Analysis of the ultimate kinematic characteristics of railgun accelerators of solids

Abstract

A comparative analysis of the dependences of the ultimate (under heating conditions) velocity on the dimensions and thermal properties of the projectile and on the length of the railgun is performed on the basis of a numerical solution of two-dimensional unsteady equationsIntroduction. One factor that hinders attainment of high velocities in the acceleration of conducting solids in railguns is the loss of metal contact between the projectile (armature) and the rails (electrodes) and transition to the electric-arc regime of short circuit. Results of numerical simulation of the electrothermal state of the armature show that the temperature of the armature is nonuniform across its section and is maximum in the regions of contact between the rear side of the armature and the rail surface, where there is concentration of current due to the velocity skin effect (VSE) [1][2][3][4]. The velocity of the projectile at which it begins to melt near the contact boundary between the rails and the armature is commonly called the critical or ultimate velocity, i.e., the velocity at which metal contact is still retained. For traditional materials, the ultimate velocity is usually about 1 km/sec, which currently limits the use of conducting solids in railguns. A considerable number of recent papers deal with the search for methods of increasing the critical (ultimate) velocity, or, in other words, decreasing the current concentration due to the VSE. Multilayer bodies with orthotropic electric conductivity, bimetallic rails whose contact side is coated with a layer of a material with high electric resistance, and other approaches (see, for example, [1,[3][4][5][6][7]) were studied. It should be noted that the potentials of composite materials for increasing the ultimate velocity have not been completely clarified. Sometimes, authors draw opposite conclusions. In particular, the statement of Dreizin [5] that a high-ohmic coating is efficient at increasing the ultimate velocity is in conflict with the conclusion of Long and Weldon [3] that such coatings are not recommended for use in railguns of solids. This can be due to the fact that different physical phenomena are considered in the analysis of the heating rate of the armature, and coating materials with different electrothermal properties are used (Mo and C).In the present work, we perform a comparative analysis of the influence of resistive coatings with different electrothermal properties on the ultimate kinematic characteristics of homogeneous and multilayer armatures in the nonarc regime of acceleration. The nonarc regime occurs when the armature does not melt during acceleration. Two physical processes that affect the variation in the temperature of the armature are taken into account: Joule heating and heat transfer. Ignoring the other phenomena responsible for heating of the armature and decrease in the effectiveness of acceleration, in particular friction, we obtain the upper bound for the ultimate velocity in the nonarc acceleration regime. The difference between the res...

Effect of the shape of metal solids on the rate of their joule heating in electromagnetic rail launchers

“…4 depend greatly on the characteristic sizes of the elements used in the finite-element approximation of Eqs. (1) and (2). These distributions are mainly qualitative (although they were obtained for almost identical characteristic element sizes) and indicate nonuniform current distribution.…”

“…The calculation results suggest that a severalfold increase in the ultimate (for heating conditions) kinematic characteristics (shown by two-dimensional simulation in [2,3]) is possible for real rail launchers of conducting solids.…”

“…In the conductors, Eqs. (1), (2), and (4) were approximated using the Galerkin weighted residual finite-element method [5], and outside the conductors, the boundary element method was employed [6]. For the two-dimensional case, this procedure is described in [7].…”

“…Obviously, if these conditions are satisfied, the velocity to which a solid of given mass can be accelerated on a given distance is also limited, and for homogeneous materials, its is, as a rule, of the order of 1 km/sec. Previous calculations [2,3] have shown that the ultimate (for heating conditions) characteristics of launchers depend largely on the electrothermal properties of the structural materials used, the kinematic parameters of the launcher, projectile mass, the acceleration dynamics determined by the shape of the current pulse, and the acceleration distance; these characteristics can be increased severalfold by using structural and composite conductors in the current-carrying units of launchers and by optimizing the current pulse shape. However, the results presented in [2,3] and in a number of other papers have been obtained by numerical simulations of launchers in a twodimensional formulation.…”

“…Previous calculations [2,3] have shown that the ultimate (for heating conditions) characteristics of launchers depend largely on the electrothermal properties of the structural materials used, the kinematic parameters of the launcher, projectile mass, the acceleration dynamics determined by the shape of the current pulse, and the acceleration distance; these characteristics can be increased severalfold by using structural and composite conductors in the current-carrying units of launchers and by optimizing the current pulse shape. However, the results presented in [2,3] and in a number of other papers have been obtained by numerical simulations of launchers in a twodimensional formulation. The question of how much the maximum current density on the armature-rail interface, the armature heating rate, and the ultimate (for heating conditions) kinematic characteristics in real launchers differ from those obtained in two-dimensional simulation has not been solved.…”