where R is the moment arm of the Y-axis thrusters, y The propellant requirements for the X and Z axes are more difficult to estimate. If they are in their nominal orientation, there is no torque and no propellant required for their control. In actuality, they will deviate from their nominal orientation. Since torques about these axes are destabilizing, some control effort will be required. For estimation purpose, 0 and 0 are assumed to undergo sinusoidal variations of ±1° amplitude with a period equal to the orbital period. Equations 2.4-26 and 2.4-28 become Annual propellant requirements are summarized in Table 2.4-1. Engine requirements are summarized in Figure 2.4-2. 2.4.4 Attitude Control System Alternatives Even though high-performance electric thrusters can provide needed attitude control torques, they have serious limitations in an SPS application. The most serious problems are large annual propellant resupply, a very complex system, contamination of the vehicle, and limited life of the thrusters. Three alternative concepts have been analyzed to assess their utility to SPS. Each has the advantage of eliminating the requirement for propellants, and result in a simpler system. Except for momentum wheels, the life of the technique is the same as the basic life of the satellite. They also offer further advantages of reducing total system mass and reducing power requirements. Canted Vehicle For an SPS vehicle, the POP axis experiences gravity gradient because the mass distribution is such that the vehicle looks "thin". These torques can be eliminated by redistributing the mass to make the vehicle look as thick as it is wide. In general, the mass cannot be redistributed within the vehicle cross section as its location is determined by other system requirements. Integrating these equations gives propellant requirements per orbit as
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