Michael J. Veracka

Characterization of the Velocity Skin Effect in the Surface Layer of a Railgun Sliding Contact

We present a characterization of contact velocity skin effect (VSEC), which is a major velocity- and efficiency-limiting effect at a railguns sliding contact. Despite enormous contact forces, the armature remains separated from the rail by a thin layer, 4 to 12 A thick. Evidence suggests VSEC is also the primary mechanism responsible for the contact voltage drop. VSEC effects are seen in both electromagnetic launcher (EML) efficiency and breech voltage. We compare theoretical predictions of system efficiency and breech voltage to experimental measurements for both a conventional and an augmented railgun. The characterization of VSEC extends our previous theoretical work in this area and provides new insights into the physics of EML operation, especially with regards to the armature and sliding contact. VSEC is a significant energy loss mechanism and heat source, possibly contributing to contact erosion and transition. We propose a similar VSEC mechanism to explain velocity saturation and efficiency roll-off in plasma and hybrid armature railguns, as well as arc restrike.

The Maximum Theoretical Efficiency of Constant Inductance Gradient Electromagnetic Launchers

The maximum theoretical efficiency of constant inductance gradient electromagnetic launchers (EMLs) is analyzed and discussed. The maximum theoretical efficiency is a parameter needed to calculate the EMLs efficiency. Constant inductance gradient EMLs include the conventional railgun, the augmented railgun, and the conventional helical launcher. The maximum theoretical efficiency of an EML is dependent on its geometry and the manner, or mode, in which it is powered. In the lossless case, the conventional railgun, the augmented railgun, and the conventional helical launcher are capable of 50% maximum efficiency when operating in constant current (CC) mode. Conventional and augmented railguns can achieve 100% maximum efficiency when operating in zero exit-current mode. While zero exit-current mode promotes high efficiency, this mode can reduce EML lifetime since it requires current levels much higher than those found in CC mode. The high-efficiency helical launcher, presented and analyzed here for the first time, combines 100% maximum theoretical efficiency with the low-current benefits of constant-current mode.

Solid-Projectile Helical Electromagnetic Launcher

Helical electromagnetic launchers (HEMLs) can operate at significantly lower currents and higher efficiency in comparison to conventional railgun and induction coilgun launchers. The HEMLs versatility is due, in part, to its large inductance gradient which is typically two to three orders of magnitude greater than a conventional railgun and can be tailored to almost any value in that range. Historically, however, HEMLs were not considered practical since they consisted of a hollow projectile (i.e., armature that is accelerated on the outside of a stator coil). This investigation demonstrates, for the first time, a 40-mm bore times 750-mm length solid-projectile HEML, where the armature is accelerated on the inside of a stator coil. The goal of this paper is to demonstrate the practicality of solid-projectile HEML concept and to measure its performance. Numerous successful tests were conducted. The highest velocity measured for a 170-g projectile was 64 m/s.

Solid-Projectile Helical Electromagnetic Launcher With Variable Gradient Stator and Magnetically Levitated Armature

The design and operation of 40- and 70-mm-bore solid-projectile helical electromagnetic launchers is presented and discussed. The 70-mm launcher demonstrates the first reported magnetically levitated zero hoop-stress armature and is tested in both constant- and variable-gradient stators. The theory of variable-gradient launcher operation is presented and shows that a properly designed variable-gradient launcher has efficiency independent of velocity but dependent on a new parameter called the launcher impedance constant. The variable-gradient launcher can also behave as an ideal launcher operating at 100% of its maximum theoretical efficiency independent of velocity if the ratio of the system resistance and impedance constant meets other conditions. This investigation also introduces the concept of launcher impedance and shows that variable-gradient launchers with high impedance are desirable in terms of efficiency.

The Specific-Force Performance Parameter for Electromagnetic Launchers

A new electromagnetic-launcher (EML) performance parameter called the specific force is presented and analyzed in this paper. The specific force is the second derivative of the EMLs force equation with respect to current and represents the force generated by the EML per unit square ampere, i.e., the EMLs current efficiency. The specific force is independent of operating current and is defined for EMLs utilizing linear and nonlinear magnetic materials. The second derivative is termed as the specific force, since it unifies the various EML geometries so that only one force equation is required. The specific force, together with the energy-conversion efficiency, can be used as criterion to evaluate and compare EML geometries for various applications. The specific force for conventional railguns, augmented railguns, conventional helical launchers, and high-efficiency helical launchers is derived in this paper. The experimental performance of conventional railguns, augmented railguns, and conventional helical launchers are also analyzed in terms of their specific-force parameters.

Automatic Multishot Operation of an Electromagnetic Launcher

The Naval Research Laboratory has developed an automatic electromagnetic launcher system for countermeasure deployment. The launcher operates at 1 Hz and can fire bursts of one to six 44-mm2 projectiles. The launcher is controlled from a computer that permits selection of the bank charging voltage and the number of shots. The present experimental setup consists of a railgun, an autoloader, and a rapid charger with computer control.

The Voltage–Current Scaling Relationship and Impedance of DC Electromagnetic Launchers

Electromagnetic launchers (EMLs) including railguns, helical guns, and coilguns have been proposed for numerous applications. However, the scaling characteristics of EMLs are unknown. This paper examines the nonlinear operating characteristics of EMLs and derives the operating voltage and current scaling relationships as determined by the mass and velocity of the projectile and the length of the launcher. The scaling relationship is derived using the kinetic power relationship common to all types of EMLs. The scaling relationship is expressed as a simple function of the projectile velocity, projectile mass, and launcher length. This paper also defines the impedance of an EML. The accuracy of the scaling relationship and the EML impedance are verified in a comparison with the experimental results of practical EMLs. The effects of resistive losses and inductive energy storage on the scaling relationship are included in the analysis.

Demonstration of a Reversible Helical Electromagnetic Launcher and Its Use as an Electronically Programmable Mechanical Shock Tester

A reversible helical electromagnetic launcher (R-HEML) that is able to accelerate and decelerate a projectile is presented and discussed. The R-HEML in this paper has a hollow-projectile geometry and has a barrel length of 750 mm and a bore diameter of 40 mm. The R-HEML is powered by a three-module sequentially fired capacitive pulse-forming network that has a total energy storage of 1 MJ. The projectile mass is typically on the order of 700 g. The largest measured acceleration is 8000 m/s2, and the largest measured deceleration is 15 000 m/s2. Pulselengths for the acceleration and deceleration pulses varied from 5 to 14.5 ms. A modified version of the R-HEML is constructed and used in an industrial setting to generate high-g shock and vibration pulses to characterize materials and devices. Utilization of the R-HEML in this manner is one of the first industrial applications of an electromagnetic launcher.

Design of Low-Current High-Efficiency Augmented Railguns

Properly designed augmented railguns (ARGs) can operate at significantly lower currents and higher efficiency than conventional non-ARGs. This paper analyzes the performance of a two-turn and a three-turn ARG showing the design parameters that are needed to achieve low-current high-efficiency operation. The ARG of this paper has a 40-mm square bore and is 820 mm long. Typical operating currents are from 150 to 300 kA with pulsewidths of approximately 6 to 8 ms. Projectile mass is typically in the range of 130 to 400 g. Projectile velocity is variable from 50 to 250 m/s. Experimental performance is compared with the theoretical predictions of the recently developed electromagnetic launcher (EML) equation. There is a good agreement between measured efficiency and theoretical predictions providing further confirmation of the EML equation. Deviations between measured efficiency and theoretical predictions are attributed to a poor sliding contact. The data also provide further evidence of the recently reported velocity-skin-effect in the contact.

The Velocity and Efficiency Limiting Effects of Magnetic Diffusion in Railgun Sliding Contacts

The authors identify and characterize a velocity and efficiency limiting effect called contact velocity skin effect (VSEC) which occurs in the interface of a railguns sliding contact. Despite enormous contact forces, the armature will remain separated from the rail by a thin adsorbed layer, on the order of 10 to 15 angstroms in this investigation. Electrical conduction through the layer is via quantum tunneling which is more resistive than classical conduction through the conductor. A comprehensive theory to account VSEC is presented and is applicable to all constant gradient electromagnetic launchers. The theory not only accounts for the velocity-limit effect, but also predicts a maximum efficiency. Theoretical parameters include the inductance gradient, system resistance, projectile velocity, and sliding contact area. Theoretical predictions are compared to experimental data from a conventional and an augmented railgun. The augmented railgun is a 3-turn augmented system with 40 mm bore x 750 mm length typically operating at 250 kA peak current with a projectile velocity of 300 m/s. The conventional railgun is 40 mm bore x 4000 mm length which typically operates at 850 kA with a projectile velocity of 1600 m/s. The characterization of VSEC predicts velocity saturation and efficiency roll-off in electromagnetic launchers and provides new insights into EML operation and physics especially with regard to the armature and its contacts.