Armature Performance Research Articles

A comparison of the wear behaviors of six elemental wire conductors

The goal of this work is to relate properties of armature materials to observed armature performance. In simple railgun electromagnetic launchers, the wear behavior of the widely-used aluminum alloys as sliding electrical contacts on copper rail conductors leads to a progressive change on the rail conductor surface as debris from the armature/rail interaction adheres to the rail. Here, the authors compare the behavior of wires of silver, copper, aluminum, nickel, molybdenum, and tantalum as armature conductors on ETP copper rail conductors. Some of these wires were also tested on molybdenum rail conductors to assess the effects of rail hardness. The emphasis is on the effect of current/contact under current densities greater than 1 kA/mm/sup 2/ on the wear behavior and eventual failure of the armature wire conductors. Characteristics of material transfer and a descriptive model are presented. The value of temperature-dependent properties such as hardness and surface tension are related to materials transfer of the wire conductors as the contact surface deforms and flows in response to specific driving current profiles. >

Dynamic bore/interaction effects on metal armature performance (railguns)

It is pointed out that metal armatures must be designed to accommodate dynamic bore-armature interaction under electromagnetic loading. High forces must be maintained to ensure low-voltage, nonarcing metal-to-metal contacts. This contact force must be maintained despite relative rail and armature motion. The authors have measured railgun barrel and armature dynamic motion and contact forces, due to electromagnetic loading. They have measured rail motion when subjected to electromagnetic loading, for several different barrel designs using laser interferometric techniques. They have also determined armature contact forces from in situ measurements using load cells. They describe the experiments, and results, and their assessment of the design requirements imposed by these data.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

An investigation of electromagnetic launch repeatability

A battery-charged capacitor power system has made long-duration (6-7 s) electromagnetic launcher (EML) experiments possible. An experiment conducted to investigate EML launch-to-launch performance consistency is summarized. A series of eight ten-shot bursts each separated by 15 to 30 min was performed in one day using a single set of bore materials. EML performance repeatability was determined to vary less than 9% with respect to an average velocity calculated for the complete experiment. While there are several factors which influence the variance observed, injection velocity decay and armature initiation delays appear to be the dominant factors. The rail resistance increase that results from the bulk heating of the rails was determined to have a relatively minor influence on performance. Armature performance for the particular EML bore configuration considered was surprisingly consistent and not likely to contribute significantly to the velocity variance observed. >

Further investigations of plasma armature performance in the Culham Laboratory HTF rail launcher

The results from an experimental investigation of plasma armature behavior in the Culham Laboratory HTF (hypervelocity test facility) rail launcher are presented. The object of this work was to gain further insight into the conditions which lead to the formation of secondary plasma arcs in the rail launcher. The railgun was operated with a 1-cm square bore formed from glass-reinforced epoxy insulators and either stainless steel or copper rails. Experimental results were obtained with projectile muzzle velocities ranging from 1.5 km/s to 3.0 km/s; two free arc shots also occurred during the test series. The results show that the launcher performed much better with copper rails than with stainless steel rails. The results also show that the glass epoxy insulators performed much better than the acetyl copolymer material previously used in the HTF.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Solid armature performance with resistive rails (railguns)

The velocity skin effect and limits which it sets to projectile velocities in solid armature railguns have been considered previously. However, the most effective type of rail design has not been analyzed in a consistent and systematic manner. The author performs such an analysis and substantiates the advantage of the use of highly resistive layers on the contact surfaces of rails. Such layers virtually neutralize the velocity skin effect and its detrimental consequences. The thermal and electrodynamical problems are solved for several combinations of rails and armatures to compare this method with other possibilities. The results show that using rails with highly resistive layers in combination with solid conductive armatures permits reaching velocities far in excess of 10 km/s in the conventional electromechanical regime of acceleration, without overheating armatures and rails. Recommendations are presented for the choice of parameters of such layers.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Large and small caliber railgun solid armatures: experimental results

Solid/transitioning armature performance results of two railgun programs is reported. Solid armature experiments continue for 15 and 90 mm bore guns. These armatures have been accelerated to over 2300 m/s. For the armatures tested, typical muzzle voltage measurements show the armature drop remains less than 50 V for velocities under 1000 to 1200 m/s; at velocities above 1000 m/s to 1200 m/s, the contact forms a hybrid armature. Armature performance data presented include muzzle voltage, gun current, armature resistance, total energy deposited in the armature, launch velocity, launch package muzzle kinetic energy, peak armature action, armature contact wear, and physical examination of recovered armatures. These data are separately summarized for each gun type and demonstrate armature survival at high current densities, bore pressures, and specific action. >

Post-shot analysis of plasma/rail interaction in a small bore load driving EML

The dynamics of an aluminium plasma structure in a small-bore railgun have been characterized using scanning electron microscopy (SEM) and Auger electron spectroscopy (AES) (depth profiling and X-ray spectroscopy) techniques on rail samples exposed to a single passage of a plasma armature driving a load. These analyses suggest the existence of a thick layer of armature material (aluminium in this case) on all segments of the exposed rail surface with composition varying from pure metallic at the breech and oxidized aluminium (Al/sub 2/O/sub 3/) in other areas. The implications of the oxidized layer to railgun operation are yet to be established. However, the presence of oxidized aluminium on the rail surface would modify both the thermal and electrical characteristics of the rail surface in ways that degrade the performance of plasma armatures and may have implications for armature instability and the evolution of secondary arcs.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Scaling study for the performance of railgun armatures

A model is developed for investigating the performance of solid, plasma, hybrid, and transitioning armatures as a function of railgun geometry and gun operating conditions. The two figures of merit used in the calculation are the armature efficiency and the maximum velocity. Effects investigated include armature parasitic mass, armature resistance, friction, ablation drag, and, for the hybrid armature, gap growth. The model is applied to study how armature performance scales with projectile mass, or correspondingly bore dimension, and with gun current per unit rail height in the hypervelocity regime from 7 to 15 km/s. The model indicates that armature efficiency generally increases with projectile mass, whereas the maximum velocity for plasma and transitioning armatures is relatively insensitive to projectile mass. Calculations are also performed to determine the sensitivity of the model's predictions to uncertainty in key parameters, such as the ablation entrainment fraction, the skin friction coefficient, and the contact potential.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The velocity skin effect on plasma armatures in railguns

The authors examine the effect of the finite current diffusion rate, 'the velocity skin effect,' on plasma armature performance and describe the physics of this effect. Model results are compared to experimental observations, and some postulates regarding the differences between observations and their model results are formulated. >

Composites for improved armature performance

The use of advanced composites to improve the performance of a railgun armature is considered. The authors discuss the theoretical basis for material selection, review the materials options, and describe the composite of choice. It is shown that composite armatures can be developed that have a mass that is 15% or less of the total mass launched by a railgun. The composite with the best performance as an armature is beryllium-lithium. The toxicity of beryllium makes beryllium-lithium an expensive material to produce and use. It is concluded that aluminum-lithium armatures can be fabricated at a reasonable cost to demonstrate the potential of tailoring the electrothermal properties of metal armature materials.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A comparison of armature performance

All electromagnetic launchers require that the launch package have an armature. The is the component in which the electromagnetic forces are generated. In this paper we examine armature as one measure of performance which permits direct comparison of metal, plasma, and hybrid armatures. Efficiency also translates directly to energy and power requirements. In this paper we examine the efficiency of metal and plasma armatures for rail guns. The operating regimes in which each type performs best are identified.