Crusader

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

Postby aturner » Tue Oct 31, 2006 11:52 am

That is a very nice launcher! I like how your use of double walled contruction for the entire chamber allows you to place each component wherever you want to. In the typical design, folks are generally limited to placement of electrodes, etc.

As for your chrony data, that is truly remarkable! I am jealous! (in a good way)

I have fired many spuds through my chrony, but for some reason have never acutally chronied GB's from a combustion launcher. And I don't recall anyone else producing this sort of data either. So I honestly have no clue what the expected velocity should be. However, I do have a similarly sized chamber and I have a few GB barrels. Next time I get a chance, I'll see if I can get a GB in the same neighborhood of velocity as what you have observed. I sure hope so, b/c that is truly exciting stuff!

Someone asked if the muzzle blast might be an issue. I can say I've never observed any effect from muzzle blast, and I almost always place my launcer within 1 or 2 feet of the chrony.

One thing you may want to do next time is take along a BB gun, and do a test fire before each real launch, just to confirm the chrony is reading roughly as expected. It just gives one more level of QC.

Just curious, did you modify your GBs to stablize their flight? Did they fly straight or erratic? If the GB's were spinning as they came out of the barrel, then I wonder if reflected light from the spinning dimples of the GB was detected by the chrony? Again, just a thought.

I am also intrugued by your plactement of the spark, use of a bigger fan, and use of Mapp. Maybe you've found a perfect combination. Thanks for sharing this!
aturner
 
Posts: 1470
Joined: Thu May 27, 2004 1:50 pm
Location: USA
 

Postby SpudMonster » Tue Oct 31, 2006 12:06 pm

Holy...

I'm blown away. Considering this is a normal combustion cannon, that, to my knowledge, is the first cannon (hybrid or not) to break mach 2, and it isn't exceptionally different from other cobustion cannons in design, I am amazed.

This makes me wonder what my cannon with a 332 CI chamber and 12' long golfall barrel would do if put on a chrony.
SpudMonster
 
Posts: 1566
Joined: Thu Aug 04, 2005 11:48 pm
Location: USA
 

Postby BigBang » Tue Oct 31, 2006 12:17 pm

Aturner - Hey, great to hear from you. No I didn't stabilize the golfballs. I did use a small lubricated cloth patch to hold them in the barrel prior to launch. I suspect they weren't spinning as they exited the barrel. Most flew fairly straight (as far as I could watch them), but one wizzed around like a bumble bee. Side to side it must have wandered about by more than 20 feet. I suspect that if I had attempted to put some back spin on the balls they would have flown even further. Wouldn't be able to find them anyway, so I'm not sure I'll try the experiment.

BTW, I discovered your BlackHawk launcher recently. I especially like the way you designed the back end of the chamber. That is much more heavy duty than a regular clean-out cap. I think I'll incorporate that design element into future builds. In addition, that will provide a way to increase the chamber size of existing cannons. Neat idea.
BigBang
 
Posts: 155
Joined: Thu Jul 31, 2003 11:45 am
Location: USA
 

Postby aturner » Tue Oct 31, 2006 3:22 pm

<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote">Originally posted by BigBang
[br]Aturner - Hey, great to hear from you. No I didn't stabilize the golfballs. I did use a small lubricated cloth patch to hold them in the barrel prior to launch. I suspect they weren't spinning as they exited the barrel. Most flew fairly straight (as far as I could watch them), but one wizzed around like a bumble bee. Side to side it must have wandered about by more than 20 feet. I suspect that if I had attempted to put some back spin on the balls they would have flown even further. Wouldn't be able to find them anyway, so I'm not sure I'll try the experiment.<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
That's nice they were flying true for the most part. I think it makes sense that the use of a patch helps reduce spin as the ball comes down the barrel...right?

<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote">
BTW, I discovered your BlackHawk launcher recently. I especially like the way you designed the back end of the chamber. That is much more heavy duty than a regular clean-out cap. I think I'll incorporate that design element into future builds. In addition, that will provide a way to increase the chamber size of existing cannons. Neat idea.
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
Thanks man! Funny thing is I think some of your builds, where the entire chamber is sleeved, helped to inspire me in the design of the extension for the blackhawk. The best thing about that design is how much easier it is to load, b/c there is actually some space around the breech of the barrel. The typical breech loader tends to tear up my knuckles as I try to load a spud, b/c there is very little clearance between the barrel and cleanout threads.
aturner
 
Posts: 1470
Joined: Thu May 27, 2004 1:50 pm
Location: USA
 

Sponsored

Sponsor
 
 

Postby jimmy » Wed Nov 01, 2006 11:34 am

The efficiency of typical heat engines are very well known. (goggle "heat engine efficiency") A gasoline engine is a heat engine and <u>many</u> engineers have spent thousands of man hours trying to eek out another percent or so of efficiency. If a person came up with a way to get 1% better efficiency out of a generic heat engine he/she would be "Bill Gates rich" in nothing flat.

30% efficiency is pretty good. Modern car engines are about that. A high performance racing engine is maybe 35%. IIRC, certain types of turbines are a fair bit better, perhaps approaching 60% efficiency. But you just can't do much better than that.

In doing the energy calculation you also have to take into account the energy lost in hot exhaust gases. If a spud gun was a "pretty good" heat engine then the temperature of the gases in the chamber (and exhausted out the barrel) after firing would be near ambient temperature. Every degree of heat in the exhaust gases represents energy not used to accelerate the projectile.

Besides, though it is a nice gun, it really isn't unusual in any way except perhaps the placement of the fan relative to the spark gap. Why would this basic design suddenly out perform <b>every other standard combustion gun ever built?</b> And not by a little, but by a factor of at least 2 in muzzle velocity? That's at least four times more energy in the projectile than any other combustion.

How many ~mach 2 combustion guns are there? Heck how many mach 1 guns, of any design, including hybrids and compressed gas guns, are there? Not many.

How many firearms fire at mach 2? Well, quite a few, but that is with a couple grams of modern powder (versus much less than one gram of MAPP), a steel barrel that can withstand a couple thousand PSI and a detonating (not just burning) combustion process.

Sorry, it just looks too much like a "perpetual motion" claim. Extraordinary results require extraordinary proof.
jimmy
 
Posts: 790
Joined: Wed Jul 30, 2003 11:02 am
Location: USA
 

Postby aturner » Wed Nov 01, 2006 11:47 am

jimmy, are you aware of any other data on the velocity of GB's fired from a combustion launcher such as this? 3800ml chamber and a 5' barrel of 1.5" SDR21? I don't recall every seeing any similar data prior to this.
aturner
 
Posts: 1470
Joined: Thu May 27, 2004 1:50 pm
Location: USA
 

Postby fullmetaljacket » Wed Nov 01, 2006 1:10 pm

<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote">Someone asked if the muzzle blast might be an issue. I can say I've never observed any effect from muzzle blast, and I almost always place my launcer within 1 or 2 feet of the chrony.<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">

I've found that even the blast from a CO2 pistol will upset certain chronographs if fired too close, resulting in muzzle velocity discrepancies of over 35%.

Forgive me but I have to agree that it's extremely unlikely that a simple combustion can achieve the same sort of velocities as a shotgun will, considering the latter typically generates around 12,000 psi of barrel pressure.
fullmetaljacket
 
Posts: 1684
Joined: Mon Jun 14, 2004 1:33 pm
 

Postby jimmy » Wed Nov 01, 2006 2:43 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]jimmy, are you aware of any other data on the velocity of GB's fired from a combustion launcher such as this? 3800ml chamber and a 5' barrel of 1.5" SDR21? I don't recall every seeing any similar data prior to this.
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
Is it really all that different than a hundred other guns? The projectile doesn't make a lot of difference, it has mass, it has friction. Indeed it is denser than a spud so you would expect a lower muzzle velocity than with an identically sized piece of starch. A GB perhaps has lower dynamic friction than a spud. But if the friction is very low then the quality of the seal is probably not very good giving significant blowbuy. Or the seal is pretty good making the friction similar to what you get with a spud.

The 1.5" barrel would be expected to be a little less efficient than a more common 2" barrel. The area is only 1.5<sup>2</sup>/2<sup>2</sup>=0.56 as much. In addition, the "pushed area" to "frictional area" is less in 1.5" than 2" so friction has more of an affect on a 1.5" barrel than on a 2" barrel.

The difference in fuel (MAPP vs. propane) isn't all that great. <a href="http://www.burntlatke.com/lpmapprussets.html">Latke's data</a> comparing the two gave ~11% higher muzzle velocities with MAPP then propane. A significant difference to be sure, but not enough to get a supersonic spud from a standard combustion setup.

The somewhat larger than normal chamber will give higher muzzle velocities. But compare this 3.8L chamber with <a href="http://www.burntlatke.com/15cb-data.html">Latke's 2.6L on a length optimized 1.5" barrel</a> and you have to be able to justify getting ~1500fps with this gun versus ~630fps with Latke's smaller chamber. The volume ratio 1.4:1 but the muzzle velocity is more than twice as high with this gun. Furthermore, Latke was using a gasket equipped wood slug that is probably lighter than a GB, so his muzzle velocities would be even slower if he was firing GBs. (I hunted all over Latke's site and couldn't find the weight of his gasket slug anywhere.)

I think it is pretty much a no-brainer that there is a problem with the posted muzzle velocities. Indeed, they are about 2.5x higher than is reasonable.
jimmy
 
Posts: 790
Joined: Wed Jul 30, 2003 11:02 am
Location: USA
 

Postby Los Frijoles » Wed Nov 01, 2006 5:44 pm

Absolutly stunning gun (w/ velocities and all). Very nice!
Los Frijoles
 
Posts: 628
Joined: Mon Nov 14, 2005 4:12 pm
Location: USA
 

Postby BigBang » Wed Nov 01, 2006 5:50 pm

Jimmy, I truly appreciate your thoughtful comments. Without a bit of controversy science is just no fun. I also agree that more data is necessary before we can close the case on this one. Nevertheless, let me throw some further considerations into the discussion.

In separate correspondence Freefall has pointed out to me that there is a problem with your accounting of recoil. There is, instead, a momentum balance which must be preserved; i.e., M1*V1=M2*V2.

Recalling that kinetic energy is 0.5*M*V^2, after a bit of algebra we can derive the following relationship:

KEgb*Mgb = KEsg*Msg (1),

where KEgb and KEsg are the kinetic energies of the golfball and spudgun, respectively, and where Mgb and Msg are the respective masses.

Next we recall that

KEgb+KEsg=(eta)*(Total energy released during combustion) (2),

where eta is the effencieny you have mentioned previously. Using equation (1) in equation (2) and rearranging we then find

KEgb = eta*(Energy Released)*Msg/(Msg + Mgb).

In most cases the mass of the spudgun will be much much greater than that of its projectile. (In my case, the Crusader weighs in at about 10lb, while the golfball comes in at a measly 46g.) Therefore virtually all of the energy goes to the projectile, or

KEgb ~ eta*(Energy Released).

Now, the total energy ET released is

ET = (Chamber Volume)*(fuel volume fraction)*(heat of combustion)

For my gun, I used a 1X stoichiometric MAPP mixture in a chamber with a volume of 3728ml, yielding ET=(3728)*(0.05)*(154.45)= 27.8kJ.

I teach probability and statistics and regularly tell my students that a single data point does not constitute a trend. I am perfectly willing to accept that the Mach 2 shot was potentially erroneous. Focusing then on the median velocity of 1679fps (511.76 mps) - which is less influenced by outliers than is the mean value - we find that the effeciency of the process comes in at a perfectly acceptable 21%. That is...

eta = (0.5)*(0.046)*(511.76^2)/27.8 = 0.21

If we instead we assume that the efficiency was 30% then we find a maximum expected speed of ~2006fps (611.34 mps).

Based on these calculations my measurred speeds are not only well within reason, they are in fact quite plausible. Why others have not achieved similar results prior to this is a mystery to me too.

I'm not tying to throw fuel on the fire, or ruffle feathers. I too am interested in getting to the bottom of this. It just fascinates me.

Have a nice evening,
BigBang
BigBang
 
Posts: 155
Joined: Thu Jul 31, 2003 11:45 am
Location: USA
 
 
PreviousNext

Return to Finished Cannons

cron