The development of rail guns has been spurred in recent years by their potential as antimissile weapons and even as replacements for the conventional cannons used by tanks and warships. But most rail guns have a fatal disadvantage: firing them generally does so much damage to the launchers themselves that they must be repaired or replaced after one or two shots.
The damage is caused by sliding. A rail gun is based on two rails that conduct electricity, rigidly mounted inside a strong steel barrel. A typical rail-gun test projectile is a plastic cube or cylinder backed by a thin sheet of metal, which remains in contact with both rails as it is shot down the bore of the gun. A strong electric current is fed to the rails, and this vaporizes the metal sheet behind the projectile, forming plasma, an electrically charged, superhot gas. The electric arc through this plasma completes a circuit, producing the electromagnetic field needed to drive the projectile forward. As the rail-gun projectile (with its plasma arc) slides between the metal rails that carry the current, the rails are burned and ripped.