This paper deals witli^Ii^^Mnear dynamics of the servo system that is used to control the gimbal position of the service propulsion engine of the Apollo service module. Particular attention is given to the actuator that is the major component of the system. This actuator is unusual for the application in that it is an electromechanical unit employing magnetic particle clutches, rather than the more common electrohydraulic device. The actuator and its equations of motion are described. The results of the dynamic analysis show that the system exhibits a phase-stable resonance near 12 cps and that the position and velocity feedback paths contain antiresonances, or "quadratic zeros," that strongly influence the achievable bandwidth. The closed-loop bandwidth of the system as presently configured is about 2.2 cps. Comparison of the derived dynamic model with results from frequency and stepresponse tests reveals good correlation, confirms the existence of these singularities, and facilitates accurate selection of stiffness and damping constants. Methods are given for using the dynamic relationships in reverse to extract from test data the values of parameters influencing response.