Satellite Structural Design - Finite Element Analysis for Satellite Structures - page 59

T ¼ KFd
where: (K) is the torque coefficient, and (d) is the stud diameter. When the stud
condition is not stated, the torque coefficient (K) is taken as 0.2. Therefore,
T ¼ 3 : 6 N.m
3.4.3 Preliminary Sizing of Structural Modules
This section describes the preliminary process used to size each module of the
primary structure of Small Sat. An iterative procedure is performed to achieve a
suitable initial design for each structural module. The loading condition used to
compute the maximum stresses and deformations for each module is the maximum
longitudinal g-load during launch. The margins of safety, MS, are calculated using
the formula given before, where the allowable yield stress (r y ) for aluminum alloy
AMg6 is equal to 150 MPa. The margins must be positive for all strength cases.
The analysis is done by ANSYS package for each module separately to calculate
the maximum deformations and stresses. Each structural module is first modeled
using arrays of beam and shell elements. The preliminary dimensions of the
structure are defined according to the arrangement of equipment. An element size
and its arrangement must provide a stiff load path. Appropriate mechanical
interfaces between structural modules must be provided. A combination of inter-
secting lines and in-plane areas is used as a graphical model for each structural
Module. These lines represent internal and external stiffeners of the Modules. They
connect the main points to support bending, torsion, and axial forces. The main
points are the locations of the eight fastening studs, equipment seats, and four
interface bolts in the base plate. The in-plane areas represent the skin, which
reinforces the structure by supporting the shear forces introduced by the interior
member connections.
For simplicity during analyses, the Small Sat primary structure is divided into
the following modules: base plate, upper frame, basis plate, basis unit walls, lower
frame, and mounting plate. The graphical models created on ANSYS package are
presented in Figs. 3.3 , 3.4 , 3.5 , 3.6 , 3.7, and 3.8 . All primary structural modules
have skin except the upper and lower frames. For the base and mounting plates, the
skin is used as a kind of environmental shield besides supporting the shear forces.
The main function of the skin in the basis unit block is reinforcing the structure
and decreasing the deformation to keep mounting accuracy of high precise
equipments. Skin is removed from the upper and lower frames to reduce structural
weight with little effect on shear deformation.
The material used during preliminary analysis is aluminum alloy AMg6 with the
following properties: modulus of elasticity (E) = 72 9 10 3 MPa, Poisson's ratio
(v) = 0.33, density (q) = 2630 kg/m 3 , yield stress ð r y Þ¼ 150 MPa ; and ultimate
stress ð r u Þ¼ 310 MPa : The ANSYS element types used for the meshing process are: