Difficulties in Control System Synthesis for Launch Vehicles Exhibiting Severe Mode Interaction 650582
An investigation was made of the problem of providing an automatic control system for a large booster subject to severe mode interaction. This interaction is defined as a strong aerodynamic coupling between the rigid body motion and one or more elastic modes of the vehicle and can appear as a flutter phenomenon between rigid and elastic degrees of freedom as opposed to “classical” flutter between two or more elastic degrees of freedom. For the booster considered, the interaction resulted in a rigid-body mode static divergence for an aerodynamically statically stable rigid configuration in the uncontrolled or forward-loop-only system. It was shown that preliminary control system design must include the significant elastic modes when mode interaction is present. A rigid-body-only synthesis will not yield “ballpark” values for the control element gains.
The investigation revealed that the active control philosophy of using auxiliary control inputs to increase the frequencies of the elastic modes sufficient to reduce the mode interaction in the forward-loop is a false notion. It is the free-free bending modes which interact with the rigid-body motion, and input forces cannot alter their mode shapes and frequencies. They can be altered only by mass and structural stiffness changes to the basic vehicle. The input forces result in changes to elastic mode coupled frequencies and damping ratios which do not effect the mode interaction phenomenon as defined herein.
Analysis has shown that it is meaningless to specify desired transient response in terms of closed-loop short-period frequency and damping ratio as a criterion for control system synthesis when the vehicle is subject to severe mode interaction. In general, the closed-loop modes cannot be identified as being elastic or rigid-body in the conventional sense. In this case new synthesis criteria are needed. Perhaps a better approach would be to specify desired time histories for the various output variables.