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Acceleration of macroparticles in an electromagnetic railgun using a metal armature is preferable for a number of reasons. The main reasons, in our opinion, are: 1) the possibility to effectively accelerate heavy macroparticles (mass from hundreds of grams to several kilograms), 2) small voltage drop across the contact rail/armature, 3) longer accelerator life in comparison to that in the case of a plasma armature.However, in this case many phenomena arise, whose physical nature is not yet clearly understood. Specifically, it is unknown what pressure along the sliding rail/armature contact would be sufficient to ensure good electrical contact, or how particle deformation influences the acceleration dynamics, etc.It was studied numerically in [1] how the properties of armature material influenced acceleration dynamics in a railgun with copper rails; the following armature materials were studied: A1, Cu, W, and Mo. Preliminary investigations have shown that the acceleration was greatly influenced by particle construction, and (to a lesser extent) by properties of the armature material.In [2] a simplified analysis was given as to how strained macrobody deformation affects the process of acceleration in a railgun. An analytical formula has been obtained for the coefficient which reduces the Lorentz force due to deformation with a corresponding increase in friction force.In this paper the finite-element method was applied to the study of one of the factors influencing the friction force at high-velocity sliding contact, namely, the total normal pressure force which presses the armature to the surface of a railgun bore. The friction force is proportional to the normal pressure. Reducing the latter reduces the frictional losses, thus raising the electromagnetic projection efficiency. The main computed results are confirmed by experimental data.1. Let us consider the factors influencing the level of normal pressure forces, using as an example a cylindrical armature (Fig. 1) moving in a round bore. Note that in this case contact between the armature and the bore occurs over some cylindrical surface of radius r0 and the normal pressure force is given by 2r Lwhere ~, z are the cylindrical coordinates; p is the pressure upon the contact surface (contact pressure); L is the length of the generatrix of the cylindrical contact surface. From (1.1) it follows that in order to reduce the total normal pressure force (and thus frictional losses) it is necessary to reduce the values of L and p or to achieve values such that the total normal pressure force N would be minimized.The contact pressure at different parts of the contact surface has various physical natures. In the section with electric contact between rails and armature the contact pressure consists of the active pressure p~, caused by electromagnetic forces, and the passive pressure pp, due to total lateral deformation of the Lyubertsy Scientific and Production Association "Soyuz," Dzerzhinsky 140056, Moscow Region.

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