The elements defining required motion of the antenna relative to the spacecraft include the nominal motion between the sun-oriented SSPS and the earth pointing antenna, the normal SSPS control system limit cycle, and the bending motion of the SSPS mast at the antenna interface. The once-a-day 360° rotation of the antenna relative to the long axis of the solar pointing spacecraft is required to maintain boresight pointing to the rectenna. Additional antenna motion in azimuth and elevation is required to compensate for normal ±1° spacecraft limit cycling. The modal characteristics of the SSPS affecting the antenna elevation drive is shown in Fig. 3.3-3. The normalized modal displacements, 0, and the normalized modal slopes, a, corresponding to each mode, are identified at the ends of the axis where the control actuators and sensors for the SSPS are located (data presented in Ref 4). Also included is the anti-symmetric mode shape used to determine movement at the antenna interface driven by a coupled firing of the 44.5N SSPS attitude controllers. Maximum antenna motion due to bending will be approximately 0.15° at a frequency of 0.018 rad/sec. Figure 3. 3-4 summarizes the system angular motion requirements for design of mechanical interfaces between the spacecraft and antenna. Control error signals will be sinusoidal with a frequency equal to that of the SSPS control system limit cycle. Antisymmetric bending motion is superimposed on the basic control motion. The minimum azimuth antenna rate is equal to orbital rate, while minimum rate in elevation is zero. Maximum azimuth rates occur just prior to SSPS jet firing and are induced by gravity gradient torques. Maximum accelerations occur at jet firings in azimuth and at peak energy points in the anti-symmetric bending oscillation in elevation. 3. 3.1.1.2 Antenna Disturbance Torques - The rotary joint configuration shown in Fig. 3.2-2 consists of four equally spaced rollers attached to the solar array mast (Ref 5). The rollers slide on a track incorporated on the rotating mast. There are two sets of rollers at each rotary joint. Each set of bearings transmits bending moments through the mast segments by normal loads in each set of rollers. The critical mast bending moments result from loads induced by spacecraft gravity gradient correction torques. These torques produce a 3600N m bending moment which results in rolling friction torque of in each set of rollers. (The Teflon coated rollers have a coefficient of friction against rolling of 0.05.) The slip ring brushes also induce frictional torques. Contact pressures between the 2 brushes and rotary joint ring will vary between for optimum power transfer. At an assumed system voltage of 20 KV and a brush current rate of of brush area is required to transfer 10 GW of power. The total normal force is 4.45 x 10 N (10 lb) at a coefficient of friction
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