Science: Energy: Lightning
Electrically Generated Ordnance Denial Sphere   (+1, -1)  [vote for, against]
Positively charged bullet gets zapped by lightning defense system.

This device fills an area surrounding it with electrically negative ions. This is done in a similar way as a household ozone generator ionizes the air for whatever purpose those fulfill. In this case it creates a conductive path. The surface of the device has a strong negative charge.

At this point the children's book on lightning that is being used as a resource while this is being worked through warns us not to play outside during lightning storms. A picture of a little boy and girl running towards a house while being followed by a small dog further illustrates this point.

A bullet or other missile fired at this device picks up a positive charge from the surrounding air during its travels. Once that bullet penetrates the electric field of ionized air surrounding the device it becomes much like the aforementioned children or puppy. The bullet is disintegrated by the high energy discharge of the device, never in fact reaching the target where warm beds and perhaps milk and cookies await.
-- rcarty, Aug 18 2011

...and if I'm standing between the bullet and the electrode when the device discharges?
-- Alterother, Aug 18 2011

No more milk and cookies for you.
-- rcarty, Aug 18 2011

So it doesn't really save me from getting shot, it just makes the situation that much more interesting.
-- Alterother, Aug 18 2011

Why must you stand in front of it? Would you stand infront of your shield in a sword fight? If you simply have to stand in front of it for some reason perhaps you could get away with wearing a nonconductive bodysuit.
-- rcarty, Aug 18 2011

In a sword fight, I (hopefully) know which direction the sword is coming from; not so with a bullet or missile. If I'm relying on this device to save me from said bullet or missile, not only am I standing very close to the naked discharge of a massive amount of electrical energy, but I'm also running the risk of the arc going right through me to get at the bullet, in which case I think I'd rather be shot.
-- Alterother, Aug 18 2011

Then this is obviously not the complicated form of suicide for you, sir.
-- Alterother, Aug 18 2011

Missile is a very general term. And [Alterother] maybe you can wear this device so there is no way of being electrocuted by it.
-- rcarty, Aug 18 2011

Surely there must be something you wouldn't mind having lightning intercept.
-- rcarty, Aug 18 2011

// maybe you can wear this device so there is no way of being electrocuted by it. //

Unless I happen to be the path of least resistance at some point. If there's one thing I've learned from working with electricity, it's that you can't make it do what you want. You have to ask very politely for its cooperation, or it will take the first opening it gets and it won't even buy you breakfast after what it does to you on the way out.
-- Alterother, Aug 18 2011

It would be okay for point defense, so long as everyone wears rubber-soled boots and stays inside the bunker.
-- Alterother, Aug 18 2011

I left out "if it worked as advertised."
-- Alterother, Aug 18 2011

Please analyze the charge on a bullet in flight.

Then please determine the likelihood of your coil discharging to said bullet with enough energy to vaporize it. Please account for where the excess charge goes once the bullet is neutralized.

Then determine the likelihood of your coil discharging to the ground. Note that this is primarily a factor voltage (very high) and distance from the emitter to the ground.

Then determine the likelihood of the coil discharging to the nearest grounded/earthed object.

Then keep this thing at least 500 meters away from me.
-- MechE, Aug 18 2011

I'm sticking with my automated miniguns for perimeter defense, thanks.
-- Alterother, Aug 18 2011

<Amused by the phrase 'the bullet is neutralised'>

IF one was able to discharge enough energy into the bullet to vapourise it in the very short time available, the resulting explosion would unleash considerably more destructive force than was contained in the bullet.

So even if you don't get zapped by the lightning bolt, your remains will be scattered unto the four winds in short order.
-- Twizz, Aug 18 2011

We would have annotated this earlier, but we were laughing too much.

What follows is highly simplified, so that the slower students (that's you, [rcarty]) have a vague chance of following.

Let us assume a projectile made of solid lead, total mass 10 grams. We will neglect (i) frictional heating of the projectile while in the barrel, and (ii) frictional heating due to air resistance, and consider only latent heat of vaporisation.

Lead has an atomic weight of 207, and a latent heat of vaporisation of 179.5 kJ·mol&#8722;1.

For a 10 gram bullet, the energy required to convert liquid lead at 1749 C to vapour is therefore 9kJ.

Just liquefying the solid bullet requires more energy still, plus the energy required to raise the liquid from 327 to 1749C. But we will ignore this as it is small compared to the vapourisation energy; to deliver 9kJ in 1 millisecond, the device must have, at a very minimum, and output capability of 9MW.

Factoring in all the possible losses in the system, a design output of 15MW from the primary energy source would most likely be the minimum required.

Practical ? Maybe.

Portable ? No.

And one question; once the discharge track has neutralised the accumulated positive charge on the projectile, why exactly does current continue to flow along the plasma track into an electrically isolated object ?

There was this bloke called Michael Faraday ...
-- 8th of 7, Aug 18 2011

... who lived in a cage. (as opposed to being just outside it and getting electrocuted)
-- Twizz, Aug 18 2011

Sorry to pile in, but say there's a solid lead object weighing 10g flying through the air at 100m/s - now imagine the same 10g of lead, only liquified - it still weighs 10g, and it is still flying right at you at 300m/s (guessing - enough to impart a sting if it hits you anyway) only now it's red-hot and on fire. Finally, imagine 10g of flaming-hot gasseous lead, almost but not quite at a plasma-temperature, very likely hotter than the surface of the sun, still flying very much in your direction, only hotter, scarier and much more energetic, still weighing an eye-watering 10g, and still heading for you at 300m/s. How exactly has raising the temperature helped?

A lead ingot of 1000g weighs the same as a molten lead ingot of 1000g, which weighs the same as a lead balloon of 1000g. Drop any one of them on your foot, and you'll break your foot. The only real difference is that in each case it's more likely to set you, your friends and the very air you breathe, on fire.
-- zen_tom, Aug 18 2011

So, I'm living inside a great big Tesla dome. Sounds like fun!

Imagine the incredible joy-buzzer you could add, (one time use only) 'let me shake your hand!' ZZZAAPPP! <smoke, body parts fly>
-- RayfordSteele, Aug 18 2011

// 10g of flaming-hot gasseous lead ... still flying very much in your direction //

No. As soon as it gasifies, it will lose cohesion and give up all its kinetic energy to the surrounding air in a very short time. The thermal energy will disperse almost as quickly, giving a slowly-rising cloud of very hot microcrystalline lead dust intermingled with equally hot air.

There's a reason why EFPs are only generated upon contact with the target; their very short range of action.

To vapourize the projectile, you need a way of conveying an appropriate amount of energy to it. This might be achieved by coherent electromagnetic radiation, or a magnetic field; although lead would be a poor candidate material for magnetic deflection. A directed plasma weapon is impractical in a planetary atmosphere using your current technology.
-- 8th of 7, Aug 18 2011

<Amused by the phrase 'the bullet is neutralised'>

To clarify, I meant electrically per 8/7's last point, not euphemistically.
-- MechE, Aug 18 2011

The doubled chickenwire approach, where the bullet creates a path of least resistance between the positive and negative meshes is more practical, as it does provide a reasonable energy path. In practice, however, I suspect that the difference in dielectric value between air and lead is not high enough that the energy levels of the two meshes would be high enough to vaporize the bullet without also being high enough to arc between the meshes even without a bullet (and thus vaporizing the mesh).
-- MechE, Aug 18 2011

However ...

That would require:

(i) That the voltage between the two mesh layers was just below that required for spontaneous discharge;

(ii) That on penetrating the mesh, the projectile effectively "bridges" the gap, causing the arc to strike;

(iii) That the power system can deliver sufficient energy to completely compromise the structural integrity of the projectile in the very short transit time available, and

(iv) Immediately (iii) is achieved, the arc is automatically quenched to save energy, and - more importantly - stop the mesh burning away.

The mesh will have to be made of a refractory metal - tungsten would be a possibility - with a fairly thick cross section to handle the currents and temperatures involved.

A series of projectiles in rapid succession, targeting one particular area of the screen, would no doubt cause both significant mechanical damage to the mesh, and severe local overheating, inevitably reducing the system's effectiveness at dealing with further impacts.
-- 8th of 7, Aug 18 2011

Why not follow DARPA's lead and shoot a bigger missile at the incoming missile?
-- DIYMatt, Aug 18 2011

Yeah, this seems like protection against only a one-time fire event. Any standard-issue machine gun would be able to defeat it, due to charge time alone.
-- RayfordSteele, Aug 18 2011

Fully agree [8th], didn't say it was realistic, just more practical.

Especially since, even if possible, I can't see any situation where 4 inches of lexan doesn't serve the purpose better.
-- MechE, Aug 18 2011

I see this scheme as best for a science fiction endeavor; maybe a cowboys and aliens type deal where the aliens smugly reside behind their screen and the savage humans blaze away ineffectually at it. Trebuchet loads of salt water would be an effective countermeasure, as mentioned.
-- bungston, Aug 18 2011

Wouldn't coating a projectile in rubber cancel its conductivity?
-- 2 fries shy of a happy meal, Aug 18 2011

And the winner of the "marked for tagline anno" contest is...And let me tell you, ladies and gentlemen, there were SEVERAL great contestants - the envelope please...

"We would have annotated this earlier, but we were laughing too much."
-- normzone, Aug 18 2011

<respectful bow>
-- 8th of 7, Aug 18 2011

[zev], are you entirely sure you're annotating the right idea ... ?
-- 8th of 7, Aug 18 2011

I was initially thinking nonmetal projectiles would get thru but actually I think they would be stopped even more effectively. Think how easily one imparts a charge to a rubbed balloon or a vinyl LP, which then attracts ferret hair from your couch or lightning from Zeus. Try imparting a charge to a plumbers wrench.
-- bungston, Aug 18 2011

random, halfbakery