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Most common mistakes on first advanced combustion cannon?

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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Most common mistakes on first advanced combustion cannon?

Unread postAuthor: chubbylad13 » Wed Nov 03, 2010 2:14 pm

Hello, im about to build my first advanced propane conbustion cannon. Im am planning on going with a under over design. Before I start building I was just wondering what are the most common mistakes people make when constructing there first cannon so I can avoid them when constructing my cannon.
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Chubbylad13
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Unread postAuthor: jrrdw » Wed Nov 03, 2010 2:26 pm

Go through the showcase section and study others and then come back and then ask specific questions.

Welcome to Spudfiles.
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Unread postAuthor: chubbylad13 » Wed Nov 03, 2010 3:29 pm

Ok, One question that came to mind was that I think I read that you are only supposed to tap holes in to a double layer of pvc. Why is this? I know Im a noob but I really appreciate your help.
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Unread postAuthor: jackssmirkingrevenge » Wed Nov 03, 2010 3:35 pm

You're not "supposed to" really, it's just a safer practice, and more important for pneumatics that are constantly pressurised as opposed to the brief pressure spike in combustions, but you're perfectly safe tapping holes in a single layer of PVC ;)
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Unread postAuthor: Gun Freak » Wed Nov 03, 2010 3:45 pm

jackssmirkingrevenge wrote: but you're perfectly safe tapping holes in a single layer of PVC ;)

But you're safer doing it through 2 layers :wink:

@chubbylad (hehe)
If you do over and under, make sure there are supports for the barrel to prevent stress on the elbows in the rear. Also, don't use dwv cleanout caps. I'd use a bushing to a ball valve for venting. Reducers are ok if they are DWV but I am more of the safe kind. My friends DWV cannon blew up yesterday upon pressurization to 75 psi, and he normally ran it at 70 psi. So, use NSF-PW if at all possible.
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Unread postAuthor: clemsonguy1125 » Wed Nov 03, 2010 6:21 pm

Make sure you stay away rom clean out caps and dont use cell core PVC. Also make sure you fan and ignition circuit is servicable.
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Unread postAuthor: Moonbogg » Wed Nov 03, 2010 7:08 pm

My first mistake was assembling the chamber and THEN drilling and installing the spark screws. They did not match up and I ended up having to scrap the entire chamber. I could have epoxied the holes and tried again, but do that stuff before gluing everything together.
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Unread postAuthor: Pete Zaria » Thu Nov 04, 2010 12:22 am

As others have pointed out, make sure the chamber is serviceable so you can adjust spark gaps or fans or etc... if needed.

I accomplished this by attaching the reducer at the "back end" of the chamber (farthest from the barrel, where my vent valve is located) with six carriage bolts instead of glue. I also use these carriage bolts as conductors to pass the voltage for both the spark system and fan to the inside of the chamber, eliminating the need to drill additional holes for wires.
This works well and seems very secure (and has survived several hundred shots) but is difficult to open: to open the chamber, you have to remove the bolts, then "tap" the reducer off of the chamber with a rubber mallet, and reverse the process to seal it back up.

You could also use threaded fittings or a union to access the chamber for maintenance.

I highly suggest a ball valve for venting the chamber - makes things much easier. I used a 2" PVC valve, works great.

I also highly suggest a breech-loadable barrel; this makes shooting a little easier and less tedious, but also allows you to change out the barrel for different diameters or lengths later.

Put some time into planning where you want to put the handles and the overall ergonomics of the gun.

Take your time and do everything right the first time whenever/wherever possible. If you don't have what you need to proceed, wait until you have the right tools/supplies instead of jury-rigging it.

Peace,
Pete Zaria.
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Unread postAuthor: King_TaTer » Thu Nov 04, 2010 9:58 am

Is this your fist time building a cannon?

Do you know how to identify pressure rated PVC pipe and fittings?

How much money are you willing to spend?

Just a few questions for you to consider. Try and figure out exactly what you would like to do and how simple or complex you want your cannon to be. This will narrow your search and help us out. Good luck and again, welcome
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Unread postAuthor: chubbylad13 » Thu Nov 04, 2010 10:20 am

King_TaTer wrote:Is this your fist time building a cannon?

Do you know how to identify pressure rated PVC pipe and fittings?

How much money are you willing to spend?

Just a few questions for you to consider. Try and figure out exactly what you would like to do and how simple or complex you want your cannon to be. This will narrow your search and help us out. Good luck and again, welcome


This is pretty much my first time building a cannon. I have build a basic spray and pray cannon, so from building that I learned some basics and I learned about pressure rated pipe.Thanks for all the tips and ill post pics in the showcase when Im done.
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Unread postAuthor: Technician1002 » Thu Nov 04, 2010 10:35 am

One common mistake not mentioned is mistakes in calculating or measuring the chamber and meter. Simple math mistakes have caused several threads on "Why won't it fire?" Nominal pipe size vs actual pipe size is one problem. The distance from the meter to the chamber can leave much of a measured charge in the pipe between the meter and chamber, not the chamber, and simply not purging the meter of air before beginning will impact the mix. A bad pressure gauge that can affect the mix more than 10 percent is less common.

Use an accurate gauge and regulator, use very short distance between the meter and chamber, and volume measure the chamber and meter with water. Double check your math.
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Unread postAuthor: chubbylad13 » Thu Nov 04, 2010 4:52 pm

This should be my last question. If my barrel is 2.5" pvc and my chamber is 4" pvc what would you guys recommend for lenghts. I was thinking about 3 feet for the barrel and 1.5 feet for the chamber, I was looking at some of the other cannons and I thought this would be about average..... What do you guys think?
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Unread postAuthor: Gun Freak » Thu Nov 04, 2010 5:26 pm

Well a 3 foot barrel sounds a bit on the short side. How about like 5 feet barrel, and a 3 to 4 foot chamber?
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Unread postAuthor: Technician1002 » Thu Nov 04, 2010 5:54 pm

I would recommend doing the math. Burnt lake has shown that a chamber barrel ratio of about 0.8:1 works well.

Decide on a barrel size say 4 feet to keep the full size reasonable. An 8 to 9 foot overall length is a little awkward for most people. A 5 foot barrel with a 4 foot chamber is way too big.

Find the volume of the 2.5 inch barrel and then calculate the size of chamber to match it with a volume of about 80% of the barrel.

In volume a 4 foot long 4 inch chamber is way too big for a 5 foot 2.5 inch barrel.

Area times length = volume of a cylinder.
Area = Pi X Radius squared.

For example a 4 foot 4 inch chamber would have a cross section of;
3.14 X 2 squared. 3.14 X 4 = 12.56 square inches.

Volume is area X length.
12.56 Sq in times 48 inches (use the same units of measure) = 602.88 Cubic inches.

Do the math for your proposed sizes.
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Last edited by Technician1002 on Thu Nov 04, 2010 6:02 pm, edited 1 time in total.
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Unread postAuthor: Gun Freak » Thu Nov 04, 2010 6:00 pm

Make it an over/under style and the length will only be 5'.
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