Crusader

Just build a cannon and want to show it off? Post it in here.

Postby BigBang » Fri Nov 03, 2006 10:13 am

<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote"><b>Originally posted by aturner</b>
..I think it is much more productive for a couple of folks to attempt to independently replicate the test, rather than argue. I can tell you that as soon as I have a free evening, I'm planning to run a few GB's though my chrony. It's got to be one of the simplest tests that could be run, b/c the ammo is relatively standard.
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Can I make a couple of suggestions to help replicate the experiment?

1) I don't know how much this affected things, but I used a small cloth patch which I lubricated well with silicone spray lubricant. This held the gb's in place prior to ignition and sealed most of the gaps. It was a very barely snug fit, though. Not tight at all. I just tore an old thin cleaning cloth I had in the garage into roughly 1"x5" strips and wrapped the gb's in them.

2) Please give MAPP a try. Some comparison shots with Propane would be interesting too.

3) I honestly think the fan made a big difference. Those little Ozark Trails fans move a lot of air! I really suspect that the fan helped the combustion reaction move through the chamber more quickly, thereby increasing performance. Please try mounting one of these fans in your chamber if you can, and leave it on during firing.

4) I am less convinced that the rearward ignition point was a major contributing factor, though it may have been. I'm not sure the chamber was long enough for the pre-compression build-up effects to be of any significance. I may be wrong, but if there is one chamber modification I would try, it would be in swaping out your fan, not changing the ignition point.

5) My gb barrel is not of the pass-through variety. I loaded from the breech, taking the barrel off each time, but no part of the barrel extended into the chamber at all. Not even into the 2" neck into which I screw the barrels. There were no obstructions in the chamber except for the fan itself.

Lastly, send me you chamber volume with your gb barrel installed and I'll run some numbers for you and see if we can come up with some expected velocities.

We all anxiously await your findings.
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Postby aturner » Fri Nov 03, 2006 10:38 am

By "thin cleaning cloth", would that be something as thin as a T-shirt?

I actually have a modular "test" launcher that allows me a lot of flexibility, including variable volumes. I'll aim for a volume close to what you used, and if anything may go slightly larger. This modular launcher allows me to easily swap out my 80mm brushless computer fan for something like the camping fan you used. I've seen those ozark fans at wally world and will pick one up. I can give MAPP a try as well. I can place the spark anywhere in my modular launcher.

If I get anything remotely close to your results, I'll be very happy! If it works, I can try to substitue the fuel and fan etc, to see which seems to make the biggest difference. Should be able to try it this weekend.
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Postby clide » Fri Nov 03, 2006 1:25 pm

If you really believe the velocity figures of BigBang's launcher then I would strongly suggest remote ignition. The pressure needed to propel a golf ball that fast from a 5' barrel is a pressure I would never use in PVC, even sch80. Especially when it needs to be created in around a millisecond or less.
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Postby BigBang » Fri Nov 03, 2006 1:34 pm

<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote"><b>Originally posted by aturner</b>
By "thin cleaning cloth", would that be something as thin as a T-shirt?<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
That's about right, maybe a little thicker. It was an old cotton dish towel - not of the plush variety.

I'll have to take another look at your website. I didn't realize your set-up was that versatile.

Good Luck!!!!!
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Postby jimmy » Fri Nov 03, 2006 2:26 pm

<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote">Originally posted by aturner
[br] Yes, but a GB in 1.5" SDR21 is nearly a float fit, depending on the specific GB and pipe. Quite different than a potato or a gasketed wooden slug. It's Apples to Oranges.<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
But a "float" fit is generally considered to be bad in a combustion gun. Too much blowbuy for one thing. And, the static friction of the projectile with the barrel is critical for allowing the pressure to build since combustion of a gas is pretty slow. Low friction means the projectile starts to move before the pressure in the chamber has risen very much. Hence, the projectile leaves the barrel long before combustion is complete and/or peak pressure is reached. I figure this has been pretty well established for combustion guns. A double beveled muzzle knife increases static (and dynamic) friction but also <u>increases</u> muzzle velocities. If the round is a "float-fit" it is not going to be very efficient. A low friction projectile fires at lower muzzle velocities than the same projectile with the proper amount of friction.

Heck, this is the basis of a burst-disk gun. The burst disk allows the gun to reach higher pressures before the projectile starts to move. The burst-disk allows the gun designer to replace the advantageous "delay caused by friction" with a more controllable, and larger magnitude, "delay until a certain pressure is reached". So, I would expect that a combustion GB gun will fire better with a fairly snug wading that raps around the GB and increases the friction significantly compared to the "float-fit" of just the GB in the barrel.
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Postby aturner » Fri Nov 03, 2006 2:39 pm

<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote">Originally posted by jimmy
But a "float" fit is generally considered to be bad in a combustion gun. Too much blowbuy for one thing. And, the static friction of the projectile with the barrel is critical for allowing the pressure to build since combustion of a gas is pretty slow. Low friction means the projectile starts to move before the pressure in the chamber has risen very much. Hence, the projectile leaves the barrel long before combustion is complete and/or peak pressure is reached. I figure this has been pretty well established for combustion guns. A double beveled muzzle knife increases static (and dynamic) friction but also <u>increases</u> muzzle velocities. If the round is a "float-fit" it is not going to be very efficient. A low friction projectile fires at lower muzzle velocities than the same projectile with the proper amount of friction.

Heck, this is the basis of a burst-disk gun. The burst disk allows the gun to reach higher pressures before the projectile starts to move. The burst-disk allows the gun designer to replace the advantageous "delay caused by friction" with a more controllable, and larger magnitude, "delay until a certain pressure is reached". So, I would expect that a combustion GB gun will fire better with a fairly snug wading that raps around the GB and increases the friction significantly compared to the "float-fit" of just the GB in the barrel.

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Jimmy, did you happen to catch where big bang said he used a cloth patch? Therefore, I was wrong, and it was not a float fit. At any rate, it's great you agree the comparison is apples to oranges.
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Postby jimmy » Fri Nov 03, 2006 4:02 pm

Bigbang

Did a little more research. Your analysis of the energy partition between recoil and projectile appears to be exactly correct. I found a Wiki page (which now I can't find!) that gives about the same analysis you presented, momentum is conserved not kinetic energy. So I stand corrected on that point.
<hr>I did dig up a couple of things.

The muzzle energy of a 0.50 caliber machine gun is 13~18 KJ (depending on the barrel etc.) with muzzle velocities approaching 3000 FPS and a slug weight of 46 grams. (Exactly the same as a GB?)

The muzzle energy of a 0.30-06, i.e., an M1 Garrand, is in the 2.7~4 KJ range (depending on the barrel length etc.).

The kinetic energy of a GB at say 1500 FPS (your gun) is 4.8 KJ, roughly one third that of a 0.50 caliber and more than a 0.30-06 M1.

This just does not seem very likely to me.
<hr>
Combustion spud guns are limited by the very slow rate of combustion of the gaseous fuel. As I mentioned in an earlier post, the slow burn rate means it is very difficult to keep the round in the barrel until maximum pressure is reached. The net result is that a heck of a lot of the energy calculated for the fuel load is wasted. I've <a href="http://home.earthlink.net/~jimsluka/Piezo_Spud.html">measured the burn rate of propane in a spud gun</a>. The time it takes to release the combustion energy is long compared to the time it takes the projectile to leave the barrel. For your gun with a 5' barrel and a 1500 FPS muzzle velocity... assume average speed is the muzzle velocity over 2 (a crappy approximation but it makes the math much simpler)...
time in barrel = 5'/750FPS = 6.7milliseconds. From my measurements with propane, combustion takes longer than that, perhaps ~50 milliseconds. For perhaps 30 mSec of that time there is enough pressure to start to move the projectile even though the pressure is well below the maximum pressure.

The only possible explanation I can come up with is that to get that kind of muzzle velocity the MAPP must be burning significantly faster than propane does. Perhaps ten times faster.
<hr>
The high muzzle velocities you measured can not be explained by the difference in energy between MAPP and propane (or butane or acetylene...).
<table border="1"><tbody><tr><td align="center">Fuel</td><td align="center">Heat of<br>combustion (KJ/mol)</td><td align="center">Combustion Equation</td><td align="center">Mol O<sub>2</sub> / Mol fuel</td><td align="center">Heat / Mol O<sub>2 </sub>(kJ)</td><td align="center">Relative<br>heat/mol O<sub>2</sub> (propane=1)</td></tr><tr><td align="center">Propane</td><td align="center">2039</td><td align="center" nowrap="nowrap">C<sub>3</sub>H<sub>8</sub> + 5O<sub>2</sub> --><br>3CO<sub>2</sub> + 4H<sub>2</sub>O</td><td align="center">5</td><td align="center">408</td><td align="center">1</td></tr><tr><td align="center">Butane</td><td align="center">2652</td>
<td align="center" nowrap="nowrap">2C<sub>4</sub>H<sub>10</sub> +
13O<sub>2</sub> --><br>8CO<sub>2</sub> + 10H<sub>2</sub>O</td><td align="center">6.5</td><td align="center">408</td><td align="center">1.00</td></tr><tr><td align="center">Acetylene</td><td align="center">1254</td><td align="center" nowrap="nowrap">2C<sub>2</sub>H<sub>2</sub> +
5O<sub>2</sub> --><br>4CO<sub>2</sub> + 2H<sub>2</sub>O</td><td align="center">2.5</td><td align="center">502</td><td align="center">1.23</td></tr><tr><td col$pan="6"><b>MAPP:</b></td></tr><tr><td align="center">   Methyl Acetylene (CH3-C=-CH)</td><td align="center">1847</td><td align="center" nowrap="nowrap">C<sub>3</sub>H<sub>4</sub> + 4O<sub>2</sub> --><br>3CO<sub>2</sub> + 2H<sub>2</sub>O</td><td align="center">4</td><td align="center">462</td><td align="center">1.13</td></tr><tr><td align="center">   Propadiene (CH2=C=CH2)</td><td align="center">1853</td><td align="center" nowrap="nowrap">C<sub>3</sub>H<sub>4</sub> + 4O<sub>2</sub> --><br>3CO<sub>2</sub> + 2H<sub>2</sub>O</td><td align="center">4</td><td align="center">463</td><td align="center">1.14</td></tr></tbody></table>
MAPP is a mixture of ~20% methyl acetylene, ~80% propadiene with a small amount of a couple other hydrocarbons (ethane, propane ...). Methyl acetylene and propadiene have the same molecular formula and very nearly the same heats of combustion so either can be used to represent the behavior of MAPP.

The important column in the table is the "Relative heat/mol O2". This value is the relative energy available from the different fuels in a constant chamber volume. MAPP only contains about 13% more energy than propane or butane.
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Postby SkweezMyCheez » Sun Nov 05, 2006 8:00 pm

<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by DR</i>
<br>
Geez, I've been trying to tell most people (for a long time now) how incredible these little fans are. - I'm really glad that someone else has also actually seen this for themselves.

Brad, what can I say?- You are always outdoing yourself. ~8)
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Anyone care to let me know about the type/brand/model/name of this fan? I would appreciate it.
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Postby BewareOfDog » Mon Nov 06, 2006 1:08 am

<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote"><b>Originally posted by SkweezMyCheez</b>
Anyone care to let me know about the type/brand/model/name of this fan? I would appreciate it.
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Ozark Trail outdoor equipment Portable Mini-Fan... You can find them at Wal-Mart, in the camping section.
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Postby markfh11q » Mon Nov 06, 2006 6:48 am

First, let me sincerely apologize for taking this long to reply.

I would like to secondly say, well done! I love those little fans as well, and you just showed me a way to mount them without having to deal with drilling and tapping perfectly straight holes into the chamber wall for allthread and then mounting a flimsy fan body onto that as well.

Regardless of how fast it shoots, it looks great! All of your cannons are so clean!
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