Contribution to the modeling, design, and experimental study of an augmentel railgun and its projectile
This thesis was supervised by Professor Jean-Marie Kauffmann of the University of Franche-Comté. The co-director at the Royal Military Academy was Dr. Johan Gallant and the experiments at Franch-German Research Institute ISL were directed by Dr. Markus Schneider. A conventional electromagnetic railgun is composed of two conducting rails connected by a projectile. The magnetic field generated by the current in the rails interacts in the projectile resulting in an electromagnetic force accelerating the projectile. The projectile used in this thesis is two brush projectile. For a conventional railgun the most efficient way to increase the force on the projectile is to increase the current in the rails. But the current density is limited. The heating of the sliding contacts between the brushes and the rails due to the Joule losses and the friction can cause the contacts and can result in contact transition which we want to avoid. One way to reduce the heating is to add more current brushes to obtain a better current distribution between the brushes. Another way is to add an extra pair of rails and create an additional magnetic field. This augmenting field allows us to increase the electromagnetic force without increasing the current through the projectile. In this thesis the current and heat distribution in a two brush projectile in a parallel augmented railgun is studied through simulations and experiments. Because the current brushes and the inner rails form a closed loop, the augmenting field will induce a loop current and influence the current distribution between the brushes. The first simulation model is a global model of the railgun in PSice which allows us to predict the global currents as well as the average temperature in the brushes, the force on the projectile and the position and the velocity of the projectile. The model takes into account the velocity skin effect and was validated based on experiments. The second model is a finite element model in ANSYS for a fixed projectile. This model allows a local study of the current and temperature distribution in the projectile. A model for the contact between the rails and the projectile is introduced. The local model is used to calculate the time-dependent coefficients for the force equation used in the global model. Both simulation models are compared and a good correspondence is found. The LARA railgun of ISL with a length of 1.5 m and a square caliber of 15 mm has been used for the experiments. A maximum of three capacitor banks was used for the non-augmented and the augmented configuration. The muzzle velocities obtained in the experiments vary between 48 and 214 m/s. To determine the current distribution between the brushes a technique proposed by [SCH05a] has been used. It is based on the measurement of the voltage between two pins in the rails in combination with the voltage in a loop. When applied to this velocity range, the eddy currents in front of the projectile hamper the interpretation of the signals and the results are not what we expected. An analytical method for the determination of the current distribution based on the voltage in the loop was introduced. The results are then compared with the results of the simulations. The maximum current in the rails found with PSpice shows a good correspondence with the experiments, the calculated decrease of the current is slightly overestimated. The errors on the velocities are less than 10%. Both simulation models and experiments show that the brush towards the breech carries the greater part of the current for the non-augmented as well as the augmented railgun. In the last part a parametric study is carried out with ANSYS for the preliminary design of an augmenting circuit for an existing railgun.