978-1-4244-7815-6/10/$26.00 ©2010 IEEE ICARCV2010 Rocket Roll Dynamics and Disturbance – Minimal Modelling and System Identification Christopher E Hann 1 , Malcolm Snowdon 1 , Avinash Rao 1 , Robert Tang 2 , Agnetha Korevaar 2 , Greg Skinner 2 , Alex Keall 2 , XiaoQi Chen 2 and J. Geoffrey Chase 2 1 Department of Electrical and Computer Engineering, University of Canterbury, Christchurch, New Zealand. 2 Department of Mechanical Engineering, University of Canterbury, Christchurch, New Zealand. Abstract— The roll dynamics of a 5kg, 1.3 m high sounding rocket are analyzed in a vertical wind tunnel. Significant turbulence in the tunnel makes the system identification of the effective inertia, damping and asymmetry with respect to roll challenging. A novel method is developed which decouples the disturbance from the rocket frame’s intrinsic roll dynamics and allows accurate prediction of roll rate and angle. The parameter identification method is integral-based, and treats wind disturbances as equivalent to a movement in the actuator fins. The method is robust, requires minimal computation, and gave a realistic disturbance distribution reflecting the randomness of the turbulent wind flow. The mean absolute roll rate of the rocket frame observed in experiments was 16.4 degree/s and the model predicted the roll rate with a median error of 0.51 degrees/s with a 90th percentile of 1.25 degrees/s. The roll angle (measured by an encoder), was tracked by the model with a median absolute error of 0.25 degrees and a 90th percentile of 0.50 degrees. These results prove the concept of this minimal modeling approach which will be extended to pitch and yaw dynamics in the future. Keywords—rocketry, roll-control, wind-tunnel, minimal modelling, integral-based parameter identification, disturbance I. INTRODUCTION AND MOTIVATION A special topic rocket systems engineering course has been developed at the University of Canterbury in 2009, and has led to a funded summer project with a view for introducing rocketry and aeronautical control into research and teaching for Mechanical Engineering/Mechatronics and Electrical Engineering students. One of the motivations for the course and research is to facilitate a possible entry into the NASA university student launch initiative (USLI) competition [1]. The goal for this competition, is to build and fly a re-usable rocket that lifts a scientific payload to as close to one mile as possible. For details on current progress including rocket propulsion, avionics and control system development see [2]. An important aspect in control system design to stabilize a rocket is the use of mathematical models to describe dynamics in all the axes. The motion of a rocket has been well documented in the literature but is typically very complex involving up to 6-DOF and therefore very complex system identification methods [3-5]. This paper looks at the development of a simple modeling and system identification method for understanding the roll dynamics and the effect of disturbances on a small spin- stabilized sounding rocket, of about 1.3 m in length and 5kg in weight. The simulation tool developed could then be used to develop and analyze various controllers to stabilize the rocket motion. The maximum velocity of this type of rocket is approximately 600 km/h and it can reach up to 2 km in altitude. Sounding rockets are recoverable research rockets, designed to take measurements and perform scientific experiments during its flight. Figure 1 shows a model of the rocket, which describes the adopted nomenclature of the system. To simplify attitude control, this research concentrates on controlling the roll of the rocket. This approach effectively decouples the roll dynamics from the pitch and yaw avoiding complex controllers and advanced analysis (e.g. [6]). Figure 1: A CAD model of the rocket To provide an intermediate step from simulation to a rocket launch, a vertical wind tunnel was built with a vacuum to suck air past a rocket airframe that was actuated by aluminum fins. Figure 2 shows the wind tunnel and a shot of the rocket airframe during operation with the fin extended.