Page 187 - Intro to Space Sciences Spacecraft Applications

174 Introduction to Space Sciences and Spacecraft Applications

Figure 8-7 provides a rudimentary comparison between dual spin and
momentum bias systems. In spin-stabilized satellites, the larger mass
satellite bus spins (approximately once per second in geosynchronous
orbits) and a smaller mass (communications antennas, remote sensors) is
despun so that it can point in an assigned direction (a point on the earth or
other planetary body being observed or communicated with). Conversely,
the 3-axis stabilization system controls the spacecraft body with one or
more internal spinning masses under the direction of the control comput-
er. Figure 8-8 illustrates this even further for a simple body stabilized
momentum bias system using a single spinning wheel to control the
spacecraft pitch axis while thrusters andor magnetic torquers provide
control forces in roll and yaw.

Moderately complex control
Moderate accuracy (1 .On to 0.1’)
Can point to a fixed location

II

Figure 8-7. Gyroscopic stabilization (dual-spin).

Yaw and roll axes are
controlled by thrusters
or magnetic torquing
Pitch axis is controlled
by varying the wheel
speed

Figure 8-8. Body-stabilized momentum bias concept.