Swoosh! It's a Railgun

By Frederic Golden

Swoosh! It 's a Railgun

A new electromagnetic launcher for earth and space

Ever since the city fathers of Florence ordered up brass cannon and iron balls in 1326 to defend themselves against the city of Lucca, in the first recorded use of explosive-powered metal artillery, gunsmiths have been trying to perfect their weapons. Guns have improved over the centuries--in range, accuracy and deadliness--but their firepower has always depended on the rapid expansion of exploding gases down a tube, which pushes the bullet forward. The maximum speed such gases--and thus the gun's projectile too --can reach is severely constrained. None of the particles in the gases can travel faster than the speed of sound through the gas, at best about 10 km (6 miles) per sec.

Rockets, which are also driven by exploding chemicals, can exceed these sonic limits because the combustion takes place in the projectile itself. But rockets also operate under handicaps. So large are the fuel requirements for reaching orbital speed of 8 km (5 miles) per sec. that no one has yet been able to place a payload into orbit totaling more than 1% of the weight of the vehicle on the ground.

More than a century ago, visionaries like Jules Verne were suggesting a better way. A bullet-shaped vehicle, they claimed, could be propelled far faster by using powerful electromagnetic fields. Now, as a result of lab work in the U.S. and abroad, the Vernean scheme shows promise of becoming a practical reality with far-reaching consequences: armor-piercing guns that can puncture the toughest steels, and perhaps a whole new era of space launchers.

The devices are called railguns, not because they sit atop railroad cars, like World War I artillery pieces, but because they consist of two parallel rails which act as both gunpowder and barrel. When the gun is fired, a powerful pulse of electricity goes down one rail. As the current surges to the other rail, it vaporizes a metallic fuse in back of the bullet, creating a cloud of electrically charged particles, or plasma. Simultaneously, it generates a strong magnetic field between the rails, like those in an electric motor. The field exerts a force against the plasma, just as it would against a motor's rotor. But instead of spinning, the plasma moves forward, guided by the rails and pushing the projectile ahead of it. Not constrained by any sonic limitation, the plasma could, theoretically at least, approach the speed of light (300,000 km per sec.).

Revival of serious interest in railguns began a few years ago, when Physicist-Engineer Richard Marshall and his colleagues at the Australian National University in Canberra updated the old concept with some notable innovations, including the plasma-creating fuse. They also increased the gun's muzzle velocity by resorting to an unusual power source: a huge homopolar electric generator which uses two rapidly spinning flywheels to build up and store electricity. In bare ly a second the Canberra homopolar de livered as many as 500 megajoules of direct current -- enough to light up a small city. Such a quick surge is essential for rapid buildup of the propelling magnetic field. Eventually, they were able to deliver the electromagnetic kick even quicker, and accelerated small plastic cubes to muzzle velocities of 6 km per second.

At the University of California's Los Alamos Scientific Laboratory and at the Lawrence Livermore Laboratory, scientists added another improvement: a magnetic flux compression generator, which increases the thrust of the magnetic field by squeezing it with a carefully directed explosive charge, a technology pioneered during nuclear weaponry research. When the gun is fired, the electric surge ignites the near end of an explosive strip placed just on the outside of one of the rails. As the detonation speeds forward, faster than the blink of an eye, it presses one rail against the other, confining the magnetic field between them in an ever smaller space and imparting still greater velocity to plasma and projectile. Teams led by Physicists Ronald Hawke and Max Fowler have fired half-inch projectiles down a railgun's square-bore barrel at an estimated 10 km per sec. They believe velocities of 150 km per sec. could be reached.

That will require much more research. One problem: single-shot railguns like the Los Alamos-Livermore machine must be painstakingly rebuilt after each firing. The projectiles also have an annoying habit of breaking apart when they leave the gun barrel. But the remarkable possibilities -- high-speed guns of almost every kind that can shoot through practically anything -- ensure continued research, financed jointly by the Departments of Defense and Energy.

Unlike rockets, missiles launched by railguns would not leave fiery, polluting exhausts detectable by satellite. In a forthcoming issue, Physics Today reports that some scientists think that railguns, firing a stream of high-velocity particles at a target of deuterium and tritium, may offer the best way yet of achieving controlled fusion, a key energy hope for the future. Perhaps the most far-reaching application involves the space colonization ideas of Princeton Physicist Gerard O'Neill. He and some colleagues at M.I.T. are already building models of kindred electromagnetic launchers that they believe could be assembled on the moon and used to propel tons of lunar ores into space for construction of solar-powered space habitats.

Melissa Ludtke Lincoln/New York

With reporting by Melissa Ludtke Lincoln/New York

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