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Gas spring in ggdt

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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Gas spring in ggdt

Unread postAuthor: daccel » Wed Dec 10, 2008 11:58 pm

What valve parameters would you use to model a sear triggered gas spring?
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Re: Gas spring in ggdt

Unread postAuthor: Ragnarok » Thu Dec 11, 2008 12:39 am

You wouldn't, because GGDT's algorithm is incapable of handling such things.
If you have reasonable skill with fluid mechanics and thermodynamics, it is however possible to figure the compression yourself, and get GGDT to handle the second part of the equation.

Still quite a bit of work though.

I have modelled a full modeller that does both halves, but I have absolutely no bleeding idea where I put the program I wrote for it.
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Unread postAuthor: daccel » Thu Dec 11, 2008 2:02 am

It is mainly the second part of the equation I'm interested in. Compression is easy enough to figure out with pre-compression and volume (ignoring heat).

I'm working on a replacement design for this http://www.spudfiles.com/forums/pneumatic-chisel-t14047.html, as I couldn't get it to work reliably.

So I'm basically trying to get ggdt to model the barrel and chamber in single tube, with the piston being both the valve and the projectile, released by a sear. I think just a generic valve with a very small opening time, but what efficiency?
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Unread postAuthor: Ragnarok » Thu Dec 11, 2008 12:21 pm

daccel wrote: Compression is easy enough to figure out with pre-compression and volume (ignoring heat).

Huge oversight. Heat is not something you can possibly ignore under these circumstances and still get a result that means anything.

Particularly with your design, you're especially facing the problem is that GGDT cannot handle those circumstances.
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Unread postAuthor: daccel » Thu Dec 11, 2008 8:28 pm

Alright I guess I will have to go the old trial and error route.

With the compression heating, does that cool down the same amount when it re-expands? If not, is there the potential to get hot enough to weaken the tube and o-rings?
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Unread postAuthor: Ragnarok » Thu Dec 11, 2008 9:32 pm

daccel wrote:With the compression heating, does that cool down the same amount when it re-expands?

Yes, provided it re-expands to the same volume, and the conditions are adiabatic (which they won't be - but at least reasonably close to should count).
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Unread postAuthor: Hotwired » Thu Dec 11, 2008 9:38 pm

daccel wrote:With the compression heating, does that cool down the same amount when it re-expands?


In a way.

In a cannon scenario the heat energy from compression is generally lost to the surroundings long before the gas gets a chance to expand again.

So when it does get released it goes well below it's original temperature as it is back to it's original volume minus the heat energy it lost.


Anyway, if you have one part of your device which is constantly compressing air it will heat up and another where it is constantly being allowed to re-expand it will cool down.

It can get enough to do nasty things to seals and plastics (hot or cold ends).
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