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Inducing spin on coil gun projectiles!

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guitarlord66

Mon Feb 14 2011, 03:39PM

Registered Member #1805 Joined: Sat Nov 08 2008, 06:29AM
Location:
Posts: 67

I decided even though I'm not too good with electronics I want to give this problem ago.

HOW TO MAKE A COIL GUN PROJECTILE STABILIZE DURING FLIGHT?

I started by researching different kinds of motors, I pulled apart an old pc fan, looked at an old brush motor and started to understand how motors work.

I found "Switched Reluctance Motors" which I think may be the key to inducing spin on coil gun projectiles...

Definition: "A reluctance motor is a type of synchronous electric motor that induces non-permanent magnetic poles on the ferromagnetic rotor. Torque is generated through the phenomenon of magnetic reluctance."

"The switched reluctance motor (SRM) is a synchronous machine which runs by reluctance torque. It has wound field coils as in a DC motor for the stator windings. The rotor however has no magnets or coils attached. The rotor of the motor becomes aligned as soon as the opposite poles of the stator become energised. In order to achieve a full rotation of the motor, the windings must be energised in the correct sequence. For example, if the poles a1 and a2 are energised then the rotor will align itself with these poles. Once this has occurred it is possible for the stator poles to be de-energised before the stator poles of b1 and b2 are energised. The rotor is now positioned at the stator poles b. This sequence continues through c before arriving back at the start. This sequence can also be reversed to achieve motion in the opposite direction. This sequence can be found to be unstable while in operation."

Ok onto my idea :)

My idea is to make 6 coils, with a rectangular prism iron or steel core, that are the same length as the projectile and are fixed around a coil gun barrel, after the acceleration coil or coils.

SwitchedReluctanceMotorIdea

Red/Orange represents enameled wire, grey resembles the core and black resembles the barrel...

These coils would all be placed around the barrel, at the same spot and they would have to be energized like this:

I'm no good with circuits... but I assumed something like this, modified to energize coils instead of LED's and run much faster would do the trick.

BikeLeftCct

The idea in my head is that when the acceleration coil is discharged, as the projectile travels down the barrel it comes in contact with these electromagnets that are energized in a circular motion and will force the projectile to spin, stabilizing flight.

What do you guys think?

[Edit: Picture size]

sidneylopsides

Tue Feb 15 2011, 12:22AM

Registered Member #3690 Joined: Mon Feb 14 2011, 10:59PM
Location: Leeds, UK
Posts: 11

Hi, this is my first post, but last night I was actually thinking over the same problem.
From what I can work out here, the rotating magnets would have to be hugely powerful to get any decent spin on the projectile as it passes them as it would be for such a brief period of time.

I was wondering about spinning the projectile up before it was fired, having a rotating magnet that holds it in place.

My other thought was darts as they wouldn't need spin, but I don't know if they'd work in a coil gun that well.

That's just what I thought last night, and seeing as I've only been looking into coil guns since the night before that, I'm probably way off the mark :p

Martin King

Tue Feb 15 2011, 12:46AM

Registered Member #3040 Joined: Tue Jul 27 2010, 03:15PM
Location: South of London. UK
Posts: 237

sidneylopsides wrote ...

My other thought was darts as they wouldn't need spin, but I don't know if they'd work in a coil gun that well.

The way to launch darts appears to be to embed them in a carrier that separates from the dart once launched.

Martin.

guitarlord66

Tue Feb 15 2011, 01:00AM

Registered Member #1805 Joined: Sat Nov 08 2008, 06:29AM
Location:
Posts: 67

I've thought about trying to make the projectile spin before its fired but while reading about it, I read that it needs to be spin to around 100,000 rpm for it to keep stability. I could be WAY off but I was thinking if the projectile is already accelerating, and went through these "switched reluctance motor" electromagnets it would give it a spin, that cause the projectile to spin faster than if it was sitting still when it first begins to spin.
I could be wayyy off but I just been thinking about it a lot. I don't want to go to all the trouble of trying to make it to find it doesn't work...

Darts in a carrier!! thats a really good idea! Sort of, or the same as whats used in railguns half the time? How the projectile is inside a two piece armature that splits when it exits the barrel?

Thanks for your input guys :) much appreciated :)

Turkey9

Tue Feb 15 2011, 03:17AM

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

You would probably have to have a fairly long "spin section" to get up to speed. If the projectile has rotational forces imparted on it for only an inch of it's path, it's not going to get > 10 000 RPM IMO. You might have better luck having the switched reluctance section before the projectile is accelerated. Either that or make the coils much longer.

guitarlord66

Tue Feb 15 2011, 03:42AM

Registered Member #1805 Joined: Sat Nov 08 2008, 06:29AM
Location:
Posts: 67

That makes sense when you think about a standard gun, the rifling goes throughout the entire barrel, and well, now I think about it this is almost magnetic rifling isn't it... I'm not sure if it requires 100, 000 RPM but thats what one guy said, but he claimed his coilgun to fire over 100m/s...

What if the coils were wrapped around a cylindrical piece of iron or steel pipe instead, and 6 were around the barrel. But this was repeated in open areas down the barrel.

AlongBarrelIdea

In this image are 2 accelerator coils and 8 small coils, pretend that the small coils are all around the barrel, making a total of 48 small coils... this is theoretical...

Either that or instead of there being 6 coils around the barrel in the same spot, split it out and stretch it out, so it spirals down the barrel.

And then I'm still stuck with the question whether these coils would induce any spin on a cylindrical projectile, or a custom projectile might need to be developed...

It would be a lot of work to build this, its just an idea a the moment so any constructive criticism or help would help a lot :)

[Edit: Again]

aarpcard

Tue Feb 15 2011, 04:46AM

Registered Member #2848 Joined: Tue May 04 2010, 05:19AM
Location:
Posts: 44

I've been looking into this as well. You're on the right track, but it needs some tweaking.

Look at the patent below.

The problem with having the coils spiraling down the barrel is that they won't induce a spin. They will merely tug the projectile back and forth along the length of the barrel. Getting an object to spin in a magnetic field is quite difficult. The only way this will work is if you have coils all around the barrel.

There are other problems too. In order to achieve flight stabilization by spin, you're going to need anywhere from 50,000 to 100,000 rpm depending on the actual projectile. The slower the projectile is traveling the faster it has to spin to be stable. In order to spin a projectile by this method and to spin it up to such high rpm you will need enormous power pulsing through the spin coils.

Look at it this way. You need to rotationally accelerate a projectile from 0rpm to 50k-100k rpm in a couple milliseconds. In order to get that effect from your spin coils, you're going to need (just an estimate) near 1000 times more power sent to them than your acceleration coils. The logistics of this is obviously a problem.

If you want flight stabilization then your best bet would be to use hollow tail projectiles instead of this.

guitarlord66

Tue Feb 15 2011, 05:33AM

Registered Member #1805 Joined: Sat Nov 08 2008, 06:29AM
Location:
Posts: 67

Thanks :) even though I kinda got shot down... haha thats why i asked. I've seen the link you gave me, I thought that was for railguns, but I didn't understand it because of the words... I haven't studied electronics yet, but I intend to soon...

So it needs to spin really really fast...
This would have been so cool if it worked hahahaha oh well time to get thinking again, unless anyone else can think of a way to get it to work?

sidneylopsides

Tue Feb 15 2011, 08:29AM

Registered Member #3690 Joined: Mon Feb 14 2011, 10:59PM
Location: Leeds, UK
Posts: 11

Quick look online and I found a small motor capable of 100,000rpm for $60, if something like that was used to spin up the round as the caps charged it might work?

The darts thing is a sabot, and I believe they are used in gas powered firearms to create a solid face against the exploding gases in the barrel, then fall away so the dart can fly freely.
As there are no gases to worry about in the coil gun, I guess you could have plain darts.

edit: I mean a fletchette I think, that brings up proper google results anyway!

guitarlord66

Tue Feb 15 2011, 08:42AM

Registered Member #1805 Joined: Sat Nov 08 2008, 06:29AM
Location:
Posts: 67

The reason I assume for the dart having an armature/casing/fletchette/sabot whatever you want to call it is for the projectile to have a larger ferromagnetic area for the accelerator coil to attract.

The motor could work, just need to think of a way for the motor to attach to the projectile... magnets wouldn't work because they wouldn't have a strong hold they would slip, or so I'd assume. maybe a gear, or locking system that disengages when you pull the trigger...

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