Literature Review - Finite Element Analysis for Satellite Structures - page 3

1.3.1 Conventional Structural Types
Primary structures are designed using many criteria that depend on the mission
requirements [ 2 ]. Conventional spacecraft integrates four basic primary structural
types:
• Skin-frame structures;
• Truss structures;
• Monocoque cylinders;
• Skin-stringer structures.
A skin-frame structure uses an interior skeletal network of axial and lateral
frames to mount exterior skin panels using fasteners or rivets [ 2 ]. The frames
support bending, torsion, and axial forces. The skin reinforces the structure by
supporting the shear forces introduced by the interior member connections. To be
stable, the ends of a skin-stringer structure must be closed with the skin. The skin
is sometimes minimized to save mass, even though the thin skin leads to some
structural instability. When the skin buckles due to shear, it transfers all additional
shears loading to in-plane tension forces at 45 o which must be supported by
the connections. The buckling modes of the skin exhibit large deformations that
make it insufficient for exterior mounted components such as solar cells. Inter-
mediate frames are used to mount equipment or increase the buckling strength of
the skin or stringers. Internal access can be difficult; therefore, removable panels
may be needed. Sandwich and isogrid panels can be used to mount components.
A Truss structure uses an array of members that can only support axial loads.
Truss members are produced independently and arranged typically in arrays of
triangles for stability [ 2 ]. The members are manufactured using extruded tubes
made of composite, metallic, or sheet metal materials. A stable truss is statically
determinate and has no excess members to introduce alternate load paths. Trusses
are generally mass-efficient when the members are configured into rectangular or
triangular cross-sectional assemblies. However, they become less efficient as the
cross-section becomes more circular or hexagonal. Buckling is typically the crit-
ical failure mode for trusses. Also, the design of the structure creates stress con-
centrations at interface mounting points, such as separation systems. Components
may be mounted both internally and externally, and the absence of shear panels
enables easy access to a payload. However, this absence of shear panels is not
helpful to spacecraft requiring body mounted solar cells. Machining a full side of
the truss from a single piece of metal is usually more economical than fabricating
and assembling individual members.
A Monocoque cylinder is an axi-symmetric shell that does not contain any
stiffeners or frames [ 2 ]. The shells are manufactured using metallic or sandwich
panels with curved sections formed by rolling. Typically, two or three curved
sections are fabricated and assembled into the cylindrical configuration. The
strength of monocoque cylinders is usually limited by its buckling strength.
The shells are most efficient when the loads are distributed evenly throughout the