Coil Guns...

[Lockednloaded]

I've been playing around with electronics a lot lately, and now i want to put them to use in a more fun way. By fun, i mean destructive; i want to build a coil gun. i have my capacitor bank all wired up with 5 disposable camera capacitors, but i have a few questions about the gun it self.
I would like to know how long my coil should be, how powerful it will be with this setup, and what would be a good projectile for a straw.

p.s.
If it is not too complicated, how do people do three separate coils with one launch tube?

[DYI]

1. Use a solenoid inductance calculator along with Barry's RLC calculator and some math to figure this out. It really depends on what you intend on launching and the exact specs of your capacitors.
2. Whatever you're launching, you could throw it considerably faster.
3. An iron rod with a diameter slightly less than the ID of the straw.

Multiple stages are achieved by using semiconductor switches triggered by sensors which "see" the projectile coming, and activate the switches at the appropriate times. It can also be done by programming the switches to fire when the projectile should be in the correct position (with the sequence started by the activation of the first switch), but for obvious reasons this can cause problems. To get any technical details on the method you'll have to ask rp181 or someone from the 4hv forum, as I've got no practical experience in the matter.

[245Tommy]

I'd at least double the amount of camera caps, even if it's efficient it probably wouldn't even go through a soda can on five caps. You can design the coil with this simulator. http://cool-science.tk/ For the projectile, just use a mild steel rod as close to the inside dia of coil as possible and about as long as the coil.

[rp181]

Using a microproccesor to time when shots is a bad idea, you want to use active sensing. Shots are too variable to use timed triggers.

The easiest way: Wire up SCRs to each coil and capacitor bank. Have a wire brush inside the barrel so when the projectile passes right before the coil, the gate signal conducts and fires the SCR. A better way is optical sensing. A simple infrared photogate is hooked up to a comparator, so when the resistance of the photodetector falls (projectile blocking the infrared beam), the comparator triggers and the output goes low. Take a look at the LM339.

NOTE: When I say the output goes low, that is right. the LM339 switches the cathode side, so when it goes low, the circuit is completed. Keep this in mind when wiring it up.
anode---->SCR gate---->series resistor---->LM339 output.

@245Tommy: Barry showed a projectile 75% as long as the coil is the best choice. Pointing the projectile decreases efficiency.

[Lockednloaded]

alright, i'll probably use more caps, but for this project, bear in mind, I'm not looking for amazing power, this is mearly my introduction into the world of electromagnetic acceleration
one other question, these things are practically silent right?

[Technician1002]

Silent is hardly the term for these.. LOL.. Think speaker coil and a massive shotgun shell going off next to it. The discharge time and thus the time things get moving is well into the audiable range. There is no exploding gunpowder, but with a blindfold, it will be hard to tell the difference.

[Lockednloaded]

Really? even at this small scale, underpowered stage? i was hoping for something quieter... oh well this will still be a fun, new project

[245Tommy]

Silent is hardly the term for these.. LOL.. Think speaker coil and a massive shotgun shell going off next to it. The discharge time and thus the time things get moving is well into the audiable range. There is no exploding gunpowder, but with a blindfold, it will be hard to tell the difference.

Did you ever build one? They are silent trust me.

[rp181]

On this scale, it is quite inaudible. The most noise is from the projectile knocking around.

With higher energies you get noise from the switch, coil contraction, etc...

If you take two pencils and hit them together lightly, that kind of what it sounds like.