Sergey V. Stankevich

Erosion of explosively compacted Mo/Cu electrodes in high-current arc discharges

The erosion of molybdenum-copper composites produced by the explosive powder-compacting method has been studied in order to analyze the potential for their use as railgun electrodes. The results of experiments have shown that the erosion of Mo/Cu compacts at the same values of current impulse (I/sub max/=180 kA, T/sub max//spl ap/50 /spl mu/s, /spl int/Idt=15 C, /spl int/I/sup 2/dt=1.7/spl middot/10/sup 6/ A/sup 2//spl middot/s) is about 10 times less then the erosion of pure copper and is about 3 time less then the erosion of pure molybdenum. The loss of mass data curve has a flat minimum for molybdenum contents of 20 to 80 wt.%.

Comparison Between 2-D and 3-D Electromagnetic Modeling of Railgun

This paper compares the results of 2D and 3D numerical calculations of current density and temperature distributions in electromagnetic rail accelerators of conducting solids with some typical armature shapes. It is shown that, for the cylindrical and saddle-shaped armature shapes, the 2-D description of the armature heating is in reasonable agreement with the 3-D description.

Three-Dimensional Numerical Simulation of the Joule Heating of Various Shapes of Armatures in Railguns

This paper reports the results of 3-D numerical simulations of the Joule heating of armatures and rails in railguns. Armatures of various shapes with homogeneous and orthotropic electrical conductivity, homogeneous rails, and rails with a resistive coating are considered. It is shown that the maximum current density is reached at the perimeter of the rail-armature interface. The current-density value and, hence, the armature-heating dynamics are significantly affected by the armature shape and the electrothermal properties of the armature and rail materials, as well as by the acceleration dynamics, which, in turn, is determined by the total current value in the electromagnetic launcher and the total mass of the projectile. The ultimate projectile velocities are obtained when the Joule heating of the rails and armature by electric current during the shot does not tend to increase the temperature at any point of the railgun above its melting point. By controlling the structure and properties of rail materials, it is possible to reach ultimate (in terms of heating conditions) velocities much higher than those with homogeneous materials.

Ultimate kinematic characteristics of armatures with orthotropic and anisotropic electroconductivity

The paper presents the results of the theoretical analysis of ultimate kinematic characteristics of multilayer armatures in railguns. The ultimate kinematic characteristics are obtained provided that the Joule heating of metallic layers by electric current in accelerating does not tend to increase the temperature in an arbitrary layer above its melting temperature. Consideration is given to the accelerated armatures consisting of metallic layers separated by insulating or weakly conducting layers. The study provides some dependencies of the ultimate velocity on the acceleration distance and armature mass resulting from a numerical solution of equations for the unsteady magnetic field diffusion and the heat conductivity in the two-dimensional statement. A possibility of increasing the ultimate kinematic characteristics is considered by changing the armature structure and properties of layer materials.

Problem of Materials for Railguns

This paper is concerned with analyzing the ultimate velocity versus projectile mass at fixed acceleration distance for various methods of decreasing the current density at the rail-armature interface. The analysis is performed by numerical solution of the system of equations of unsteady magnetic-field diffusion and unsteady heat transfer in a two dimensional formulation. Homogeneous and multilayer projectiles and homogeneous rails and rails with a resistive coating are considered. It is shown that the ultimate kinematic characteristic of railgun accelerators of solids can be considerably increased by changing the structure and thermal properties of the projectile and electrode materials.

Ultimate kinematic characteristics of composite solids

This paper considers the ultimate (under heating conditions) kinematic characteristics of composite solid bodies accelerated by a unsteady-state magnetic-field pressure. The accelerated sheet comprises two layers: a layer of a composite material consisting of a mixture of two materials with different electrothermal properties, and a homogeneous material layer. Variation of the electrical properties of the composite layer with the coordinate is achieved by changing the volume concentration of its constituent materials. For an exponential magnetic field rise, an analytical solution is obtained for the problem of finding the optimum variation in the volume concentration of the composite constituents to attain a maximum increase in the ultimate velocity of the sheet. Numerical simulation showed that the optimum structure of the sheet calculated using analytical relations is nearly optimal for different pulse shapes of the accelerated magnetic fields. The possibility of considerably increasing the ultimate velocity through the use of composite layers compared to the ultimate velocities for the homogeneous materials constituting the composite is shown analytically and numerically. For an Fe-Cu-Cu sheet, this increase can reach a factor of two to three.

Search for new possibilities of attaining high launching velocities

The paper is concerned with analyzing the ultimate velocity versus the projectile mass at fixed acceleration distance for various methods of decreasing the current density at the rail-armature interface. The analysis is performed by numerical solution of the system of equations of unsteady diffusion of a magnetic field and unsteady heat transfer in a two dimensional formulation. Homogeneous and multilayer armatures, homogeneous rails, and rails with a high-resistive layer are considered. The results reported in the paper show that use of a resistive coating on the conductive side of rails is highly efficient at decreasing a current concentration on the rear side of the armature due to the high-velocity skin effect. This ensure a considerable decrease in the heating rate of the armature near the contact boundaries. As a result, the maximum velocity to which armature can be accelerated with retention of solid metallic contact with rails in a channel of a given length can be increased by a factor of 24, and the kinetic energy can be increased by a factor of 4-16 compared to the case of using rails without coating. Use of a multilayer armature with orthotropic electrical conductivity in combination with a resistive coating on the contact side of the rails allows one to attain high velocities and energy characteristics of the armature at short acceleration distances.

New materials and technologies for pulsed power research and applications

Composites produced by explosive welding (bimetallic and multilayer materials) and composites with disordered structures produced by explosive compaction of powders were studied from the viewpoint of using them in railgun electrodes. Data on ablation are presented as a function of the electric current parameters. Results of metallographic and X-ray analysis of the post-shot electrode surfaces are reported. It is shown that using composite materials, it is possible to decrease the erosion of electrodes and increase the critical current density above which the erosion processes tend to increase considerably. The use of the method of explosive compaction of powders to produce high-temperature insulators and the barrels of railgun accelerators of projectiles is discussed.

Thermal limitations in a rapid-fire multirail launcher powered by a pulsed magnetodhydrodynamic generator

The operation of rapid burst firing multirail electromagnetic launchers of solids is numerically simulated using unsteady two-dimensional and three-dimensional models. In the calculations, the launchers are powered by a Sakhalin pulsed magnetohydrodynamic generator. Launchers with three and five pairs of parallel rails connected in a series electrical circuit are considered. Firing sequences of different numbers of solid projectiles of different masses is modeled. It is established that the heating of the rails is one of the main factors limiting the performance of launchers under such conditions. It is shown that the rate of heating of the rails is determined by the nonuniformity of the current density distribution over the rail cross-section due to the unsteady diffusion of the magnetic field into the rails. Calculations taking into account the unsteady current density distribution in the rails of a multirail launcher show that with an appropriate of the mass of the projectiles (up to 800 g), their number in the sequence, and the material of the rails, it is possible to attain launching velocities of 1.8–2.5 km/s with moderate heating of the rails.

Rail Heating in a Rapid-Fire Multirail Launcher Powered by a Pulsed MHD Generator

The operation of electromagnetic multirail launchers of solids in the mode of rapid-fire sequential launching of several projectiles has been studied by a combined 2-D and 3-D nonstationary numerical simulation. A Sakhalin type pulsed magnetohydrodynamic generator is used in the calculations as a power supply for the launchers. Launchers with three or five pairs of parallel rails connected into a series electrical circuit are considered. The acceleration of different numbers of projectiles in a burst and for different masses of projectiles is simulated. It has been found that the heating of the rails is a major factor that limits the operation of the launchers in these modes. An essential feature that determines the rate of rail heating is the nonuniform current density distribution over the rail cross section due to the nonstationary diffusion of the magnetic field into the rails. The calculations taking into account the nonstationary distribution of currents in the rails of a multirail launcher have shown that an appropriate choice of the mass of projectiles, their number in a bursts on the order of five projectiles with a firing frequency on the order of 200 Hz, it is possible to accelerate projectiles weighing up to 800 g to velocities 1.8–2.5 km/s without the rails melting.