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Electromagnetic Railgun Blasts Off

  1. Feb 8, 2008 #1

    SF

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    Last week at the Naval Surface Warfare Center, in Dahlgren, VA, a seven-pound bullet emerged from a truck-sized contraption at seven times the speed of sound and sent a visible shockwave through the air before crashing into a metal bunker filled with sand. With 10.6 megajoules of kinetic energy, this aluminum slug was propelled not by explosives but by an electric field, making this the most powerful electromagnetic railgun ever fired.

    http://www.technologyreview.com/Infotech/20164/
     
  2. jcsd
  3. Feb 8, 2008 #2

    mgb_phys

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    So about 1/4 of the explosive energy of an equivalent sized conventional shell.
     
  4. Feb 8, 2008 #3
    Railguns are still reallly cool technology, but I don't like that the military is doing them.
     
  5. Feb 9, 2008 #4

    mgb_phys

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    Also the stated use doesn't match the technology. Railguns are very fast and can deliver moderate amounts of energy over short ranges so are ideal for close quarter defence such as shooting down inbound missiles/shells.
    The articles suggest that they are a replacement for cruise missles which deliver a 1000kg warhead 1000km away flying at low level in a relatively stealthy way.
    Railgun projectiles are generally line of sight, you certainly can't steer them to ground hug like a cruise missile, you could hit a distant target by firing a ballistic path but at mach 8 it would almost be in orbit.
    Apart from the amount of energy you would need to do this, the outgoing round would be like a shuttle launch and would announce your position to everyone on the planet, in addition a ICBM like flight path arriving near certain countries could make them very nervous.

    It is probable that this is either a justification for a meag project in the hope of getting enough funding for a more limited aim or simple pork barrelling. Like Boeings 747 based star wars laser.
     
  6. Feb 10, 2008 #5
    Seems to me that the stated use exactly matches the technology. I didn't see anything in the article (that SF posted) about replacing cruise missles. They want to replace the MK45, a sea-to-land gun.

    Also, while the test resulted in 10.6 Megajoules, the gun is designed for 32 Mj, and their goal is 64 Mj.
     
    Last edited: Feb 10, 2008
  7. Feb 10, 2008 #6
    You are quite on point about the range issue. Railguns are being considered for long (>10 km) shots. It's the basic USS Iowa situation except the Iowa used BIGGG shells at low velocity whereas a railgun would use small projectiles at high velocity, the M16 idea with the same drawbacks about deflection.
     
  8. Feb 10, 2008 #7

    SF

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    What about sniper rifles? Can this technology be adapted to such small sizes?
     
  9. Feb 10, 2008 #8
    That's probably not practical anytime soon. The power cord running up to the sniper's nest would be a dead giveaway. Plus, there's the time delay problem. Current technology works pretty well. Maybe in the future, when battery energy density is significantly greater, this could be looked at again.
     
  10. Feb 10, 2008 #9
    No, it couldn't be used for a rifle, etc. because they used huge supercapacitors, (check out Powerlabs), way to heavy.
     
  11. Feb 11, 2008 #10
    Would'nt want to try to shoulder the recoil of sniper railgun lol esp at 10MJ :P
     
  12. Feb 11, 2008 #11
    There shouldn't be much recoil, I'd think, because the projectiles is being pulled by the magnetic field.
     
  13. Feb 11, 2008 #12

    mgb_phys

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    It doesn't matter - if you throw mass m with velocity v, the gun is going backward with velocity v (m/M).
    Of course it helps that although v is large the mass of the railgun is pretty huge.
     
  14. Feb 11, 2008 #13
    Speaking of all that, why did the gun in the demo video have such a heavy breechblock?
     
  15. Feb 11, 2008 #14

    mgb_phys

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    The point is that at roughly supersonic speeds with a considerable fall of shot you only have to fire a naval shell at 30 deg to hit a target 20km away.
    If the projectile is going at mach 10-20 in a straight line - the only way to hit a target 20km away is to fire it almost straight up so it goes up 100km and falls back on a balastic missile type trajectory. Since it is then going to be falling at terminal velocity you lose a lot of the advantage of firing it at mach20 in the first place. These are great close range ground to air weapons but do not replace cruise / BFO guns for over the horizon stuff.
     
  16. Feb 11, 2008 #15
    Hey, I'm just repeating the Navy's story that they want to do ship-to-shore up to 200 miles. They know what the trajectory will look like.
     
  17. Feb 11, 2008 #16

    Mech_Engineer

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    I don't really think this is true... A shell going at large supersonic speeds is still subject to projectile motion and aerodynamic drag.

    After some very dirty calcs, a shell fired at Mach 10 from an initial height of 15m (deck of a ship) could be fired at an angle of 1 degree to hit a target about 20km away, with a flight time of about 6 seconds and a maximum vertical height of 150m. Longer flight trajectories are even harder to estimate since errors in the drag approximation compound over the time integrated; but it looks like shooting at the same speed and an elevation of 15 degrees would yield a range of about 75 km (with a maximum altitude of 7500m) and a final projectile speed of Mach 9.5. Whether the numbers I calculated are correct or not is irrelevant however; the bottom line is superfast railguns would have to shoot at very low angles to achieve the same usable range as conventional weapons, but can achieve much higher maximum ranges.

    Now LASER weapons would be impossible to use for over-the-horizon attacks from a ground-based system, because they actually do travel in a straight line.
     
    Last edited: Feb 12, 2008
  18. Feb 11, 2008 #17

    mgb_phys

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    I thought the current Mach10 tests could only go a very short distances and to reach >10km you needed much higher velocities.
    Not sure how you calculate the atmospheric drag of somethign doing mach 20-30!

    The trouble with very low angles at sea is that waves tend to get in the way!
     
  19. Feb 11, 2008 #18

    Mech_Engineer

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    Well I'm probably messing up some part of the atmospheric drag (mechanical engineer, not aero), because it looks like it would be quite easy to achieve 50km with the projectile they tested in the article (mach 7, 7 lb slug). I do know one thing, getting a 40lb slug to go 200mi would take a fantastically powerful railgun, on the order of 100-300 MJ I would think...

    Well, in my previous post you can see that the maximum flight altitude would be 150m, well above any waves. Shooting at an initial height of 15m isn't unreasonable, and that would clear you of most of the waves anyway.
     
  20. Feb 16, 2008 #19

    mheslep

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    Ultracaps might change that. 5KW/kg now.
     
  21. Feb 16, 2008 #20

    mheslep

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    Sure they can. You fire an explosive projectile so that terminal velocity KE is not a concern. Recall that Sadam Hussein was working on the same idea w/ traditional chemical launch 'supergun' cannons until the Israeli's killed his designer. Several 100KM range, long enough to hit Israel from Iraq was the goal. Yes the barrel is pointed at high angles which gets the round up above most of the atmosphere for most of the trip. Problem is that the barrel has to grow to extraordinary lengths (~50M) to allow the explosive gas expansion time to accelerate the projectile before it leaves the barrel and becomes ballistic. Railgun tech. doesn't have that limitation.
     
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