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coaxial combustion

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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coaxial combustion

Unread postAuthor: Eddbot » Sat Jan 12, 2008 6:18 pm

just wondering if anyone has ever made a coaxial combustion, and if so, how did it work out
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Unread postAuthor: BigGrib » Sat Jan 12, 2008 6:35 pm

Co axial combustions are easy you dremel down the ridge in the bushing to push the barrel all the way into the combustion chamber. if you are using metered propane you must remember to take the volume of the barrel out of the volume of the chamber when doing the math for your meter.
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Unread postAuthor: sjog » Sat Jan 12, 2008 6:55 pm

Why would any one make one?
What are the pros and cons?

Shorter overall length?
No breach loading.

Performance?
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Unread postAuthor: Novacastrian » Sat Jan 12, 2008 7:02 pm

Yes i have made a coax combustion, as said above i made it that way so i could load ammo and fuel through the same place at the rear of the chamber. It doesn't really make the cannon shorter by much, maybe 6 inches. Performance actually drops a little bit i think, probably due to the flame front having to travel around the barrel inside the chamber.
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Unread postAuthor: sjog » Sat Jan 12, 2008 7:22 pm

OK I get it. On performance I can understand that too.
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Unread postAuthor: Eddbot » Sat Jan 12, 2008 10:01 pm

sjog wrote:Why would any one make one?
What are the pros and cons?

Shorter overall length?
No breach loading.

Performance?


just because i can

how is it only shorter by 6 inches, my spray and prey is 5 feet, if i made the barrel flush with the chamber, it'd only be 2 feet long, i would have a shorter barrel, but the combustion space would be smaller so it's relatively proportional, and if i had multiple ignition points the flame front would travel faster around the back
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Unread postAuthor: Novacastrian » Sat Jan 12, 2008 10:09 pm

The only reason i said 6 inches is because i was referring to my cannon.
You can actually shorten the barrel by nearly the full length of the chamber- my chamber was only around 8 inches long. I ignited mine from the rear but like you mentioned if you have multiple spark gaps you may negate any problems in that regard. Most of the combustions i have made have the barrel go into the chamber about 1/3 of the way, i just like doing it like that!
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Unread postAuthor: BigGrib » Sat Jan 12, 2008 10:14 pm

it all depends on how you build it. if you put the barrel all the way down the chamber yeah you will save yourself from lugging however many feet of extra length of barrel since it's in the chamber. as far as performance goes i've never made one so i wouldn't know how it'd effect the performance, but as far as the flame front having to go down the back of the chamber and then turn and go out the barrel, f@ck pressure is pressure and it is equal on all surfaces, so i don't see how it'd be a big performance killer.

the only thing i can find a problem with is evacuation of the chamber if you were to build an advanced combustion, since you'd have the valve at the back and the barrel at the back venting completely might be a tricky bit. but overall i think it's a good idea, i mean my blue blaster golfball cannon all together with the barrel attached is 9 1/2' long. if i'm firing it myself i have to take the barrel off after every shot to reload it. i think if it were a co-axial it's be nice to have the shortened length with roughly the same power.
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Yea, that's definitely going to get you at least a tazer.

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Unread postAuthor: Eddbot » Sun Jan 13, 2008 5:19 am

if i had some really tiny computer fans, like 1/2 to 3/4, then i could put three or four around the barrel in the combustion chamber, but then there could be a problem with the fans getting in the way of the flame and getting blown out of position, predicaments *shift+arrow left* *copy-paste* predicaments :roll:
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Unread postAuthor: bluerussetboy » Sun Jan 13, 2008 12:13 pm

for venting you can add a tee to the muzzle end of the chamber and put your fan in the lower part of the tee.
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Unread postAuthor: jimmy101 » Sun Jan 13, 2008 12:17 pm

BigGrib wrote:it all depends on how you build it. if you put the barrel all the way down the chamber yeah you will save yourself from lugging however many feet of extra length of barrel since it's in the chamber. as far as performance goes i've never made one so i wouldn't know how it'd effect the performance, but as far as the flame front having to go down the back of the chamber and then turn and go out the barrel, f@ck pressure is pressure and it is equal on all surfaces, so i don't see how it'd be a big performance killer.

But, if the barrel is in the chamber you need a longer chamber to get to the same chamber volume. I would think that a 3"ID chamber with a 2"ID barrel would require that the chamber be roughly twice as long to get to the same chamber volume as you would need for a standard configuration. Of course, the bigger the ID of the chamber, or the smaller the OD of the barrel, then the less you would have to boost the chamber length to get to a particular chamber volume.

Pressure does work in all direction but gas flow is sensitive to changes in direction (since gas has mass) and to flow restrictions. So the decrease in gas flow might lower the performance of a coaxial a bit relative to a standard configuration.

The thing I would worry the most about in a coaxial would be the increased heat loss you would get with nearly twice the surface area per unit volume of chamber that the coaxial would have relative to a standard configuration. Heat loss is a big performance killer in combustion guns. Standard combustion guns probably loose 50% or more of their chamber energy as heat lost to the chamber walls. Any significant increase in the heat lost may really lower the performance.

And, as others have posted, it is trickeir to get a fan and spraks into a coaxial and thoroughly venting a coaxial is probably a bit harder than a standard configuation.
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