Electrothermal-chemical Research Articles

Influence of high loading density charge configurations on performance of electrothermal-chemical (ETC) guns

Experiments have shown that an electrically-generated, ablation-stabilized plasma is an effective ignition stimulus in electrothermal-chemical (ETC) guns. The current generation of ETC concepts in the United States utilizes a small amount of electrical energy during the ignition process of the order of the chemical energy in conventional igniters. Thus, in order to obtain the significant muzzle kinetic energy increase that has been demonstrated in the US with ETC guns in comparison to fielded systems, a new generation of energetic materials has been developed with increased impetus and improved packing properties. The propelling charge loading density (propellant mass/initial free volume in the combustion chamber) his risen concurrently in direct fire applications from the conventional 0.85-0.95 g/cm/sup 3/ to a value up to 1.35 g/cm/sup 3/ in experimental firings. The increase in loading density and concomitant decrease in initial ullage in the combustion chamber changes the sensitivity of the charge and the multi-phase properties of the interior ballistic flow. In this paper the authors present comparisons between high and low density cases for a number of ballistic parameters. Experimental data are also examined to compare the pressure gradient in conventional guns versus high loading density cases.

Measurement of the properties of plasma from ETC capillary plasma generators

Essential to the optimization of plasma generators for electrothermal-chemical (ETC) guns is an understanding of the physics of their operation throughout the entire plasma cycle. Identification and measurement of the physical quantities within the plasma parameter space are central to such an understanding. Presented within this paper are results from a series of tests in which a capillary plasma generator was discharged into air at atmospheric pressure. The process is explored from the start of the electrical discharge, including details of the conditions within the capillary to the exit of the plasma from the nozzle of the plasma generator and its expansion into free space, X-radiography, fast-frame photography and time resolved spectroscopy have been used together to build-up a basic understanding of the operation of the capillary plasma generators used. The state of the plasma initiating wire, the plasma temperature, emissivity and velocity on exit from the plasma generator and at various locations from the plasma generator nozzle are examined. Estimates for the total radiative power loss from the plasma are also made. Results are compared with the predictions from a one-dimensional model of capillary plasma generators.

Electrothermal-chemical synthesis of nanocrystalline aluminum nitride using a metal vapor pulsed plasma jet

A relatively new process, electrothermal-chemical (ETC) synthesis, is proposed and tested for synthesizing nanocrystalline aluminum nitride (AlN) and aluminum powders. The ETC synthesis employed a plasma gun especially adapted for material synthesis. The plasma gun is powered by high magnitude current pulse (100 kA flowing for 1.2 ms) and produces pulsed, high-velocity metal vapor plasma jets. The pulsed plasma jet is directed into a reaction chamber which is filled with room temperature atmospheric pressure nitrogen (N/sub 2/) or helium (He) gas reacting with the metal vapor plasma jet. Transmission electron microscopy and X-ray diffraction have been applied to characterize the synthesized materials and confirmed that the material contained nanocrystalline aluminum (Al) and AlN whose particle size ranging 30-120 nm.

Green Farm Electric Gun Research and Development Facility: the final chapter

The Green Farm Electric Gun R&D Facility has been operated by Maxwell Technologies Inc. (MTI) for the Defense Threat Reduction Agency (DTRA) since it's inception in 1986. MTI continued to operate this workhorse facility for thirteen years under the technical oversight of DTRA until its closure in 1999. The facility supported a wide variety of both electromagnetic (EM) and electrothermal chemical (ETC) weapons concepts research and component development for the Department of Defense. In the paper, DTRA and MTI present their successes, achieved milestones, lessons learned, and recommendations for re-establishing the gun facility and its 32 MJ pulsed power system. Topics include the design of the facility, the analysis and engineering behind the 90 mm Single Shot Gun (SSG), and the 32 MJ power supply, system performance characteristics, diagnostics and instrumentation.

Plasma-propellant interactions studies: measurements of heat flux produced by hydrocarbon ablation-supported plasmas

An investigation is underway to determine the mechanisms which contribute to the enhanced performance of electro-thermal chemical (ETC) ignition systems. Measurements of the plasma properties are difficult due to the hot, low density, ionized plasma flow which interfered with the data acquisition system. The present study utilizes an infrared (IR) camera to measure the temperature of both a copper disk and propellant grains exposed to a plasma. Use of the IR camera minimizes the electrical noise interference experienced in past experiments. The copper disk was used to determine the total energy flux produced by the plasma and the propellant grains were used to determine the energy flux absorbed by the propellant. A larger average energy appears to be transferred to JA2 compared to M30 propellant with similar plasma energy inputs. While it is likely that this is due to the higher absorption coefficient of JA2, the JA2 experiments also showed a significant amount of particles on the propellant face which may have contributed to the higher energy measured.

Account of interior ballistic parameters of the launch by slurry/liquid propellant with ETC-ignition

A mathematical model and algorithms of the basic problem of interior ballistics of the electrothermal-chemical (ETC) launcher on slurry/liquid propellant (SP) is presented. Simplifying assumptions permit the equations to be reduced to the numerical integration of a quasilinear equations system in partial derivatives of a hyperbolic type for functions of two arguments, with appropriate boundary conditions. Calculated and experimental results are compared.

Hard coatings of metals and ceramics with a new electro-thermal-chemical gun technology

A new spray technology for producing hard-coatings, using an electro-thermal chemical (ETC) gun is presented. The powder particles of the coating material are accelerated by conventional solid or liquid propellant combustion products. A pulsed-plasma jet, provided by a confined capillary discharge, ignites the propellant and controls the combustion process. The powder particles are accelerated to velocities calculated to be over 1000 m/s. A high throughput and high powder consumption efficiency is obtained due to the very high carrier gas density and velocity. The plasma jet enables control of the gas temperature, and consequently influences the powder temperature. The technology capabilities have been demonstrated using several material systems such as sintered carbides to obtain coatings on stainless steel and other substrates. In order to evaluate the coating device performance, the coatings obtained were characterized chemically and physically, and a physical model describing the complex processes taking place within the gun was developed. The model is able to predict the main propelling gas and particle parameters, in particular, their acceleration and heating processes.

Energy distribution and radiative heat loss in electrothermal-chemical (ETC) closed chambers

This work investigates the impact, on electrothermal-chemical (ETC) closed chamber data analysis results, of treating the plasma energy and mass in a more detailed manner than is generally considered; in addition, the extent of energy loss due to radiation in the ETC closed chamber is addressed. An experimental ETC closed chamber data is analyzed to determine the impact of treating the plasma and the net radiation loss more exactly than currently implemented. Major observations are: (1) ionization energy is minimal; (2) dissociation energy should not be included in the data analysis; (3) high heat losses in ETC firings are expected; and (4) approximately 20% of the total electrical energy may be lost due to radiation.

Experimental characterization of plasma effects on energetic materials for electrothermal-chemical launch applications

Energetic electrical plasmas play a fundamental role in the electrothermal-chemical (ETC) propulsion concept, where they are used to provide energy conversion from an electrical power source to the combustion chamber of an ETC launcher. Recent research in the areas of large-caliber ETC gun experiments as well as small-scale plasma experiments have demonstrated the ability to alter the propellant combustion process and the chemical or physical properties of propellant samples exposed to energetic electrical plasmas. Such results have warranted continued investigations into the underlying processes involved in propellant and plasma interactions, specifically in the behavior of exposed propellant samples to plasmas operating at varying degrees of electrical power. Experimental characterization has been performed with various samples of energetic materials including JA2, CL20, RDX, M9, M30, M43, KELP, and BAMO-AMMO (BA), which were allowed to interact with plasmas having different electrical power levels. The plasma power level ranged from 60 MW to a maximum of 264 MW, and total energy transferred to the plasma capillary ranged from 17 to 70 kJ. Propellant samples were analyzed using a Fourier transform infrared spectroscopy (FTIR) technique, which was used to determine the effects of plasma exposure on the energetic materials. Preliminary results from the experiments indicate that some propellant formulations are easily altered (either chemically or physically) or ignited by a plasma while others are less sensitive to the plasma exposure. In addition, the effects of the plasma are found to be minimized with the introduction of protective films even for plasmas of high power and energy levels.

The UK electric gun programme in 1998

The UK is undertaking research into both electromagnetic (EM) and electrothermal-chemical (ETC) propulsion for future weapon systems. In the EM field, efforts have been concentrated in the railgun armature area. Recent work has tried to reduce the parasitic mass of base-push solid armatures, since this leads directly to lower launch energies and greater overall system efficiency. The aim is to understand the many property requirements needed to allow velocities over 2000 ms/sup -1/ with full mass payloads. The use of multi-material armatures has been shown to be beneficial both theoretically and through examination of hardware recovered after firings at Kirkcudbright. Conditions at high velocities and energies have been found to be vastly different from those at more moderate levels, and support the UK decision to tackle the problems in these difficult regimes. Recent ETC research effort has spanned the topic from the fundamental to the systems' levels. The fundamental plasma-propellant interactions continue to be studied. Enhanced gas generation rates (EGGR), both during and in some cases after the electrical discharge, have been studied in a wider variety of propellant types and geometries. The scaling of plasma generators has been investigated. A plasma jet type of plasma generator has been operated successfully in open-air at a peak electrical power of 3 GW. The concept of muzzle velocity control of artillery guns known as Smart Gun has also been studied. Analysis indicates that the concept has the potential to significantly improve artillery precision.

Simulation of plasma radiation in electrothermal-chemical accelerators

The energy transport by radiation in hot, dense plasmas-as produced in the temperature/density range of electrothermal-chemical (ETC) guns-is the dominant effect in the energy balance. For this reason, radiation is an important factor in the initiation, control and optimization of the ignition and combustion processes of propellant charges by a plasma in such devices. Results of published experiments also indicate a strong interaction between the plasma radiation and the surrounding propellant and combustion products. For the optimization of ETC guns, a program has been developed using the approximate method of "partial characteristics" for the solution of the radiative transfer equation (RTE) under the assumption of local thermodynamic equilibrium (LTE). The computation of the RTE depends on frequency dependent absorption, emission and scattering coefficients, which describe the interaction of the radiation field with matter. Therefore, the IABG has developed the program POCAL which calculates absorption, emission and scattering coefficients for molecular bands, atomic and ionic lines as well as continuum radiation using a model of detailed configuration accounting. Having computed the radiation transport, this solution may then be used in the conservation equations and to establish the direct transfer of radiative flux to the surrounding propellant charges.

EML technology research in China

In the last 20 years, electromagnetic launch (EML) technology has been developed in the US and Europe, and the electromagnetic (EM) launcher has been regarded as a prospective equipment of propulsion. In the early 1980s, experiments on EML technology were done in some Chinese laboratories, and a lot of work has been finished in order to systematize the theory of EML technology in some universities and institutes. One result is that the Chinese book 'Principles of Electric Guns' was one of the first publications of its kind in the world. Now, there are seven organizations which are participating in the research or experiments in China. They are the Southwest Institute of Fluid Physics (SWIFP), the Institute of Plasma Physics (IPP) of the Chinese Academy of Sciences, Nanjing University of Science and Technology (NUST), Huazhong University of Science and Technology (HUST), Ordnance Engineering College (OEC), Xidian University (XU) and Institute 208 of Chinese Ordnance Industry (I208COI). The invention patents relating to the EML technology have been applied for; three books which directly relate to the EML technology have been published; some papers have been translated, compiled and published internally. The 15 papers submitted to this symposium involve the rail gun, the coil gun, the electrothermal-chemical (ETC) gun, pulsed power conditioning, energy storage, and an overview.

The interaction of electrothermally supplied energy with compact solid propellants

The realization of the electrothermal-chemical (ETC) technology depends on the electrothermally controlled combustion of solid propellants with increased loading density. In order to design a powder charge and an appropriate converter system of electrical energy for an ETC-gun setup, detailed knowledge of the interaction between the electrical energy input and the combustion of the powder is necessary. Basic investigations have been performed at TZN with various closed vessels to understand the fundamental parameters that influence the combustion of compacted solid propellants. The paper presents the results obtained in experiments in a 500 ml closed vessel with compacted JA2-powder in the form of disks and electrothermal energy input. The setup of a closed vessel is given and characteristic experiments as well as their parameter variations are described and discussed.

Analysis of high-temperature materials for application to electric weapon technology

High-power and temperature pulsed-power electronics can be exploited by future military combat systems using advanced electric weapon concepts such as electrothermal-chemical (ETC) and electromagnetic (EM) gun technologies. The results of experiments conducted demonstrate the electrical behavior of SiC and metal ohmic-contact layers as a function of thermal stress. It has been determined from these experiments that both titanium (Ti) and tantalum (Ta) metalization structures will provide a stable electrical ohmic-contact with n-type SiC at elevated temperatures for short bursts that are considered relevant for pulsed-powered electric weapon technologies. The Ti-SiC structure exhibited a stable current-voltage (I-V) characteristic to as much as 800/spl deg/C for a 10-min burst, while Ta metalizations provided a stable I-V characteristic on SiC even after a temperature burst of 1000/spl deg/C for as long as a 3-min interval. For samples of n-type, 4H SiC, metalized with (Ti), the standard deviation in resistance (resistivity) of the measured samples is less than 0.17 ohms for a sample having an average resistance of 4.45 ohms. The Ti-SiC sample was exposed to an elevated temperature range of 300-1,120/spl deg/C. For the Ta contact on SiC, the standard deviation in resistance is 0.05 ohms for a sample having an average resistance of 4.25 ohms over a temperature range of 600-1120/spl deg/ C. The experiments showed that for both Ti and Ta metalized SiC samples, the change in resistivity of annealed samples is between 3.8% and 1.2% compared to the average values of sample resistance based upon the I-V measurement technique used.

An analysis of ballistic parameters/mechanisms affecting efficiency in electrothermal-chemical (ETC) guns

Historically, electrothermal-chemical (ETC) gun firings have failed to achieve ballistic performance as measured by ballistic ratios (BRs) commensurate with those for conventional powder guns. In this paper, the ETC ballistic cycle is examined in an attempt to identify possible aspects of the ETC process leading to the lower-than-expected BRs. Results indicate that at least a partial answer may be the energy required to evaporate and dissociate the polyethylene and metal electrodes of the plasma capillary, state changes which occur in the combustion gases due to the higher temperatures of the plasma and radiative energy losses.

Parametric study of high energy plasmas for electrothermal-chemical propulsion applications

Theoretical calculations are performed with a one-dimensional (1-D), steady state, isothermal computer plasma model to define plasma output parameters for various input electrical energies and capillary radii of relevance to the electrothermal-chemical (ETC) propulsion concept. Three capillaries of 1.92, 4.75, and 7.0 mm radius, and a fixed length of 11.84 cm, were chosen for this study with input currents between 30 and 350 kA. Plasmas are categorized according to their total power and energy levels (based on a 3-ms pulse width) and are compared with respect to their resistance, exit pressure, and core plasma temperature. The input power ranges from 0.17 to 1.89 GW, for input energies from 0.49 to 5.80 MJ, which is considered suitable coverage for ETC ignition through ETC enhanced propulsion concepts. The study shows that the range of resistance, pressure, and temperature are 12.8-195 m/spl Omega/, 19.8-2000 MPa, and 2.9-13.5 eV, respectively, for the chosen capillary geometry. Flow conditions for plasma calculations include choked (no pressure boundary) and unchoked (450-MPa pressure boundary) for some calculations. Results from the computational model and interpretations from the perspective of capillary implementation into ETC propulsion concepts are also included.

Recent advances in electrothermal-chemical gun propulsion at United Defense, L.P

Electrothermal-chemical (ETC) propulsion uses an electrical energy pulse as a means of initiating and, in some cases, controlling the combustion of propellant charges. The electrical energy pulse is generated by a pulse-forming network (PFN) and is dissipated within the gun in the form of a plasma. The high-energy density plasma resulting from the electrical energy dissipation has been demonstrated to be highly effective in ignition of solid and liquid propellant charges. Variations in electrical dissipation rate have been shown to affect propellant gasification rate and, therefore, pressurization throughout the ballistic cycle. This paper discusses recent development results at United Defense, L.P., in the areas of ETC solid propellant ignition and performance improvement, ETC temperature compensation, and a high-performance hybrid liquid/solid propellant ETC charge.

Demonstration of a modular, repetitive-fired 1/4-MJ PFN for ETC guns

A compact pulse forming network (PFN) storing 1/4 MJ at 16 kV has been assembled and tested to validate its performance as a driver for an electrothermal-chemical (ETC) gun. The PFN features a triggered vacuum gap output switch; shielded inductors at energy density near 10 MJ/m/sup 3/, with low external magnetic fields; 25 MJ/m/sup 3/ metallized electrode, PVDF dielectric capacitors; and it computer-based controller. The PFN was repetitively fired at 1/3 Hz in 5-shot bursts. Variability of the output energy over the burst was less than 2%. Energy density for the overall PFN module was 1.2 MJ/m/sup 3/ and the energy transfer efficiency predicted for a 12-module PFN system to an ETC gun like load was 85%.

On the feasibility of mounting an electrothermal chemical gun system on a combat vehicle

The US Army Research Laboratory (ARL) has completed a study on the feasibility of mounting an electrothermal chemical (ETC) gun pulser on an M1 Abrams main battle tank, as a possible variant for the next generation of this weapon system. The ETC gun will provide enhanced lethality without increasing weapon caliber or propellant requirement. This was part of a larger study involving other US Army research elements. The ARL approach examined capacitor and inductor based energy storage and pulse forming networks. With adequate prime electrical power, available from a thermal engine driven generator, the above systems were smaller in size than alternative rotating machine based systems, and can sustain rapid fire as long as they have projectiles. These systems also have the advantage of modular construction, allowing the components to be placed wherever there is space available in the tank. This resulted in a majority of the system being capable of being placed in the tank turret, which had the advantage of an easy coupling of the gun pulser to the gun breech. This system passes continuous charging power through slip rings to the pulse forming network in a fully rotatable turret, avoiding the dubious practice of passing pulse power through sliding contacts.

Consideration of pressure gradients in electrothermal-chemical (ETC) guns

In this paper we explore possible physical causes for lower performance than anticipated in experimental electrothermal-chemical (ETC) firings using a one-dimensional code coupled to a plasma code to account for interaction between the rate of plasma flux and external pressure. Gradients in physical variables such as pressure are examined in comparison to conventional solid propellant guns.