In science fiction, the use of gunpowder-based weapons is generally portrayed as something from a savage past, with technology having long since moved on to more civilized types of destructive weaponry, involving lasers, microwaves, and electromagnetism. Instead of messy detonating powder, energy-weapons are used to near-instantly deposit significant amounts of energy into the target, and railguns enable the delivery of projectiles at many times the speed of sound using nothing but the raw power of electricity and some creative physics.
Of course, the reason that we don’t see sci-fi weapons deployed everywhere has arguably less to do with today’s levels of savagery in geopolitics and more with the fact that physical reality is a very harsh mistress, who strongly frowns upon such flights of fancy.
Similarly, the Lorentz force that underlies railguns is extremely simple and effective, but scaled up to weapons-grade dimensions results in highly destructive forces that demolish the metal rails and other components of the railgun after only a few firings. Will we ever be able to fix these problems, or are railguns and similar sci-fi weapons forever beyond our grasp?
The Lorentz Force
The simplest way to think about a railgun is as a linear motor. At its core it consists of two parallel conductors — the rails — with an armature that slides across these rails as it conducts the power between the two rails. This also makes it the equivalent of a homopolar motor, which was the first type of electric motor to be demonstrated.
In the photo on the right you can see a basic example of such a motor, with the neodymium magnet providing the magnetic field and the singular wire the current that interacts with the magnetic field. Using the right-hand rule that was hammered into our heads during high school physics classes we can thus deduce that we get a net force.
With this hand-held demonstration the screw will rotate when current is passed through the wire. For stand-alone homopolar motors with the magnet on the battery’s negative terminal and a conductor loosely placed on the positive terminal while touching the magnet, the Lorentz force will cause the wire to rotate around the battery.
We can visualize this interaction between the current-carrying wire (I), the magnetic field (B) and resulting force vector (F) in such a homopolar motor fairly easy, but how does this work with a railgun?
Rather than a permanent magnet or a complex electromagnet on each rail using many windings, a single current loop is used in a railgun. This means that massive amounts of currents are pumped through one rail, which induces a sufficient strong magnetic field.
The projectile, playing the role of the armature, is located inside the generated magnetic field B, with the current I coursing through the armature, resulting in a net force F that will push it along the rails at a velocity that’s proportional to the strength of B.
Crudely put, the effective speed of a project launched by a railgun is thus determined by the applied current, so unlike it’s close cousin, the coilgun, there is no tricky timing requirement in energizing coils in a sequence.
This also provides some hints as to what major obstacles with railguns are, starting with the immense currents that have to be immediately available for a railgun shot of any significant size. If this is somehow engineered around using massive capacitor banks, then you run into the much more significant issues that have so far prevented railguns from being widely deployed.
Most of this comes down to wear and tear, because going fast comes with certain tradeoffs.
Making Big Stuff Go Fast
Theoretically you can just scale everything up: creating railguns with larger rails and larger armatures that can launch larger projectiles with increasingly faster speeds. This has been the impetus behind various railgun projects across the world, with notable examples being the railguns developed and tested by the US and Japan.
Railguns were invented all the way back in 1917 by French inventor André Louis Octave Fauchon-Villeplée, when the issue of the massive electricity consumption kept further research on a fairly low level. Even the tantalizing prospect of a weapon system capable of firing at velocities of more than 2,000 m/s couldn’t get into deployment during the time that Nazi Germany was working on their own version.
Ultimately it would take until the 1980s for railgun designs to become practical enough to start testing them for potential deployment at some point in the future, seeing a surge of R&D investment for it and other new weapon systems that could provide an edge during the Cold War and beyond.
Yet despite decades of research by the US military, no viable design has so far appeared, and research has wound down over the past years. Although both China and India are testing their own railgun designs, there are no signs at this point that they haven’t run into the same issues that caused the US to mostly cease research on this topic.
Only Japan’s railgun research seems to so far offer a viable design for deployment, but their focus is purely defensive, for countering ballistic and hypersonic missiles in a close-in role. The size is also limited to the current 40 mm prototype by Japan’s Ministry of Defense ATLA agency.
Physical Reality
In a perfect world with zero friction and spherical cows, railguns would be very simple and straightforward, but as we live in messy reality we have to deal with the implications of sending immense amounts of currents through a railgun barrel. A good primer here can be found in a June 1983 report (archived) by O. Fitch and M. F. Rose at the Dahlgren Naval Surface Weapons Center in Virginia.
Much of this comes down to efficiency as you scale up a basic railgun design. The two main factors are basic ohmic resistance (ER) and system inductance (ES). These two factors limit the kinetic energy (EK) and set the losses (EL) of the system, with the losses being in the form of thermal and other energies.
Reducing these losses is one of the primary points of research, and factors like the rail design and alloys as well as the switching of the current pulses play a role in affecting final efficiency, and with it durability of the railgun’s ‘barrel’.
Naturally, that was all the way back in 1983, and since then a few decades of technical and material science progress having occurred. Or so one might be led to believe, if it wasn’t for current research papers striking a rather similar tone. For example Hong-bin Xie et al. in a 2021 paper as published in Defence Technology.
This review article covers the common issues of rail gouging, grooving, arc ablation, and other problems, as well as the current rail materials in use today and their performance characteristics.
Many of these issues are somewhat related, as the moving armature rarely maintains a perfect contact with the rails. This results in arcing, localized heating, ablation, and grooving due to thermal softening. All of these effects result in a rapidly degrading rail surface, and higher currents result in more rapid degradation and even worse contact with subsequent shots.
Various rail metal alloys have been or are being tested, including Cu-Cr, Cu-Cr-Zr and Cu/Al2O3, replacing the pure copper rails of the past. None of these alloys can resist the pitting and other wear effects from repeated railgun firings, however. This has pivoted research towards various coatings that could limit wear instead, such as molybdenum (Mo) or tungsten (W).
Fields of research involve electroplating, cold spraying, supersonic plasma spraying and laser cladding, using a wide variety of coatings. The authors note however that these rail coatings have only begun to be investigated, with success anything but assured.
Defensive Benefits
Quite recently railguns have surged to the forefront in the news cycle courtesy of certain ill-informed fantasies that also involve destroyers which identify as battleships. In these feverish battleship dreams, railguns would act as a kind of super-charged version of the 16″ main guns of the Iowa-class, the last active battleships in history.
Instead of 16″ shells that ponderously arc towards their decidedly doomed target, these railguns would instead send a projectile at a zippy 2-3 km/s towards a target. As tempting as this seems, the big issue is as we have seen of repeatability. The Iowas originally had a barrel life of a few hundred shots before their liner had to be replaced, but this got bumped up to basically ‘infinite’ shots after some changes to their chemical propellant.
A single Mark 7 16″ naval gun fires twice per minute, and this is multiplied by nine if all three turrets are used. The range of projectiles launched included high-explosive, armor-penetrating, and even nuclear shell options, with a range of 39 km (21 nmi) at a leisurely ~800 m/s. To compete with this, a naval railgun would need to be able to keep up a similar firing rate, feature a similar barrel or at least acceptable barrel life, and have a longer range for a similar payload effect.
At this point railguns score pretty poorly on all these counts. Although range of a projectile falls between that of a missile and a Mark 7 naval gun’s projectile, barrel life is still poor, power usage remains very high and the available projectiles at this point in time are basically just relying on their kinetic energy to cause harm, limiting their functionality.
Taking all of this into account, it would seem that the Japanese approach using railguns as a very responsive, close-in weapon is extremely sensible. By keeping the design as small-caliber as possible, reducing rail current, and not caring about range as long as you can hit that hypersonic anti-ship missile, they seem to be keeping rail erosion to a minimum.
Since the average missile tends to perform rather poorly after a 40 mm hole appears through it, courtesy of it briefly sharing the same physical space with a tungsten projectile, this might just be the defensive weapon niche that rail guns can fill.
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