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Comment by ourmandave | Hacker News Reader

I watched the video with the slow motion shot of the projectile coming out with a large ball of fire behind it. It then goes on while the casing falls away.

But I don't understand, why is there a fireball if it's all electromagnetic?

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The projectile has so much kinetic energy that it turns everything it touches into plasma. That includes the air it travels through. At the point of launch there is also very substantial material ablation that feeds a plasma bloom.

In fact, one of the primary engineering challenges in the design of a rail gun is minimizing ablation of the gun itself across firings. Rail guns slowly eat themselves so you want a design where (1) the ablation is slow enough that you can still get many shots off before it is non-functional and (2) the ablation is localized in easily, cheaply, quickly replaceable parts.

Probably from dumping 32 megajoules of energy into the rails/barrel. I wouldn't be surprised if part of it is melting/igniting the actual hardware itself and part of it is due to the friction of the projectile against the air.

Remember that this is still a prototype and one of the big concerns since day one has been lifetime of the system. Essentially, it's probably still melting its own hardware (at least to some degree) every shot.

32 MJ is actually not all that much... it's almost exactly the energy in a single litre of gasoline (44 MJ/kg, 32 MJ/l).

The important thing about a railgun is not the total power put into it but that the power is dumped into the projectile continuously over the course of acceleration, which makes much more efficient use of energy than conventional weapons, which produce large initial accelerations followed by rapidly decreasing gains as the projectile moves along the barrel and the hot gases behind it expand (the limit of conventional technology is the so-called "light gas gun" that uses hydrogen as a propellant to maximize the speed of sound in the accelerating medium and produce a relatively flat pressure curve along the barrel.)

well, that gives you an idea just how much chemical potential energy there is in gasoline. its pretty incredible stuff.

It's a pretty incredible amount of energy. Just think if you put a gallon of gas in a car than hit the petal, then lets take away (or at least mostly minimize so that the car has same aerodynamics as a artillery shell) air resistance for fun. The car accelerates until it burns through a gallon of gas, so for maybe, what 10-30 minutes depending on the vertical it's constantly accelerating, I can even imagine what speed it could get up to if you discount mechanical limitations of the engine/tires/etc, but let's say at least 1000 mph, conservatively. Then car runs into a brick wall. That is the amount of energy in a gallon of gas.

its way more energy than that! car engines aren't even close to 100% efficient. all the heat that blows out your exhaust pipe is energy that the engine couldn't capture. ditto for heat of friction on the mechanical components, and heat that radiates into the air from the engine block.

For fun, KE = (1/2)mv^2 = 32 MJ -->[mathing intensifies]--> a 1,000 kilogram car moving at about 565 mph carries 32 MJ of energy, which is what 1 liter of gasoline carries.

A gallon is about 4 liters, so at 100% efficiency it could get a 1,000kg car to ~1,130 mph or a 1,500kg car to ~925 mph.

For reference, the same calculations tell us a quarter-pounder with cheese could get a 165 lbs (75kg) human up to 540 mph.

now that's fast food!

The projectile is moving so fast the friction against the air makes enough heat to create a ball of plasma

I wouldn't be entirely surprised if the breakdown voltage of whatever is in there (I'm guessing air...) would be exceeded, thus ionizing it, either.

I think the gun is specifically designed so that won't happen, since the current has to go through the projectile (else you're just "short circuiting" the weapon)

Oh, fair enough. I'm not familiar with how railguns work. I paid a quick visit to Wikipedia and the current is supposed to go through the armature of the projectile, so breakdown of the air inside the shaft is detrimental.

> One big question this video begs is, what causes the giant fireball? Rail guns are supposed to be powered solely by electricity, and don't use explosives of any kind for propellant. Babb told PopSci the answer: The flames are from pieces of the projectile disintegrating; the 7-pound slug is jammed so firmly between the rails that when it's fired, pieces shear off and ignite in the air.

http://www.popsci.com/military-aviation-space/article/2008-0...

I'm pretty sure that's the air exploding as the round pushes it out of the way

I'm curious as well.

My first guess was that the explosions are coming from the projectile punching through those walls they put up, or the speed of a projectile superheating the air through which it's travelling to the point of igniting the air around it.

The latter guess seems way off base, but I'd love to learn exactly why!

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