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Mass flow and De Laval nozzles for your spuddy

A place to ask general spud cannon related questions.
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Unread postAuthor: Gippeto » Sun Dec 07, 2008 12:39 pm

That's interesting.

To make it work with a "spuddy" level of tech, (if I understand this) you would in essence, build a conventional high pressure launcher (co-axial), with a second piston in the barrel.

On the end of the barrel you would need a burst disk and another barrel with projectile.

A one shot wonder. But ohhh, what a shot!


It occurs to me that if one were to leave out the burst disk, and use a tighter fitting projectile, this is more or less a springer air gun. (A substantially more powerful one)

Perhaps, at a lower power level (not 5000-33000fps :shock: ), the piston could be made to survive.

I may play with this idea in the future. :)

Thank you D. Hall

On the other hand;

I believe that if I were simply using a burst disk instead of a piston, I would already be getting the reading I'm looking for.

I want to use a single piston for this project.
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Unread postAuthor: D_Hall » Sun Dec 07, 2008 6:22 pm

Gippeto wrote:To make it work with a "spuddy" level of tech, (if I understand this) you would in essence, build a conventional high pressure launcher (co-axial), with a second piston in the barrel.

On the end of the barrel you would need a burst disk and another barrel with projectile.

A one shot wonder. But ohhh, what a shot!

More or less correct, but you've got me confused with your "second piston" statement.

Think of it like this... A "traditional" pneumatic made of steel with a threaded barrel. It could be used like the normal pneumatic that it is, but if you loaded it with a spud and then screwed on a smaller barrel. The smaller barrel would have a burst disc and and a BB.... Now you've got a 2 stage design.

It occurs to me that if one were to leave out the burst disk, and use a tighter fitting projectile, this is more or less a springer air gun. (A substantially more powerful one)

Correct.
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Last edited by D_Hall on Sun Dec 07, 2008 10:52 pm, edited 1 time in total.
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Unread postAuthor: Ragnarok » Sun Dec 07, 2008 6:27 pm

D_Hall wrote:More or less correct, but you've got me confused with your "second piston" statement.

I assume the first piston refers to a piston valve used to provide the 400 psi into the driving side of the compression piston (the 2nd piston Gippeto refers to)
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Unread postAuthor: Gippeto » Sun Dec 07, 2008 8:42 pm

A correct assumption. :)
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Unread postAuthor: Gippeto » Mon Dec 08, 2008 12:27 am

I apologize for the double post.

Re: De Laval nozzles and wind tunnels

D Hall, are you heating the air only when you are trying to get mach 3+?

My understanding was that at mach 1-2 heating the air was not necessary.

Of course, I don't have one at the office. :(

Thanks.

Edit: Some further research yielded a proper name for the idea I was getting to. It's called a Ludweig tube.

If you can picture a Ludweig tube, without the vacuum chamber, that's where I was originally heading thought wise. (sized to be hand held of course.)

Nothing new, just new to me. :)
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Unread postAuthor: D_Hall » Mon Dec 08, 2008 2:41 am

Gippeto wrote:D Hall, are you heating the air only when you are trying to get mach 3+?

That would be a safe statement. I'm not aware of us having ever run our tunnel at lower Mach numbers. 'Taint what it's designed for and all that.

My understanding was that at mach 1-2 heating the air was not necessary.

I ran some quick numbers. Starting at 400 psi, if you push that through a nozzle, you'll start having CO2 condense out (read: chunks of dry ice in your flow) at about Mach 1.5. That's perfect, ideal conditions.

And do I need to mention that for this to happen that air is COLD?
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Unread postAuthor: Gippeto » Mon Dec 08, 2008 10:13 am

Ok. Thanks.

I think I'll stop beating my head against that particular wall now. :lol:
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Unread postAuthor: Hotwired » Mon Dec 08, 2008 11:12 am

D_Hall wrote:<img src="http://physci.llnl.gov/Organization/HDivision/Research/GasGun/Images/Nellisbx.gif">

Just replace "burning gasses" with 400 psi air and "hydrogen" with ambient air, and you oughtta have a supersonic-capable gun powered by nothing more than 400 psi air.


One of these days I'll get round to doing this properly. Tried it out last year.

Image

It was actually made as an accessory to my signature cannon roughly like this:

Image

I was quite disappointed in the result to be honest, it was merely equal to or lower than if I'd attached the 4mm barrel directly to the 15mm valve port (that caused a barrel blowoff once...).

Full topic on UKSGC.

Or the more interesting result page

I'm still convinced it could work with the following:

Higher pressure
Threaded connections (the compression fitting eventually failed after being whacked with the piston)
A tighter sealing piston
Stronger burst disks

It is a lot of hassle to get a large bore subsonic launcher to do small bore supersonic but the whole setup only has to be a barrel replacement.
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Unread postAuthor: Lentamentalisk » Mon Dec 08, 2008 4:30 pm

What did you use for your piston, and what was the burst pressure of your burst disk? for this to work well, as stated above, you need a very heavy piston, and the burst disk must have a similar burst pressure to the max pressure you pressurize you gun to, though experimentation is necessary.
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Unread postAuthor: Hotwired » Mon Dec 08, 2008 5:32 pm

I didn't do this just now, I did it almost 2 years ago:

Hotwired in Feb 2007 wrote:Actually if the piston has sufficient momentum and the barrel is sealed enough the pressure can rise above the original pressure.


The piston was a solid 14mm dia x 20mm long polycaprolactone slug.

I didn't do much more testing after the compression fitting holding the smaller and larger barrel together and also tripling as the burst disk holder failed.

I'd need to go through the improvements list to rebuild a pneumatic ram barrel that is likely to be successful.
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Unread postAuthor: starman » Mon Dec 08, 2008 6:53 pm

I recall reading tech articles about the SHARP gas gun projectat Lawrence Livermore back in the 90's...an L shaped version of the gun D_Hall described.

I have often thought about this and the other light-gas gun projects from the 60's as really being the parentage of our relatively meager by comparison, hybrid cannons....or at least close cousins.... :wink:
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Unread postAuthor: jackssmirkingrevenge » Tue Dec 09, 2008 1:05 am

Hotwired wrote:I was quite disappointed in the result to be honest, it was merely equal to or lower than if I'd attached the 4mm barrel directly to the 15mm valve port (that caused a barrel blowoff once...)


Hadn't we come to the conclusion that for a pneumatic piston made in this way, you can only achieve as much pressure as you have acting on the piston?

What you need is a piston of a larger diameter doing the pushing that is connected by a rod to the piston doing the compressing.
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Unread postAuthor: Ragnarok » Tue Dec 09, 2008 1:12 am

jackssmirkingrevenge wrote:Hadn't we come to the conclusion that for a pneumatic piston made in this way, you can only achieve as much pressure as you have acting on the piston?

No, because the piston has momentum. It's perfectly possible to develop a higher pressure than you started with, but it has to be a smaller volume of that pressure.
It's much like a boost converter works in electronics.
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Unread postAuthor: jackssmirkingrevenge » Tue Dec 09, 2008 1:21 am

Ragnarok wrote:No, because the piston has momentum.


True, but the fact that its accelerating in the face of ever increasing resistence from the gasses which are being compressed means that it's better to have a much more substantial piston on the pushing end.
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Unread postAuthor: Ragnarok » Tue Dec 09, 2008 1:40 am

Springer air rifles get away with it. The peak force generated by the pressure created by the piston's compression vastly exceeds the greatest force put on the piston by the spring.

Actually, a better example is a Gas Ram rifle. It's my recollection that they use pressures of the order of a few hundred psi in the rams, but easily generate thousands in the compression chamber.

I'd say the compression side of the ram can easily exceed that on the driving side by some fairly significant amount, but that you need to take a considerable volume hit to make that possible.
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