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Science Fiction (not): record-setting EM rail gun test


Joe Frickin' Friday

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Joe Frickin' Friday

WOW.

 

Electromagnetic rail gun sets muzzle velocity record

 

Great video at end. Sparks from the gun at 0:13 suggest they're pretty much at the edge of what their equipment can do right now!

 

For reference, muzzle velocity of an Iowa-class battleship projectile (1900-2700 pounds) is about Mach 2.6. The rail gun is pushing a 20-pound projectile to mach 7; assuming energy equivalence, it would only be able to get a 2000-pound projectile up to about mach 0.7. So I guess it's not terribly impressive for launching big shells, but if you're interested in a short time-of-flight, or insane armor-penetrating speeds, the rail gun starts to look pretty attractive.

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Mitch, that's pretty neat, however...

 

It is still a ballistic weapon, so,

  • I wonder what the remaining velocity of a 20lb projectile would be after 20 miles. That can't be computed without knowing the ballistic coefficient of the round, so we're out of luck figuring that out. The one they use in the clip is square-fronted and my guess is that it's to to kill the velocity quickly in this experiment.
    This weapon depends on velocity for its impact energy. A lightweight projectile (and 20 lbs is lightweight for a cannon) bleeds off velocity very quickly (but long and skinny is better than short and fat for velocity retention.) A heavier round (to compensate for the loss of velocity) would call for an increase in launch energy that I bet would be disproportional to the weight.
  • No doubt, accuracy would be increased since time of flight would be shorter but they would still call for ballistic calculations. The equation changes, of course, if they are firing a smart round.

 

Fascinating stuff, though.

 

Pilgrim

 

 

 

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Danny caddyshack Noonan

(.5)*mass*velocity*velocity is how one arrives at kinetic energy.

 

Although the BB gun has mass, it doesn't have much velocity compared to the railgun. It does possess much more chemical energy in the round though. The railgun has the squared difference in the velocity component to give it much more kinetic energy.

 

Another difference is chemical energy required to launch a BB shell and electrical energy for a railgun. You have to carry the chemical energy in shells with a defined amount of storage. The railgun just has to find a way to store or create the electrical energy. That right there is the challenge. Windage and elevation don't play very much at mach 7 and up when you are within a few miles.

Railguns are largely insesitive munitions in a very broad sense. Depending on the source of electrical energy, there isn't a magazine to cook off.

 

I think I recall something weird happening in upper mach numbers to ballistic coefficient (there is an old haze of Reynolds and Nuessolt numbers) that makes railguns attractive besides not having to carry any energetic components in the round. It is hit-to-kill.

 

Dang, slacking at work time running out.

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I believe two of the goals of the rail gun research are longer range with a 200 mile target and perhaps more importantly the advantages of electrical vs. chemical energy as Peter mentioned.

 

This could in theory allow the gun to continuously charge via the ships batteries/powerplant, although I have read that the Navy said this is still years of battery development away.

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Really interesting article. What spiked my curiosity is that it does not explode on impact but instead obliterates the intended target. Its a 20Lb shell traveling at mach 8. If it hits an object that is much larger than the shell with no explosive elements, how does it obliterate the object. Another cool thing is that Its traveling at about 1.3 miles a second which is fast, but still it is only 1/186000th of the speed of light. Another issue I see is that with almost 90 seconds of flight time to reach its target, (100 miles) and as it is traveling above the speed of sound, it does by default give off a warning signal through the sound wave, allowing the target time to manipulate the target away from the quoted pinpoint accuracy. The other thing is if an object is traveling 8 times faster than the speed of sound, is the hearing or detection factor of the sound wave delayed by 7/8ths in distance behind the moving object. Does the sound wave get smaller or larger as the speed increases in multiples of the speed of sound.

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Joe Frickin' Friday
Really interesting article. What spiked my curiosity is that it does not explode on impact but instead obliterates the intended target. Its a 20Lb shell traveling at mach 8. If it hits an object that is much larger than the shell with no explosive elements, how does it obliterate the object.

 

By virtue of the kinetic energy it's carrying. No doubt a 20-pound projectile - even one traveling at Mach 7 - won't do as much damage as a 2000-pound shell filled with high explosive, but it's nonetheless impressive what raw kinetic energy can do. Tungsten darts of appropriate mass, fired at high speed, are able to penetrate tank armor without the benefit of high explosives:

 

http://en.wikipedia.org/wiki/Kinetic_energy_penetrator

 

That article mentions an American-made anti-tank weapon, a 9-pound tungsten dart maybe 1" diameter and a couple of feet long, fired (with chemical propellant) at a muzzle velocity of about mach 5.5. This isn't a prototype: the KEW-A1 has been in service for many years now.

 

 

Another issue I see is that with almost 90 seconds of flight time to reach its target, (100 miles) and as it is traveling above the speed of sound, it does by default give off a warning signal through the sound wave, allowing the target time to manipulate the target away from the quoted pinpoint accuracy. The other thing is if an object is traveling 8 times faster than the speed of sound, is the hearing or detection factor of the sound wave delayed by 7/8ths in distance behind the moving object. Does the sound wave get smaller or larger as the speed increases in multiples of the speed of sound.

 

The shock wave manifests as a cone behind the projectile, same as the sonic boom behind an airplane:

 

800px-Sonic_boom.svg.png

 

The projectile will reach its target before its sound waves do. The only hope for a target would be to observe the inbound projectile on radar early enough to take some kind of action.

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Danny caddyshack Noonan

Another issue I see is that with almost 90 seconds of flight time to reach its target, (100 miles) and as it is traveling above the speed of sound, it does by default give off a warning signal through the sound wave, allowing

 

It's actually more insidious than that. Without moving itself, the target moves due to rotation of the earth. Artillery takes this into account usually.

 

BTW, this thing is going to run out of energy long before 90 seconds. I won't be surprised if they ultimately put a booster or sustain motor into it.

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Joe Frickin' Friday

I've seen it suggested elsewhere that a weapon with such a high muzzle velocity may have limited battlefield utility. At very high speeds, the ballistic arc is much more shallow than at low speeds, so you can either fire line-of-sight at short-range targets, or ballistic at long-range, over-the-horizon targets - but firing ballistic at medium range targets, it will be difficult to "drop" rounds in on the target because of obstructions in the (low-arc) path of the projectile.

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I've seen it suggested elsewhere that a weapon with such a high muzzle velocity may have limited battlefield utility. At very high speeds, the ballistic arc is much more shallow than at low speeds, so you can either fire line-of-sight at short-range targets, or ballistic at long-range, over-the-horizon targets - but firing ballistic at medium range targets, it will be difficult to "drop" rounds in on the target because of obstructions in the (low-arc) path of the projectile.

 

Maybe that's why it appears the Navy is the sponsor of the gun in the link ... outside of a few pesky islands, not many obstructions on the ocean.

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The only hope for a target would be to observe the inbound projectile on radar early enough to take some kind of action.

 

Mach 7 is about a mile a second. It has a range of 100 miles. So all is over in bit over 100 seconds. Radar could certainly detect and track it, but what kind of meaningful action can be done in that short time? Perhaps another gun round? Likely, the radar will tell you that you are doomed.

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Joe Frickin' Friday
Mach 7 is about a mile a second. It has a range of 100 miles. So all is over in bit over 100 seconds. Radar could certainly detect and track it, but what kind of meaningful action can be done in that short time?

 

Evacuate?

 

 

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The only hope for a target would be to observe the inbound projectile on radar early enough to take some kind of action.

 

Mach 7 is about a mile a second. It has a range of 100 miles. So all is over in bit over 100 seconds. Radar could certainly detect and track it, but what kind of meaningful action can be done in that short time? Perhaps another gun round? Likely, the radar will tell you that you are doomed.

 

The answer to that is

Metal Storm.

 

Wikipedia Article.

 

This video contains several iterations of firing at increasing rates, so stick with it until the end. One rate is a theoretical million rounds per minute for a small burst of 180 rounds.

 

Point is, the rail gun fires a ballistic round. Ballistic paths are completely predictable so, with further development that requires no magic breakthroughs, a Metal Storm gun could put a cloud of rounds into the path of the incoming shell, just as they do for missiles now with the 20mm CIWS. Since the incoming round is a kinetic energy projectile any induced miss is as good as a mile, sort of. I do wonder, though, what would be the result of its impact with water at that kind of speed close alongside a vessel.

 

Pilgrim

 

 

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Joe Frickin' Friday
I do wonder, though, what would be the result of its impact with water at that kind of speed close alongside a vessel.

 

Speed of sound in water is almost five times what it is in air - in other words, an airborne projectile traveling at Mach 5 would make a pretty spectactular splash when it hit the water, but I wouldn't expect much of a waterborne shock wave, at least not one capable of damaging a big vessel.

 

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