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first hybrid- a few questions

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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first hybrid- a few questions

Unread postAuthor: burgerace69 » Sat Jul 05, 2008 3:55 pm

I am deciding to make a hybrid with a piston instead of a burst cap and i had a few questions.
1) With a PVC schedual 40 gun how high should I take the psi up to before combusting the gas?
2) is a typical charcoal lighter strong enough to ignite the gasses? (considering the compressed water vapor in the chamber)
3) are pistons commonly used in replacement for burst caps?

When im finished with my diagram on photoshop ill upload it and you guys can critique.
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Unread postAuthor: LikimysCrotchus5 » Sat Jul 05, 2008 4:06 pm

1. Never use pvc for a hybrid
2. If you mean a bbq/piezo igniter then yes, but only up to certain mixes
3. No, there is only one piston hybrid and that is _Fnord's (Link Please) and since they are a little more complex and have a different safety factor, they are not easier to build.
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Unread postAuthor: burgerace69 » Sat Jul 05, 2008 4:20 pm

does this have to do with the temperature changes? fast heating/cooling on the pvc that makes it unsafe?
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Unread postAuthor: SpudFarm » Sat Jul 05, 2008 4:23 pm

you don't seem to know what you are doing.

look at the hybrids on the forum and get ideas.
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Unread postAuthor: burgerace69 » Sat Jul 05, 2008 4:37 pm

thats true. i have however had alot of experience with combustion and pneumatics and i dont like the idea of the piston valve so i decided to make a gun using the reverse of a piston valve. ill upload the diagram as soon as i am able to finish it hopefully nowish.
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Unread postAuthor: SpudFarm » Sat Jul 05, 2008 4:46 pm

i think you should add my msn address. i can really help if we can talk a little better
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Unread postAuthor: burgerace69 » Sat Jul 05, 2008 4:51 pm

ill talk later i am being scotch taped by little cousins now
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Unread postAuthor: psycix » Sun Jul 06, 2008 6:47 am

PVC is just plastic. Hybrids are devices which carry extremely high pressure peaks.
Almost all PVC hybrids in history failed after some time.
Its just not safe.
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Unread postAuthor: SpudFarm » Sun Jul 06, 2008 6:54 am

hmm psycix: i think you should remove your sig before he makes it from a bottle :)
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Unread postAuthor: jimmy101 » Sun Jul 06, 2008 12:49 pm

1) With a PVC schedual 40 gun how high should I take the psi up to before combusting the gas?
1X is a reasonable limit. Worst case scenario is that the valve doesn't open. The chamber pressure will peak at ~120 PSIG giving you a bit over a 2X safety factor assuming pressure rated pipe and fittings and typical IDs. The peak pressure will drop by 50% in ~0.1 second.

2) is a typical charcoal lighter strong enough to ignite the gasses? (considering the compressed water vapor in the chamber)
Assuming you mean a BBQ piezo, the answer is yes if the gap is small enough and the electrodes are pointy enough.
Why is there water vapor in the chamber?

3) are pistons commonly used in replacement for burst caps?
No.
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Unread postAuthor: Mr.Sandman » Sun Jul 06, 2008 1:01 pm

mate if you use pvc on your hybrid chamber then you better write your will at the same time. i can almost guarentee that if you use pvc on your chamber you will die or be horribly injured. not trying to scare you or anything but that is the cold hard truth( more like explosive hot truth) about hybrids. anyways check out the wiki if you need more info about hybrids, its what it is there for.
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Unread postAuthor: burgerace69 » Sun Jul 06, 2008 1:52 pm

ok thanks. ill have to think about this one. if i can figure out how to upload my diagram maybe it's a good idea. But the initial intention of my plans for this gun were not high pressure, but rather, fast release/ immediate force on the ammo. If i keep a low pressure in the chamber (30-40psi) and keep the spring at a low compression force to mirror the 30-40 psi then it should open relatively easily causing (hopefully) minimal pressures in the chamber.
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Unread postAuthor: frankrede » Sun Jul 06, 2008 2:04 pm

psycix wrote:PVC is just plastic. Hybrids are devices which carry extremely high pressure peaks.
Almost all PVC hybrids in history failed after some time.
Its just not safe.
No my behemoth
Which was taken up to 3x often and on a few occasions 4x
http://www.spudfiles.com/forums/first-h ... t7037.html
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Unread postAuthor: burgerace69 » Sun Jul 06, 2008 4:31 pm

so im just curious, but why dont more people use pistons? it seems like a pretty decent idea (btw i have decided that the chamber be made of steel- everything else can be PVC, dont worry i wont kill myself now)
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Unread postAuthor: SEAKING9006 » Sun Jul 06, 2008 5:02 pm

In a hybrid, you want to release the gasses at their peak combustion pressure, not at the storage pressure. Also, nobody uses pistons because springs are either unavailable, expensive, or they just plain won't work at the pressures used. Pistons used in pneumatics are set so the tank does not push on them, and are actuated by a pilot valve rather than the chamber pressure. A burst disc is INFINITELY more usable and design-friendly. Also, setting the spring for the storage pressure won't accomplish anything. It will simply leak gasses very slowly until the pressure is low enough, just lower than the spring force. I really suggest you take some kind of physics class or at least do some kind of online learning thing to learn about pistons and springs, and how they work. If you aren't completely sure how something functions, don't try to incorporate it into something that will experience super high pressure spikes and temperatures over 150 degrees Fahrenheit.
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