Research On The Trajectory Stability And Cavity Characteristics Of Highspeed Water Entry Projectiles

Weapons such as missiles, air-dropped torpedoes and rocket assisted torpedoes will go through water-entry process when attacking the underwater targets. Only the nose of the projectile is contact with the water, while the other parts of the projectile are surrounded by a cavity during the water entry process, which will lead a great trajectory deviation and projectile attitude deflection when there is a relatively small underwater hydrodynamic disturbance. Since the last parameters of the projectiles at the sailing with cavity stage will act as the initial parameters of the full wet sailing stage, in order to ensure a satisfied trajectory characteristic after water entry, it becomes necessary to take researches on trajectory stability and cavity characteristics of projectiles at the sailing with cavity stage.Firstly, the six degrees of freedom trajectory equation of projectiles at the sailing with cavity stage were established basing on the momentum theorem and momentum theorem, and the forces acting on the projectiles were analyzed. Secondly, the water entry processes of four typical nose shape cylindrical projectiles were recorded by a high-speed camera. The changing law of velocity attenuation, mass center lateral offset, attitude angle of the projectiles and the developing process of water entry cavity with the nose shape, initial velocity and angle of the projectiles were obtained through experimental images data. The changing law of instantaneous drag coefficient at the sailing with cavity stage and initial pressure wave generating in the moment of impacting water were also investigated basing on the experiments and simulation analysis. Finally, the closure mechanism of water entry cavities were researched, the expansion model were established basing on the energy conservation theory and potential theory respectively. The cavity expansion process, motion of cavity wall and deep closure characteristic of the cavity were researched in the extreme conditions. Although the results were based on some hypothesis, the research methods were generic.The results obtained in this paper can enhance our understanding to the movement of projectiles and cavity expansion laws during high speed water entry. Besides, it can provide experimental support and theoretical guidance to the study of trajectory characteristics during the sailing with cavity stage.