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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. Bu

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?

bigstumpy 12y ago
I'm pretty sure that's the air exploding as the round pushes it out of the way
nerfhammer 12y ago
The projectile is moving so fast the friction against the air makes enough heat to create a ball of plasma
- weland 12y ago
  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.
  - darkmighty 12y ago
    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)
    - weland 12y ago
      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.
JonLim 12y ago
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!
ethanbond 12y ago
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.
- tjradcliffe 12y ago
  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.)
  - metaphorm 12y ago
    well, that gives you an idea just how much chemical potential energy there is in gasoline. its pretty incredible stuff.
    - deeviant 12y ago
      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.
jandrewrogers 12y ago
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.
vilhelm_s 12y ago
> 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/2008-02/navy-tests-32-megajoule-railgun http://www.popsci.com/military-aviation-space/article/2008-0...

bigstumpy 12y ago

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

nerfhammer 12y ago

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

weland 12y ago
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.
- darkmighty 12y ago
  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)
  - weland 12y ago
    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.

JonLim 12y ago

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!

ethanbond 12y ago

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.

tjradcliffe 12y ago
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.)
- metaphorm 12y ago
  well, that gives you an idea just how much chemical potential energy there is in gasoline. its pretty incredible stuff.
  - deeviant 12y ago
    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.

jandrewrogers 12y ago

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.

vilhelm_s 12y ago

> 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/2008-02/navy-tests-32-megajoule-railgun http://www.popsci.com/military-aviation-space/article/2008-0...