Electromagnetic Force, Jerk, and Electric Gun Projectile Design

As railgun projectiles have become more sophisticated, attention is being given to railgun produced electromagnetic forces that affect the mechanical loading of the projectile. The rate at which a launch package is accelerated directly relates to the stresses developed in the projectiles structural members. Acceleration rate is defined as jerk. This paper will define jerk in terms of electromagnetic parameters and give simple examples of stress amplification with increasing acceleration rate. For a given barrel caliber and length, railgun and projectile structural limitations produce a family of curves which define a range of allowed muzzle energies. Maps of ideal performance will be presented based on idealized current waveforms modified to observe railgun and projectile structural limitations. Once an acceptable current waveshape is identified, the pulsed power source must be modified to produce the required output. In a capacitor system, the series Inductance may be designed to produce. the appropriate waveshape; in the compulsator system, the inductance variation and initial firing angle of the machine may be adjusted to meet current rise requirements, and; in the railgun system driven with inductive stores, the opening switch function has to be modified to produce the desired acceleration profile. The second part of this paper describes a modification to explosive opening switches which allows control of the developed voltage of the switch and therefore, the rate of rise of current in the railgun. Experimental data demonstrating this capability on the Balcones 60 MJ power supply will be presented.