Questions about projectile
Pinkamena, Wed Feb 29 2012, 07:40PMHello!
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?
Re: Questions about projectile
Legit_bacon, Wed Feb 29 2012, 08:17PM
AHAHA!!! well that would be 42 of course, unless it was about 11...
Legit_bacon, Wed Feb 29 2012, 08:17PM
4. Lastly, what is the airspeed velocity of an unladen swallow?
AHAHA!!! well that would be 42 of course, unless it was about 11...
Re: Questions about projectile
Saz43, Wed Feb 29 2012, 10:19PM
(section 9)
Saz43, Wed Feb 29 2012, 10:19PM
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. 1. Is there such a thing as a "perfect" length to diameter ratio?
(section 9)Pinkamena wrote ...
4. Lastly, what is the airspeed velocity of an unladen swallow?
Over 9000 quadratic furlongs per acre4. Lastly, what is the airspeed velocity of an unladen swallow?
Re: Questions about projectile
Forty, Wed Feb 29 2012, 10:48PM
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.
Forty, Wed Feb 29 2012, 10:48PM
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.
Re: Questions about projectile
Turkey9, Thu Mar 01 2012, 12:27AM
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.
Turkey9, Thu Mar 01 2012, 12:27AM
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.
Re: Questions about projectile
Pinkamena, Thu Mar 01 2012, 11:58AM
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.
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?
Pinkamena, Thu Mar 01 2012, 11:58AM
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.
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 ...
(section 9)
Interesting. I'll go with a long and slender projectile then.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. 1. Is there such a thing as a "perfect" length to diameter ratio?
(section 9)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.
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?
Re: Questions about projectile
Forty, Thu Mar 01 2012, 05:51PM
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.
Forty, Thu Mar 01 2012, 05:51PM
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.
Re: Questions about projectile
Pinkamena, Thu Mar 01 2012, 08:22PM
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.
Pinkamena, Thu Mar 01 2012, 08:22PM
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.
Re: Questions about projectile
Turkey9, Fri Mar 02 2012, 12:54AM
EDIT I meant to say cylindrical, not conic. My point was that you shouldn't have a tip at all on the projectile.
Turkey9, Fri Mar 02 2012, 12:54AM
EDIT I meant to say cylindrical, not conic. My point was that you shouldn't have a tip at all on the projectile.
Re: Questions about projectile
Pinkamena, Fri Mar 02 2012, 02:42PM
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?
Pinkamena, Fri Mar 02 2012, 02:42PM
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.
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?
Re: Questions about projectile
Saz43, Fri Mar 02 2012, 07:26PM
The cylindrical projectile would probably have slightly better magnetic coupeling with the coil, but I think the losses that come with sharpening the last few centimeters are going to be miniscule. The sharpened tip gives you greater advantages, such as less drag and more importantly, more penetrating power. I like to build my coilguns to do as much damage as possible, and the sharp tip is necessary to get the killing power.
Saz43, Fri Mar 02 2012, 07:26PM
The cylindrical projectile would probably have slightly better magnetic coupeling with the coil, but I think the losses that come with sharpening the last few centimeters are going to be miniscule. The sharpened tip gives you greater advantages, such as less drag and more importantly, more penetrating power. I like to build my coilguns to do as much damage as possible, and the sharp tip is necessary to get the killing power.
Re: Questions about projectile
klugesmith, Fri Mar 02 2012, 08:29PM
Here are three other ideas for making laminated projectiles to reduce eddy currents.
1. Glue together a stack of "I" laminations from a transformer core, to make bar with a square cross section. Could wrap with fiberglass for more strength.
2. If you can get a sufficiently thin sheet of steel, roll it into a very thick-walled tube. OK to leave a steel mandrel in the center (like a sardine-can key?) The spiral cross-section can't conduct circumferential eddy current, if the turns are insulated from each other. For example, use painted metal from a "tin" cookie container.
3. Use a dense bundle of straight steel wires, bonded with epoxy. Could use glass tubing or a test-tube as sacrificial mold -- make barrel to fit projectile, instead of the other way around. Vacuum methods might help to get good resin penetration.
Have you got your hands dirty yet? How about making and characterizing a coilgun with traditional solid-steel projectiles? Then you would have a reference system, against which to compare the alleged benefits of eddy-current suppression or "external iron".
klugesmith, Fri Mar 02 2012, 08:29PM
Here are three other ideas for making laminated projectiles to reduce eddy currents.
1. Glue together a stack of "I" laminations from a transformer core, to make bar with a square cross section. Could wrap with fiberglass for more strength.
2. If you can get a sufficiently thin sheet of steel, roll it into a very thick-walled tube. OK to leave a steel mandrel in the center (like a sardine-can key?) The spiral cross-section can't conduct circumferential eddy current, if the turns are insulated from each other. For example, use painted metal from a "tin" cookie container.
3. Use a dense bundle of straight steel wires, bonded with epoxy. Could use glass tubing or a test-tube as sacrificial mold -- make barrel to fit projectile, instead of the other way around. Vacuum methods might help to get good resin penetration.
Have you got your hands dirty yet? How about making and characterizing a coilgun with traditional solid-steel projectiles? Then you would have a reference system, against which to compare the alleged benefits of eddy-current suppression or "external iron".
Re: Questions about projectile
Turkey9, Sat Mar 03 2012, 06:54PM
Yeah I may vote for a no sharpened tip projectile, but all of mine are sharpened!
Have you thought of using JB weld? As far as I know it's not conductive but is impregnated with steel filings so you would already be part of the way there. Plus, JB weld is some of the strongest epoxy I know of. It is actually used to fix cracks in the pistons in engines where my step dad works. You might not have a problem with chipping.
Turkey9, Sat Mar 03 2012, 06:54PM
Yeah I may vote for a no sharpened tip projectile, but all of mine are sharpened!
Have you thought of using JB weld? As far as I know it's not conductive but is impregnated with steel filings so you would already be part of the way there. Plus, JB weld is some of the strongest epoxy I know of. It is actually used to fix cracks in the pistons in engines where my step dad works. You might not have a problem with chipping.Re: Questions about projectile
Forty, Sat Mar 03 2012, 09:31PM
I already suggested J-B weld :P Their website said it was insulating, so that's good. I'll try it out myself sometime soon.
I've tried the I laminations before. If you use pieces from several transformer sizes then you can form more of a rounded shape by putting the widest sections in the middle and decreasing the width as you stack them up (might provide slightly better fill factor that way if you choose to surround the laminations with epoxy to make it round.) You could also just leave it as a square and use square holed transformer bobbin to make your coil on. A square barrel could be tricky though...
I've considered the wire segment idea before because i saw the same sort of core used in a classroom demo device. Can't think of a source material to use though.
the sheet metal spiral is a new one. I wonder if that ultraperm shielding they're always trying to sell on goldmine would work well. Just a thought. Would those super permeability materials even be good to use?
Oh and directly comparing two different projectiles using the same coil setup wouldn't be the best comparison unless you could somehow get their size and mass to be equal. I think what you'd have to do is find the optimum coil/initial position for each projectile and then compare the efficiency (using the same bank and switching.)
There's gotta be a better way than that to compare the materials though. Something with a permanent magnet maybe? or use it in the core of an electromagnet and do something with it? any ideas?
I agree that the projectile should be pointed if you intend to shoot it at anything hard. If the front is flat, then a ding or hard impact could misshape the edge and cause it to catch on the inside of the barrel.
Anyways, Pink have you tried anything yet or are you still just theorizing?
Forty, Sat Mar 03 2012, 09:31PM
I already suggested J-B weld :P Their website said it was insulating, so that's good. I'll try it out myself sometime soon.
I've tried the I laminations before. If you use pieces from several transformer sizes then you can form more of a rounded shape by putting the widest sections in the middle and decreasing the width as you stack them up (might provide slightly better fill factor that way if you choose to surround the laminations with epoxy to make it round.) You could also just leave it as a square and use square holed transformer bobbin to make your coil on. A square barrel could be tricky though...
I've considered the wire segment idea before because i saw the same sort of core used in a classroom demo device. Can't think of a source material to use though.
the sheet metal spiral is a new one. I wonder if that ultraperm shielding they're always trying to sell on goldmine would work well. Just a thought. Would those super permeability materials even be good to use?
Oh and directly comparing two different projectiles using the same coil setup wouldn't be the best comparison unless you could somehow get their size and mass to be equal. I think what you'd have to do is find the optimum coil/initial position for each projectile and then compare the efficiency (using the same bank and switching.)
There's gotta be a better way than that to compare the materials though. Something with a permanent magnet maybe? or use it in the core of an electromagnet and do something with it? any ideas?
I agree that the projectile should be pointed if you intend to shoot it at anything hard. If the front is flat, then a ding or hard impact could misshape the edge and cause it to catch on the inside of the barrel.
Anyways, Pink have you tried anything yet or are you still just theorizing?
Re: Questions about projectile
ZakWolf, Sat Mar 03 2012, 09:47PM
I wanted to give this a "shot" (sorry) and im going to use steal filings and shape lock.
Im going to put it to a section cut from the barrel for the mold, and as it cools shape some stabilizing fins on the back.
just my two cents
ZakWolf, Sat Mar 03 2012, 09:47PM
I wanted to give this a "shot" (sorry) and im going to use steal filings and shape lock.
Im going to put it to a section cut from the barrel for the mold, and as it cools shape some stabilizing fins on the back.
just my two cents
Re: Questions about projectile
Pinkamena, Sun Mar 04 2012, 06:10PM
Just theorizing :)
I'll be building it the summer, and I am taking my time planning it.
Pinkamena, Sun Mar 04 2012, 06:10PM
Forty wrote ...
SNIP
Anyways, Pink have you tried anything yet or are you still just theorizing?
SNIP
Anyways, Pink have you tried anything yet or are you still just theorizing?
Just theorizing :)
I'll be building it the summer, and I am taking my time planning it.
Re: Questions about projectile
Yandersen, Fri Oct 05 2012, 04:25PM
Did anyone thought about or tried to make a projectile from a piece of thick-wall iron pipe, length-cutted? Something like 10mm in outer diameter, 6mm inner, 15mm in length. Just another theoretical idea, but IMHO such projectile may be accelerated with higher efficiency, have lower eddy loses and be more stabile in air. If velocity is high, screw-thread on the inner part may give it some spinning also due to the compressed air going through it. Sure, making such projectile require special equipment, but maybe anyone have it? =>
Yandersen, Fri Oct 05 2012, 04:25PM
Did anyone thought about or tried to make a projectile from a piece of thick-wall iron pipe, length-cutted? Something like 10mm in outer diameter, 6mm inner, 15mm in length. Just another theoretical idea, but IMHO such projectile may be accelerated with higher efficiency, have lower eddy loses and be more stabile in air. If velocity is high, screw-thread on the inner part may give it some spinning also due to the compressed air going through it. Sure, making such projectile require special equipment, but maybe anyone have it? =>
Re: Questions about projectile
Yandersen, Wed Dec 05 2012, 02:49AM
Guys, I want to buy a machine for making projectiles out of iron rod. Any suggestions? Who has it already - where the hell did you get one?! =(
Yandersen, Wed Dec 05 2012, 02:49AM
Guys, I want to buy a machine for making projectiles out of iron rod. Any suggestions? Who has it already - where the hell did you get one?! =(
Re: Questions about projectile
Yanom, Wed Dec 05 2012, 03:06AM
Thought about this. It seems that you'd want your projectile mass closer to the wall of the coil, so it could be hollow....
and shorter projectiles are better right? Is that true up to a point or would it be good to have a coin-shaped projectile shorter than it is wide?
Yanom, Wed Dec 05 2012, 03:06AM
Yandersen wrote ...
Did anyone thought about or tried to make a projectile from a piece of thick-wall iron pipe, length-cutted? Something like 10mm in outer diameter, 6mm inner, 15mm in length. Just another theoretical idea, but IMHO such projectile may be accelerated with higher efficiency, have lower eddy loses and be more stabile in air. If velocity is high, screw-thread on the inner part may give it some spinning also due to the compressed air going through it. Sure, making such projectile require special equipment, but maybe anyone have it? =>
Did anyone thought about or tried to make a projectile from a piece of thick-wall iron pipe, length-cutted? Something like 10mm in outer diameter, 6mm inner, 15mm in length. Just another theoretical idea, but IMHO such projectile may be accelerated with higher efficiency, have lower eddy loses and be more stabile in air. If velocity is high, screw-thread on the inner part may give it some spinning also due to the compressed air going through it. Sure, making such projectile require special equipment, but maybe anyone have it? =>
Thought about this. It seems that you'd want your projectile mass closer to the wall of the coil, so it could be hollow....
and shorter projectiles are better right? Is that true up to a point or would it be good to have a coin-shaped projectile shorter than it is wide?
Re: Questions about projectile
Yandersen, Wed Dec 05 2012, 11:54AM
The hole through projectile will increase efficiency only for coils with few layers of wire. For most efficient shape (3x outer to inner diameter) it will not add any significant benefit as field inside such coils is almost constant all over the hole.
I am not completely sure about overshortening the projectile. The shorter the coil the better, yes, but overshortening projectile by making it's length close to diameter or even shorter may be not efficient even for such short coils. And the shorter the projectile the less stable it in the air.
Yandersen, Wed Dec 05 2012, 11:54AM
The hole through projectile will increase efficiency only for coils with few layers of wire. For most efficient shape (3x outer to inner diameter) it will not add any significant benefit as field inside such coils is almost constant all over the hole.
I am not completely sure about overshortening the projectile. The shorter the coil the better, yes, but overshortening projectile by making it's length close to diameter or even shorter may be not efficient even for such short coils. And the shorter the projectile the less stable it in the air.
Re: Questions about projectile
Steve Conner, Wed Dec 05 2012, 12:50PM
It's called a hacksaw and file... :P
Steve Conner, Wed Dec 05 2012, 12:50PM
Yandersen wrote ...
Guys, I want to buy a machine for making projectiles out of iron rod. Any suggestions? Who has it already - where the hell did you get one?! =(
Guys, I want to buy a machine for making projectiles out of iron rod. Any suggestions? Who has it already - where the hell did you get one?! =(
It's called a hacksaw and file... :P
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