjected to the total weight of the initial design. Detailed finite element models were established to validate the structural responses. By contrast, the optimized design presents lower bottom surface temperature and better thermal buckling characteristics, performing a better balance between thermal insulation and load-bearing constraints.

An inverse method for real-time estimation of aerothermal heating for thermal protection systems of space vehicles

Space vehicles experience extremely high aerothermal heating during atmospheric entry which necessitates the use of appropriate thermal protection system (TPS). Effective design of TPS and the health monitoring systems (HMS) for space vehicles requires accurate flight data during atmospheric entry process. While direct measurement of the surface heat flux is a very challenging task, an alternative approach is to use the measured temperature values from the inner layers and solve the associated inverse heat conduction problem (IHCP) in order to estimate the surface heat flux. In the present paper, a solution approach is developed based on filter form of Tikhonov regularization method for near real-time calculation of surface heat flux in a one-dimensional medium consists of three layers that represents an integrated thermal protection system (ITPS). The solution is evaluated through numerical test cases developed in ANSYS and using experimental data from the literature. A parametric study is also conducted in order to understand the effect of sensor location (two layers and three layers models) as well as the effect of temperature dependent material properties on the performance of the solution. It is found that the developed solution estimates the surface heat flux with an average RMS error of about 1.96 and 3.44% for the two layer models with constant and temperature dependent material properties respectively. For the three layer model, the average RMS values are found for the constant and temperature dependent material properties as 2% and 4.44% respectively. It is also shown that the developed solution can evaluate the surface heat flux with 17 and 80 s delay for the two and three layers domain respectively, facilitating a near-real time operation for the inverse solution algorithm that can support the development of HMS for space vehicles or other industrial applications with the need for heat flux monitoring. The proposed solution technique is fast, accurate and very convenient to implement even for complex problems involving large temperature variations and temperature dependent material properties.

Thermomechanical Analysis of a Bio‐Inspired Lightweight Multifunctional Structure

This work investigates the thermal and mechanical performance of a bio‐inspired lightweight structure, which is inspired by the porous (up to 93% porosity) internal skeleton of cuttlefish, also known as cuttlebone. The potential of using this structure as an air‐cooled heat sink for electronic devices and an integrated thermal protection system (ITPS) is tested and systematically compared with three conventional structures based on wavy, pyramid, and cylindrical pin fin geometries. When cooling a hot spot (100 × 100 × 2 mm3) subjected to heat flow (250 W), and given the same solid volume fraction, the wavy and cuttlebone structures reduce the maximum temperature to 57 °C, which is 13 and 19 °C lower than for the pin fins and pyramid structures, respectively. Concerning the ITPS application, the cuttlebone exhibits the most isometric deformation with a 0.062 mm maximum standard deviation among its four corners. Moreover, the cuttlebone outperforms the other structures in terms of compressive strength (20%, 17%, and 219% increase compared with the wavy, pyramid, and pin fin structures, respectively). More importantly, the simulations indicate that the cuttlebone structure undergoes a graceful failure process due to the sequential buckling of the vertical walls compared with other structures.

Coupled ablation and thermal behavior of an all-composite structurally integrated thermal protection system: Fabrication and modeling

Structurally integrated thermal protection system (ITPS) was investigated, which has both load-carrying and thermal protection capacities. A novel all-composite sandwich panel with corrugated core was fabricated using the hot press molding method. According to the surface ablation and transient heat transfer theory, an approximate analytical model for predicting the coupled ablation and thermal response was presented for the composite sandwich panel. The effects of pyrolysis reactions, phase transitions, evaporation of quartz fibers, generation of decomposition gases, as well as surface ablation were considered in the mathematical model. After the heat conduction equations had been rewritten by separation of variables and further homogenized by a linearized approach, the governing differential equations were solved using Helmholtz formula and orthogonal expansion technique. A thermal exposure experimental test was performed to validate the approximate analytical model. The monitored temperature–time profiles in the thickness direction are in good agreement with the simulated results. The developed all-composite ITPS successfully protects any payload from an exposure to heat fluxes in the 300–500 kW/m2 range during approximately 2 min.

A multi-layer integrated thermal protection system with C/SiC composite and Ti alloy lattice sandwich

In this paper, a multi-layer integrated thermal protection system (ITPS) is proposed and compared with two existed ITPS. This multi-layer ITPS consists of a carbon fiber reinforced silicon carbide matrix composite (C/SiC) panel, a glass wool layer and a titanium (Ti) lattice sandwich filled with glass wool. In order to satisfy the aerodynamic shape requirement of hypersonic vehicles, a curved ITPS structure is concerned in this study. Multi-layer curved ITPS and single-layer curved ITPS made of Ti alloy and C/SiC composite are firstly analyzed and discussed under typical aerodynamic heating and pressure loads. Numerical results show that the proposed multi-layer ITPS provides efficient thermal insulation and load bearing properties compared with the C/SiC sandwich ITPS and Ti alloy sandwich ITPS. Secondly, multi-layer curved ITPS samples are fabricated. Experimental tests and numerical validation are carried out to evaluate the heat transfer characteristics of the ITPS samples.

Design and Analysis of Multi-layer Integrated Thermal Protection System based on Ceramic Matrix Composite and Titanium Alloy Lattice Sandwich

In order to satisfy the aerodynamic shape requirement of hypersonic vehicles, a curved multi-layer integrated thermal protection system (ITPS) is developed in this paper. The ITPS consists of a ceramic matrix composite layer, a lattice sandwich layer and a glass wool layer. The curved ITPS samples are fabricated. Then experimental test and numerical simulation are carried out respectively to evaluate the heating insulation effect of the ITPS samples. In addition, a series of numerical simulations are conducted to investigate the influences of lattice configuration and gradient layer layout on the heating insulation and load bearing capability of the ITPS.

Thermal and thermomechanical performance of actively cooled pyramidal sandwich panels

Previous research has proved that pyramidal core sandwich panel is an excellent kind of passive Thermal Protection Systems (TPSs) for hypersonic aerospace aircrafts suffered from in-service environment. To explore its further insulation performance and load-bearing capability, four kinds of actively cooled pyramidal core sandwich panels are designed and investigated in this paper. Convection flow mechanism for cooling channels is described in details. Influences of the coolant velocity, channel height ratio and flow direction of the two layers on thermal performance are firstly studied and discussed in an appropriate way. Analysis of thermomechanical performance is then performed. The effect of the cover board thickness on thermomechanical performance is also studied. In consequence, a light-weight, structural and thermal integrated thermal protection system (ITPS) has been designed to protect hypersonic aircrafts from intense radiation heat flux in service.

Topology optimisations for integrated thermal protection systems considering thermo-mechanical constraints

With both thermal insulation and load-bearing capabilities, integrated thermal protection systems (ITPS) can effectively improve structural efficiency, and thus are promising TPS concepts for future hypersonic vehicles. However, the connection structure of ITPS must adhere to the constraints of both low thermal conductivity and high mechanical performance. The layout design of the connection structure is key to successful applications of ITPS. Based on the principle of minimum net heat transfer rate and minimum strain energy, a topological optimisation method for ITPS was established to reduce both equivalent conductivity and maintain structural stiffness. Volume constraints were considered to maintain structural efficiency. ITPS designs were reconstructed from the optimised layout, and corresponding thermo-mechanical analyses were carried out. Compared to the initial design, optimised ITPSs indicate lower values in maximum back temperature, maximum deformation, and maximum component stresses. Topological optimisation method can provide a good balance between thermal insulation and load-bearing constraints in ITPS designs. With additive manufacturing technologies, these designs show promising applications.

Thermal protection system integrating graded insulation materials and multilayer ceramic matrix composite cellular sandwich panels

Current integrated thermal protection systems (ITPS) are generally devised with single layer metal sandwich panel and uniform insulation material, inevitably leading to limited thermal insulation effect and high risk of buckling failure. By contrast, here, ITPS, which incorporates graded insulation materials and multilayer ceramic matrix composite cellular sandwich panels, is devised to raise both thermal insulation effect and buckling strength. The effective thermal and mechanical properties are theoretically established. Transient heat transfer characteristics are numerically calculated. The results reveal that ITPS with specific graded insulation materials can present lower temperature and better temperature distribution uniformity than those of ITPS filled with uniform insulation material, demonstrating that graded insulation layers can firmly enhance the insulation effect. Besides, the insulation effect of ITPS, in which the densities and thermal conductivities of graded layers are taken arranged from low to high, is much better than those of ITPS which have opposite sequence. Moreover, the buckling strength is demonstrated to be significantly improved through the approach of integrating multilayer sandwich panels. The thermal short effect can be also suppressed, since it is demonstrated that the multilayer sandwich panels in the ITPS can reduce the temperature on bottom facesheet and raise the temperature distribution uniformity.

Structural and thermal analysis of integrated thermal protection systems with C/SiC composite cellular core sandwich panels

Current studies of the integrated thermal protection system (ITPS) are mainly focused on the metal corrugated core sandwich panels, resulting in low service temperature and high weight level. Here, by contrast, ITPS based on lightweight C/SiC composite sandwich panels with corrugated, pyramid as well modified pyramid cores are proposed for purpose of raising the service temperature, and lowering the weight level. As the basis, the structural analysis identifies the critical relative density of the failure models in the proposed ITPS, giving a guide to avoid the buckling failure. The effective thermal conductivity, which characterized the ability of heat conduction, is theoretically derived and numerical verified. Moreover, under typical aerodynamic heat and pressure, preliminary sizing analysis figures out the accepted dimensions of the ITPS with the fulfillment of both thermal and mechanical constraints. A new evaluate index: effective specific heat Ceff is proposed to compare the ITPS quantitatively and objectively. The comparison firmly confirms that the areal densities of the ITPSs with the proposed C/SiC pyramid and modified pyramid sandwich panels are much lower than those of the reported ITPSs with metal corrugated core sandwich panels. Meanwhile, the effective specific heat of the proposed ITPS is much higher than that of the reported ITPSs, revealing that the proposed ITPS incorporates the advantages of lower areal density and higher service temperature up to 1600 °C compared with the reported metal corrugated core ITPS.