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Propane Combustion and Tank sizes

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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Propane Combustion and Tank sizes

Unread postAuthor: broken_system » Mon Nov 16, 2009 11:20 am

Hey guys, first time on the forum! I just had some questions maybe someone can help me out with. I am working on a design for a semi-automatic propane combustion system, I have the basic design worked out for cycling the components to allow for semi-auto fire, however I am concerned about the heat generated, and the PVC materials that are to be used. Can PVC hold up to the head generated from a propane combustion?

I am concidering using some aluminum tubes or steel tubes for the combustion chamber then finding a way to cool this down, however I am still worried about the barrel melting lol. I am toying with the idea of a C02 cooling system however ideally I would like to have the gun operate purely on Propane.

I have been trying to find small propane tanks (say the same size or close to a 12g C02 powerlet) but so far no dice. Anyone know of a small container, or powerlets that can be bought in bulk?

Thanks guys!
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Unread postAuthor: starman » Mon Nov 16, 2009 11:33 am

Unless your cycle rate is something like 1 shot every 30 seconds and you have lots of fresh air flow in between, I wouldn't fool around PVC and fast cycle rate semi-auto. Heat is an enemy to PVC...room temperature is where it likes to stay.....derating begins almost immediately above that.

Effective CO2 cooling systems are going to be too complex to be practical on a small scale cannon. Just go with a metal setup.
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Unread postAuthor: Willdebeers » Mon Nov 16, 2009 12:59 pm

I agree. Or just build a pneumatic setup, like clide did.
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Unread postAuthor: POLAND_SPUD » Mon Nov 16, 2009 1:31 pm

does it really have to be a combustion semi auto ??
as will mentioned above it would be easier to build semi auto pneumatic (easier doesn't mean it's something easy)

but if you want something really challenging then go for it


could you post more details of the gun, please?
What principle are you going to use to remove combustion gasses after each shot and then draw in fresh air ot the chamber ??

I have been trying to find small propane tanks (say the same size or close to a 12g C02 powerlet) but so far no dice. Anyone know of a small container, or powerlets that can be bought in bulk?
so little propane won't be very practical.... probably you should be fine with propane tanks for brazing and soldering

I am considering using some aluminium tubes or steel tubes for the combustion chamber then finding a way to cool this down, however I am still worried about the barrel melting lol. I am toying with the idea of a C02 cooling system however ideally I would like to have the gun operate purely on Propane.
aluminium seems like a good material for the barrel but probably it would be easier to use iron for the chamber (because threaded connections & fittings are easily available)

if you want to achieve high ROF it might be a good idea to weld a sort of radiator to the chamber... in this way the chamber would dissipate heat much faster... I am not sure what material you can use for it though... my guess is that steel sheet or corrugate steel sheet would be ok
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Unread postAuthor: broken_system » Mon Nov 16, 2009 2:28 pm

Thanks for the input guys. I am actually aiming for a smaller scale system to launch smaller projectiles. The idea to cycle the combustion chamber lies within the idea of having a bolt that would cycle forward to allow the expanding gas to escape, and have a spring return that would also link to a disk inside the combustion chamber, combines with some check valves to allow fresh air to enter inside.

As for the cooling system, the idea would be to have a high heat-conductive metal for the working bits (combustion chamber, upper tube bolt housing) then either solder or weld very thin gills evenly spaced appart in a ziz-zag pattern. This would allow the head to be conducted by the thin fins, and allowing C02 to expand (therefore cooling), and rushing through the fins, and venting at the end point of the cooling system. Each shot would have to release a small amount of C02.

If I did go for a cooling system like this the combustion system wouldn't totally be purely driven by propane, but it should work well. It might take a while (busy with college to get ready for univ.) but when i start putting together a working prototype I'll post pics and schematics. Until then I am trying to gather as much info as possible.

Technically Propane cools down when expanded just like C02, however having propane used to cool down hot fins might be counter-productive in the whole "not having the thing blow up in my face" department lol.
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Unread postAuthor: POLAND_SPUD » Mon Nov 16, 2009 4:34 pm

Technically Propane cools down when expanded just like C02, however having propane used to cool down hot fins might be counter-productive in the whole "not having the thing blow up in my face" department lol.
this is a pretty good idea... you could wrap the whole chamber in a coil of thin copper tube and run propane through it

but still I think it would be easier to forget about co2 and propane cooling and just solder or weld small fins all over the chamber


you could also put a larger PVC pipe around the chamber, seal it and fill with water...

...or use a metal pipe instead and fill the volume between the two pipes as a propane tank... each shot would consume some propane and thus cool it... at the same time combustion would produce heat... the only question is what would happen in the long run ??
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Unread postAuthor: broken_system » Mon Nov 16, 2009 4:50 pm

"...or use a metal pipe instead and fill the volume between the two pipes as a propane tank..." That definetly could work, I would just be paranoid that the heat generated from the firing chamber could set off the propane (even though it would have the incorrect ratio of oxygen to propane).

Ideally I will have a stacked tube design, the lower being the combustion chamber, and the upper being the housing for the bolt and leading to the barrel. The propane would be channeled in by some small tubing attached to a pin valve so that when pressure is appied to the pin, the propane is dumped into the combustion chamber until the pressure equalizes and no more propane can enter (and somewhere before and during this process vent the combustion chamber to ensure proper O2 to Propane mixture).

So anyways, after the propane and oxygen are in the firing chamber, the pin valve from the propane shuts, sealing the chamber, then the ignition would combust the fuel, expanding the gas and near the end of the combustion chamber would be a pressure release valve, when the pressure builds up enough to open the valve the gas would rush through up to the bolt-housing, pressing the bolt forward (to seal the round off inside the barrel), then the bolt would allow the gas to pass through, firing the round through the barrel.

After the round is out and the remaining gas leaves the barrel (no pressure remaining), the bolt would return by spring, allowing the next round to be chambered (maybe belt fed, maybe hopper fed). At the same time the bolt will be connected to a rod that will push a disk into the combustion chamber with some check valves behind it to allow fresh air to vent into the combustion chamber (at some point before or during fuel combustion, this disk would return to once again seal the chamber)

Ive actually figured out a theoretical way that I could make it fully automatic however my concern again is cooling. I've toyed with ideas similar to what Poland Spud was saying, however replacing water cooling with oil or even anti-freeze, but I figured this would be very messy, especially for basic maintenance.

*edited for dumb spelling mistakes
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Unread postAuthor: POLAND_SPUD » Mon Nov 16, 2009 5:05 pm

That definetly could work, I would just be paranoid that the heat generated from the firing chamber could set off the propane (even though it would have the incorrect ratio of oxygen to propane).
It's not possible but yeah I agree that it probably isn't safe as the amount of heat generated by combustion will be higher than the cooling effect of expanding propane..

with each shot the temperature of propane would increase and so would the pressure... some members could probably calculate how fast would the temperature increase


Do you want the gun to be hand-held ?? that's an important issue... if it doesn't have to be then you could add as much water around the chamber as you like
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Unread postAuthor: broken_system » Mon Nov 16, 2009 5:12 pm

Yeah I'm trying to have it so I could carry it around. Thats why I wanted to go for a small Propane (say 12g powerlet). The idea was to have a stock, mount the small propane receiver inside or underneath the stock, then have a relatively small body of the gun (maybe 12' long by 1.5"-2" wide by 2.5"-3" tall). then have a barrel length of about 20". The rounds I would be firing would be no larger than 3/4".

So technically the whole gun could be run off:
-one 12g C02 powerlet for cooling (should I go this route)
-a small propane tank (say the equivalent of a 12g C02 powerlet)
-a battery (most likely 9v hooked up to some capacitors for ignition).

I would estimate that a small projectile with a low mass (but high enough to keep KE) would allow me to shoot a relatively high amount of rounds off the fairly small fuel requirements!
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Unread postAuthor: broken_system » Fri Nov 20, 2009 11:04 am

Also a quick question, if I were to use macro-line or hard-line to allow the propane gas to enter the combustion chamber from the tank, should I be worried about back flow of the combustion through the macro-line or hard-line?

Basically in between the propane tank and combustion chamber is going to be a valve that will be pushed in via a pump-arm, hammer, etc havent decided yet. So that will allow the chamber to fill with propane (and o2 from another source), then that valve will close before combustion, but should it not close before combustion what are the chances of the flame travelling through the tiny hard-line and cause an explosion?

My guess would be that since the propane is dumping into the chamber from the macro-line there is no 02 inside therefore not a proper ratio to allow for ignition but what do you guys think?
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Unread postAuthor: ramses » Fri Nov 20, 2009 4:04 pm

you could use a check valve. the tank won't ignite, but any regulator you have would most likely not like the sudden spike in down stream pressure. Check valves are $5. Regs can get really expensive.
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Unread postAuthor: broken_system » Fri Nov 20, 2009 4:08 pm

Right on, a check valve would definetly do the trick, question though I did some searches online and couldnt find any small'ish check valves, do you think its possible to make a small check valve with some small tubing, some spring and a plug?
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Unread postAuthor: ramses » Sat Nov 21, 2009 8:05 pm

there are almost always 1/4" brass or stainless check valves on ebay. (which doesn't want to work right now so no link.)
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Unread postAuthor: broken_system » Tue Nov 24, 2009 11:58 pm

Thanks guys, one more question about the actual combustion. When the expanding gas rushes out the barrel, does this not create a vacuum? When it does, assuming a way to open a vent inside the combustion chamber, could this not cause the combustion chamber to vent the waste and allow enough oxygen to enter for the next combustion?
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Unread postAuthor: Technician1002 » Wed Nov 25, 2009 12:52 am

broken_system wrote:Thanks guys, one more question about the actual combustion. When the expanding gas rushes out the barrel, does this not create a vacuum? When it does, assuming a way to open a vent inside the combustion chamber, could this not cause the combustion chamber to vent the waste and allow enough oxygen to enter for the next combustion?


Nice in theory, but the chamber/barrel becomes a resonant cavity. It much more vibrates, than go to a vacuum and flow in fresh air. It happens way too fast to draw in a full chamber of fresh air. Look in the pulse jet forums on chambers that do pull in fresh air through a valve. If the porting is big enough and the valve light enough with a long barrel, it might work, but you would sacrifice power to the projectile to use the projectile exit as a draw of vacuum. Most spudguns are still under considerable pressure on projectile exit so the projectile accelerates the entire length of the barrel.
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