Nonlinear output feedback control and simulations for launch vehicles

The launch vehicle is an autonomous system and thus the control design plays a crucial role for the mission success. The flight Software, which runs on the On Board Computer, cyclically executes the GNC (Guidance Navigation and Control) algorithms to control the launcher from lift-off up to payloads release. The control design for a launcher vehicle presents several difficulties due to the complexity of the system. Some of the reasons that discourage from performing and implementing complicated control laws are: • The intrinsic instability of the vehicle • The presence of several disturbances (e.g. aerodynamic loads and elastic modes) • The high variation of the structure (time varying properties) • The high number of states derived from the equations of motion • The algorithms computational time These are ones of the main reasons that justify the use of "gain scheduling" approach for this kind of control problems, that consists in linearizing the model around specific trajectory instants, designing the respective controller and checking the system properties by means of the well-known LTI tools. Even If this approach has been proven to work in the industry, it leaves always some amount of uncertainty due to the approximation forced on the original system (the nonlinear and time varying nature of the launcher are not properly considered). In order to be able to design a control algorithm for this kind of system it is fundamental to derive reduced orders models, depending on the particular phase under analysis (so the modeling aspect is always crucial for the control engineer). Differently from the standard LTI methods, the nonlinear control theory allows to take into account more representative models. Nonlinear control feedback linearization requires however full state feedback information and an accurate plant modeling in order to work. To overcome the usual lack of knowledge on all the system states an observer is added in the control architecture resulting in a Nonlinear Output feedback control scheme for the attitude regulation problem. This controller has better performances under specific conditions since it allows to work with a nonlinear model instead of a linearized one. However, a real improvement for launcher control design would be to focus on design methods and stability metrics that are capable of handling without restrictions the time varying nature of this system.