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My first combustion - A few questions

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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My first combustion - A few questions

Unread postAuthor: Pax » Sat May 15, 2010 9:10 pm

Question 1: How much danger is involved with combustion? I'm not going to be dumb with it...But if its not a beginner thing then I'm not going at it ,I've already made a pneumatic and I'm looking for something i can run without an air compressor, and I'm looking for something simple...

Question 2: Based on the size of the one below, How loud will it be? (If there is any other factors then state them if you will) because i live in an urban area and I'm trying to go under the radar, Combustions are considered real firearms where i live


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The bigger pvc is 1 and 1/2" and the smaller is 3/4", and im going with the simple piezo igniter + hairspray = boom

Edit: And its not finished yet, I dont have the igniter, the cap, And i havent glued everything, Will i need a cooling fan?
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Last edited by Pax on Sat May 15, 2010 9:11 pm, edited 1 time in total.

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Unread postAuthor: clemsonguy1125 » Sat May 15, 2010 9:11 pm

Its not more dangerous than a pneumatic, what are you using for fuel
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Unread postAuthor: Pax » Sat May 15, 2010 9:13 pm

The bigger pvc is 1 and 1/2" and the smaller is 3/4", and im going with the simple piezo igniter + hairspray = boom


And maybe axe deodorant
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Last edited by Pax on Sat May 15, 2010 9:20 pm, edited 1 time in total.

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Unread postAuthor: mark.f » Sat May 15, 2010 9:19 pm

How loud it is depends on the C:B ratio and fueling. What are the lengths of the chamber and barrel?

If you are concerned about noise and wish to lower the C:B ratio, repeatability is key. I think you should switch to propane fueling. Nothing fancy... all you need is a syringe and a torch head to do it.

The mixture fan is also key to repeatability, so I would say use one. You might be hard-pressed to find one that fits in 1-1/2" PVC, though.

Anymore questions just ask.
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Unread postAuthor: Gun Freak » Sat May 15, 2010 9:21 pm

Looks good, will be relatively quiet also.
Might want to lengthen the barrel a bit.
You probably won't need a fan for such a small cannon... maybe its bigget than it looks but you can just mount a little hand pump through a hole in the chamber for ease of use to vent the chamber.
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Unread postAuthor: Pax » Sat May 15, 2010 9:22 pm

Thanks, What exactly do you mean by repeatability? and is there any sort of guide or somthing on the propane thing?

And how powerful will it be? Ima shoot oak bullets with a homemade rifled barrel Any distance/speed estimates?
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Unread postAuthor: Gun Freak » Sat May 15, 2010 9:24 pm

I never said repeatability. I said relatively quiet, meaning pretty quiet.
Im not sure if there is a guide on propane metering, try searching it.
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Unread postAuthor: Pax » Sat May 15, 2010 10:27 pm

I remember seeing a way you can make an ignitor from a bic lighter, Can anyone elaborate? I couldnt find much when i searched, I swore i saw something like that somewhere
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Unread postAuthor: Gun Freak » Sat May 15, 2010 10:53 pm

If you rip the lighter apart, you can get the ignitor out of it. I did it successfully for my mini. At least that is so for long nosed lighters... don't know about the little ones.
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Unread postAuthor: theBOOM » Sat May 15, 2010 11:22 pm

Yeah what gun freak said, if you're going to use these igniters from lighters don't make the connection permanent, by that I mean don't glue it onto your cannon or anything beacause they tend to malfunction if used alot.

If I where you I'd just get a piezo bbq igniter, I've had mine for about 2 years and it's still sparking.

As for fueling, ask your mom or go to the pharmacy and buy yourself a 10 ml syringe. Get some propane or butane fuel and just fuel it with the syringe. It's quite simple, message me if you need help.

Adding a fan would significantly increase the performance and reliability of the cannon but it might be kind of hard to incorporate it onto the 1" PVC.
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Unread postAuthor: starman » Sun May 16, 2010 1:53 am

With the relatively short barrel shown here, this will be fairly loud....perhaps in the .22 long rifle shot range. The more massive your projectile will cut down on the loudness to some degree. Doubling or tripling your barrel length will cut down the loudness significantly.

Always treat any spudgun as you would any firearm. Combustion cannons are no more inherently dangerous than pneumatic ones.
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Unread postAuthor: jimmy101 » Sun May 16, 2010 10:50 am

For info on hacking a piezo based butane lighter see: http://www.inpharmix.com/jps/Hacking_lighter.html

The fan isn't used for cooling, it is used to mix the fuel and to help vent the gun after firing (you have to get rid of the combustion products and flush fresh air into the chamber before the next shot).

Your design will be fairly loud, at least firecracker loud.
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Unread postAuthor: mark.f » Sun May 16, 2010 11:25 am

Thanks, What exactly do you mean by repeatability? and is there any sort of guide or somthing on the propane thing?


Alright, to reduce noise, the easy thing to do would be to lengthen the barrel while leaving the chamber alone (lowering the C:B ratio).

However, with inefficient fueling like hairspray, or inconsitent shots due to bad fuel mixing, lowering the C:B ratio could hurt performance.

For propane, simply multiply your chamber volume by 4% and then fill the syringe to this volume. Inject through a cracked-open cleanout cap and close quickly, then mix and fire.
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