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Hollow projectiles?

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Barry

Wed Jan 07 2009, 03:03PM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

blackgrunge wrote ...

... people may be picking up some extra velocity when using a ferromagnetic ring or hollow cylinder is that when the projectile leaves the coil the capacitors aren't done completely discharging and the inductor is still producing a magnetic field. In doing so eddy currents are created in the ring and it is pushed away from the coil as well as pulled at first by the coil.

Nice thought but it doesn't work that way. On entry, the eddy currents will reduce the magnetic attraction by some percentage. On exit they work the same, the magnetic attraction is the same dominate factor as on entry, and eddy currents will reduce that attraction by the same percentage.

In theory it would be possible to both attract on entry, and repel on exit. But to do so requires changing the frequency. You'd need a low frequency pulse to attract, as usual. Once the projectile reaches the midpoint change to a much higher frequency so that the skin depth becomes much thinner, thereby becoming an induction launcher. The coil won't 'see' the inner ferrous core of the projectile.

In practice a double-mode launcher is a bad idea. All the factors that make a good switched-reluctance launcher will make a bad induction launcher, and vice versa. It might work but it will never work very well.

Cheers, Barry
Barry greets 2009 warmly, gives her a hug and offers her a drink. Or two. And hints of fun things to come. "Let's get cozy," he says.

rp181

Wed Jan 07 2009, 03:47PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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Posts: 1529

Awhile back someone proposed a 2 part projectile(

, steel and aluminum to do what you said . look up that thread to see what people said.
The eddy currents exert a force on the projectile, but the net force is still zero assuming a even pulse.

Barry

Thu Jan 08 2009, 03:01PM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

From another conversation, rp181 wishes to clarify his answer with:

"By even i meant a sine wave where the peak was when the projectile was in the center. This would also require a constant speed on the projectile though. I found the thread: Using both induction and reluctance for coilgun on Sun Apr 16 2006"

Thanks rp! interesting reading that arrives at about the same conclusion.

hotcrazyfruit

Sun Jan 25 2009, 08:33PM

banned on 5/26/2009
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Barry wrote ...

For flight stability we need the center of pressure to be located behind the center of mass. Or spin-stabilize the projectile.

that only happens at speeds higher than mach 1, or supersonic rounds. which is hard to do unless you have a really large capacitor bank, a good efficiency and something to shoot at xD.

hotcrazyfruit

Sun Jan 25 2009, 08:34PM

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and your thinking of the skin effect. and that happens on high frequency electricity. it all goes to the outside of the wire basically. "to the skin". i think.

Barry

Mon Jan 26 2009, 01:14AM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

hotcrazyfruit wrote ...

Barry wrote ...

For flight stability we need the center of pressure to be located behind the center of mass. Or spin-stabilize the projectile.

that only happens at speeds higher than mach 1, or supersonic rounds. which is hard to do unless you have a really large capacitor bank, a good efficiency and something to shoot at xD.

It's dead simple -- even low-speed arrows and shuttlecocks use 'feathers' to stabilize flight. It works by putting most of the air resistance (center of pressure) behind the center of mass. The prevents tumbling and, if you angle the feathers slightly off-axis, it even provides a small spin to keep it moving relatively straight.

I'm not sure what effects you might be thinking of in regards to supersonic speeds. I hope it's a problem I need to deal with soon but, alas, it looks like it could be awhile for me.

Cheers, Barry
I want to die in my sleep like my grandfather, not screaming and yelling like his passengers.

hotcrazyfruit

Mon Jan 26 2009, 08:43PM

banned on 5/26/2009
Registered Member #1877 Joined: Mon Dec 22 2008, 02:03AM
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Posts: 147

Barry wrote ...

hotcrazyfruit wrote ...

Barry wrote ...

For flight stability we need the center of pressure to be located behind the center of mass. Or spin-stabilize the projectile.

that only happens at speeds higher than mach 1, or supersonic rounds. which is hard to do unless you have a really large capacitor bank, a good efficiency and something to shoot at xD.

very true, the comment about the skin effect was to another poster, but i remembered that at speeds higher than mach one, the center of resistance can be put behind the actual bullet. kind of the same concept with the arrow and shuttle cock, only the center of resistance is pushed behind the bullet. and thus creating extreme stability in the projectile, even more so than an arrow.

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