36000 km operational phase; orbit transfer, plane changes and attitude control Attitude control is discussed in a later section. This section will concentrate on the propulsion system designs for drag compensation and orbit raising. Drag Compensation Propulsion system design is driven by the ‘delta V' (velocity increment) required to be imparted on the spacecraft to perform a given maneuver. The delta V required for drag compensation for a spacecraft of this size is considerable. This is illustrated in Figure 10.4.8. As can be seen, the drag is very sensitive to temperature and hence solar activity. The propulsion system must therefore be designed for the worst case (highest delta V) and will be understressed for most of the time. When chemical thrusters are used for this function, they induce undesirable attitude disturbances. Some degree of ‘throttle-ability' is required, and this is more easily achieved through the use of electric propulsion. Figure 10.4.8 Drag Compensation Velocity Increment Requirements Electric propulsion systems are, however, very low thrust devices. At this altitude (350 km) and with this size of structure, the drag force is very high. For ‘worst case', the drag force is calculated below. where: For LEO spacecraft, particles are incident at [] (see later), accommodated for a short period of time on the spacecraft surface and e-emitted at the ‘thermal velocity', [] (corresponding to the spacecraft surface temperature).If we assume very low specular reflection (low thermal velocity) then it can be shown that:
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