Electromagnetic guns

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General Atomics Electromagnetic Railgun prototype.

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.

Applications

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. 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 speeds of more than about 2 to 3 km/s, the dynamic pressures of impact are so much higher than the material strength of anything involved in the collision that both the projectile and the target can be considered to act as a fluid. Mechanical strength ceases to be relevant. A hypervelocity dart impacting at these speeds will penetrate as if it were a jet of liquid of the same length and density. In any such impact, the materials will splash, violently, throwing fragments and debris. This violence will creating a blast that can gouge craters and damage nearby materials, equipment, and personnel.

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.

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 (and some designs, such as railguns, 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.

In space, there is no aerodynamic drag to worry about. Meanwhile, typical relative speeds between spacecraft range from several km/s to several tens of km/s, potentially much more for spacecraft boasting advanced fusion thrusters, beam-driven spacecraft, or plasma sail spacecraft. This means that the problem is not getting the impact to occur at high speeds – you get that for free. Rather, the main problem is getting your projectile to intercept your target. If your target and you are separating at a higher speed than your gun's projectiles go, you won't hit. If your target can thrust to get more speed than your projectile before it arrives, it also will not hit. Because space is big, even a very fast projectile can take a long time to reach its target. If the projectile is not guided, this means that any maneuvering on the part of the target will cause the projectile to miss. This is why serious consideration of electromagnetic guns for space warfare will design around shooting guided rocket projectiles that can home in on a target.

Sometimes it is useful to have a gun that can shoot projectiles really fast for purposes other than shooting other people. Electromagnetic guns can be used for the field of hypervelocity impact science, which is useful for developing protection and mitigation schemes for satellites and other spacecraft against micrometeorites. Some fusion energy schemes rely on impact fusion, where you collide a hypervelocity projectile with a stationary target, at least one of which contains fusion fuel. These sorts of studies often require impact speeds well in excess of 5 km/s. While the standard go-to instrument for reaching these speeds are light gas guns, some electromagnetic guns have been developed that can get things going this fast and faster.

No method of acceleration can ever be 100% efficient. Ohmic heating, eddy currents, time varying mechanical strain as the projectile is subject to changing magnetic forces during launch; these and others will all dissipate some energy in the projectile. This additional energy will cause the projectile to heat up. When the kinetic energy of the projectile is so much higher than the energy needed to completely vaporize it, even small inefficiencies can lead to a large enough temperature spike to disable, warp, or melt the projectile. This means that there is an upper limit to the speed of a launched projectile before the projectile is destroyed. The particular speed limit depends on the implementation – see the description of the electromagnetic rocket gun below for a design that uses this waste heat to its advantage to go even faster!

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.

Launch assist

Building rockets to launch stuff into orbit is hard. One of the things that makes it hard is that you need to lift the propellant you'll need for later on in the burn in your initial stages of ascent. This is all encapsulated in the Tsiolkovsky rocket equation showing how your initial launch mass grows exponentially with the final speed you need

$M_{0}=M_{f}\exp(\Delta v/v_{e})$

where $M_{f}$ is all the stuff you want to get delivered to orbit, $\Delta v$ is how much speed you need to add to your rocket from the launchpad to get it into orbit, $v_{e}$ is the speed of the exhaust coming out the back of your rocket, and $M_{0}$ is the total mass of your rocket including $M_{f}$ but also all the extra propellant you need to use to boost the rocket up to that $\Delta v$ speed. For something like a hydrolox rocket engine with $v_{e}\approx$ 4 km/s, and $\Delta v$ to reach low Earth orbit (including atmospheric losses) something like 8 km/s, you'd need something like six times the mass of your rocket payload in propellant.

All this means that any little boost you can give to cut down on $\Delta v$ can give substantial savings on launch weight and cost. If you could shoot your rocket out of a railgun at 2 km/s, you could cut the amount of fuel nearly in half. These kinds of arguments have motivated many designs for launch guns to shoot stuff toward space. This does not necessarily have to be a railgun, it could be a coilgun or even a conventional chemical combustion gun.

There are limits to this idea that go beyond merely the technological speed limits of a railgun. The most severe is that such a space gun can never launch something directly into orbit. An orbit is an ellipse with one focus on the center of the planet. This means that any orbital ballistic trajectory which starts from the ground will intersect the ground again before a full orbit is completed. To get around this, you will need to perform additional thrust maneuvers above the ground – a trick that can't be done directly with a ground-based launch gun and generally relies on rockets. A space gun can still give some initial speed that will eventually help get the payload into orbit, it can give a boost that will loft the rocket up above the thick part of the atmosphere where the rocket will work more efficiently and the majority of the atmospheric drag is past, and it could potentially launch stuff on escape trajectories that leave the planet forever.

Launch with an impulse delivered only at ground level puts a payload on an orbital trajectory that will always intersect the ground. It may be a way to get from Cape Canaveral to Boliva (as shown) but can't put you in an orbit that lets you stay in space.

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.

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 $K=1/2\,{\vec {p}}\cdot {\vec {v}}=p\,v/2$. This shows that 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 impulse will be produced (however, we 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).

The gun may gain the same recoil impulse as its projectile, but its recoil energy is much less. For a gun of mass $M$ recoiling with impulse $p$, its energy will be $p^{2}/(2M)$. You can see that the heavier the gun, the less energy it will have on recoil. This is one of the reasons that shooting a heavy gun feels like it has less recoil than a light gun with the same cartridge, and why heavy guns are less likely to leave bruises. Holding a gun snugly effectively increases the gun's mass, helping to reduce the felt recoil. The impulse that throws off subsequent shots during rapid fire may be the same, but it will hit less hard.

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 electromagnetic gun 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 an electromagnetic gun 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.

Credit

Author: Luke Campbell