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Propane, Pure Oxygen, and PVC

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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Unread postAuthor: ammosmoke » Sat Jun 16, 2007 1:50 pm

Well, nevertheless it does concern me. All one has to do is accidentally make a more powerful ratio, or get carried away, and it will explode. Shrapnel is bad for you. Oxygen added creates vast increases in power and expansion rates. The pipe may be able to withstand pressure that high under normal circumstances, but not a spike that rapid. I would say use metal please. And don´t get carried away with your ratios. Also, much more heat is produced, which is damaging to pvc. It may work for a while, but the high heat will soon compromise its integrity.
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Unread postAuthor: boilingleadbath » Sat Jun 16, 2007 3:42 pm

propane-O2 mixtures are a couple orders of magnitude more prone to DDT than propane-air mixtures, at least from the studies I've seen.

Which, in short, means that DDT occurs very early with propane/O2 mixtures - within a foot, certainly, though this will depend upon pipe diameter.

For this reason, I think it'd be a good idea to not run a propane-O2 mixture, even in steel pipe. (although it certainly could be done... gasseous detonations <i>are</i> containable. Otherwise, research into PDEs wouldn't be very mainstream.)

...it's for the best, anyways; 3x hybrid will give you better performance.
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Unread postAuthor: frankrede » Sat Jun 16, 2007 10:37 pm

sandman wrote:
spudthug wrote:sch is usually used for hybrids and i dont think a propane/oxy mix could generate the 400 psi (estimate) that a 4x hybrid would generate would it?

if you do make it out of sch 80 make sure u dont go above 2 inch for the chamber because 3 inch may not be able to withstand the blast


yep, my 3" sch 80 is rated to 370 psi, and my 2" sch 80 is rated to 400 psi, so only the 2" is safe
oh dear my hybrid has 4" sch 80:(
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Unread postAuthor: azurex » Sun Jun 17, 2007 5:20 pm

I'm new here, not sure what all the acronyms are such as DDT, or PDE.

Also, I'm planning on a 4 inch diamter chamber and a 2 inch diamater barrel. Weight and cost are also a concern, It's metal, but i want to be able to move it, and steel is heavy, what about aluminium.
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Unread postAuthor: Fnord » Sun Jun 17, 2007 6:22 pm

DDT= deflagration to detonation transistion,
Basically, when the flame front of exploding gas reaches supersonic speeds, it conpresses gasses in front of it and causes extreme pressure spikes.

PDE= pulse detonation engine,
I havent looked into these, but one could logicaly infer it is a type of [jet] engine that uses DDT gas explosions to produce a massive amount of thrust.

Quarter inch aluminum should be fine.
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Unread postAuthor: Jumpin Jehosaphat » Sun Jun 17, 2007 7:32 pm

A gun that could contain DDT would be fun.
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Unread postAuthor: SpudBlaster15 » Sun Jun 17, 2007 8:52 pm

Small diameter steel pipe may be able to withstand DDT, as is evidenced by this graph from the Gexcon Handbook:

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The maximum pressure spike in this case was approximately 50 bar (725psi). Small diameter steel pipe should easily withstand this pressure level, but I am unsure of whether it could withstand the rate at which this pressure is produced.
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Unread postAuthor: Velocity » Sun Jun 17, 2007 9:03 pm

From what I have learned on these forums, DDT sounds better, but it really isn't. Unlike deflagration, detonation releases energy in a tremendously powerful energy spike. However, this energy spike does not impart as much energy to the projectile. Deflagration is a slow release of energy (compared to detonation). More energy is imparted to the projectile, and therefore greater performance. It seems to me like detonation is just an unnecessary risk. However, I am sure that at some point, detonation will become more powerful than deflagration, and therefore at this point detonation will be necessary to have better performance than deflagration.

...Although, good luck finding components rated to a high enough pressure to test this theory. I have no idea how powerful detonations can get.
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Unread postAuthor: ammosmoke » Mon Jun 18, 2007 8:08 pm

Lets just say that you could probably create a powerful enough mixture to turn a sch 40 steel pipe into a bomb. Not due to the pressure, but due to the steep spike, the pipe cannot always contain such a rapid blast. For example, you can stand on an empty pop can, but if you fall from as little as ten inches, you will crush it. This isn't quite the same thing, because of G forces involved.
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Unread postAuthor: azurex » Mon Jun 18, 2007 9:33 pm

Ouch, even steel? Doesn't Aluminum stretch a little bit? Then maybe it could take the spike?

What if I use SCH 80 steel? SCH 80 aluminium?

Also, do i need two meters, one for propane, one for O2, or can i find some sort of T fitting that will allow me to connect two bottles?

The diemensions i want to use are 4 inch diameter chamber 2 inch diameter barrel. Not sure about lenghts yet. SInce metal in threaded and not glued together, i may use different size barrels depending on what i want to shoot, potatos, tennisballs, soda cans.

Regardless, I will use a remote ignition, and be behind a bigass wall for the first several launches.
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Unread postAuthor: VH_man » Mon Jun 18, 2007 9:38 pm

haha stand behind the wall for ALL launches............... but post pics/vids.........

and use the thickest pipe you can possibly find............ preferably brass because it holds the most pressure........
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Unread postAuthor: Kenny_McCormic » Mon Jun 18, 2007 11:34 pm

spudthug wrote:sch 80 is usually used for hybrids and i dont think a propane/oxy mix could generate the 400 psi (estimate) that a 4x hybrid would generate would it?

if you do make it out of sch 80 make sure u dont go above 2 inch for the chamber because 3 inch may not be able to withstand the blast

its not so much how much pressure is generated its more about the burn rate of the fuel and the shock sensitivity of pvc
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Unread postAuthor: Insomniac » Tue Jun 19, 2007 4:56 am

Why not just make a decent hybrid? It will be about the same power, but the nitrogen in the air will slow down combustion and keep it below ddt. With propane and oxy you are asking for trouble, unless you make it out of thick steel or make a small gun (smaller chambers are usually stronger) If you decide to make an oxy propane gun anyway, you will need to do extensive testing with it to ensure that the chamber can cope.
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Unread postAuthor: azurex » Tue Jun 19, 2007 2:35 pm

Um, I've looked around at the hybrid forums, but I still don't know what a hybrid gun is. Is it just fuel and air put in the chamber under pressure and then ignited?
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Unread postAuthor: Binder17 » Tue Jun 19, 2007 7:01 pm

I have thought of this idea before. It is extremely dangerous though. Adding pure oxygen to anything that burns spells out danger. I would be sure that all your calculations are correct and then somehow test the mixture in a mock-up chamber. A mock-up chamber would be a PVC combustion chamber and barrel(optional) and finding a way to ignite it at a distance. The problem is the potential for something to go wrong. The ratio for oxygen to propane at standard pressure is 1:5, five being the oxygen.

And a hybrid is a spudgun that uses air pressure to compress a fuel/air mixture in the chamber. Most of the time a burst disk between the barrel and chamber to contain the pressure and is broken through on ignition.
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