Military Develops Electric Slingshot

May 8, 2008
The Navy’s goal of replacing gunpowder with electricity got one step closer when it successfully test-fired its latest rail gun.

Engineers at the Naval Warfare Center in Dahlgren, Va., recently fired their 10.64-MJ rail gun, launching an aluminum projectile out of the muzzle at 2,530 m/sec. It’s part of the Navy’s project to arms its ships with this new type of weapon.

The Navy’s goal
Over the next decade, the Navy plans to deploy newer ships that generate significantly more electricity. The Arleigh Burke Class destroyers, for example, were first deployed in 1991 and carry 7.5 MW of electrical generating power. The Admiral Zumwalt Class destroyers, which had its first ship just recently commissioned, will carry more than 10 times that figure, 78 MW. While much of that power will go to propulsion and life support, the ship’s crew will be able to divert power to newer systems such as rail guns when needed, unless the ships is traveling at flank speed. (It is estimated that the ship’s generators would have to burn 3 gallons of fuel to generate enough power for one rail-gun firing.) So it appears the Navy is building in the capacity to handle the rail gun’s enormous appetite for power with its next-generation all-electric warships.

The Navy envisions a rail gun that can put a 30-lb round on targets 200 miles away, and at up to 10 rounds/min. These rounds would not be the solid aluminum projectiles the military currently uses during R&D. Actual rounds would carry GPS and inertial navigation, along with guidance, turning the dumb rounds into smart weapons. The military has already tested individual guidance and navigation components, as well as aerodynamic control surfaces, at up to 100,000 gs, so developing a guidance system for rail-gun ammo is not a showstopper.

The advantages of a rail gun are obvious. Besides increasing the range of Navy’s current gun from 13 to 200 miles — the rail-gun rounds also get to the target in a hurry, at up to Mach 7. The trajectory is also much higher, into the stratosphere actually. So a rail gun can hit targets on the opposite side of steep mountains.

Rail-gun rounds, which should cost about $10k apiece, will not carry chemical explosives as propellants or in the warhead. The round’s destructive power stems from its kinetic energy and it relies on electricity for propulsion. This makes the rounds inherently safer than current shells and rockets and does away with the need for special handling and the possibility of rounds exploding during loading or in the magazine. It also eliminates dud rounds because there is no fusing or warheads, and rail guns will not leave unexploded ordnance on the battlefield.

The lack of propellant and warhead also means rail-gun rounds will be smaller and ships will be able to carry more of them. It is estimated a destroyer could carry 2,400 rounds for a rail gun instead of 600 rounds for the Advanced Gun System currently installed on Zumwalt-class destroyers.

There are plans for at least three different types of rounds. The largest will be for hard targets such as bunkers, buildings, and prepared positions. A rod-type round, probably made of tungsten for increased mass, will be for targets such as tanks and other armored vehicles. Pellet-dispensing antipersonnel rounds will be used to rain hundreds, perhaps thousands of lethal, high-speed pellets against massed troops.

A few remaining issues
One engineering hurdle that needs to be conquered involves the power supply. Rail guns need a short burst of high power. The recently fired gun, like all others, relied on capacitor banks to supply the high-energy pulse. The likelihood engineers will find a way to reduce the size and weight of these banks is not too promising given that they haven’t made much progress on those goals over the last decade. Capacitors also suffer from limits on how long they can hold a maximum charge and the fact they degrade after repetitive operation. One potential solution is the pulsed alternator. Current designs use a large spinning flywheel to store large amounts of kinetic energy that can be siphoned off in increments. And each increment would be enough to fire the rail gun.

In the mid-1980s, the military built a pulsed alternator nicknamed the “Iron-core Compulsator.” The device was a six-pole, rotating-field machine that could store 40 MJ of kinetic energy at 4,800 rpm. Engineers could coax ten 1-MJ, 2-msec pulses from it. More R&D into pulsed alternators could yield a lighter, more-compact power source for rail guns. They could also make rails guns suitable for armored vehicles.

Some of the electricity used to fire a rail gun gets converted to heat, about 15 MW of it. And this heating is not uniform. It is much higher at the front of the rails, where the armature is not traveling as fast. There’s also a potential for high-temperature arcing when a pulse is first sent through the rails. This arc can spot weld the armature in place and lead to catastrophic failures. To keep the barrel or rails from melting, liquid nitrogen or water could be used. This would add complexity, but the size and mass penalties could be absorbed by warships. Tanks and other armored vehicles, however, might would be hard pressed to house the same cooling and power systems.

Keeping the front end or breech of the rails cool has been solved by pressing the rails together with enough force to maintain good electrical contact. And if the round is kept moving along the rails before and during the initial pulse, welding is no longer a problem.

Another issue with the rails is that the armature leaves gouges in them as it races between them. (A similar effect is seen on the rails of rocket sleds.) Military engineers are confident that hypervelocity gouging can be prevented with the right choice of materials for the rails and armature. Reduced-scale, full-velocity experiments confirm this.

Like most guns, when fired, the muzzle blast is bright and loud. To limit the enemy’s ability to pinpoint the rail gun based on muzzle blast, military planners want to reduce the gun’s optical and audible signatures. So far, they have been able to design a muzzle shunt that reduces muzzle blast by three orders of magnitude.

Other issues that will need to be resolved but don’t seem to present insurmountable technical challenges are rail life and recoil. The military wants to get as many shots per rail as possible and is currently trying to squeeze 60 out of a test rig. One solution is to consider the rails consumable, and lets ships just carry replacements. For recoil, the Navy says it will try several methods but shrouds them and much rail-gun technology in secrecy. md

A projectile fired from the electromagnetic rail gun at the Navy’s Surface Warfare Center, Dahlgren, Va., on January 31 of this year had a velocity of 2,520 m/sec. The rail gun was fired at 10.64 MJ, a record for rail guns.

The aluminum slug, caught by high-speed camera, travels at 2,530 m/sec toward the target.

Banks of capacitors stored electrical power and released it in a controlled burst to power the recent rail-gun test firing. The Navy is working with engineers from several companies, including BAE Systems, Boeing, Draper Labs, and General Atomics to develop the weapon.

In a rail gun, two electrically conductive rails carry a flowing current that travels up the positive rail, across the armature, and down the negative rail. The net magnetic force is directed toward the end of the rails. This pushes the armature which, in turn, pushes the projectile.

Test engineer Andrew Wyman shows Chief of Naval Operations Admiral Gary Roughead an aluminum test round for the rail gun while standing in the concrete-protected target area.

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