Railgun projectile bursting through a metal plate in a Navy test system. The projectile is the dark metal bar in the center-right of the image.
Image courtesy of the Office of Naval Research

U.S. naval and military planners are salivating  at the possibilities of railguns.  Railguns are electromagnetic artillery weapons whose projectiles have muzzle velocities not limited by the speed of propellant gasses, but are limited only by the amount of electrical current used to propel them. Traveling at hypersonic speeds in excess of mach 5, the projectiles do not require an explosive filler to effect their damage. It is simply their very high kinetic energy that gives them their killing power. Also because of their very high speeds, railguns are suitable for terminal point antimissile defense, conceivably even against Intercontinental Ballistic Missiles (ICBMs).

What a Railgun Is

I wrote briefly about railguns  in conjunction with how we might defend against hypersonic glide vehicles (HGVs) being developed by Russia and China, in the post Defenses Against Hypersonic Glide Vehicles. Instead of expanding gasses inside an artillery tube propelling the projectile, a magnetic force acting on a current, often called a Lorentz force, propels the projectile. The basic idea behind the railgun is illustrated below.

Consider the figure on the far right. Two long, parallel conducting rails are in electrical contact with a sliding armature that is perpendicular to both rails. When a voltage difference is applied between the two rails at the ends labeled “+” and “-“, a current directly proportional to the voltage difference flows down one rail, shunted across the sliding armature, and then back the reverse direction along the second rail to the other pole of the potential difference. If you were to take the thumb of your right hand and point it in the direction of the current, curling your fingers around the rail, the curled fingers would point in direction of the magnetic field lines produced by the current, which are approximately circular around the conducting rail. This is called the “right hand rule,” and the magnetic field lines produced are illustrated as the blue circles surrounding the rails. Note that in the figure, this produces magnetic fields that all point upwards in-between the rails, and downwards outside the rails in the plane defined by the rails and the armature. The magnetic fields from all parts of the current loop add to each other constructively inside the current loop. Maxwell’s electromagnetic field equations tell us the amplitude of the produced magnetic field is directly proportional to the amplitude of the current.

In addition, a magnetic field produces a force on the current, the direction of which can be determined by yet another right hand rule: If you point the fingers of your right hand in the direction of the current and then curl them in the direction of the magnetic field, the thumb will point in the direction of the force, as illustrated in the figure. The rails and the power supply are clamped down so that they are not allowed to move. The armature however is allowed to slide precisely in the direction the force on it is pointing, causing it to accelerate in the parallel direction of the rails. If we attach some kind of projectile to the armature, or even better make the armature part of the projectile, such that the projectile is released when the armature reaches the end of the rails, the projectile is accelerated together with the armature to be released into free-space at the end of the rail gun. Because the magnetic field is directly proportional to the current magnitude, and the Lorentz force is directly proportional to both the magnetic field and current being accelerated, the acceleration of the projectile down the rail gun is directly proportional to the square of the current: double the current and you get four times the acceleration.

The result is a gun whose muzzle velocity is not limited by the speed of expansion of a burning gas, but by the amplitude of the current that can be forced through the rails and the sliding armature. Although there are problems to be answered by materials science, the biggest problem is obtaining power supplies large enough to produce the needed currents.

To persuade you that the rail gun is something that really exists and is not something out of science fiction, watch the video produced by CBS News below. In it the Chief of the Office of Naval Research, Rear Admiral Matthew Klunder, notes that the cost of a rail gun projectile is about 1/100th the cost of more conventional missiles used against aerial threats, costing many millions of dollars each. Each projectile of the type he picks up costs approximately $25,000. Why does it cost that much? Klunder tells us it has an electronic guidance system that allows it to change direction slightly to match changes in trajectory of the target. If it were to be a mere slug of metal it could cost orders of magnitude less. If the target is another ship, relatively slow moving compared to an HGV, the Navy might indeed use projectiles that are just solid metal. Also, Klunder notes that a Navy warship could carry many hundreds (probably thousands) of such warheads due to their very small size and weight.

In addition, check out the video below of a single test of the BAE system to get an appreciation for the immensity of the weapons system itself.

Navy Applications

As you may have gathered from the video above,  the Office of Naval Research has taken the lead role among the armed services in developing the railgun. They have even developed a special new class of guided missile destroyer, on which they can emplace railguns. The first ship in this class, the USS Zumwalt (DDG-1000) is a very striking looking ship, with composite materials and angles to its superstructure to make it radar-stealthy.

This ship was initially designed with Tomahawk cruise missiles, RIM-162 Evolved SeaSparrow missiles, Standard Missiles, and two 155 mm gun Advanced Gun Systems (AGS) as the main armament. As with all too many advanced systems built for the armed forces these days, the ship was being designed and built at the same time as much of its armament and sub-systems were being developed. The SPY-3 multipurpose radar system, electronics, computers, composite material superstructure, and its Integrated Power System are all state-of-the-art. The problem with jam-packing so much new technology in a single new ship class is that all that expenditure for research and development has to be amortized over the first order for ships. As the costs mounted for the Zumwalt class, the Obama administration scaled back the original order from 32 to three ships. This inflated the cost per ship to a mind-blowing $4 billion!

And it is not just the initial sticker price that blows the mind; some costs of operation are also out of hand. Consider the two innocent-looking boxes on the front deck of the ship. Those were each to house one of the two 155 mm gun AGS. They were to fire rocket-assisted shells with a pinpoint accuracy (circular error probable of 50 meters — 160 feet — or less) at a range of around 60 miles. Each round would cost a very pricey $800,000!

As it turns out, the contractor for the AGS, the European company BAE Systems, is also the Navy’s prime contractor for the development of its railgun. Now suppose the Navy were to swap the railgun for the AGS. Even with an electronic guidance system to change control surfaces on each projectile, each round would cost about $25,000, quite a savings over the $800,000 AGS rounds of around 97%.  At the same time a railgun system should provide comparable capability. For less-taxing anti-ship applications or stationary targets at moderate ranges, just a dumb bolt of metal costing only a few hundred dollars might suffice.

Army Applications

The Army also sees railguns as an economical possible solution  to a very big problem: a new generation of Russian tanks capable of delivering tactical nuclear rounds from their main guns. According to unconfirmed reports, Russia has decided to upgrade their T-14 Armata main battle tank with a new 152 mm main gun that can fire a tactical nuclear round.

Without some means of neutralizing a nuclear armed T-14, NATO could find itself in a disastrous battle field position if Putin decided to take back the Baltic Sea states and Eastern Europe. Reportedly, the T-14 has a new active protection system that possesses explosive reactive armor that can stand up to the world’s most advanced anti-tank shells and missiles. Also reportedly, the T-14 eventually will be a completely automated drone that would be remotely controlled. Such a main battle tank could be close to an insuperable tactical problem on wherever battlefield it was used.

To counter these and other tactical threats, the Army is developing a concept for a vehicle-portable railgun system with projectiles that would travel anywhere from Mach 3 to Mach 6. The projectile that would be used, the Kinetic Energy Projectile (KEP) would be an actual shell with an explosive filler. However, as Randy Simpson, a weapons programs manager at Lawrence Livermore National Lab, explained it, the KEP “take(s) advantage of high terminal speeds to deliver much more energy onto a target than the chemical explosives they carry would deliver alone.”

Maj. Gen. William Hix, the Army’s director of strategy, plans and policy, said the Army’s work on railguns was still conceptual, that an actual program for developing a system was not yet in existence. He said that “We’re looking at ways we might — key, might — use that capability in one of our existing launch platforms as part of the weapons suite that we have.”  The Army Tactical Missile System, made by Lockheed Martin was currently the main contender for the launcher of a KEP,  but the use of the Navy’s railgun is also being seriously considered.

Almost exactly one year ago, General Atomics Electromagnetic Systems demonstrated a railgun system shown below at the U.S. Army’s home for artillery, Ft. Sill, Oklahoma.

Notice the portable power supplies just behind the railgun. The big problem with any portable system like this is the amount of power that can be supplied to the rail-armature circuit to drive the projectile, and how long it would take to recharge for the next shot. Perhaps a portable fission reactor? Compare the size of these power supplies with that for the Navy BAE test railgun below.

The power supplies for this BAE test weapon are below and to the rear of the actual weapon. Notice how much more massive they are than that for the General Atomics system.

Future Applications

The limiting factor  of any railgun system is its power supply. For conventional artillery the power propelling the projectile is compactly stored in chemical propellant bags. That makes the movement and displacement of conventional artillery batteries much easier than if they had to drag around huge electrical power supplies. On the other hand, if you can solve the power supply problem, and can keep the electrical impedance of the rail-armature circuit to an absolute minimum, you can achieve a projectile muzzle velocity limited only by the amount of current you can drive through the basic circuit. Hypersonic muzzle velocities have been physically demonstrated. This dependence on large power supplies is more easily solved on board large naval ships, especially with nuclear reactors. The dependence would be far more limiting with mobile Army systems, with the size of power supplies limited by what could be transported by a realistic vehicle.

The ideal system on land would be one where you never would have to move the weapon, and where the power supplies could made as large as possible. For this reason, point terminal phase defense against ICBMs would be an ideal application. I would not be surprised if in coming years we witness railgun ICBM defenses sprouting all around our major cities.

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