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coilguns...accuracy?

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LAN_403

Quantum Singularity

Wed Feb 15 2006, 06:22PM

Registered Member #158 Joined: Sun Feb 12 2006, 09:53PM
Location: Central Ohio
Posts: 282

joe doh wrote ...

But should you not figure out how to give your coilgun enough power before you worry about accuracy? Normal coilguns with a few 10s of m/s muzzle velocity do not need to be accurate as the projecile just drops to the ground after a few meters :P

Yes. Fins and other similar forms of stabalization depend on air pressure against them to work properly. I am a bowhunter so I am pretty good with fletchings

A lower velocity cg will probably not benefit from fletchings. I do however plan on building a powerful one. I'll start small, but I already have caps available from small photo-flash up through large maxwells.

Also you say one of the differences with cg and firearm is the bullet shape - cg being long and skinny because it needs to match the coil, and a riffle being short and stubby. This is true... but why does the coil have to be so long? I am sure there is an optimum size for a combo, but I have seen alot of good coils that were fairly stubby, and alot of riffle rounds that are fairly long 1" - 1.25" long. I will need to do more research though before I decide on a projectile and coil size. I dont see a problem with large caliber cg round so long as it is very permeable and light. As long as the field density remains about the same. This would be difficult though, and its so much easier to use things like steel rod, etc. Great ideas so far though.

I have been reading the old forums for hours trying to soak up all thats been discussed in the last few years. There is so much more info nowadays then when I first tried a cg.

Simon

Thu Feb 16 2006, 07:12AM

Registered Member #32 Joined: Sat Feb 04 2006, 08:58AM
Location: Australia
Posts: 549

Having fins will stop you from having the armature fit snugly into the coil. That'll detract from efficiency and effectiveness.

Dr. Shark

Thu Feb 16 2006, 01:51PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

Ah, you are a bow hunter, you know exactly what I'm talking about. But lets not get OT here... From my understanding (which I limited as I am not really into coilguns) the attainable stage veloicty of a reluctance coilgun is not limited by the size of your caps, but saturation of the armature. For good iron this is something like 2 Tesla, and that kind of sets an upper limit on the force that you can have acting on the armature to a few Newton. Compare this with a rifle where you have 3000bar on 1cm squared giving you 3kN of force. Also you are just sucking the projectile into the coil which gives you an effective acceleration distance of less than an inch, as the magnetic field falls off very quickly. This is actally the reason why you want to have a long coil. Again compare to the 20in or so of a rifle.

Since you seem to have big caps at your disposal, maybe you you go for an induction coilgun rather than a reluctance-based one. There is a fairly good description of one by "pulslaser" in the projects board. If you want to have a look at his hompage

I can translate the stuff for you if you have questions about the design.
With this design you are not limited by saturation, basically you can have the same force acting on you projectile as you would have on a coin when you do coin shrinking. It takes big pulsecaps, but it seems you have a bunch of Maxwells, yeah? This design also eliminates suck-back as the projectile is pushed out and not pulled in, so timing is less critical. I think it is generally agreed that for high energies, this is the better design.

Maz

Thu Feb 16 2006, 02:41PM

Registered Member #111 Joined: Fri Feb 10 2006, 01:04AM
Location: Menasha,Wisconsin
Posts: 65

i have three thoughts,
longer coils, longer pulses with longer projectiles, they would have less tendancy to spin.

cutting slots into the projectile almost like threads, but augar style and much more steep.

creating an airfoil, while it wouldnt help with efficiency, it wouldnt tumble and it would travel further.

Quantum Singularity

Thu Feb 16 2006, 04:15PM

Registered Member #158 Joined: Sun Feb 12 2006, 09:53PM
Location: Central Ohio
Posts: 282

Simon wrote ...

Having fins will stop you from having the armature fit snugly into the coil. That'll detract from efficiency and effectiveness.

That is true. But as a hunter again, sometimes accuracy can mean alot more than energy and velocity. I havent started to build my cg yet so I havent set any specific goals at this point.

Joe Doh, if by 'induction coilgun' you are refering to a flat spiral with a round conductive non-ferous object placed by it then perhaps you didnt notice my avatar? Click the link below to my homepage too, my navigation bar is on the left side and scroll down to "EMP Ringlauncher". I am not 100% sure on terminology. And that other link doesnt help much (I am not German or whatever that language is).

Maz, I had some trouble undertsanding exactly what you said but I assume your talking about putting riffling (grooves, notches, etc.) on the projectile. This might not do anything for a projectile moving 100m/s or slower. Just not enough air pressure against the riffling to get it to spin much. If cg's start getting into 500m/s and faster then it might be worth while.

Self Defenestrate

Thu Feb 16 2006, 04:18PM

Registered Member #87 Joined: Thu Feb 09 2006, 01:36PM
Location: San Jose
Posts: 191

The dot I aimed for was about 3-4mm round. Spinning the projectile before its fired sounds like a good idea, as the magic gyroscope effect would kick in and keep it from tumbling. A dremel and a few hard rubber bits could keep it aligned with the coil.
As for fins, I wouldnt have anything hanging off the back, nor protruding from the edge. Instead, go for a torpedo type of fin setup, where the fins are aligned with the body, and the body tapers off. It wouldnt be too hard to do either.

aahz

Fri Feb 17 2006, 12:38AM

Registered Member #186 Joined: Thu Feb 16 2006, 07:22AM
Location:
Posts: 42

If you attached a long, feathered length of string to the back (probably 2-3 times the length of the projectile or more) it should keep it from tumbling. Of course you're adding dead weight to the projectile, and creating drag while in flight...but at least you wouldn't create any air-gaps.

Dr. Shark

Fri Feb 17 2006, 08:49AM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

Hey Tristan, I thought the pictures in the link would be self explanatory, but it seems they are not, so why dont you just check the thread here

(ups, its not in the projects board after all). An induction coilgun is very similar to a ring launcher, but uses solenoid coils instead of pancakes and slugs instead of disks. So you are still going to get all the accuracy that you need

Again, spinning the projectile is NOT going to work for a long thin projectile, it is going to destabilize it, you can read that in any basic ballistics book. Having some kind of tail is going to work much better, and added weight will be minimal. With the slug projectile in an induction coilgun spinning would help a lot, but I don't see a way how it could be done consistently so you might aswell stick with the tail/fletchings.

Quantum Singularity

Fri Feb 17 2006, 09:41PM

Registered Member #158 Joined: Sun Feb 12 2006, 09:53PM
Location: Central Ohio
Posts: 282

Ahh I C... so the induction cg does have some advantages (no suckback, lighter projectile like aluminum, perhaps wont try to crush the barrel,etc.). So what are the main disadvantages? I would think with these advantages there would be far fewer ppl building reluctance(is that trhe correct term?) cg's and more building induction cg's. Perhaps there less effecient? Anyhow, I'll probably need to focus on just one type of cg this year, but I still have a couple months to decide, so go ahead and persuade me please

Simon

Sat Feb 18 2006, 01:23AM

Registered Member #32 Joined: Sat Feb 04 2006, 08:58AM
Location: Australia
Posts: 549

This is interesting work. Do you have a particular application/goal in mind?

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