For all the centuries of refinement that separate a modern rifle from a Renaissance arquebus, the basic idea has not changed. That idea is to convert the chemical energy stored in an explosive into kinetic energy stored in a speeding projectile.
But there is nothing to say that chemicals have to provide the kick. Indeed, the physics of chemical explosions put a limit on what a conventional gun can realistically accomplish. In an attempt to get around those limits, America’s navy is trading explosives for electricity and working on a railgun, a weapon designed to hurl shells at seven times the speed of sound. Prototype systems at the Naval Surface Warfare Centre, in Virginia, have been firing since 2006. If all goes according to plan, the first firing tests on board ship will take place next year.
Railguns are a staple of science fiction. The idea dates back to 1919, when a French inventor called André Louis Octave Fauchon-Villeplée filed a patent for an “Electric Apparatus for propelling projectiles”. Unlike other sci-fi staples, such as laser blasters and particle-beam weapons, railguns are conceptually simple—simple enough, indeed, that a hobbyist can build one at home (many do, and upload footage of their creations to YouTube).
As the name suggests, a railgun dispenses with the enclosed barrel employed by explosively propelled artillery in favour of a pair of electrically conductive rails (see diagram). This creates a linear electric motor. The motor’s moving part—its armature—sits between the rails and carries a projectile. When someone presses the trigger, current flows up one rail, through the armature and down the second rail. This generates a set of magnetic fields that accelerate the armature, and thus the projectile, forward along the rails and propel it out of the muzzle of the gun.
That, at least, is the theory. Building a useful weapon has proved tricky in practice, for such a device requires a great deal of power. The currents involved—millions of amps—are difficult to generate, and they place huge stress on the system. The same force that flings the projectile out of the gun also tries to force the rails apart. The faster the muzzle velocity, the stronger the rails must be, both to avoid buckling and to resist erosion from the friction created by the accelerating armature. Early attempts at building a railgun were plagued by the need to replace the rails frequently, sometimes after every shot.
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