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Gas expansion rate?

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Gas expansion rate?

Unread postAuthor: Moonbogg » Thu Apr 16, 2009 3:19 am

Hey guys,
I am thinking up a cannon and I need to make sure it makes practical sense. I want it to have the barrel of a full sized combustion and the same power, or more than that of a full sized combustion, but with a smaller chamber. I want it to have 6 small chambers so thats why they have to be smaller. I want to use hybrid mixes to compensate for the smaller chamber's volume.
The gun needs to be able to shoot spuds. After all, its a spudgun. But won't a hybrid mix of just about any X power expand too fast for a spud and break it apart? It wouldn't be a big deal if the chambers were large enough to deliver decent power under 1X. That way it could be used as a combustion for spuds and a hybrid for harder objects, like the Venom I made.
I have a feeling it depends on C:B ratio a lot as well. Just wanted your thoughts and opinions. Of course, when it comes to harder objects, such as limes etc, it will be pretty strong I believe under 3 or 4X.
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Unread postAuthor: psycix » Thu Apr 16, 2009 4:05 am

Throwing another 2000 bucks in it arent you?

Yes a powerful hybrid can obliterate spuds inside the barrel. Maybe use golfballs. More consistent too!

For higher mix hybrids, C:B ratio can be lower. Its like the same with pneumatics: half the chamber size, double the pressure and the gun will still get stronger.

My largest worry is how you are going to swap and connect the chambers, without leaking fuel mixture, without blowing up and without restricting a lot of flow.
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Unread postAuthor: Moonbogg » Thu Apr 16, 2009 4:49 am

psycix wrote:My largest worry is how you are going to swap and connect the chambers, without leaking fuel mixture, without blowing up and without restricting a lot of flow.


Yeah, sounds challenging doesn't it? Thats why it would be really awesome to pull it off and to have it clean and wire free. Back to the subject, I might have to experiment with pvc to see how the spuds hold up. If it can't shoot spuds then its not a spud gun.
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Unread postAuthor: Hubb » Thu Apr 16, 2009 7:34 am

With my hybrid, I was able to fire a spud at 3x. Anything higher than that would make soup. But, this is using a 1" barrel and has a chamber volume of around 20 cubic inches.

I'm not sure if it was from the blast itself or if it was from the burst disk (probably a combination of both) that destroyed the spud. If it was the burst disks that shredded the tater, then you could probably go with a piston hybrid. That would probably actually be easier than your giant burst disk idea too. Another idea, if you are intent on using a tater as a round, is to go with a sabot of some sort.

Anyway, good luck with it.
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Unread postAuthor: POLAND_SPUD » Thu Apr 16, 2009 8:45 am

if I were you I would either
-switch to a pneumatic for rapid loading and firing.. even the simplest breech loading gun could fire several shots per minut
- or build a semi auto combustion/hybrid.... I suspect that the amount of work needed to build a 6 shot hybrid is more less the same as building a fully semiauto one.. so what's the point ??
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Re: Gas expansion rate?

Unread postAuthor: Technician1002 » Thu Apr 16, 2009 11:22 am

Moonbogg wrote:Hey guys,
I want it to have 6 small chambers so thats why they have to be smaller.


If this has 6 burst disks, the first one to rupture may delay the rupture of the other ones to keep peak pressure down. On the other hand if all the spark gaps were fired in a daisy chain so a shock wave hit all 6 at the same time, or combined to a single burst disk, it may work as a shaped charge. Interesting concept. As others stated, the combining plumbing could pose all kinds of flow restrictions. It's much easier to build a single open chamber of the same volume and more likely to be higher power.

***Edit*** I think too show, the extra surface needed for the multiple chambers provides more heat removal path limiting the heat expansion. A single spherical chamber is most efficient in this regard.
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Unread postAuthor: psycix » Thu Apr 16, 2009 12:48 pm

Yeah, sounds challenging doesn't it?

No, more like a PITA. :P


@Technician1002
He means to fire every chamber independently. As in 6 different shots without having to refuel in the meantime. Not at the same time.
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Unread postAuthor: Moonbogg » Thu Apr 16, 2009 7:18 pm

psycix wrote:Throwing another 2000 bucks in it arent you?


Oh no. Not a chance. This will be a very simple, easy to achieve design no matter what. I will do it so it is achievable financially lol. I will be asking for advice and recommendations along the way. Thats how I was able to make the Venom a nice gun, with everyone's advice. Expensive for sure, but a treat.
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Unread postAuthor: MountainousDew » Thu Apr 16, 2009 7:55 pm

As far as I can tell your basic problems are:
-Potato blowing up in the barrel
-Figuring out how to keep the gases sealed while rotating the 6-shooter
-And others that I had in my head, but then forgot in the midst of typing this...

Even though the Venom was/is amazing and awesome, and I don't doubt your ability to conquer these problems as you are describing your gun design I think all of the problems you are going to face is by going with a piston type hybrid, which Hubb017 said.

Oh yeah, and I'm kinda new to advanced spud guns like you have created, so if I'm totally wrong in what I'm saying just disregard it and move on to the bigger & better things you're planning on.

Edit: And I don't know anything about gas expansion rates either...
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Unread postAuthor: jook13 » Thu Apr 16, 2009 10:18 pm

My belief is that once you go to a barrel that fits golfballs nice, you may never want to go back to shooting taters. I say forget potatoes and go for a higher mix and do some serious damage.
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Unread postAuthor: Moonbogg » Thu Apr 16, 2009 11:56 pm

Imagine multiple chambers, each holding their own pressurized gasses yet they get their fuel from a single inlet. Each chamber can have a high pressure check valve in the back, or something along those lines, that lets gas in but not out. After they are fueled, the connecting port is disconnected from the propane tank and there is no fuel between the chambers any longer to blow.
each chamber can be the same diameter as the barrel and they can simply snap firmly into place and be rotated by hand. Each shot just grab a nice knob and rotate and click in place for the next shot.
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