Robust Control of a Conventional Aeroelastic Launch Vehicle

Long and slender bodies, such as (small) conventional launch systems, may suffer from an unwanted coupling between the rigid body and its flexible modes. The current research treats the launch vehicle as a flexible beam with lumped masses to account for the subsystems and fuel, using a three-dimensional assumed-modes method with longitudinal and lateral effects. Given the response of a simple proportional-derivative controller as benchmark, the performance of an Incremental Non-linear Dynamic Inversion (INDI) controller and a system based on Simple Adaptive Control is studied for a number of distinct manoeuvres. Of the three, the INDI controller has a superior performance and is not affected much by the inclusion of engine dynamics and flexible modes. Properly designed filters in the feedback loops show that the rigid-body response can be decoupled from the flexible-body motion, although controller gains need to be adapted to this new configuration. A second advantage is that structural vibration is reduced. Finally, the inclusion of gyroscopes, placed far away from the launcher's centre of mass and which suffer from the effects of aeroelasticity, does not lead to a large performance degradation if both the pitch-rate and pitch-angle feedback signals are properly filtered. To just counter the effects of flexibility, band-pass filters are best suited. However, the effect of flexibility on the gyroscope output is best compensated by notch filters.