with the boom extended combined to require an estimated 30 days to achieve stability [Pettus, 1973].

Further prelaunch analysis yielded an improved procedure incorporating several attitude acquisition strategies:

1. Gravity-gradient capture at a boom length of approximately 1 m could be achieved by first allowing the spacecraft to despin under the influence of gravity-gradient and damper torques and subsequently extending the boom approximately 0.5 m when pitchasrollsaO [Repass, el al., 1975].

2. Pitch, but not roll, librations could be removed by a sequenced boom retraction and extension as described in Section 19.5.2; therefore, roll librations would need to be removed before boom extension.* Roll librations could be removed by the damper if gravity-gradient stabilization could be achieved before boom extension to 6.5 m because, at a boom length of 1 m, the damping time constant is only 13 hours [Davis and Yong, 1975].

3. After gravity-gradient stabilization, the acceleration of the wheel at the proper point in the pitch libration cycle, when the wheel acceleration reaction torque and gravity-gradient restoring torque cancel, could minimize subsequent librations [Pettus, 1973].

'When a 4- to 8-day (tamping time constant dominates the dynamics.

These strategies were incorporated into the acquisition sequence outlined in Table 19-3 and were implemented using a combination of passive stabilization based on gravity-gradient torque and active, open-loop, commanding using realtime graphic displays. After spacecraft separation, the attitude data indicated a slow spin, 1 deg/s, about the body / axis and the boom and damper magnet were released to permit the spacecraft to despin. Figure 19-18 illustrates the theoretical attitude behavior during the despin (i.e., events 3 to 4 in Table 19-3).

The damper magnet is driven through the Earth's magnetic field by the spacecraft's orbital motion. As described in Section 19.5.2, this induces the magnet to spin about the orbit normal at a mean inertial rate of 2 rpo. Consequently, the reaction torque on the spacecraft damps roll motion and induces the spacecraft to spin at a steady-state pitch rate of 1 rpo. Equation (19-96) describes this pitch motion, where up = 0.087 deg/s for a 1-m boom length. Representative attitude solutions are illustrated in Fig. 19-18. Closed trajectories about pitch 0 and 180 deg represent proper and inverted capture, respectively; open trajectories for positive

0 0

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