Re: Coil length
Signification, Sun Apr 12 2015, 03:34PM

The typical answer to this question is something like "no, don't waste your time with this", but I recently need exactly the same thing and believe it can work--well! I will post on a bit later since I am working on it NOW! I --usually-- design the projectile to be between 50% and 100% of the coil length, but, IMO, there are many other valid possibilities.

Please give more details of your setup plans and design parameters...wire size, capacitance, SCR shutoff, etc.

Re: Coil length
Gregary Boyles, Sun Apr 12 2015, 05:11PM

No concrete plans as yet. Just trying to understand the concepts and toy with thought experiments.

I wonder if it would work best with thin wire, high voltage and low current or thick wire, lower voltage and high current.

I guess you would you are effectively doing is creating a tesla coil like winding as opposed to a flyback transformer like winding.

It would be interesting to compare the simulation of the magnetic fields in both cases.

Do you know of any good tutorials on how to use that femm software?

I downloaded it but I can't quite figure out how you are supposed to use it.

Re: Coil length
DerAlbi, Sun Apr 12 2015, 05:30PM

Long coils come to mind to everybody who starts with CGs... its discussed here over and over. Just scroll down a little bit and read "Coilgun - 1 long 1 layer coil as long as the barrel? Good, Bad?"-Thread to get a start of the disadvantages. No reason to avoid the forum search or Google search for coilgun concepts. There are tutorials out there.

I wonder if it would work best with thin wire, high voltage and low current or thick wire, lower voltage and high current.

Its both the same.

I guess you would you are effectively doing is creating a tesla coil like winding as opposed to a flyback transformer like winding.

The coils timeconstant is afaik better f the coil is multilayerd insteads of long and singlelayered.

Do you know of any good tutorials on how to use that femm software?

The turorials out there arent so nice but they teach you how to use the shitty userinterface. When it comes to simulations: you must know the physics and should read about the math behind it.. its all about the border-conditions. However for a start i just would google for pictues of simulation results. this stuff is so basic, it should be done before. Hiowever i dont expect you to read any information out of it.

You should really start to read ALL of Barrys work. This guy is amazing! He presents all his experience and explains the results and everything. Just read. Yes, that takes hours, but it will save you many more time than spent after you worked though the pages.

Re: Coil length
Signification, Sun Apr 12 2015, 05:41PM

I was thinking of very thick wire, perhaps even paralleled layers, to increase the current, and very large capacitance to slow the discharge giving the B-field time to 'grab' the slug, overcoming it's inertia. Maybe tuned for a half-cycle discharge to allow that huge SCR pulse-current and easy shutoff at the end of the cycle. But right now, these are just 'blind thoughts'--but have good gut-feelings.

Re: Coil length
Ash Small, Sun Apr 12 2015, 06:52PM

I seem to remember repeatedly readingthat the best coil shape for a CG (and a lot of other applications as well) is when the coil diameter is three times the width.

You can arrange several of these in line along the barrel, and use an optical trigger to trigger each one to 'fire' as the 'slug' enters each coil.

This is obviously a 'multi-stage CG'

Others can probably give more detail.

EDIT: typo corrected

Re: Coil length
Signification, Sun Apr 12 2015, 11:38PM


Sorry, but I seem to be missing something here: what do you mean by:
"when the coil diameter is ee times the width."?

Re: Coil length
Uspring, Mon Apr 13 2015, 09:56AM

I looked at some stuff in Barrys work. So here my first question:
He states in the chapter about saturation:

The magnetic saturation point puts a limit on the maximum effective strength to drive your coil. There will be no benefit at all if you exceed this, only wasted heat.

Force on the projectile is m * dB/dx. When saturated, m won't increase anymore with rising B, but dB/dx still does. So why is a larger B completely wasteful?
AFAIK, saturation occurs at some tens of amp-turns per cm for e.g. steel. Even a moderately powerful coil seems to be much beyond that. So all of these are wrongly designed?

Re: Coil length
DerAlbi, Mon Apr 13 2015, 10:00AM

actually yes. without saturation, the force increases quadratically with current, with saturation it rises only linear.
Of course resistive losses rise by square-law..so you want the force to rise the same way... and with saturation you dont have that.

Its all a tradeoff... if you want to be a cool kid that shoots stuff on youtube that shatters easily anyway then you want maximize outputpower... in a basic design this leads to bad efficiency. I f you want to be cool on the number-side,you wont be recognized for it, but you will win efficiency

Re: Coil length
Ash Small, Mon Apr 13 2015, 10:31AM


I meant to type 'three'. Sorry about the typo. Not sure what happened, some kind of wireless keyboard 'glitch'. Thanks for pointing it out. I should check my posts more carefully.

There is a 'three to one' optimum ratio for a lot of coils, but I don't remember all the detail. It's been discussed here before several times.

Re: Coil length
Uspring, Mon Apr 13 2015, 10:59AM

Makes a lot of sense, since you want the length, where dB/dx is large to be big, not the length inside the coil, where B is almost constant. Depends somewhat on whether this is the first stage or a later one.
Also, the L/R ratio, which describes the length of time, which the field takes to die down, is roughly proportional to the copper cross section of the coil, i.e. the area of copper you see, when you saw through the coil perpendicular through its axis. If you want that to be large, you end up with short fat coils.

Re: Coil length
Ash Small, Mon Apr 13 2015, 08:40PM

From memory, the length should equal the inner diameter, and the outer diameter should be three times the length, so that when you saw through it, the two sections of coil that you see are square.

If I get a chance, I'll try to find some references. There's a name for this shape coil, but I don't remember that either.

Re: Coil length
Uspring, Tue Apr 14 2015, 08:17AM

I meant cutting through it perpendicularly to the axis.

Re: Coil length
BigBad, Tue Apr 14 2015, 08:23AM

Yup, but at high speed this no longer applies so much because resistive losses are small compared to the mechanical power you're generating, so you have to be careful with the first few stages and save energy for the later ones. On the later ones you can go further beyond saturation without a lot of penalty, within reason, and reducing the number of stages you need.

Re: Coil length
Signification, Sun Apr 19 2015, 11:21PM


Please let me know if you find any references--or have any clues where to search for such pertaining to single-stage. I have looked but can't find any like this. I see why this one would be for multi-stage only...If you plan to launch a 1/4" projectile from one stage with these guidelines, the coil's dimensions would be quite unusual: OD=3/4" ID=1/4" AND Length=1/4" ! Am I correctly interpreting this? If you had the OD vs the LENGTH rule backward this would become: LENGTH=3*OD. Which looks like better dimensions for a single-stage coil.

Re: Coil length
Ash Small, Mon Apr 20 2015, 12:20AM

I think what you suggest is impractical, although you may correct me.

If the coil ID is 1/4", the OD of the barrel is 1/4", this sounds impractical to me, although you may have a specific application in mind.

If the projectile is 1/4" diameter, and wall thickness is 1/16", then you have 3/8" ID for the coil, this still sounds a bit on the small side to me.

I was visualising something like a half inch bore ID, 1/4" wall thinkness, coil of 1 inch ID, 3 inch OD and 1 inch long, although this can be varied a bit, as long as the proportions remain constant.

This will only work for a multi-stage coil gun.

Maybe you coul go to 1/4" projectile diameter, 1/8" wall thickness, 1.5" coil OD and 1/2" coil length.

What I mean is if the coil is as long as it is high (ie square cross section), this seems to be very efficient for lots of applications, not just coil guns.

This is just something which has 'gelled' in my mind from reading lots of posts on this forum. It also seems to be efficient for other types of coils, as well as coil guns.

I'm sure there is a name for this 'coil topology', but the name currently escapes my memory.

Re: Coil length
Signification, Mon Apr 20 2015, 04:26AM


I think we are misunderstanding each other here: From what you originally stated. You said:

"THE LENGTH SHOULD EQUAL THE INNER DIAMETER, AND THE OUTER DIAMETER SHOULD BE THREE TIMES THE LENGTH"

From this I get, using as an example, a 1/4" diameter projectile, the following: Coil ID=1/4" Coil OD=3/4" and coil length=1/4"??? I am assuming:...the coil ID = the barrel OD. And very thin walls, therefore, coil ID~projectile diameter (1/4"). This is what I found odd for a coil.

PLEASE correct me if I am misinterpreting the the above assumption (written in all caps).

EDIT: I re-read your last reply and saw that you were not ignoring "VERY THIN" wall thickness--I was--when I am able to reply without double-posting, I think we will be in understanding.

Re: Coil length
Ash Small, Mon Apr 20 2015, 09:30AM

I found a link to a thread from two years ago, started by Yanderson, who hasn't posed here for some time, but he did do quite a bit of exermentation regarding coil efficiency.

You can see the dimensions of his coil fit exactly the ratios I described.

I seem to remember there was another thread here he discussed why he felt that this was the optimum shape for a multi-stage coilgun coil.