Pneumatic / Combustion Performance Disconnect


Postby D_Hall » Mon Sep 25, 2006 10:02 am

Not reall a pnuematic question. Not really a combustion question. So I threw it in the hybrid area! In any event....


As I've always understood it, pneumatics are generally acknowledged to have superior performance to simple combustion guns. Assuming that this is the case:

1) Pneumatics generally operate at or below 100 psi. Yes, there are exceptions, but I'm talking *generally*.

2) A 1x combustion gun generates (in theory) approximately 100 psi.

1 + 2 = contradiction of the "pneumatics are better" experience. Why? Well, for starters combustion guns don't have valve restrictions as pneumatics do. In addition, the heated gases of a combustion launcher should allow for even easier energy transfer.

I can only surmise that the difference between theory and observed performance is a result of slow combustion processes. The literature indicates that the flame propagation rate of stoichiometric propane/air mixtures at 1 atm is a mere 12 fps. This rate will no doubt change with pressure (although I can find no useful data on this). Admittedly, the addition of flame holders in the chamber (ie, introduction of turbulence) can accelerate the flame propogation rate to hundreds of meters per second, but this acceleration doesn't happen instantaneously.

In any event, I was just wondering what the consensus was concerning propane combustion in the time domain.
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Postby jimmy » Mon Sep 25, 2006 4:37 pm

1) Pneumatics generally operate at or below 100 psi. Yes, there are exceptions, but I'm talking *generally*.

I would say they generally operate at 120 PSIG. But that is close enough.

2) A 1x combustion gun generates (in theory) approximately 100 psi.

In practice they don't get anywhere near 100 psi. And I think you are correct in ascribing the difference to the slowness of the combustion process. From spark to exiting from the barrel is about <a href="http://home.earthlink.net/~jimsluka/Piezo_Spud.html#new_3-4-06">50mS</a>. Figure 20mS of that nothing much is happening, combustion is starting but the temp and pressure hasn't risen very much yet. That leaves 30mS for combustion to complete once the process has really gotten going. But, once pressure is enough to overcome static friction the shell starts to move. Now, the question is, which happens first, (1) shell leaves barrel or (2) combustion completes?

It is easy enough to back calculate an "effective chamber pressure" from the muzzle velocity, barrel ID, mass of shell and estimate of static and dynamic friction. (Probably do it in GGDT). I think you'll find the the effective pressure in the chamber is ~50 PSIG or less for a typical combustion gun.

I've always figured that the "0.8 C:B ratio" rule is a reflection of this basic characteristic of using burning fuel in air to propel a projectile.
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Postby joannaardway » Mon Sep 25, 2006 5:17 pm

As my chemistry teacher said only today:

"If you have a theory and data, and they don't match, which one is wrong? The theory."

(Unless you refer to Douglas Adams, in which case "The guide is definitive, reality is frequently inaccurate")

So your initial theory is an over simplification.

I'm trying to work on this right about now, but delays are affecting things.

The current observation is that the pressure falls dramatically (I don't have the exact figures right now), more than what just the spud's movement would have, in a distorted Drop, plateau, drop, plateau manner, presumably created by movement of flame fronts - hopefully that makes sense.
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Postby sgort87 » Mon Sep 25, 2006 7:54 pm

Hahah, good old Doug Adams. I just finished reading The Guide last week.
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Postby Freefall » Mon Sep 25, 2006 8:23 pm

The most plausible explanation that I've heard concerning the lower performance of most pneumatics is that the spud begins moving well before the combustion process reaches completion, thereby resulting in an effectively expanding chamber volume.

By the time combustion has completed:
1: The combustion volume is significantly larger than V<sub>0</sub>.
2: Work has been done on the spud, thus T<sub>final</sub> is lower than what it would be if the expansion were isentropic.
3: A small amount of heat has been lost to the chamber walls.

Therefore, maximum real-world pressure is significantly lowerthan it would be in a sealed, insulated vessel.
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Postby D_Hall » Mon Sep 25, 2006 8:47 pm

Fair enough. But it begs the follow up question: What do the combustion (and hybrid) boys *DO* about it? A number of things pop into my mind as possible solutions.

1) Multiple ignition points.
2) Turbulence inducers (read: flame holders).

I've read of both of these being used in these circles but the big one that comes to mind - jet ignition - I've never seen mentioned. Does nobody use this? If not, why not?
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Postby clide » Mon Sep 25, 2006 9:32 pm

The idea of a jet ignition has come up a few times. One of the problems that have been discussed with it is getting a good fuel mix in the jet area as it is usually a relatively long, small diameter tube, or a larger area that passes through a small opening.

DR has experimented with a few ideas that are a bit like jet ignition. I can't remember his exact results, but here are a few of the threads.
http://forums.spudtech.com/topic.asp?AR ... C_ID=11333
http://forums.spudtech.com/topic.asp?AR ... IC_ID=9011
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Postby D_Hall » Mon Sep 25, 2006 11:33 pm

<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote">Originally posted by clide
The idea of a jet ignition has come up a few times. One of the problems that have been discussed with it is getting a good fuel mix in the jet area as it is usually a relatively long, small diameter tube, or a larger area that passes through a small opening.<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">

HUH?

It's trivial.

Basically you just build a gas-fired rocket motor that operates in a pulse mode. No big deal.

At the risk of sounding like an arrogant arse....

https://www2.nawcwd.navy.mil/techTrans/ ... 6&patID=76

('Tis a method patent for a jet igniter. Your's truly was the inventor.)


edit: All I did was a simple back of the envlope... I picked a pressure to run fuel and O2 at. I then said:

(O2Pressure*O2OrifaceArea) / (FuelPressure*FuelOrificeArea) = StoichiometricRatio.

Or something very close to that. I may have gone wild and used Bernoulli's to calculate injection volumes a bit closer. The point is, however, that it was crude... And it worked first try.
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Postby BewareOfDog » Tue Sep 26, 2006 1:02 am

Holy Shit!... Finally, a decent topic!
Pneumatics, in my opinion, do indeed kick ass! But a supercharged combustion stomps a pneumatics dick in the dirt. (Please excuse the redneck mentality)

The problem with hybrids, is finding the correct ratio for your barrel length. I use to build all of my hybrids at a 0.7:1 ratio, but have found that extremely low C:B ratios resulted in a dramtic increase in performance.

Burning all of the available fuel too early will only result in poor performance, as there is little to nothing left to <b>continue</b> to push the projectile down the barrel, once the projectile has started moving.

The second generation hybrid is (by far) the best launcher that I've ever fabricated. At a 5 atm compression, I can punch a golfball through 4 inches of wood... But what's sad is that I've only a limited amount of knowledge when it comes to fine-tuning a launcher.

I base all of my results and so-called "theories" on experience. And by experience, I mean; simple trial and error. Flamefront acceleration
is great for an absurdly large C:B ratio, as it will crreate the most pressure in the shortest amount of time...

But what good does it do when you burn most of the fuel before the disc even ruptures? Imagine a garden hose pressurized to 75 psi, and shutting off the hose bib before opening the nozzle at the end of the hose; You get a strong "spurt", and nothing else.

Detonation of your fuel (in a 1 atm fuel mixture) will not cause your launcher to fail, as previously theorized. And getting a disc to hold back a 3atm mix until it ruptures just below peak pressure will only result in a huge decrease in performance... (as explained above).

Jet ingition, multiple ignition, spark strips... The only thing they's be good for is to increase the chances of igniting a poorly distributed or improper air/fuel mixture.
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Postby D_Hall » Wed Sep 27, 2006 9:03 pm

Really, that just sounds like poor burst diaphram design.
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