Satellite Configuration Design - Finite Element Analysis for Satellite Structures - page 31

Fig. 2.8 Basis block case of

Small Sat

Basis unit walls

Basis plate

grouping the basis block case and its equipment is called the basis unit block.
Computer aided design can be invaluable for identifying interferences and trying
various arrangements for equipment, so Mechanical Desktop computer package
(MDT) is used to issue the current configurations.
Before starting the configuration process of the basis unit block, there are some
constraints which should be taken into account. The star sensor must be turned by
49 o from Zenith direction in the positive Y-axis. One of the angular velocity
meters (gyro) is redundant, which requires to be mounted at a skewed axis.
Therefore, two brackets are designed to provide mounting constraints for both star
sensor and the redundant gyro. To minimize the required surface area for mounting
equipment, another bracket is used to collect the three pieces of magnetorquers in
three perpendicular axes. Each angular velocity meter must be connected to one of
the interface units, so each pair is located as close as possible to each other to
reduce cabling lengths. The magnetometer must be installed at sufficient distances
from high magnetic field components, so a distance not less than 0.6 m must
separate it from magnetorquers, and not less than 0.3 m from the nearest reaction
wheel. Figure 2.9 shows the final packaging arrangement for the basis unit block
of Small Sat.
By reviewing Fig. 2.9 , the basis plate carries the star sensor with its bracket,
magnetorquers with their bracket, and the Z-direction reaction wheel on one side,
while the other side carries MBEI, Z-direction gyro, skewed gyro with its bracket,
and interface unit of skewed gyro. The first wall located at the positive Y-axis
carries two reaction wheels in the Y-direction, one of them acting as a redundant.
The second wall located at the positive X-axis carries the X-direction reaction
wheel, X-direction gyro, and the interface unit of the Z-direction gyro. The third
wall located at the negative Y-axis carries the Y-direction gyro, and both interface
units of Y-direction gyro and X-direction gyro. The magnetometer can be mounted
directly on the basis plate or on the third wall with the help of a bracket. The first
idea is more reliable because using a bracket will decrease mounting accuracy of
the magnetometer. Therefore, the basis plate should be designed to provide high
accuracy mounting for the most critical equipments like MBEI, star sensor, and
magnetometer. It is clear that there is no equipment mounted on the fourth wall
located at the negative X-axis. The reason for this is to produce a free space for
other equipment which is relatively big and need special constraints on their
locations.