Thermal Protection System Panel Research Articles

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.

Thermomechanical Performance of Bio-Inspired Corrugated-Core Sandwich Structure for a Thermal Protection System Panel

A skin structure for thermal protection is one of the most interesting components that needs to be considered in the design of a hypersonic vehicle. The thermal protection structure, if a dense structure is used, is heavy and has a large heat conduction path. Thus, a lightweight, high strength structure is preferable. Currently, for designing a lightweight structure with high strength, natural materials are of great interest for achieving low density, high strength, and toughness. This paper presents bio-inspired lightweight structures that ensure high strength for a thermal protection system (TPS). A sinusoidal shape inspired by the microstructure of the dactyl club of Odontodactylus scyllarus, known as the peacock mantis shrimp, is presented with two different geometries, a unidirectionally corrugated core sandwich structure (UCS) and a bidirectionally corrugated core sandwich structure (BCS). Thermomechanical analysis of the two corrugated core structures is performed under simulated aerodynamic heating, and the total deflection and thermal stress are presented. The maximum deflection of the present sandwich structure throughout a mission flight was 1.74 mm for the UCS and 2.04 mm for the BCS. Compared with the dense structure used for the skin structure of the TPS, the bio-inspired corrugated core sandwich structures achieved about a 65% weight reduction, while the deflections still satisfied the limits for delaying the hypersonic boundary layer transition. Moreover, we first fabricated the BCS to test the thermomechanical behaviors under a thermal load. Finally, we examined the influence of the core thickness, face-sheet thickness, and emittance in the simulation model to identify appropriate structural parameters in the TPS optimization. The present corrugated core sandwich structures could be employed as a skin structure for metallic TPS panels instead of the honeycomb sandwich structure.

Thermomechanical Behavior of Superalloy Thermal Protection System Under Aerodynamic Heating

This study investigated the thermomechanical behavior of a superalloy thermal protection system panel using a combination of experiments and numerical simulation. A superalloy thermal protection system panel was created by sandwiching a thermal insulation material and two superalloy plates with the support of spacer structures. The applied temperature profile was calculated from aerodynamic heating data. The thermal protection system panel was tested for three thermal cycles (approximately three missions). Thermal deformation of the thermal protection system panel was measured using a three-dimensional digital image correlation method. The results indicate that the presented thermal protection system panel provided good thermal protective performance under a thermal loading of 1023 K (750°C) with a heat-shielding index of 78% in the steady-state condition. The crucial finding is the permanent deformation of the thermal protection system panel after the first thermal cycle test. The thermomechanical coupled simulation results reveal the occurrence of a permanent deformation due to the plastic deformation of the spacer supporting structure, which was also observed in this Paper’s experiment. This study offers an indispensable understanding of the thermomechanical response of the thermal protection system panel, which is crucial for preventing damage to the thermal protection system panel and for modifying the structural design of the supporting structure in the thermal protection system panel.

Determination of distribution of heat-conducting material concentration in protective layer of thermal protection system panel

Paper presents the problem of heating the damaged insulation of an orbiter. Changes of the insulation?s thermal properties, made by adding conductive material of high value of specific heat in a form of a dope to the protective layer, were examined. An iterative algorithm determining a variable of dope concentration in the material was developed. Insulating material LI900 was used for calculations. Determination of distribution of conductive material concentration was made for materials which, after verification, demonstrated the most beneficial effect on protective properties of the modified insulation layer. Change of properties was to enable time extension of the LI900 insulation tile heating up to the maximal temperature and, additionally, to lowering this temperature.

A Modified Time Reversal Method for Guided Wave Detection of Bolt Loosening in Simulated Thermal Protection System Panels

In this work, a modified time reversal method is proposed for guided wave detection and localizing loosened bolt in a complicated multibolt-jointed structure. Different from the traditional time reversal guided wave method, the response signal due to a tone burst input received at the healthy state is time reversed and recorded as a standard reemitting signal. In the detection process, this recorded standard signal is used for all damage cases to yield time reversal-focalized reconstruction signals. This largely improves the sensitivity of the focalized signal to damage state. In this paper, the peak amplitude of the focalized wave packet in the reconstructed signal is calculated and utilized as tightness index. By bonding PZT transducers at different joint locations inside the structure, multiple tightness indices, where each tightness index presents the correlation between the current joint condition to its healthy condition at the joint, can be obtained. To analyze a large number of tightness indices, a principle component analysis method is introduced, and a neural network-based loosening detection method is proposed. The proposed method is experimentally validated in a simulated double-layer bolt-jointed thermal protection system panel. Experimental results illustrate that the proposed method is effective to identify and localized the bolt loosening in complicated multibolt-jointed structure. The detection and identification of the location of multibolt loosening is realized.

An approach for bending and transient dynamic analysis of integrated thermal protection system with temperature-dependent material properties

A C0 higher-order layerwise finite element model combined with homogenization techniques for bending and transient dynamic analysis of integrated thermal protection system (ITPS) panel is presented. The ITPS panel is homogenized as an equivalent sandwich plate, and a reduced temperature field for the entire panel is derived. An advanced analytical equivalent model based on energy approach and Castigliano’s theorem for the mechanical properties of the trapezoidal corrugated core is presented, which considers the effects of temperature-dependent material properties. The proposed layerwise theory which is developed for a sandwich plate, assumes linear, quadratic and cubic polynomial distributions of displacements for middle layer and linear polynomial distributions of displacements for top and bottom layers. And the effect of transverse normal strains is taken into account in the layerwise theory. A four-noded isoparametric element with 16 degrees of freedom per node is employed to model the equivalent sandwich plate. The accuracy of the proposed method is verified by comparison with the results by three-dimensional (3D) finite element analysis. It has been shown that the proposed method requires significantly less computational effort and agrees well with the finite element results.

Assessing Hypersonic Boundary-Layer Stability in the Presence of Structural Deformation

This work investigates the effect of two-dimensional surface deformations on hypersonic boundary-layer stability. The deformations are obtained from previous research and represent the characteristic response of integrated thermal protection system panels due to combined aerodynamic and thermal loading. Boundary-layer stability is assessed using linear stability theory and the parabolized stability equations for several different cases, such as deformation direction, deformation location, multiple deformations in series, structural boundary condition, surface temperature, the combined effect of Mach number and altitude, and deformation-mode shape. The broad set of results provides important insight into the conditions where thermomechanical compliance can promote, or even possibly delay, hypersonic boundary-layer transition.

Pulsed Thermography and Ultrasonic Non-Destructive Evaluation of Corrugated Metallic Thermal Protection System (MTPS) Panel

Space vehicles that re-enter earth’s atmosphere require thermal protection system (TPS) to safeguard them from intense aerodynamic heating. Depending on the type of re-entry (lifting/ballistic), trajectory, duration of flight in atmospheric regime etc., the heat flux may vary from 40kW/m2to 180kW/m2. In our studies on metallic TPS (MTPS), corrugated core is joined to either side of the skins by brazing. The brazed joint is evaluated using both pulsed thermography (PT) and ultrasonic testing. PT images show clear visualization of joint and defects. Also it is faster and easier. Ultrasonic testing is done using pitch-catch and pulse-echo techniques, which shows many limitations. In literature, it is reported that for brazed joints using ultrasonic testing, defect location can be identified by faster decay pattern of multiple echoes (compared to that of good location). In case of PM 2000 panel, reversal of the patterns is observed. This new phenomenon is verified with the support of PT. It is also found that for inconel panel, the decay patterns are as reported in literature.

Dimensionality Reduction Approach for Response Surface Approximations: Application to Thermal Design

of the maximum bottom face temperature is needed. The finite element model used to evaluate the maximum temperature depended on 15 parameters of interest for the design. A small number of assumptions simplified the thermal equations, allowing easy nondimensionalization, which together with a global sensitivity analysis showed that the maximum temperature mainly depends on only two nondimensional parameters. These were selected to be the variables of the response surface approximation for maximum temperature, which was constructed using simulations from the original nonsimplified finite element model. The major error in the two-dimensional response surface approximation was found to be due to the fact that the two nondimensional variables account for only part (albeit the major part) of the dependence on the original 15 variables. This error was checked and reasonable agreement was found. The two-dimensional nature of the response surface approximations allowed graphical representation, which we used for material selection from among hundreds of possible materials for the design optimization of an integrated thermal protection system panel.

Bayesian Wavelet Methodology for Damage Detection of Thermal Protection System Panels

This paper presents an intelligent computational methodology for loose-bolt detection in thermal protection panels, considering uncertainties in sensed data. The proposed methodology is based on the integration of a dynamic artificial neural network, wavelet signal analysis, and Bayesian probabilistic assessment. A dynamic fuzzy wavelet neural-network model is employed to perform the multiple-input/multiple-output nonparametric system identification of the panel using time-series data obtained from the panel under a healthy condition. The trained model is used to predict dynamic responses of the structural system under unknown conditions. Both predicted and sensed-time-history data are decomposed into multiple time-frequency resolutions using a discrete wavelet-packet transform method. The wavelet-packet component energy is computed in terms of the decomposed coefficients and used as a signal feature to detect loose bolts. The effectiveness of the selected features is assessed using both cross-correlation and cross-coherence metrics. The multivariate comparison in damage detection is handled by an interval-based Bayesian hypothesis-testing approach. The methodology is implemented to detect one loose bolt of a prototype thermal protection system panel with four mechanically bolted joints using experimental data collected at the U.S. Air Force Research Laboratory from seven different sensor configurations.

A study on metallic thermal protection system panel for Reusable Launch Vehicle

A Ni-based superalloy honeycomb thermal protection system (TPS) panel has been fabricated. And a curved Ni-based superalloy honeycomb sandwich has also been fabricated. The preliminary thermal insulation results of a fabricated Ni-based superalloy honeycomb TPS panel (the areal density of this panel is 6.7kg/m2 and total height is 32mm) indicate that the maximum temperature of the lower surfaces of the panel is lower than 150∘C when the temperature of outer surface is held at 650∘C for 30min. The flatwise tensile strength and compressive properties of a fabricated Ni-based superalloy honeycomb sandwich coupon was studied at room temperature. A multilayered coating has been developed on the surface of the superalloy honeycomb TPS panel for environmental protection and thermal control. The oxidation weight-change results show that the weight change of the Ni-based superalloy honeycomb sandwich with the oxidation resistant coating is extremely small at 1100∘C in air for 10h. The emittance layer of the multilayered coating imparts an emittance in excess of 0.85 during exposure at 850∘C, which was at least 14% greater than that of the substrate with oxidation resistant alone.

Impact Loads Identification in Standoff Metallic Thermal Protection System Panels

This article describes two experimental approaches for the identification of transient impact loads applied to a metallic composite thermal protection system (TPS) panel. The critical nature of the TPS to the survival of the vehicle and crew warrants the development of a technique that can detect, quantify, and locate transient impact loads. This information will provide an opportunity to identify specific panels that are more or less prone to damage from transient impacts. Furthermore, a historical database of impact loads encountered can be retained for use in the development of statistical models that relate impact loading to panel life. The two techniques presented herein are a physics-based approach, where the panel is assumed to behave as a rigid body at low frequencies, and an inverse frequency response approach, where the system impedance matrix is measured, from which the loads acting on the panel can be estimated using the measured response. The validity of approaches is verified experimentally, and it is shown that the inverse frequency response method is capable of detecting, locating, and quantifying transient impact loads to a high degree of accuracy.

Detection of bolt loosening in C–C composite thermal protection panels: I. Diagnosticprinciple

A concept demonstrator of the structural health monitoring (SHM) system was developedto autonomously detect the degradation of the mechanical integrity of the standoffcarbon–carbon (C–C) thermal protection system (TPS) panels. This system enables us toidentify the location of the loosened bolts, as well as to predict the torque levels of thosebolts accordingly. In the process of building the proposed SHM prototype, efforts havebeen focused primarily on developing a trustworthy diagnostic scheme and aresponsive sensor suite. In part I of the study, an attenuation-based diagnosticmethod was proposed to assess the fastener integrity by observing the attenuationpatterns of the resultant sensor signals. The attenuation-based method is basedon the damping phenomena of ultrasonic waves across the bolted joints. Themajor advantage of the attenuation-based method over the conventional diagnosticmethods is its local sensing capability of loosened brackets. The method can furtherdiscriminate the two major failure modes within a bracket: panel-joint loosening andbracket-joint loosening. The theoretical explanation of the attenuation-basedmethod is performed using micro-contact theory and structural/internal dampingprinciples, followed by parametric model studies and appropriate hypothesis testing.

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.

3307 超高温真空条件におけるCMC耐熱パネルの加熱試験と熱弾性解析評価(S84-2 耐熱構造・材料(2),S84 耐熱構造・材料)

High-temperature heating tests of CMC thermal protection system (TPS) panel were carried out by using the vacuum heating test equipment with the infrared lamp, and the temperature and displacement of the panel were measured at several points. The transient values of temperature, displacement and stress were calculated by using FEM with taking into account cavity radiation and the nonlinear stress-strain relation of the material. Calculated values by FEM indicate relatively good agreement with the experimental results.

Development of Advanced Metallic-Thermal-Protection System Prototype Hardware

A new metallic thermal-protection-system concept has been designed, analyzed, and fabricated. A specific location on a slngle-stage-to-orbit reusable launch vehicle was selected to develop loads and requirements needed todesign prototype panels. The design loads include ascent and entry heating rates, pressures, acoustics, and accelerations. Additional design issues were identified and discussed. An iterative sizing procedure was used to size the thermal protection system panels for thermal and structural loads as part of an integrated wall construction that included the thermal protection system and cryogenic tank structure. The panels were sized to maintain acceptable temperatures on the underlying structure and to operate under the design structural loading. Detailed creep analyses were also performed on critical components of the panels. Four 18-in.-square metallic thermal-protection-system panels were fabricated. A lightweight, thermally compliant support system to connect the thermal protection system to the cryogenic tank structure was designed and fabricated.

Methodology for Supersonic Panel Flutter Analysis of Thermal Protection System Seals

A methodology is presented for the e utter analysis of the seal of the thermal protection system panel of the X-33 Advanced Technology Demonstrator test vehicle. The seal is simulated as a two-dimensional cantilevered panel with an elastic stopper, which is modeled as an equivalent spring. This cantilever beam ‐spring model under the aerodynamic pressure at supersonic speeds becomes an impact nonlinear dynamic system. The e utter analysis of the seal is, thus, carried out using time-domain numerical simulation with a displacement stability criterion. The e utter boundary of the seal is further verie ed with a family of three traditional and one nontraditional panel e utter models. The frequency-domain method that applies eigenanalysis on the traditional panel e utter problem was used. The results showed that the critical dynamic pressure could be more than doubled with the properly chosen materialforthebase stopper. Theproposed methodology can beeasily extended to three-dimensional panel seals with e ow angularity. Nomenclature [aa], [Aa] = element and system aerodynamic ine uence matrices b = beam width Ca = aerodynamic damping coefe cient fdg = element nodal displacement vector E = Young’ s modulus Fe

Numerical Simulation of Metallic Thermal Protection System Panel Bowing

Numerical simulation of the thermoelastic response of metallic thermal protection panels is presented. The panels, which arebeing designed foruseon the windward surface of theX-33 e ighttest vehicle, deform into convex and concavebowedsurfacesdueto thermalgradientscausedbyaerodynamicheating.Threenumerical models, for the e owe eld, forthein-depth heat transfer, and for the thermoelasticdeformation, are coupled in sequence to yield the transient response of the metallic panel. The aerothermal loads are derived from computational e uid dynamic solutionsandareprescribedasadistributionfunctionwithmaximumbowheightasthegoverningparameter.Finite element models are used to simulate the thermal and structural responses. The coupled simulation is compared to a single-pass uncoupled solution. Results show negligible feedback between the structural deformation and the deformation-induced perturbation of the aerothermal heat load. Nevertheless, signie cant temperature variations on the surface of the panel are produced. The deformations induce lateral temperature gradients that increase the thermal stress within the panel. Finally, it is shown that panel bowing does not appreciably alter the trajectory integrated heat load.

Acoustic testing of high-temperature panels

Results are presented of a series of thermal-acoustic tests conducted on the NASA Langley Research Center Thermal-Acoustic Test Apparatus to (1) investigate techniques for obtaining strain measurements on metallic and carbon-carbon materials at elevated temperature; (2) document the dynamic strain response characteristics of several superalloy honeycomb thermal protection system panels at elevated temperatures of up to 1200 F; and (3) determine the strain response and sonic fatigue behavior of four carbon-carbon panels at both ambient and elevated temperatures. A second study tested four carbon-carbon panels to document panel dynamic response characteristics at ambient and elevated temperature, determine time to failure and faliure modes, and collect continuous strain data up to panel failure. Strain data are presented from both types of panels, and problems encountered in obtaining reliable strain data on the carbon-carbon panels are described. The failure modes of the carbon-carbon panels are examined.