Dwayne Surls

Design, Construction, and Testing of an Inductive Pulsed-Power Supply for a Small Railgun

Advances in high-power-density batteries have rekindled interest in using inductive store as a pulse compression system. Although these batteries are considered very power dense, they lack over an order of magnitude of power density to drive a deployable electric gun. However, one can add an inductive circuit to a battery bank to make a hybrid system that has a much higher power density than batteries alone. A battery-inductor hybrid pulsed-power supply boasts several advantages over pulsed alternators, as inductors are static and relatively easy to cool. Inductors are potentially more energy dense than capacitors, making a battery-inductor hybrid pulsed-power supply an attractive alternative to capacitor-based pulsed-power supplies. The opening switch has been a major obstacle in previous inductive store projects, but in simulation, a new circuit topology-the Slow Transfer of Energy Through Capacitive Hybrid (STRETCH) meat grinder-greatly attenuates the problem. This paper discusses the design, construction, and testing of a small-scale STRETCH meat grinder system, which was successfully used to power a miniature railgun

Stretch Meat Grinder: A Novel Circuit Topology for Reducing Opening Switch Voltage Stress

The slow transfer of energy through capacitive hybrid (STRETCH) meat grinder is an inductive- capacitive current multiplication circuit that reduces switching requirements and achieves a high degree of current multiplication while possessing an energy density approaching that of a purely inductive system. Initially, the STRETCH meat grinder operates like a single-stage meat grinder; it increases the current through an inductor by switching out a coupled inductor. However, during switching in generic meat grinder circuits, leakage flux caused by imperfect coupling and the sudden change in current induces a voltage across the opening switch well beyond what modern solid-state switches can handle. The STRETCH meat grinder mitigates these problems by using a capacitor to recapture the energy in the leakage flux and to slow down the turnoff of current in one of the inductors. The energy from the leakage flux is then used to reverse the current on the turned-off inductor, thereby further increasing the current multiplication. A system comprising several STRETCH meat grinders in parallel can develop currents in the mega-ampere range without exceeding the capabilities of solid-state switches. Such a system could be used to power a railgun.

Prediction and verification of electromagnetic forces in helical coil launchers

Calculating the circuital parameters and electromagnetic force production between two coupled coils is critical when modeling helical coil launchers (HCLs). Computer tools that calculate the self inductance, mutual inductance, and inductance gradient of a coupled-coil pair are developed for the PSpice circuit simulator. The inductance values and gradient are used to model a short (16.3-mm length), small-bore (19.4-mm diameter) HCL. The HCL is constructed in the laboratory and its performance is measured. The HCL accelerated nominal 16-g projectiles up to 170 m/s using a 6-kJ capacitive energy store. The highest overall electric-to-kinetic conversion efficiency was 5.4%. Comparisons are made between the theoretical and experimentally measured launcher parameters. In general, there is good agreement between the predicted and measured HCL parameters.

Dynamic Load and Storage Integration

Modern technology combined with the desire to minimize the size and weight of a ships power system are leading to renewed interest in more electric or all-electric ships. An important characteristic of the emerging ship power system is an increasing level of load variability, with some future pulsed loads requiring peak power in excess of the available steady-state power. This inevitably leads to the need for some additional energy storage beyond that inherent in the fuel. With the current and evolving technology, it appears that storage will be in the form of batteries, rotating machines, and capacitors. All of these are in use on ships today and all have enjoyed significant technological improvements over the last decade. Moreover, all are expected to be further enhanced by todays materials research. A key benefit of storage is that, when it can be justified for a given load, it can have additional beneficial uses such as ride-through capability to restart a gas turbine if there is an unanticipated power loss; alternatively, storage can be used to stabilize the power grid when switching large loads. Knowing when to stage gas turbine utilization versus energy storage is a key subject in this article. The clear need for storage has raised the opportunity to design a comprehensive storage system, sometimes called an energy magazine, that can combine intermittent generation as well as any or all of the other storage technologies to provide a smaller, lighter and better performing system than would individual storage solutions for each potential application.

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.

Modification and Testing of a Battery-Inductor Repetitive Pulsed Power Supply for a Small Railgun

Pulsed power supplies for electromagnetic launch have unique requirements, including very low impedance, relatively long pulse discharge times, and high stored energy. The three primary types of pulsed power systems for electromagnetic launch have been capacitive, inductive, and rotating machine. This paper describes recent results in the design, construction, and testing of a pulsed power system based on an inductor that is charged by batteries.

Measurement of High-Strain-Rate Adiabatic Strength of Conductors

The current-carrying conductors in electromagnetic launchers and rotating machines are exposed to high stress and thermal loads that last for a few milliseconds, with temperatures ranging up to the melting point of the materials. Even though equilibrium behavior of materials at high temperature is well characterized, nonequilibrium, short-duration behavior is not well understood. Properties for short-duration exposure to high temperatures (a quasi-adiabatic process) are crucial to understanding the transient physics of railgun components, since use of equilibrium (isothermal) properties would entail compromise in evaluation of stress and strain fields. These errors can grow when considering multiple applied pulses, as in cycle life evaluation . In this paper, we describe an experiment that will help characterize such properties near railgun operating conditions. We present preliminary data obtained for copper

Operational Limits of a Commercial Gate Turn-Off Thyristor for Inductive-Store Systems

This paper details the testing of a commercially available gate turn-off thyristor (GTO) for application in inductive-store power supplies. GTOs have been used as a component of inductive-store systems directly and as part of compound switches. These GTOs are generally available commercially and typically have extensive datasheets. However, the inductive-store application differs significantly from conventional switching applications, which significantly reduces the utility of the datasheets; GTOs are typically used at 100-1000 Hz in a fairly continuous manner, whereas in an inductive-store system, a GTO may be conducting for a relatively long time before doing a single turn off.

All-electric rifle-caliber launcher with permanent magnet augmentation

The authors discuss the design, construction and performance of an all-electric rifle-caliber railgun. The railgun, pulsed power supply and injector weigh approximately 38 kg. The barrel is 0.5 m in length and has a 5 /spl times/ 10 min rectangular bore. Neodymium-iron-boron permanent magnets provide the augmentation field. Projectiles ranging in mass from 0.5 to 1 g were fitted with both sliding and plasma contacts and were tested in the railgun. The sliding contact projectiles proved to be the most efficient. The pulsed power supply consisted of ten electrolytic capacitors arranged in a type B Guillemin network that were charged to a maximum of 450 V. The maximum railgun current was approximately 15 kA over a 1 to 4 ms time period. The maximum measured projectile velocity was 450 m/s with a 0.5 g projectile. Projectile materials were brass, tungsten, aluminum, titanium, stainless steel, copper, graphite and copper-impregnated graphite and the rail materials were brass, copper, copper-chromium and copper-impregnated graphite. The copper-impregnated graphite was the best material for both the rail and the projectile because of its low erosion rate and relatively low electrical resistivity. The results show that the highest measured railgun efficiency was slightly less than 1 %.

Electromagnetically Driven Expanding Ring Experiments for Strength Studies

Most high-temperature mechanical properties of metals are available for isothermal conditions obtained after heating the specimen for several hours. However, in pulsed-power applications, materials are adiabatically heated by rapid deposition of energy. Experimental evidence from electron beam heating indicates that high- temperature mechanical properties significantly depend on the rapidity and duration of heat deposition. We have designed an experimental apparatus to apply heat using a short-duration electric pulse in an expanding ring experiment originally developed by Gourdin et al. [1], [2]. While earlier experiments were primarily concerned with obtaining high-strain-rate strength and fragmentation data, our primary goal is to obtain high-temperature data under pulsed heating conditions. The experiment uses a primary coil powered by an RC circuit designed to be critically damped to induce a current pulse in a thin ring of specimen that expands and fragments due to electromagnetic forces. The induced current heats the sample prior to significant expansion of the ring. Current in the primary and secondary are measured using Pearson and Rogowski coils. We used a VISAR to measure the rings expansion speed and a high-speed camera to capture its dynamic fragmentation. Data generated will quantify the rate of heating sensitivity of material properties in commonly used materials for development and validation of appropriate constitutive equations.