A study of spacecraft reaction thruster configurations for attitude control system

Reaction thrusters (RTs) are used as an alternative to momentum exchange devices when disturbance torques exceed the control authority of momentum exchange devices. The reaction control system (RCS) can employ some rocket thrusters to provide attitude control during the thrusting or coast phase. Within the control loop, the RCS's target could be either achieving and keeping a certain attitude or controlling the rate of an attitude change. In the coast phase, some tasks such as preacceleration, settling of liquid propellant, damping of structural vibrations, providing a velocity increment to separate stages and payloads, and carrying out orbital and nonorbital maneuvers may be included in its functions. The propulsion perturbation torques, whose size is relatively large, are primarily produced because of the center of mass offset, which itself is produced as a result of static unbalance, transient gas flow phenomenon, and nozzle cant angle misalignments. Nozzle cant angle misalignments are produced because of manufacturing tolerances and pressurizations, which should be compensated by the attitude control system. In addition, rocket thruster plume impingement against surrounding structure or components produces sizable disturbance torques and cross-coupling effects that degrade the dynamic stability and increase the duty cycle that should be corrected by additional thrusters to restore vehicle attitude. During each maneuver, there are always undesirable angular rotations in consequence of some errors and uncertainties in the various components of a spacecraft. The attitude control system has to be capable of compensating these imperfect effects of the mechanical system.