Satellite Structural Design - Finite Element Analysis for Satellite Structures - page 57

From the previous discussion, it is clear that fastening studs are mainly used in
mounting and connecting satellite structural modules. They are pre-loaded with a
compressive load to perform this function. This pre-load compression prevents
lateral sliding of the structural modules. This function depends on the static
friction criterion. Therefore, the amount of longitudinal compression load gener-
ated from applying torque to each stud must be sufficient to overcome the most
critical lateral load affecting the satellite in all phases.
The static friction coefficient (l s ) between two solid surfaces is defined as the
ratio of the tangential (lateral) force (F) required to produce sliding divided by the
normal force between the surfaces (N),

l s ¼ F = N
When the tangential force overcomes the frictional force between the two
surfaces, the surfaces begin to slide relative to each other. In this case, the sliding
frictional resistance is different from the static frictional resistance. The coefficient
of sliding friction is expressed using the same formula as the static coefficient and
is generally lower than the static coefficient of friction. For dry surfaces, the
coefficient of friction is independent of the surface area. In Small Sat case,
structural modules are separated by thin steel washers which work as thermal
isolators besides helping in overcoming surfaces irregularity between structural
modules. The friction coefficient in this case is l = 0.3 between aluminum and
steel in static dry case. To prevent relative sliding, the friction force (f) must be
greater than the maximum lateral force (F l max ) during all satellite loading phases.
The friction force is equal to the friction coefficient (l) multiplied by the net
normal force (N net ), which is the total longitudinal compressive force (F c ) gen-
erated by applying a torque to the studs plus the longitudinal force (F a ) generated
during the maximum lateral force case.

9
=

f [ F l max
f ¼ l N net
N net ¼ F c þ F a

... Friction Equations

;

By reviewing Sect. 3.4.1 , the most critical lateral acceleration occurs in case 1
during road transportation in the container. This acceleration is the resultant shear
acceleration (N l ), which can be considered uniform along the satellite body. So the
maximum lateral force due to the total gross mass of the satellite is equal to:

F l max ¼ 256 46 : 28 ¼ 11847 : 68N

where (256 kg) is the total predicted gross mass of the satellite plus a 25 % growth
allowance, which is a historical average. In case 1, there are two situations for
axial loading one downward and the other upward. From friction equations, the
most critical situation is the upward axial loading. So the longitudinal (axial) force
is equal to: