FLONE's question redux

Just starting to spud and wonder how long your barrel should be? Want to know how to build a better cannon? This is just the place! If you think your question is one of a newcomer, please post it in here. If your question is posted elsewhere, and it's a newcomer's type question, it will be moved in here.

Postby FLONE » Wed Nov 29, 2006 11:14 am

THANK YOU FOR ALL THE REPLIES, AND I RESPECT THE EXPERIENCE OF ALL THE REPLIERS.

UNDERSTAND THERE ARE A LOT OF FACTORS INVOLVED BEFORE THE SPUD LEAVES THE BARREL, AND A CHRONOGRAPH IS THE FINAL ANSWER.

JUST A STRUGGLE TO THINK THAT A 200 CUBE 6" DIAMETER CHAMBER ATTACHED TO A 2" BARREL MIGHT NOT PUSH A SPUD ANY FASTER THAN A 200 CUBE 4" DIAMETER CHAMBER ATTACHED TO THE SAME 2" BARREL.

SOUNDS LIKE THE PERFECT EXCUSE TO BUY A CHRONY!!!!!!
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Postby jimmy » Wed Nov 29, 2006 1:55 pm

I suspect that you are chasing a wild goose here...

As long as the change in diameter between the chamber and the barrel isn't too severe it won't make much difference. For example, a 2" barrel on a 3"D versus 4"D chamber. If the total chamber volume is constant (by changing the length of the chamber) I seriously doubt you can measure a performance difference. There is probably a small difference but the difference is less than the shot-to-shot variability of the typical spud gun. This is for "reasonable" changes in diameter between chamber and barrel. If you choke a 4"ID chamber down to a 1/2"ID barrel then flow restriction would be much more significant. But even in this severe case it is largly irrelevant since a 1/2"ID barrel is going to suck just because it is so small.

Regarding the quote attributed to DR, to wit;
<i>"I believe that the reason as to why a 0.7:1 or 0.8:1 ratio works best for a metered-propane Combustion launcher is because the burning gases are "choked" as they are forced to go through a smaller opening.
This "bottleneck" of the burning gas allows the gas a sufficient amount of time to generate the maximum amount of pressure that they can."</i>

I doubt this is true. The "bottleneck" may allow a higher chamber pressure but I doubt it is significant. Besides, any advantagous affect of the "bottleneck" would be counter acted by a dis-advantagous affect of limiting mass transfer into the barrel once sufficient pressure has built up and the spud starts to move.

The best bet for the basis of the "0.8 C:B" rule is a balance between the slow combustion rate and static friction between the spud and the barrel.
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Postby boilingleadbath » Wed Nov 29, 2006 4:05 pm

I'm not aware of any data that helps us make a definite statment here; the latke tests where done using chambers with fairly constant aspect ratios.

That said, it appears that combustion takes a fairly long time - probably on the order of 5-10 ms in latke's L3 (propane, multiple sparks, chamberfan, ~4x14"). (note the factor of two difference in my values - based on a rough visual analysis of the latke data, numerous estimates... salt them heavily)
My feeling is that, if one could burn the fuel instantaneously, preformance would increase substantialy - maybe even a couple dozen percent!

...and a fatter chamber could shorten combustion time - but then, so could more ignition points. (and they'd do it cheaper...)
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Postby jimmy » Wed Nov 29, 2006 9:53 pm

Combustion is indeed slow, in the 40~50mS range. Interestingly, the peak combustion pressure in a closed chamber is nearly independent of the chamber volume but the burn time is dependent on the chamber volume.

It is possible to "burn the fuel instantaneously", that is what detonation means. Acetylene, and to a lesser extent most other spudgun fuels, can detonate. Detonation is generally considered to be a bad thing in a typical PVC gun since the pressure spike is much higher than in the normal deflagration mode. I think most people equate "detonation" in a spud gun with a "spud gun-grenade" and the loss of body parts.

Multiple ignition points <i>may</i> help but I suspect they really only speed up the slowest phase of the combustion process and result in little, if any, change in muzzle velocity.

My personal "pet theory" says that there would be little or no measurable difference between a 3"D chamber and a 4"D chamber (at constant total chamber volume). You only start to see signifficant affects of chamber ID when the ID is small, say an inch or less. With a long and very narrow chamber too much heat is lost to the chamber walls.
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Postby boilingleadbath » Thu Nov 30, 2006 6:04 pm

If combustion took 40-50 ms to compleate, we wouldn't be getting the type of pressure curves we see.
For instance, here's a graph of pressure vs. projectile position using latke's 3/4" b:c ratio tests:

http://img.photobucket.com/albums/v611/ ... svspos.png

(ignore the error bars and the pink line)

Note how it peaks WELL before 14 ms.
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Postby Freefall » Thu Nov 30, 2006 6:11 pm

FLONE -

Please check your email.
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Postby spudproof » Sun Dec 03, 2006 5:11 am

this whold say that a 3" chamber give less peformance than a short 4" one!?

YES OR NO
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Postby jimmy » Sun Dec 03, 2006 11:07 am

<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote">Originally posted by spudproof
[br]this whold say that a 3" chamber give less peformance than a short 4" one!?

YES OR NO
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
If the total volume is the same, I doubt you can measure the difference between a 3"ID and a 4"ID chamber.
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Postby jimmy » Sun Dec 03, 2006 11:24 am

<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote">Originally posted by boilingleadbath
[br]If combustion took 40-50 ms to compleate, we wouldn't be getting the type of pressure curves we see.
For instance, here's a graph of pressure vs. projectile position using latke's 3/4" b:c ratio tests:

http://img.photobucket.com/albums/v611/ ... svspos.png

(ignore the error bars and the pink line)

Note how it peaks WELL before 14 ms.
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
Boiling: Where did your data come from? I wasn't aware that Latke had a pressure transducer or had any ability to record the pressure versus time in the chamber during firing.

If the pressure vs. time profile was calculated from just the muzzle velocity versus barrel length data then your "total combustion time" results will be way off. In particular, the time it takes for the pressure to rise high enough to overcome static friction will be missing. The time for the pressure to rise high enough to get the projectile to move is significant, probably in the 20~30mS range.

Lets see, for a 3/4" barrel the barrel cross section is ~0.44in<sup>2</sup>. To overcome 10 pounds (WAG) of static friction the chamber pressure need to exceed (10pounds/0.44in<sup>2</sup>)=22.7PSIG. It takes a "long" time for the pressure to rise that high.

I based my "40-50mS" total combustion time on the <a href="http://home.earthlink.net/~jimsluka/Piezo_Spud.html">pressure signature I measured in the combustion chamber using a piezo transducer</a>.
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Postby boilingleadbath » Sun Dec 03, 2006 2:39 pm

Yeah, it's calculated via barrel length and muzzle velocity. I geuss this means that it's not acctualy total combustion time... but rather combustion time after the projectile starts to move.

That said, I'm not so certain about your about your analysis of your pressure curve:
1) You have the projectile exiting in ~6 ms. This makes for somewhat less than 1000 fps. (EVBEC has your launcher estimated at 275-350 fps (and your launcher is fairly close to the test launchers), which puts the begining of movement of the projectile around the second peak in the raw peizo data (~.043 seconds on your chart))

2) You have a spike in the integrated pressure curve when the projectile is partway down the barrel, which you have marked "apple begins to move (assuming I'm correct about #1). SO DO I, although mine occurs earlier (at about 5-6ms post-movement instead of 14ms).
I think this pressure spike is due to the combustion in the extreamly turbulent barrel flow - which might be rapid enough to reverse the downward pressure trend at that point in time. Assuming this is the case, it makes scence that latke's spike occured earlier, because his launcher had two ignition points, although I'm not sure how to explain why latke's is so much bigger (about 8% vs. about 3%).
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