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Reid

Tue Jun 30 2009, 01:10AM

Registered Member #2064 Joined: Sat Apr 04 2009, 09:26PM
Location: NJ, USA
Posts: 16

although the various essays I have read appear inconsistent, it appears that a projectile has a 'saturation limit', that is that it is incapable of absorbing more than so much energy without having an opportunity to dissipate its magnetic 'charge'. abet without access to a computer, while on a long car trip, I did some theoretical numbers and "designed" a quad-stage coil gun styled after the tau pulse rifle from WH 40k.

the final (theoretical) exit velocity was about 400+ m/sec, which is 45J for a 1g projectile.... requiring about .002 farads @ 10% and 660v.

it doesn't make sense practically to dump 450j into a projectile in one stage, it would obviously saturate and become hugely inefficient, so Im asking one question:

IS IT POSSIBLE TO CALCULATE THE SATURATION OF A GIVEN MOLAR MASS OF IRON?
if so, how?

or do you guys just do trial and error?
thanks,

ps, you guys told some n00b (I know I'm one) that you couldn't design a coilgun for a specific velocity a while ago, I know this has to do w/ saturation, but try this formula

(((eff./200)*C(V^2))/mass)^.5
should be the theoretical velocity of a projectile with a given efficiency, voltage, mass and capacitance.

kromsson

Tue Jun 30 2009, 05:27AM

Registered Member #1444 Joined: Tue Apr 15 2008, 06:55AM
Location:
Posts: 20

is basically a railgun/coilgun

I'm not familiar with the specific project, but it is pointless to be BOTH. It would probably be either a coilgun OR a railgun. And for what's more, what is the plasma chamber doing there? Is it an Electro Thermal Gun maybe ?

but try this formula

Your formula is obviously correct. However, saturation has an effect in efficiency (eff. in your equation).

'saturation limit', that is that it is incapable of absorbing more than so much energy without having an opportunity to dissipate its magnetic 'charge'

Not stated very correctly and clearly. Saturation is not about energy, it is about magnetism.
When the external field is applied to the projectile, it becomes a magnet itself (otherwise there would be NO force upon it by the field!). The magnetism that is induced on the projectile is increased, if you increase the external field. However, there is a saturation limit to this, so the magnetism of the projectile will stop increasing after this limit.
The force applied to the projectile is proportional to BOTH the external field, And the induced filed of the projectile. So, below the saturation limit, the force increases much more if you increase the current that produces the field (xI^2). After the saturation limit, it only increases proportionaly (xI^1). This is the saturation effect.

Basically it means that it is better to have a high saturation material for the projectile, so that the filed that will be induced on to it will be greater (thus better eff.). But even above the saturation limit (and you will DEFINITELY be above it if you use a large field) the projectile is drawn or pushed proportionally to the current you use to power the coil.

Not much meaning is there to try and calculate the saturation limit, as the fields are difficult to calculate anyway, just use the highest saturation material (metglass or Mumetal I think), that you can get your hands on (probably iron...)

it doesn't make sense practically to dump 450j into a projectile in one stage, it would obviously saturate and become hugely inefficient,

It makes perfect sense. Saturation is not about energy (the 450J), it is about the field you apply and the mass and the material of the projectile. The best however would be to try and tune every stage of your gun to the limit of saturation of your projectile, thus saving the extra energy for the next stage. On the other hand, switching many stages on is VERY difficult above some speed...

Proud Mary

Tue Jun 30 2009, 07:03AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Hello Reid,

can you please read our rules on posted image sizes to which you agreed when you joined our forum,

Please take steps to change this. Thank you,

Harry

Moderator

Reid

Tue Jun 30 2009, 04:11PM

Registered Member #2064 Joined: Sat Apr 04 2009, 09:26PM
Location: NJ, USA
Posts: 16

kromsson wrote ...

I'm not familiar with the specific project, but it is pointless to be BOTH. It would probably be either a coilgun OR a railgun. And for what's more, what is the plasma chamber doing there? Is it an Electro Thermal Gun maybe ?

Not stated very correctly and clearly. Saturation is not about energy, it is about magnetism.
When the external field is applied to the projectile, it becomes a magnet itself (otherwise there would be NO force upon it by the field!). The magnetism that is induced on the projectile is increased, if you increase the external field. However, there is a saturation limit to this, so the magnetism of the projectile will stop increasing after this limit.
The force applied to the projectile is proportional to BOTH the external field, And the induced filed of the projectile. So, below the saturation limit, the force increases much more if you increase the current that produces the field (xI^2). After the saturation limit, it only increases proportionaly (xI^1). This is the saturation effect.

Basically it means that it is better to have a high saturation material for the projectile, so that the filed that will be induced on to it will be greater (thus better eff.). But even above the saturation limit (and you will DEFINITELY be above it if you use a large field) the projectile is drawn or pushed proportionally to the current you use to power the coil.

Not much meaning is there to try and calculate the saturation limit, as the fields are difficult to calculate anyway, just use the highest saturation material (metglass or Mumetal I think), that you can get your hands on (probably iron...)

It makes perfect sense. Saturation is not about energy (the 450J), it is about the field you apply and the mass and the material of the projectile. The best however would be to try and tune every stage of your gun to the limit of saturation of your projectile, thus saving the extra energy for the next stage. On the other hand, switching many stages on is VERY difficult above some speed...

Ok, thanks for the response and the EXCELLENT clarification on saturation.
my apologies to the mods and to you for the confusion over the picture, it was a fictional design for a fictional plasma rifle.

so your position is that it makes sense to simply ignore saturation.

seeing then that I would like to build a multistage design of the specs listed in post no. 1, how would you decide on the number of coils to be used?
How would you space them?

thanks,

big5824

Tue Jun 30 2009, 08:06PM

Registered Member #1687 Joined: Tue Sept 09 2008, 08:47PM
Location: UK, Darlington
Posts: 240

You need to use FEMM or a similar modeling app to calculate how many tesla you have in the projectile, then compare this with a BH curve for the material and see if you are over the saturation "bump"

kromsson

Wed Jul 01 2009, 08:46AM

Registered Member #1444 Joined: Tue Apr 15 2008, 06:55AM
Location:
Posts: 20

Saturation as I said before, is not much of an issue.

If you are using a LARGE field to attract the projectile, your projectile will definitely be saturated ANYWAY - otherwise your filed is VERY small...

Now the discussion on coils, is quite a long one.

You ask about the number, and the spacing. Well the number is free up to you, the sky is the limit - theoretically. In practice, it is VERY difficult to initiate a coil (close its cicuit that is) at the right moment - you need a sensor, be it optical, or inductive, or mechanical or otherwise. And this requires some higher knowledge of electronics.

The best result would be if you could feed the coil with current when the projectile is VERY near the first turn of the coil, and then the pulse length should be VERY short, and after half a cycle is completed the circuit of the coil should open and remain open. This should happen at the exact time the projectile is approaching the center of the coil. Using an SCR and a bypass diode, this can be easilly done.But you need to measure the exact Inductance of the coil (L) using an LC-meter, and then calculate the time constant of the pulse.

See here:

You don't need to go through all the math, what you need is the frequency f. T=1/f is the time to complete a full cycle - but that would mean that the current will go to zero, then back from the coil to the capacitor - and thus the projectile will be drawn back to the coil after it has gone through it - suckback!

So, T/2 = 3.14 x sqrt(LxC) is half the cycle, at this moment the current goes to zero and the SCR will open the circuit and remain open, thus avoiding the suckback.

You should tune the coil (the inductance L that is) to be small, and such that the T/2 time is similar to the time since the coil is initiated, and up to the projectile reaching the center of the coil.

Now for the coil design, AGAINST all projects I have seen, i firmly believe that the coil should be VERY short - actually no more than 5-6 turns. For a discussion on this, we need a differnet thread though.

I suggest you tune ONE coil first - which is already very difficult - and then decide about the sensors and tuning the next stages.

(Then again, for a serious ballistic speed, I suggest you move to railguns, or even better, ETGs).

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