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RESEARCH SECTIONS:
Integrated
Thermal Protection System
Micro
Mechanics and Homogenization
Composite
Materials and Structures
Multi-Fidelity
Design Response Surface Integrated Thermal
Protection System
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
Micro Mechanics and
Homogenization 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
Reverse
homogenization for a 2D plate Composite Materials and Structures 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.
Multi-Fidelity Design
Response Surface 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
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