Electromagnetic guns

We can use electromagnetism to make things move. The most familiar way to do this for many of us is to use a rotary electric motor, which turns electric power into the rotary motion of a shaft. This can be use to spin a drill bit or a saw or turn the wheels on a car. But not all electric motors are rotary in nature. Any rotary motor can be unrolled, so to speak, to turn it into a linear electric motor. Now you are using electric power to move an object back and forth. if you only move it forth, and you move it forth very quickly, and you don't bother to catch the forward moving part when it leaves the motor, you have a gun.

Common features of electromagnetic guns

There are many different kinds of electric motors. They all have gun versions. Different kinds of electromagnetic guns have different advantages and disadvantages. But there many features that are similar about them as well.

Charging equipment

An electromagnetic gun requires high power to be delivered in a very short pulse, on the order of a millisecond long. For artillery or naval canons, this can result in an instantaneous power of tens to hundreds of gigawatts! Unless the electromagnetic gun is directly plugged in to the full electrical output of a major regional power generating station (and you are willing to cause blackouts when it fires), you won't be able to directly deliver that kind of power from your main power supply. Instead, you will need to gradually build up energy over time, storing it in some kind of equipment that can deliver the stored energy in a very fast pulse. Common ways to do this include charging up capacitor banks or spinning up a compulsator (basically a flywheel attached to a generator). At least one research program used a massive inductor to store the energy. Other potential energy accumulation systems include other varieties of flywheel energy storage as well as superconducting magnetic energy storage (which is still an inductor, but a particularly useful form of inductor if you have the tech to make it convenient).

In most cases, the energy accumulator system will store enough energy for one shot. Your rate of fire will depend on the time it takes for your primary power supply to build up enough energy for one shot.

Another option are explosively pumped flux compression generators, which store the energy as high explosives and give a one-time surge of power when the explosives are detonated to drive your generator. Because the generator and associated equipment do not usually survive this process, it can be a rather expensive method to power your gun. Combined with the need to carry a magazine that could potentially explode, this starts to eat into many of the potential advantages of electromagnetic guns over conventional guns.

Flywheels, capacitors, and inductors all generally give a sudden surge of power at the beginning that decays over time. An explosively pumped flux compression generator, on the other hand, delivers a pulse that ramps up from zero to maximum power right at the end of the pulse. However, what you usually want is a specific pulse shape for your particular engineering design – for example, a constant power level for the duration of the shot. This is accomplished with a pulse forming network, which are inductors and capacitors connected in series (or certain kinds of transmission lines) to give the desired pulse profile.

Self forces

The same magnetic forces that push on the projectile will also push outward on the accelerating machinery of the gun (it's barrel, if you will). In order to keep the barrel from bursting, it will be important that it is built with enough structural reinforcement to hold it together.

Recoil

Newton's third law of motion stipulates that whenever you push on something, it pushes back on you just as much. In a closed system, you would get no net movement. If something gets pushed away, the rest of the system recoils back in the opposite direction.

In a conventional firearm this recoil comes from the hot high pressure gas pushing the bullet down the barrel. The pressure of the gas pushing back on the breach face gives the gun its kick. In electromagnetic guns, this recoil force comes from the same interactions of currents and fields in the gun that push back on the gun as pushes out the projectile. In a railgun, the current in a high field might push the projectile one way, but that current loop must close somewhere and the field will push back on the rest of the current loop in the gun. In an induction coilgun, the electromagnet in the barrel that pushes on the induced electromagnet in the projectile is in turn pushed back by that same induced electromagnet. A ferromagnetic coilgun, the same interaction occurs in the electromagnet in the stator and the permanent magnets in the armature.

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. In math-speak, the recoil impulse will be $-{\vec {p}}$, where ${\vec {p}}=m\,{\vec {v}}$ is the momentum of the projectile with $m$ the projectile mass and ${\vec {v}}$ the projectile velocity.

This impulse will be similar in magnitude to the recoil produced by a chemical propellant firearm with the same ballistics, 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.

Ammunition

For an electromagnetic gun, the projectile and sabot (if present) make up the entirely of the ammunition. There's no powder, no casing, no primer. This is expected to make electromagnetic gun ammunition rather more compact than that for equivalent conventional firearms – one claim has a railgun able to store over six times the number of shots in the same volume as conventional ammunition.

Safety

One advantage suggested for electromagnetic guns 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, an electromagnetic gun 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 an electromagnetic gun or not.

Shot consistency

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 many designs of electromagnetic guns, resulting in more consistent exterior ballistics and improved accuracy.

Cost considerations

One of the motivations for recent naval railgun development is the low cost per shot of railguns compared to missile systems. A guided railgun projectile from a naval cannon might cost $25,000, compared to $500,000 to $1,5000,000 for conventional missiles. The argument is that a large naval railgun could strike targets within 300 km at a rate of perhaps six to ten rounds per minute; because 85% of the world’s population resides within 300 km of shore a ship could deliver strikes to most targets at a fraction of the cost. The same general arguments are likely to apply to any other electromagnetic gun.

High speed

Test of a railgun-fired hypervelocity projectile penetrating a series of metal plates. The shot is moving from right to left. A video of test firing is available here.

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. An electromagnetic gun, 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 electromagnetic guns contenders 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, experimental railgun programs for military weapons development commonly reach speeds of 2 km/s to 2.5 km/s. Speeds of 3, 3.5, and even 6 km/s are often touted, and some experimental railguns have launched solid (albeit plastic) projectiles at up to 10 km/s, but for practical systems near of being fielded such speeds have been stubbornly aspirational rather than actual.

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 not 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.

As an example, the M795 155mm artillery shell contains 10.8 kg of TNT as explosive filler, giving it an explosive energy yield of approximately 45 MJ. The shell itself has a mass of 46.8 kg. The same energy could be achieved without explosives if the shell impacted at a speed in excess of 1.4 km/s.

The usual expression for the kinetic energy is $K=1/2\,m\,v^{2}$. Using ${\vec {p}}=m\,{\vec {v}}$ for the momentum we can express the kinetic energy as $K=1/2\,{\vec {p}}\cdot {\vec {v}}=p\,v/2$. We can now see another benefit of high speed – for constant kinetic energy, the magnitude of the recoil impulse is $p=2\,K/v$. So the faster the projectile is launched, the less recoil will be produced (however, we again must caution that the terminal effects of the projectile depend on more than just kinetic energy and it is a mistake to think that two projectiles with the same $K$ but different $m$ and $v$ will have the same effect on target).

It is worth considering that for use on a planet, it doesn't really do much good to get an electromagnetic gun beyond about 2 to 2.5 km/s. At those speeds, the projectile will hit hard enough to explode anyway and, as already mentioned, is where the penetration is going to be maximized. If you go faster, you will lose more energy early on to aerodynamic drag if the projectile has to go through an atmosphere. in addition, you will have more problems with rail wear. For a given energy-per-shot budget, it will often be advantageous to keep the projectile speed at 2.5 km/s or less but increase the mass to match your energy output. Neglecting drag, a projectile launched at 2 km/s can go 200 km; one launched at 2.5 km/s can go over 300 km. When you include drag, the distance decreases. The more massive the projectile, the less drag will affect its trajectory and the farther it will go – giving yet another reason to favor mass over additional speed. The hypervelocity projectile program gives a range of 185 km for a railgun-launched HVP dart; the railgun this was designed for is commonly reported to shoot projectiles at around 2 km/s, suggesting that for big naval guns like this and narrow aerodynamic darts, drag will not have too much of an effect. Such arguments do not necessarily apply to guns designed to work in space, however.

A projectile going at 2 km/s or more will explode. If it is a dense dart, it might punch through a considerable thickness of material in the process, but it will produce a significant bast in the process. Common media depictions of hypervelocity guns leaving nice neat holes with no collateral damage to nearby objects are not accurate.