The analytical model of the electrical current transfer between an armature and electrodes in a rail accelerator is considered. The model is based on the solution of the magnetic field diffusion and energy equations written in the integral form, with approximation of the sought functions in the one-parametrical form. The system of algebraic equations for the definition of the critical speed of a projectile, at which the contact surface of an armature begins to evaporate, is obtained. The comparison with the known experimental data has shown that the presented method has good accuracy. The influence of the basic physical factor-the conductivity of contacting materials on the critical speed has appeared nonlinear and ambiguous. A large number of computations over a wide range of the determining dimensionless groups, has shown that in order to increase the critical speed, the conductivities of the contacting materials should strongly differ, irrespective of which material is the armature and which is the electrode. From the practical point of view, it is recommended to make the armature out of graphite, if electrodes are made of a good conductor.
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