Electromagnetic Railgun Blasts Off

A supersonic bullet is fired with a record-breaking 10 megajoules of muzzle energy.

Show of force: A shockwave is produced (above) as a supersonic bullet emerges from the navy's electromagnetic railgun, which was tested late last month. Credit: U.S. Navy

Multimedia • Watch the railgun fire a seven-pound bullet. • See the flames produced by the bullet.

Last week at the Naval Surface Warfare Center, in Dahlgren, VA, a seven-pound bullet emerged from a truck-sized contraption at seven times the speed of sound and sent a visible shockwave through the air before crashing into a metal bunker filled with sand. With 10.6 megajoules of kinetic energy, this aluminum slug was propelled not by explosives but by an electric field, making this the most powerful electromagnetic railgun ever fired. The device is part of the navy's railgun development program.

While propellant-driven shells have been mainstays of naval warships for the past hundred years, the cost and safety issues related to storing explosive materials have driven engineers to seek alternatives like the electromagnetic railgun. "There are physical limits to what you can do with gunpowder," says Charles Garnett, the manager at Dahlgren, referring to the maximum velocities that explosions can produce. A railgun could eventually send a 40-pound slug 200 miles in six minutes--10 times the range of the navy's primary surface support gun, the MK 45--and it could be used to support Marine troops engaged in land-based operations.

"A lot of people think a railgun is not going to make a lot of noise," Garnett says. "It's electrically fired, and they expect a whoosh and no sound." In reality, when the bullet emerges, it lets out a crack as electricity arcs through the air like lightning.

The railgun gets its name from two highly conductive rails, which form a complete electric circuit once the metal projectile and a sliding armature are put in place. When current starts flowing through the device, it creates a powerful electromagnetic field that accelerates the projectile down the barrel at 40,000 gs, launching it in a matter of milliseconds. Aerodynamic drag along with a million amps of current heats the bullet to 1,000 °C, igniting aluminum particles and leaving a trail of flame in its wake. The researchers estimate the muzzle energy based on the mass and velocity of the bullet in the barrel and from precisely timed x-ray snapshots during flight.

"What's important," says Garnett, "is that this is the first step on the way to building a tactically viable system with 64 megajoules of energy."

The previous experimental railgun record of 9 megajoules had been set 15 years ago by a team at the University of Texas at Austin funded by the U.S. Army. But the Texas railgun was operating at the upper end of its capacity, while Garnett says that the new gun has been designed to handle up to 32 megajoules, and the ultimate goal of the project is to build a 64-megajoule model.

Jon Kitzmiller, an expert in electromechanical systems at the University of Texas, who worked on an earlier railgun project, says that the navy team is "going to have considerable difficulty getting [to 64 megajoules], but it's certainly achievable." He says that the navy's budget of $40 million a year secured through 2011 proves that it is serious about making the gun a reality in the next 15 to 20 years. The previous effort was derailed by funding constraints.

One of the biggest challenges, says Kitzmiller, will be in designing a power supply that can handle multiple shots. "In order to store multiple 64-megajoule shots on a capacitor bank, you would need an aircraft carrier full of capacitor banks," he says. One solution, Kitzmiller and Garnett agree, is a system of rotating pulsed alternators, called compulsators, rather than traditional capacitors.

Other challenges include developing a projectile guidance system that can withstand 40,000 gs--twice the acceleration of current systems--and building a gun barrel that can withstand the force and heat produced by repeated firings. The same force that drives the bullet out of the barrel also tears the rails apart. The Dahlgren prototype looks nothing like a typical gun, and parts will frequently have to be replaced.

"Firing a gun once or twice [makes it] a novelty," says Garnett. "Firing it a thousand times [makes it] a weapon."

http://www.technologyreview.com/Infotech/20164/