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LAN_403

Pinkamena

Wed Feb 29 2012, 07:40PM

Registered Member #4237 Joined: Tue Nov 29 2011, 02:49PM
Location:
Posts: 117

Hello!
I am currently 3 or 4 months into designing my coilgun, and I have begun putting some real thought into the projectile, and so I have some questions.

1. Is there such a thing as a "perfect" length to diameter ratio?

2. What would be the best material for casting? I have already decided a long time ago that I want to make my projectile using iron filings and some sort of material suitable for casting. Resin has crossed my mind, but that seems awfully fragile.

3. Regarding the front end of the projectile, I had thought of getting it machined from a piece of iron. That way it won't just be dead weight, but will be accelerated as well as the rest of the projectile. Problem is, I'll get eddie currents if the head is solid. So I was thinking of putting slits into it like this, and then filling them with some sort of hard, nonconductive material. What do you think of that idea?

4. Lastly, what is the airspeed velocity of an unladen swallow?

Legit_bacon

Wed Feb 29 2012, 08:17PM

Registered Member #4034 Joined: Thu Jul 28 2011, 10:41PM
Location: somewhere in the Southern hemisphere
Posts: 138

4. Lastly, what is the airspeed velocity of an unladen swallow?

AHAHA!!! well that would be 42 of course, unless it was about 11...

Saz43

Wed Feb 29 2012, 10:19PM

Registered Member #1525 Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294

Pinkamena wrote ...

1. Is there such a thing as a "perfect" length to diameter ratio?

Probably not. But a long, slender projectile will develop a weaker demagnetizing field, which works against the magetnizting field of the coil.

(section 9)

Pinkamena wrote ...

4. Lastly, what is the airspeed velocity of an unladen swallow?

Over 9000 quadratic furlongs per acre

Forty

Wed Feb 29 2012, 10:48PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

1,843,200 gallons per square inch fortnight

(20mph)

edit: your projectile tip looks very nice. A powdered iron projectile would be good so long as you can find a
way to keep it from shattering every time it hits something. Perhaps a thin walled shell could be cut on a lathe from
plastic (or teflon), and then that filled with your iron mixture. You'll gain some air gap but at least the rounds can be fired more than once. A longer, dart like projectile would be something cool to see, and with the denser steel tip at the front, you might gain a little stabilization from it.

Turkey9

Thu Mar 01 2012, 12:27AM

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

Unless you're going to utilize some sort of stabilizing technique, your projectile isn't going to need a nose cone. I would also imagine that it would perform better with a purely conic shape. With a nose cone or a tapered tip, you get an air gap for the first little bit of acceleration that will hurt efficiency.

If you do go with the nose cone, but slot it, why not just make a single piece and slot the entire projectile? Sounds overly complicated to me.

Pinkamena

Thu Mar 01 2012, 11:58AM

Registered Member #4237 Joined: Tue Nov 29 2011, 02:49PM
Location:
Posts: 117

Turkey9 wrote ...

Unless you're going to utilize some sort of stabilizing technique, your projectile isn't going to need a nose cone. I would also imagine that it would perform better with a purely conic shape. With a nose cone or a tapered tip, you get an air gap for the first little bit of acceleration that will hurt efficiency.

If you do go with the nose cone, but slot it, why not just make a single piece and slot the entire projectile? Sounds overly complicated to me.

Thanks for the reply. I'll go with a conic shape then. The reason I am not making the projectile out of a single piece, is because I want to have better control of the dimensions. If I cast it myself, I can easily control the dimensions, and make many projectiles. Besides, I do not have access to machinery for working with metal.

Saz43 wrote ...

Pinkamena wrote ...

1. Is there such a thing as a "perfect" length to diameter ratio?

Probably not. But a long, slender projectile will develop a weaker demagnetizing field, which works against the magetnizting field of the coil.

(section 9)

Interesting. I'll go with a long and slender projectile then.

Forty wrote ...

Your projectile tip looks very nice. A powdered iron projectile would be good so long as you can find a
way to keep it from shattering every time it hits something. Perhaps a thin walled shell could be cut on a lathe from
plastic (or teflon), and then that filled with your iron mixture. You'll gain some air gap but at least the rounds can be fired more than once. A longer, dart like projectile would be something cool to see, and with the denser steel tip at the front, you might gain a little stabilization from it.

Now where to find teflon sheets... Good idea though.

But do you guys have any insight to what sort of material I should mix the iron filings into? As I said, resin would work, but it seems a little too fragile. Are there any sort of materials that when it hardens, becomes extremely durable?

Forty

Thu Mar 01 2012, 05:51PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

I was thinking more of a teflon rod that you'd cut to length and then bore out a cavity from the end using a lathe or carefully with a drill. Delrin could work too.
I think the problem with the bonding material for the filings is that you want as much iron and as little filler as possible. So with such a small amount of the bonding agent dispersed all over within the slug, it doesn't have sufficient strength to hold together.

Anyone know how powdered iron toroids are made? Are they sintered or something?

Another shell idea would be to use a conductive material with a single slot along the length. you could stick that in a mold (so it doesn't open up) and then pack it with your iron epoxy mix. A material source that comes to mind (because I was just looking at them yesterday for a barrel) would be an aluminum arrow shaft. They're approx 0.3" diameter, not sure if you wanted larger than that.

something like J-B Weld might be more durable than standard epoxy.

Pinkamena

Thu Mar 01 2012, 08:22PM

Registered Member #4237 Joined: Tue Nov 29 2011, 02:49PM
Location:
Posts: 117

Instea of using teflon, wouldn't it be better to use a composite material like carbon fiber?
And regarding iron powder toroids, I saw a person on youtube who mixed the iron with standard fiberglass resin.

Turkey9

Fri Mar 02 2012, 12:54AM

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

EDIT I meant to say cylindrical, not conic. My point was that you shouldn't have a tip at all on the projectile.

Pinkamena

Fri Mar 02 2012, 02:42PM

Registered Member #4237 Joined: Tue Nov 29 2011, 02:49PM
Location:
Posts: 117

Turkey9 wrote ...

EDIT I meant to say cylindrical, not conic. My point was that you shouldn't have a tip at all on the projectile.

I think it should. Not for better aerodynamics, but because the projectile will shatter too easily if it doesn't have a solid tip that can distribute the force of impact across the entire area of the projectile. I'll make the projectile out of a fragile resin/iron powder mix, remember?

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