Orbit and Attitude Control

As we saw in Chapter 8, the attitude control subsystem borrows the services of the propulsion subsystem to do the job of attitude control. The small thrusters, fired in opposed pairs (see Figure 8.4), are used as control torquers. These small rocket engines, grouped in clusters, around the spacecraft, are referred to as the spacecraft's secondary propulsion system.

The orbit control function, which we discussed in Chapter 3, is also handled by the spacecraft's secondary propulsion. We found that when the spacecraft is launched into the ideal mission orbit, it does not stay there, unfortunately. Perturbing forces due to drag, the Sun's, Moon's and Earth's gravity, and light pressure cause small changes in the orbit, which must be controlled if the mission orbit is to be maintained. To correct the orbit, small changes in the spacecraft's orbital speed are required, and we have seen that this can be achieved by firing small rocket engines onboard the spacecraft. Two small thrusters are fired, not in opposite directions to produce a torque, but this time in the same direction to produce a small AV to correct (or control) the orbit, as shown in Figure 9.2.

Figure 9.2: Two thrusters are fired in the same direction, to produce a small change in orbital speed to control the orbit against perturbations.
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