SpudFiles

I am looking to build an advanced combustion cannon (you know; propane injection, fan, and all that good stuff). The problem I got is that I don't want it to be too big because I want my kids to be able to shoot it.

I figured on using a 1" barrel with a 2" chamber. I put these sizes in the ratcalc and it figured that for a .8:1 ratio, the chamber would be 6" and the barrel 30". I think that may be too long for what I am looking for.

I thought about using a 1.5" pipe for the chamber. This would allow for a shorter barrel to get the ratio.

The problem I have is that if I use a 1.5" chamber, I'm concerned about the power because, after all, I'm still a guy.

Does anyone have any experience with a cannon this size and, if so, does it have decent power for a "guy's toy"?

Could you not make it standard size and then make a small tripod to rest it on for your kids? I'm sure they'd much rather you made a big one

the thing with going small is you have to be very precise in you fuel meter measurements

I'm concerned about the power because, after all, I'm still a guy.

Love your reasoning Don't worry, with a fan in the mix the power should be there.

I made a cannon about the size you are talking about, maybe even a little smaller- Anyway i used one of those little pc fans but it only survived for about 10 shots if that, i don't know if it was on it's way out already.
It had lots of power though, your's should be fine with nice fitting ammo.

Thanks guys. I mainly build pneumatics (after an incident with my first combustion, which can be read about in that "cannon accident" thread). However, after seeing some of the outstanding creations on this site, and then building my mini, I want one so bad. It should be posted by next weekend.

Another question:

In this combustion, I want to put a camera flash ignition with a homemade induction coil for the spark (I figured this would be harder to do than just sticking on a stun gun, and I always like a challenge).

I was wondering if anyone has made one of those homemade induction coils from this site and, if so, does it work good?
http://www.sciencebuddies.org/mentoring ... p027.shtml

Another question:
I looked at the advancedspuds.com site. It said something about using a check valve on the chamber to aid in rapid venting. My question is if the fan opens the check valve to vent the chamber, won't it open it again when mixing the fuel? I'm pretty sure it does so is there a solution to this?

hubb017:

Making a homebrew step-up transformer shouldn't be too hard (but a bbq piezo is a heck of a lot simpler, safer, and probably cheaper).

The cite uses bare copper wire for the primary so it is important that the primary coils don't touch each other. With insulated primary wire you can omit the PVC insulation between the two coils (and it'll work better without the thick hunk of PVC in there), the primary can be wound right on top of the secondary. A layer or two of electrical tape might be a good idea between the primary and secondary.

Obviously, you don't need the Neef vibrator. The setup in the cite is also called a buzz coil. Ford's used to use this as an automotive ignition system way back when. This basic circuit has also been used in cheapo hand held, battery operated "Sears Tower" radiation detectors.

12G wire for the primary is several gauges to small (the wire diameter is way bigger than needed). 15 to 20G or so wire should be able to handle the power that a photocap will supply. Since the cap supplies current for much less than 1 second you can use much smaller wire than what would be required for a continuously operating system. 30G for the secondary would be good. RadioShack sells (or at least they used to) a set of three spools of magnet wire; 20, 24 and 30G IIRC for $5.

Somewhere I have the equations for choosing the wire size for this type of coil. If you want I'll dig them up.

The photocap is 300~320V so a 30:1 turns ratio will give you ~9KV which should be more than enough for an ignition system.

The 2Meg resistor is probably not needed, neither is the capacitor. You probably will need a diode across the coil. When the current stops and the magnetic field collapses the coil will send a big negative voltage pulse back into the flash charging circuit. The photcap might be able to absorb that pulse, if not, it'll fry the transistor(s) in the circuit.

The power/fire switch is tricky. Most switches won't last very long at the 300V and tens (hundreds?) of amps the photocap will supply. A "trigatron" is probably the best way to go. (IIRC, "Trigatron" is SixMHz's name for it.) Basically, you leave the entire photoflash circuit intact, including the Xenon flash bulb. The primary of your coil is wired in series with the flash tube. The "switch" is basically the flashtube itself, the trigger is the trigger on the flash board for the tube. This setup completely avoids having to have a high power switch, any old switch will handle the flash board's trigger circuit.

If you do make a homebrew step-up transformer then please post it. If it works out well it would make a great "How To" and/or Spud Wiki page.

As to the check valve and venting, I don't think the fan is strong enough to pull the check valve open, IIRC you have to help it with a small push. When the gun is loaded the operation of the fan doesn't put any load on the check valve, it's a closed chamber, running the fan just moves the gases around it doesn't generate a significant pressure gradient. You might want to take a look at Burnt Latkes' vent valve.

from here.