Experimental results from CEM-UT's single shot 9 MJ railgun

A 10-m-long, 90-mm bore railgun has been designed and fabricated. During the test program, a number of solid armatures and projectile packages have been tested in a 50-m-deep vertical test range. The experiments are powered by six homopolar generator (HPG) charged inductive stores, sequentially staged to provide the desired acceleration profile. Prior to testing, computer simulations are run to determine the preferred current profile and predict system performance. During projectile flight, high-speed films, X-rays, muzzle volts, and velocity/acceleration profiles are recorded along with power supply operating parameters. Postshot diagnostics include bore wear analysis and armature and target recovery. Comparisons of predicted and recorded shot performance are also made. On selected tests, an energy balance is performed to determine efficiencies of the various components. A summary of all 90-mm gun shots is presented along the critical data collected from selected tests     

Thermal analysis of fiber armatures

A detailed time-dependent thermal model using temperature-dependent material properties has been developed and implemented in the Westinghouse railgun simulation code to estimate interface temperature for fiber armatures. Fiber and rail materials are selected to protect the rails by confining deformation and melting to the armature. Heat generation at the interface, composed of electrical contact and friction heating, is calculated with a novel friction model using static and hydrodynamic terms. Projectile injection velocity and starting current waveform guidelines are given to protect the breech end of the gun, where the armature is moving slowly. Results are presented for typical railgun conditions as a parametric study which includes precooled armatures. Computer runs were performed for both copper and aluminum fiber armatures on copper, tungsten, and molybdenum rail surfaces. The results indicate that full film lubrication of fibers is unlikely for the typical railgun systems currently in use. However, melting of the fiber ends of actual armatures is common and is probably due to bouncing arc contact of the fibers with the rails     

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.     

The stability and the conditions for initiation of electric-arc discharges in railguns

Characteristic physical features of plasma formations in high-velocity sliding electric contacts are treated, in particular, those in railguns with both electric-arc and metal armatures. Comparison is made of the effect of a number of MHD instabilities (superheating, Rayleigh-Taylor, and erosion-dynamic) on the working capacity of plasma armature. It is demonstrated that the armature compactness may be disturbed both in the case of fairly intensive abrupt drop of current and in the case of increase in the associated mass of the armature, which is defined by the processes of erosion and ablation of the electrodes, walls, and dielectric projectile. The theory of transition of metal contact is generalized in view of the presence of transition electric resistance and external magnetic field.     

Soft acceleration of a metal plate by a rail gun plasma

The following situation is considered: A dense plasma armature which has been accelerated to a high velocity in a rail gun is allowed to impact a stationary metal plate. At impact, a shock wave is transmitted into the plate and a shock wave is reflected back into the plasma. For very high plasma velocities, the subsequent behavior depends primarily on the plasma density profile just prior to impact. The appropriate partial differential equations are analyzed in plane geometry and the conditions required to prevent spallation of the plate are determined. These results are compared with the "plasma only" armature plasma profiles predicted by Sloan, and it is shown that the conditions for not spalling the metal are well satisfied. The typical rail gun makes use of a low mass plasma armature to continuously accelerate a more massive solid projectile down the length of the rails. It has been suggested that substantially higher velocities and energies may be achievable with a "plasma only" rail gun-which has no solid projectile, but which has an armature mass comparable to the mass of a solid projectile. Subsequent to the acceleration, the momentum of this high velocity plasma must be transferred to a solid projectile. One way to do this is to simply allow the plasma to collide with the projectile. For this to be of interest, the projectile should not shatter and the energy transfer should be reasonably efficient. The purpose of the paper is to analyze an idealized model of such a collision.     

The potential of hydrogen armature experiments

Unlike aluminum or copper plasma armatures, hydrogen armatures of the same temperature and electron density are virtually transparent to ultraviolet and are totally transparent to soft x-rays (∼ 20 eV). Thus, it may be feasible to observe significant interior plasma dynamics with an appropriate experimental device. Some possible experiments of this type are discussed. The typical rail gun armature is aluminum or copper and at typical number densities of 10²⁰- 10²¹/cm³andTsim 3-5eV is opaque to radiation well into the kilovolt x-ray region. As a result, spectroscopic diagnostics cannot provide any information about the interior of the plasma. The principal contribution to the short mean-free path for light absorption in metal plasmas is the bound-free transitions. By contrast, at these densities and temperatures, hydrogen is almost completely ionized and thus the only significant absorption mechanism is inverse bremstrahlung. This leads to a mean free path for 10 eV radiation in hydrogen which is in the range of a centimeter. Thus, hydrogen armatures are transparent in the soft x-ray region and if a hydrogen armature rail gun can be developed, soft x-ray spectroscopy can be used to explore the internal physical phenomena. Two phenomena which are conjectured to exist in rail guns are arc spots and MHD or fluid turbulence. In the following, we suggest experiments in hydrogen armatures which should detect these phenomena if they are present.     

United Kingdom EM gun programme

The authors review the progress in the RARDE (Royal Armament Research and Development Establishment) electromagnetic gun (REMGUN) program. They cover research on both railgun and LIA (linear induction accelerator) technology, including work on modeling, projectile/barrel interactions armatures, switching, and instrumentation. They also describe the major research facilities. Progress is reported in the identification and development of single-shot switches, integrated switch/barrel concepts, armatures and pulse power supplies as well as the integration of railgun components. LIA research has concentrated on launcher design and projectile/barrel interactions, highlighting the problems of pulse power systems for this form of electromagnetic launcher     

Railgun performance with a two-stage light-gas gun injector

Results obtained with the HELEOS (hypervelocity experimental launcher for equation of state) railgun, which uses a two-stage light-gas gun (2SLGG) as an injector, are presented. The high-velocity 2SLGG injector preaccelerates projectiles up to ~7 km/s. The high injection velocity reduces the exposure duration of the railgun barrel to the passing high temperature plasma armature, thereby reducing the ablation and subsequent armature growth. The 2SLGG also provides a column of cool, high-pressure hydrogen gas to insulate the rails behind the projectile, thereby eliminating restrike. A means to form an armature behind the injected projectile has been developed. In preliminary tests, the third-stage railgun has successfully increased the projectile velocity by 1.35 km/s. Extensive diagnostics have been used to determine the behavior of the armature and track the launcher's performance. Insome cases, velocity increases in the railgun section have been achieved, which are in close agreement with theoretical predictions, whereas in other experiments deviations from theoretical have been observed. The reasons for and implications of these results are addressed. Recent tests are reported     

A comparison of armature performance

All electromagnetic launchers require that the launch package have an armature. The armature is the component in which the electromagnetic forces are generated. In this paper we examine "armature efficiency" as one measure of armature 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.     

Mechanical design aspects of the HYVAX railgun

The hypervelocity experiment (HYVAX) railgun (Fig. 1) is designed to produce projectile velocities greater than 15 km/s in a 13-m-long, round bore gun. The HYVAX gun incorporates a modular design enabling it to operate in either a distributed energy-storage mode or a single-stage mode. The gun is composed of seven O.3-m-long power input modules and nine 1.2-m-long accelerating modules. The gun is designed for a 100-shot life. To accommodate this, the bore may be enlarged from an initial diameter of 10.8 mm to a final diameter of 12.7 mm. This will allow the bore to be refinished several times during the life of the gun. To minimize mechanical and arc damage to the gun between bore refinishing operations, the gun will incorporate a low pressure helium projectile injector. Projectiles will be injected under vacuum at 350 m/s. The gun will be operated at a peak current and voltage of 600 kA and 6 kV respectively. The gun will undergo three phases of testing. The first phase will be the characterization of the gun's performance using a 3.0-m-long section of the gun comprising two power modules and two accelerating modules. This testing will be accomplished using two of the seven capacitor bank modules shown in Fig. 1. The second test phase will use a distributed power configuration and seven capacitor bank modules, as shown in Fig. 1, to demonstrate a velocity of 15 km/s with a 1-g projectile. The predicted performance of the gun for this test phase is illustrated in Fig. 2. In the third phase of testing we will use a magnetic flux compression generator (MFCG) to power the gun with a goal of demonstrating a velocity of 25 km/s.     

High-efficiency, medium-caliber helical coil electromagnetic launcher

Research progress in the development of a 40 mm/spl times/750 mm helical-coil electromagnetic launcher (HCEL) is presented and discussed. Significant technical problems that have been solved in this research include efficient stator commutation methods and the ability to simultaneously implement high-inductance gradient armatures. The HCEL is able to launch a 525-gram projectile to a velocity of 140 m/s. Power for the HCEL is derived from a 62.5 kJ sequentially fired pulse forming network (PFN) of 900 V (maximum) electrolytic capacitors. The experimentally measured HCEL efficiency of 18.2% is substantially greater than a conventional or augmented railgun of similar scale (i.e., equivalent mass, bore-size, and velocity). The HCEL's high launch efficiencies result from its 150 /spl mu/H/m inductance gradient, which is approximately 300 times greater than the inductance gradient of a conventional railgun. HCEL computer model predictions are given and compared to experimentally measured HCEL and PFN parameters including peak current, inductance gradient, acceleration time, parasitic mass ratios, and electrical-to-kinetic conversion efficiency. Scaling relationships for the HCEL are also presented and used to predict launcher operation at higher velocity and with a larger diameter bore size.     

The mechanical design of armatures

The authors describe a mechanical design approach for metal armatures which is based on preventing contact arcing. The minimum pressure required to prevent arcing is combined with the mechanical characteristics of the armature to define its safe operating envelope. The design goal is to provide an envelope which will permit armature operation over the range of current and armature deflection expected during launch. The procedure has been used to design successfully a 15-mm armature     

The CEM-UT rapid-fire compulsator railgun system-recent performanceand development milestones

Twenty-seven compulsator-powered railgun experiments have been performed, including a 1.0 MJ discharge at 3510 r/min. In this test, a 724 kA current pulse accelerated an 80 g, aluminum armature to 2.05 km/s, thus exceeding the projectile velocity goal at 73%-rated machine speed. Furthermore, operation with a single gun barrel has been achieved using a parallel path, solid-state closing switch to deliver 132 kA to the railgun injector. The latest data are presented from the rapid-fire compulsator railgun facility. Included is a discussion of the energy transfer, power output, and system efficiency during a 1.0 MJ discharge. Also shown are the injector current, voltage, and di/dt curves for this test which were used in the design of the solid-state closing switch. Results of railgun experiments using the solid-state switch are analyzed     

Hypervelocity macroparticle accelerator experiments at CEM-UT

Several railgun experiments designed to accelerate projectile masses of 2 to 5 g to velocities greater than 6 km/s were performed. Two parallel rail-type accelerators with 12.7 mm square bores were used for the experiments. One gun is 2 m long, has molybdenum rails and alumina ceramic insulators. The other is 1 m long, has molybdenum rails and granite insulators. The greatest velocity achieved to date during the experiments was 5.1 km/s. During the test program, the following ideas to enhance launcher performance were tested: stiff-gun structures to reduce plasma leakage and rail movement, refactory bore materials to reduce ablation and frictional losses, and prefilling the gun bore with gases which will eliminate precursor arcs. After three experiments utilizing the 2 m long launcher, with peak currents ranging from 660 to 780 kA, a gun barrel comprised of 96% pure alumina ceramic insulators and 99.9% pure molybdenum rails has survived with minimal damage and no degradation of seals     

Strain rate effects for aluminum and magnesium alloys in finite element simulations of steering wheel armature impact tests

This paper addresses the strain rate effects for aluminum and magnesium steering wheel armatures when they are subjected to dynamic impact tests. Two geometrically different steering wheel armatures, a three spoke proprietary aluminum alloy armature and a four spoke magnesium alloy (AM50A) armature, underwent experimental impact testing. The testing conditions for each armature were different; testing with the aluminum alloy armature involved impacts with a deformable chestform and the magnesium armature experienced impact tests with a rigid plate. Finite element models of all testing apparatuses were developed for both testing conditions and numerical simulations were conducted based on the experimental method employed. Strain rate effects for the aluminum alloy were considered using the Cowper–Symonds constitutive relation and a Johnson–Cook material law was utilized for the magnesium alloy. Simulations were conducted with and without strain rate effects considered. The comparison between the experimental and numerical methods illustrate that there is only a minor change in the numerical testing results with the inclusion of strain rate effects, however, a better correlation between experimental and numerical methods occurs.     

安装差动胶泥缓冲器的舰炮检验弹入膛仿真分析

舰炮检验弹由于结构不同于实弹,入膛时反弹速度过大,造成相关零部件断裂和关闩故障。故设计采用差动胶泥缓冲器吸收过大的动能以保证检验弹的入膛可靠性。为评估差动胶泥缓冲器在检验弹上的缓冲效能,以及分析检验弹的入膛运动规律,提出一种结合缓冲器流体数值分析(computational fluid dynamics,CFD)和入膛多体动力学分析(multi-body dynamic,MBD)的仿真分析方法。计算结果表明:此方法能够准确计算和预测缓冲器的缓冲效能,检验弹的缓冲器满足设计技术要求;静压过程在活塞后方形成多个涡旋,而冲击过程流线平稳无涡旋;黏性阻抗分力随压缩速率降低先增大后减小。研究不仅为胶泥缓冲器在舰炮检验弹上的应用提供理论支撑,还为胶泥缓冲器本身的研究提供新的分析方法。     

Experimental launcher facility - ELF-I: Design and operation

The ELF-I system has recently become fully operational and the experimental program begun. This system uses a 36 kJ, 5 kV capacitor bank as a primary pulse power source. When used in conjunction with a 5 μH pulse conditioning coil, a 100 kA peak current and ∼10 ms wide pulse is obtained. The launcher barrel is 2 m long with a 12.7 mm square bore. With the launched mass in the range of 3 to 7 g velocities in excess of 1 km/s have been obtained. An extensive array of diagnostic instruments have been installed on the system to obtain measurements of current, breech and muzzle voltage, in-bore velocity profiles, and terminal velocity. Problems associated with electromagnetic interference and high common-mode voltages have been addressed and solved. A three station, 150 kV, flash x-ray system is operational for obtaining in-bore photographs of the projectiles. Details of the mechanical and electrical design and the instrumentation system will be presented. The experimental program, now underway, is directed towards obtaining information on armature-rail interactions at sliding velocities up to 1 km/s for both metallic and plasma armatures. One particular problem being addressed is that of current transfer in the breech where the sliding velocity is very low. Experimental results and their analysis are presented.     

Observation and analysis of current carrying plasmas in a railgun

Observation of current-carrying plasma arcs driving solid projectiles in round-bore and square-bore railguns have been made with magnetic induction coils. Both free-running precursor arcs as well as projectile armature arcs are observed. Qualitative behavior of the plasmas is inferred from data and compared to basic physical models. The non-uniform nature of the force on the armature is investigated. Experimental details as well as difficulties in calibration and in quantitative analysis of magnetic probe signals are discussed. Evidence suggesting evolution in the structure of free-running arcs is presented.     

Effect of in-bore gas on railgun performance

Acceleration of a projectile in a nonevacuated railgun bore produces a series of shock waves traveling through the gas in front of the projectile which retards the projectile's motion. A model is presented which describes the three components of this retarding force-the force required to accelerate the gas to the projectile velocity as it is entrained by the shock front, the force required to continue to accelerate previously shocked gas as the projectile accelerates, and the force required to overcome the viscous drag which arises from the interaction of the shocked gas and the gun tube. The authors address the relative contributions of the three components of the force and the significance of the retarding force when compared to the net accelerating force. The validity of the strong shock approximation for computing the retarding force is discussed