Effect of Fin Cant Angle on Coning Stability of an Unguided Artillery Rocket

Abstract

This paper presents the flight stability studies of an unguided artillery rocket with curved wrap-around fins. Due to asymmetry in the flow field around the fin, curved wrap-around fins unlike the planar fins exhibit special aerodynamic characteristics such as induced roll moment, roll reversal through supersonic Mach number, excessive side force and out-of-plane moment. The out-of-plane moment if not carefully controlled can lead to severe coning motion of the rocket causing flight instability and catastrophic failures. Such an artillery rocket which encountered failures in several flight tests due to poor dynamic stability is considered for the study. The aerodynamic characteristics of the rocket with different fin configurations is obtained from wind tunnel tests and analyzed to identify the parameters causing the instability. Coning motion stability analysis of the rocket is carried out for all its configurations to identify the existence and behavior of the instability. The results obtained from the analysis shows that the stability of the rocket improved, by reducing the out-of-plane moment. Hence, a suitable curved fin configuration is proposed with reduced fin cant angle, to reduce the out-of-plane moment and improve the dynamic stability of the rocket. Trajectory simulation of the rocket with both the fin designs is carried out to visualize the improvement in the performance of the rocket. Flight tests of the rocket with reduced fin cant angle was carried out and the configuration achieved the desired range with excellent flight stability and consistency.