High velocity linear induction launchers

Electromagnetic launchers (EMLs) have received great attention in the last decades because of their potential application to a variety of energy, transportation, space, and defense systems. Particularly, they can serve as kinetic weapons, such as ground-based and naval artillery, space-based anti-missile guns, Earth-to-Orbit launcher, and mass transportation. The main advantage is that EMLs can accelerate projectiles to hyper velocities, i.e. velocities greater than those achievable with conventional cannons. The Linear Induction Launcher (LIL) is an air-cored electromagnetic coil launcher operating on the principle of the induction motor. Polyphase excitation of the coils constituting the barrel is designed to create an electromagnetic wave packet, which travels with increasing velocity from the breech to the muzzle. The projectile is a hollow conducting cylinder (sleeve) carrying the payload within it. Relative motion (slip) of the wave packet with respect to the projectile induces azimuthal currents in the sleeve that interacts with the exciting magnetic field to produce both propulsive and centering forces. This paper deals with the design of a high velocity linear induction launcher with muzzle velocity up to 6000 m/s. It addresses the design specifications of the launcher and utilizing a projectile weighing 1 kg. In the paper, the design specifications with simulation results for the phase voltages, the currents, the velocity, and the temperature rise of the sleeve are presented.     

Fast electromagnetic launchers

A general discussion of the form of the force equation for fast electromagnetic launchers, and of their energetics, is presented. It suggests that a class of launch devices whose total inductance decrease during the launch cycle has a number of attractive features, especially the potential for high electrical-to-mechanical energy conversion efficiency. Examples of specific launcher concepts in this class are given.     

Development of hypervelocity electromagnetic launchers

Interest has increased substantially during recent years in the application of electromagnetic launch (EML) technology for a variety of purposes. In part this increased interest is due to the recent availability of compact pulsed power supplies suitable for driving such launchers. Also, several successful EML experiments have provided encouraging results.

The history of electromagnetic launch is reviewed, the current status of the railgun is presented, and plans for the next generation of electromagnetic launchers are discussed.     

EMET technology for rail launchers

The EMET concept is a marriage of electrothermal plasma jet technology with rail accelerators using plasma armatures. By injecting a structured plasma immediately behind a moving projectile prior to the current pulse, the plasma armature properties can be highly controlled. Parameters of interest are the armature mass and length, molecular weight, specific heat ratio gamma, and temperature. Proper control of these parameters leads to control of problems facing rail launchers such as wall ablation and viscous wall drag. In support of EMET, a Material Test Facility (MTF) has been developed for performing basic physics and materials research on hypervelocity launchers, by making direct measurements of the plasma pressure and jet velocity in a 1 cm bore. These measurements are then compared with theoretical models for various types of plasmas, in order to understand and eliminate barrel ablation. The paper discusses measurement techniques used on MTF, and the approaches being taken to develop EMET in the laboratory.     

Rail launchers to reach hypervelocity

In this paper we present a theoretical study of a 80 mm round bore railgun which allows us, by a current distribution along the projectile, to accelerate a long rod penetrator with a fineness ratio of 30 up to a muzzle velocity of 2500 m/s with an overall efficiency greater than 30%.

This study was started because an optimal impact velocity which allows a given depth of penetration to be reached with a minimum kinetic energy exists for all the targets (homogeneous, composite, structured or reactive). Two years ago we showed that this impact velocity is always greater than 2300 m/s for a heavy alloy penetrator with L/D = 30. For these velocities the electromagnetic rail launchers may have efficiencies over 35 % when classical powder guns have efficiencies about 20 %.     

Computational design of hypervelocity launchers

The andia ypervelocity auncher (HVL) uses two-stage light-gas gun impact techniques to launch flier plates to velocities in excess of 10 km/s. An important requirement in designing successful third stage techniques for impact launching fliers to such velocities is detailed understanding of the interior ballistic performance of the third stage. This is crucial for preventing melt and fracture of the flier plates during the extraordinary accelerations that they undergo (10⁹ g). We seek to optimize HVL launch conditions in order to achieve two major goals: first, to maximize the potential launch velocity for a given flier, and second, to allow different flier configurations. One tool that we have applied in studying HVL performance are multi-dimensional wave propagation codes, particularly the Sandia Eulerian code CTH. Recently this work has culminated in a major contribution to HVL design, namely the capability to launch “chunk” fliers. The initial phases of design development were solely devoted to CTH computations that studied potential designs, identified problems, and posed possible solutions for launching chunk fliers on the HVL. Our computations sufficiently narrowed the design space to the point that systematic experimental progress was possible. Our first experiment resulted in the successful launch of an intact 0.33 gram titanium alloy chunk flier to a velocity of 10.2 km/s. The thickness to diameter ratio of this flier was approximately 0.5.     

Design limitations on ultra-high velocity projectile launchers

The maximum velocity attainable in a gasdynamic gun is limited by the maximum sound speed in the driver gas. For a conventional 2-stage light gas gun, the limit is 10 km/s. Higher velocities are possible, but probably not without the destruction of the gun barrel. As long as this occurs on a time scale longer than the residence time of the projectile, a useful system may still result. Using a newly developed computer code called IGUN, we have evaluated the performance of several multistage designs capable of achieving ultra-high projectile velocities. The main problem is in maintaining the integrity of the projectile. Our calculations indicate that 20 km/s should be achievable without fracturing the projectile. If it is only required to retain near original areal density, velocities in excess of 30 km/s appear feasible.     

The way ahead [EM launchers]

The author discusses the history and future of electromagnetic launchers, focusing on linear induction machines for producing standstill forces, for propelling high-speed vehicles, and as accelerators for producing kinetic energy. He refers to his own experience to illustrate the points made. Conclusions concerning fruitful directions for future research are drawn     

Pulsed power requirements for electromagnetic launchers

Both linear (railgun) and coaxial (mass driver, etc.) electromagnetic launchers (EMLs) are treated as time-varying impedances to determine the relationships between acceleration force, payload velocity, and power supply voltage and current. These relationships are then examined in the light of electromagnetic parameters associated with each EML type to establish a basis for determining and comparing power supply requirements for various EMLs.     

Rail and armature current distributions in electromagnetic launchers

The current and magnetic field distributions in the rails and armature of an electromagnetic launcher are obtained in closed form for the steady state. These solutions assume that the armature moves with a steady velocity and account fully for the two-dimensional skin effect caused by the relative motion between the rails and the armature. Both solid and laminated armatures are considered. It is found in the case of the laminated armature that the phenomenon can be described by a single dimensionless parameter,frac{ell}{w}frac{sigma_{o}}{sigma_{r}}sqrt{frac{uell}{pieta_{r}}}.     

Critical velocity for rails in hypervelocity launchers

This paper is related to the dynamic and strength analysis and optimized design of hypervelocity electromagnetic launchers. Projectile motion along the rails at critical velocity is associated with damaging resonant regimes. These regimes reveal increased displacements and stress that can lead to failure of a launcher. To calculate critical velocity and to visualize dynamic deformations of the launcher we have developed two alternative approaches utilizing analytical and finite element models. The first approach employs a closed form analytical solution for critical velocity that is based on the Bernoulli–Euler model of a beam resting on an elastic foundation and subjected to a moving load. The critical velocity is expressed as a function of geometric and material parameters of the rail and equivalent stiffness of the supporting structure. The stiffness of the supporting structure is found from a 2D finite element model. In the second approach, we employed beam finite element and 3D solid finite element models to visualize and measure the “natural” and “forced” bending waves traveling along the rails. These approaches helped to better understand the transient resonant dynamic processes and offered insight on how to alter the launching device materials and geometry to reduce the critical-velocity effects.     

Review of impact fusion concepts [EM launchers]

The authors review the main features of the impact fusion concept in which gram-sized metallic projectiles from electromagnetic launchers collide around heavy-hydrogen fusile gas to produce thermonuclear plasma whose pressure brings the projectiles to rest, providing a pulse of fusion power during the `turnaround' of the projectiles. They discuss the concepts of nonmagnetic as well as magnetically insulated, impact fusion, showing how its potential advantages as an inertial confinement system could be realized. Of particular interest is magnetically insulated impact fusion, where the decreased plasma heat loss may allow lower impact velocities, consistent with the nearer-term state of the art of electromagnetic launchers. Also considered is the concept of mechanical helicity injection that allows both convenience and flexibility in producing the final magnetic configuration     

Elastic waves and solid armature contact pressure in electromagnetic launchers

Electromagnetic launchers suffer a phenomenon referred to as armature transitioning: when the armature and rails suddenly lose contact with each other, damage can occur to the armature and the rails of the launcher. In this paper, we explore transient elastic waves as a possible explanation for the transitioning of solid armatures in electromagnetic launchers. We use a finite-element code to model the transient dynamics of a typical electromagnetic launcher guide rail. We found that dynamic rail deflections caused by the movement of the armature are similar in magnitude to those caused by the magnetic field, and that the contact pressure between the armature and the rails changes dramatically when the speed of the armature reaches the critical velocity of the rails.     

The use of hypervelocity launchers to explore previously inaccessible states of matter

The dynamic response of materials at very high pressure and temperature is important in a wide variety of scientific and programmatic applications. Thermodynamic regions of interest include the high-pressure response of condensed phases and the highly expanded states of materials. Since these regimes are normally studied with flat-plate impact techniques, it is necessary that projectile velocities achievable with highvelocity launchers be sufficient to access the desired thermodynamic regions. In this paper, we discuss the equation-of-state regions that would be accessible with a plate impact capability of 15 km/s and summarize the status of a hypervelocity launcher under development which will provide the required velocity capability.     

Current scaling of projectile velocities with and without asnowplow effect in electromagnetic launchers

Scaling laws are studied, with and without a snowplow (SP) effect, for macroparticle projectile velocities in electromagnetic launchers (EMLs) as a function of the peak EML current. Analytical formulae to predict the instantaneous position and velocity have been derived for both phases of rising and constant EML currents. The SP effect altered the scaling law of the short-pulse muzzle velocity (ν_xw), but not of the long-pulse muzzle velocity (ν_xm). The absolute values of both ν_xw and ν_xm are, however, seriously reduced in the SP regime, depending on the newly introduced parameter, ∈, which is a mass ratio of the armature/projectile system to the gas in the barrel. The critical ∈ s with which the SP effect alters the muzzle velocity and time by more than 10% are presented. Numerical examples are given for possible applications of EMLs on calibrated impact damage testers and impact thermonuclear fusion, where short (1 m) and long (1 km) barrels are to be used for projectiles of several grams to achieve the maximum velocities of 1 and 130 km/s, respectively     

A finite-element method for the prediction of joule heating ofconductors in electromagnetic launchers

The authors describe a method based on a magnetic vector potential development for calculating the joule heating of the conductors in arbitrary launcher geometries. The method accounts for the effects of varying conductivity of the launcher's materials and can be implemented using available two-dimensional finite-element diffusion codes. Conductor temperature distributions as functions of time for a selected launcher geometry are provided to demonstrate the method     

Ohmic heating losses in cryoresistive pulsed energy storage aluminium inductors for electromagnetic launchers

Analytic procedures are developed for calculating the ohmic heating loss for various cryoresistive magnetic energy storage coil configurations that are used to deliver periodic short huge bursts of energy to electromagnetic launchers (railguns). Simple geometries such as long solenoids, continuous winding toroids, low aspect ratio dipoles or straight wire are considered as examples. The effect of cabling on reducing eddy current losses is addressed in detail.