Nonlinear Study of an Attitude Control System for Orbital Space Vehicles

For the present decade it appears the most precise and accurate technique for attitude control of space vehicles consists of actuation with momentum devices. The majority of investigations using this form of attitude control generally assume a zero motor lag, linearized approximation of the equations of motion and control, elimination of the first order gyroscopic cross-coupling effects, and complete indiference to the second order gyroscopic cross-coupling effects. In some cases these assumptions are justified; in other cases they are not. This investigation is concerned with precise three-axis attitude control of an orbital space vehicle. The equations of motion are general and designed to encompass many of the parameters that will influence an orbital space flight and will not be constrained by specific vehicle geometry, construction, orientation or trajectory. The primary source of actuation is a set of three orthogonal reaction wheels which provide control torques and integrate the disturbance torques along each axis. Because of the gyroscopic effects of the rotating masses an interdependency exists between the various axes. The extent of these effects are investigated. In addition to wheel control, the system features a reaction jet system for desaturation of the wheel momentum when any wheel has reached a prescribed maximum speed. Significant points investigated are: 1. The problem of reaction wheel motor damping and its effect on the system steady state and transient behavior 2. The effectiveness of first and second order decoupling in reducing gyroscopic coupling between axes 3.