Robust and adaptive TVC control design approaches for the VEGA launcher

This thesis addresses the design of the atmospheric control system of a launch vehicle. During the ascent-flight phase, the launch vehicle is heavily impacted by wind-induced structural loads and generally exhibits a flexible behaviour characterised by several resonant modes that can generate large oscillations and lead to instability. In this challenging scenario, the control system must ensure stability to guidance commands while satisfying very demanding and tight performance requirements in the presence of parameter dispersions. Based on the above, the atmospheric ascent-flight of a launch vehicle represents a challenging control problem, which is traditionally addressed using a classical design approach. Although there is a rich heritage and experience in applying classical control solutions to the launcher problem, several practical limitations are recognised. With the current industrial state-of-practice it is hard to achieve stability and performance robustness characteristics along the atmospheric phase. In addition, this strategy results in a very time-consuming design, tuning and validation process. Considering the above limitations and also the increasingly competitive launch service market, more methodological synthesis techniques must be proposed to extend the actual control system capabilities as well as to facilitate the control design task. In this context, this thesis proposes a synthesis framework based on robust control techniques. In particular, the capabilities of the structured H∞ and Linear Parameter Varying (LPV) synthesis techniques are explored for the design of the atmospheric control system of the European VEGA launcher. It is shown that these robust control approaches can provide a direct trade-off between robustness versus performance, reduce tuning effort across launch missions and has the capability to simultaneously handle multiple performance requirements and also to explicitly include system uncertainties in the design. This thesis also explores adaptive features for the atmospheric VEGA control system with the aim to evaluate its performance and robustness properties. The main goals of the proposed adaptive scheme are to improve the performance in dispersed conditions and to provide recovery and prevent the loss of the vehicle in extreme off-nominal conditions. Finally, a comparison between the proposed structured H∞, LPV and adaptive controllers are provided.