Rail guns for the Navy?
Via email, a reader asked me to comment on Belmont Club's post about the next-generation of US Navy destroyers, the DDX. In particular he wanted some comment on the DDX's main battery, a rail gun using not chemical propellant, but electro-magnetism.

The electromagnetic rail gun which is being developed for employment in the Navy's next class of destroyers, the DDX, allows the entire ship's power output to be directed into an acceleration device which will shoot a projectile at anywhere from Mach 7 to Mach 16 clear out of the earth's atmosphere onto targets hundreds of miles away. They will be devastating.

To put things in perspective, our current 5-inch gun has a muzzle energy of 10 megajoules. ... In contrast, naval rail guns will achieve muzzle energies from 60 to 300 megajoules. ... Research indicates that a notional first-generation naval rail gun could deliver a guided projectile with an impact velocity of Mach 5 to targets at ranges of 250 miles at a rate of greater than six rounds per minute.

... An important advantage of rail guns is the ability to exploit the high kinetic energy stored in the projectile ... One test demonstrated that the release of the rail gun projectile's kinetic energy alone would create a 10-foot crater, 10 feet deep in solid ground, and achieve projectile penetration to 40 feet.

Since the shells will be solid darts, a destroyer will carry 10,000 rounds in its current magazine space... .

Now, this is not a new idea. Rail guns, so named because they don't require an enclosed tube to fire, but use, well, a rail instead, were envisioned decades ago, and were even postulated for awhile as a means of routine space launch. Science-fiction author Robert Heinlein made rail guns the main weapon of revolutionary moon colonists in his novel, The Moon is a Harsh Mistress.

They work by using a pulse-modulated, electro-magnetic field to propel a projectile down a guide rail. Magnetic impellers are at regular distances along the guide rail and pull the payload along the rail, then are turned off as the next impeller starts to pull it. With ample power and a long enough rail, such as the DDX system, enormous velocities can be attained.

My reader was doubtful that solid shot could achieve desired effects on the target. At normal speeds, say that of current naval artillery, he is right. But hypervelocity shot is in fact explosive on impact. Back in the mid-1980s, when the Strategic Defense Initiative was started, I read a book by one of the projects military commanders on the subject of Project Thor. Project Thor was named after the Norse god, whose legendary hammer could smash anything.

What Project Thor envisioned was orbiting satellites of long, aerodynamically-efficient rods of solid, dense metals. The solid rods were bombardment weapons. As I recall, each rod would be about 20 pounds, but when de-orbited would strike a target with the equivalent of 40 kilograms of high explosive (all this from memory, mind you). That's about the same as an eight-inch artillery shell.

A hundred or more Thor rods could be targeted together, resulting in an enormous destructive effect.


So the DDX rail gun's target effectiveness would be very high. The only technical problem I foresee springs from the fact that high-velocity guns shoot very flat trajectories. This means that if a short-range target is masked by terrain - on the other side of a hill, for example - a flat trajectory can't hit it.

The solution is "high angle fire," raising the muzzle to the point that as the rail's elevation increases, range decreases. You can do this with a garden hose with a jet nozzle. Turn it on full and point it a few feet in front of you. As you raise the nozzle, the water falls farther away, but at some point as you raise the nozzle, the waterfall will move back toward you. That's high angle fire.

High angle fire achieves a very high apogee, or in ballistic terms, "maximum ordinate." This increases the shot's circular-error probable (CEP), the normative distance from target center into which, statistically, 90 percent of the shots will fall. The variations of fall come from the fact that all ballistic launches have random deviations called "jump" that can be statistically accounted for but not individually predicted.

However, if the DDX's rail gun's projectile is guided, then that alleviates the CEP problem, for then the line of flight is not actually ballistic. The other matter is whether the rail gun's launch speeds are variable. Conventional howitzers are designed for variable velocities so that high angle fire goes as low as possible and still remain fairly quick. But a, say, Mach 12 launch nearly straight up, to hit a terrain-masked target only a few miles distant, will take several minutes to reach the target. For tactical targets, that is unacceptably long. But I assume that the engineers know this and have designed the rail gun for effective high-angle fire.

by Donald Sensing, 7/6/2004 10:41:48 AM. Permalink |