THE RAIL GUN

There are many reasons, not all of them military, for accelerating chunks of matter to ever higher velocities. Elementary particles can be accelerated to within a hair of the speed of light, but macroscopic matter could, until recently, be significantly accelerated only either by rocketry (which takes a long time to accelerate its load to a high velocity), or by a gun.

But the gun has, in a sense, reached perfection: certain guns can shoot projectiles with a velocity close to the theoretical limit. A gun accelerates its projectile by the expansion of an exploding gas in the barrel. But no matter how long the barrel and how powerful the explosive, the gas cannot expand much faster than the velocity of sound. That does not mean the velocity of sound in cold air (which is comparatively slow), but the velocity of sound in the expanding gas, which depends on its chemical composition and temperature. By choosing the right gas and a high temperature (preheating the gas in a "two-stage" gun), it is theoretically possible to reach muzzle velocities in excess of 5 km/sec, (11,190 mph), and guns approaching that limit have, in fact, been built.

But for some applications that is still far too slow. It is, for example, not even half the velocity needed to escape the gravitational field of the earth (11.2 km/sec), i.e., to shoot something into orbit.

The only part of an electromagnetic force we shall need here is the one utilized in almost every electric motor: when a current flows through a magnetic field, a force acts on the conductor carrying the current; its direction is perpendicular to both the field and the current. For example, if a current (I) flows along a fixed rail through a movable cross-piece as in the figure and back through the other fixed rail, that current will give rise to a magnetic field B directed downward, and the force on the crosspiece will be as indicated, so that it will push a projectile forward along the rails.

Although the principle is there, in this primitive form the rail gun would probably not achieve the muzzle velocity of a BB gun. Yet two improvements will make it surpass the explosive-charge gun. First, the friction of the cross-piece against the rails can be eliminated by making the former of a fuse-like material, so that it melts at the first surge of current; the current keeps on flowing in the form of an arc discharge, which moves forward just as the cross-piece did, but without friction.

Second, the forward force is proportional to the square of the current; to achieve very high velocities of the projectile, currents of the order of millions of amperes ("amps") are needed. (The current flowing through an industrial 3 HP electric motor is about 10 amps.) It is possible to achieve tens of thousands of amps by a number of methods such as storing electrical energy in banks of capacitors and then shorting them through the rail gun. But that is not enough, and an entirely new method was devised to raise the current to almost one million amps: at the moment when the current surges through the rail gun, its two rails are shot together with explosives.