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My cannon actually "sucks"

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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My cannon actually "sucks"

Unread postAuthor: blackspud » Mon May 28, 2007 8:27 am

i recently built quite a large combustion cannon. i used an 18 litre water butt for the combustion chamber which takes about 4 seconds of hairspray to fill and a 6ft barrel which has a 3inch diameter. For the ignition i used a bunsen burner sparker.
But when i fire it the cannon seems to "suck" in a load of air after the discharge. This sucking is so strong it actually sucks dust and other debris down the barrel and into the chamber meaning i have to constantly clean the sparker to keep it firing. Have any of you guys ever had this problem? If so how could i solve it?
any suggestions would be appreciated
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Unread postAuthor: chaos » Mon May 28, 2007 8:38 am

new chamber that doesn't flex as much

*fixed*

it happens as well when u make small combustion guns out of coke bottles, they barrel is probably choking it a bit to.
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Unread postAuthor: VH_man » Mon May 28, 2007 9:10 am

cut down the barrel lengh. thats all i ahve to say.
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Unread postAuthor: mega_swordman » Mon May 28, 2007 11:31 am

Is the sucking actually a problem? Apart from cleaning the sparker, I would think it would be really cool for it to be sucking in debris. This is caused by the fact that all the air that was inside the camber pushed out. The chamber would then be a vacuum and suck in air. This sounds like a normal situation with larger cannons like yours.
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Unread postAuthor: blackspud » Mon May 28, 2007 3:42 pm

thanks guys,if only i could try these suggestion :(
this afternoon my cannon exploded. The chamber was completely destroyed but the barrel is ok. the sparker was even blasted a good 30 ft away. im ok but it did mannage to burn my arm and give me a bit of shellshock. i think i may try to make a metal one next as i have just aquired a welder :D
thanks for the help anyway
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Unread postAuthor: Fnord » Mon May 28, 2007 3:55 pm

lemme guess, DWV?
Glad to hear you werent hurt. take some pictures of the damage if you can. Did you do anything to cause it to explode?
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Unread postAuthor: blackspud » Mon May 28, 2007 4:11 pm

DWV? unfortuneately i didnt get any pictures but ill try and show you my new design somtime. no i had fired it many times before but this was only the second time i had fired apple out of it. perhaps the friction between the apple and the barrel was to great? although when i think about it i did put an unusual amount of hairspray in :(
so do you think i should go back to plastic or resort to the safety of metal?
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Unread postAuthor: Fnord » Mon May 28, 2007 4:32 pm

DWV stands for drainage, waste, vent. Pvc that says dwv and has no pressure rating on it is not designed to hold pressure, and (as you can see) will eventually rupture. The friction the projectile has with the barrel shouldnt cause a normal pvc or abs chamber to explode.

You'll be fine with plastic as long as you use pressure rated stuff next time. Look for pipe that says nfs-pw ### psi at 73F. where ### will be a number like 240.
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Unread postAuthor: frankrede » Mon May 28, 2007 7:29 pm

_Fnord wrote:DWV stands for drainage, waste, vent. Pvc that says dwv and has no pressure rating on it is not designed to hold pressure, and (as you can see) will eventually rupture. The friction the projectile has with the barrel shouldnt cause a normal pvc or abs chamber to explode.

You'll be fine with plastic as long as you use pressure rated stuff next time. Look for pipe that says nfs-pw ### psi at 73F. where ### will be a number like 240.
Wow.
Just because a pipe is DWV doesn't mean it is a time bomb.

and if you read his post you would see that his chamber is a water bottle.
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Unread postAuthor: zeigs spud » Wed May 30, 2007 2:28 pm

WROUNG PLACE TO POST THIS!!!!!!! ~where did the report button go?!~

anywho this is from the left over hot gas still in the chamber cooling rapidly and then when the gasses cool they "shrink" and then make a bit a a vacume within the chamber sucking in air, it is normal, but for it to eb moving things, perhaps your barrel is too long. that is often the case for me.


EDIT: wow exploded eh? any chamber that is flexing in and out alot is unsafe..
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Unread postAuthor: iknowmy3tables » Wed May 30, 2007 3:11 pm

no I think it must have been cellcore that is horible but could you get pic of the gun, sorta brings a new meaning to damage pics
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Unread postAuthor: jimmy101 » Thu May 31, 2007 12:42 pm

So by "18 litre water butt" you mean the chamber was made from one of those 5 gallon clear plastic water jugs used on water coolers? (18L is ~5 gallons, ~1100 in<sup>3</sup>.)

Then you cut a hole in it to install the sparker?

Not surprised it failed after a few firings.

You might try doing some research before you build, and then stand next to, something that is supposed to contain an explosion.

BTW: The presence of "DWV" on a pipe means essentially nothing. Pressure rated pipe frequently has DWV printed on it.
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