Moving mass trim control for aerospace vehicles

A moving mass trim controller is proposed to increase the accuracy of axisymmetric, ballistic vehicles. The moving mass trim controller differs from other moving mass schemes because it generates an angle of attack directly from the mass motion. The nonlinear equations of motion for a ballistic vehicle with one moving point mass are derived and provide the basis for a detailed simulation model. The nonlinear equations are linearized to produce a set of linear, time-varying autopilot equations. These autopilot equations are analyzed and used to develop theoretical design tools for the creation of moving mass trim controllers for both fast and slow spinning vehicles. A fast spinning moving mass trim controller is designed for a generic artillery rocket that uses principal axis misalignment to generate a trim angle of attack. A slow spinning moving mass trim controller is designed for a generic re-entry vehicle that generates a trim angle of attack with a center of mass offset and aerodynamic drag. The performance of both moving mass trim controllers are evaluated with the detailed simulation. VER the years, techniques for controlling the flight character- istics of missiles and re-entry vehicles (RV) have gravitated to systems that deliver relatively large amounts of control authority. For certain missions, such as an air-to-air missile or an RV designed to evade defenses, a large lateral acceleration capability was required. The technologies used to perform these missions ranged from ac- tuated canards, elevons, and flaps to jet interaction, thrust vector control, and a variety of other techniques.1 Because of the mission requirements for large maneuvers, systems that provided modest amounts of control capability were of little or no value. However, a new mission for accurate artillery rockets and RVs that utilize existing assets has prompted a renewed interest in simpler control techniques that produce small maneuvers. One such control technology is moving mass control. This tech- nique has previously been evaluated in conjunction with other con- trol methods such as the moving mass roll control of an aerodynam- ically asymmetric RV.2'3 A more direct application of moving mass control technology is the moving mass trim controller (MMTC). The MMTC generates a trim angle of attack (AOA) on an axisym- metric, ballistic vehicle directly from the motion of the mass. It is a novel, lightweight, low-cost retrofit to spinning ballistic vehi- cles that require modest flight-path corrections to obtain increased accuracy. Over 10 years ago, initial studies of the MMTC were performed by Regan and Kavetsky4 at the U.S. Naval Systems Warfare Cen- ter. Regan and his co-workers devised a single-shot MMTC that would provide modest range corrections near the target. At Sandia National Laboratories (SNL), the MMTC was an outgrowth of the deconing device test (DOT) described by White and Robinett.5 The DDT provided an initial glimpse of the effects of principal axis mis- alignment (PAM), roll rate, and center of mass offset. The MMTCs developed at SNL address the issue of roll rate, static margin (SM), PAM, and center of mass offset. The trim AOA for a fast spinning vehicle is generated by a PAM, whereas a slow spinning vehicle with a small SM relies on a center of mass offset to create a trim AOA resulting from aerodynamic drag. This paper derives the gen- eral nonlinear equations of motion for a one-moving mass system,