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The first device that could be called a railgun was invented in 1918 by Louis Octave Fauchon-Villeplee.  His “Electric Apparatus for Propelling Projectiles” resembled a linear motor[3], which have applications such as maglev trains.  His device consisted of two parallel busbars and a projectile whose wings contacted the bars.  A force was induced by the magnetic field surrounding the apparatus.  This force was used to launch the projectile.    During World War II, the German’s designed, but never built, a railgun that was supposed to launch projectiles at a speed of 2,000 m/s.  When this was discovered after the war, it was published in a report that the device itself was theoretically possible, but the power required could illuminate half of Chicago.  At this time, some scientists and engineers believed that railguns could be used to replace conventional weapons and accelerate objects to almost any speed.  Even as time and technology has progressed, this dream has not been realized to that extent[2]. 





A railgun is composed of several components:  two parallel rails, a power source, and a moving armature with a projectile attached to the front end of it.  The power supply provides a current that flows into the postive rail, through the armature, and back through the negative rail in the opposite direction.   Using the right hand rule, the magnetic field (B-field) on the negative rail flows in the opposite direction of the positive rail.  These opposing magnetic fields create the magnetic repulsion force that moves the armature.  This force is called the Lorentz Force.  The formula for Lorentz Force[5] is:





where dL/dx is the inductance gradient of the rails, and I is the current flowing through the rails.  As can be seen in the formula, the dominating factor in producing the force and, in turn the exit velocity of the projectile, is the current that the power supply is producing.

The velocity of the projectile can be estimated using the following equation[2]:


where u is the initial velocity and m is the mass of the projectile and armature.  Sometimes the velocity is initialized with compressed air.  This helps to prevent the armature from melting, or welding itself, to the rails due to the very high current flowing through the rails.  Once again, current is the dominating factor in the exit velocity. 



The materials railguns have to meet certain requirements.  The current can be on the order of one million Amperes.  This means that the rails must be made out of very durable materials that can not only withstand the large amount of current generated by the power supply and the heat associated with producing that current, but also the violent accelerations and friction forces of the projectile and armature[1].  There is some debate on what other forces the rail needs to stand up to.  Some say that the recoil force acts on the breech of the railgun.  Others say that it acts along length of the rails.  The rail is also pushed by the magnetic field, just as the projectile and armature are.



Another material and design consideration is that the power supply must be able to produce a large amount of current for a useful period of time.  As of February 2008, the most energy used in a rail gun is 32 million joules.  In order to store this energy, most rail guns use capacitors or compulsators.  


Railguns as weapons



ddxRailgun.jpgRailguns are being heavily pursued for use as weapons[1].  A large proponent for their use is the lack of explosives.  Because railguns can fire solid projectiles at speeds up to 3500m/s (or Mach 10), the kinetic energy of the projectile, which is exponentially related to its’ velocity, can be superior to the energy yield from an explosive projectile of higher mass.  This amounts to lower cost per projectile and larger storage capacity on vehicles compared to conventional projectiles such as the tomahawk missile.  By firing at high velocities the railguns are capable of greater range with less drop due to gravity and wind.




Railgun Testing



250px-Railgun_usnavy_2008.jpgSeveral full scale railgun models have been built and tested.  The United States military is funding railgun experiments.  The United States Naval Surface Warfare Center demonstrated an 8 Mega Joule (MJ) railgun that fired 3.2 kg projectile in October 2006 as a prototype for a proposed 64 MJ version to be deployed on Navy ships.  Since then, BAE Systems has delivered a 32 MJ prototype to the Navy.  Most recently, on January 31st 2008, the U.S. Navy tested a railgun with a muzzle velocity of 2520m/s and an energy of 10.64 MJ.  The final version is expected to have a muzzle velocity of over 5800m/s and is expected to be ready in 2020-2025.





Problems with Railguns


Railgun technology has many uses for military and propulsion uses however they also face many serious issues.


Power Supply: Generating the power required to launch projectiles from railguns at high velocities requires a large amount of current which must be built up and stored in capacitors prior to firing[6].  Capacitors used in computers and other common household electronic devices are very small.  Capacitors used in large railguns can take up entire rooms.

Melting: The high velocity of the armature sliding on the rails creates intense friction which along with the resistive heating (the heat generated from the inherent resistance of a material to an electrical charge) of the rails creates intense heat.

Repulsion: The current in each rail moves in opposite directions which results in opposite magnetic fields.  The opposite field repel the rails away from each other causing damage to the rails.

Because of the damage from heat and repulsion, current railguns can only fire a certain number of times before the rails need to be replaced.  The wear and tear issues along with the large power requirements have retracted from the practicality of implementing railgun systems.





1. http://en.wikipedia.org/wiki/Railgun

2. http://www.powerlabs.org/railgun2.htm

3. http://en.wikipedia.org/wiki/Linear_motor

4. http://www.projectrho.com/rocket/rocket3x.html

5. http://en.wikipedia.org/wiki/Lorentz_force

6. http://science.howstuffworks.com/rail-gun1.htm


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