Max L. Blosser

Micromechanical Analysis of Composite Corrugated-Core Sandwich Panels for Integral Thermal Protection Systems

termsA44 andA55 werecalculatedusinganenergyapproach.Usingtheshear-deformableplatetheory,aclosed-form solution of the plate response was derived. The variation of plate stiffness and maximum plate deflection due to changing the web angle are discussed. The calculated results, which require significantly less computational effort and time, agree well with the three-dimensional finite element analysis. This study indicates that panels with rectangular webs resulted in a weak extensional, bending, and A55 stiffness and that the center plate deflection was minimum for a triangular corrugated core. The micromechanical analysis procedures developed in this study were used to determine the stresses in each component of the sandwich panel (face and web) due to a uniform pressure load.

(Student Paper) Analysis and Design of Corrugated-Core Sandwich Panels for Thermal Protection Systems of Space Vehicles

A preliminary design process of an integral thermal protection system (ITPS) has been presented. Unlike the conventional TPS, the ITPS has both thermal protection as well as load bearing capabilities. The objective of this research work is to establish procedures and identify issues in the design of an ITPS. Corrugated-core sandwich construction has been chosen as a candidate structure for this design problem. An optimization problem was formulated as part of the design process with mass per unit area of the ITPS as the objective function and different functions of the ITPS as constraints. The optimization problem was solved by developing response surface approximations to represent the constraints. Response surface approximations were obtained from finite element (FE) analyses, which include transient heat transfer analyses and buckling analyses. A Matlab code (ITPS Optimizer) has been developed for generating the response surfaces, which has the capability to carry out hundreds of FE analyses, automatically, in conjunction with ABAQUS. Accurate response surface approximations could be obtained for the peak temperatures of the ITPS structure. It was found that response surface approximations for the smallest buckling eigen value of the whole structure were inaccurate. Therefore, the buckling modes were separated and similar buckling modes were grouped together. One response surface approximation was obtained for the smallest buckling eigen value of each group. The preliminary design process for the ITPS generates a design with areal density of approximately 10 lb/ft. Even though the ITPS has much higher load bearing capabilities, it is still on the heavier side when compared to conventional TPS (typical weight 2 lb/ft). New design changes have been proposed as part of the future work to make the ITPS lighter than the current design.

Thermal Force and Moment Determination of an Integrated Thermal Protection System

This paper is concerned with homogenization of a corrugated-core sandwich panel, which is a candidate structure for integral thermal protection systems for space vehicles. The focus was on determination of thermal stresses in the face sheets and the web caused by through-the-thickness temperature variation. A micromechanical method was developed to homogenize the sandwich panel as an equivalent orthotropic plate and calculate the equivalent thermal forces and moments for a given temperature distribution. The same method was again used to calculate the stresses in the face sheets and the core. The method was demonstrated by calculating stresses in a sandwich panel subjected to a temperature distribution described by a quartic polynomial in the thickness direction. Both constrained and unconstrained boundary conditions were considered. In the constrained case the plate boundaries are constrained such that there are no deformations in the macroscale sense. The unconstrained case assumes that there are no force and moment resultants in the macroscale. The results for stresses are compared with that from a three-dimensional finite element analysis of the representative volume element of the sandwich structure, and the comparison was found to be within 5 % difference. The micromechanical analysis, which is less time consuming, will be useful in the design and optimization of integral thermal protection system structures.

METALLIC THERMAL PROTECTION SYSTEM PANEL FLUTTER STUDY

Panel flutter analysis results of a reusable launch vehicle metallic thermal protection system [Ref.l] panel are presented in this paper. Panel flutter is a self-excited aeroelastic vibration. The unsteady aerodynamic theory used in this study is the first- and second-order piston theory. The analysis capability is incorporated into a commercial finite element code with the goal of providing a useful tool for aerospace engineers —MSC/MARC [Ref.6]. The capability consists of three parts: linear flutter analysis, linear flutter analysis with arbitrary flow direction, and nonlinear hypersonic flutter analysis. Structural nonlinearities are not considered. The current paper covers both panel flutter analysis tool development as well as parametric panel flutter study of a specific metallic TPS configuration. The resulting analytical tool was used to analyze a metallic TPS being built for the X-33 experimental vehicle. The results show that nonlinear, hypersonic aerodynamic terms were found to have an insignificant effect on the panel flutter predictions for the X-33 TPS. The honeycomb sandwich that comprises most of the outer surface of the X-33 TPS panel was found to have large margins of safety for panel flutter for all flight conditions. However, the outer overlapping, panel-to-panel seals were predicted

Fundamental Modeling and Thermal Performance Issues for Metallic Thermal Protection System Concept

A study was performed to develop an understanding of the key factors that govern the performance of metallic thermal protection systems for reusable launch vehicles. A current advanced metallic thermal protection system concept was systematically analyzed to discover the most important factors governing its thermal performance. A large number of relevant factors that influence the thermal analysis and thermal performance system were identified and quantified. Detailed finite element models were developed for predicting the thermal performance of design variations of the metallic thermal protection system concept mounted on a simple, unstiffened structure. The computational models were also used, in an automated iterative procedure, for sizing the metallic thermal protection system to maintain the structure below a specified temperature limit. A statistical sensitivity analysis method, based on orthogonal matrix techniques used in robust design, was used to quantify and rank the relative importance of the various modeling and design factors considered in this study. Thermal performance was shown to be sensitive to several analytical assumptions that should be chosen carefully. Of the design factors studied, the heat capacity of the underlying structure was found to have the greatest effect on thermal performance. Therefore the structure and thermal protection system should be designed concurrently.

Two-Dimensional Orthotropic Plate Analysis for an Integral Thermal Protection System

plate theory. The two-dimensional plate deformations are used to obtain local integrated thermal protection system stresses through reverse homogenization. In addition, simple beam models are used to obtain local facesheet deformations and stress. The local stresses and deflections computed using the analytical method were compared with those from a detailed finite element analysis of the integrated thermal protection system. For the integrated thermal protection system examples considered in this paper, the maximum error in stresses and deflections is less than 5%. This was true for both mechanical and thermal loads acting on the integrated thermal protection system.

Optimal Functionally Graded Metallic Foam Thermal Insulation

Optimum density profiles that minimize heat transmission through a metal foam thermal insulation under onedimensional steady-state conditions are investigated. The effective thermal conductivity of the foam is derived in terms of cell parameters and the temperature. Maximizing the temperature at the outside wall of the insulation minimizes the heat conduction through the insulation because this maximizes the radiated heat. An optimality condition is derived, and the optimization problem is reduced to that of an ordinary, but a nonlinear differential equation, which is solved numerically. The optimum density variation through the thickness of the insulation for a given incident heat flux and the transmitted heat are presented for graded and uniform foams with open and closed cells. For open-cell foams, functional grading of the foam density can reduce the heat transfer through the foam for given thickness. Conversely, for a specified amount of heat transmission through the foam, the functionally graded foam insulation can be made thinner than uniform density foam insulation.

Minimum mass design of insulation made of functionally graded material

The problem of steady state heat conduction in a functionally graded open-cell metal foam thermal insulation is studied. The mass is minimized by varying the solidity profile for a given thickness. An optimality condition is derived and the optimization problem is reduced to that of an ordinary, nonlinear differential equation, which is solved numerically. The results include optimum cell size variation through the thickness of the insulation for given aerodynamic heating and the corresponding temperature distribution. It is shown that for a given thickness using a functionally graded insulation is predictably lighter than uniform one INTRODUCTION Metal foams [1] and advanced metallic thermal protection systems [2] are being investigated for use in multifunctional structures for reusable launch vehicles. Such multifunctional structures would insulate the vehicle interior from aerodynamic heating as well as carry primary vehicle loads. Varying the density, geometry, and/or material composition from point to point within the foam can produce functionally graded materials (FGM) that may be superior to uniform materials. To develop and test FGM for thermal protection systems, it is important to develop an understanding of what material property distributions offer significant efficiency gains. Satchi Venkataraman et al. [3] developed criterion for minimizing heat conduction through an open-cell titanium foam with variable cell size through its thickness. For a fixed inner wall temperature and foam thickness the outside wall temperature is maximized. Maximizing the outside wall temperature maximizes the heat radiated at the surface and therefore corresponds to minimizing the transmitted heat. The current study seeks to identify density profiles that may yield large improvements in weight efficiency compared to materials with uniform density. These results will be then used to direct research into improved modeling of FGM that will be used to refine the initial optimization. Finally, it is hoped that the results could be used to direct testing of promising configurations. The thermal protection systems (TPS) problem is inherently a transient one. However, we are first solving the simpler steady state problem to gain understanding of the effects of using functionally graded insulations. In this paper an optimality criterion is derived for minimizing the mass of an open-cell titanium foam with variable cell size through its thickness. The effective thermal conductivity of the foam is a function of temperature, pressure, properties of the foam material, and the foam geometry. The objective of optimization problem is to determine the density distribution that minimizes the mass of a titanium foam of given thickness for a fixed inner and outer wall temperatures. The optimality condition is developed and used to obtain the optimum density profile. The minimum mass obtained using a functionally graded foam and uniform density foam are compared to illustrate performance payoffs provided by optimization of graded foam properties. 43rd AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Con 22-25 April 2002, Denver, Colorado AIAA 2002-1425 Copyright

Thermal-stress-free fasteners for joining orthotropic materials

Hot structures fabricated from orthotropic materials are an attractive design option for future high speed vehicles. Joining subassemblies of these materials with standard cylindrical fasteners can lead to loose joints or highly stressed joints due to thermal stress. A method has been developed to eliminate thermal stress and maintain a tight joint by shaping the fastener and mating hole. This method allows both materials (fastener and structure), with different coefficients of thermal expansion (CTEs) in each of the three principal material directions, to expand freely with temperature yet remain in contact. For the assumptions made in the analysis, the joint will remain snug, yet free of thermal stress at any temperature. Finite element analysis was used to verify several thermal-stress-free fasteners and to show that conical fasteners, which are thermal-stress-free for isotropic materials, can reduce thermal stresses for transversely isotropic materials compared to a cylindrical fastener. Equations for thermal-stress-free shapes are presented and typical fastener shapes are shown.

Evaluation of Integrated Sandwich TPS design with Metal Foam Core for Launch Vehicles

§Two kinds of thermal protection systems for launch vehicles are compared for the launch and reentry phases of the mission. One is the integrated sandwich structure with titanium foam core as insulation, and the other is a structural panel with Safill™ insulation (fibrous alumina insulation) design. The weights of b oth designs are minimized subject to temperature constraints, stress constraints or both. The sandwich structure is analyzed by the method of Fourier expansion combined with the Galerkin method. Global buckling, shear crimping and face wrinkling are invest igated for the integrated sandwich structure for the launch. It is found that for temperature constraint only, integrated sandwich design tend to require thick insulation , while Safill design requires thin structure . Shear crimping is most critical among a ll the three failure modes we studied in integrated sandwich design.