Strength Analysis - Finite Element Analysis for Satellite Structures - page 116

Table 4.3 Maximum Von Mises equivalent stresses and yield margins of safety for structural
modules in each design load case
Structural modules r e (MPa) MS y

Design case Design case

«case 1» «case 2» «case 1» «case 2»

Base plate 108.69 69.03 0.38 1.17

Mounting plate 16.34 31.97 8.18 3.69

Basis plate 51.93 70.4 1.89 1.13

Basis walls 30.56 52.97 3.91 1.83

Upper frame 91.14 58.75 0.65 1.55

Lower frame 29.55 63.04 4.08 1.38

Launch vehicle adapter 92.44 55.78 0.62 1.69

q

ð r x r y Þ 2 þð r y r z Þ 2 þð r z r x Þ 2 þ 6 ð s xy þ s yz þ s zx Þ

r e ¼ 1

p 2

where:

T f r g¼ stress vector ¼ r x ½ r y r z r xy r yz r zx

The maximum stress values of r e in the structural modules in each design load
case are given in Table 4.2 . This table also shows the yield margins of safety, MS y .
The margins must be positive for all strength cases to satisfy safety criteria. The
values of the maximum displacements under operational loads are given in
Table 4.3 . The maximum value of displacement in any structural module must not
exceed 3 mm to avoid component collapse.
By reviewing the results of the yield margin of safety listed in Table 4.3 ,itis
found that the first load case has the most critical influence on the base plate, upper
frame, and launch vehicle adapter. On the other hand, the mounting plate, basis
plate, basis walls, and lower frame are more influenced by the second load case.
The minimum values of margin of safety for each structural module satisfy the
safety criteria. The mounting plate and basis walls module are statically over safe.
However, the size of these modules will not be diminished to reduce structural
weight because of many technical reasons. Manufacturing process techniques have
limitations and constraints on the sizes of structural modules. Satisfactory per-
formance of the satellite requires verifying pointing and alignment requirements
for sensors and highly precise equipments (Mounting Accuracy), which are
installed on both the basis plate and basis walls modules. Therefore, these modules
require high stiffness to provide the desired mounting accuracy. The natural fre-
quency constraint imposed on Small Sat structure is satisfied with a small margin
as shown in next section. Reduction of the dimension of these two structural
modules or any other one will lower the first natural frequency leading to unac-
ceptable structural design. Finally, fatigue and damage accumulation checks may
also prevent any size reduction in structural modules (Table 4.4 ).