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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 EML's 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 Iossless 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. Index Terms--Coilguns, electromagnetic launching, linear motors, railguns.

Helical electromagnetic launchers (HEMLs) can operate at significantly lower currents and higher efficiency in comparison to conventional railgun and induction coilgun launchers. The HEML's 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 x 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. Index Terms--Coilguns, electromagnetic launching, linear motors, railguns.

We present a characterization of contact velocity skin effect (VSEC), which is a major velocity- and efficiency-limiting effect at a railgun's 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. Index Terms--Coilguns, electromagnetic launching, linear motors, railguns.

Railguns involve both static and sliding electrical contacts, which must transmit the large transient electrical currents necessary to impart high forces onto a projectile for acceleration to hypervelocity. Static electrical contacts between metals initially take place through small asperities, or 'a-spots', distributed over the contact area. The voltage developed across the interface is directly related to the contact temperature and force, the number of a-spots, the thermophysical and mechanical properties of the contacting materials, the current history, and any interfacial materials that may be present. To physically simulate some of the conditions attained within a railgun, a pulsed current static electrical contact experimental facility has been developed at the Naval Research Laboratory. This facility employs a 500-kN capacity servohydraulic load frame equipped with an electrically insulated load train to establish a contact force on interfaces between metals through which a pulsed current is transmitted. The time dependent evolutions of the voltage drops across the interfaces, as detected by probes pushed into the contacting materials, are recorded during a 40-kA peak current pulse having a 300-[micro]s rise time with peak current densities on the order of 50 kA/[cm.sup.2]. The interface stack is assembled from a 12-mm outside diameter annular disk of metal with a 6.3-mm hole which is compressed between two hollow pedestals of a second metal. The evolution of the voltage drop across an interface during a pulse will be described as a function of initial contact pressures, current density, and polarity for dissimilar (AI/Cu) metal contacts. Thermal effects on the surfaces, including localized melting of the interlace materials, were also investigated. Index Terms--Dissimilar metals, electrical contacts, pulsed current, railgun.

We present efficiency and scaling relationships for dc (i.e., noninduction) constant inductance gradient electromagnetic launchers. We derive expressions for electromagnetic force, efficiency, back-voltage, and kinetic power in terms of electrical circuit parameters. We show that launcher efficiency is a simple function of armature velocity and the launcher's characteristic velocity. The characteristic velocity characterizes the launcher and is the product of two new parameters: the mode constant and launcher constant. Mathematically, the launcher must operate at its characteristic velocity for 50% maximum efficiency. The mode constant reflects the manner in which the launcher is powered and its maximum efficiency. The launcher constant reflects the geometry of the launcher. We consider two modes of operation: constant current and zero exit current operation. We develop the ideal electromagnetic launcher concept and define it as operation at 100 % maximum efficiency at all velocities. We also develop the concept of same-scale comparisons, that is, that electromagnetic launcher comparisons should be done with equal bore diameter, launcher length, projectile mass, and velocity. Finally, we present a comparative analysis based on experimental data of same-scale constant gradient electromagnetic launchers for conventional railgun, augmented railgun, and helical gun launchers in terms of the launcher constant, inductance gradient, bore diameter, bore length, system resistance, and armature (i.e., projectile) velocity. Index Terms--Coilguns, electromagnetic launching, linear motors, railguns.

The design, development, and testing of a medium-bore helical coil electromagnetic launcher (HCEL) that can accelerate a 500-gram mass to 100 m/s with an efficiency of 13.2% is presented and discussed. The HCEL has a bore size of 40 mm and a length of 750 mm. The measured HCEL inductance gradient of 150 mH/m is more than a 100 times larger than a conventional railgun. The HCEL is powered by a sequentially fired pulse forming network consisting of four 15-kJ electrolytic capacitor banks. Typical PFN V-I characteristics are approximately 15-kA peak current at 550 V for 8-ms pulse length. A computer model of the HCEL launcher is constructed in PSPICE and accurately predicts launcher performance within 25%. Theoretical and numerical predictions of launcher performance are compared with experimental measurements, including projectile velocity, launcher voltage, launcher current, and inductance gradient. Index Terms--Coilguns, electromagnetic launching, pulse power systems.

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 EML's force equation with respect to current and represents the force generated by the EML per unit square ampere, i.e., the EML's 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. [ABSTRACT FROM AUTHOR]