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this might be a silly question but...

A place to ask general spud cannon related questions.
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this might be a silly question but...

Unread postAuthor: beergut » Sun Dec 31, 2006 8:04 am

I was drawing up a parts list for my hybrid last night and I got to thinking...

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... So, if it only takes maybe 60 - 90psi to start moving that potato, which in effect also enlarges the combustion chamber, which also at the same time lessens the amount of pressure in the chamber due to the extra volume created as the potato moves up the barrel... why would pressures in the cannon ever exceed say... 200-250 psi? I would think that by the time the combustion reached the 250psi mark, that potato would be far gone allowing the chamber to vent completely. I guess what I'd need to know is, does the explosion expand equal in all directions (like ice freezing) or follow the path of least resistance, in this case, the potato... The potato in essence acts as a pressure relief valve for the combustion chamber, and should really never let the pressures in the chamber get too high above what it takes to push that soon to be mashed potato.

So I's assume that by using different gas or a mixture of gas and oxy, you're really just changing the combustion velocity, and not necessarily increasing the pressure a whole lot (sorta like 0-60 time in 2 different cars). And once the pressure reaches the point to break the potato free from the bottom of the barrel, which at that point means the potato is traveling as fast as the expansion of the explosion... I come back to my first question above... :?

I'm ready to learn... teach me! (but be nice!)
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Unread postAuthor: jrrdw » Sun Dec 31, 2006 8:22 am

I can only guess, i think it's because the chamber is allready pressureized. the extra pressure from the gasses going off pushs the pressure to the peak, whatever that may be.

Iv'e wondered how fast is the transision from the exact peak untill the declien?
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Unread postAuthor: medievalman » Sun Dec 31, 2006 8:30 am

i think that if you have the correct c:b ratio, the pressure is greatest just before the spud leaves the barrel. the gasses burn, and expand completely just before the spud leaves the barrel, so the pressures are greatest then. and at that point, they would be pressing equally on all sides of the chamber and barrel, searching for an exit, but as soon as the potato leaves, all the gases will rush out of the barrel, instantly relieving the pressure, so there will be a very high pressure, for a very short time.
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Unread postAuthor: joannaardway » Sun Dec 31, 2006 10:51 am

If you have the right C:B ratio the pressure is zero when the spud leaves the barrel. The highest pressures are (hopefully) attained when the spud is near it's start point.

The pressure is more or less equal in all directions, but the gas flow will follow the potato. (Two seperate things)

The speed at which the pressure is reached is critical, rather like valve opening time on a pneumatic. The quicker you reach your optimum pressure, the more time the spud will be experiencing maximum acceleration for.

To use your car analogy, imagine two cars on a 1/4 mile drag race. If one accelerates faster, then it will be going faster over the finish line (ignoring top speeds, which spuds don't really have). Translate that into a 4 foot barrel and a spud, and better acceleration will equal better muzzle velocity.

The question about what maximum pressure is reached is different depending on the projectile. However, except in extreme circumstances, the flame front speed is massively faster than the projectile (your hypothesis assumes a lesser speed than that), so maximum pressure is reached before the projectile has moved a notable distance, so the effects are neglible.

In the case of a ping pong ball, which is so light that small forces can create vast accelerations. Under typical spudgun forces, the acceleration can exceed 10,000G. In those cases, the maximum pressure might be noticably lower, and achieved later, but in the case of a weightier spud, then maximum pressure is not vastly different to what a sealed propane combustion would achieve.
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Unread postAuthor: beergut » Sun Dec 31, 2006 2:03 pm

That sounds like it makes good sense. So, I'll do a search for the formula to calculate the proper C:B ratio for a 5' 2.5" barrel. I like a smaller diameter barrel because it easier (and cheaper :wink: ) to find projectiles. Also easier to reach supersonic speeds :D

I have access to a ton of steel pipe (not iron), all with at least 1/4" to 3/8" thick wall. I think I'll do my hybrid all from steel, with welded seams, so I can use a pressurized chamber with an oxygen / acetylene combo and get some real power... I wanna hit the moon ~ Alice!
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Unread postAuthor: Insomniac » Sun Dec 31, 2006 8:46 pm

Don't pressurise acetylene!!!!!!!!!!!! it becomes unstable and will self-detonate above 15 psi!

Oh and in hybrids it's not only the high pressure that limits the type of gas you put in. If the mix is too high, DDT (deflagration to detonation transition) can occur. This means that the flame front moves faster than the speed of sound, causing a shockwave that makes a huge pressure spike that will rupture most chambers ( I don't know if a metal chamber will be able to cope or not, but a PVC chamber will shatter with the force of the shockwave)
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Unread postAuthor: rna_duelers » Sun Dec 31, 2006 8:47 pm

Dont we all.I guess this is war beergut,me and you hybrid build off.Hope you can handle.Steel chamber does sound nice :wink: .I have a dis-used oxygen bottle ratted to 12,000psi and I have access to a large lathe and a person that can weld pressure vessels so war on!

How about the goal of this duel is to who can attain the highest velocity with acurate measurments.

Do you accept?
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Unread postAuthor: Insomniac » Sun Dec 31, 2006 8:53 pm

Rna, how are you going to stop the acetylene detonating before you fire it???? Or is that the way it will be fired?
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Unread postAuthor: beebs111 » Sun Dec 31, 2006 8:56 pm

that is the reason that we use burst disks, that way the pressure can build up behind the ammo and be released after it hits a higher pressure. without a burst disk, the actual pressure in the chamber probably wouldn't break 100 psi
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Unread postAuthor: rna_duelers » Sun Dec 31, 2006 8:58 pm

Well I plan not to use acetylene although that does sound very fun.I plan to use MAPP and oxidizers mixes.

I might actualy give that a try.Pressurising the acetylene till it detonates/deflagrates.
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Unread postAuthor: Insomniac » Sun Dec 31, 2006 9:02 pm

Why not use hydrogen/oxygen??? Its powerful and it is fairly safe ( :lol: well safer than acetylene anyways :lol: )

I used electrolysis to make small amouts of hydrogen/oxygen in the right amouts to combust, it was pretty powerful and loud for the amount of gas. I'll draw up a diagram in a second.

Edit: heres the diagram
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Unread postAuthor: beergut » Mon Jan 01, 2007 9:03 am

rna_duelers
I accept your challenge... :D
I will begin a strenuous regiment of dieting and exercise in preparation of this challenge, I will study, learn and master the ancient techniques of the Chinese art of demolition... there will be simulations and drills, some may parish, some may run, but for those who stay the ground, for the brave and idealized, for the many for whom will forever be remembered by virtue of their cunning tactics and savvy logic, there will be riches, there will be honor, there will be finalization of their sacrifice and recognition of their bravery, there will be mountains and oceans named after you and celebrations so grand in your honor.

but it’ll have to wait until my hangover goes away... :wink:


About the acetylene… if it blows at 15 psi, then why does my 3 foot tall acetylene tank gauge read 60 psi when I crack it open? and the line pressure is over 15 psi…

I do have some reading up to do before I build one of these advanced gas cannons, I’ll need to put together a nice list and do some math homework.
Should be real fun though.
I once filled the chamber of one of my other cannons (sch 40) with acety. when the starting fluid wouldn't ignite... well when I hit the sparker it put me on my a$$ and I'd swear I hit the moon with the "duct-tape-beer-can" I shot. The cannon was fine though (lucky for me I guess)
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Unread postAuthor: Insomniac » Mon Jan 01, 2007 5:22 pm

Maybe acetylene needs air to self-detonate???

http://www.ccohs.ca/oshanswers/chemical ... press.html read the bit on dissolved gasses.
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Unread postAuthor: beergut » Mon Jan 01, 2007 6:18 pm

the acetylene is mixed with the O2 through the torch nozzle and doesn't explode... :?
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Unread postAuthor: rna_duelers » Mon Jan 01, 2007 6:58 pm

I also have had my dis-beliefes that acetylene detonates in a low pressure environment.

Insomniac-It would make sence that O2 be present in air or not,for the detonation of acetylene wouldnt it.

Ohh well I guess this is going to be a dog fight to the end,with strategic blows and strikes.I am now cracking open my steroids and parcticing the art of invisability(not sure why :? ) This is war(while listening to This Is War by Ill nino)
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