Propane failing to ignite reliably.

Boom! The classic potato gun harnesses the combustion of flammable vapor. Show us your combustion spud gun and discuss fuels, ratios, safety, ignition systems, tools, and more.
uninc4life2010
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Sun May 24, 2020 11:27 pm

Hello,

I have been using a blue canister of Bernzomatic propane as fuel. My propane fill chamber is 1/2" ID and 12" long with an integrated pressure gauge. My combustion chamber is 3" ID and 24" long. I get reliable combustion when I use pure oxygen to supplement my propane fuel, but using only propane hasn't proved to be very reliable from an ignition standpoint. I'm filling my auxiliary fill chamber to about 12.5 psi propane before opening the valve into the combustion chamber, but even then, I'm not getting reliable ignition. For reference, I've tried using lemons and sand-filled socks as projectiles, both of which seem to provide a good seal between the barrel and the combustion chamber. My ignition system is a simple BBQ ignitor.

What could be the problem? Am I using too rich of a propane mixture?

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Moonbogg
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Tue May 26, 2020 2:27 am

I saw your other post. I see the oxygen bottle sitting there. I also see the igniter right next to the cannon with a short wire, meaning you have to be close to fire it. You are going to die. You can't use propane and oxygen in a PVC cannon. You are going to die or someone else is going to get mangled when this thing explodes.
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hectmarr
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Tue May 26, 2020 11:15 am

YES, as Moonbogg says and the project is dangerous. A weapon that has a mixture of air and gas made of PVC is in itself dangerous. If you use oxygen, it is much more dangerous. I've seen a PVC pipe explode live and the fragments fly everywhere ... Build the camera out of sturdy metal.
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KannonKing
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Wed May 27, 2020 1:39 am

I have to agree, using oxygen is a dangerous additive. If you are adding more than smidge, you're in danger if you finally get the mixture right and it ignites properly. Propane will only ignite between 2.5% and 9.2% by volume with air at sea level, so you may well be under fueling it. Get it all sorted out with no added oxygen and no extra pressure before you try to literally jump the gun.

That said, your chamber barrel ratio is a bit off, so don't expect to put anything into orbit.

http://www.burntlatke.com/jpg600/15cb-graph.gif

You're off the bottom of that graph... lots of bang and muzzle flash, but the sock has barely gotton going before it's out of the barrel.

I did a full two days and hundreds of rounds just to calibrate the fueler for Big Red. All my shots with oxygen were conducted with a face shield and 12 feet of separation.

You have a chamber fan? I don't see a battery box... that will be a HUGE help in getting consistent ignition while you test with propane or MAPP first, then you'll have to adapt the mix as you add a TINY bit more O2.

If you keep trying to make it go bang with more and more fuel and oxygen, invest in a good D&D insurance plan, or at least a remote trigger.
uninc4life2010
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Wed May 27, 2020 9:30 am

KannonKing wrote:
Wed May 27, 2020 1:39 am
I have to agree, using oxygen is a dangerous additive. If you are adding more than smidge, you're in danger if you finally get the mixture right and it ignites properly. Propane will only ignite between 2.5% and 9.2% by volume with air at sea level, so you may well be under fueling it. Get it all sorted out with no added oxygen and no extra pressure before you try to literally jump the gun.

That said, your chamber barrel ratio is a bit off, so don't expect to put anything into orbit.

http://www.burntlatke.com/jpg600/15cb-graph.gif

You're off the bottom of that graph... lots of bang and muzzle flash, but the sock has barely gotton going before it's out of the barrel.

I did a full two days and hundreds of rounds just to calibrate the fueler for Big Red. All my shots with oxygen were conducted with a face shield and 12 feet of separation.

You have a chamber fan? I don't see a battery box... that will be a HUGE help in getting consistent ignition while you test with propane or MAPP first, then you'll have to adapt the mix as you add a TINY bit more O2.

If you keep trying to make it go bang with more and more fuel and oxygen, invest in a good D&D insurance plan, or at least a remote trigger.
Is your C:B ratio in terms of the volume of each?
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Moonbogg
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Thu May 28, 2020 7:08 pm

The first good combustion cannon that I built was powerful enough to make me crap my pants and it worked reliably. I made it out of ABS plastic pipe, but many use pressure rated PVC. The way I did it was I first researched the designs here and asked a bunch of questions. Take a look at over-under designs in the combustion section. 4" chamber by about 18" long and a 2" barrel about 60" long is a good combo resulting in about a 1:1 chamber to barrel ratio, which is pretty good for a combustion. About 0.8:1 seems to be optimal, but around 1:1 is better I think because there will be enough gas energy left over to deliver a properly loud bang. Going with a larger chamber will only waste energy potential without increasing velocity very much at all.
Chamber fans are basically critical for good, reliable performance in a combustion cannon. Dual spark gaps by making a spark strip powered by a stun gun seems to help as well, so long as fuel is measured accurately each time with a meter pipe and a fuel tool as featured on the burntlatke site or HGDT featured here. Look for designs in the combustion section. You can get extreme, consistent performance every shot if done right without using any oxidizers. People often think they need oxidizers to get big power. I suspect the reason they think that is because they haven't learned the art of optimizing a combustion cannon properly. Trust me, you won't go looking for oxidizers as soon as you take your first shot with a proper combustion cannon. You won't believe it.
To do it reasonably safely, you have to research and ask questions. There is no other way. You have to come to understand how they work and how to make one yourself by analyzing cannons in the combustion section and asking questions until you understand exactly what needs to be done. A properly made propane combustion cannon from PVC or ABS rarely fails, but it still happens occasionally. Most of the failures I've personally seen or read about have resulted from people using non-pressure rated components, doing a bad glue job, or doing a bad design/build which resulted in too much stress being placed on an elbow or some other area of the cannon, usually from recoil or as the barrel tries to lift up and move during shooting. The barrel needs to be well secured to the chamber in an over-under design. Over-unders are good because you can get a more powerful cannon with less overall length because the barrel wraps around and runs along the top of the chamber.
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Fri May 29, 2020 9:51 pm

uninc4life2010 wrote:
Wed May 27, 2020 9:30 am
Is your C:B ratio in terms of the volume of each?
Yes
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Thu Jun 04, 2020 10:56 pm

The importance of the C:B ration is kind of over rated. If you look at the data, the velocity goes up as the chamber goes up for a fixed size barrel. Eventually the chamber get too big but that is up near C:B ratios of >5:1 or something. So bigger is generally better. The 0.8 value is actually more about efficiency than performance. Since fuel is so cheap, and making a 50% bigger chamber costs very little, there is no reason to limit the chamber volume to hit the 0.8 ratio so the gun is fuel efficient. Guns are generally really really really inefficient.

A big chamber is really helped by a chamber fan and multiple sparks.

How well does you ammo seal the barrel? A very loose fit can trash your fuel ratio, and of course it also lowers performance by allowing too much blow-by when firing.
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KannonKing
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Mon Jun 15, 2020 3:14 pm

Easiest way to understand C:B ratio is that if the barrel is too short, the projectile is still accelerating when it leaves the barrel (sub optimal), and if the barrel is too long, NEGATIVE pressure behind the projectile will slow it down before it exits the barrel ( if your barrel is very long, and your projectile/barrel seal is very good, the projectile will stop and actually come back down the barrel)
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Mon Jun 15, 2020 4:50 pm

KannonKing wrote:
Mon Jun 15, 2020 3:14 pm
Easiest way to understand C:B ratio is that if the barrel is too short, the projectile is still accelerating when it leaves the barrel (sub optimal), and if the barrel is too long, NEGATIVE pressure behind the projectile will slow it down before it exits the barrel ( if your barrel is very long, and your projectile/barrel seal is very good, the projectile will stop and actually come back down the barrel)
But this is from the point of view of a fixed chamber volume (that's what Latke did) and doesn't really address changing the chamber volume for a fixed barrel. In the latter case making the chamber bigger boosts performance up until CB ratios are above about 1.5. The efficiency drops but the performance measured as the muzzle velocity increases.

And, the change in performance between say CBs of 0.5 and 1.5 is small and unless you are precisely metering your fuel, and have consistent ammo, the difference between 0.5 and 1.5 is basically unmeasurable.
http://www.inpharmix.com/jps/Optimal%20 ... 0Size.html
spud_wiki/index.php?title=Chamber_to_barrel_ratio
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