Moving-Mass Roll Control System for Fixed-Trim Re-Entry Vehicle

Abstract

The ability of a moving-mass roll control system to control the roll attitude and trajectory of a simple fixed-trim re-entry vehicle using a linear roll autopilot is investigated. The governing equations of motion of the coupled vehicle-moving-mass two-body system, which form the basis of the study, are presented and discussed. An analysis of these equations reveals that dynamic and aerodynamic cross-coupling is produced by the operation of this control system; however, some vehicle design techniques are employed to mitigate these effects. The mass effectiveness parameter, obtained through linearization of the moment equation, is shown to be the key system design parameter. The design of an optimal linear roll autopilot using modern state-space methods is presented in detail, along with an optimal-gain-vector scheduling function. A nonlinear seven-degree-of-freedom simulation of a typical mission profile trajectory demonstrates the ability of the linear autopilot to effectively control the vehicle's roll attitude and trajectory. Dynamic, aerodynamic, and inertial cross-coupling do not appear to significantly degrade the control system's response.