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About this Article
Written by: Branden Lee
Written on: April 23rd, 2012
Tags: aerospace engineering, electrical engineeringm, energy & sustainability, mechanical engineering, physics, security & defense, transportation, chemical engineering
Thumbnail by: U.S. Navy/Wikimedia Commons
About the Author
Branden Lee was a sophomore majoring in mechanical engineering at the University of Southern California. He grew up in San Marino, CA. His enthusiasm for physics and the Transformers movie inspired him to write this paper. He enjoys watching and playing sports during his free time.
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Volume XIV Issue III > Rail Guns: From Sci-Fi to Reality
A rail gun uses magnetic and electric forces to accelerate a projectile. Parallel rails extend along the length of the firing chamber of the rail gun powered by capacitors. With the power generated by the magnetic fields contained in rail guns, objects can be launched at incredible speeds. The result is a destructive force. The rail gun, though, still has a variety of other applications. Some of these applications are revolutionary and ground breaking. However, there are still many obstacles to overcome. Advancements must be made in rail gun technology. By understanding the physical concepts behind the rail gun, one can grasp the promise that the technology holds.

Introduction

Gunpowder has long been the primary choice of propellant used in firearms since its invention in ancient China. Conventional weapons have been designed based upon the principle of expanding gases to fire projectiles. However, these weapons have physical limitations. Expanding gases can only propel rounds at specific speeds, thus scientists and engineers have been developing a weapon which utilizes a totally new source of propellant: electricity and magnetic forces. This weapon is the rail gun. Although the concept of electric-powered weaponry has been around since the early 20th century, the rail gun has become a reality in recent years.

Design and Theory

A rail gun is essentially an electric circuit which comprises three types of parts: a power source, a pair of rails, and an armature, which is placed on a boat [1]. The power supply provides the rail gun with the needed current to produce the required forces to propel the projectile. Rail guns usually work with currents in the millions of Amperes. For perspective, the average 60-Watt light bulb only contains 0.5 Amperes of current. The parallel rails and the armature are made of conductive metals. The armature is used to bridge the circuit between the two rails to keep current flowing. It also houses the projectile to be fired.
In electricity, current flows from the positive terminal of the battery to the negative terminal [2]. In Fig. 1 of a rail gun circuit, the current flows from the positive terminal of the power source up the positive rail, across the armature, and back down the negative rail to the negative terminal of the power source. Current running through a wire has an intrinsic magnetic field associated with it [2]. In a rail gun, the rails act as the wires of the circuit. Magnetic fields are created around these rails. The magnetic force lines run in a counterclockwise and clockwise direction around the positive and negative rails, respectively. Therefore, in between the two rails, the net magnetic force is upwards.
Tosaka/Wikimedia Commons
Figure 1: The flow of current through a rail gun.
The projectile is acted upon by the Lorentz Force (F). The Lorentz force is the product of the current (i) flowing through the armature and the magnetic field (B). The Lorentz force is the driving force which propels the projectile. The magnitude of the Lorentz Force is determined by the equation F=i*L*B [2]. As such, the Lorentz Force can be magnified by increasing the current or length (L) of the parallel rails. When the projectile is fired, it exits the ends of the rails. In doing so, the circuit is broken, and all current flow ends.

Power Supply

Rail guns require a large power supply to generate the necessary forces to accelerate the fired projectile. Capacitors need to store enough electric charge until a large enough current has built up [1]. These capacitors usually are many cubic meters in size. A problem arises when a rail gun with a high firing rate (6 rounds/min) requires as much power as needed for ship propulsion [3]. These power requirements can be met with the all-electric warship with an integrated power system (IPS). IPS is a highly efficient electric power generation and distribution system. With this system, a ship will be able to manage its power and switch between the rail gun and propulsion.

Intense Heat

Electric current that passes through conductive material must take resistance into account. Materials usually have an internal resistance. The current excites the molecules of the material, which causes heating [1]. This heat is hot enough to melt the rails of the gun. In order for rail guns to be practical, a solution to this intense heating must be found. Some proposed solutions offer cryogenic cooling or the improvement of the conductivity of the material in the rails. The Navy proposed the idea of using liquid nitrogen or a saltwater heat exchanger to cool the rails in order to avoid the problem of welding.
The armature, which houses the projectile, must be in physical contact with the rails at all times to keep the electric circuit complete. Sometimes, the heat dissipated by the rail gun is hot enough to weld the armature and rails together, leaving the rail gun inoperable. A possible remedy for this problem is Metal Vapor Arcing (MVA) [4]. In this set-up, a thin metal foil is placed on the back of a non-conducting projectile. When current flows through the foil, the foil vaporizes and turns into plasma (ionized gas). The plasma is still conductive, which allows the current to flow through, keeping the circuit intact. However, there are drawbacks to MVA. After several shots, metallic residue would begin to build up on the rails due to the plasma cooling back to a solid state. Until a viable cooling option or design is found, intense heating will be one of the main obstacles in the advancement of rail guns.