Doyle Motes

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.

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.

Thermal Analysis of High-Energy Railgun Tests

This paper describes temperature measurements made on the high-energy medium-caliber launcher at the Institute for Advanced Technology. Simulations performed in Maxwell 3-D and E-Physics showed that Joule heating from current diffusing into the rails accounts for most of the temperature rise in the conductors. Temporal skin effects increase thermal dissipation significantly over what would be expected by the ohmic losses under fully diffused conditions. Based on this analysis, Joule heating is the overwhelmingly dominant source of heating in low-speed tests. As the velocity of the armature increases, Joule heating remains the dominant source of heat; however, additional mechanisms-which may include frictional heating, arcing energy, aluminum deposition, and temperature-dependent properties-are required to more satisfactorily explain the temperature profile obtained.

Disrupting Armature Ejecta and Its Effects on Rail Damage in Solid-Armature Railguns

Material ejected from aluminum armatures at the rail-armature interface has been identified as a mechanism that degrades both rails and insulators in a railgun, significantly reducing the bore lifetime. With the goal of controlling the onset of armature ejecta, a series of single-shot tests was conducted in a small railgun with a bore cross section of 22 × 44 mm. The tests utilized channels of various sizes and geometries machined into the rail contact surface of the armatures to see if ejecta could be controlled. These tests identified several channel patterns as having the potential to delay the onset of armature ejecta. A series of multiple-shot tests was subsequently conducted in a larger railgun having a bore cross section of 38 × 76 mm. The goal of these experiments was to see if the channel patterns that delayed armature ejecta had a significant impact on rail erosion at start-up. Three test series were conducted. In the first series, three armatures with a nested circular channel pattern were tested. In the second series, three armatures with a large centrally located channel were tested. Both armature designs used equivalent contact areas. These results were then compared to a standard armature contact face with no modifications in a third test series.

Development of a Plasma Railgun for Affordable and Rapid Access to Space

The Institute for Advanced Technology (IAT) of The University of Texas at Austin (UT) is developing a plasma-driven railgun to launch low-mass projectiles of roughly 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: 1. using magnetic augmentation to reduce power dissipation in the plasma, 2. using high-purity alumina insulators to raise the ablation resistance of the bore, 3. using pre-acceleration to prevent ablation of the bore materials at low velocity, and 4. using a synchronously driven, distributed power supply to electrically isolate stages.

Developments in making space access rapid and affordable using a plasma railgun

For the last four years, the Institute for Advanced Technology has been working on the development of a plasma driven electromagnetic launcher (EML), for economic access to space [1]. The research is focused on overcoming setbacks experienced in the early developmental days of plasma-driven EMLs, which prevented researchers from obtaining muzzle velocities in excess of 6 km /s [2]. The possibility of achieving muzzle velocities in excess of 7 km/s with an EML make its use attractive and cost-efficient means for launching small (∼ 10 kg) microsatellites into low Earth orbit. For that reason, the research being performed is funded as part of a multidisciplinary university research initiative (MURI) by the United States Air Force Office of Scientific Research (AFOSR). In the summer of 2007, a muzzle velocity of 5.2 km/s was achieved with no evidence of restrike arcs or bore ablation, the effects of which are believed to limit the velocity of plasma railguns to no more than 6 km/s. Since then, a series of modifications have been made to the railgun bore to improve its performance and lifetime. Some of those modifications, and the experimental results obtained as a result, are discussed here.

Measurements of Electrical Specific Action to Melt for Brass and Aluminum Alloys

This paper describes a novel method for determining the specific action to melt the metals, and reports the values for action to melt that measure for several elements and three alloys: aluminum 2024, aluminum 6061, and C27400 brass. We electrically heat small diameter wires (127

A Railgun System for Supersonic Launch of 120 mm Mortars

\mu{\rm m}

An Investigation of Internal Gas Dynamics for an Electrothermal Launcher

) to the point of vaporization using a slow regime exploding wire experiment. Using high-resolution voltage and current data, we compute the derivative of electrical resistivity with respect to specific electrical action. Features in the plot of this derivative clearly show the onset of melting for many of the materials we tested. We compare our results for copper, silver, aluminum, molybdenum, and titanium to those published by Tucker and Toth in the 1970s. Our data agree with their published values for silver and molybdenum, but not with those for copper, aluminum, and titanium. This paper presents our results and discusses possible reasons for the discrepancies between some of our measurements and those of Tucker and Toth.

A Study of Electrothermal Launcher Efficiencies and Gas Dynamics

Summary form only given. This paper reports on the design and fabrication efforts of a supersonic, high-mass electromagnetic launcher. The program goal was to efficiently accelerate an existing M933/934 mortar round to over 400 m/s. The basic electromagnetic architecture chosen was a railgun, with a detailed system study undertaken to determine the optimal railgun configuration. Operational limits required that the overall launch package have a mass of less than 18 kg and a length of less than one meter. The peak acceleration was required to be less than 10 kG, and no launch package components could be discarded in flight. In addition, the program required a laboratory demonstration, which when coupled with the schedule and budget constraints, required that the existing IAT electromagnetic launch facility (ELF) be capable of providing the prime power.