Therefore, some other method of thickening the vehicle is required. Figure 2.4-6 is a schematic representation of an SPS vehicle whose two halves have been rotated or canted, through an angle 6. The moments of inertia of the canted vehicle can be written in terms of the moments of inertia of the uncanted vehicle as where the prime denotes the canted vehicle, is the distance from the vehicle center of mass to the center of mass of one side and M is the vehicle mass. Note that as 3 increases Ix' and lz ' decrease while 1^' increases. The gravity gradient torque about the Y-axis then varies as and the torques about the Y axis decrease as 3 increases. Proper choice of 3 results in no gravity gradient torques about the Y axis. From Equation 2.4-36, the proper 3 is found from Torques can be eliminated only if the cant angle is small. Otherwise, performance of the remainder of the system may be unduly compromised. Figure 2.4-7 illustrates the required cant angle for several vehicles. Note that the longer and narrower the vehicle, the smaller the required cant angle. In cases where gravity gradient torques cannot be eliminated for acceptably small angles, torques can still be reduced. Canting has several apparent advantages, the most important being it is passive. For the reference vehicle other subsystems would probably not be adversely affected by the small cant angle. Attitude control propellant requirements would be reduced by 47 percent. Momentum Storage Use of momentum storage devices is a common technique for attitude control of today's spacecraft. It has the advantage of eliminating effluents and being potentially lightweight for long duration missions. However, existing devices
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