I was hoping for some feedback on my oxy-propane combustion cannon.

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uninc4life2010
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Sun May 24, 2020 3:21 am

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I built a standard combustion cannon with an auxiliary fill chamber and an integrated pressure gauge. I was disappointed with the performance when I used only propane, so I decided to add an additional valve that leads to an oxygen gas cylinder. I found that the best performance was when my auxiliary fill chamber registered roughly 15 psi of propane followed by roughly 40 psi of pure O2. Although this mixture ratio provided the most power, it scared me a little bit. It was quite loud, there were a lot of flames, and I completely lost sight of the sand-filled sock that I used as a projectile.

Since O2 canisters are a bit expensive, I experimented with using supplemental air from a bike pump. Since air is only about 20% oxygen, more had to be added to compensate. I found that 15 psi of propane in the auxiliary chamber with about 60 psi of air supplied by the bike pump performed similarly to the 15 psi propane/40 psi oxygen mixture.

My concern is that SCH 40 PVC isn't strong enough to withstand the pressure and heat generated by a propane/O2 combustion mixture. Would it be wise to rebuild this cannon with SCH 80 PVC or even metal pipe?

I'm using a 12" segment of 1/2" ID metal pipe as my auxiliary fill chamber. Does anyone have recommendations for an ideal propane/oxygen ratio? Are there better fuels to use than propane when supplementing the combustion reaction with pure oxygen gas or compressed air? I haven't experimented with butane or acetylene gas yet, and I'm not sure if it's even worth it.
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mrfoo
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Mon May 25, 2020 5:51 am

Personally, I wouldn't use PVC for anything involving compressible fluids at pressure, certainly not for combustion chambers, and doubly not so for anything involving oxy-gas mixtures. But that's just me, I like to keep the parts of my body complete and attached where they are.

For air and propane, you want something like 24:1 air:propane by volume, or 15:1 by mass (from memory, running pulse jets, you might want to double check). Not sure of the stoichometry for propane and pure oxygen. The internet knows all, though.
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Mon May 25, 2020 9:24 am

Oxygen is a huge liability. Not many people really seems to understand that. I've personally been close to an incident which killed a young man with a wife and 3 little kids at home. The smoke had not yet cleared when I walked through the area wondering what the hell just happened. The root cause was people not understanding what oxidizers are or what they can do. They could've accomplished the same thing with plain ol' air and the young man would've went home to his family instead of dying on the way to the hospital.
I have been working with oxy-fuel equipment for nearly 40 years and I have no interest in keeping useful amounts of oxygen around my place. I burn all my metals with plasma, which passes plain ol' air through an electric arc.
Oxygen not only makes fuel gas combust at a temperature sufficient to cause steel to actually burn (burn is the correct word. When oxy-fuel cutting you can start your cut then shut the fuel gas off and continue just fine because the steel has become the fuel), but it makes pretty much everything combust at a temperature sufficient to burn steel.
The largest non-nuclear bombs in use today use plain ol' air as the oxidizer.
If you have not spent time working with air-fuel mixtures you should. It still does not guarantee your safety but you may be surprised at the results.
Since spud guns are cyclically loaded devices by nature you should also have a firm grasp of the concept of material fatigue, which is the main reason spud guns need to have a significant amount of extra strength built in and should be inspected frequently for signs of cracking, stretching, thinning, de-laminating, etc.
If I was interested in making an oxy-fuel spud gun I would look for an M4 Sherman tank barrel lol.

Be Safe, and best of luck.
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Tue May 26, 2020 2:22 am

You built what we call a "hybrid" cannon. It uses a measured, pressurized mixture of propane and air and is then ignited. How do you keep the pressurized air and fuel in your chamber before igniting it? It typically requires a piston design or a burst disc. Also, pressurized fuel/air mixtures or oxidized fuel mixtures are usually too much for plastic to handle for any length of time. I have two cannons made of aluminum that work with pressurized air and propane and I will never be completely comfortable with them, even after using one of them reliably for nearly 12 years now. I'd never consider using PVC for something like this unless you plan for it to explode on you. If you expect it to fail, then that's fine and you can remote fire it while taking cover from a distance. For the record, I'd fully expect it to explode on you.
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jackssmirkingrevenge
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Sat May 30, 2020 5:40 am

uninc4life2010 wrote:
Sun May 24, 2020 3:21 am
"I was hoping for some feedback on my oxy-propane combustion cannon."
The general consensus as you have probably realized is "don't"

Ditch the oxygen and get it to work properly with propane first.

Syringe fuel metering is an easy way to start:

hectmarr wrote:You have to make many weapons, because this field is long and short life
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Thu Jun 04, 2020 10:38 pm

What Jack said, but with a chamber that big a syringe is going to have to be huge. Is that a meter pipe system? if it is and you think the performance is lacking I would look at 2 things. (1) Is you mixture correct? Your fuel volume using air at 1 ATM is 4% of the chamber volume. (2) Big long chamber will take a while for fuel to mix, do you have a chamber fan?


And, using more than 1X air-propane in sch40 PVC is probably not a good idea.
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jackssmirkingrevenge
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Sat Jun 06, 2020 6:06 am

jimmy101 wrote:
Thu Jun 04, 2020 10:38 pm
What Jack said, but with a chamber that big a syringe is going to have to be huge.
eBay to the rescue!

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hectmarr wrote:You have to make many weapons, because this field is long and short life
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Mon Jun 15, 2020 4:58 pm

The biggest I usually see is 150 ml. That's big enough for 1x in a 3.75 liter (229 in^3) chamber, which is a pretty big chamber.
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