Trevor Watt

Experimental Results From a Two-Turn 40 mm Railgun

The Institute for Advanced Technology (IAT) has tested a high-inductance-gradient series-connected two-turn railgun. The primary goal of these experiments was to launch a dual-armature launch package in the two-turn configuration while maintaining electrical isolation between the rail and armature pairs. Two tests with the dual-armature launch package on a single-turn configuration independently examined the performance of the launch package. Results of those tests showed good thermal and structural performance of the armature assembly and no melt transfer between the armature contact regions. Five tests were then conducted on the two-turn rail configuration. During the second test, the crossover that connects the two rail pairs at the breech suffered arcing damage at the rail contacts. A new crossover was fabricated, and all subsequent tests were successful with no crossover arcing. The final system had a propulsive inductance gradient of 1.8 muH/m, with projectiles successfully launched in excess of 400 m/s at linear current densities up to 35 kA/mm.

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.

Effect of Geometry Change on Armature Behavior

Diffusion-controlled erosion of armatures, which is accentuated by the velocity skin effect, gives rise to nonuniform current conduction and consequent nonuniform heating. Earlier computational studies have highlighted the role played by armature geometry in current density and temperature distribution. Recently, we designed modified C-shaped armatures, called saddle-shaped armatures, to study the effect of geometry change on armature erosion and its consequent effect on rail deposits. The trailing edge and the throat of the armature were curved to better align with the magnetic field, thus reducing the nonuniformity of current density distribution. A front tab was also provided to reduce current concentration and heating at the leading edge of the contact area. Low-speed tests, in which armatures were recovered and compared with recovered conventional C-shaped armatures, were conducted. Similar experiments at speeds above 2 km/s were conducted and compared with corresponding C-shaped armature experiments. It was found that in conformation with our hypothesis, the saddle-shaped armatures recovered from low-speed shots showed less erosion both on the contact surface and in the throat area. The high-speed tests showed that the contact voltage for saddle-shaped armatures was significantly lower than that for conventional C-shaped armatures. This implies less energy loss at the rail-armature interface and potentially less lateral force generated during contact arcing

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

The Design and Testing of a Large-Caliber Railgun

A large-caliber railgun was developed to demonstrate the supersonic launch of 120 mm projectiles. A trade study that evaluated over 70 different railgun configurations and geometries resulted in the selection of a high-inductance-gradient, multi-turn configuration as the best overall choice. Subscale tests were performed on both the railgun and launch package configurations. A full-scale laboratory system was installed that included a dedicated high-current, large-caliber breech and gunline. A full-scale launcher was successfully tested to beyond the design conditions.

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.

A Study of Magnetic Sawing in an Aluminum Bar

Magnetic sawing is a phenomenon that can severely damage electrical conductors. In this process, high local current densities melt the conductor and eject the liquid metal through magnetic forces. This is exacerbated at the interface between dissimilar metals. To further characterize magnetic sawing, we used a new technique with alternating strips of copper conductors and G-10 insulators placed onto an aluminum alloy bar. When a current of 1.7 MA was applied to the copper strips and aluminum, the bar experienced severe magnetic sawing. We repeated the experiment with different current pulses to study the effects of electrical action. Results show that increasing the action greatly increases the severity of the damage

The Effects of Surface Coatings on the Onset of Rail Gouging

In solid-armature railguns, hypervelocity gouging damage can significantly limit the useful life of rails. Most solutions to this problem have involved the use of hard rail materials, which tend to be poor electrical conductors. However, it has also been observed that precoating the conductors of a railgun can significantly delay the onset of gouging. This is usually accomplished by launching several aluminum-alloy armatures on a set of rails below the gouging threshold. When an aluminum-alloy armature is launched at high speeds on copper-alloy rails, a thin (tens of microns) layer of aluminum is deposited on the surface of the rail. Subsequent tests at higher speeds often result in the delay or absence of gouging at speeds where it is normally expected to occur. However, this effect is not particularly robust, since gouging can still occur with sufficient lateral loads. In the experiments reported here, tests were conducted on UNS C15725 (Glidcop Al-25) copper rails to examine the effect of a controlled precoating applied with an electroplating process. Aluminum thicknesses of 2, 5, 25, and 50 μm were tested, providing a well-characterized aluminum layer as opposed to the rough variable-thickness layer typically deposited with an aluminum armature. Results of the experiments and analysis are reported in this paper.

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

Magnetic Repulsive Forces on Armature Contacts

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