Francis Stefani

Parameters for an electromagnetic naval railgun

The United States Navy is considering the electromagnetic (EM) railgun as a future candidate for long-range shore bombardment missions. This brief study evaluates the gun and the pulsed power supply for this application. Approximate parameters are derived for a notional system that includes the projectile, launch package, railgun, and pulsed power components.

Experiments to measure gouging threshold velocity for various metals against copper

Hypervelocity gouging is a form of damage that can occur to surfaces in sliding contact at high relative velocity. Gouges, which are in the form of teardrop-shaped craters, have been observed on rocket sled tracks, in light gas gun barrels, and in the bore of railguns. One aspect of gouging that has not been adequately explained is the existence of a minimum velocity (or threshold velocity) for a given material pair below which gouging does not occur. This paper reports a series of experiments to test the hypothesis that the onset of gouging is governed by the hardness of the harder material and by the density and sound speed of both materials. In the tests, samples of various metals were accelerated to 2.2 km/s while in direct sliding contact with CD110 copper rails. The samples were carried in a Lexan polycarbonate forebody, modified to apply normal loads of 40-80 MPa to the sample/rail interface. The portion of the armature directly in line with the samples was cut away to avoid contaminating the gouge track. Visual inspection of the resulting gouges was used to establish a gouging threshold for each metal. The tests were conducted in the 40 mm square bore electromagnetic launcher at the Institute for Advanced Technology (IAT). Metals tested include AISI 1015 steel, silver, molybdenum, pure copper, tungsten, nickel, magnesium, and 7075 aluminum alloy. The results of the experiments show the existence of a straight line fit between hardness of the harder material and the shock pressure for a normal collision at the gouging threshold velocity.

Numerical modeling of the velocity skin effects: an investigation of issues affecting accuracy [in railguns]

This paper explores the factors that affect the accuracy of numerical analysis of railguns with motion using Electromechanical Analysis Program in Three Dimensions (EMAP3D), a Lagrangian finite element method (FEM) code that models thermal and electromagnetic diffusion into conductors with moving interfaces. In situations involving sliding electric contact between conductors, EMAP3D solves a series of velocity-dependent diffusion problems in which the armature moves to different axial positions that satisfy the equations of motion far the armature. This paper develops two relationships between time step size and solution accuracy. One relationship is a lower bound on time step size, based on the need for linear brick elements to represent accurately the exponential-like spatial variation of current density as currents diffuse into conductors. The other relationship is an upper bound, which limits the motion of the armature to distances on the scale of the current distribution around the armature. Because the upper bound decreases with increasing velocity, the two bounds eventually converge, at which point accurate solutions to problems involving motion are no longer possible for given mesh dimensions. The velocity at which accurate solutions are possible can be increased by increasing the resolution of the mesh. At present, a practical limit to simulating realistic railgun problems is less than 500 m/s. Because this limit is set by rapidly advancing state-of-the-art computer hardware, the prospects for achieving higher velocities in the near future are good.

IAT Armature Development

This paper provides a brief overview of railgun armature development undertaken at the Institute for Advanced Technology (IAT) and elsewhere over the last decade. The fundamental physics issues that govern the armature requirements are described. These include the operating requirements, minimum parasitic mass, material action limits, contact interface pressures, electromagnetic skin effects and current nonuniformities, magnetic sawing, launch package interactions, material surface treatments, melt lubrication, gouging, and transition to arcing contact. Different bore geometries-square, rectangular, round, augmented, -turn-are also described and require matching armature and launch package designs. Novel designs are discussed, including forward tabs, magnetic obturators, splined armatures, fiber contacts, pseudoliquid armatures, plasma, and hybrid armatures.

Investigation of Damage to Solid-Armature Railguns at Startup

This paper describes work investigating a rail damage mechanism observed in solid-armature railguns at the Institute for Advanced Technology, The University of Texas at Austin. The damage occurs in the starting section of rails and is not associated with high-speed phenomena such as hypervelocity gouging or transition to arcing contact. The damage, which we call grooving, is localized to the region of the rails adjacent to the insulators. In this paper, we describe grooving damage observed in multiple tests using copper rails. In one series, in which we tested up to 20 shots on one pair of copper rails, we obtained grooves on the order of a millimeter deep and several millimeters wide. We present evidence that grooving is caused by liquid erosion and is not a result of plasma heating or mechanical deformation

Numerical modeling of melt-wave erosion in two-dimensional block armatures

The majority of published work on the transition to arcing contact in solid-armature railguns has focused on modeling the current melt wave, a localized front of molten material that erodes the perimeter of the armature. The most frequently cited models are based on two-dimensional (2-D) analytical formulations in which the dominant source of current concentration is due to the velocity skin effect (VSE). This paper compares the predictions of three such models with a 2-D numerical simulation that includes the effects of VSE, melting, and loss of molten material.

Electrodynamics of the current melt-wave erosion boundary in a conducting half-space

An analytic model of the current melt wave in a one-dimensional stationary conductor has been developed to gain insight into the complex problem of melt-wave erosion contact wear in railgun armatures. Unlike two-dimensional models with motion, in which the dominant driving mechanism for the erosion front is current concentration from the velocity skin effect, in one dimension, the driving mechanism is a concentration of current caused by the electrodynamics at the melt-wave erosion boundary. Specifically, as molten material is ejected, current is driven into the remaining solid material at the interface, resulting in a local concentration of current and joule heating at the interface. We derive an expression for the velocity of the melt-wave front in one dimension, and assess the importance of this effect by comparing the erosion speed we obtain with erosion speeds predicted by previous models. The comparison suggests that the electrodynamics of the moving melt-wave boundary has an insignificant effect on melt-wave erosion in solid armature railguns, and as such can be justifiably neglected.

A turbulent melt-lubrication model of surface wear in railgun armatures

This paper describes a melt-lubrication model of the liquid film interface in solid-armature railguns that includes the effects of turbulence. The liquid film is modeled as a high-speed Couette flow with viscous heating. The model focuses strictly on mechanical wear and does not include magnetohydrodynamic body forces or Joule heating. The effects of turbulence are incorporated by using Prandtls mixing-length theory and semiempirical methods. Turbulent viscosity is added to the laminar viscosity for the solution of the momentum equations. Thermal resistance is accounted for at the thermal contact between the melt film and rail. Expressions are obtained for the quantities of film thickness and melt wear rate. Results from the model are compared with experimental data that measured high-speed mechanical wear of 7075 aluminum sliding against ETP copper for face pressures ranging from 45 to 150 MPa in railguns. Although the model reproduces general trends in the data, it predicts melt speeds that are significantly greater than measured; in this regard, a previously published laminar model provides a better fit. However, given the shortcomings of both models, it is quite possible that a purely thermal hydraulic formulation is inadequate to the task of modeling high-speed, high-pressure mechanical wear, and that viscoplastic processes in the slider should be considered next.

Experiments With Armature Contact Claddings

This paper summarizes experiments to investigate the possibility of delaying the onset of transition by using armatures with high melting temperatures. The rationale for the experiments was that armature materials with high melting points might outperform aluminum by reducing melting and erosion at the rail-armature interface. The tests used a novel technique that consists of placing thin claddings on the rail-contacting surfaces of the armatures. Two low-speed tests with copper armature claddings proved that claddings affixed to the trailing arms can survive a railgun launch. These tests were followed by four tests at about 2 km/s that used titanium armature claddings. These tests showed that titanium-clad armatures can perform well; however, this performance cannot be maintained beyond the first shot because the titanium armature claddings cause significant damage to the rails

Noise component in muzzle voltage traces

The muzzle voltage in a solid-armature railgun is an important diagnostic because it can provide information about the state of the rail/armature interface. Large changes in amplitude in the muzzle voltage usually indicate a transition from nonarcing to arcing contact. Medium and small amplitude changes are also observed in the muzzle voltage. Although the meaning of these is less understood, it has been suggested that these too might contain information about the condition of the rail/armature interface. This paper reports on an experiment in which features of the muzzle voltage trace were correlated to physical features of the launcher that most likely perturbed the inductance gradient of the gun. Although we could not conclusively determine which specific physical features make the majority of the contributions, our results suggest that physical features of the railgun do account for aspects of the secondary structure in our muzzle voltage traces.