Railguns

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A railgun is a projectile weapon that uses high electric currents to push a projectile between two rails. The potential advantages of a railgun are high speed projectiles. Disadvantages include rail wear, the need to store large amounts of energy, and the equipment to produce pulses of extreme currents.

Working principles

An electric current creates a magnetic field that circulates around it. If you have two parallel conductors carrying current in opposite directions, they both produce a field that points in the same direction between them, amplifying the field in that direction (likewise, outside the two wires the fields point in opposite directions making the field weaker there and causing it to fall off faster than the field from a single wire).

A magnetic field exerts a force on any electric currents going through it. The force is in the a direction perpendicular to both the magnetic field and the current, and is proportional to both.

The magnetic field (magenta) circulating around a cross sectional plane perpendicular to the direction of an infinite line of current (green).	The magnetic field (magenta) circulating around a cross sectional plane perpendicular to the direction of two infinite line of current in the opposite directions (green).	The force on a current due to a magnetic field.

The basic idea for building a railgun is to take two parallel conductive rails. Short the two rails with a conductive projectile near the breach. Apply a pulse of very high current, that will run down one rail, through the projectile, and back up the other rail. The current-carrying parts of the rail make a high magnetic field between them. This field pushes on the current flowing through the projectile, which launches it down the rail. As long as the projectile shorts the two rails, it experiences the force and is accelerated faster.

A simplified diagram showing the workings of a railgun.

The magnetic field can be enhanced if the railgun uses a ferromagnetic barrel around its rails. This in turn will increase the force on the projectile and improve the railgun efficiency and performance.

More engineering details here - calculate energy, efficiency as a function of current, permeability of barrel.

Safety

One advantage suggested for railguns is that without the need for reactive propellants, ships and ammunition storage warehouses will be significantly less hazardous if hit by enemy fire. In operation, a railgun might only store enough energy for one shot in volatile fast-discharge energy storage devices such as capacitors. The fast discharge energy storage would be recharged by a generator between shots. On a vehicle, this could be the same generator used for electric traction, thus allowing a unified power supply system for the drive train and weapons. While hits to the fuel or generator are still possible, this hazard is inherent to the operation of the vehicle and is present whether the vehicle is armed with a railgun or not.

High speed

In a conventional firearm, the propulsion of a projectile becomes increasingly inefficient as the projectile moves at speeds close to the speed of sound in the propellant gas. A railgun, lacking propellant gas, does not have this problem. As long as the projectile maintains electrical contact with the rails the projectile can be accelerated. This makes railguns a contender for launching high speed projectiles, potentially attaining speeds that would otherwise require an exotic and complicated chemical propellant firearm like a light gas gun. For comparison, artillery will tend to get to around 0.6 to 1 km/s and a tank firing an anti-tank APFSDS dart might reach speeds approaching 2 km/s. Meanwhile, a recent program developing a railgun for the U.S. Navy could launch projectiles at up to about 3.3 km/s (CITE, check if true, other reports say only Mach 6 ~= 2 km/s. NEED TO FIND PRIMARY DOCUMENTS HERE!!!).

At around 3 km/s, the kinetic energy of a projectile will match the energy released by the detonation of the same mass of TNT. This means that a separate warhead is un-needed. The energy liberated by the projectile slamming into a target at more than 3 km/s will produce a bigger explosion than if it were filled with explosives – particularly because artillery shells need to be built extra sturdy to survive launch leaving relatively little space for the warhead. It is worthwhile to note that penetration doesn't increase when speeds get over 2 km/s. A faster speed can flatten a larger area but if your goal is punching through armor any energy used getting faster than 2 km/s at the target is energy wasted. Note that this is speed at the target. Because a projectile launched in atmosphere will suffer from aerodynamic drag in flight, when shooting at distant targets you may need higher speeds at the muzzle to get the desired terminal performance.

In order to maintain electrical contact with the rails the projectile must either keep a sliding physical contact with the rails or strike an electric arc to the rails. An electric arc is arguably the worse of the two options, as each shot will be arc-welding the rails and will produce excessive rail wear. A sliding contact is no worse than any conventional firearm with the bullet maintaining a sliding pressure seal with the barrel. But as speeds get higher and higher, a sliding contact produces more and more barrel wear. A high speed projectile can be expected to significantly reduce rail life compared to the barrel life of a modern firearm. (CITE navy railgun, ~100 shots per barrel & not all that high speed compared to modern artillery, not clear if these were all full power shots) (Disposable rails? Liquid metal rail contacts?)

High speed wear on the rails and projectile will produce vaporized material that are ejected from the barrel on launch, producing a loud muzzle blast and muzzle flash. Much like modern firearms, this will indicate to observers that the weapon was fired and can help to localize its location, either directly by the flash or from dust and debris kicked up by the blast.

As current flows through the projectile, electrical resistance will heat it up. Thus, some fraction of the energy delivered for the discharge will go into raising the temperature of the projectile. At high enough speeds, this inefficiency will deposit so much heat that the projectile will be affected, either warping, partially or fully melting, or vaporizing. Warping or partial melting will adversely affect accuracy, complete melting or vaporization will prevent the projectile from reaching its target. (CITE likely upper achievable speed due to melting, EMRG paper?)

Shot consistentcy

In modern artillery, variance in timing of ignition and burning rate of the powder can produce differences in muzzle speed and delays of launch of the projectile. This can result in decreased accuracy. These will not be present in a railgun, resulting in more consistent exterior ballistics and improved accuracy.

Self forces

The same interaction between the magnetic field and the current that gives a strong force on the projectile also acts on the current flowing through the rails. This produces a strong force that acts to push the rails apart. If the rails are pushed apart, electrical contact with the projectile will be broken and the rails might get permanently damaged if warped beyond their elastic limit. A consequence of this is that railguns will not have bare exposed rails. Instead, the rails will be contained within a strong barrel structure that can support the forces pushing on the rails to minimize strain on the rails and keep the gun from bursting or warping. Sadly, common artistic interpretations of railguns with a pair of exposed unsupported rails will not work; nor will the rails have electric arcs buzzing between them for reasons outlined above – namely, avoid arc welding of the rails. Railguns may be high current, but they are low voltage devices.

Recoil

The circuit containing the current in the rails and projectile must be closed on the other end of the current loop. The magnetic forces push on this just as much as they do on the projectile, producing recoil in accordance with Newton's second law of motion. The total recoil impulse (momentum transfer) will be the mass of the projectile that is launched out the end of the barrel times the speed of the projectile as it is launched. This will be similar in magnitude to the recoil produced by a chemical propellant firearm, with one important difference. A considerable portion of the recoil from a chemical propellant firearm comes from the hot gases jetting out of the barrel, acting like a rocket. The amount differs depending on the interior ballistics of the firearm but a contribution of very roughly 30% to the recoil is typical. The railgun will lack these propellant gases, so that the recoil will be somewhat less than that of an unmodified firearm. However, the same reason it lacks this additional recoil does not allow a railgun to use a muzzle brake, which can reduce the recoil of a firearm even further.

Recoil as a function of speed for constant KE.

power supply, pulse forming network, lack of casing & propellant & primer

efficiency of navy's electromagnetic rail gun ~30% (CITE this)

helical railguns

hybrid railguns-coilguns (using electromagnets to increase the B field)

plasma railguns

plasma rails & why they don't work