Anyone fancy discussing Rail Guns?

[chris_uk_83]

Hopefully embedding this video works, if not click this link to watch something extremely cool:

http://www.youtube.com/watch?v=PhifbbjwSPI

This is a real life rail gun, a gun with (theoretically) no maximum projectile velocity. This means that if anyone can make it work properly they'll have a projectile weapon with limitless energy, which could decimate anything.

The problem is, how do you stop the projectile melting and sticking to the inside of the barrel. Anyway, I think they're cool .

[chris_uk_83]

Anyone know how to embed videos?

[Simetrical]

Embedding fixed.


Railgun projectiles cannot have unlimited speed; they are limited by the speed of light, since they have mass. The Wikipedia article gives 20 km/s as a high speed achieved for a small projectile, a far cry from light's 300,000 km/s, or even Earth's 30 km/s around the Sun.

They also can't have infinite energy, which would violate mass/energy conservation. Typical energy usage appears to be on the order of megajoules, where a single kilowatt-hour is equal to 3.6 megajoules. I.e., a single shot might use a few hours' worth of a small house's electricity supply, probably only seconds' or minutes' worth of an air-conditioned shopping mall or such.

A megajoule can accelerate twenty grams of matter to a speed of 10 km/s if perfect efficiency is attained; in practice, it will only be some fraction of that. To accelerate to 20 km/s, you would need four times the energy, not just double (K = mv²/2).

As noted, there are limits to this: expending a million times the energy will not accelerate you quite to 20,000 km/s, and a hundred million times will probably get you quite short of 200,000 km/s. As you approach the speed of light, the energy required increases at such a rate that it would require infinite energy (which is impossible) to actually get to lightspeed.

Railguns have a number of interesting benefits, as noted in the Wikipedia article. They'll probably supplant chemical guns someday, but we aren't there yet.

[Syron]

the US navy is currently running tests on ship-borne versions designed to offer similar bombardment capabilities to tomohawks

[chris_uk_83]

Sorry, I said limitless velocity, and as you point out, I meant limitless energy. This assumes of course that you can create a power supply that provides limitless energy (which we all know you can't do.......yet).

And these things would be great on ships, which can handle the size of the power supply needed to fire them. You'd punch a hole straight through any ship's hull if you could give the armature (projectile) enough energy. Add that to the fact that your ship becomes less vulnerable because you no longer have to carry high explosive ammunition for your guns.

I'm interested, have the US military managed to overcome the melting projectile sticking to the barrel and rendering it useless after a coupe of shots yet?

[Kretchfoop]

Sorry, I said limitless velocity, and as you point out, I meant limitless energy. This assumes of course that you can create a power supply that provides limitless energy (which we all know you can't do.......yet).

And these things would be great on ships, which can handle the size of the power supply needed to fire them. You'd punch a hole straight through any ship's hull if you could give the armature (projectile) enough energy. Add that to the fact that your ship becomes less vulnerable because you no longer have to carry high explosive ammunition for your guns.

I'm interested, have the US military managed to overcome the melting projectile sticking to the barrel and rendering it useless after a coupe of shots yet?

I saw a show on TV that talked about railguns for a bit. I can't remember which channel though, probably Discovery. Anyways there was about a 20 minute or so segment and they mentioned the earliest they thought they could be operational for military purposes was in 30 years. But I don't know how old the show was and I saw it a few years ago so it could be closer to 20 now, I don't know. I'm not sure if they were talking about naval railguns or something else.

[LSJ]

I've always been interested in railguns. Sure, they have limitations, and the massive energy required to hurl a large projectile is quite limiting itself, but with a proper compact energy supply they can be really dangerous and fun. A homemade railgun with the proper sabot round and voltage can pierce the armour of an M1 Abrams. The only problem is being good enough to make it properly and accurately, and not getting killed by the capacitors. When charged they're like defibrilators.

[Wild Bill Kelso]

So I assume since there is no explosive force involved a railgun would be a suitable space weapon. Also , would you really want to be lifiting chemical ordanance into space? It would be bad enough being lifted to space by a controlled explosion, then to also have to worry about your cargo exploding. How does interia work with these things? Am I correct to say that it throws projectiles rather than pushes them?

[chris_uk_83]

If by 'throw' you mean like a trebuchet or a catapult does then no, it pushes them. Have you ever done the experiment at high school where you put a wire in a U-shaped magnet and then turn on a current in the wire? The wire jumps up out of the magnet (or is pushed down into it depending on the direction of the current). It's the same principle at work in the railgun (though it's configured slightly differently). So technically the magnetic field on the rails 'pushes' on the magnetic field on the projectile I suppose.

If you're talking about inertia and space weapons I assume you're wondering about recoil effects, in which case there isn't a difference between throwing and pushing. Newton tells us that every action has an equal and opposite reaction so if you send a projectile in one direction with a certain amount of momentum, the firing/throwing device recoils with the same momentum in the opposite direction.

As for suitability as a space weapon, I agree. The ammunition is lighter than bullets/shells so it's easier to lift into space and, as you say, it's not explosive so you don't need to worry about that. It's also a relatively simple construction (compared to something like a laser) so it's rubust enough to survive the journey.

[Simetrical]

Sorry, I said limitless velocity, and as you point out, I meant limitless energy. This assumes of course that you can create a power supply that provides limitless energy (which we all know you can't do.......yet).

The kinetic energy of the projectile would be limited too, at the very least by the energy in the universe (obviously, assuming known physical laws hold). Slightly more practically, I expect that there would be limits on the amount of energy you could store in the same place before the energy density becomes large enough to spontaneously form a black hole, although I could be badly misunderstanding that aspect of relativity. So still not strictly limitless, although undoubtedly the limits are higher than for chemical propellants.

And these things would be great on ships, which can handle the size of the power supply needed to fire them. You'd punch a hole straight through any ship's hull if you could give the armature (projectile) enough energy.

Don't rule out advances in material physics: carbon nanotube ships would be much more resistant to such things, and probably in the decades to come we'll find even more exotic things. Also, assuming that the projectile remains far below the speed of

I'm interested, have the US military managed to overcome the melting projectile sticking to the barrel and rendering it useless after a coupe of shots yet?

Not last I heard. A few hours of repair are still needed after every shot, as I recall.

So I assume since there is no explosive force involved a railgun would be a suitable space weapon.

Suitable in what way? It would have the advantage that it doesn't require fuel to be brought in from anywhere (given that simple electricity can be generated by solar power, nuclear fission, laser beams from remote space stations, or other methods), but it still needs projectiles. Lasers don't even need those, which makes them arguably more suitable.

How does interia work with these things?

Inertia works as it always does: the momentum of the projectile must be countered by an equal and opposite change in momentum of the rest of the craft (equivalent to Newton's Third Law). Of course, this might be a rather large backward force, but of course it can always be countered by expelling another projectile in the opposite direction at the same speed.

Am I correct to say that it throws projectiles rather than pushes them?

There's not really a difference between throwing and pushing, that I can see.

As for suitability as a space weapon, I agree. The ammunition is lighter than bullets/shells so it's easier to lift into space and, as you say, it's not explosive so you don't need to worry about that. It's also a relatively simple construction (compared to something like a laser) so it's rubust enough to survive the journey.

How much of a bullet's mass is the propellant? I'm guessing on the order of half. So that's substantial but not massive savings. I don't think lasers have a problem surviving a few g's, do they?

[chris_uk_83]

The kinetic energy of the projectile would be limited too, at the very least by the energy in the universe (obviously, assuming known physical laws hold).

Well quite, though there are a number of theoretical ways of providing massive amounts of energy such as harnessing zero point energy. I should point out that this is only theoretically possible though and is unlikely to happen any time soon, if ever.

I expect that there would be limits on the amount of energy you could store in the same place before the energy density becomes large enough to spontaneously form a black hole

The way I understand it, the only thing that causes a black hole to form is a high concentration of mass. The gravitational field of an object is governed by its mass, and a black hole is simply a body whose Schwarzschild radius (the radius at which light cannot escape from the body) http://en.wikipedia.org/wiki/Schwarzschild_radius is greater than its physical size. I don't think energy density has anything to do with it (though I am open to being proven wrong on this).

I don't think lasers have a problem surviving a few g's, do they?

No, lasers can be made to be quite robust and survive a lot. But if you break the cavity on a laser you can't repair it easily. Laser cavities have to be a very precise length (to the order of wavelengths of light) and you need extremely well polished and reflective mirrors. At least you do for a laser with enough power density to do any damage to a spaceship (I love how this discussion has got into the realms of space war now).

You can make solid state lasers (which you find in CD players, laser pens etc.) which are incredibly resistant to damage, but their power density is too low to use them as a weapon.

click here for a quick overview of lasers

I daresay you'd be able to carry spare parts and could have the equipment to rebuild them, but I believe a railgun would be simpler to repair due to it's relative simplicity. Though I agree with your point about lasers not needing ammunition. I imagine though that by the time we're worrying about equiping spacecraft with weaponry we'll have come up with a better way to get into space than rockets, and none of this will actually matter!

Oh, and sorry to keep referencing Wikipedia, I do have other sources, but Wikipedia is easy to find and provides a quick overview without confusing the reader with too much unnecessary detail (and maths!).