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GGDT Supersonic Prediction?

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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GGDT Supersonic Prediction?

Unread postAuthor: saefroch » Tue Oct 26, 2010 7:35 pm

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This seems a bit optimistic to me... what do you think?
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Unread postAuthor: jackssmirkingrevenge » Tue Oct 26, 2010 7:40 pm

Highly optimistic, especially given the disparity between bore diameter and projectile diameter.
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Unread postAuthor: lozz08 » Tue Oct 26, 2010 9:40 pm

I agree, from what I can tell GGDT does not seem to decrease the energy enough when you decrease projectile size for a given bore.
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Unread postAuthor: wngovr » Wed Oct 27, 2010 8:49 pm

Ive been working on something close to this size lately and two things came to my attention on yours.

The chamber length is pretty long. I was planning to use a chamber the same O.D. as yours but only 6 to 8 inches long. Im planning to use a hand pump though.

Also I dont know what you are planning to use for a projectile but a steel ball for a .364 bore size would weigh about 3.5 gm. .24665 is almost nothing. A 177 pellet weighs twice that.
I dont think your round would go more than a hundred feet down range.
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Unread postAuthor: saefroch » Thu Oct 28, 2010 6:50 am

The round I put in for that simulation is HDPE.
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Unread postAuthor: D_Hall » Thu Oct 28, 2010 10:00 am

Yeah, GGDT is crap in the supersonic region. Even I acknowledge that. :)

Basically the early versions handled transonic poorly on the slow end. I did some reading, thought I had a handle on why, rewrote some stuff... And now it's bad on the other side of the equation.

I've done more reading. Think I have a handle... But haven't had the time to go back in and redo everything.

Note that next iteration HGDT and GGDT will be a single integrated package.
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Unread postAuthor: Technician1002 » Thu Oct 28, 2010 10:16 am

@ D_Hall

In regards to SOS and air cannons, do you think it is possible to go supersonic using a combination of a large bore piston valve in combination with high pressure as low as 200 PSI and a DeLaval Nozzle?

From my reading it looks like if we start at about 15 Bar, hit the nozzle and drop to 7 bar, we could in theory have supersonic flow with 100 PSI muzzle thrust.

The velocities would be somewhere near 700 FPS through the piston valve, transonic in the nozzle throat, and supersonic in the barrel.

http://en.wikipedia.org/wiki/De_laval_nozzle

Of course this should only work with low mass projectiles or none.

I ask this as I may try to implement this on the 2.5 inch QDV to launch a golf ball. Due to the low pressure drop through the nozzle the throat to nozzle will be a low ratio. It would go from approximately 2.5 inches in, to 1.5 at the throat to 1.7 exhaust into the golf ball barrel providing an expansion of less than 2. This would leave high exhaust pressure for acceleration of the golf ball. The goal would be to have high acceleration the entire length of the golf ball barrel instead of the typical drop in acceleration due to flow restrictions near the local SOS in typical valves.
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Unread postAuthor: Ragnarok » Thu Oct 28, 2010 10:42 am

Technician1002 wrote:DeLaval Nozzle

And, as I was telling you in the chat the other day, you've misunderstood how DeLaval nozzles work.

They convert HEAT to flow velocity, not pressure. If you assume temperature is the same before and after the nozzle, you get the following:

Mass flow rate has to be the same before and after the nozzle. So, a doubling of velocity means a halving of density. Simple.

Total energy also has to be the same. But, if you double velocity that means that energy quadruples, so to get the same kinetic energy before and after, the density of the gasses would have to quarter.

That means the two halves don't match up. Hence, energy needs to come from somewhere to provide the extra kinetic energy needed to maintain the mass flow rate. This has to come from the temperature of the gasses. So, temperature is not the same before and after the nozzle.

As temperature is practically the defining element in speed of sound (Strictly, the equation makes it proportional to the square root of the ratio of pressure to density, but the ratio of pressure to density in a given gas is defined by temperature), this means that the actual speed of sound falls across the nozzle.

Across the nozzle, velocity increases - as does Mach number (because of both velocity increase and SOS decrease) - or at least, it MAY increase.
It's very easy for it to just turn into a choke point and reduce overall flow. Unless you've got the heated (and higher SOS) gasses so that the flow in the throat of the nozzle is just barely or approaching sonic (remember this is literally Mach 1 - the speed of sound in the gasses at that point. Not what velocity you normally think Mach 1 is, specifically the speed of sound), that's what it's going to be - a choke point.

This means that a DeLaval nozzle only works when the gas flow before the nozzle is quite considerably subsonic - by the standards of its own temperature, of course.

As I was trying to say in the chat, DeLaval nozzles are designed to be used with heated gasses, and convert that heat to flow velocity (So, yes, they do lose pressure, but as a result of a temperature drop, not a density one). Their use with normal temperature gases is practically non-existent - they just act as a choke point.
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Unread postAuthor: Technician1002 » Thu Oct 28, 2010 11:01 am

Thanks for the explanation. Most of it makes sense. I do know the nozzle is a low loss device, so how much of a choke it will provide is still unknown, just as a venturi is a low loss device. As such I was looking for a way to get the KE of a 2.5 inch subsonic flow into a nozzle to convert to a supersonic flow after the nozzle where the nozzle is only used to change the cross section of the barrel from large to smaller and preserve the energy of the flow in the smaller barrel. This could result in a moderate pressure drop and low resulting temperature drop (and local SOS) with the possibility of supersonic flow.

I know that compressors on turbofan jet engines are capable of supersonic exhaust. I'm looking for a way to achieve the same thing in a barrel using compressed air. It would preserve the energy of the compressed air with low loss. I am not looking for over unity energy gain, only using existing energy efficiently.

It would be like my accelerating AA batteries. With the sabot and large barrel, I had high velocity. With the small barrel and less energy, the impact was not as impressive. Instead of a sabot, I am looking to take the energy from a large barrel and put it into a smaller barrel with the mechanical advantage from the cross sectional area change.

I do know that I am getting exhaust velocities in excess of 0.75 Mach in my current valves at less than 100 PSI. In reducing the diameter of the flow with low loss, a drop in pressure and temperature would result in higher velocity. The question is can it exceed Mach 1.

High energy splat
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Low energy splat
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Unread postAuthor: Gippeto » Thu Oct 28, 2010 1:43 pm

We've touched on the subject of DeLaval before.

http://www.spudfiles.com/forums/mass-fl ... 16641.html

I went with 11 degrees as a nozzle angle in the bb experiment.
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Unread postAuthor: Technician1002 » Thu Oct 28, 2010 3:30 pm

Thanks for the link. I'll play some with some conservation of energy theory and see what it would take to transfer the energy of compressed gas transfered to a projectile near Mach 1.

I guess I need to look into possible ways to accelerate the projectile while keeping the main mass of air from accelerating while taking it's energy of compression and transferring it.
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