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Why do railguns produce an "explosion"?

  1. Feb 15, 2015 #1


    On 0:15 in the video above, you can clearly see an explosion just when the projectile in launched from the end of the barrel. However, as I understand it, railguns are purely powered by electromagnetic launchers and does not use any kind of explosives to launch the projectile.

    Many comments on Youtube suggest that the explosion is caused by air friction/shock heating but I do not think that is the case since I have seen videos of vehicle undergoing atmospheric reentry and they do not "explode" like how the projectile in his video does. Also, the projectile is not glowing from the heat and no re-entry plasma trails are visible but I could be mistaken.
     
  2. jcsd
  3. Feb 15, 2015 #2

    SteamKing

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    The article below describes in a little more detail than various You Tube comments what happens when the rail gun is fired:

    http://www.technologyreview.com/news/409497/electromagnetic-railgun-blasts-off/

    The extreme velocity of the projectile causes quite a bit of heating to occur, not only to the projectile itself but also to the surrounding air.

    I think you are overlooking the fact that a space craft re-entering the atmosphere does so in a gradual fashion, i.e. the craft assumes a certain shallow angle w.r.t. the horizon before actually entering the atmosphere. The density of the upper atmosphere is also quite low compared to what you find closer to sea level.
     
  4. Feb 15, 2015 #3

    A.T.

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    Because they control the vehicle to avoid that. Here is what an uncontrolled entry looks like:

     
  5. Feb 15, 2015 #4
    Thanks for the replies.

    In the video above, the meteor leaves behind a trail and brightly glows from the heat. Why does the explosion abruptly stops after the projectile leaves the barrel? Shouldn't it also glow and leave a fiery trail behind as well?
     
  6. Feb 15, 2015 #5
    Well, after the explosion there is a slower implosion. Some very damped ringing possibly too. All that isn't as violent and loud as the initial explosion.
     
  7. Feb 15, 2015 #6

    A.T.

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    The meteor disintegrates and leaves material behind it.
     
  8. Feb 15, 2015 #7
    I bet you the "barrel" of the gun is in a sealed chamber and the projectile is breaching the seal at Mach 5 or so, hence the explosion. Some light gas guns do this to isolate gases with different speed of sounds.
     
  9. Jan 28, 2016 #8
    The projectile does not "explode" upon leaving the gun. What you see are the sabot sections falling away from the projectile. The undamaged projectile continues its trajectory.
     
  10. Jan 28, 2016 #9
    It's not a real detonation (explosion) or even a deflagration (burning).

    It's just the venting of hot gases. Hot gases glow just like hot lava from a volcano or hot coils on an electric stove.

    The muzzle flash of conventional firearms can demonstrate a similar effect.
     
  11. Jan 28, 2016 #10

    davenn

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    except that IS the result of a detonation !
     
  12. Jan 28, 2016 #11
    Not really. The only detonation in a conventional firearm is a tiny amount of chemical in the primer when struck by the firing pin that is only intended to ignite the nitrocellulose based gunpowder. The bullet does not even move unless the nitrocellulose is ignited. The burning of the nitrocellulose (gunpowder) is a subsonic deflagration (burning) that occurs over about a 2 millisecond time period as the bullet moves down the barrel rather than a supersonic deflagration that would take less than 100 microseconds (0.1 milliseconds) given the typical dimensions cartridge cases.

    A firearm is the simplest version of an internal combustion engine. Just as a burning gas-air mixture forces a piston in a car engine, a burning nitrocellulose based powder forces a bullet out of the barrel. The typical rise time of burning gunpowder in a rifle chamber is 0.4 milliseconds. Real detonations have pressure rise times under 1 microsecond representing a true shock front.
     
  13. Jan 28, 2016 #12

    davenn

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    yes really !!

    T
    of course :rolleyes::rolleyes: and that is the real detonation


    yes and the flame is from the burning powder :wink::rolleyes:

    we all know how a firearm works

    but of course this is all irrelevant in the discussion of the railgun
    so lets please get back on topic

    D
     
    Last edited: Jan 28, 2016
  14. Jan 28, 2016 #13
    Please stop obfuscating the real differences between burning (deflagration) and true detonation.

    See:
    https://en.wikipedia.org/wiki/Deflagration
    and
    https://en.wikipedia.org/wiki/Detonation

    The reaction of the nitrocellulose in a conventional firearm is a deflagration.


    The question at hand is whether the glowing gases that appear out of the front of a rail gun are attributable to an explosion (a detonation), burning (a deflagration), or something else.

    You don't need a detonation or a deflagration for gases to glow. All you need is heat. Just as electric coils on a stove will glow simply because they are hot, so will gases emerging from the muzzle of a rail gun or a conventional firearm. Look up the formulas for blackbody radiation. Gases are not perfect blackbodies, but hot gases will glow due to similar mechanisms. We can discuss the mechanisms that heated the gases in each case, but the bottom line is hot gases coming out of a barrel emit radiation in the visible range simply because they are hot. There need not be an ongoing chemical reaction (deflagration or detonation).
     
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