Search

Experiments with the objective to stimulate combustion of solid propellant through addition of electric energy are reported. In this paper, the idea of stimulating combustion by conducting an electric current through the flame is tested. The current leads to resistive heating of the flame, which adds to the reaction heat, and thus amplifies the heat feedback to the unburnt propellant. The electric conductivity of the flame must be sufficiently high in order to reduce random delay and to increase controllabllity of the electric discharge. The propellant composition has to be modified to meet this requirement. Thermochemical calculations give the equilibrium state and composition, which are used to estimate the flame conductivity. Calculations are carried out for several propellant compositions: some being considered for preparation, while others are intended solely for comparison. The calculation results indicate that, given a high flame temperature, a substantial increase in conductivity can be obtained by doping the propellants with alkali compounds. A double-base propellant doped with potassium nitrate has been used in the experiments so far. The experiments are performed in a closed vessel where a propellant slab is pinched between two copper plates connected to a pulsed power supply. Results presented include current and voltage recordings, yielding the added electric energy, and pressure measurements from which the burn rate can be determined. Index Terms--Alkali-doped propellant, closed vessel, electrothermal-chemical, ETC.

The 'smart gun' is an artillery concept that uses pure electrothermal (ET) energy as a means of compensating for in-bore projectile velocity variation with the goal of achieving consistent muzzle velocity. Small-scale preparatory tests (30-mm caliber) and full-scale (155-mm caliber) firings are described which demonstrate that an ET discharge can be used to augment the velocity of projectiles during their down-bore travel. The smart gun consists of three component parts: a means of measuring the progress of the projectile as it moves along the bore, a means of predicting the muzzle velocity, and a means of correcting for any predicted muzzle velocity variation. Work has progressed on the former two items and some details are highlighted here. However, this paper's primary purpose is to report on the success in using ET energy to achieve in-bore velocity augmentation. Full-scale down-bore velocity enhancement was achieved using a 1.5-MJ 'boost pulse' of ET energy at the breech approximately 3 ms after peak pressure. The pressure wave generated caught the projectile during its down-bore travel, causing the projectile to undergo a velocity increase of 13 m/s above that which would have been achieved without the boost. Mathematical modeling using the demonstration results suggest that 20 m/s of velocity correction may be achieved for a 155-mm caliber top-zone firing with the use of a 'boost' discharge of approximately 3 MJ of electrical energy. Index Terms--Electrothermal guns, muzzle velocity control, muzzle velocity prediction, smart gun.

The compulsator stator armature winding includes multiple conductor layers. Each conductor layer is made of insulation wraps and transposed aluminum litz wire bundles with epoxy potting compound between the wires and the wire bundles. For a typical pulse-duty application, based on a room-temperature electrical loss calculation, the temperature rise in the aluminum winding is approximately 30 [degrees]C per shot. Adequate active cooling must be provided such that the stator insulation can survive thermally under a certain pulse repetition rate. The thermal management is quite challenging since the heat loss in the litz wires needs to be conducted through multiple insulations before reaching the coolant passages. As a result of the low equivalent thermal conductivities of the stator winding in the transverse direction, without compromising the overall structural integrity of the winding, the coolant passages have to be allocated as close as possible to the heat sources. For a given electric gun firing mission, four different cooling configurations have been investigated and analyzed by using water ethylene glycol mixture as an active liquid coolant. In this paper, thermal analysis cooling parameters, transient stator winding temperature distributions, and hot-spot temperature histories are presented, discussed, and compared. Index Terms--compulsator, transposed conductor wire bundles, stator armature winding cooling.

With the use of electrothermal-chemical ignition in conjunction with a fire out-of-battery (FOOB) recoil system, the occurrence of a FOOB firing fault mode is significantly reduced, but not eliminated. The application of magneto-rheological (MR) dampers for mitigating the fault modes associated with FOOB (hang-fire, prefire, and misfire) are examined. This is completed using a mathematical model developed to predict the behavior of MR dampers at larger calibers. The results indicate that the MR damper is able to effectively mitigate the FOOB fault modes. Index Terms--Active recoil, fire out-of-battery recoil (FOOB), magneto-rheological fluids, MR damper.

Proposed ship-based long-range electromagnetic guns require input energies greater then 100 MJ, at peak powers in excess of 50 GW, and at average power in the tens of megawatts. This paper describes a conceptual pulsed-power system for driving these types of guns. A generic design was developed based on state-of-the-art power-supply, capacitor, and switching technologies. Results of modeling and parametric studies are presented, and key issues that impact system size and configuration are explored. An assessment is made of the feasibility of integrating this type of system into an electric-drive ship. Index Terms--Circuit modeling, pulsed-power, railgun.

We have been developing a two-stage plasma armature railgun as the first step to produce a multistage railgun system, which is considered as a promising tool to achieve hypervelocities. For successful operation of the two-stage railgun, three technical problems are addressed: the insulation between the two segmented rails, the formation of, and correct timing of, the ignition of the plasma armature in the second-stage railgun. Of these problems, the correct timing of the ignition of the plasma armature of the second-stage railgun will most significantly affect the performance of the two-stage railgun. A large increase in the projectile velocity has been obtained in the second-stage railgun by solving these technical problems, from 1.7 to 5.8 km/s. Index Terms--Breakwire system, rapid rising of plasma armature velocity, two-stage railgun.

The integral containment electromagnetic (ICE) gun constitutes a structural containment design method that has been proposed to achieve desirable prestressing of railguns. The principal purpose of this method is to minimize the maximum strain excursion of the rail containment structure during firing stresses to maximize the fatigue life of lightweight armament. The ICE gun may be considered a means to achieve an 'autofrettage' effect from a laminated steel disk railgun construction where the disks are strained upon assembly to achieve the desired prestress of the containment structure. Index Terms--Prestress, railgnn, structural containment.

The application of electric guns in combat vehicles require pulsed power supply systems with a compact design and a high reliability. Other investigations show that capacitive pulsed power supply (PPS) systems will fulfill these requirements for electrothermal-chemical (ETC) guns. In these PPS systems, high energy density capacitors and high power switches are critical components that have yet to be realized. This paper gives an overview of the latest results obtained from Rheinmetall's investigations on high energy capacitors and optically activated semiconductor switches being developed for ETC PPS systems. The basic technical requirements to be fulfilled by these components are presented. The final setup and the design data of a 1.9-2 MJ/[m.sup.3] discharge capacitors and a 22 kV/40 kA closing switch are described. Results obtained from full scale testing are presented. Basic considerations on other applications of these components and their scaled performance data conclude the paper. Index Terms--ETC, high energy density capacitor, pulsed power supply system, semiconductor.

The use of compulsators as a compact power supply has the potential to be used in electrogun applications. Although compulsators have been developed for electromagnetic railguns for some years, the use of compulsators in the power supply of electrothermal-chemical guns (ETCGs) may give rise to problems such as providing the proper current waveforms and matching of the capillary load. Of particular importance is the nonlinear characteristic resistance of the capillary load. This paper reports on a staged power supply system based on a compulsator for ETCG applications and describes the working process of the simulative system. The whole system is composed of a compulsator, a high-voltage capacitor, a capillary, a high-current thyristor switch, a power diode, and a control unit. The high-voltage capacitor is used for igniting the capillary arc, and helps to reduce the output voltage of the compulsator. Models of a compulsator and a capillary are considered as a whole to simulate the system performance. These models can be used to explain the discharge process of the capillary and provide the theory for the parameter choices and the power supply design. Experiments demonstrate that the staged power supply can meet the current and voltage requirements of ETCG applications. Index Terms--Capillary, compulsator, electrothermal-chemical guns, pulsed power technology.

A new type of capillary with closing switch properties is proposed as a realistic alternative to the conventional capillary in electrothermal (ET) launchers. This capillary is designed as a plasma switch with a trigger electrode (TE) in place of an exploding wire. The capillary plasma switch (CPS) has dual functions of plasma generator or ignition for chemical medium, plus as a switch for the charged circuit, which considerably simplifies the power supply system and meets the requirements of a compact system. The conventional closing switches in the power system for ET launchers can be eliminated. In our work up to now, the special emphasis for CPS has been on the design of the TE. Experiments were carried out to investigate the electrical characteristics of CPS. This paper describes details of experimental testing of devices under a wide range of operating conditions and example waveforms are illustrated. The experiments proved that the CPS was suitable for ET launchers and can offer several advantages over the conventional capillary such as repetitive operation and compactness. Index Terms--Capillary, electrothermal launchers, plasma switch.

The design of an integrated launch package (ILP) for an electromagnetic railgun is considered. A code incorporating analytical engineering expressions for thermal and mechanical loads is used. The armature linear current density and armature and rail pressures are used to define the range of solutions based on mission requirements and subprojectile criteria. Characteristics for large- and reduced-scale launcher and ILP solutions are presented, which are consistent with mission requirements. Single-taper sabot configurations are used to illustrate the methodology. Index Terms--Electromagnetic propulsion, projectile, railgun.

The utilization of high-speed compulsators is central to the success of the electromagnetic pulsed power launch systems under development. The efficient use of these high-energy storage devices depends, in turn, on high-strength carbon composite bands for attaining sufficient speed, and thus energy levels, without disintegrating. While advances in material science allowed significant improvements in the performance of the latest generation of compulsators, a growing concern is their vulnerability to ballistic impact. A comprehensive experimental study using a scaled but realistic version of an operational compulsator is presented. A set of two test rotors consisting of several layers, including a simulated coil section and an outer carbon confining shell, was tested. The rotors were spun to approximately 18 000 r/min in a highspeed test facility, and shaped charges were fired at the outer shell at normal incidence. The results of a set of experiments are presented, showing damage ranging from benign to complete failure, depending on the impact energy level. The experiments indicate that significant impact energies do not necessarily result in complete failure. Observed rotor behavior is compared with predictions. An evaluation of observed failure modes indicates that good engineering can optimize the construction of composite confining bands in compulsators to resist a certain level of high-energy impact without complete failure. Index Terms--Carbon fiber, coil band, compulsator, confining band, impact damage, shaped charge, spin test.

Pulsed power generators being developed for applications in electromagnetic launch operate at high speeds and are intended to deliver megajoules of energy within a few milliseconds. The self-inductances of the rotor and the stator, [L.sub.r], [L.sub.s], and the mutual inductance, M ([theta]), are required to model the performance of these machines. In general, three-dimensional (3-D) field computations are required for the calculation of inductances, and the integration of the stored energy has to be carried out over an infinite domain. The domain of integration could be reduced to just the volumes of current-carrying conductors, using equivalent expressions for the total energy involving the dot product of the current density and the magnetic vector potential. A semianalytic code, Faraday, has been developed using this method for the computation of [L.sub.r], [L.sub.s], and M ([theta]). The 3-D topology of conductors with rectangular cross sections in the rotor and the stator is described by a set of discrete rectangular segments (parallelepipeds) with specified current directions. The segments are continuous along the axis. Analytic integral expressions for the total energy of differential current-carrying segments are used to calculate the total energies and inductances. The inductances [L.sub.r], [L.sub.s], and M ([theta]) for a model pulsed disk alternator were computed using Faraday. The results compare favorably with experimental data. Index Terms--Coupling constants, mutual inductance, pulsed power generators, self-inductance.

The University of Texas Center for Electromechanics (UT-CEM), as a member of the Defense Advanced Research Projects Agency (DARPA) Flywheel Safety and Containment program, has developed a lightweight containment system for high-speed composite rotors. The containment device, consisting of a rotatable, composite structure, has been demonstrated to contain the high-energy release from a rotor burst event and is applicable to composite rotors for pulsed power applications. UT-CEM recently conducted a burst spin test of a composite flywheel inside this composite containment device at Test Devices Incorporated (TDI), Hudson, MA. The most important aspect of this design is that the flee-floating containment structure dissipates the major loads (radial, torque, and axial) encountered during the burst event, greatly reducing the loads that pass through the stator structure to its attachments. The design results in significant system-level weight savings for the entire rotating machine when compared to a system with an all-metallic containment. Of equal interest to the containment design, the experimental design and instrumentation was very challenging and resulted in significant lessons learned. This paper describes the containment system design, rotor burst test setup, instrumentation for measuring loads induced by the burst event, and a detailed explanation of the successful containment test results and conclusions. Index Terms--Containment, flywheels, measurement, mechanical energy storage, pulse power, road vehicle power systems, rotating machines protection, safety.

Air core compensated pulsed alternators are being developed as compact power supplies for tactical electromagnetic gun systems. The field coil of the compensated pulsed alternator is a critical component that establishes the excitation magnetic field. Since the machine is air cored, the number of ampere-turns required from the field coil are significant, especially considering that the excitation flux density near the armature is typically in the range of about 3 to 4 T and that near the field conductors are in the range from 7 to 10 T. The rotating nature of the field coil requires that it be light so it can be supported under centrifugal loads. This implies that the current density in the field coil conductors is quite large. One way of overcoming the excessive heating associated with the high current density is to charge the field coil fast (in tens of milliseconds). However, charging the field coil too fast also results in transient losses due to proximity and skin effects. These must be accounted for in the design and simulation of these machines. During the discharge into the load, transient currents flow in the armature winding. These currents produce magnetic fields that interact with the field coils and produce additional losses. This calculation is complicated by the fact that there is relative motion between the conductors. This paper describes a two-dimensional numerical analysis that has been conducted to evaluate the losses in the field coil under these two dynamic conditions. The method integrates circuit analysis with finite-element calculations. This approach is essential because the field coil conductors have a connectivity that must be described in the calculations. The motional aspects have been accounted for by choosing a reference frame on the rotor and carrying out the calculations in the d-q frame attached to the field coil. The results of this analysis are presented, along with the means of optimizing the field coil and machine geometry in view of the transient losses. Index Terms--Air-core, eddy currents, Park's transformations, pulsed electrical machines.

A mathematical model for a homogeneous (over regions of distinctly different electrophysical properties) simulation of phenomena in a railgun launcher is constructed. The necessity of the use of three-dimensional (3-D) approximation is shown. The algorithm for defining boundary conditions is developed, so that a given total launcher current is equal to the current through a region of simulation. Numerical calculations with different ways of specifying boundary conditions are compared. The results of 3-D numerical simulation of railguns of different configurations are presented. Particularly, the muzzle-fed railgun is described in detail. Index Terms--Computational electromagnetics, inductance per unit length, modeling, railguns.

An electromagnetic system that operates both as a rail and as a coil launcher is proposed, and its performance is analyzed. The device has a composite stator that consists of two rails properly slotted in order to allow the presence of a system of coils. The armature consists of a conductive slab sliding between the rails. Another system of slots is present in the armature where a system of short-circuited coils is placed. A source of constant voltage is connected to the rails and the current flowing in the armature through the sliding contacts produces a thrust force on it. If a proper system of currents able to produce a traveling wave of flux density in the region between the rails is used to feed the barrels, the induced currents that flow in the armature's short-circuited coils produce a further thrust force on the armature itself. The analysis of the behavior of this launcher is performed via a computer code based on an integral formulation and a tool that analyzes the phenomena related to the velocity skin effect; the interactions between the two systems of currents on the armature and the currents on the stator (rails and barrels) are investigated. The thermal behavior of the device has been taken into account by a simple adiabatic model since the short operating time allows one to neglect heat diffusion in the conductive parts of the system. A comparison between the velocities, respectively obtained by separately feeding rails and coils and the system with the combined feeding, has been performed. Preliminary results of the analysis show that this device, because of the presence of two systems of thrust forces acting on the armature, can be successfully used in the acceleration of heavy masses at relatively high velocities, the maximum achievable speed being limited by thermal and mechanical stresses. Index Terms--Coil launchers, induction launchers, rail launchers, sliding contacts.

A railgun launcher requires very high current pulsed power over the period of a few milliseconds. For laboratory systems, capacitor banks have traditionally been used to provide this energy, but field applications require a system with significantly higher energy storage density. Rotating machines that store the energy in the form of rotational kinetic energy and can quickly convert that energy to high current electrical energy have been designed and built. These low-impedance, multiphase, multipole synchronous generators are referred to as pulsed alternators. The ac output of the pulsed alternator is rectified to provide dc to power the railgun. The design of the rectifier set and control circuitry is very dependent on the alternator characteristics; to facilitate the design and evaluation of the overall pulsed power system, a modeling tool which accurately represents the performance of the pulsed alternator while allowing easy changes to the external circuitry and controls is needed. The direct-quadrature, or Park's, transformation is commonly used to model synchronous generators for simulation, but such models are better suited for modeling disturbances around a steady-state operation point in a power system. A pulsed alternator may deliver half of its stored rotational energy in just a few mechanical cycles. This results in very high discharge torques and large armature reactions that are difficult for many commercial simulators and synchronous generator models to handle properly. In addition, the transient nature of the discharge can require that electromagnetic interaction with nonwinding structures in the generator be modeled to accurately predict the performance of the system. This paper describes a manner in which these pulsed alternators can be accurately modeled. The simulation platform SABER was chosen because of the robust modeling engine, the ease of integrating mechanical components, and the large library of existing models for a wide range of electrical components. The validity of the model is established by comparing it to experimental data gathered from the subscale alternator built by the Center for Electromechanics at The University of Texas under the Focused Technology Program.

A railgun bore with a rectangular cross section was found to be advantageous for launch of a hypervelocity kinetic energy penetrator (KEP). Since guns are rarely used to execute a mission with a single type Of round, it was important to assess the feasibility for rounds other than a KEP. An analysis is presented to support the operation of a mass-stabilized round with a high-explosive (HE) payload from the same rectangular bore cross section. Design criteria and two bounding cases are provided to illustrate the effectiveness. Removal of material from the bore insulators can provide space for the mass-stabilized projectile base diameter. The resultant HE round provides an increased capability as compared tO conventionally launched HE rounds in velocity, armor penetration, and payload capacity. Furthermore, launch of kinetic energy and HE munitions from the same launcher provides increased mission capability. Index Terms--Electromagnetic propulsion, projectile, railgun.

Many advances in electromagnetic (EM) railgun and power supply technology have been made in recent years. Laboratory experiments with railguns have demonstrated muzzle velocities of 2-3 km/s and muzzle energies 3> 8 MJ. The extension of this technology to the muzzle velocities ([greater than or equal to] 7500 m/s) and energies ([greater than or equal to] 10 GJ) needed for the direct launch of payloads into orbit is very challenging, but may not be impossible. For launch to orbit, even long launchers C>1000 m) would need to operate at accelerations > 1000 gees to reach the required velocities, so that it would only be possible to launch rugged payloads, such as fuel, water, and material. A railgun system concept is described here and technology development issues are identified. Estimated launch costs could be attractively low (< $600/kg) compared with the Space Shuttle (>$20 000/kg), provided that acceptable launch rates can be achieved. Further evaluations are needed to establish the technical and economic feasibility with confidence. Index Terms--Launch, railgun, space, system.

Focus toward advanced mobile tactical configurations for railgun power supplies has resulted in the evolution of five compulsator generations in the past 15 years. Unfortunately, the rapid maturation of the technology has tended to dilute the relative importance and application base of previous generations. Technical variance between generations has been significant, including air-core or iron-core, rotating armature or field, single-or multiphase, solid rotor or shell, and externally excited or self-excited. It is useful, therefore, to review and classify the important distinctions between the generations of machines, thus allowing proper selection of the one best suited for a given application. This paper provides an overview of the evolution of compulsators developed at The University of Texas at Austin Center for Electromechanics. Features of past and present configurations are discussed, such as machine topology, method of excitation, basic switching methods, discharge pulse shapes, and potential energy and power densities. A list of potential applications is generated for all disciplines within the armed forces. This information is used to create an application guide that can be used to select appropriate compulsator options for the mission given. An extensive list of references is also provided. Index Terms--Air-core, alternator, composite, compulsator, electric gun, high performance alternator, power supply, pulsed alternator, railgun.

The U.S. Navy is considering developing an electromagnetic railgun for use on future ships for long-range shore bombardment missions. The goals are to provide support for ground forces in a timely fashion, increase the ship-to-shore standoff distance, and improve ship survivability in combat situations. This paper describes the parameters of a notional railgun design that may be capable of supporting the Navy's needs. The Naval Surface Fire Support mission requires a railgun capable of firing high-energy projectiles for ranges of 300-500 km with a firing rate of up to 12 rounds per minute. The notional system described here is intended to meet these requirements while providing the ability to take advantage of the integrated electric drive architecture to be used on the next generation destroyer. Several important technology issues will need to be addressed before the feasibility of such a system can be demonstrated. These issues are identified and discussed. Index Terms--Naval, railgun, study, system.

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 [micro]H, while the remainder maintained their original 60-[micro]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. Index Terms--Pulsed power, railgun, railgun experiments with capacitor-based power supply.

A laboratory repetitive inductive storage power supply (ISPS) for the ignition of an electrothermal-chemical (ETC) gun is described. It is designed for delivering to an electrothermal load an energy of 400 kJ in a shot under the assumption that this energy suffices for an effective ignition of advanced consolidated propellants accounting for temperature compensation. The ISPS is battery-based. The battery has a peak power of 5 MW and is able to charge a cylindrical coil to 700 kJ. Fringe fields of the coil with and without ferromagnetic screens in static and transient conditions are analyzed using numerical simulation. A conclusion is drawn that a screen of reasonable weight cannot reduce the stray field below a susceptibility level of electronic devices that should be shielded individually. A hybrid repetitive opening switch (OS) rated 50 kA, 4 kV, 70 [micro]s breaking time is described. A commercial vacuum circuit breaker serves as the first stage. At a desired moment, it is opened, and the current is transferred to the second stage, which is an integrated gate-commutated thyristor (IGCT) that finally breaks the current. The IGCT turnoff is assisted by an inverse current injection into the second stage. The main benefit of this technique is a dramatic reduction (by a factor of ten) in the quantity of semiconductor devices in the second stage. The OS design, assisted by PSpice simulations coupled with field analysis of nonlinear magnetic components is described. Test results (1) are presented. Overall, this work supports an earlier conclusion that an 0.5-MJ ISPS, capable of delivering the first round of an eight-shot burst in 0.3 s, will occupy 0.6 [m.sup.3] net volume and weigh 1000 kg. Index Terms--Electrothermal gun, fringe field, inductive storage, opening switch.

The physical and chemical phenomena involved in the ballistic process, both in conventional guns and in electrothermal-chemical (ETC) guns, are very complex, so that optimization of gun performance requires a detailed understanding of the influence of different parameters. The initial stage of propellant ignition is critical to the rest of the process, in terms of both performance (muzzle velocity, repeatability) and safety. A better understanding of the ignition stage is thus very important in achieving improved performance. In this paper, we propose a new method and ignition sensor aimed at providing direct experimental information about the propellant ignition process. We further report on measurements of the ignition delays, and the propagation of the 'ignition wave,' performed using these sensors in experimental devices such as closed bombs and gun simulators. Index Terms--Ignition delay, plasma, sensors.

In an electrothermal-chemical (ETC) gun, electrical energy from a storage device is used to influence the combustion processes that occur within a conventional gun chamber. This typically occurs via a dense, highly nonideal plasma. The ability to simulate an ETC gun system is dependent upon a prediction of the output from the plasma generator. Improvements to a plasma generator computer model have been made as a result of understanding their operation, which has led to significant improvements in the ability to simulate plasma generator operation. However, there is still some way to go before a truly predictive capability can be achieved. This paper is concerned with some improvements to the computer simulation codes and the experimental work that lies behind these improvements. Of particular interest is the mode of the wire explosion process, where the experimental wire ablation rate is far lower than expected from simulations due to anomalous current transfer around the condensed portions of the wire. A new hypothesis is explored that attempts to explain this anomaly. This hypothesis relies upon an electrically insulating metallic vapor transition zone resulting from thermal gradients at the interface between the condensed and plasma phases of the exploding wire. Application of these ideas explains the wire fragmentation process and has also been applied to other areas of capillary plasma generator operation. Further, this hypothesis, if borne out, may have implications for other fields of research such as lightning strikes, electromagnetic gun plasma armatures, and electric armor disruption of shaped charge jets. Index Terms--Capillary plasma generator, electrothermal-chemical (ETC) launcher, exploding wires, wire fragmentation.

Surface characterization of energetic materials and their components has been a useful tool for resolving many research and quality control issues. An important application for gun propulsion is the understanding of plasma-propellant interactions (PPIs) in order to exploit plasma ignition, which offers the potential for rapid, controlled ignition of high-energy density charges. Toward this end, surface characterization using Fourier-transform infrared (FTIR) microscopy has been performed on propellants exposed to plasma in extinguished closed-bomb and open-air plasma experiments. These experiments are being performed to elucidate the properties of plasmas that enhance energy transfer and ignition, ultimately with the goal of designing an electrothermal-chemical (ETC) igniter-propellant charge for the next-generation weapon for the Future Combat System. However, analytical techniques capable of low detection limits for propellant degradation products are needed for characterizing the nature (e.g., radiation, convective heat transfer, hot particle flow, etc.) and distribution (in a packed bed or charge to ensure uniform and repeatable ignition) of plasma interaction with the propellant. Knowledge of such phenomena is valuable for validating simulations of plasma dynamics and optimizing igniter design. Reflectance spectroscopy has been used previously in our laboratory in closed-bomb studies to characterize the morphology and chemical composition of the grains from extinguished closed-bomb and open-air plasma experiments, but degradation products had proved difficult to detect. Recently, the same samples were reanalyzed using point-to-point contact attenuated total reflectance (PPC-ATR) in order to improve diagnostics for plasma-propellant interaction. Commercially available attachments have been designed to exert greater pressure on the sample for greater contact with the ATR crystal, improving sensitivity. With PPC-ATR, it was possible not only to detect but to identify the components of JA2 most likely associated with plasma degradation. Decomposition products were limited to less than a millimeter into the sample, involved secondary carbon denitration, and apparently did not migrate, strongly suggesting that plasma-propellant interaction involved denitration of nitrocelluose (NC). A chemical species with infrared absorption at ~1570 [cm.sup.-1] is apparently specific to plasma ignition (i.e., not observed with conventional ignition) and could serve as a marker compound for the extent of plasma interaction in charges with ETC igniters. Index Terms--Attenuated total reflectance, electrothermal-chemical, ETC, JA2, plasma, plasma-propellant interaction.

Solid propellant electrothermal-chemical (SPETC) gun concepts have evolved over the past decade. A common difficulty experienced by many of the 'large-scale' applications of SPETC technology has been the excessive electrical energy and power requirements needed to produce significant performance benefits. However, one aspect of SPETC gun technology that has attracted a lot of interest is in the area of plasma ignition due to its lower power requirements. It has been shown by various groups that plasma ignition offers benefits over a conventional gunpowder ignition system at large scale. High-energy electrical discharges previously used for ignition can potentially lead to adverse internal ballistics. Effort has recently been directed at reducing the ignition energy stimulus with the result that electrical discharge energies of a few tens of kilojoules are now routinely used to ignite large caliber gun system propellant charges. This not only gives improved internal ballistics but also reduces the cost, size, and complexity of SPETC gun components. An understanding of the energy transfer mechanisms involved during the ignition phase, which allows the electrical energy to be transferred to the surface of propellants, is central to reducing the electrical energy requirements, and these mechanisms are the subject of this paper. There is some evidence for radiative energy transfer being the dominant transfer mechanism for some higher power plasma igniters. However, with low power density and low energy ignition, the radiant flux is negligible in the timescale under consideration, suggesting that radiation is not the dominant process. Supporting experimental evidence from measurements of the radiant flux incident upon the propellant grain surface is offered. These measurements have been performed with the use of embedded optical fibers in translucent propellant formulations. The light collected by these fibers is input to an absolutely calibrated time-resolved spectrograph. A new hypothesis is put forward, which proposes that the energy transfer mechanism for low-power SPETC ignition is dominated by a process of metallic vapor deposition. Modeling and experimental results are presented, which support this hypothesis. Index Terms--Capillary plasma generator, emission spectroscopy, plasma ignition, SPETC launcher.

It is important to understand the physical processes contributed by the plasma injector in the development of an electrothermal-chemical (ETC) gun. To investigate basic properties of a capillary discharge, an injector was discharged in open air. Comparing the measured and the calculated plasma resistivities, the state of power balance was investigated. The capillary plasma discharged in open air showed large ejection of plasma without power-balancing, while the capillary plasma produced in the ETC gun experiment reaches near balanced steady-state as the pressure of the combustion chamber becomes appreciable. The ejected flames were monitored with a high-speed camera. Typically the observed flames were about 50 cm ~1 m long and 25 cm wide. To estimate temperature and density, a preliminary measurement of time-resolved spectra in the visible wavelength region was done. Index Terms--Capillary discharge, electrothermal chemical propulsion, plasma resistivity.

The performance of electrothermal-chemical (ETC) guns, besides being dependent on the total energy used, is also influenced by the time frame over which electric energy is deposited. This means the current pulse shaping is an important factor in increasing the pressure pulsewidth within the limit of the gun design pressure. For experiment design, it is necessary to predict the load resistance to supply the required shape of current pulses into the gun load. But it is difficult to identify the load resistance theoretically because the dynamic interactions of plasma with propellants should be understood. In this paper, it is suggested that the measured current and voltage can be utilized to develop an approximated model of the load resistance. It is also shown that the load resistance depends on the discharging voltage and current. Experimental tests have been performed with several shapes of current pulses to validate the resistance model. Index Terms--Electrothermal-chemical (ETC), load resistance, time-varying system parameter.

Two electrothermal-chemical gun systems of 20- and 60-mm calibers have been operated with different pulse power supplies at 35 and 10 kV, respectively. Dissimilarities of the injected current behavior during plasma ignition by exploding wires are reviewed. The effects of the varied plasma energy and increased loading density of consolidated propellant charges are analyzed with regard to the resulting pressure development. The improvement of the firing performance with the larger caliber still remains less satisfying. However, empirical regression functions have been found that approximate the kinetic muzzle energy as a function of the maximum pressure in the combustion chamber and can be useful for predictions. Index Terms--Electrochemical devices, electrothermal launching, guns, plasma applications.

The ignition of solid propellants by plasma jets with high temperature and density has been suggested as an improvement over conventional primers. Plasma ignition may enable the ignition of propellant charges with high loading density and/or low sensitivity, which is difficult using conventional primers. In addition, plasma ignition can provide a better control of the ballistic process, improving repeatability and enabling compensation for the propellant temperature effect. Due to the typically large volume and weight of electrical energy storage devices and pulsed power systems, which increase with the required energy, it is important to reduce the amount of plasma energy needed for ignition. In this paper, we present and analyze experiments performed in a gun simulator as well as a series of 105-mm gun firings. The aim is to improve the understanding of the plasma-propellant interaction at low plasma energies, in order to reduce the energy needed for a proper propellant ignition and smooth ballistic process. Index Terms--Ignition delay, plasma, sensors.

The realization of proper interior ballistic performance in a large-caliber electrothermal-chemical (ETC) gun requires a well-defined interaction of the plasma ignition system with ETC-tailored propellants and charge designs. Before testing the interior ballistic cycle in a large-caliber gun, basic investigations in closed vessels and firing simulators have to be performed. In order to conduct these investigations, a 70-mm firing simulator has been designed and fabricated at Rheinmetall, Unterluess, Germany. The paper gives an overview on the setup of the 70-mm firing simulator. In the following, the design of the propellant charge to be tested is described briefly. Results obtained from preliminary interior ballistic simulations on the charge setup are presented. An overview on the test program is given and results of firings at different propellant temperatures are described and evaluated. Index Terms--Electrothermal-chemical (ETC) guns, experiment, interior ballistics, plasma ignition, simulation.

In order to maximize electrothermal-chemical (ETC) gun performance, the power supply for the ETC gun driver needs to have a capability of producing a flexible current pulse. This current pulse forming can be realized by a combination of current pulses from multiple modularized capacitor banks. For this purpose, a 2.4-MJ pulse power system (PPS) has been developed. The 2.4-MJ PPS consists of eight modular 300-kJ capacitor banks (CBs). The 300-kJ CB modules are designed to operate independently and have a charging voltage of 22 kV and a current capability of 150 kA. Operation characteristics of 2.4-MJ PPS have been investigated. Various shapes of current pulse have been tried to a dummy resistive load and an ETC gun load. In this paper, we discuss some problems that take place during multiple modules operation and suggest solutions for these problems. We also show some experimental results when the 2.4-MJ PPS was applied to resistive dummy load and a 30-mm ETC gun load. Index Terms--Capacitor bank, ETC gun, 2.4-MJ pulse power system, 30-mm electrothermal-chemical (ETC) gun, 300-kJ pulse power module.

Assessment of the output of plasma devices and comparison with thermodynamic calculations makes it possible to improve the efficiency of these devices. The experiment is based on two capacitor banks (1 and 20 kJ) and on a small plasma torch used in electrothermal-chemical (ETC) basic studies. The behavior of a plasma igniter is studied using spectroscopic measurements to obtain the electronic density from [H.sub.[alpha]] line Stark width. The temperature is obtained from Boltzmann plots on copper and iron atomic lines. Under the local thermodynamic equilibrium assumption, the electron density ([N.sub.e]) depends on the electron temperature ([T.sub.e]) via Saha's law ([N.sub.e] [varies] [T.sup.3/4.sub.e]). A check on this behavior permits validation of the thermodynamic equilibrium assumption, and the pressure of the emitting area is obtained by using Dalton's law. The flight time and the overpressure generated by the plasma igniter can be compared at the Sedov-Taylor blast wave theory. Finally, we give a comparison of the pressure of the emitting area found using Dalton's law and also by applying the Sedov-Taylor blast wave model. Index Terms--Blast wave, electron density, emission spectroscopy, electrothermal-chemical (ETC), plasma arc, pressure, Sedov-Taylor.

A series of experiments has been performed to contribute to the process of fully characterizing the interaction between a hot plasma igniter and gun propellant such as occurs in electrothermal-chemical (ETC) guns. Diagnostics include pressure generation records for different interaction geometries and chemical analysis of residual propellants following plasma exposure. It is found that ignition times vary substantially by propellant even within similar classes of materials. Subsurface chemical activity has been recorded. Index Terms--Electrothermal-chemical (ETC), ignition, propellant.

A precise and continuous measurement of the velocity of projectiles inside the barrel would permit to improve the understunding 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 x 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. Index Terms--Armature, Doppler-radar, railgun.

Experiments have been carried out to explore the role of chemical effects in the plasma-propellant interaction (PPI) process. The experiments involve varying the chemical composition of a confined, ablatively 'fed' plasma, and then observing the response of the propellant (JA2, M9, or M30) via the recorded pressure traces. The combustion is terminated by rapid depressurization, allowing recovery and chemical analysis. In situ light emission measurements provide information on the relative temperatures of the plasmas investigated, as well as on the relative radiation levels at the propellant surface. The standard polyethylene (PE)-fed plasma contains carbon, hydrogen, and aluminum (the exploding wire is aluminum). A carbon-rich plasma (without hydrogen) is formed with a graphite (Gr) or lampblack (LB)-fed plasma, and a hydrogen-rich plasma with an aluminum hydride (Al[H.sub.3])-fed plasma. In addition, ammonia triborane (ATB), another hydrogen-rich reducing agent, has been tested. The results show widely varying extents of plasma-feeding for the different plasma chemistries, higher plasma temperatures (and longer duration of high temperature and radiation) for the chemistries that feed the plasma least, and post-discharge propellant combustion rates that suggest competition between the opposing radiative and pressure-producing properties of the additive. Index Terms--Chemistry, plasma, propellant ignition, radiation.

We measure some characteristic parameters to investigate the electrical discharge in different types of insulators in-the energy range 150-500 J. By varying the type of insulator and the confining geometry, we quantify their specific influence on the pressure, the dissipated energy, and the ablated mass. These considerations are useful to increase the knowledge of the physical mechanisms which play a role in the high-current plasma-wall interaction. In agreement with previous theoretical works, we show two opposite behaviors between polyethylene and polyoxymethylene in terms of ablated mass and graphite formation. Index Terms--Ablation, graphite formation, plasma-wall interaction.

This paper describes experiments to measure melt-wave erosion (MWE) damage in railgun armatures. C-armatures were launched on the Medium Caliber Launcher at the Institute for Advanced Technology (IAT), Leander, TX, and recovered by 'soft catching' in a tube filled with cotton rags. Recovered armatures were mechanically profiled to measure erosion depth as a function of position on the contacting surfaces. Although recovered armatures provide only a final snapshot of the erosion process, one can show how erosion proceeds by recovering armatures at different stages of the launch. This type of study requires the use of a crowbar switch at the breech of the railgun to rapidly shunt current out of the armature, thereby interrupting the erosion process at the desired point in the launch. We describe the experimental setup and summarize the results of 11 experiments. The experiments investigated the effect of linear current density (ranging from 35 to 45 kA/mm) and rail material (copper and aluminum) on MWE damage. Lastly, we present some thoughts on contact transition mechanisms that, in some cases, prevented the collection of MWE data. Index Terms--Current melt-wave soft catch, electromagnetic launcher, melt-wave erosion, perimeter erosion, railgun, transition to arcing contact.

As the technologies of rotating power supplies and thyristor switches advance, new methods of measuring various signals must be developed. The University of Texas at Austin Center for Electromechanics (UT-CEM) has developed new methods to measure compulsator position and speed and has made advancements in health monitoring of the thyristor switches. Technological advancements in machine design have enabled dramatic increases in machine speeds, which in turn increase electromagnetic interference. It is essential to a successful operation to have a method (reliable within this harsh environment) of measuring speed and sensing rotor position to generate gate signals for thyristor switch modules. As the modules switch larger amounts of current and voltage, it is correspondingly important to monitor the switching process, so that any damage to the system caused by a fault condition can be minimized. This paper describes sensors developed at UT-CEM to monitor speed, sense rotor position, and detect fault conditions. Index Terms--Compulsators, current measurement, electromagnetic launching power supplies, rotating machine measurements.

For new tactical gun systems to have the potential be integrated into future lightweight combat vehicles, mass optimization is a key design feature. In an electromagnetic launcher during normal solid metal armature operation, the magnetic field pressure forces the conducting rails outward, applying an asymmetric load to the bore containment. This loading can be accounted for in the containment design. When a solid armature transitions, a gas pressure is developed, causing additional force to be applied to the insulator and the containment in the insulator direction. This force generates requirements for additional strength in the insulator and bore containment and creates an additional mass penalty. Quantification of transitioning armature pressure allows for optimization of the launcher design. In this paper, we describe the technique used in measuring these in-bore pressures. Detailed are the test setup, hardware, instrumentation, and data acquisition equipment used for the experiments. Additionally, initial testing results of measured pressure are presented here along with some comparisons to theoretical average pressures across the bore area. Index Terms--Hybrid armatures, in-bore pressures, plasma armatures, pressure measurements, pressure probes, pressure tests, Titan, transitioning armatures.

This paper deals with a numerical modeling of the phenomena involved in sliding contacts in homopolar disk generators. Because of the velocity skin effect, the current distribution in sliding contacts is uneven and mainly concentrated in the rear part of the brushes. This can cause melting of the portion of the brush in the devices characterized by high currents. The knowledge of the distribution of the currents is important in order to avoid melting. An integral equivalent network formulation previously developed for the analysis of moving conductors has been modified to take into account the presence of sliding contacts. The numerical model presented is used to analyze the current distribution in a homopolar disk generator. Index Terms--Homopolar generator, integral equations, moving conductors, sliding contacts.

Finite-element (FE) analyses in electromagnetic launch usually involve coupled electromagnetic, thermal, and structural equations. The number of unknowns to be solved could range up to a million and beyond. Parallel solution using massively parallel processing systems (MPPs) is an obvious approach toward reduction of computation times. Beowulf clusters are inexpensive parallel platforms compared with the supercomputing MPPs such as the Cray T3E. They could also serve as dedicated systems. The results from parametric analyses of a sliding electrical contact problem using an FE software EMAP3D and a Beowulf cluster are reported here. Index Terms--Beowulf clusters, computational electromagnetics, finite elements, massively parallel processing, sparse matrix solutions.

In this paper, a new method for the field analysis in tubular coilguns by wavelets on the interval and equivalence theorem is proposed. By adopting the equivalence theorem, the computational domain is restricted to the conductive region while expanding the space variables by wavelets on the interval. We obtain a storage of the unknowns in a matrix form instead of the usual vector form yielding a savings of memory storage and CPU times. The solution is then obtained by a standard time-stepping method. Index Terms--Equivalence theorem, wavelet expansion.

This paper presents two numerical approaches to modeling melt-wave erosion in conductors. The first uses a one-dimensional finite-difference formulation and provides a starting point for understanding the effects of applied field and material properties on erosion speed. The second approach involves manipulating material properties in the three-dimensional (3-D) finite-element analysis (FEA) code EMAP3D. Because EMAP3D cannot eject material from the computation in the same manner as the finite-difference code, a 'pseudoerosion' algorithm was devised to render cells above the melting temperature nonconducting. A comparison of the two approaches shows results that are in good agreement for depth of magnetic diffusion, current density, and temperature rise, as well as for depth and speed of erosion. These results are encouraging because they suggest that EMAP3D could be used to model melt-wave erosion in realistic 3-D armatures. Index Terms--Accuracy, finite difference, finite element, melt-wave erosion, time-step size.

Pulsed power generators of the drum type utilize radial components of magnetic fields created by field coils located on a rotor to induce voltage on stator coils located at larger radii. Conventionally, two-dimensional (2-D) field analyses are used, and further, the field is expressed as a harmonic series and the fundamental part of the radial field is used for initial designs. Analytical series expressions for 2-D potentials and fields are utilized in this paper to derive analytical series expressions for the self-inductances and mutual inductances between the rotor and stator coils in the shape of partial sectors. The magnitudes of higher order terms are compared with that of the fundamental component. The effects of parameters such as the aspect ratios of conductor sections on the inductances and coupling constants are evaluated. Index Terms--Computational electromagnetics, coupling constant, mutual inductance, pulsed power, self-inductance.

This paper analyzes some physical effects that occur on the electrode surface in plasma-armature rail launchers when the linear current density is higher than the critical value. The melt on the electrode surface is found to be unstable. An analysis of the possible development of a number of instabilities showed that under experimental conditions, Rayleigh-Taylor and Kelvin-Helmholtz instabilities and magnetohydrodynamic instabilities due to the interaction of the current with the self-magnetic field can develop over times much smaller than the discharge time. As a result, the electrode material can enter the plasma armature, thus limiting projectile velocity. X-ray recording of the electrode surface confirmed the presence of small-scale inhomogeneity and ejection of the electrode material from the surface. Under certain conditions, the emergence of conducting metal jets from the electrode surface was detected. Index Terms--Arc discharges, electromagnetic launching, magnetohydrodynamic instabilities.

A two-dimensional model, developed previously to investigate the transport of electromagnetic fields and energy in solid-armature railguns, is simplified to study exclusively heating within the interior of the armature. The simplifying assumptions, made in order to reduce the time required for calculations, include neglecting the effects of velocity as well as the details of the physics occurring at the rail-armature interface. These assumptions are not believed to be particularly restrictive when the intent of the calculations is to investigate regions of the armature that are far removed from the rails. The governing equations, appropriate under this set of assumptions, are presented. The model is then applied to analyze current and heat transport in a double-taper sabot armature, a type of armature that is currently under active study and is known to be valuable in launching high-density tactical payloads. In particular, calculations are undertaken to investigate the manner in which a variation of certain geometrical parameters that define the shape of the armature affect the time evolution and spatial distribution of temperature, fields, and forces within the armature. Results of the calculations are studied carefully and explained by basic physical principles. Index Terms--Electromagnetic gun, electromagnetic launch, electromagnetic propulsion, railgun.

In a linear induction launcher, both the barrel and the projectile are of finite length. This causes the force-displacement curve to have a jagged shape. The reasons are: the barrel is partitioned into sections; end effects are present; dc components exist in the input currents; and the drive coils are of finite width. The first three factors were dealt with in our previous publications. This paper describes how to improve the performance of a four-section linear induction coil launcher, powered by synchronous generators, by redesigning the drive coils so that they may be fed by a five-phase source rather than the customary three. The paper presents the drive-coil design criteria, the reasoning behind the selection of the firing sequence for each of the four sections, and the improved accelerating force curve. Index Terms--Coil launchers, efficiency, linear induction launchers, polyphase currents.