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This paper describes a series of experiments to study the efficiency of an electrothermal (ET) launcher. The launcher serves as a preaccelerator for a plasma-driven hypervelocity railgun experiment at the Institute for Advanced Technology. The ET launcher is designed to accelerate small polycarbonate projectiles to about 1 km/s. The objective of this paper was to understand how pulse duration affects the efficiency with which the pressure of the ET discharge is coupled to the projectile. This paper consisted of multiple tests for which the total energy of the discharge was kept constant, but the pulselength was varied. The dependent variable in the tests was the muzzle velocity of the projectile. The experimental results are presented and compared with gas-dynamic considerations. Index Terms--Electrothermal (ET) launcher, gas dynamics, plasma-driven railgun.

The Institute for Advanced Technology (IAT) is developing a plasma-driven railgun to launch low-mass projectiles of 5-10 g to a velocity in excess of 7 km/s. Accomplishing this goal requires overcoming the problem of bore ablation, which has been linked to an observed velocity ceiling of about 6 km/s in plasma-armature launchers. Bore ablation is a direct consequence of the intense heat radiated by plasma armatures. Controlling bore ablation requires a coordinated approach that includes magnetic augmentation to reduce power dissipation in the plasma, high-purity alumina insulators to raise the ablation resistance of the bore, and pre-acceleration to prevent ablation of the bore materials at low velocity. This paper describes the consequences of excessive bore ablation, the rationale for the IAT experiment, and results obtained from the hardware that has been designed and tested to date. Index Terms--Electromagnetic launch, hypervelocity, plasma, railgun.

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. Index Terms--Armature, cladding, railgun, titanium, transition to arcing contact.

Transition to arcing contact in solid-armature railguns has been linked to several mechanisms. Recent work suggests magnetic blowoff forces arising from perimeter erosion as a possible significant mechanism. Three-dimensional finite-element (FE) models were constructed to analyze the repulsive contact forces resulting from perimeter erosion. Results show that magnetic blowoff occurs only when there is a loss of more than 98% of the contact area. Such gross perimeter erosion is unlikely and is not found in recovered armatures. Magnetic liftoff from perimeter erosion does not appear to be a causative mechanism in armature transition. This computational study also provided a platform for testing two FE codes, which showed very good agreement overall. Index Terms--Finite element (FE), perimeter erosion, repulsive forces.

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. Index Terms--Grooving, liquid erosion, solid-armature railgun.

This paper describes a continuation of earlier reported work in which C-armatures were launched and recovered for the purpose of studying melt-wave erosion (MWE) damage on the contact surface of the trailing arms. The purpose of the experiments reported here is to begin linking experimental results to theoretical predictions of the erosion process. Specifically, we sought to test a claim made in many of the two-dimensional (2-D) analytical models of MWE that the speed of the erosion front should be proportional to velocity and to the square of current. To this end, we performed two series of tests in which velocity and current were varied while controlling other variables. Peak currents ranged from 400 to 1200 kA, and exit speeds ranged from 100 to 700 m/s. Results reported include mass and volume of eroded material measured by mechanical profilometry and qualitative observations of the damage. The erosion rates are in reasonable agreement with the scaling relations predicted by 2-D theory. The erosion patterns, however, are qualitatively different from 2-D predictions. Index Terms--Melt-wave erosion, profilometry, railgun, recovered armatures.

Heating at the root or throat position in C-shaped armatures is a concern because it can lead to cracking, melting, and premature failure. This paper presents results from experiments in which railgun armatures made of 7075 aluminum alloy were recovered after being launched at various levels of electrical action. Inspection of the recovered armatures showed reproducible patterns of damage at the root position that correlated with the severity of the launch conditions. Damage ranged from deformation and cracking at low levels of electrical action to melting to ejection of material and magnetic sawing at higher levels of action. There appears to be a threshold beyond which damage begins to propagate rapidly into the armature. The temperature rise in the armature was modeled with the three-dimensional (3-D) finite-element analysis (FEA) program EMAP3D. Our results show that it is possible to predict the onset of severe damage without modeling the complex phase-change behavior of the materials. Index Terms--C-shaped armatures, electromagnetic launch, finite-element analysis, melt-wave erosion, root-radius melting.

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 Prandtl's 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. Index Terms--Armature wear, melt lubrication, Prandtl's mixing-length theory, railgun, solid armature, turbulence.

This paper describes a series of experiments and their numerical simulations in which cracks were created in conductors by using high-transient magnetic fields. The tests were conducted on small samples of 7075 aluminum alloy in a test fixture that placed the samples in series with a high-voltage capacitor bank and constrained the samples from moving. Notches were machined in the samples at various depths for the purpose of creating an initiation site for the cracks. After each pulse, the samples were unmounted and examined under an optical microscope. The peak current was varied to examine its effect on crack tip behavior. It was found that the crack tip extended linearly, was blunted by cavitation (blowholes formed due to intense heating), or bifurcated--depending on the level of the peak current. Multiple pulses were also applied to one sample to study the effects of repeated pulsing on crack extension. Finite-element computations were performed using the three-dimensional finite-element analysis program EMAP3D in conjunction with the stress code DYNA3D. The computational results confirm most of the features observed in the experiments. Index Terms--Cavitation, crack bifurcation, crack propagation, crack tip heating, electromagnetic pulse, finite-element analysis.

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. Index Terms--Inductance gradient, muzzle voltage, solid-armature railgun, transition into arcing contact.

This paper describes experiments to measure melt-wave erosion (MWE) damage in railgun armatures. C-armatures were launched on the Medium Caliber Launcher at the Institute for Advanced Technology (IAT), Leander, TX, and recovered by 'soft catching' in a tube filled with cotton rags. Recovered armatures were mechanically profiled to measure erosion depth as a function of position on the contacting surfaces. Although recovered armatures provide only a final snapshot of the erosion process, one can show how erosion proceeds by recovering armatures at different stages of the launch. This type of study requires the use of a crowbar switch at the breech of the railgun to rapidly shunt current out of the armature, thereby interrupting the erosion process at the desired point in the launch. We describe the experimental setup and summarize the results of 11 experiments. The experiments investigated the effect of linear current density (ranging from 35 to 45 kA/mm) and rail material (copper and aluminum) on MWE damage. Lastly, we present some thoughts on contact transition mechanisms that, in some cases, prevented the collection of MWE data. Index Terms--Current melt-wave soft catch, electromagnetic launcher, melt-wave erosion, perimeter erosion, railgun, transition to arcing contact.

This paper presents two numerical approaches to modeling melt-wave erosion in conductors. The first uses a one-dimensional finite-difference formulation and provides a starting point for understanding the effects of applied field and material properties on erosion speed. The second approach involves manipulating material properties in the three-dimensional (3-D) finite-element analysis (FEA) code EMAP3D. Because EMAP3D cannot eject material from the computation in the same manner as the finite-difference code, a 'pseudoerosion' algorithm was devised to render cells above the melting temperature nonconducting. A comparison of the two approaches shows results that are in good agreement for depth of magnetic diffusion, current density, and temperature rise, as well as for depth and speed of erosion. These results are encouraging because they suggest that EMAP3D could be used to model melt-wave erosion in realistic 3-D armatures. Index Terms--Accuracy, finite difference, finite element, melt-wave erosion, time-step size.

The 'magnetic obturator' is an armature concept developed by IAP Research in the late 1980s. Advantages of the magnetic obturator include lower mass than conventional 'C-type' railgun armatures (approaching the theoretical minimum mass), compatibility with mid-drive sabots, and the potential for nontransitioning operation. The Institute for Advanced Technology (IAT) at The University of Texas at Austin began testing magnetic obturators in 1997. Our tests showed that magnetic obturators can operate very well, but not yet as reliably as C-type armatures. This paper discusses experiments conducted to understand and resolve several recurrent problems that include difficulties associated with start-up, high contact pressures leading to local yielding of copper rails, and susceptibility to develop intermittent arcing contact before transition. This paper reports our progress in addressing these three areas and presents recommendations for further research. Index Terms--Armature, railgun, transition, wear.

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. Index Terms--Current melt-wave, electrodynamics, moving boundary, solid armature erosion.

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, tangsten, 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. Index Terms - damage, gouging threshold velocity, high-speed sliding contact, hypervelocity gouging, railgun, wear.

An important consideration in the design of solid armatures is whether they maintain good solid-on-solid sliding contact during the initial portion of a launch, when the magnetic pressure is too low to provide sufficient loading of the contact faces. This paper analyzes the complete electrical/structural startup behavior of a 'C-shaped' armature in the 40-mm, square bore, laboratory launcher at the IAT, using a pair of linked 3D finite element codes. A first analysis solves for the electromagnetic loading on conductors using the EM code EMAP3D. A second analysis uses the hydrocode DYNA3D to solve the dynamic response of the armature, rails, insulators, and railgun containment using the results from EMAP3D as forcing functions. Peak displacements transverse to the direction of motion are found to be on the order of 0.1 mm. Index Terms - DYNA 3D, dynamic structural analysis hydrocode, electromagnetic analysis, finite element analysis, finite element method, solid armature.

The HART facility is a flexible multi-bank system for testing independently augmented railgun concepts. The HART I launcher has a 1.27-cm square bore with GlidCop rails and Al2O3 insulators. The bore components and augmentation turns are supported by G-10. Features which make HART I well suited for pursuing research on the approach of independent augmentation are its bore design, which can accommodate a variety of insulator types in tile or strip form, and the densely placed B-dot instrumentation. Ex-bore velocity tracking is provided by light gates, cw x-rays, and breakwires. The modular power facility provides a high degree of flexibility in waveforms and timing. The use of parallel coaxial cables in the pulse-power circuits simplifies the configuration of the capacitor banks into individual or parallel arrangements, with inductors chosen to provide desired pulse shapes. Tests are planned by using simulations to choose the primary and augmenting currents and their independent timing. Independent augmentation allows hypervelocity projectile tests to be fired at low plasma armature currents. With low-ablation insulators, this reduces wear on the bore and allows many tests to be conducted before retiling. Both projectile and free-running arc tests were conducted on the gun with the objective of verifying augmented operation. Free-running arc tests are initiated with an in-bore spark gap. Using a helium preinjector and wire fuses, projectiles have been launched with the absence of secondary arcs.

This paper explores the factors that affect the accuracy of numerical analysis of railguns with motion using Electromechanical Analysis Program in Three Dimensions (EMAP3D) [1], 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 for 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. Index Terms--Accuracy, diffusion, time-step size, velocity skin effect.

Conducting shields are often used in pulsed rotating machines to protect the surrounding electronic devices and humans from exposure to high oscillating magnetic fields. These conducting shields are subjected to rotating magnetic fields due to excitation currents in rotor windings. This induces eddy currents, which reduce the magnitude of the operating component of magnetic fields (radial component in drum machine and axial component in disk machine), and thus reduce the mutual inductance between rotor and stator windings. The eddy currents also induce drag torques on the rotor windings, which results in a degradation of machine performance. In this paper, both disk and drum pulsed alternators are investigated by the finite element code EMAP3D. The effects of conducting shields on drag torques and the operating components of the magnetic field are studied, and the results are presented as functions of the gap between the rotor and the shield. Index Terms--Computational electromagnetics, electromagnetic launch, pulsed power, rotating machines, shields.

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. Index Terms--Bombardment, long-range, railgun.

An important research problem for solid armature railguns is understanding the sudden transition from low-voltage sliding electrical contact between the rails and armature to high-voltage contact involving a plasma arc. Although an armature can fail in many ways and thereby 'transition,' two distinct transition mechanisms currently limit the performance of solid armature electromagnetic (EM) launchers. One mechanism, broadly termed 'wear-induced transition,' results from uneven or excessive loss of material from the contact interface. The other mechanism is associated with a rapid reduction in driving current (negative dI/dt). A series of experiments have shown the waveform-induced transition mechanism is independent of the state of wear of an armature. This paper explores this mechanism, which we call 'electrodynamic transition,' and seeks to explain the cause of transition as an electromechanical instability that disrupts the liquid film interface between armature and rail. Using 3-D finite element analyses (FEA), we observed the development of localized forces at the edges of the armature as the driving current drops rapidly. This behavior, we believe, may cause molten material to be ejected from the armature-rail contact region. Index Terms--Arcing contact, armature, electromagnetic launcher, railgun, sliding electric contact, transition.

This paper describes the first step toward a model of the liquid film at the rail-armature interface in solid armature railguns. The model considers high-speed Couette flow with viscous heating. It does not include magnetohydrodynamic body forces or Joule heating. The focus of the model is on coupled fluid dynamics and multiphase heat transfer. The formulation is similar to the analysis of melt lubrication in rotating projectile bands. However, our first principles model allows the possibility of solidification while the armature is passing, a feature that has been missing from previous analyzes. The model is moderately successful at reproducing results of experiments that measured high-speed mechanical wear of 7075 aluminum sliding against electrolytic tough pitch (ETP) copper for face pressures ranging from 45 to 150 MPa. Discrepancies between calculated and experimental results are discussed. Possible causes include 1) inadequacy of the laminar flow approximation, 2) uncertainties in modeling the complex phase change behavior of aluminum alloy 7075, and 3) thermal contact resistance at the rail interface. Index Terms--High-speed Couette flow, liquid film interface, melt lubrication, rail-armature interface, solid armature erosion.