Reducing the cost of launching a space vehicle is one of the vital requirements of the space industry. Reducing the cost of delivering a pound of payload into space by an order of magnitude is one of NASAs main objectives. The space vehicle's thermal protection system (TPS) is one of the most expensive and critical systems of the vehicle. The TPS protects the vehicle structure traveling at hypersonic speeds through a planetary atmosphere from damage due to aerodynamic heating. It occupies huge acreage on vehicle exteriors and accounts for a significant part of the launch weight.
Apollo Command Module (Source: http://images.jsc.nasa.gov/lores/S68-55292.jpg)
One potential way of saving weight is to have a load-bearing TPS that performs some structural functions. One such concept called the Integrated Thermal Protection System (ITPS).
Integrated Thermal Protection System (ITPS) uses a corrugated-core sandwich structure. A sandwich panel is a three layer element composed of two thin flat faces separated by a thick, lighter, and flexible core. The thin flat faces are high in stiffness when compared to the low average stiffness of the thick core. Sandwich constructions are frequently used because of their high bending stiffness to weight ratio. The corrugated core keeps the face sheets apart and stabilizes them by resisting vertical deformations, transverse shear strains, curvature in the longitudinal direction, and enables the structure to acts as a single thick plate. Empty spaces in between the webs can be packed with insulation material (e.g., SAFFIL) to block the internal radiation coming from the top face sheet. It is expected that by suitably designing the corrugated-core sandwich structure, a robust, operable, weight-efficient, load-bearing TPS can be developed.
Corrugated-core Sandwich Structure concept for Integrated Thermal Protection System (ITPS)
3D High-Fidelity Model:
One fourth of the ITPS panel containing half the total number of unit cells is modeled using the ABAQUS finite element (FE) software. Thermal and pressure load is applied and linear stress analysis is done on the 3D model. The procedure used finite element analyses to construct response surface approximations (RSA) of the critical constraints. The RSAs obtained were of high fidelity; however they required large computational time. This cost is expected to increase by at least an order of magnitude when uncertainties are taken into account and added as additional variables for obtaining a robust design.
a) The ITPS Panel b) Typical mesh and boundary conditions for the high fidelity 3D finite element model of one fourth of the panel
Micromechanical analysis of a unit cell is performed to determine the structure's extensional, bending, coupling shear stiffness, stresses, and unit cell behavior. Thus, the sandwich panel as an equivalent thick plate which is homogeneous, continuous, and orthotropic. A FE based homogenization procedure is developed to obtain the equivalent plate properties, and also for calculating the stresses in the web and face sheets from the 2D analysis results (reverse homogenization). The FE based homogenization and analysis of the 2D plate is very less expensive compared to that of 3D. Methods are developed to calculate the equivalent A, B and D matrices, transverse shear stiffness A44 and A55 and thermal forces NT and MT for a given through the thickness temperature distribution.
a) One fourth of the ITPS panel, b) Typical mesh and boundary conditions for the low fidelity 2D finite element model
Aerodynamic Pressure Load 101 kPa Edges symmetric
about y-axis Panel Edges
Aerodynamic Pressure Load 101 kPa
Edges symmetric about y-axis
Reverse homogenization for a 2D plate
The biggest challenge of an ITPS is that the requirements of a load-bearing member and a TPS are contradictory to one another. A TPS is required to have low conductivity and high service temperatures. Materials satisfying these conditions are ceramic materials, which are also plagued by poor structural properties like low impact resistance, low tensile strength and low fracture toughness. On the other hand, a robust load-bearing structure needs to have high tensile strength and fracture toughness and good impact resistance. Materials that satisfy these requirements are metals and metallic alloys, which have relatively high conductivity and low service temperatures.
The top face sheet is a hot structure and ceramic composites such as SiC/SiC composites are candidate materials. The web could also be made of similar composite. From weight efficiency point of view the bottom face sheet will be wade of graphite/epoxy composite. Thus there are several design variables-geometric and material property variables. The design of such a TPS will require several thousand analyses to obtain a minimum mass design. When the uncertainties in properties, dimensions and loads are taken into account, the computational costs will be prohibitively high. Hence there is a need to develop efficient methods for analysis and design of ITPS for future space vehicles.
A three-dimensional RSA FE method of the ITPS is the expensive. The low fidelity model RSA method is less accurate but inexpensive. We can reduce the computation time and also improve the accuracy of the 2D by fitting it with a high quality surrogate, which will then be corrected by the use of a small number of high fidelity 3D FEA. Fitting the difference or the ratio between the high fidelity analyses and the low fidelity surrogate with a response surface approximation allows construction of the so-called correction response surface (CRS). The entire approach, known as multi-fidelity, usually allows use of significantly fewer high fidelity analyses for a given accuracy.
Flow chart describing Multi-fidelity approach