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Transonic and supersonic pneumatic gun directory

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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Transonic and supersonic pneumatic gun directory

Unread postAuthor: btrettel » Sun Jul 26, 2009 9:46 pm

I've been interested in the accurate modeling of transonic and supersonic guns since a few months ago. There's not much cold hard data for guns like spud guns out there, and while I have a few things bookmarked, I know I'm using wrong search terms and am missing at least a few things.

Post links, detailed descriptions, test results, GGDT files, and such for any pneumatic guns here that shoot faster than Mach 0.7 or so (~ 800 fps). Test results and reasonably accurate numbers for the parameters a simulation like GGDT asks for are most helpful.

Hybrid data would be helpful too, but not as much so as pneumatic data. I don't intend to tackle hybrid simulation any time soon if ever.

At the moment I have a reasonable idea of how to model the associated effects, but 1) it's complicated and 2) I still have much more to learn, especially about numerical methods involved. Gathering data in the mean time seems appropriate, especially given that others might find such a thread useful.
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Unread postAuthor: jackssmirkingrevenge » Sun Jul 26, 2009 11:42 pm

Here's my own contributons in 6mm calibre:

piston launcher

burst disk launcher
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Unread postAuthor: Technician1002 » Mon Jul 27, 2009 12:26 am

Light projectiles in my launchers are over 700 FPS. Chronograph and video confirms it. No projectiles clocked supersonic yet, but supersonic airflow is seen. Next project is an attempt at it.

Larger valve and higher pressure may do it. GGDT predicts supersonic. Moving to lighter projectiles runs the speeds supersonic in GGDT. Long barrel, high pressure, large valve, and larger diameter barrel is required for the higher speeds.

http://www.spudfiles.com/forums/ggdt-and-in-tank-quick-dump-valve-performance-t18090.html

http://www.spudfiles.com/forums/quick-dump-valve-t17858.html

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Door dented with a marshmallow
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Golf Ball.jpg
Golf ball model on 3 inch QDV with 2 inch barrel and sabot driven golf ball at 200 PSI. Dead space is for venturi transition to 2 inch for high COEF
Marshmallow.jpg
Over 1500 FPS is predicted in GGDT for a full size marshmallow.
Last edited by Technician1002 on Mon Jul 27, 2009 5:13 am, edited 8 times in total.
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Unread postAuthor: Gippeto » Mon Jul 27, 2009 12:53 am

6mm

http://www.spudfiles.com/forums/x-1-1-m ... 16607.html

1/2" type m copper (9/16" bore)

750fps with wadded 3 gram marble on 400psi co2, 830 fps on 400psi air.

http://www.spudfiles.com/forums/piston- ... 13478.html
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Unread postAuthor: inonickname » Mon Jul 27, 2009 4:26 am

15.5mm bore, wadded marble, rifled..

I'd bet my left leg that the Q-tip guns pulled this with ease, bordering on mach but they nolonger exist, unfortunately.
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my gun 2.PNG
With hydrogen pumped with stirrup (derived from NaOH + Al)
my gun 2.PNG (30.56 KiB) Viewed 1510 times
my gun 1.PNG
400 Psi air (from stirrup)
my gun 1.PNG (30.76 KiB) Viewed 1510 times
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Unread postAuthor: btrettel » Tue Jul 28, 2009 6:48 pm

Thanks for the replies.

I might be contacting some of you in the future for some more specific measurements, but this is a good start. Let me reiterate that a combination of measured data (not and detailed (i.e. everything GGDT asks for and then some) specs of the gun are most helpful.

Of course, I'll be doing some tests of my own, but one has limited time and an even more limited budget these days, so additional results are greatly appreciated.
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Unread postAuthor: jeepkahn » Wed Jul 29, 2009 8:43 am

Are you trying to model trans and supersonic pneumatics, or are you trying to figure out how to make a pneumatic go supersonic???

I ask because "according to ggdt" almost all of mine should be well beyond SS, and while all of them are consistently measured in the high transonic region 950-1100 fps, the only one that measures supersonic is the decimater, and there are several of us trying to figure out how it's actually achieving what shouldn't be possible given the gas being used.

To get a pneumatic to go supersonic with air or co2, it takes more than just pressure and efficient valves, it takes things like venturi effects, compression heating, pressure front collisions(my theory on deci's performance), projectile "stiction", projectile seal, and projectile initial postion.(the last 2 having effect on compression heating the gas)

I think modeling performance will be tough, but modeling what it takes to get SS performance will be tougher...
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Unread postAuthor: inonickname » Wed Jul 29, 2009 9:05 am

That's a common complaint with GGDT, it's overly generous near the sound barrier. If it sounds implausible then it's good to doubt it.
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Unread postAuthor: jeepkahn » Wed Jul 29, 2009 9:07 am

inonickname wrote:That's a common complaint with GGDT, it's overly generous near the sound barrier. If it sounds implausible then it's good to doubt it.


See the Decimater thread for GGDT vs Real readings on the decimater...
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Unread postAuthor: btrettel » Wed Jul 29, 2009 10:07 am

I'm looking to both write an accurate simulation and make supersonic guns.

From what I've read, reasonably accurately modeling transonic/supersonic performance should be relatively simple given that the gun fits certain specifications (that most do unless they have significant bends from the valve to the projectile). Such a simulation will be "1D". The pressure, density, velocity, etc. of the gas can vary as a function of the length down the barrel. Reality varies in 3D, so this is an assumption, but it already is enormously more complex than what BAGS (my simulation) and GGDT (to the best of my knowledge) do, which are best described as 0D. I don't know about Apocalypse--Rag does seem to know more than I do so I wouldn't be surprised if he took a similar approach to my planned 1D approach.

Note the emphasis on relatively. Having knowledge of what it would take to do a full CFD model, this "mini CFD" approach seems very manageable, but it is very significantly more complicated than the existing approaches.

(I did put a few things here about what approaches I could take computationally, but I honestly don't know much about this so I'll wait until I'm less ignorant.)

As for a couple things you mentioned, I'm not quite sure "compression heating" factors into this. Gases get hotter generally only when energy is added to them (i.e. when compressed with a piston). Where would the extra energy to cause an increase in gas temperature come from? Free expansion of a gas (i.e. through a valve) without heat transfer is approximately isenthalpic, which means for certain simplifying assumptions the temperature is constant. Now, energy is being extracted from the gas in a spud gun (to accelerate the projectile), so it'd get colder, not hotter.

I could see something that might be described as "compression heating" happening if the gas had a lot of kinetic energy and suddenly slowed down a lot--the KE could go into thermal energy. I'm unsure of how significant an effect that could have in spud guns. My approach should factor this in if it happens to the best of my knowledge (the energy equation of the Euler equations should handle it).

The Venturi effect is taken care of in the valve flow equation. I've actually only seen valves treated as orifices in what I've read if you want some more specifics about how to do this. The flow is based on a flow coefficient, which usually is the coefficient of discharge, and is based on empirically measured data because finding the vena contracta is difficult experimentally and through simulation. Through some math you can make whatever sort of coefficient you want that corresponds to whatever you want (i.e. valve efficiency [which I believe is the same as the coefficient of discharge except as a percent], Cv, Kv, etc.)
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Unread postAuthor: D_Hall » Wed Jul 29, 2009 10:39 am

btrettel wrote:As for a couple things you mentioned, I'm not quite sure "compression heating" factors into this. Gases get hotter generally only when energy is added to them (i.e. when compressed with a piston). Where would the extra energy to cause an increase in gas temperature come from? Free expansion of a gas (i.e. through a valve) without heat transfer is approximately isenthalpic, which means for certain simplifying assumptions the temperature is constant.


The correct term would be "adiabatic heating" but I assure you it's a very real phenomenon (we've had explosions at work caused by it. :( ). I don't know how relevant it is to spud guns but follow me if you will....

Do you agree that a volume of air, if suddenly compressed, gets Really Hot?

Assuming so...

Imagine a volume of air compressed suddenly by a cylinder. Voila, a diesel engine!

OK...

Now imagine that the cyclinder is driven not by a crank shaft but by another chamber of air. Same effect, no?

OK...

Now imagine that the cylinder is nothing more than a micro-thin diaphram separating the two air masses. Same effect, no?

OK....

Now get rid of the diaphram.



The energy comes from the high pressure gases being introduced into the chamber. The high temperatures come from the low pressure gas being compressed.

I assure you, it's very real.

Again, I don't know if it's a first or even second order effect inside a spud gun, but I have seen explosions driven by it.
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Unread postAuthor: btrettel » Wed Jul 29, 2009 11:05 am

I know about adiabatic heating. I've even derived many of the relationships for adiabatic processes just to see what other assumptions they make (the PV^k = constant relationship for isentropic [i.e. adiabatic + reversible] processes requires calorically perfect gas assumptions and is "quasi-steady"). I don't see how free expansion (i.e. no work, no heat transfer) can cause a significant change in temperature aside from turning kinetic energy (or gravitational energy, or energy released from a reaction, etc.) into thermal energy.

Anyway, in your example one gas would get colder and the other would get hotter if one were compressing another (this is the combination of adiabatic expansion and compression). Both don't get hotter as it seems to be implied--that would violate the conservation of energy.

Edit: To make this more explicit, I doubt that there's a significant change in temperature from simply letting one chamber expand into another in most cases. Adiabatic compression and expansion (i.e. work adding energy which in turn increases temperature) is not what I'm arguing against.

Edit again: After some more thought, yes, adiabatic (in the strictest sense) heating or cooling might be acceptable names for heating as a result of a reduction in kinetic energy, etc., but it's not what I usually think of when I think adiabatic heating; I think work increasing temperature.
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Last edited by btrettel on Tue Aug 18, 2009 8:59 am, edited 7 times in total.
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Unread postAuthor: Technician1002 » Wed Jul 29, 2009 11:27 am

jeepkahn wrote:
inonickname wrote:That's a common complaint with GGDT, it's overly generous near the sound barrier. If it sounds implausible then it's good to doubt it.


See the Decimater thread for GGDT vs Real readings on the decimater...


If you look at the construction.. the chambers have 90 degree bends next to the valve. At near supersonic, I would predict extreme loss of energy in turbulance and if it was a liquid you could see, cavitation. At near SOS, the edges of the 2 90 degree turns and the head on collision in the T would form a shock shadow of low pressure that chokes off the high pressure flow to a small area effectively reducing the area with flow.. In short, lower, much lower coef at near SOS.

It's layout design was not designed to provide efficiency at supersonic speeds. Actual testing simply shows it. Think shock waves and mach diamonds at these speeds and where the air will lose energy when it crashes into an internal obstruction on it's way to the barrel. There is more to supersonic design than just C/B ratio. GGDT can't model these walls to high speed flow.

On this discussion...
Gases get hotter generally only when energy is added to them (i.e. when compressed with a piston). Where would the extra energy to cause an increase in gas temperature come from?


In the laws of thermodynamics neither energy nor mass is either created or destroyed. Think about it..

Now look at the gun performance near supersonic. There was energy lost that wasn't transfered to the projectile as kinetic energy.. Where did it go? It went to HEAT.

Simply stirring water adds heat. Stirring air also adds heat. Water falling over Niagara falls gains about 0.2 degrees F from the energy. (quote from foggy memory. Hunting reference)

http://id.mind.net/~zona/mstm/physics/m ... nergy.html

Unfortunately at the moment I can't find the page where the math was done on how much gravitational energy is released by the elevation change over the falls and the amount of energy required to heat the water.

If someone can find the page.. please post it.
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Unread postAuthor: Ragnarok » Wed Jul 29, 2009 3:25 pm

Given the questions, I'd better answer them.

The transonic calculation of Apocalypse v1 was based on data from my cannons, using formulas based around Mach number and Reynolds number.
It wasn't hugely scientific, but it started out as me studying patterns in these things, and then slowly turned into a modeller.
It got good results, but I couldn't tell you WHY it got good results, which meant I had an "it works, but we don't know how" situation, which isn't really acceptable.

So, I went for a second attempt, unsurprisingly titled v2.0, which as btrettel correctly reasons, models gas temperature, pressure, velocity, density, makeup, etc. as a function of displacement along the barrel.

I'm looking at the possibility of Apocalypse v3.0, which would (if I decide to go ahead) be a 2D simulator, modelling these parameters as a function of both displacement in the barrel, and the distance from the barrel's longitudinal axis, then assuming the barrel to be perfectly rotationally symmetrical. So, it would need simplification for rifled barrels, but I CBA to fuss around with more than that.

~~~~~

HEAL is safely transonic, but I've just realised all my test chronograph data for it is on my old hard drive - which I don't have handy.

However, possibly more useful to you will be the data I'll need to reap on HEAL 2 (for the purpose of one of my projects), because its more powerful and efficient design will mean it has the capability to get that much closer to Mach.
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Unread postAuthor: inonickname » Thu Jul 30, 2009 4:35 am

Moar the merrier?

Simulation of my next-gen Q-tip gun..decided to take the idea further. I'm expecting it to perform much better than the last. Smaller pilot volume (much smaller), lighter piston, faster pilot flow..it will be a cup shaped piston much smaller than the chamber (co-ax).. (the piston will be much lighter than it is, and P.V. will be lower than there..)

With hydrogen it exceeds the U.K. legal limit for air rifles..with a Q-tip..
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Plain 'ol air..
next gen q-tip gun.PNG (31.31 KiB) Viewed 1326 times
next gen q-tip gun hydrogen.PNG
Beware, I'll smack your 12 ft/lbs legal air rifle in the face ;p
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