Scott Levinson

Numerical modeling of the velocity skin effects: an investigation of issues affecting accuracy [in railguns]

This paper explores the factors that affect the accuracy of numerical analysis of railguns with motion using Electromechanical Analysis Program in Three Dimensions (EMAP3D), 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 far 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.

High-Resolution Projectile Velocity And Acceleration Measurement Using Photonic Doppler Velocimetry

This paper describes the new photonic Doppler velocimetry (PDV) technique for measuring time‐resolved projectile velocity and acceleration profiles for full flight of the launch package in a two‐stage light‐gas gun (LGG). This technique is shown to provide excellent temporal and spatial resolution in measurement. Two non‐linear acceleration stages were observed for the first time on the LGG. When the gun bore was at atmospheric pressure, a strong opacity was observed during hypervelocity projectile motion >2200 km/s.

Preliminary investigation of microwave telemetry on an EML projectile

This paper describes a preliminary experimental investigation to convey acceleration measurements, made on-board an electromagnetic (EM) launched projectile, using microwave telemetry techniques. The focus of the investigation is to extend the Hardened Subminiature Telemetry and Sensor Systems (HSTSS) technology for use in EM launch environments. Many aspects of the HSTSS technology have already been successfully demonstrated in high-G, high-pressure launch environments in conventional gun systems. In addition, microwave transmission in stationary experiments, where the telemetry package was exposed to relatively high magnetic fields and EM transients in a railgun launcher, has been demonstrated. However, microwave telemetry during an EM launch has not been demonstrated until now. In this work, setback acceleration measurements obtained with an on-board accelerometer were processed by HSTSS electronics and transmitted simultaneously during launch via an antenna located in front of the EM launch package. While only the test conducted at 9-kG peak acceleration provided the most useful data, detailed analyses suggest that mechanical problems-rather than the microwave telemetry process itself-were probably responsible for difficulties experienced at higher accelerations. This paper reviews the experimental methodology and discusses the results of the investigation.

Muzzle arc control using an inductive shunt

Many electromagnetic launcher applications require that no electric arc occurs at the muzzle when the armature exits. Various techniques have been proposed to satisfy this requirement, including a muzzle shorting switch or a resistor connected across the muzzle. This paper examines the prospects for using a fixed inductive short circuit across the muzzle to eliminate muzzle arcing. The design of an inductive muzzle shunt is conceptually simple. During launcher operation, magnetic flux passes through an armature due to its finite resistance. The muzzle shunt traps this flux in the barrel, causing part of the input current to be diverted past the armature to the muzzle shunt. As the armature approaches the muzzle shunt, this flux is compressed into the shunt inductance. If the inductance of the shunt is chosen correctly, then at exit all of the current will be in the shunt and the armature current will be zero. The process of compressing the flux into the shunt can occur quite rapidly as the armature approaches the shunt, resulting in minimal loss of effective launcher length. In addition, the augmentation effect of the shunt reduces the action integral of the armature current, which can result in increased performance. An experimental demonstration of muzzle shunt arc suppression was performed on a 25-mm bore railgun. The shunt design is described and current waveforms are presented that demonstrate armature current interruption at exit.

IMPROVED BAR IMPACT TESTS USING A PHOTONIC DOPPLER VELOCIMETER

Bar impact tests, using the techniques described elsewhere in this symposium, were used to measure compressive and tensile strengths of borosilicate glass, soda lime glass, and a glass ceramic. The glass ceramic was 25% crystalline spinel, furnished by Corning Inc. There are two measures of compressive strength: the peak stress that can be transmitted in unconfined compression, and the “steady state” strength. For borosilicate glass and soda lime glass, these values were similar, being about 1.8 and 1.5 GPa, respectively. The glass ceramic (25% spinel) was almost 50% stronger. Tensile failure in the glass and glass ceramic takes places via surface flaws, and thus tensile strength is an extrinsic, as opposed to intrinsic property.

DAMAGE EVOLUTION IN BALLISTIC IMPACT OF GLASS PLATES

High‐velocity impact onto a layered glass target produces a very extensive damage pattern exhibiting many distinct morphologies. High‐speed photography reveals failure waves and cracks that move at acoustic velocities. These prompt features evolve into a complex final damage pattern that includes needle fragments around the penetration cavity, radial cracks at mid distance, and dicing cracks near the edges.

X-Ray Tomography to Measure Size of Fragments from Penetration of High-Velocity Tungsten Rods

Behind-armor debris that results from tungsten rods penetrating armor steel at 2 km/s was studied by analysis of recovered fragments. Fragment recovery was by means of particle board. Individual fragments were analyzed by x-ray tomography, which provides information for fragment identification, mass, shape, and penetration down to masses of a few milligrams. The experiments were complemented by AUTODYN and EPIC calculations. Fragments were steel or tungsten generated from the channel or from the breakout through the target rear surface. Channel fragment motions were well described by Tate theory. Breakout fragments had velocities from the projectile remnant to the channel velocity, apparently depending on where in the projectile a fragment originated. The fragment size distribution was extremely broad and did not correlate well with simple uniform-fragment-size models.

COMPARISON OF THEORY AND MEASUREMENTS OF A TWO‐STAGE LIGHT‐GAS GUN

A comparison of techniques for obtaining projectile velocity history on a two‐stage launcher and discuss gun code accuracy vis‐a‐vis pressure gauges and the new photonic Doppler velocimetry (PDV) technique is presented. The PDV technique itself is described in a companion paper. The PDV records were differentiated to compute acceleration and, hence, base pressure. Two acceleration episodes are revealed in the data. Base pressure values were compared with measurements from stationary pressure gauges and with predictions of a standard two‐stage gun code. The agreement with the pressure gages was satisfactory. Code predictions did not account for the two acceleration stages. However, for the main acceleration episode, the predicted base pressure is in good agreement with the smoothed pressure computed from the PDV record. Both the gauge records and PDV contain short‐time pressure spikes which are apparently real. Therefore, use of computed base pressure for projectile design may lead to failures if the proje...

Development of a Pumped LC Tank Circuit for Use in an Adiabatic Pulsed Inductive Heating Application

In any high-energy pulsed power experiment, the metallic conductors are expected to heat up significantly due to resistive losses. In the pulsed case, the effects of local heat transfer are decreased due to the limited thermal diffusion time, so the process is considered to be adiabatic rather than isothermal. Previous results indicate that the high-temperature mechanical properties of metallic conductors significantly depend on the rapidity and duration of heat deposition. With this in mind, it is important to understand the mechanical properties of metals heated rapidly so that the correct mechanical properties are considered when designing high energy experiments. An expanding ring experiment has been performed at the Institute for Advanced Technology (IAT) to test such mechanical properties. The experiment uses a primary coil powered by a near-critically damped RC circuit to induce a current pulse in a thin specimen ring that expands and fragments due to electromagnetic forces. So that the heating time is minimized, an inductive heating source has been developed to rapidly heat the specimen ring. Temperatures as high as the materials melting temperature can be reached within a few milliseconds, prior to the application of electromagnetic expansion forces. The source employs a pumped LC tank circuit with a resonant frequency of roughly 25 kHz to induce a current in the ring. The current in the primary and secondary coils are measured using Pearson and Rogowski coils. A high-speed infrared camera is used to measure the temperature of the ring specimen during heating.