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Hollow projectiles?

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mogallin

Thu Jan 01 2009, 04:17PM

Registered Member #1883 Joined: Thu Dec 25 2008, 01:58PM
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Posts: 15

I heard somewhere that inside a conductor, at least if it is hollow, the will be no magnetic field and all charge will be on the outside. If this is the way it is, would it mean that a hollow projectile will be pulled towards the magnetic field just as good as a solid projectile?

rp181

Thu Jan 01 2009, 04:20PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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look at what i found:

Barry

Thu Jan 01 2009, 05:58PM

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

Yes, hollow and irregular projectiles are pulled towards the magnetic field exactly like solid regular projectiles. As far as the forces involved from magnetism go, the shape is irrelevant.

But for other considerations, the general shape is fairly important. I'm still really struggling with a 'final' answer but I have learned a few things along the way...

A long object works better than a very short one. The reason is that shorter objects need a faster pulse than long ones, and designing a very fast discharge is more difficult and stresses parts more than a long pulse.

A thicker rod can absorb more energy than a thin pin or nail. The reason is that the larger cross-section will magnetically saturate at higher applied field strength.

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

Round conductive objects will suffer eddy currents, so it's beneficial to have a longitudinal break in the skin.

The bottom line ... exit velocity depends on the summation of the force on projectile at each point as it moves through the coil, which in turn depends on the instantaneous current and the 3d projectile shape. So the waveform of the discharge has to be factored in. Not a simple problem. So I take a simple approach by whacking off lengths of 1/4" welding rod and fire away.

I hope others add to this list!

Happy New Year
Barry screeches to a stop, reaches over to open the door and roughly pushes 2008 to the curb. Then he drives away.

rp181

Thu Jan 01 2009, 06:05PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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Spin stabilization probably wont work on most coilgun's, the low speed will made it difficult to spin (by means of rifling).

mogallin

Thu Jan 01 2009, 07:41PM

Registered Member #1883 Joined: Thu Dec 25 2008, 01:58PM
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Posts: 15

Didn't quite get all of that...

But iI did understand some of it. Thanks!

CyrusTheGreat

Thu Jan 01 2009, 07:55PM

Registered Member #537 Joined: Sun Feb 18 2007, 07:22PM
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I've tried using a hollow projectile, but it didn't turn out so well. The efficiency and kinetic energy were crud. However, it did go pretty fast because of the reduced mass. I imagine that if I had used a thicker projectile with a smaller hole in the middle it would have worked better.

Cheers!

Turkey9

Fri Jan 02 2009, 02:39AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

I also tried a hollow projectile on my six stage cg. The velocity went up, but the energy stayed about the same so there was still the same efficiency. My cg was pretty low energy per stage so the projectile didn't come close to saturation so i don't know if that was a factor. The hollow projectile did make a nice whistling sound when it was fired

blackgrunge

Fri Jan 02 2009, 06:27PM

Registered Member #1889 Joined: Mon Dec 29 2008, 07:36AM
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To answer Barry's question and give an explanation for why some people would be seeing improvements is that your not only picking up the force of attraction but also of repulsion. As everyone knows eddy currents cause a projectile to emit an EM field that repels it from the coils, thus the reason for eddy currents being bad (and that it wastes energy). This idea can be seen if you have a hollow copper cylinder and but it slightly of center of a coil and pulse a current through it. The change in voltage induces eddy currents in the copper ring and causes it to produce a magnetic field that repels the one being created by the coil. The reason that 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.

rp181

Fri Jan 02 2009, 06:29PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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He said the energy output was about the same, So i am thinking it was just a normal trade off between mass and speed.
Arn't slots is projectiles supposed to lower eddy currents?

blackgrunge

Tue Jan 06 2009, 10:22PM

Registered Member #1889 Joined: Mon Dec 29 2008, 07:36AM
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Posts: 55

He's not talking about slotted, he's talking about hollow. The reason that the energy outputs were about the same is because the force he lost in the attraction portion of it was then picked up in the gain repulsive forces due to eddy currents. If he were to do this on a much larger scale he would realize that more and more energy would be lost because he would be losing a lot more attractive force and the resistance and the inductance of the hollow projectile would waste the repulsive energy necessary to balance things out again. A better way to think about it is the difference between the attractive impulse and the repulsive impulse becomes greater and greater as magnetic field strength increases.

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