Forget "pew pew pew," the boomsticks are here

[Cray]

The USN is progressing in its railgun testing and hopes to have one of two finalists on a ship in 2016:
http://arstechnica.com/information-techn...un-to-sea/

Direct to video:
https://www.youtube.com/watch?feature=pl...tszh_6Sl1s

One of the interesting features in that video (to me) is the projectile's flight: It has a nice, controlled spin thanks to its fins. Or the sabot. Or the barrel. Its spinning when it emerges from its sabot, so I'm not quite sure what started the spin. Anyway, it looks like the quadrail musket is here.

Another is at the beginning: the railgun recoils. Great googly moogly, I don't know how many internet arguments I've been in trying to communicate that, yes, electromagnetic projectile accelerators will experience recoil, too. Just because magnetic fields are involved doesn't exempt them from Newton's laws. I might have to do some thread necromancy...

[stevebowers]

I assume that the spectacular cloud of plasma that forms during a railgun test is a result of the missile travelling through the atmosphere



If a railgun were fired in the vacuum of space, this plasma would not be visible. When I made the models of the recent Gung-Ho ships I considered equipping them with railguns; but it seemed likely that a firing railgun would be more-or-less invisible, wheras a very bright laser could illuminate any ambient gas clouds, especially in a combat situation.

[Cray]

I assume that the spectacular cloud of plasma that forms during a railgun test is a result of the missile travelling through the atmosphere

Nope. If it was atmospheric effects, the nose of the project would be sheathed in plasma, too, like a shooting star. Rather, what's happening is that railguns generate a great deal of electrical arcing and ablation of the projectile (and sometimes the rails). Plasma pressure is a non-trivial source of propulsion for the projectile. If you look at the sabots (link below), you'll see the stern is cupped to ride on that gas (well, plasma) pressure.

For atmosphere effects, notice the second post in this thread by a "Kyler." His third picture clearly shows the trace of a shockwave. Meanwhile, his second picture shows a close-up of a projectile where none of the front end is engaged in plasma effects. It's just trailing off the stern.
http://www.kbismarck.org/forum/viewtopic...=36&t=2919

There's a closeup of the sabot in this link:
http://nextbigfuture.com/2011/04/general...pdate.html

But the most awesome site on railguns is this link, a hobbyist who builds these. He has a lot to say about the plasma pressure and arcing:
http://www.powerlabs.org/railgun.htm

"my previous attempts at building Rail Guns were never very successful: they would either weld the projectile to the rails, or blow apart from plasma pressure"

His post on "Rails, Rail Enclosure, and Armature Design" goes into the problem of plasma pressure, projectile design, and evaporation. Further down, you see some railgun firing sequences (and videos) generating incredible muzzle flashes, which are explained in the link.
http://www.powerlabs.org/images/shotstrip.jpg

If a railgun were fired in the vacuum of space, this plasma would not be visible.

The plasma from the evaporating projectile would be highly visible. You wouldn't get as much oxidation from a burning aluminum projectile, but the metal would still be glowing white-hot.

This guy also filmed a demo for the Discovery Channel and ran into a problem (discussed near the end of the page):
http://www.powerlabs.org/railgun2.htm

This armature, much to my dismay, EXPLODED inside the barrel, creating an incredible blast and a shower of fragments from the muzzle.
Since the gun was designed to withstand over ten thousand PSI, no damage of any sort occurred and filming went on as planned. The exploding armatures looked spectacular on camera, but didn't do much more electrical efficiency. After several shots were fired carbon vapor deposited itself on the insulators and created a rail-to-rail flashover condition. A further analysis revealed redeposited copper vapor all over the rail channel.

[stevebowers]

Sounds as if railguns are damaged significantly each time they are fired. How can this be minimised?

[Cray]

Sounds as if railguns are damaged significantly each time they are fired. How can this be minimised?

"Carefully."

The Powerlabs site details a lot of design elements used to protect the rails: soft projectiles (or sabots), Teflon coatings, insulation in the right places to avoid inappropriate shorting, etc. It also seems to help to maximum conductivity and thus lower resistive heating.

Ideally, the rails will be wear-resistant, hard, and resistant to arcing erosion. A lot of that is sort of a tall order because even minimal alloying can dramatically reduce conductivity. You can get away with something oxide dispersions (hard particles mechanically mixed into the base metal without chemical mixing), or a small percentage of elements like cadmium and beryllium for copper alloys. Cadmium helps copper with arcing resistance in oxidized environments.

Also, it might not be a bad idea to try to minimize or eliminate rail-projectile contact. You can coat most of the projectile in something like Teflon and achieve electrical contact between the rails and projectile with a deliberate electrical arc. (In a vacuum, inject some gas.)

I don't know their specific techniques, but the US Navy railguns seem to be further along in wear resistance than hobbyists. They are after long-lived, low-cost weapons and barrel wear is an old problem for naval guns. (Though steel barrels don't have to worry about electrical conductivity requirements.)

In the worst case, you might try sacrificial rails. After every few rail shots, eject the rails and slide in some new ones.

Coil guns largely avoid this wear problem, but coil guns seem to be limited to 1-3km/s velocities. Railguns aren't nearly so constrained in their muzzle velocities.

[Ithuriel]

Also, it might not be a bad idea to try to minimize or eliminate rail-projectile contact. You can coat most of the projectile in something like Teflon and achieve electrical contact between the rails and projectile with a deliberate electrical arc. (In a vacuum, inject some gas.)

Nah. You'll reduce some of the wear, but you've still got what basically amounts to a plasma explosion inside the barrel of your gun, and there aren't many materials that will stand up to that.

I wonder if it would be possible to build the rails out of a dense, cold plasma. If it were, not only would you eliminate rail wear, but at the point at which you charge up your rails your gun barrel will emit an ominous glow. Totally worth it.

In the worst case, you might try sacrificial rails. After every few rail shots, eject the rails and slide in some new ones.

I believe that is exactly what the naval railguns will do. It'll still be cheaper than a bunch of cruise missiles or antiship missiles.

Coil guns largely avoid this wear problem, but coil guns seem to be limited to 1-3km/s velocities. Railguns aren't nearly so constrained in their muzzle velocities.

Railguns give you better bang for their size. Coilgun-based surface-to-orbit launch systems have had workable designs since the late 70s, but they're huge... lengths measure in kilometres (up to 100s of km for delicate cargo like people).

A coilgun equivalent to the US navy design in terms of projectile speed and mass (using some very quick and dirty figures, courtesy of Project Rho and Luke Campbell) would need a barrel longer than 100m, even if it were using a load of very fancy superconductors, and you'd still have to work hard to stop waste heat from destroying the projectile before it left the barrel.

[Cray]

Nah. You'll reduce some of the wear, but you've still got what basically amounts to a plasma explosion inside the barrel of your gun, and there aren't many materials that will stand up to that.

Sounds like a challenge.

I bet an answer could be found in coatings before you need to get to cold plasma armatures. There are some low friction, arc-resistance coatings in the electrical industry today like cadmium oxide.

In the worst case, you might try sacrificial rails. After every few rail shots, eject the rails and slide in some new ones.

I believe that is exactly what the naval railguns will do. It'll still be cheaper than a bunch of cruise missiles or antiship missiles.

The Navy probably doesn't mind because their battleship guns only had a barrel life of a few hundred rounds. Barrel wear is a known issue for them.