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About the rail gunEdit
A railgun is a purely electrical gun that accelerates a conductive projectile along a pair of metal rails. Railguns use two sliding or rolling contacts that permit a large electric current to pass through the projectile. Railguns should not be confused with:
- Coilguns, which are more commonly known as Gauss guns (which in their own right should not be confused with the Gauss Cannon). Are contactless and which use a magnetic field generated by external coils arranged along the barrel to accelerate a magnetic projectile.
- Railway guns, which are very large artillery pieces placed on railway tracks and predominantly used in and between the American civil war and the Second World War.
The U.S. Navy has tested a railgun that accelerates a 7-pound projectile to seven times the speed of sound. As of February 2008, the largest known energy used to propel a projectile from a railgun was 32 million joules
A railgun consists of two parallel metal rails connected to an electrical power supply. When a conductive projectile is inserted between the rails (from the end connected to the power supply), it completes the circuit. Electrons flow from the negative terminal of the power supply up the negative rail, across the projectile, and down the positive rail, back to the power supply.
This current makes the railgun behave similar to an electromagnet, creating a powerful magnetic field in the region of the rails up to the position of the projectile. In accordance with the right-hand rule, the magnetic field circulates around each conductor. Since the current is in opposite direction along each rail, the net magnetic field between the rails (B) is directed vertically. In combination with the current (I) across the projectile, this produces a Lorentz force that accelerates the projectile along the rails. There are also forces acting on the rails attempting to push them apart, but since the rails are firmly mounted, they cannot move. The projectile slides up the rails away from the end with the power supply.
A very large power supply providing, on the order of, one million amperes of current will create a tremendous force on the projectile, accelerating it to a speed of many kilometers per second (km/s). 20 km/s has been achieved with small projectiles explosively injected into the railgun. Although these speeds are theoretically possible, the heat generated from the propulsion of the object is enough to rapidly erode the rails. Such a railgun would require frequent replacement of the rails, or use a heat resistant material that would be conductive enough to produce the same effect.
In relation to railgun physics, the magnitude of the force vector can be determined from a form of the Biot-Savart Law and a result of the Lorentz force. It can be expressed mathematically in terms of the permeability constant (μ0), the radius of the rails (which are assumed to be circular in cross section)(r), the distance between the centerpoints of the rails(d) and the current in amps through the system (I) as follows:
The formula is based on the assumption that the distance(l) between the point where the force (F) is measured and the beginning of the rails is greater than the separation of the rails (d) by a factor of about 3 or 4 (l > 3d). Some other simplifying assumptions have also been made; to describe the force more accurately, the geometry of the rails and the projectile must be taken into consideration.
Proposed and invented by Gerard K. O'Neill, the mass-driver uses a similar concept of magnetic acceleration to "catapult" spacecraft into orbit.