Thermal Protection System Structures Research Articles

Probabilistic design and optimization of thermal protection system with variable thickness based on non-uniform aerodynamic heating

Thermal Protection System (TPS) is a critical component of space vehicles to protect them from damage by extreme aerothermal heating conditions. The uncertainty of the aerothermal heating conditions, including approaching velocities, atmospheric density, pressure, etc., significantly affects the aerothermal heating imposed on the TPS, vastly changing the initial design conditions. In this study, we developed a new uncertainty quantitative (UQ) analysis method to comprehensively consider all the uncertainty of TPS materials, structures, and aerothermal heating conditions. To address the challenges of a massive amount of input and determined optimization parameters, we first induced the response surface method and Monte Carlo algorithm to produce a large number of analysis samples for Probabilistic Design. Then, the key affecting factors, not all the input design parameters, were chosen for optimization with sequential optimization and reliability assessment (SORA) strategy by a sensitivity analysis. With such improvements, the optimization for UQ analysis becomes practical. We used this new UQ method to analyze the effects of uncertainty of the aerothermal heating conditions on the thermal protection layer. A variable thickness design is proposed to adapt the non-uniform aerothermal heating conditions for the TPS. The results show that significant light-weighting can be achieved without the reliability penalty. The optimized structure also reduces internal temperature gradients and facilitates a more uniform temperature field distribution for the TPS. This study provides a new UQ engineering design approach for the system with many designed parameters.

Parameterized Reduced-Order Models for Probabilistic Analysis of Thermal Protection System Based on Proper Orthogonal Decomposition

The thermal protection system (TPS) represents one of the most critical subsystems for vehicle re-entry. However, due to uncertainties in thermal loads, material properties, and manufacturing deviations, the thermal response of the TPS exhibits significant randomness, posing considerable challenges in engineering design and reliability assessment. Given that uncertain aerodynamic heating loads manifest as a stochastic field over time, conventional surrogate models, typically accepting scalar random variables as inputs, face limitations in modeling them. Consequently, this paper introduces an effective characterization approach utilizing proper orthogonal decomposition (POD) to represent the uncertainties of aerodynamic heating. The augmented snapshots matrix is used to reduce the dimension of the random field by the decoupling method of independently spatial and temporal bases. The random variables describing material properties and geometric thickness are also employed as inputs for probabilistic analyses. An uncoupled POD Gaussian process regression (UPOD-GPR) model is then established to achieve highly accurate solutions for transient heat conduction. The model takes random heat flux fields as inputs and thermal response fields as outputs. Using a typical multi-layer TPS and thermal structure as two examples, probabilistic analyses are conducted. The mean square relative error of a typical multi-layer TPS is less than 4%. For the thermal structure, the averaged absolute error of the radiation and insulation layer is less than 25 °C and 6 °C when the maximum reaches 1200 °C and 150 °C, respectively. This approach can provide accurate and rapid predictions of thermal responses for TPS and thermal structures throughout their entire operating time when furnished with input heat flux fields and structural parameters.

Design of LP and CP microstrip antennas covered by lossy thermal protection system

Low-temperature antenna system that covered by a dedicated thermal protection system (TPS) is an indispensable component of hypersonic vehicle. However, electromagnetic interactions between the installed antennas and the TPS tend to impair the antenna's performance, such as the resonant frequency, radiation efficiency, and polarization purity. To mitigate these negative effects of placing a thick, lossy TPS in close proximity to the microstrip patch antennas, this article proposes a concept of near-field matching technique. The magnetic field distribution of the installed antenna is calculated in the near field, followed by adopting a surface fitting method to determine the optimal shape of a modified TPS. Two patch antennas with linear polarization (LP) and circular polarization (CP) are analyzed by this method, respectively. It is discovered that different TPS shapes suit LP and CP antennas, respectively. Both experimental and numerical results show that the proposed TPS structure improves the impedance matching and radiation properties of the microstrip antennas simultaneously. The proposed design achieves a harmonious balance between the electrical and thermal protection performance, surpassing that of a traditional flat TPS. To verify the design, a prototype CP antenna is fabricated and measured.

New insights on the ablation mechanism of silicon carbide in dissociated air plasmas

Thermal protection system (TPS) applied in space vehicle is mainly composed of silicon carbide (SiC) and related components. When exposed in re-entry flows, the SiC based TPS may suffer from severe ablation if the surface temperature is beyond a critical threshold (generally ≥1600°C). Unveiling the ablation mechanism of SiC is thus vitally important to define the borders of service and to help optimize the TPS structures in re-entry environments. Motivated by this, this work uses an inductively coupled plasma wind tunnel to generate the dissociated air plasmas and explores the ablation behaviors of several SiC materials (including SiC ceramic and SiC fiber reinforced SiC matrix composites). Some new insights are acquired based on the detailed analysis of the surface characteristics prior to and after ablations. First, the role of surface roughness on the critical ablation temperature of the SiC based composite is put forward. Second, a ‘from-point-to-surface’ ablation process due to the surface roughness during the short ‘temperature jump’ is proposed, which is an important supplementary to the existing ablation model of SiC; Finally, a smoother surface with less defects is concluded to efficiently enhance the ablation resistance of the SiC based TPS in re-entry environments.

Design, Fabrication, and Testing of Metallic Thermal Protection Systems for Spaceplane Vehicles

This study presents the design and testing of a metallic thermal protection system (TPS) for future spaceplane vehicles. A conceptual design of the developed metallic TPS panel consisting of an outer sandwich structure, thermal insulation, inner support brackets, and a base framelike structure was redesigned from the metallic TPS panel developed by NASA. The thermal load was selected from a previous study, which was equivalent to the highest surface temperature of 1050 K. Nine metallic TPS prototype panels and supporting hardware structures made of 304 stainless steel were initially prepared for thermomechanical testing in room conditions. A noncontact thermal–mechanical measurement method was employed to measure full-field temperatures and deformations of the inner base structure and supporting hardware structures. The results indicated that the proposed design satisfied the requirements of temperature and permanent deformation limits. Panel-to-panel gaps showed a significant effect on the temperature rise in the inner base structure. The performance of the prototype proved the feasibility, reusability, and durability of using low-cost 304 stainless steel for TPS structures up to four thermal cycles, and may achieve more in further tests. Results provide valuable data for evaluating designs at the early stage of TPS development and testing in space conditions.

Dynamic Characteristics and Damage Detection of a Metallic Thermal Protection System Panel Using a Three-Dimensional Point Tracking Method and a Modal Assurance Criterion.

A thermal protection system (TPS) is designed and fabricated to protect a hypersonic vehicle from extreme conditions. Good condition of the TPS panels is necessary for the next flight mission. A loose bolted joint is a crucial defect in a metallic TPS panel. This study introduces an experimental method to investigate the dynamic characteristics and state of health of a metallic TPS panel through an operational modal analysis (OMA). Experimental investigations were implemented under free-free supports to account for a healthy state, the insulation effect, and fastener failures. The dynamic deformations resulted from an impulse force were measured using a non-contact three-dimensional point tracking (3DPT) method. Using changes in natural frequencies, the damping ratio, and operational deflection shapes (ODSs) due to the TPS failure, we were able to detect loose bolted joints. Moreover, we also developed an in-house program based on a modal assurance criterion (MAC) to detect the state of damage of test structures. In a damage state, such as a loose bolted joint, the stiffness of the TPS panel was reduced, which resulted in changes in the natural frequency and the damping ratio. The calculated MAC values were less than one, which pointed out possible damage in the test TPS panels. Our results also demonstrated that a combination of the 3DPT-based OMA method and the MAC achieved good robustness and sufficient accuracy in damage identification for complex aerospace structures such as TPS structures.

Fatigue life prediction in frequency domain using thermal-acoustic loading test results of titanium specimen

High supersonic vehicles are exposed to high temperature generated by aerodynamic heating. Thermal protection system structures are used on the skin of the fuselage and wings to prevent the transfer of high temperatures into the interior of the vehicle. Thin skin panels can be exposed to acoustic loads by high power engine noise and jet flow noise, which can cause sonic fatigue damage. Therefore, it is necessary to examine the behavior of supersonic/hypersonic vehicle skin structures under thermal-acoustic loads and to predict fatigue life. In this paper, thermal-acoustic testing of titanium specimens under thermal-acoustic load was performed. The response stress history of the specimen was obtained, and the fatigue life was predicted using the time and frequency domain fatigue life prediction method. The effect of the mean stress on the predicted results of the time and frequency domian fatigue life was analyzed. Stress history was generated using a sine series of random phases from stress PSD without phase information. The fatigue life in the generated stress history was predicted using the time and frequency domain fatigue life prediction methods. As the temperature increased, the mean stress of the response stress and the error in the frequency domain fatigue life prediction results increased. The error in the frequency domain fatigue life prediction results with the mean stress effect were greatly reduced by considering the completely reversed stress.

Optimal Sensor Placement of Health Management for Thermal Protection System on RLV

When a Reusable Launch Vehicle (RLV) travels through the atmosphere there should be strong aero heating around it, so a Thermal Protection System (TPS) would be used to protect the vehicle structures from aero thermal ablation. However, the TPS structure may fail more frequently than normal vehicle structures because of the severe environment and load conditions, while a TPS failure can even lead to a catastrophic breakdown of vehicle structures. To keep the RLV safe, the TPS must be monitored real-time. To learn the heat transfer inside a ceramic TPS tile finite element models are built, and computation made with them indicates that temperature monitoring and frequency monitoring of the TPS structure should be effectual for fault detection. Health management for TPS is one of the key technologies for space vehicle. The paper puts forward a method that analyze and valuation the system states by monitoring the key function index sign of TPS. The concept of health management for Thermal Protection System of space shuttle and the principles of health monitoring are introduced. The principles of sensor selection and layout optimization method for TPS Health Management are discussed.

Thermal-Acoustic Fatigue of a Multilayer Thermal Protection System in Combined Extreme Environments

In order to ensure integrity of thermal protection system (TPS) structure for hypersonic vehicles exposed to severe operating environments, a study is undertaken to investigate the response and thermal-acoustic fatigue damage of a representative multilayer TPS structure under combined thermal and acoustic loads. An unsteady-state flight of a hypersonic vehicle is composed of a series of steady-state snapshots, and for each snapshot an acoustic load is imposed to a static steady-state TPS structure. A multistep thermal-acoustic fatigue damage intensity analysis procedure is given and consists of a heat transfer analysis, a nonlinear thermoelastic analysis, and a random response analysis under a combined loading environment and the fatigue damage intensity has been evaluated with two fatigue analysis techniques. The effects of thermally induced deterministic stress and nondeterministic dynamic stress due to the acoustic loading have been considered in the damage intensity estimation with a maximum stress fatigue model. The results show that the given thermal-acoustic fatigue intensity estimation procedure is a viable approach for life prediction of TPS structures under a typical mission cycle with combined loadings characterized by largely different time-scales. A discussion of the effects of the thermal load, the acoustic load, and fatigue analysis methodology on the fatigue damage intensity has been provided.

Numerical Simulation of Heat Transfer Characteristics for Two Metallic TPS Structures

Two structures of metallic thermal protection system(TPS) for hypersonic vehicle were presented. One model was a multi-layer construction and the other has cavities in the metallic layer. Numerical simulations were conducted on the three-dimensional TPS models using CFD software of Gambit and Fluent. Two heating temperatures of 1073K and 773K with constant temperature and isothermal boundary conditions were considered. Heat transfer was treated as single conductivity and thermal radiation effect was not involved. The results of simulation investigation showed that: The metallic layer had poor capability to restrict the heat conductivity. Heat was easier to transfer across the bracket into the internal part of the TPS. The ability of cavities in metallic layer to resist heat conductivity was limited. The temperature-heating time variation pattern was similar for different external heating temperature. Internal cooling was important for the TPS. The thermal radiation effect on the TPS would be focused in further research.

Analysis of sandwich TPS panel with functionally graded foam core by Galerkin method

The method of Fourier analysis is combined with the Galerkin method for solving the two-dimensional elasticity equations for a sandwich thermal protection system (TPS) insulation panel with foam core subjected to transverse loads. The variation of the Young’s modulus through the thickness is given by a polynomial in the thickness coordinate and the Poisson’s ratio is assumed to be constant. The Fourier series method is used to reduce the partial differential equations to a pair of ordinary differential equations, which are solved by using the Galerkin method. The method will be useful in analyzing functionally graded TPS structures with arbitrary variation of thermomechanical properties in the thickness direction. The analysis was also performed using sandwich plate theory. Significant differences were found in the results suggesting that the sandwich theory may not be suitable for the analysis of thick sandwich TPS panel.