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Electromagnetic Launcher [EML]

An Electromagnetic Launcher [EML], which is sometimes referred to as an electromagnetic railgun, uses electricity to propel a projectile. Electromagnetic launchers have been widely investigated because of the potential to achieve velocities exceeding those that can be practically attained in powder and other thermodynamic guns, in which the maximum velocity is limited by the specific energy of the known propellants. If high enough velocities could be obtained efficiently, electromagnetic launchers would have important military, scientific and commercial applications. There is another class of electromagnetic accelerators called "coaxial launchers". These launchers are basically linear synchronous motors and are well known in the prior art. There is no physical contact between the object being accelerated and the accelerating tube. In some designs, the electric-to-kinetic operating efficiency can exceed 99%. Coaxial electromagnetic accelerators suffer no deterioration and can be used an unlimited number of times without breakdown. Unfortunately, the efficiency and overall performance of coaxial launchers is not very high for small calibre bores.

An EML typically consists of two parallel "rails," which are made from a material that conducts electricity, and which are attached with a narrow space between them to form a barrel, similar to the barrel of a typical gun, The projectile that is propelled from an EML has attached to the back of it an "armature," which is made from a material that conducts electricity. An EML propels a projectile with a very high electric current that is applied to one of the rails, and the current arcs across the rear of the projectile causing all or part of the armature to vaporize into a high temperature gas or "plasma," A magnetic field created by the plasma interacts with the current in the armature and exerts a force on the rear of the projectile, driving it to the length of, and out of, the barrel.

Electromagnetic accelerators designed for launching projectiles at hypervelocities must be evacuated in order to eliminate the disturbing effects of atmospheric drag. Of course, when operating in the vacuum of space, this is not a problem.

KE propulsion systems include the categories of power supplies, power conditioning and storage, and launcher and barrel. Advanced energy and power conditioning technologies are essential to operating KEWs that have a capability exceeding conventional munitions. Improvements in batteries, capacitors, inductors, pulse compression networks, homopolar generators (HPGs), and compulsators can support this goal. Batteries store energy in the chemical bonds formed by reactions between the electrodes and electrolytes and tend to have a relatively high internal resistance that restricts their rapid discharge. The newer lithium batteries have lower internal resistance for faster discharge capabilities and can store higher energy densities than normal batteries. Capacitors store energy in the electric field set up across their dielectric by the disposition of positive and negative charges. Capacitors amplify current, charging at low currents and discharging at high currents (same voltage level). Electrolytic capacitors are relatively small-energy, low-voltage devices (700-2,000 V). Capacitors are available with voltage ratings up to 100 kV. Inductors store energy in the magnetic field set up around a current-carrying conductor and can generally store higher energy densities than capacitors. Inductors essentially amplify the voltage while keeping the current constant. However, since they have shorter time constants than capacitors, they internally dissipate stored energy faster. Cryogenically cooling the inductor drastically reduces its internal resistance but still only increases the time constant from milliseconds to seconds, whereas capacitors can store energy for 1,000 sec or more. Pulse forming networks (PFNs), consisting of capacitors and/or inductors connected by appropriate switches, accept input power at a low level and store and sequentially discharge this power at subsequent stages [at a higher power level and a shorter (compressed) pulse time]. An HPG is a variant of a direct current (DC) machine. In its simplest form, it is a Faraday disc rotated in an axial magnetic field. The rotor is brought up to speed by an externally applied torque, and KE is stored in the rotating disc. Then, the external drive is removed, and the HPG becomes a generator as the conductive disc rotates in the magnetic field and the external circuit load is applied. A compensated pulsed alternator, or compulsator, is a variant of the simplest alternating current (AC) synchronous machine, which has its output generated in a rotating armature winding (usually in the 10- to 15-kV range). The compulsator incorporates an additional stationary winding, which is connected in series with the rotating armature winding. This compensates for the internal inductance at one point in each cycle (usually at peak voltage, where the windings are opposed and the inductance is a minimum), at which point the internal impedance is minimum and a fast (submillisecond) pulse of current output is generated. Early railguns used metal armatures. During acceleration, the metal armatures were heated by the current pulse. If the contact is ideal and the current in the metal armature is distributed uniformly, there exists a theoretical velocity limit due to thermal degradation of the armature material. The theoretical velocity limit for a copper or an aluminum slug-shaped, unloaded armature having a length of about 1 cm is several tens of kilometers per second. The theoretical limit increases proportionally to the armature length. Experimentally, the sliding contact experiences arcing at velocities about or below 1 km/s. No increase of velocity with armature length is found.

Thunderbolt EML, was a ground-based demonstrator operated by the Defense Nuclear Agency. The goal of the Thunderbolt program is to continue development of the Thunderbolt EML to enable the Strategic Defense Initiative Organization (SDIO) to deploy a spaced-based kinetic energy weapon system.

The Thunderbolt EML is fired by injecting the projectile by means of a single-stage light gas gun, A gas gun is used because it reduces the barrel erosion that occurs when the projectile is electromagnetically accelerated, When the projectile enters the EML from the gas gun, it is traveling at about 1 kilometer per second, At the entry point into the barrel, current is applied as described above and the projectile is electromagnetically accelerated.

In previous EML experiments, the maximum velocity attained after electromagnetic acceleration has been 6 kilometers per second, The focus of this particular contract is to determine why there is a 6 kilometers per second ceiling after electromagnetic acceleration and to design or redesign the Thunderbolt EML to 10-kilometers per second velocity. The 6-kilometers per second ceiling experienced in the Thunderbolt EML is caused by erosion of material from the inner walls of the barrel that occurs when the projectile is electromagnetically accelerated, as the ablated mass adds to the plasma mass already present and reduces the projectile acceleration because of the extra mass and drag of this material. Research on various EMLs supports the hypothesis that radiative ablation is a significant factor as well.

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Page last modified: 21-07-2011 13:04:25 ZULU