SpudFiles

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

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.

cut down the barrel lengh. thats all i ahve to say.

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.

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
thanks for the help anyway

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?

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?

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.

_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.

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..

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

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 in³.)

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.