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High pressure steel piston valved rifle in progress.

Built a pneumatic cannon? Then post it here! This section is for completed, finished cannons that you have built. Please include pictures and information.
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Unread postAuthor: qwerty » Sun Nov 22, 2009 5:12 am

Jhalek why not just use GGDT instead of claiming these "over SOS" figures your Chrono is wrong. It's not shooting a 1 inch bouncy ball over the SOS more like 400fps probably.
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Unread postAuthor: Ragnarok » Sun Nov 22, 2009 10:49 am

qwerty wrote:Jhalek why not just use GGDT instead of claiming these "over SOS" figures.

Because GGDT is equally capable of giving false supersonic claims. Personally, any velocity GGDT claims will be over 300 m/s, I read as "expect no more than 300 m/s".

I may find in practise I will get a higher velocity, but I don't go out expecting it.
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Unread postAuthor: jhalek90 » Sun Nov 22, 2009 2:01 pm

Ragnarok wrote:Until you have reliable evidence that you actually achieved that velocity, it is perfectly reasonable for us to argue against your claim, simply based on the reliability of previous claims of that velocity.


So, what your telling me is that gasses wil not expand faster than there SOS?
The hotter the gas, the faster its SOS... right? or am i wrong?

So, pressure causes gas(Air) to heat up and gain a higher SOS... correct?

The rapid expansion of gas (through a valve ext) cools it, and thus lowers its SOS and therefore the speed at which it can expand?

Meaning that the air in my cannon can only expand as fast as its SOS at any given time... and the only way i would get higer FPS is to raise the gases SOS??

Please correct me if i am wrong.
And thank you for the info. You truly know what your talking about.
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Unread postAuthor: jrrdw » Sun Nov 22, 2009 4:43 pm

@jhalek90, are you refering "SOS" as = Speed Of Sound?
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Unread postAuthor: jmadden91 » Mon Nov 23, 2009 8:21 am

Lol obviously :roll:

@Jhalek90: Nice gun mate congrats
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Unread postAuthor: Ragnarok » Mon Nov 23, 2009 9:23 am

jhalek90 wrote:So, what your telling me is that gasses will not expand faster than there SOS?

Yes. If at any time a flow starts to go supersonic, its own shockwave will instantly slow it back down.

SOS is the eventual limit on these things.

The hotter the gas, the faster its SOS... right? or am i wrong?

Correct. This is the main reason why the vast majority of supersonic cannons on this site are hybrids. Not because of the extreme pressure, but because of the extreme heat of the gases.

The actual increase in speed of sound comes when a gas has a high pressure for its density. This can arise either because of high temperatures, or a lighter gas, like steam, helium or hydrogen (and to a lesser extent, pure nitrogen instead of air).

So, pressure causes gas(Air) to heat up and gain a higher SOS... correct?

Compressing air (the more rapidly, the better) causes it to heat up. Pressure alone will not result in a hot gas.

The compressing is typically done by some kind of piston - this can either be solid (as in a springer), or one made out of gas itself (as in supersonic PCPs).

The rapid expansion of gas (through a valve ext) cools it, and thus lowers its SOS and therefore the speed at which it can expand?

Precisely. It's this cooling that generally means it's very rare to see a pneumatic put out a velocity of more than about Mach 0.95 (in the surrounding air, not the pneumatic's own gas).

Now, we have seen some supersonic pneumatics, by virtue of extreme pressure and lucky design causing heating of the gas in the dead volume, and actually making the launcher's internal SOS higher than the atmospheric SOS. Still, I stress the importance of pressure on achieving this effect.

Meaning that the air in my cannon can only expand as fast as its SOS at any given time...

As an upper limit. Obviously, if the projectile is (reasonably) airtight, then the gas can't expand faster than the projectile is moving, whatever Mach number that may be.

and the only way i would get higer FPS is to raise the gases SOS??

Higher max velocity, as I explain above. Although increasing the gas' speed of sound will help to some extent in a cannon that's shooting subsonic, the real bonuses come by raising the SOS velocity, and the velocity where transonic flow occurs.

Transonic is an inefficient area. As the name "transonic" suggests, the flow is fluctuating between where parts of the flow are subsonic and other parts are trying to go supersonic (and as I said at the start, thus creating a shockwave that slows them back down again).

To illustrate the efficiency issue, although it's not quite the same thing, you can see the effects of transonic flow on the drag co-efficient of various shapes on the graph below.
The regime tends to occurs between about Mach 0.8-1.2, but it depends on exact projectile shape (or in the case of launchers, internal design)

Image

As is evident, the transonic regime results in a very large increase in drag coefficient, which actually makes it, per metre of flight, the most wasteful velocity regime for a projectile in terms of percentage of velocity retained. (It's also a region that negatively impacts accuracy because of the sonic shocks causing instability in the projectile.)
Drag coefficient slowly recovers past the transonic area, but it's still behind subsonic flow.

Similar things happen in launchers, causing lots of losses in the transonic region. So, by being able to bring up the transonic velocity, you can yield notably improved performance in the 900-1100 fps range. By being able to bring up the SOS, you can potentially get past the sound barrier in the surrounding air.

Still, I remind you, it's not called the sound barrier because it's easy to beat...
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Unread postAuthor: jhalek90 » Wed Nov 25, 2009 12:24 pm

Alright. thank you for the info. :)

Now, i am going to google this.... but a PCP gun has an air piston?? how in the world does that work??

TO GOOGLE!!!!!!

So from what i understand.... and air piston is a secondary piston that litterally shoves the air behind the projectile...

So valve piston fires... and that shoves the second "Air piston" behind the expanding air, causing the air to be samwhiched between the air piston and the projectile... compressing the air, thus heating it up, raising hte pressure, and raising its SOS.

Also, i cant find a pcp gun that uses less than 2000psi.... so this seems fairly impractical for use in any homemade gun... not impossible... but impractical.

So ragnarok... am i correct??
Aslo... is it possible that i am reaching the max velocity for the air in my valve at its current Sos... i guess my question is... is it possible that the air in my valve is moving near its own sos?

I know i could improve the valve... but as it is... is the air moving as fast as its own sos?? or close to it maybe ??
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Unread postAuthor: Technician1002 » Wed Nov 25, 2009 12:53 pm

jhalek90 wrote:Alright. thank you for the info. :)

Now, i am going to google this.... but a PCP gun has an air piston?? how in the world does that work??

TO GOOGLE!!!!!!

So from what i understand.... and air piston is a secondary piston that litterally shoves the air behind the projectile...

So valve piston fires... and that shoves the second "Air piston" behind the expanding air, causing the air to be samwhiched between the air piston and the projectile... compressing the air, thus heating it up, raising hte pressure, and raising its SOS.

Also, i cant find a pcp gun that uses less than 2000psi.... so this seems fairly impractical for use in any homemade gun... not impossible... but impractical.

So ragnarok... am i correct??
Aslo... is it possible that i am reaching the max velocity for the air in my valve at its current Sos... i guess my question is... is it possible that the air in my valve is moving near its own sos?

I know i could improve the valve... but as it is... is the air moving as fast as its own sos?? or close to it maybe ??


SSSSssshhh.. You are giving away my plans to go supersonic with my current build. :D There is a reason for the huge fast valve.
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Unread postAuthor: jrrdw » Wed Nov 25, 2009 1:05 pm

jmadden91 wrote:Lol obviously :roll:

@Jhalek90: Nice gun mate congrats


No, not to me from the way his post is worded. :roll:

From Ragnarok's answer, I can tell what he ment to say.
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Unread postAuthor: Ragnarok » Wed Nov 25, 2009 4:51 pm

jhalek90 wrote:...but a PCP gun has an air piston?? How in the world does that work??

No, when I say "air piston", I mean it rather literally.
The air coming from the main valve acts as a piston, compressing (and heating) the gas ahead of it in the dead volume.

There's a laboratory device known as a "shock tube" that tends to work in the same way, using air itself as a piston.

...is it possible that the air in my valve is moving near its own sos?

I couldn't tell you, I don't know the specifics of your launcher well enough.

The actual speed gases move in your launcher will be dependent on the velocity of the projectile and the internal SOS.
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Unread postAuthor: Brian the brain » Wed Nov 25, 2009 5:42 pm

So ragnarok... am i correct??
Aslo... is it possible that i am reaching the max velocity for the air in my valve at its current Sos... i guess my question is... is it possible that the air in my valve is moving near its own sos?


Your valve ( and so far all that I have seen or made myself)
Does not allow 100% flow.

At best, even if the piston seals the barrel directly, only about 90 % flow can be reached, due to turbulation around the entry of the barrel.

If it would have a cone, to allow more air to smoothly transfer into the barrel, it would be more likely to reachs the limits.
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Gun Freak wrote:
Oh my friggin god stop being so awesome, that thing is pure kick ass. Most innovative and creative pneumatic that the files have ever come by!

Can't ask for a better compliment!!
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Unread postAuthor: jhalek90 » Wed Nov 25, 2009 8:13 pm

Okay... cone like velocity stack?

as is... have a slow chamfer from the port into the barrel?

I was thinking about filing the port all the way to the barrel with weld, and then using a chamfered drill bit to make a velocity stack... how much of an improvement could i expect from something like that??

Edit: i found a new cool looking sealing face... a chamfered neoprene washer... its abot 1/8 in thick at the base... and about 1/4 in thick in the center... with a flat side... is very stiff neoprene

here is a photo.
Image
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Unread postAuthor: Ragnarok » Wed Nov 25, 2009 9:35 pm

jhalek90 wrote:Okay... cone like velocity stack? as is... have a slow chamfer from the port into the barrel?

This will offer an improvement in launcher performance (from the simple view that flow through the valve will improve).
That doesn't (necessarily) mean that you'll get near launcher SOS with the projectile.

Myself, I mean to try and experiment with near SOS performance using 3vo (when it gets finished, of course).
Hopefully, the data from what I've got planned will cast some more light on the area and enable me to improve my simulations of near Mach velocities.

(Although I intend for 3vo to be a typically subsonic launcher, I'm intending on doing some shots with hydrogen, for various reasons.)
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Unread postAuthor: jhalek90 » Wed Nov 25, 2009 9:45 pm

I cant wait to see it.

So, now i have another problem. MY schrader valve keeps blowing out... ive been through 3 in the last 2 days.

It seems to only blow after the valve is opened at over 300 psi.
i mean... it holds 300 psi fine... but when i dump the air it destroys the schrader valve... any suggestions??
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Unread postAuthor: jhalek90 » Tue Dec 08, 2009 5:49 pm

Update:

Got a quick disconnect... works good.... rebuilt the piston... seals better... proabbly like... 99%
pressure tested to 400 psi.. F**King LOUD!!!!!!! i cant even describe.

Ragnarok:
http://www.youtube.com/watch?v=EqljnJB9 ... re=related
this helped me understand alot of what you were talking about.
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