This research focuses on dynamic modeling and ascent flight control of large flexible launch vehicles such as the Ares–I Crew Launch Vehicle (CLV). A complete set of six–degrees–of–freedom dynamic models of the Ares–I, incorporating its propulsion, aerodynamics, guidance and control, and structural flexibility, is developed. NASA's Ares–I reference model and the SAVANT Simulink–based program are utilized to develop a Matlab–based simulation and linearization tool for an independent validation of the performance and stability of the ascent flight control system of large flexible launch vehicles. A linearized state–space model as well as a non–minimum–phase transfer function model (which is typical for flexible vehicles with non–collocated actuators and sensors) are validated for ascent flight control design and analysis.

This research also investigates fundamental principles of flight control analysis and design for launch vehicles, in particular the classical “drift–minimum” and “load–minimum“ control principles. It is shown that an additional feedback of angle–of–attack can significantly improve overall performance and stability, especially in the presence of unexpected large wind disturbances. For a typical “non–collocated actuator and sensor” control problem for large flexible launch vehicles, non–minimum–phase filtering of “unstably interacting“ bending modes is also shown to be effective. The uncertainty model of a flexible launch vehicle is derived. The robust stability of an ascent flight control system design, which directly controls the inertial attitude–error quaternion and also employs the non–minimum–phase filters, is verified by the framework of structured singular value (μ) analysis. Furthermore, nonlinear coupled dynamic simulation results are presented for a reference model of the Ares–I CLV as another validation of the feasibility of the ascent flight control system design.

Another important issue for a single main engine launch vehicle is stability under malfunction of the roll control system. The roll motion of the Ares–I Crew Launch Vehicle under nominal flight conditions is actively stabilized by its roll control system employing thrusters. This dissertation describes the ascent flight control design problem of Ares–I in the event of disabled or failed roll control. A simple pitch/yaw control logic is developed for such a technically challenging problem by exploiting the inherent versatility of a quaternion–based attitude control system. The proposed scheme requires only the desired inertial attitude quaternion to be re–computed using the actual uncontrolled roll angle information to achieve an ascent flight trajectory identical to the nominal flight case with active roll control. Another approach that utilizes a simple adjustment of the proportional–derivative gains of the quaternion-based flight control system without active roll control is also presented. This approach doesn't require the re-computation of desired inertial attitude quaternion. A linear stability criterion is developed for proper adjustments of attitude and rate gains. The linear stability analysis results are validated by nonlinear simulations of the ascent flight phase. However, the first approach, requiring a simple modification of the desired attitude quaternion, is recommended for the Ares–I as well as other launch vehicles in the event of no active roll control.

Finally, the method derived to stabilize a large flexible launch vehicle in the event of uncontrolled roll drift is generalized as a modified attitude quaternion feedback law. It is used to stabilize an axisymmetric rigid body by two independent control torques.