Interplanetary Spacecraft Controllers Using Thrusters

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This paper is concerned with the design of attitude controllers for interplanetary spacecraft. The first controller is for Rhumb-line precession using almost periodic train of thruster pulses at a constant phase with the Sun. Earlier relationships are expanded and interlaced coherently. One such precession, along with its ungrowing nutation loops, is demonstrated on a Mercator plot. The second controller is for nutation damping. For spacecraft with products-ofinertia, a control scheme is formulated that involves bandpass filters for removing the constant biases from the transverse angular rates. Four phase angles, ninety degrees apart in a nutation cycle, are determined analytically whereat the positive or negative control impulses about the transverse axes are inducted to damp the nutations. The third controller is for spacecraft attitude control during aerobraking for planetary orbit insertion. The controller seeks and maintains a zero aerodynamic torque orientation of the spacecraft in that portion of the orbit where aerobraking is effective. Since this orientation may correspond to a constant, positive bias angle of attack measured from the instantaneous velocity vector turning at a nonuniform pitch rate, a pitch angular acceleration command profile is developed for parameterizing a thruster controller. The fourth, and the last, controller in the paper is for soft landing. It comprises (a) a longitudinal velocity controller employing a velocity-altitude reference trajectory, a gravity turn, and a constant velocity phase for soft impact; (b) a lateral velocity controller, requiring pitching the spacecraft at varying rates; and (c) a reaction jet controller for tracking the commanded pitch angle and rate until touchdown.