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Supersonic Aluminum Cannon- Piston Valve Help

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Unread postAuthor: jackssmirkingrevenge » Mon Jul 04, 2011 5:49 am

inonickname wrote:Surprised noone's mentioned how unsafe this is yet. You have HEAPS of oxygen in that chamber, which can very well lead to destruction of seals, oils, etc. and can be very undesirable. 1000 PSI with ~71% O2 could equate to in excess to the equivalent of a 200x hybrid mix if the circumstances are right (oil, lubricant, plastic, seals, temperature etc). That much oxygen in your chamber at that kind of pressure can be a HUGE hazard.


*gasp* hadn't realised!

Do NOT use such high concentrations of oxygen as propellant!


Here's why:

Before:

Image

After:

Image

Another example:

Before:

Image

After:

Image
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Unread postAuthor: vakimo » Mon Jul 04, 2011 12:09 pm

sorry if i didnt make this clear, but im using an welding oxygen tank filled with compressed air, not oxygen.

Tech, what did you mean by "Lap the valve together"?

Thanks to all, lot of good advice here! will be trying much of this shortly.
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Last edited by vakimo on Mon Jul 04, 2011 12:33 pm, edited 1 time in total.
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Unread postAuthor: inonickname » Mon Jul 04, 2011 6:38 pm

vakimo wrote:sorry if i didnt make this clear, but im using an welding oxygen tank filled with compressed air, not oxygen.

Tech, what did you mean by "Lap the valve together"?

Thanks to all, lot of good advice here! will be trying much of this shortly.


You said an equal volume of compressed air before. I.e. an oxygen and compressed air mix. So you're only using compressed air?
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Unread postAuthor: vakimo » Mon Jul 04, 2011 7:10 pm

yes, only compressed air. I meant the tank has an equal volume as the gun, so 2000psi of tank pressure equalizes to only 1000 in both the gun and the tank.
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Unread postAuthor: jackssmirkingrevenge » Mon Jul 04, 2011 8:43 pm

In that case,

Image
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Unread postAuthor: vakimo » Tue Jul 05, 2011 12:03 pm

will do :D
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Unread postAuthor: Technician1002 » Tue Jul 05, 2011 1:11 pm

Lapping is a polishing step where two surfaces are ground together with an abrasive in between. This wears off the high spots on both surfaces until they mate perfectly giving a good seal. This process is best known in the automotive industry as the finishing step after grinding valves and the valve seat to shape. Lapping is done last to get a good valve seal. Remember, they don't use rubber. It is a metal to metal seal. No gaps for leakage allowed.

You are finished when the wear pattern seen at the end of the video is even all the way around on both mating surfaces.

[youtube]http://www.youtube.com/watch?v=fhXsH12Rg6s[/youtube]
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Unread postAuthor: Tobin » Wed Jul 06, 2011 11:43 am

inonickname wrote:Surprised noone's mentioned how unsafe this is yet. You have HEAPS of oxygen in that chamber, which can very well lead to destruction of seals, oils, etc. and can be very undesirable. 1000 PSI with ~71% O2 could equate to in excess to the equivalent of a 200x hybrid mix if the circumstances are right (oil, lubricant, plastic, seals, temperature etc). That much oxygen in your chamber at that kind of pressure can be a HUGE hazard.

I'm surprised Tech didn't mention it, as I understand he's fairly well versed in the hazards of oxygen. I'm sure he can back me up on this.

Not only that, but having that much oxygen in your chamber is bad for velocity. If you want to reach mach the heaviest prominent gas you probably want in your chamber would be N2, or a very hot mix of H2O and CO2 (hybrid).


Im not into hybrids so but i always thought that a 10x mix is 10 bar + fuel before ignite?
If that correct, i wonder, 200x mix, how can that be equivalent to a pneumatic 1000psi in oxygen?

How big is the performance differensebetween air and oxygen?
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Unread postAuthor: Mimikool » Wed Jul 06, 2011 1:20 pm

The different will be nigligible i think, like the diference between CO2 and air.

I thinks 200x would be most powerful i think, because, we often use burst disk with a hybrid, its the best fire system after vavless system in theory.

And the pic of pressure is reacher very quiqly, while pistons design are limited because the pressure increase progressively in the barrel.

But you don't understand one thing i feel, 200x mix means you have 200bar is the chamber, and 1000psi it's 70bar...
And and hybrid with 200x, the pic is greater than 1500bar i think.
Or it's me i don't understand i juste read te last post ^^

Perhaps you want increase the power with oxygen, which have greater relax speed

But ou can't compensate for a difference of 130bar in changing the gas, like i said, it's almost negligible.

EDIT: I think i understood nothing and reply like a crap at your post, after read it again.. sorry ^^

So i thinks i understand, he wants to say that oxygen is bad for some parts, redox i thinks, and it's concentrate gaz, so it's equal to air at 200x ^^ And it have a less speed relax.
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Unread postAuthor: ramses » Wed Jul 06, 2011 2:30 pm

Okay, to clarify, the 1x = 1 bar absolute only applies to air. In reality, even that falls apart because each x of mix requires an additional 0.62 PSI of propane.

Air is about 20% oxygen. An X is more precisely defined as the number of times more partial pressure oxygen has than an atmospheric combustion. For that reason, it is possible to have a 5x oxy-propane hybrid at just a touch over atmospheric pressure (because air is 1/5 oxygen, and oxygen is 5/5 oxygen)

A simple way to relate X to pre-ignition pressure with oxygen is to multiply the absolute pressure by 5. So, 70 bar of oxygen-propane is roughly 350x.

It is worth noting that for a given X as defined above, an air-propane mix will always produce more pressure than an equivalent oxy-propane mix. GasEQ predicts approximately 18 bar per bar of oxy-propane mix.

Ignition of lubricants would generally take place thorugh dieseling, which is, for all practical purposes, DDT.
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Unread postAuthor: Tobin » Fri Jul 08, 2011 9:02 am

Mimikool wrote:The different will be nigligible i think, like the diference between CO2 and air.

I thinks 200x would be most powerful i think, because, we often use burst disk with a hybrid, its the best fire system after vavless system in theory.

And the pic of pressure is reacher very quiqly, while pistons design are limited because the pressure increase progressively in the barrel.

But you don't understand one thing i feel, 200x mix means you have 200bar is the chamber, and 1000psi it's 70bar...
And and hybrid with 200x, the pic is greater than 1500bar i think.
Or it's me i don't understand i juste read te last post ^^

Perhaps you want increase the power with oxygen, which have greater relax speed

But ou can't compensate for a difference of 130bar in changing the gas, like i said, it's almost negligible.

EDIT: I think i understood nothing and reply like a crap at your post, after read it again.. sorry ^^

So i thinks i understand, he wants to say that oxygen is bad for some parts, redox i thinks, and it's concentrate gaz, so it's equal to air at 200x ^^ And it have a less speed relax.


Haha, the text i quoted, I took it as he compared a 200x mix with a pneumatic gun that is charged with 70bars oxygen.
And the pictures of exploded weapons abova that has ben loaded with oxygen. That wy i wonder how much more pure oxygen is compared to air in a pneumatic.
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Unread postAuthor: inonickname » Fri Jul 08, 2011 11:30 am

Tobin wrote:
Mimikool wrote:The different will be nigligible i think, like the diference between CO2 and air.

I thinks 200x would be most powerful i think, because, we often use burst disk with a hybrid, its the best fire system after vavless system in theory.

And the pic of pressure is reacher very quiqly, while pistons design are limited because the pressure increase progressively in the barrel.

But you don't understand one thing i feel, 200x mix means you have 200bar is the chamber, and 1000psi it's 70bar...
And and hybrid with 200x, the pic is greater than 1500bar i think.
Or it's me i don't understand i juste read te last post ^^

Perhaps you want increase the power with oxygen, which have greater relax speed

But ou can't compensate for a difference of 130bar in changing the gas, like i said, it's almost negligible.

EDIT: I think i understood nothing and reply like a crap at your post, after read it again.. sorry ^^

So i thinks i understand, he wants to say that oxygen is bad for some parts, redox i thinks, and it's concentrate gaz, so it's equal to air at 200x ^^ And it have a less speed relax.


Haha, the text i quoted, I took it as he compared a 200x mix with a pneumatic gun that is charged with 70bars oxygen.
And the pictures of exploded weapons abova that has ben loaded with oxygen. That wy i wonder how much more pure oxygen is compared to air in a pneumatic.


It's well known how much more pure oxygen is compared to air :wink: oxygen is of course, circa 100% oxygen and air contains ~20.95% oxygen.
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Unread postAuthor: Technician1002 » Fri Jul 08, 2011 11:49 am

One very important caution when replacing air with oxygen. Going from a 1X to "5X" by using Oxygen and expecting 5X the power is a way to get seriously injured. The Pure oxygen does not contain about 80% inert gas that takes heat from the combustion to slow the process. The chance of Detonation is very high in an Oxygen atmosphere. Remember that Oxygen Fuel mix is NOT the same as a high mix hybrid with the same resulting mass of Oxygen. The two may have the same heat of combustion with the same fuel and oxygen mass, but the rate of burn will not be the same.

Safety first. I want you around another day to post your results.
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Unread postAuthor: Mimikool » Fri Jul 08, 2011 12:45 pm

But you spoke about pneumatic cannon, so i understood you want to shoot with oxygen only, at 70bar, and compare to an hybrid 200x.

So now you speak about hybrid, at 70x but withnure oxygen, and compare with air hybrid at 200x.

With oxygen pure, there is 5 times more fuel, the power is similar to an air hybrid at 5 times the mix of the hybrid with oxygen. In your case 350x air hybrid.
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Unread postAuthor: ramses » Fri Jul 08, 2011 5:53 pm

Mimikool wrote:But you spoke about pneumatic cannon, so i understood you want to shoot with oxygen only, at 70bar, and compare to an hybrid 200x.

So now you speak about hybrid, at 70x but withnure oxygen, and compare with air hybrid at 200x.

With oxygen pure, there is 5 times more fuel, the power is similar to an air hybrid at 5 times the mix of the hybrid with oxygen. In your case 350x air hybrid.


The power is not similar; this reaction produces ~3.4 bar/x, whereas air/propane produces ~6.8 bar/x. It also runs the risk of detonating, which typically destroys the launcher and whoever happens to be holding it.
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