MULTIAXIS TRACKING AND ATTITUDE-CONTROL OF FLEXIBLE SPACECRAFT WITH REACTION JETS

The objective of this paper is to show that integral pulse frequency modulation (IPFM) reaction jet controllers can perform, with precision: multiaxis tracking of moving objects, attitude control of flexible spacecraft under orbit-adjustment forces, single-axis slews, and vibration suppression. Two sets of position, rate, and acceleration command profiles for multiaxis tracking are presented: one for a payload initially facing the zenith and the commands based on a pitch-roll sequence; the other for a payload facing the nadir and the commands based on a roll-pitch sequence. The procedure for designing an IPFM controller for tracking a given, inertial acceleration command profile is summarized. Important elastic modes of a spacecraft are identified according to their spontaneous attitude and rate response to the minimum impulse bit of the thrusters. The stability of control-structure interaction is shown to be governed by the ratio of the moments of inertia of the flexible to the rigid portions of the spacecraft. If this ratio is below unity for each axis, the spacecraft is stable; otherwise it is not. The stability inequality for symmetric elastic modes, when they induce attitude motion because of a moment arm from the vehicle mass center, is formulated. Extensive numerical results conclusively establish that the IPFM controllers are eminently suitable for the tasks mentioned above.