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I am about to venture into my first combustion cannon. I'm not ready to tackle a hybrid yet. I have had an idea kicking about for a long time and it's time to build.

I like designing things to see if problems can be fixed without creating a whole bunch more. The cannon is to be simplicity in itself. I length of 4 inch ABS with a solvent glued end cap and a 1/4 inch close nipple installed for fueling/venting and ignition. It can be used as a noise cannon (bird control) or a 4 inch nerf ball launcher of high speed.

The problems to solve are purging, fueling, mixing, and blowing off caps.
How to do ignition through a nipple is another subject not covered here. This topic is fueling.

With a long chamber, I was thinking that the purge and fuel/fresh air could be done in a single cycle. It has been long known how to meter propane with a metering tube. Is there any reason not to do both?

A pressure regulator is a valve attached to a diaphragm. When the diaphragm is moved one way the valve opens and when moved the other it closes. A spring on one side applies pressure and gas is delivered to oppose the force of the spring.

If the regulator is modified by removing the spring, it should regulate at near 0 PSI. If we apply a pneumatic pressure instead of spring pressure, the regulator should be "adjustable" and track the pneumatic pressure.

With that in mind, if the air tank in the drawing is filled to 1 or 2 bar, the fuel should regulate to match it as long as the demand isn't higher than the source pressure.

Once charged the air supply and fuel can be shut down and the system discharged. As the air pressure drops and expands, so does the fuel providing a proper mix through the entire discharge cycle.

A 1 bar cycle would provide enough for a chamber the same size as the charging system. A 2 bar cycle would provide enough mix to fill a chamber 2 X the size or provide 2 fills if the system delivered a 1 bar volume each time. At 3 bar for example it should be good for 3 rapid fire reports dropping 1 bar of pressure each shot.

To reduce mixing spent gas with the fresh charge, it would feed the chamber from the side to reduce mixing. Aimed skyward, the fresh cold mix should be denser and try to remain stratified in the chamber.

Does this have hidden problems other than the obvious size needed for the air metering tube?

Other uses could be for very small pill bottle cannons that are hard to get a proper mix.

I'm not sure how having an air meter is simpler than a basic ball valve and fan intake. However, I do believe it will work just fine if you wanted to pursue it. It will make for a heavier cannon.

If you're going to use atmospheric air, the biggest issue is making sure you fully purge the previous shot's spent gases. You may want to "meter" maybe a 1 1/4 chamber's worth of air just to make sure. That would also mean wasting some propane as well but it may be necessary.

Would a plastic fan inside a combustion chamber not get roasted by fire and end up being damaged over a short time?

http://www.spudfiles.com/spud_wiki/inde ... hamber_fan

Spudwiki wrote: Will my fan survive the combustion?

If you think that your fan might get burned or melt, don't worry, it wont. Since the heat in the combustion chamber lasts no more then a few milliseconds the fan doesn't have time to heat up or melt. As stated above, your fan should not be too close to the barrel. If its more to the back, it wont get blown to pieces due to the extreme flow.

A fan may have a somewhat shortened lifespan inside a combustion chamber, but it still can survive for many years. So for combustion cannons: Yes, your fan will survive.

In hybrids fans last shorter due to the more extreme environment. High-mix hybrids can break their fans on the first shot. Luckily, those hybrids don't need fans anyway. When you pressurize the fuel mixture with air after you have put in the fuel, the blow-in of air already mixes the fuel. Also because of DDT dangers you don't want to have turbulence anyway.