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possible very low x basketball hybrid?

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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possible very low x basketball hybrid?

Unread postAuthor: isban » Fri Nov 28, 2008 11:45 pm

a while ago i gave my basketball cannon propane fueling. while that was all well and good, i couldnt help but wonder if i could push it a little further. i was thinking about maybe just a 2x capable setup. i would have to basically overhaul the whole thing, but i think it would be worth it.
i just came up with this idea and want to see if it makes sense.
it would involve building a burst disc holder inside the cannon. it would be one ring permanently attached to the cannon and then another ring that could be bolted down with material sandwiched between.
im just wondering what would be a good material to make the rings out of as well as the burst disc material.
do you think this would work? any suggestions?
i hope the picture helps a little bit just ask if it doesnt.
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Unread postAuthor: starman » Sat Nov 29, 2008 12:15 am

You could build the rings out of plywood. The front ring closest to the basketball will need to be fully sealed onto the barrel, probably with epoxy and mounted with screws as you show. The back ring will need to be able to be fished out and have a burst disk reinstalled into it. You show it the other way, fishing out a ring from the barrel side.

The big challenge will be finding a burst disk that large that can withstand 15 psi but break reliably somewhere between 30 - 80 psi. For that reason, I would suggest building your ring down to a smaller hole, say 4 or 5". Even that will be challenging but much easier than coming up with a full 9" burst disk. A 9" disk will have to hold back 4 times the absolute force of a 4.5" disk at the same psi so would have to be significantly stronger.

However, I like the idea. I would also suggest a ball stop, possibly built with another ring further up in the barrel. You don't want the ball messing around with your disk in there...ie. tearing it.
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Unread postAuthor: jon_89 » Sat Nov 29, 2008 12:44 am

I dont know if it will work but oh my god if it does it will be sweet. :lol:
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Unread postAuthor: D_Hall » Sat Nov 29, 2008 1:05 am

starman wrote:However, I like the idea. I would also suggest a ball stop, possibly built with another ring further up in the barrel. You don't want the ball messing around with your disk in there...ie. tearing it.

I suspect that any disc capable of standing up to 15 psi on a diameter the size of a basketball is NOT going to be adversely affected by the basketball.
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Unread postAuthor: starman » Sat Nov 29, 2008 1:58 am

D_Hall wrote:
starman wrote:However, I like the idea. I would also suggest a ball stop, possibly built with another ring further up in the barrel. You don't want the ball messing around with your disk in there...ie. tearing it.

I suspect that any disc capable of standing up to 15 psi on a diameter the size of a basketball is NOT going to be adversely affected by the basketball.


Yeah this is true. Still, stablizing the ball up off of the disk should have the effect of better shot consistancy.
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Unread postAuthor: spudamine » Sat Nov 29, 2008 6:06 pm

I've recently been playing around with a 2.5" busrt disk setup and had some success with plastics, 1.5mm thick polystyrene sheet went at about 70psi but the same thickness of acryic (perspex) didn't go upto 150psi. Double or triple the thickness and you should be good to at least 15psi with a 9" setup. It also has a nice failure mode, exploding into many fragments which are shot out of the barrel with a sound similar to breaking glass :D

You could hold a smaller burst disk in larger rings, but you would then have more leverage acting to pull your rings out of the chamber wall
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Unread postAuthor: isban » Sat Nov 29, 2008 10:57 pm

yeah i was thinking about the tradeoffs of a larger hole needing a better disc and a smaller hole needing more structural support. i think it would be pretty easy to go in the middle with about a 6" hole. thats only 2" around the inside and i need space for the nuts and bolts.
my experience with plywood has been that it tends to kinda wear down especially in a situation like this with repeated installing/uninstalling. would something else be more sturdy/reliable?
ill check out the plastic sheet stuff. i think it will take a lot of experimenting to get a reliable disc.
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Unread postAuthor: starman » Sat Nov 29, 2008 11:41 pm

isban wrote:my experience with plywood has been that it tends to kinda wear down especially in a situation like this with repeated installing/uninstalling.


How many installs/uninstalls are you expecting? How many real shots are you planning for this thing. Tends to wear down? What are you using it in, a wood chipper? A couple of coats of polyurethane should protect those rings, basically forever. What other material did you have in mind that would be easier to work with?

Try 5/8" of cabinet grade plywood to cut out your rings.
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Unread postAuthor: isban » Sun Nov 30, 2008 4:32 pm

maybe i was just using crappy plywood. it was whatever i could scavenge for free at school. i guess i should invest in good plywood or maybe like particle board.
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Unread postAuthor: VH_man » Sun Nov 30, 2008 7:53 pm

I would use PVC sheet. Either that or 1/2 inch polycarbonate. Just seems to me it would work better.
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Unread postAuthor: Pete Zaria » Sun Nov 30, 2008 8:30 pm

I can't condone putting holes through PVC pipe (for your burst disk holder system) and then using it as a hybrid cannon... Heck, I say never put a hole through PVC except where fitting and pipe overlap, and even then, I try to avoid it.

If you do this, test fire it remotely several times first... I doubt anything very bad will happen, since blowing the basketball out of the barrel should require a lot less force than rupturing a pipe wall, but it could happen.

Peace,
Pete Zaria.
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Unread postAuthor: starman » Sun Nov 30, 2008 9:10 pm

isban wrote:maybe i was just using crappy plywood. it was whatever i could scavenge for free at school. i guess i should invest in good plywood or maybe like particle board.


I wouldn't use particle board for this although MDF board should work just fine. Treat it like you would cabinet grade birch plywood.

If you were using roofing or flooring grade 3/8" plywood, I can understand why you may have had trouble with it. There are lots of voids and generally poor quality wood used in that product.
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Unread postAuthor: spudamine » Mon Dec 01, 2008 3:17 pm

I can't condone putting holes through PVC pipe (for your burst disk holder system) and then using it as a hybrid cannon... Heck, I say never put a hole through PVC except where fitting and pipe overlap, and even then, I try to avoid it.

If you're worried about this you could buy a coupler, cut off a slice of it, slide it down your barrel and glue it on. You can then put your screws through a double thickness, I used this technique to sleeve the whole chamber of my hybrid, its solid as a rock.

MDF is quite weak in the plane of the sheet there might be some danger of your screws splitting it if you screw into the edge.
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Unread postAuthor: starman » Mon Dec 01, 2008 3:49 pm

spudamine wrote:MDF is quite weak in the plane of the sheet there might be some danger of your screws splitting it if you screw into the edge.


I wouldn't expect the screws to do the holding in MDF, rather, the epoxy cement. The screws would be used mainly as stabilizing and positioning pegs and therefore could be done with fewer of them.
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Unread postAuthor: jon_89 » Mon Dec 01, 2008 5:56 pm

What kind of pipe are you planning on using?
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