Literature Review - Finite Element Analysis for Satellite Structures - page 8

1.4 Methods of Enhancing Structural Efficiency

Several methods are available to enhance the structural properties of spacecraft
[ 2 ]. The most suitable method may vary depending on the design task. Three of the
most widely used methods are presented: sandwich structures, multifunctional
structures, and isogrid. All of the technologies and their benefits for satellite
structural designs are examined.

1.4.1 Sandwich Structures

Sandwich structures are often used in skin-frame designs and solar panels. A
sandwich structure consists of two thin face sheets attached to both sides of a
lightweight core. The design of sandwich structures allows the outer face sheets to
carry the axial loads, bending moments, and in-plane shears, while the core carries
the normal flexural shears. Sandwich structures are susceptible to failures due to
large normal local stress concentrations because of the heterogeneous nature of the
core/face sheet assembly. Component mounting must therefore use potted inserts
to distribute the point loads from connections. Sandwich panel face sheets are
usually manufactured using aluminum or graphite/epoxy composite panels. The
core is typically manufactured using a honeycomb or aluminum foam

construction.
Honeycomb sandwich paneling is the lightest option for compressive or
bending loading in specific applications. Honeycomb sandwich cores are manu-
factured using thin strips formed into honeycomb cells. The honeycomb geometry
is nonisotropic, with greater stiffness in the longitudinal direction. However, the
core acts nearly isotropically for in-plane loads when assembled in a sandwich
configuration. The disadvantages of using honeycomb cores are the potted inserts
required for mounting and the thermal inefficiencies. These inefficiencies stem
from the low thermal conductivity of the adhesive layers used in construction, and
make honeycomb prohibitive in optical and mirror aerospace applications.
Aluminum foam sandwich panels use a porous aluminum foam material for the
core. The flexural shear stiffness dominates the overall panel stiffness for relatively
small panels (i.e., less than 50 inches). Therefore, the core design is an integral part
of the sandwich panel design for small spacecraft. The shear stiffness of foam core
sandwich panels is generally less than that of honeycomb core sandwich panels of
equal mass. However, radial ribs and shear rings may be embedded in the core to
overcome the low shear stiffness. A major benefit of aluminum foam construction
is an increase in thermal efficiency because the core may be brazed to aluminum
face sheets rather than epoxies. Brazing provides a continuous thermal path
through the material, which benefits applications such as cryogenic mirrors and
solar arrays.