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How big should my combustion chamber be?

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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How big should my combustion chamber be?

Unread postAuthor: JT07 » Fri Feb 27, 2009 12:45 am

Newb with a newb question here.

I'm building my first cannon and I've bought and assembled everything except the PVC. (perhaps I'm building backwards?) I've built the propane meter pipe and now need to know how long to make the combustion chamber if I use 3" SCH 40. I also plan on making this an over/under design so I guess I have to take the 180 degree bend into account when calculating the chamber length/volume.

As you can see from the picture, the meter pipe is 4" long 1/4" with two 1/4" ball valves. I measured the total volume with water and it came out to 15 milliliters. I've tried to use the online calculators but had no luck.

Any help would be much appreciated. Thanks.

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Unread postAuthor: starman » Fri Feb 27, 2009 1:38 am

This is a smallish meter so you are looking at a smallish chamber. You have done well so far getting your meter volume accurately. This is more important the smaller your chamber is.

Have you studied all of the metering section at BurntLatke? Are you having difficulty with any particular concept?

I don't want to design the chamber size for you...you need to figure that out on your own. However, know that you will want to inject a propane amount that is 4.2% of the volume of your chamber. Based on bars of pressure (14.7 psi), for every bar of propane you fill in your meter you will have 15 ml of propane. So if you were to fill your meter to 2 bar (29.4 psi) you would have a 30 ml shot of propane...3 bars, 45 ml propane.

Note because of cold weather concerns you should design a meter that fills at around 60 psi, roughly 4 bars. That would give you about a 60 ml shot of propane. You happen to have a handy meter size...the psi of gas in it will fairly closely equal the volume in ml!!

Now use the 4.2% figure and 60 ml meter capacity to work backward to get your chamber volume size.

BTW, Welcome to Spudfiles! You're off to a good start.
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Last edited by starman on Fri Feb 27, 2009 1:22 pm, edited 1 time in total.
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Unread postAuthor: jimmy101 » Fri Feb 27, 2009 12:20 pm

Welcome to the forum and the meter looks good so far.

You might be wondering what Starman meant by "because of cold weather concerns you should design a meter that fills at around 60 psi".

Take a look at the Spudwiki page on propane. There is a graph that shows the pressure in the propane cylinder as a function of temperature. On a cold day at ~35F the pressure in the cylinder is about 60 PSIG. So when it is that cold your meter can't be pressurized above about 60 PSIG.

If you are not planning on shooting when it is that cold (or if you keep the propane cylinder near 70F) the cylinder can pressurize your meter to higher pressures. At 70F the cylinder is at 110 PSIG.

As to how big your chamber should be ...
* assuming your limiting the meter to 60 PSIG (which I think is unreasonable low, I would figure on 90 PSIG or so, which increases your chamber volume by 50% compraed to the 60 PSIG limit)
* assuming you don't want to double-inject fuel (which allows the chamber to be twice as big as a single injection)
* the chamber size is about right for a handheld gun. Could be perhaps 50% bigger and still be reasonable as a handheld.

Even though this will be a moderate sized gun it'll still have considerably more muzzle energy than say a 0.22 rifle, so it ain't no toy. There will be enough recoil load that you must brace the barrel to the chamber. The recoil load will put too much stress on the elbows if the elbows are all that links the chamber to the barrel.

You are of course installing a chamber fan? The fan will boost performance more than the meter, or any other mod of a 1X combustion gun.
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Unread postAuthor: JT07 » Fri Feb 27, 2009 3:31 pm

Thanks for the info guys. The BurntLatke site is actually where I got the inspiration to build a propane injection cannon. The only concept I did not fully understand was the bars of pressure until it was explained in the above posts.

This is what I get:
A 60 ml shot of propane would need a 1428.57 ml chamber. With 3" SCH 40, the length would have to be 12.3" Taking the 180 degree bend into account, I would estimate the length would be cut down to 10" but I would measure with water again to be sure.

I'm from Toronto so I might be shooting this thing in the cold. 60 psi sounds like a good idea. Besides, I think my first cannon should be "moderate" power.

And yes I'm putting in a chamber fan. Already have the control box made for it. I'll try to post pics when its all done.
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Unread postAuthor: jimmy101 » Fri Feb 27, 2009 5:42 pm

JT07, sounds like you've got it figured out.

Don't forget the bracing.
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Unread postAuthor: trigun » Fri Feb 27, 2009 6:16 pm

YES a new member that actually doesn't want to be spoon fed. :cheers:
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Note epoxy cure time very important barrels are hard to remove from the wall
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