Railgun Experiments Research Articles

In Situ Current Distribution Measurements in C-Shaped Armatures

The knowledge of the current density distribution in C-shaped armatures during a railgun experiment can be of crucial importance in the understanding of transition mechanisms and optimization of the electrical contact between rails and armature. That is why we applied during single-shot experiments with C-shaped armatures the measurement technique described, in order to deduce the spatial and temporal current density distribution of C-shaped armatures during the experiment. Based on our experimental results, we observe nonuniform current density distributions in the armature, including areas with high current densities and very low current densities. We compare our findings with the state of the post shot rail surfaces and to strengthen our interpretation we performed 3-D simulations using the COMSOL Multiphysics program.

Causes of Damage at Electromagnetic Railgun’s Initial Position and Corresponding Improvement Measures

Damage at an electromagnetic railgun’s initial position during launch affects the life of the rail and launcher as well as the launch speed and efficiency. Therefore, this study investigated such damage through electromagnetic railgun experiments. Analyses of metallographic specimens and surface composition as well as 3-D scanning and reconstruction of damaged parts indicate that the damage is a thermal effect mainly caused by transition ablation. Transition ablation problems caused by insufficient initial contact pressure, large rail spacing, and insufficient rail strength are analyzed and corresponding improvement measures are suggested. Finally, the effectiveness of these measures is verified through experiments to guide the design, manufacture, and application of electromagnetic railguns.

A Four-Stage XRAM Generator as Inductive Pulsed Power Supply for a Small-Caliber Railgun

A four-stage XRAM generator is built and then applied to a railgun having a caliber of 15 × 15 mm. The XRAM generator is based on 1-mH Brooks coils and on standard high-power thyristors which are employed as opening switches according to the ICCOS countercurrent commutation principle. The coils of the generator are charged with a current of 10 kA, which corresponds to an energy of 200 kJ. The current is successfully commutated and a pulse with a maximum current of 40 kA is applied to the railgun. Consequently, a projectile with a total weight of 33 g is accelerated to 157 m/s within the railgun length of 2.1 m. The semiconductor switches are exposed to a steep voltage rise due to the muzzle flash when the projectile leaves the railgun without their critical limit being exceeded. Thus, the successful operation of the XRAM generator using semiconductor switches during railgun experiments is demonstrated. Furthermore, we are able to show in a further experiment that the muzzle flash of the railgun can be significantly reduced by switching the XRAM back into charging mode when the projectile leaves the bore. This method also allows the conservation of the energy that remains in the coils after the exit of the projectile from the railgun.

Current Distribution and Contact Mechanisms in Static Railgun Experiments With Brush Armatures

The use of brush armatures for accelerating railgun projectiles allows muzzle velocities in excess of 2.5 km/s and contact transition velocities of at least 1.8 km/s in middle-caliber launchers to be obtained. The technology developed at ISL is based on the use of multiple brush armatures. Asymmetric wear has been observed in dynamic launch experiments when the brushes were separated in shot direction. This indicates the presence of asymmetric current distribution. In this paper, mechanisms determining the current distribution of multiple brush armatures are investigated using a static approach. Experiments were performed using a section of the ISL EMA3 railgun with 270-mm-long rails. It had a caliber of 15 mm x 30 mm and was connected to a 150-kJ capacitor bank. Three series of experiments focusing on specific parameter variations are presented. The current distributions were recorded using small Rogowski coils placed around the brushes. Magnetoresistive sensors were used for highly local measurements of magnetic fields in the vicinity of the brushes. Results reveal insight into the contact behavior of multiple brush armatures. More specifically, phase transitions at the brush-rail interfaces can be observed. This is typical in dynamic experiments when the current commutates from one brush to another. The experiments are discussed with the help of PSPICE simulations.

Analysis of Energy Conversion Efficiency of a Capacitor-Based Pulsed-Power System for Railgun Experiments

To analyze the stored-to-kinetic energy conversion efficiency, a transient electric-circuit model for a railgun system is built using the electrical circuit analysis software SIMPLORER in this paper, and the electric parameters of the model are defined with measurement values of the actual system. Many results of simulations and experiments are also presented, and the results show that the equivalent series resistance of the coaxial cables and the rail resistance are major factors on the energy conversion efficiency. In the end, some methods are proposed to improve energy conversion efficiency of the pulsed-power supply system.

Evaluation of Solid Armature's In-Bore Position, Velocity, and Current Distribution Using $B$-Dot Probes in Railgun Experiments

In this paper, the measurement of the in-bore position and velocity of the armature is performed using B-dot probes. Nine B-dot probes are installed in the rail-to-rail insulator at different distances from the breech. Based on some simplifications of the current flowing in the armature, the magnetic flux formula of the probe is derived. It is found that when the armature passes by the probe, the ratio of magnetic flux of the probe and armature current reaches to its peak value. Thus, using the integrated B-dot probe signals and armature current, we can obtain the armature's position as a function of time, and then the velocity can be obtained. Compared with zero-crossing method, this method considers the influence of di/dt to the signals of B-dot probe, and is more theoretically precise. The B-dot probe signals are also used to analyze the distribution of the armature current in launch process.

The Use of Electronic Components in Railgun Projectiles

This paper deals with experiments and calculations performed in order to investigate the influence of the electromagnetic hardening of payloads in a railgun. This is a complex task: besides the large amplitudes of the in-bore magnetic fields due to the pulsed current, the exit of the projectile from the muzzle and the consequences of plasma arcs have to be considered. At the muzzle the magnetic induction can drop from several Teslas to zero within some microseconds, leading to very high induced voltages and electric fields in the metallic parts of the projectile. On the other hand, the electric contact established by solid armatures tends to develop into electric arcs at high velocities during the launch. These plasma arcs as well as the closing switch transients of the railgun circuit are a source of electromagnetic radiation in a broad spectral range. Some electronic devices were selected and tested with static setups corresponding to the previous conditions. In a first phase a series of static railgun experiments (no projectile movement) was performed. In a second phase, static experiments simulating the muzzle exit conditions were carried out. Finally, the influence of electromagnetic waves emitted during railgun experiments on electronic devices was investigated, using a static setup with a conventional spark gap.

Magnetic Diffusion in Railguns: Measurements Using CMR-Based Sensors

Magnetic diffusion is of great importance in the domain of electromechanical energy conversion since it can be a performance-limiting mechanism. This paper deals with magnetic diffusion in the case of the rails of a railgun. The experiments benefit from the use of a new type of high magnetic-field sensor based on thin ( < 1 mum) manganite films, which exhibits a colossal magnetoresistance (CMR) effect and has very small sensitive areas (e.g., 0.5 mm times 50 mum). Some basics about CMR and the design of the sensor are given. Several sensors were used and calibrated in the course of static transient railgun experiments. Here, varying spatial field distributions due to the use of different rail materials were recorded. Magnetic-field measurements during railgun operation with projectile velocities exceeding 1000 m/s have been successfully performed. By comparison with 3-D finite-element calculations, interesting results concerning magnetic diffusion have been worked out.

A Study of Electrothermal-Launcher Efficiencies and Gas Dynamics

This paper describes a series of experiments to study the efficiency of an electrothermal (ET) launcher. The launcher serves as a preaccelerator for a plasma-driven hypervelocity railgun experiment at the Institute for Advanced Technology. The ET launcher is designed to accelerate small polycarbonate projectiles to about 1 km/s. The objective of this paper was to understand how pulse duration affects the efficiency with which the pressure of the ET discharge is coupled to the projectile. This paper consisted of multiple tests for which the total energy of the discharge was kept constant, but the pulselength was varied. The dependent variable in the tests was the muzzle velocity of the projectile. The experimental results are presented and compared with gas-dynamic considerations.

Structural Mechanics of Railguns in the Case of Discrete Supports

In this paper, numerical calculations concerning the dynamic behavior of a railgun are presented. At the first stage, the structural problem can be decoupled from electromagnetic phenomena as well as from the local projectile behavior. The magnetic pressure repelling the rails from each other and expanding with the speed of the projectile serves as a boundary condition for purely mechanical calculations. The particularity of the investigation is represented by the type of railgun housing. For some years, the ISL has been using laboratory housings of an open design allowing, for instance, to take flash radiographs during launch. The repelling forces are mainly taken by discrete supports in the form of steel bolts. These bolts are connected to bars made of glass fiber-reinforced plastics, while the rails are mounted on these bars. A 2-D finite-element model of a complex housing was developed in this paper. Bars, including rails, are described by plane-stress elements, while bolts, playing the role of discrete elastic supports, are presented by truss elements. The model is implemented using the ANSYS code. Deformation properties of the rail section and elastic supports are examined by considering a static solution, assuming constant loading. Differences between 2-D and conventional beam models being of importance for this kind of problem are briefly discussed. Transient analysis was performed for a set of constant loading velocities (600-1600 m/s) and for the experimentally derived transient-loading profile. The latter was obtained in the railgun experiments performed using the ISL-railgun EMA3 with a caliber of 15times30 mm2 and typical muzzle velocities up to 1600 m/s as well as peak currents of about 600 kA.

Highly Local Measurements of Strong Transient Magnetic Fields During Railgun Experiments Using CMR-Based Sensors

The accurate measurement of the magnetic field distribution during electromagnetic launch experiments is an ambitious task. Loop sensors are widely used for detecting the change in magnetic flux. However, this technique is mostly used only for qualitative purposes, e.g., for triggering various devices. This paper deals with the use of another type of high magnetic field sensor based on thin (<1 mum) manganite films, which exhibit the colossal magnetoresistance (CMR) effect. This sensor measures the magnitude of the magnetic induction and can have very small sensitive areas (e.g., 0.5 mmtimes50 mum). Some basics about CMR and the design of the sensor are given. Several sensors were used in experiments performed with the ISL-launcher EMA3 (Eprim =0.6 MJ, l=3 m, cal=15 mmtimes30 mm). Transient magnetic field profiles with rise times of approximately 50 mus and amplitudes up to 4 T were recorded. The results obtained with the CMR sensors are compared with those of conventional loop sensors. Also, some metrological peculiarities due to high-frequency coupling to the detector circuit are mentioned. The highly local measurements of these CMR sensors were validated by results obtained from 3-D finite element (FE) calculations of the magnetic field distributions

Down-Slope Contact Transition in Railguns

Most railgun experiments exhibit transition to high-voltage contact after the applied current starts to decrease from its peak value. A statistical examination of existing data revealed that such contact transitions occur at approximately 80% of peak current during down-slope. While several plausible explanations exist for the cause of such transition, we demonstrate through numerical simulation that a likely cause of this transition mechanism is a diffusion-controlled process that reduces the contact pressure below the threshold value necessary to carry the applied current. The current down-slope was found to result in a nonuniform body-force-density distribution in the armature such that the contact pressure at the armature-rail interface decreases (eventually becoming zero) even though the total electromagnetic force resulting from the applied current is still compressive. This event's diffusive nature suggests that transition can be delayed by controlling the diffusion time, either by judicious material grading or increasing the diffusion depth

Railgun experiment and computer simulation of hypervelocity impact of lexan projectile on aluminum target

Results of hypervelocity impact tests at an electromagnetic launcher with plasma armature are described. The tests were performed with a thick duralumin disk of 80–100 mm diameter and of 40 mm height impacted by a lexan cylinder of 1.5–3 g mass. The projectile velocity amounted to 4–5.5 km/s. The launcher operational characteristics are described. In the tests, various configurations of the target fracture were obtained including cratering and spall cracks, disks and cones. The computer simulations were accomplished by means of a 2D axisymmetrical SPH code using interparticle contact algorithms. In these algorithms, stresses and velocity at the contact points of Lagrangian particles are computed by the linear Riemann solver. For description of material failure, a model based on a threshold stress criterion was used. The main features of material distribution and target fracture are reproduced in simulations.

Doppler-radar: a possibility to monitor projectile dynamics in railguns

A precise and continuous measurement of the velocity of projectiles inside the barrel would permit to improve the understanding of the dynamic behavior of railguns. Friction-related questions, the mechanical stability of projectiles, or mass loss of armatures are problems to be mentioned in this context. One possibility to monitor the projectile dynamics is the use of a conventional Doppler-radar. In this paper, the experimental setup of a W-band Doppler system and its application at railgun experiments are described. First results obtained using a 15/spl times/30 mm/sup 2/ rectangular caliber railgun are presented. The maximum muzzle velocity for the series of experiments is about 1300 m/s. Depending on the quality of the electrical contact between rails and armatures, a very good agreement between the projectile dynamics measured by the Doppler-radar and the information given by several B-dot signals recorded at several positions along the barrel is obtained. If no electrical arcs are formed during the shot, the projectile is properly detected until it reaches the muzzle of the launcher (l=3 m). Some aspects of friction are discussed to illustrate the evaluation of the data obtained.

A 4.5-MJ pulsed power supply for railgun experiments

A 4.5-MJ capacitor-based pulsed power supply (PPS) has been installed at the U.S. Army Research Laboratory (ARL), Aberdeen Proving Ground, MD, for railgun operations. The system consists of 18 independent modules, each with an energy capacity of 250 kJ. Half of the modules were modified from their original condition to a pulse-forming inductance of approximately 24 /spl mu/H, while the remainder maintained their original 60-/spl mu/H inductance. Another modification included a pneumatic-operated shorting system added to enhance the safety of the system. Simulations were conducted for a variety of load conditions using a Spice-based code. Predicted currents and velocities are in reasonable agreement with measured quantities. The complete system allows the electromagnetic launch of hypervelocity launch packages to a downrange distance of 4 km.

A three-dimensional finite element model for thermal effect of imperfect electric contacts

There are two phenomena that have significant influence on the behavior of the imperfect electric contacts (ImPEC) region, electric diffusion and thermal effects. This paper presents a three-dimensional (3-D) finite element (FE) model for simulating the thermal effect of ImPEC using contact-area-resistance and a surface heat flux source at the contact interface in terms of local variables. The dependence of the contact-area-resistance is investigated and incorporated into a numerical model used for simulations. A contact resistance experiment was performed to establish two constants that appear in the ImPEC model. The properties of the interface obtained from both the simulation and experimental studies are incorporated into numerical simulations of railgun experiments conducted at Eglin AFB using a C-type armature. The electromagnetic (EM) FE code EMAP3D was used for all of the simulations.

Recent muzzle-fed railgun experiment on metal armature for arcless acceleration

The authors have continued the experimental and theoretical analysis of acceleration with metal armatures and muzzle-fed railguns, which earlier produced arcless accelerations up to V/spl sim/2.5 km/s. They had to carry out a lot of work both for improving the structure of the channel itself and to obtain a better understanding of electrodynamic processes. A series of experimental measurements were performed on channel (10 mm/spl times/10 mm) deformation at various current levels. The mathematical model providing the 3-D simulation of processes in an electrodynamics accelerator channel includes the Maxwell system of equations in a quasi-stationary approximation, the heat conduction equation and the equation for kinematic characteristics of the accelerated armature. For systems of a large calibre, the authors suggest an improved channel design.

Numerical Simulation of Mechanical Effects in Composite Structures by the Finite Element Method

In electromagnetic railgun experiments at ISL, the composite projectiles are accelerated up to 2000 m/s with a maximal acceleration of 7×106 m/s2. The maximum forces typically will have been achieved after 200 μs. One consequence of this dynamic load is a strain rate in the order of ε˙=2000 s−1, a rate at which metals are also giving up their static properties. The aim of research at ISL is the construction of a fiber-reinforced sabot adapted for dynamic bearing. FE simulations support the construction process as well as allowing a detailed study of the mechanical effects occurring during the acceleration of the sabot. The most important problem of dynamical simulation of composites is an adequate material model. We show in this paper, that the classical homogenized laminate models are insufficient to be used in dynamics. To develop an appropriate material model, we investigated selected materials statically as well as dynamically and we compared the experimental results with those of their numerical simulation. For a first material model approach, we made quasi-static experiments with a special sabot geometry. The quasi-static investigations have revealed that different conditions, e.g., lateral support, involve a completely different system answer. We found that a specimen without radial strain has more than the double strength of a specimen that allows radial strain, whereby the system answer still remains linear elastic and only very little internal delamination occurs. A second step toward a material model consists of dynamic experiments with specimens in a simple cylinder geometry on a Split Hopkinson Pressure Bar test facility. The specimens are made of fiber-reinforced epoxy, as well as metal-matrix composites. The dynamic experiments proved the strain as principal failure criterion for dynamic bearing, whereby the failure strain depends upon the momentary strain rate. Failure strain is also a historical function, so that the deformation history is important. We can show that the viscous polymer matrix plays a very important role for the dynamic toughness of reinforced materials. When the material fails, the stress is far beyond that of static failure: up to two times the static strength has been measured. Experiments with real sabots in railguns have shown that the failure occurs a relatively long time after the stress has achieved its maximum.

The railgun experiments on head-on collision of different mass bodies in 10-15 km/s range

Existing techniques allow the launch of solid imparters up to only about 10 km/s. A way of doubling the effective velocity for collision experiments is to use two accelerators to produce a head-on collision. However, light-gas guns and conventional EM launchers with pre-accelerators are not suitable for this mode of operation due to the difficulty of synchronizing the projectile motion when travelling through long bores (from a few meters to tens of meters). The compacted plasma armature railgun (RG) developed at the Ioffe Institute has a much shorter bore because it operates at a constant-along-the-barrel acceleration close to the maximum value allowed by the projectile strength or electrothermal explosion of rails. Acceleration of a typical 1 cm body reaches /spl sim/1-5 MGees, so that 7 km/s is achieved in 56 cm bore. The short acceleration length makes it possible to synchronize the motion in two counter-firing RGs. We have done experiments with two sets-ups, one using two RGs having identical square bores of 2 mm across and the other using an 8 mm square bore RG opposed to a 2 mm bore. Exploiting a rotating-mirror camera and a laser we have photographed collision processes at relative velocities of 4.9+5.1=10 km/s and 5.9+4.5=10.4 km/s, respectively. The potentials of this approach are far from being exhausted.

Small bore railgun activities at KSL

A series of railgun experiments was performed with the view of preventing secondary arcs in using a plasma armature. Several pairs of small holes were drilled through rail insulators to expel neutral gas which consists of bore materials vaporized by high temperature plasma. These small holes are referred to as spout holes. When the railgun was operated with them, secondary arcs seldom occurred, and projectile acceleration performances were considerably improved. Three series of successive shots experiments were also performed to investigate the influence of bore damages on the acceleration performance. In each series, bore components were only cleaned, and were never refurbished or reamed. Damages of bore components were also evaluated after ten shots. The results of copper-tungsten alloy were quite satisfactory, as the projectile velocities were relatively steady at each repetitive shots, and the amount of erosion was very small. >