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WTF is wrong with me?

Unread postAuthor: D_Hall » Tue Sep 22, 2009 10:17 pm

So somehow I've been suckered into writing yet another simulation....

Anybody up for 2LGGDT?

Yes, I'm in the exploratory phase for writing a sim for 2 stage light gas guns. Ugh. I can't decide whether this one will be a piece of cake or the death of me!
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Unread postAuthor: starman » Tue Sep 22, 2009 11:35 pm

Sure thing, go for it. I've been pondering an "afterburner" stage and its potential effect for some time now, unscientifically though. I keep going back to the thought that the volume of both chambers would eventually become one final chamber and the combustion in the second stage would have to take the both chambers size into account.

This assumes a destroyed burst disc that connects the stages. If you instead replaced that disc with a super fast acting "check valve" or pressurized piston valve, you might be able to isolate the second chamber but I'm dubious about that working fast enough.

One option would be to have a smaller combustion chamber become an "ignitor" for a much larger chamber, the first chamber becoming less relevant in the final chamber size. Just not sure this will offer much advantage over and standard hybrid design.
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Re: WTF is wrong with me?

Unread postAuthor: Technician1002 » Tue Sep 22, 2009 11:54 pm

D_Hall wrote:So somehow I've been suckered into writing yet another simulation....

Anybody up for 2LGGDT?

Yes, I'm in the exploratory phase for writing a sim for 2 stage light gas guns. Ugh. I can't decide whether this one will be a piece of cake or the death of me!


I would like to have a model. If it could cover not only light gas, it would cover air compression heated in a gas spring. I'm looking to build a gas spring compressed by a piston launched by my current 3 inch QDV to compress and thus heat air in a 3 inch tube with a transition to a golf ball barrel to launch it supersonic if possible. Would this be difficult to model?

Another varient I am interested in would be the same setup without the piston. an air blast traveling near SOS would meet a transition zone slowing compressing and heating it, to then re-expand in a smaller diameter at supersonic speed for light projectiles such as marshmallows.

I'm trying to model both but my math skills are not up to the task. I'm afraid the errors may cause a high speed piston impact instead of slowing to a stop in the compression zone.

I'm looking to start with close to 200 PSI dumping into the 3 inch barrel and then transitioning to almost 1/2 the diameter for a 4X compression as the near SOS wave transitions to the smaller diameter. Modeling of the transition zone temperature, piston decelleration distance (with and without an actual piston), and resulting expansion speed, temperature, and projectile acceleration in the small diameter would be what I am looking for.

This would be designed so intentional dead space provides warm (ambient) air that is compression heated by the main cannon discharge resulting in a light gas from compression heating. A possible varient would be possible diesel ignition near the final projectile with a paper towel dampened in lamp oil.
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Unread postAuthor: D_Hall » Tue Sep 22, 2009 11:54 pm

Huh? You've totally lost me.

edit: That was directed at starman (now I need to go back and read Tech's response....derned posting at the same time as somebody else!).



edit2: @Tech, The air driving piston may be doable. The simple mixing of air? I don't have anywhere near the fluid mech skills to do that one. I'll have to think long and hard about program architecture though. In this case, the initial model wouldn't be to model a random 2 stage light gas gun, but rather to model a very specific one at the office. It's the old story... Old Guy is expert at operating gun. Old Guy has nervous breakdown when his wife dies. Old Guy is never quite the same and for some reason can't get the gun to operate worth a damned anymore. Old Guy is deemed no longer reliable regarding his knowledge of the gun. Young Guy who is known to be a gun freak (uh, that's me) is asked to step in and figure out WTF is going on with the gun. First step (IMHO) is to start modeling it....
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Unread postAuthor: starman » Wed Sep 23, 2009 12:17 am

Picture 2 chambers connected end to end with each other with a burst disc transition, and the other end of the second chamber connected to the barrel breach via yet another burst disc.

The first chamber of some fueling configuration ignites, blows into the second chamber of some (possibly different) fueling configuration. The second chamber's ignition then blows the second disc and drives the projectile.
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Unread postAuthor: Technician1002 » Wed Sep 23, 2009 12:21 am

D_Hall wrote:Huh? You've totally lost me.

edit: That was directed at starman (now I need to go back and read Tech's response....derned posting at the same time as somebody else!).



edit2: @Tech, The air driving piston may be doable. The simple mixing of air? I don't have anywhere near the fluid mech skills to do that one. I'll have to think long and hard about program architecture though. In this case, the initial model wouldn't be to model a random 2 stage light gas gun, but rather to model a very specific one at the office. It's the old story... Old Guy is expert at operating gun. Old Guy has nervous breakdown when his wife dies. Old Guy is never quite the same and for some reason can't get the gun to operate worth a damned anymore. Old Guy is deemed no longer reliable regarding his knowledge of the gun. Young Guy who is known to be a gun freak (uh, that's me) is asked to step in and figure out WTF is going on with the gun. First step (IMHO) is to start modeling it....


Simply modeling a piston captured in a barrel (cylinder) compressing air with a transition zone to a smaller barrel with a projectile in it would be all that is needed. That would be fantastic. I could then find the rest position of the piston, the volume needed between the piston and projectile and thus figure the maximum length of the transition cone without the piston hitting it. To get the piston up to speed can be spring, air cannon, or combustion.

I understand the issue of fluid dynamics and thought this may be too hard to model, but i will do some test building and magnetic chrono the results. It will be fun to test several volumes of dead space to see if it really works. It may mix and simply not work.

My initial test will be a golf ball barrel with a transition zone attached to a 3 inch QDV cannon with a 7 gallon tank. I'm looking to see if this can exceed GGDT's prediction. Either way, I think this will get a golfball supersonic. I'm just trying to do it with a reasonable length barrel.
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Unread postAuthor: D_Hall » Wed Sep 23, 2009 12:32 am

I will say this though... I've never heard of a 2SLGG that doesn't destroy it's piston on every single shot. It's simply accepted that the piston is a consumable item. Granted, it's not like I've traveled the world looking at 2SLGGs, but... Hmmm... I'd say I've seen 8 of 'em and gotten the opportunity to play with 3.
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Unread postAuthor: Technician1002 » Wed Sep 23, 2009 1:00 am

D_Hall wrote:I will say this though... I've never heard of a 2SLGG that doesn't destroy it's piston on every single shot. It's simply accepted that the piston is a consumable item. Granted, it's not like I've traveled the world looking at 2SLGGs, but... Hmmm... I'd say I've seen 8 of 'em and gotten the opportunity to play with 3.


This is not high power to achieve Mach 3 or more. This is basicly a verson of the arrowgun. The intent is to use an air spring as a "lever" to move energy effeciently from a larger slower mass to a lighter faster mass by means of a dimeter transition and gas spring.

http://www.paintball2000.de/news/details.asp?artieklid=3104

I'm looking to make one on steroids to get a golf ball supersonic powered by an air cannon.
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Unread postAuthor: starman » Wed Sep 23, 2009 2:17 am

Oh...you ARE talking about real live 2 stage light gas guns. Sorry, my eyes saw what you wrote but my brain translated to multi-stage combustion....my bad...

Carry on... :roll:
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Unread postAuthor: Technician1002 » Wed Sep 23, 2009 2:37 am

starman wrote:Oh...you ARE talking about real live 2 stage light gas guns. Sorry, my eyes saw what you wrote but my brain translated to multi-stage combustion....my bad...

Carry on... :roll:


The glare of your avitar may have done it. :D
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Unread postAuthor: jackssmirkingrevenge » Wed Sep 23, 2009 2:39 am

Sticking with spudguns, what are the chances of translating the setup below into a pneumatic or combustion powered device, substituting the propelling charge?

Image

If so, would it offer any significant advantage over a concentional valved pneumatic charged with a light gas instead of air or CO<sub>2</sub>?
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Unread postAuthor: Technician1002 » Wed Sep 23, 2009 2:59 am

Jack, that is my current project. It is one of the reasons I am building a 3 inch 7 gallon QDV cannon to launch a golfball. There will be a 3 inch to golfball transiton and either an air dead space or a piston. I need to model it properly so the piston doesn't break things instead of stopping by compression. If I can't get the model to work I may try it without a piston, but still use a dead space for compression heating and inertial mass transfer of energy.
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Unread postAuthor: jackssmirkingrevenge » Wed Sep 23, 2009 3:37 am

Personally I think you'd be better off using a golfball barrel on a QDV launcher and using pressurised helium directly as your propelling gas.
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Unread postAuthor: D_Hall » Wed Sep 23, 2009 8:44 am

jackssmirkingrevenge wrote:Sticking with spudguns, what are the chances of translating the setup below into a pneumatic or combustion powered device, substituting the propelling charge?

I've seen it done to reasonable effect. Note the word reasonable. That gun would shoot some stuff very quickly. No doubt there. But it was MASSIVE compared to it's projectiles (even by 2SLGG standards).

If so, would it offer any significant advantage over a concentional valved pneumatic charged with a light gas instead of air or CO<sub>2</sub>?

Only if you're willing to work with very high pressures. If not, you'd be better off just moving to a hybrid. Actually, scratch that. You WOULD be better off just moving to a hybrid.

Seriously, if you want to run some experiments for the gee whiz factor, I've wondered about a hybrid wherein somebody pulls a vacuum on the chamber and then fills it with propane, oxygen.... And helium. Basically the fuel/oxy would just be there to heat the helium (which would obviously compress it... blah blah blah).
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Unread postAuthor: btrettel » Wed Sep 23, 2009 10:44 am

I've only skimmed this thread, but you should find the links on this page to be extremely helpful D_Hall: http://trettel.org/bags/other-models.html

You might find this to be the most interesting (and may even have a printed copy already available to you): http://www.dtic.mil/srch/doc?collection=t3&id=AD0475660

The high muzzle velocities will require more complicated modeling much like what I plan for BAGS 2 and what Rag has done with Apocalypse. The simplest way is to assume velocity, density, pressure, etc. varies down the length of the barrel (this is 1D).

There are a number of ways to solve an unsteady compressible 1D problem... the books I have listed primarily use the method of characteristics, as does Rag's Apocalypse. I'm been thinking about going that route myself, but I really don't have the patience for it. Ragnarok must have significantly more patience than me because the MoC is a PAIN IN THE ASS to work with. I'm sure that when it works well, it works well, but it's another layer of complexity in an already complex problem. I can give you some good references in addition to what I already have listed if you want to go this route.

For that reason what'll be released will likely use what's called a finite difference of finite volume scheme... which aren't too difficult to implement but you run into the problem of the scheme being a "black box" in that you don't really know how it works or when it's working well. So I have much highly mathematical reading ahead of me. The end result surely will be worthwhile.
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Last edited by btrettel on Thu Sep 24, 2009 8:08 pm, edited 1 time in total.
All spud gun related projects are currently on hold.

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