The initial velocity of fragment from cylindrical casing, which detonates at one end along the central of the casing, is the key issue in the field of explosion technology and its protecting. Most of the formula available can predict the initial velocity or the velocity of fragments at middle part of the cylindrical casing with greater length-dimeter ratio (L/d>2). However, when the length-diameter ratio is less than two, the initial velocity of the cylindrical casing filled with explosives will have a big difference. In the present work, a numerical simulation model acknowledged by X-ray radiography experiments was used to determine the influence of the length-diameter ratio. The formula was built on top of the Gurney formula and made use of a correctional function to account for the effect of the length-dimeter ratio. The formula was further acknowledged with the established numerical simulation model. The results indicate that the calculation formula can accurately predict the initial fragment velocity with different length-diameter ratio (L/d⩽ 2).

In the ballistic performance testing field of human body protection and armor protection, aiming at the demand of sub-caliber launch of assembled ammunition with the mass range of 4 ~ 16 g and velocity range of 280 ~ 500 m/s, establish the mathematical model of interior ballistic with MATLAB, calculate the interior ballistic state, and analyze the influence law of the change of charge parameters and projectile mass with the launch velocity. The results show that the propellant does not burn completely as the small-mass projectile leaves the barrel; under the same chamber volume, the projectile velocity increases with the charge mass, and this trend gradually decrease with the increasing chamber volume; under the same charge mass, the projectile velocity decreases with the increase of chamber volume, and this trend gradually increases with the increasing charge mass; under the same projectile mass, the projectile velocity decreases with the increase of charge mass, and this trend remains basically unchanged as the projectile mass increases. The analysis and calculation results are verified by the firing test. The results show that although there are differences between the actual charge quality and the calculation results, the obtained velocity changing trend is basically consistent with the trend revealed by the calculation results. The analysis and verification results can provide guidance for understanding the influencing factors affecting the launch of sub-caliber low-speed projectile, and have positive significance for improving the ballistic performance testing ability and promoting the development of the protection industry.

The water-entry behaviours of projectiles with hemisphere-head were studied experimentally and theoretically, focusing on projectile dynamics and drag coefficient. Based on equivalent cone theory proposed by Baldwin, the equation of drag coefficient was developed to describe the resistance of hemispherical head projectiles from the time when the projectile contacts the water to the time the hemispherical head is submerged. A set of experimental apparatus was set up to record the water-entry process by high-speed photography, the variation of resistance during water-entry was obtained experimentally. Compared the acceleration calculated by conventional method, new method and experimental results, the new method is more consistent with the experimental value.