Control system synthesis for a launch vehicle with severe mode interaction

The phenomenon of mode interaction between rigid-body and elastic motion of a large launch vehicle is described and illustrated. For severe interaction, control system synthesis by the usual approach of assuming pitch attitude and rate feedback and use of conventional servo analysis techniques is not suitable. By use of a new method based on the state variable formulations of modern control theory, a simple practical control system is synthesized without the trial and error associated with servo analysis methods. Specified closed-loop frequencies and damping ratios, including active control of structural mode dynamics, are achieved using only the booster thrust vector control. No frequency tracking filters or feedback signal discrimination is needed. The only feedback signals necessary are those provided by angular displacement and acceleration sensors. Five simple first-order passive filters are the only compensation network dynamic elements required.