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I'm returning to this hobby after a multi-year hiatus. My first foray was a spray-and-pray propane fueled cannon with BBQ ignition, two large nails for electrodes, and hilarity.
Now I want to build a new cannon with the design objective being to build a very simple device using a few well established performance-enhancing features with a singular goal of maximum noise.
The basics of the concept are already in place: 4" chamber, cleanout cap, piezo ignition, chamber fan, and pipe-metered propane with a quick disconnect to keep the entire meter and tank separate from the cannon.
But I have some real noob questions before I get started on the build:
1) How do I optimize barrel length and diameter for maximum noise level? I'm aware that a too-short barrel will cause a louder report when firing projectiles. I do not desire to fire any projectile at all (beyond some wadding at the base of the barrel to keep the chamber sealed) but want to have enough barrel length to keep the action at a safe distance from the operator.
2) What's the optimal ratio of chamber length to diameter for single point ignition of propane?
3) Do I need to use a burst disc to get a loud report?
Thanks for taking the time to read and I appreciate any and all feedback (even if just to tell me I didn't search enough )
Research bird cannon. Designed to meet your objectives..
Would take a good look at these.
http://www.spudfiles.com/forums/triple- ... 15211.html
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Liberalism is a mental disorder, reality is it's cure.
Thanks for the advice. I looked around and found some bird cannon diagrams: http://www.suttonag.com/ZonGunDiagrams.html
Looks like the chamber and barrel are the same diameter, separated by an "iris" of about half the diameter. It also appears that ignition is single spark from the rear of the chamber.
I guess the purpose of the iris is to increase chamber pressure? In that case should I change from a 3" down to a 2" chamber exit?
You guys definitely have some high standards here .. guess I'll have to pick up some krylon to make it official.
The ring is to define the chamber area where fuel and air are mixed. Without it the end of the chamber is indistinct creating a large area of variable fuel density resulting in poor combustion and diminished sound levels.
This retaining of the mixed fuel air results in less spilled fuel, better efficiency and less pollution from incomplete combustion.
In a breeze, air is less likely to stir the air in the barrel far beyond the ring disrupting the fuel density beyond in the chamber.
Do you think the ignition is at the rear to increase the tolerance to breezy weather, or does placing it there improve noise making performance?
At this point I'm planning to insert the electrodes through the doublewall at the back of the chamber, providing they would clear the fan. But if it would perform better in the center, I'm pretty comfortable putting them through the singlewall there. I'm not using foamcore pipe and pressures should be relatively low since no projectile will be driven.
Speaking of electrodes, is stainless a good material? The only downside I can think of is that soldering could be tough. In the worst case I can use my MAPP torch to get them good and hot, and install them after soldering.
In the past I just used some big nails. Now I'll be metering the fuel. Is a stoichiometric mixture more difficult to ignite?
It's the easiest to ignite.
Have you considered making a hybrid? Doesn't need to be high mix, pressure rated PVC can certainly take a 2-3x mixture especially if you don't have a barrel and projectile that's going to increase the pressure spike in the chamber after disk rupture.
For noise cannons it does improve the noise as the initial combustion without a projectile to restrict flow results in expulsion of unburned gas which then ignites in an unconfined air fuel explosion. See air fuel explosions for this effect. This is why many combustion cannons are much noiser than the air cannon counterparts that may be much higher power. The muzzle flash is a big source of noise. Suppressors try to contain this by diverting and cooling it before the gas exits the muzzle.
If you lit the cannon from the barrel end, the expulsion would have been burnt gas resulting in no or little muzzle flash.
Technician, thanks for your post, and the illustrative videos.
I've finished building the metering unit. I used a monkey wrench to remove the spout from a standard propane torch and installed an adapter to pipe threads in its place (Watts part LFA-23 for reference). I connected that to a typical air pressure regulator, then a ball valve, then the metering pipe, then the final ball valve. The regulator came with a gauge which I installed into the metering pipe. Cheap regulators aren't exactly accurate, and moving the gauge to the metering pipe makes it easy to know exactly what you're getting.
The chamber is mostly constructed. I installed a quick-disconnect coupler in the base to receive the fuel, and two stainless steel screws to function as electrodes. They are positioned at the back of the chamber, in the doublewall just ahead of the cleanout.
The chamber will be mated to a 3" barrel which right now is about 2.5 feet in length. I want to keep the action at a safe distance from the operator. I may still go with the stun gun approach to allow remote ignition but for now am sticking with piezo.
The first shots will be at less than stoichiometric just to get an idea of how the system will behave and I'll ramp it up from there.
Thanks again for all your advice, it's really helped keep me on the right track.
if you want noise then you should make a pvc end cap with a string atachted to the barrel and i mean paracord or it will fly and that should allow the gasses to burn up for high sound
Don't look any furher, go dutch
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OK, it's built. I went with an "electronic" BBQ lighter, which is basically a baby stun gun.
I wired the ignition and fan to a 4-wire modular telephone jack and use a 50 foot phone cord to connect to a remote control box.
Firing the chamber open-ended with saran wrap covering the exit it makes a very deep concussive boom but I wouldn't call it loud.
I put on a 3" barrel about 2.5 feet in length and that raised the tone a bit but still not exactly loud.
Now, it could be that having the cannon pointed away from me plays a big role, but I'm thinking that stepping down to a 2" barrel might really improve things.
OK, progress has been made.
Put a 4" to 3" bushing on the end of the chamber, then a short section of 3" pipe. Made a burst disc from masking tape (2 layers and I'll try more eventually) and then wedged a 3" to 2" reducer on the end of that with a 2' long 2" barrel.
Fires with a nice crack =)
Now that the fundamentals are in place, I'm thinking about JSR's hybrid recommendation.
Can I use my existing cannon, with burst disc in place? Or will the threaded cleanout be a dealbreaker?
I can easily use my existing meter to add a metered amount of air.
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