Using LiPo batteries in coil guns

Hello all, this is my first post!

I have been theorizing for a while about coilguns, and have been doing calculations.
here is how it goes:
A single Lithium polymer battery can hold up to 3.3 Ah and discharge at about 30c (safely). We multiply 3.3Ah by 30c and get 99 A of continuous discharge. That’s a LOT of current, but, it is still not enough to fire a good projectile. So, I’m planning on having four of these hooked up in parallel to give out 396A. This is quite a bit of dangerous current, so my wiring will have to be gauge-perfect. The magnetic field that this produces is about 9.9 teslas, as  twice much as your average MRI machine. Yes, I’m utilizing a miniature MRI to shoot stuff. If you want to know how much force an MRI puts out, here is a great video to show you https://www.youtube.com/watch?v=6BBx8BwLhqg. I am planning on having a 2,000 turn coil, with the average distance from the projectile being 1cm. The ammunition is going to be BB bullets, which means our cross-sectional area is at least 7.65*10-4m2. Using those numbers, the initial force on the BB is 3,015,030N. This is enough to lift up 25 semi-trucks. On paper, this seems magnificent. However, I’m accounting for about 2% efficiency, taken into account at this step, giving us 60,300N. This is still an enormous force to take to a .5g BB I’m implementing the Impulse to be .05s using standardized E = F * t,  this gives us 3,015 joules of energy. Huh. that’s way more than we should expect. Let’s take into account the 2% efficiency rating again, and we get a more realistic number: 60 joules. 60 joules is ridiculous for a BB pellet. We’ll use Ek = 1/2*m*V2 to solve for V. I got about 1,553 m/s. This is faster than most guns shoot regular bullets. I will be using a 2-stage setup, and will make sure to  use proper electrical equipment.

My question to you is; How accurate are these calculations? if I build this thing, what can I excpect?

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I take it you are using a 30 caliber ball barring.

You are missing a number of things, Timing, ballistic coeficency, and saturation just to mention three.

Time from 0 to mid coil at max acceleration after that the power must be off or the coil will pull the projectile back in, this limits the time you can transfer power to the projectile. Now .05s and how far, from 1 cm from coil to center of coil, at the speed of sound that is 17 m. At 1553 m/s that is 77.65 m to center of coil in .05 s.

Ballistic coeficency, in front of your projectile there will be a plow wave, behind the projectile there will be a vacuum, and the interaction of these in the confined space of a barrel, these actions slow your projectile.

Saturation, eddy currents heat your projectile and your projectile looses its magnetic properties. To much energy for the mass and it does nothing for your acceleration because it is not used, more energy more mass. Your projectile is small for the energy you are trying to put into it.

Moving a projectile in a coil gun is like moving the armature in an induction motor.

What you need to do is spread the power over time and distance like the rotating field of the stator of an induction motor.


Qcks2 years ago

Ehhh.... coilguns... I'm not an electrical engineer, but there's some things.

First, you need to consider how much you loose to eddy currents. Just because something is magnetic does not mean ever bit of electrical energy put into the system = physical force. Magnetic fields require time to form. Simply dumping current into a coil, especially if there isn't a sustained current over time, results in huge losses in heat (due to inductive heating caused by eddy currents).
Even if you make sure your wires is rated properly for the amperage you're going to dump into the coil, you still lose that energy to heat. The only advantage is that your coil gun doesn't turn into a molten/burning pile if it's wiring is rated correctly. All that energy is still lost to heat.
Being that we're talking eddy currents... Magnetic lines do not come into exitence instantly. nor do they act on a substance instantly. Magnetic fields act on things over time. so.. time = 0 and time = (the amount of time it takes the bb to move down the length of the barrel) might be 2 very different things.

After that... you're not providing use with adequate information to check your math. 2000 turns is all well and good, but is this an air core, or is it wound around a metal barrel? Is there a ferrite jacket around the coil? Anything that effects the energy in the electromagnetic coil effects this math. I'm ignoring the effect of using adequately thick wire to meet the current you're describing, since that many amps would make a single 2000 turn winding very long.

Coil guns are kinda nifty, but you have to be mindful in how you take advantage of them. They require more finesse then simply dumping current into them.

bjohnson45 (author)  Qcks2 years ago

The eddy currents are the reason I took the 2% eff into account. The solenoid has a thin film of carbon on the inside.

Qcks bjohnson452 years ago

That actually helps quite a bit....
So your electromagnetic coils will reach saturation much more readily, which means that the magnetic field will setup more quickly, but it's still not capable of being instantaneous. You may lose energy not just to heat, but also to physical force being exerted on the projectile in some other direction then down the barrel of the gun. This will be especially true if you're using an magnetizable bb. There are naturally occuring magnetic domains in the bb that will interact with the coil; this can be good because it can put spin on the bb, effectively imitating rifling, but if th bb spins at something like 2,000,000 revolutions per minute, that might be a problem.

I'm only distantly aware of this stuff cause i was looking to make an induction furnace not too long ago, and i find coil guns interesting.

A better setup for most coil guns would be to aim for instantaneous break down, rather then instant alignment of the magnetic field (which solenoids kinda do).
All inductive coils resist changes in current. So, if you slowly trickle current from one liPo battery through a coil, the magnetic field will have an opportunity to properly come into existence and up to full strength (aka saturation). This would allow you to store more energy in your inductive coil.
If you go this route, the hall effect sensor wouldn't be looking for the projectile to exit the coil, but enter it, since the inductive coil will remained charged for a few brief period of time while the bb, which should be fast moving, accelerates through the coil. The advantage to this is it would actually let you take advantage of the inductive coil's basic properties, rather then work against them. Remember, inductive coils resist changes in current. So turning the current to the coil off, would result in the magnetic field breaking down, in an attempt to maintain that flow of current. This is known as an inductive spike.

I would recommend all of afrotech's vids, but this one is particularly useful

bjohnson45 (author)  Qcks2 years ago

Okay, tell me if I'm understanding correctly.

You suggest that I produce a higher Emf (Electro-Motive force) by turning the circuit / coils OFF quickly instead of turning them ON quickly?

Qcks bjohnson452 years ago

Yes. That is exactly what i am suggesting. If your magnetic fields have time to come into saturation, they will be stronger, which is what generates the actual EmF.

caitlinsdad2 years ago

Have you factored in the cost of replacing the batteries when they go on fire? Rapid discharge of Li-po batteries can damage the battery.

bjohnson45 (author)  caitlinsdad2 years ago

The 30c rating is for continous discharge.

I'm planning on making a 6v cooling system with heatsinks on the batteries that will be powered by normal batteries.

Paralleling LiPo can be dangerous. Unless the cells are very closely matched, or have current sharing resistors, you are likely to end up with very large circulating currents in the cells, potentially wildly more than 30C