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this might be a silly question but...

A place to ask general spud cannon related questions.
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Unread postAuthor: FiveseveN » Mon Jan 01, 2007 9:01 pm

@beergut: If what you're saying is true, how come outdoor explosions exist? The combustion gasses should just leak out into the atmosphere, right? Think about it. Where there is mass, there is momentum. Air molecules have momentum, too. It's what causes shockwaves, sonic booms, lift, determines the speed of sound and other kinky stuff.
Your chemistry is off, too.
For the last time, acetylene doesn't need an oxidizer, be it oxygen or whichever, to self-detonate under pressure.

And what the crack is invisability? Invisibility? Invincibility? Need-to-go-back-to-grade-school-ability?
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Unread postAuthor: pyromanic13 » Mon Jan 01, 2007 9:49 pm

FiveseveN wrote:@beergut: If what you're saying is true, how come outdoor explosions exist? The combustion gasses should just leak out into the atmosphere, right? Think about it. Where there is mass, there is momentum. Air molecules have momentum, too. It's what causes shockwaves, sonic booms, lift, determines the speed of sound and other kinky stuff.
Your chemistry is off, too.
For the last time, acetylene doesn't need an oxidizer, be it oxygen or whichever, to self-detonate under pressure.

And what the crack is invisability? Invisibility? Invincibility? Need-to-go-back-to-grade-school-ability?


how is it possible that something can combust without oxigen? acetylene is stored at a pretty high pressure in my metal shop..
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Unread postAuthor: FiveseveN » Mon Jan 01, 2007 11:21 pm

Aaaaargh, for Christ's sake. What happens to acetylene under pressure is not combustion. It's spontaneous breakdown, like with carbonic acid (bad example but it's 6 AM and it was the only unstable thing I could think of).

Oh, and, as it was stated before, it CAN be kept under considerable pressure as long as it's dissolved in acetone and the tank is filled with an inert porous material.
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Unread postAuthor: Slugfoot » Tue Jan 02, 2007 12:41 pm

I don't think it's a good idea to be feckin' about with acetylene. It's dangerous stuff and people here just don't seem to know enough about it to use it safely.

Btw.. Acetylene can also ignite upon contact with copper or brass with a high copper content.
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Unread postAuthor: pyromanic13 » Tue Jan 02, 2007 1:00 pm

FiveseveN wrote:Aaaaargh, for Christ's sake. What happens to acetylene under pressure is not combustion. It's spontaneous breakdown, like with carbonic acid (bad example but it's 6 AM and it was the only unstable thing I could think of).

Oh, and, as it was stated before, it CAN be kept under considerable pressure as long as it's dissolved in acetone and the tank is filled with an inert porous material.


k
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Unread postAuthor: boilingleadbath » Tue Jan 02, 2007 3:09 pm

I'd avoid acetelene, and just operate your cannon at a rather high mix of air-propane.

1) C#C's DDT run-up distance (distance the flame front takes to transition to detonation) at 1 atm is alot shorter than propane's.
2) The DDT run-up distance probably decreases faster than propane as the initial pressure increases - and hydrogen is in the same boat.
3) DDT is bad.
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Unread postAuthor: beergut » Tue Jan 02, 2007 4:19 pm

hey... wait a minute! Why is everybody quoting me for something rna_duelers said (FiveseveN & pyromanic13 )!!! :?

Listen, I've been fooling around with acetylene for 10 years... blowing things up and so on... it's really not that scary. I have used it a dozen times in sch40 pipe and never had a single problem (which is kinda scary). It's surly not as deadly as fff black powder pressurized in a combustion chamber, and I've been making my own fireworks just as long... Actually, that's how I found this site, from buying chemicals and "stuff" online to make explosives and fireworks... fine (fff) black powder requires no oxygen to explode and is very shock sensitive under pressure, more so then any gas in a chamber. So if someone wants to build an acetylene cannon... then so what, as long as they take video and have health (and life :wink: ) insurance.
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Unread postAuthor: FiveseveN » Tue Jan 02, 2007 5:46 pm

Neah, I wasn't quoting you, I was just referring to your initial idea in my 1st post.
Black powder (and any other solid or liquid explosive) contains its own oxidizer (potassium nitrate). Of course it doesn't need oxygen to go boom.
Well, although we've steered a long way from the initial question, I hope you've found your answer. If not, there's always Wikipedia: http://en.wikipedia.org/wiki/Fluid_mechanics
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Unread postAuthor: beergut » Tue Jan 02, 2007 8:44 pm

I'm very familiar with black / flash powder and the many nitrates that give it the explosive properties. There are dozens of oxidizers that can be mixed with powder to provide oxygen. Although acetylene may not need oxy to combust, it does make for a hotter, more unstable combustion. Look what happens to it when oxy is added when using cutting torches... it goes from a bright orange to a blinding blue / white.

I'm not sure what you're referring to here:

"If what you're saying is true, how come outdoor explosions exist? The combustion gasses should just leak out into the atmosphere,"

I didn't say that it needed to be contained in order for it to combust... I implied that during the combustion, at a certain point the pressure would equal that of the friction (pressure) holding the potato in place and simply push it out, and since the potato is just pressed down the tube, it shouldn't take too much pressure to move it, it simply acts like a pressure relief valve. I don't know what the velocity difference of exploding propane / acetylene is, and this is the key # to figure out how strong you need to build a combustion chamber / barrel. (example: black powder -1300 ft/sec or RDX 27,400 ft/sec)

-and-

"Where there is mass, there is momentum"

I'm not sure I understand the context in which you use the word "momentum". Just because an object has mass doesn't mean it has momentum. Momentum is a result of mass x velocity (p=m*v). Unless you are referring to it's potential and not kinetic energy. In any case, there is no momentum until the mass starts to move (in macro physics anyway, quantum mechanics is a whole different monster, things disappear and reappear with out moving all the time).

I appreciate all your input, but you seem very confrontational and I get the feeling you're trying to prove something, at least in the responses you've given me in my last two "new topics"... very condescending.

While being book (or wikipedia in this case) smart is great, a solid foundation to base an actual career on, it's not to be preached. I can tell you first hand that what happens out in the field is quite different then what's printed in "the" book.


Now, on another note, FiveseveN...
Your website is absolutely awesome! I spent an hour on it!
I'd actually like to purchase "tea of blue"
I'll be in touch.
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Unread postAuthor: FiveseveN » Wed Jan 03, 2007 7:48 am

I was being condescending chiefly with pyromaniac13 and rna_duelers, probably a little frustrated as well. I guess it's just a communication issue.

Let's take this thing one slice at a time:

The pressure should only get as high in the combustion chamber as it needs to be in order to push the "potato" out the barrel

That's what I was attacking, because it's not true.
Take a perfectly good solid combustion cannon, put half a pound of C4 in it and set it off. Will it blow up? How come, if the pressure should only be as high as it needs to push the potato out of the barrel? I'm willing to bet that the potato will still be in the barrel when it'll be blown to shreds :D
The pressure on the chamber's walls will not be equal to the pressure on the back of the projectile (for a split second, but enough to destroy the chamber) because due to momentum, air acts like an elastic.
The same things apply under normal conditions. A spud gun is a large system, in terms of fluid dynamics. There's plenty of room for variations in temperature, which lead to variations in pressure. Just think about how much the geometry and dynamics of the flame front affect performance.

PS: Thanks, but sorry, Tea of Blue is not available in print. I only have it in 900X598 resolution (anything above 6''X8'' will look pretty bad).
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Unread postAuthor: beergut » Wed Jan 03, 2007 6:15 pm

I understand what you are saying... Which is why I was wondering what the velocity of acetylene (combustion) is... If I recall from my military demo days, c-4 & m112 detonate at a rate of around 27,000 feet/sec (I think the only thing faster is petn or rdx) compare this to black powder at, I think around 1300 feet/sec. So while c-4 has such a fast velocity (from boson to L.A.in 9 minutes!), it actually cuts through material, like a steel beam...gun powder actually stresses it until it breaks.
I'm not certain but I cant imagine acetylene detonating faster then black powder (but I may be wrong on this) and if the velocity isn't to fast, say around where butane is, then maybe if the mass of the potato is not too much it can actually "get out of the way" before any structural damage is done to the pvc...and for every inch that potato moves up the barrel, that's another 280psi (sch 40) to contain the pressure of the combustion. right? :? There's a lot of math of course to figure all this out... :(
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