Crusader

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

Postby Velocity » Fri Oct 27, 2006 10:00 pm

Oh yeah, I forgot to mention: I nominate this cannon for the range class
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Postby An Apple Pie » Fri Oct 27, 2006 10:08 pm

I second that nomination for the range class

Actually I think there should also be something like a "Velocity Class" or something. If so it should definatly go in that. But for the current classes range fits this one.
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Postby clide » Sat Oct 28, 2006 1:17 am

I don't question that the data collected is what the crony actually showed you, but I do question the crony's measurements.
You seem quite close to the crony in the video, is it possible that it was capturing the muzzle blast from the gun? Did you try it from further back at all? Also maybe checking that the crony is measuring correctly by something you know the approximate speed of.

It's a very nice gun and all, but I would think a gun shooting a golf ball over 1600 fps would be very difficult to hold onto, at least more so than it appears in the video. I'm sure you understand my skepticism

Also, a quick and dirty GGDT with very optimistic parameters would suggest over 600 psi needed for many of those shots and over 1200 psi for the mach 2.
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Postby jimmy » Sat Oct 28, 2006 11:30 am

Very nice gun and results.

There is clearly a problem with your chrony data. There isn't enough energy in MAPP (or acetylene, or propane or...) burning in air to get your spud moving that fast.

3800ml chamber. For propane that would be 150 ml fuel. (150ml)(mol/22400ml)(2,219 kJ/mol) = total energy in combustion ~15KJ.
(I used 2,219 kJ/mol as the propane heat of combustion. MAPP isn't that different.)

An efficient heat engine gets 30% of the heat converted into mechanical energy. A gun that recoils can never get more than 50% of the useable energy into the projectile since 50% of the energy goes into recoil. So, about 15% of the total energy is useable, roughly 3KJ.

Figure 100g spud, moving at 2000 fps (610m/s)
KE of spud = (0.5)(0.1kg)(610)^2 = 18.6 KJ

Your spud, if it is moving ~mach 2, has ~6 times more energy than was available in the fuel given typical efficiencies of a heat engine.
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Postby BewareOfDog » Sun Oct 29, 2006 1:44 pm

You know what?
WHO honestly gives a fat flying [intercourse] about all this number crunching?

The fact remains that Brad here has created yet another outstanding example of workmanship and design!

This makes me very curious about building a new launcher and placing the ignition point well <b>behind</b> the fan, rather than in fromt of it, as always.

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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Postby BillyBobJoe » Sun Oct 29, 2006 2:44 pm

<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote">Originally posted by jimmy
[br]Very nice gun and results.

There is clearly a problem with your chrony data. There isn't enough energy in MAPP (or acetylene, or propane or...) burning in air to get your spud moving that fast.

3800ml chamber. For propane that would be 150 ml fuel. (150ml)(mol/22400ml)(2,219 kJ/mol) = total energy in combustion ~15KJ.
(I used 2,219 kJ/mol as the propane heat of combustion. MAPP isn't that different.)

An efficient heat engine gets 30% of the heat converted into mechanical energy. A gun that recoils can never get more than 50% of the useable energy into the projectile since 50% of the energy goes into recoil. So, about 15% of the total energy is useable, roughly 3KJ.

Figure 100g spud, moving at 2000 fps (610m/s)
KE of spud = (0.5)(0.1kg)(610)^2 = 18.6 KJ

Your spud, if it is moving ~mach 2, has ~6 times more energy than was available in the fuel given typical efficiencies of a heat engine.
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">

Remember, he was not shooting a 100g spud, but a 45 gram golfball, which brings kinetic energy down to 8.3KJ - still not a good match but closer to the estimated.

I really don't know what to say in this argument. I trust Bigbang, but the numbers say otherwise. Maybe, as Clide said, the muzzle blast was caught? Jimmy - how accurate are those efficiency figures of a heat engine? And is 50% of the energy lost on recoil every time - wouldn't it depend on how it was held and how much movement there is?
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Postby BigBang » Mon Oct 30, 2006 10:21 am

Hey Gang,

Ironically it seems that I'm the only one who can't see my own videos. For some reason nothing at Putfile seems to launch for me. I'm glad you all can see them, though.

Clide - Intriguing idea about the muzzle blast interfering with the data. I was about 2 feet back from the first sensor in all shots. Do you think this was not far enough? I pretty much took my lead on this from images of chrony set-ups at Latke's and ATurner's websites.

On the other hand, after playing with the videos for hours I've noticed that I never see a muzzle flash. I don't recall seeing one the day of the shoot, and there is not one visible in the videos. With a C:B of 1.67 (pretty ineficient by traditional standards) I would have expected a pretty noticable muzzle flash. I've seen them with all my older guns designed for C:B=1.5. My first pass guess has been that turbulence due to that humming little fan, and the rearward spark placement somehow combined in a nonlinear fashion to cause complete combustion to take place much more quickly than usual - thereby increasing pressure and velocity. (Humm, if this is true, would we have to rethink the "optimum" C:B value?)

Jimmy - very intriguing comment!!! I'll crank through your numbers when I get a chance. I may not get to it today, though. However, if some nonlinear process (as alluded to in some of the D_Hall threads over in the hybrid section) has taken over, then first order appoximations may not hold. I suspect that if the numbers are correct, the pressures developed in the chamber are well beyond what I myself have predicted previously, for example.

Back when FuzzyScorpion's hybrid detonated (we think), I recall some discussion that detonation (a non-linear process) was an overall less efficient process than deflagration. However, since the energy was released more quickly and violently, peak pressures were much higher than expected. Not that my gun grenaded, but if some nonlinear process has been initiated, our prior intuition may not apply.

I love this stuff - making experiment and theory balance. There is never progress without a wild theory or unexpected experimental results now and then. This part of spudgunning is probably what holds my attention the longest.

BTW, has anyone ever noticed how bad MAPP smells?

Thanks,
BigBang
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Postby BigBang » Mon Oct 30, 2006 3:01 pm

Well, I still can't see my own videos, but I did take the opportunity to chase down some of Jimmy's numbers. Here is my analysis, keeping in mind that I am by no means an expert in this sort of thing:

Chamber volume with golfball barrel = 3728ml
Stoichiometric MAPP volume fraction in air = 5%
MAPP heat of combustion = 154.45 [J/ml] (check <a href="http://www.twi.co.uk/j32k/protected/band_3/jk49.html">HERE</a> and <a href="http://www.weldreality.com/gasdata.htm">HERE</a>)

Total estimated combustion energy released = (0.05)*(3728)*(154.45)=28.79 kJ.
15% of that is 4.32 kJ.

Golfball mass = 45g (0.045kg)
Median velocity = 1679fps (511.76 m/s) - eliminates outliers in the data!

Median kinetic energy = 0.5*(0.045)*(511.76^2)= 5.89 kJ.

If the net efficiency (including recoil) of the process is 20%, then the predicted and measured energies are essentially the same. I take this to mean that my data is not out of the realm of possibility.

If we (somehow) take into account increased combustion pressures due to the precompression of gasses caused by the rearward ignition point, and the likely increased burn rate caused by turbulence in the chamber I'm pretty much thinking the velocities I got were indeed a reflection of reality.

Fascinating stuff, huh!

BigBang
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Postby Barasackee » Mon Oct 30, 2006 10:01 pm

Do you actually fire it like the first picture, with the sling? if so, I should do the same for mine so I don't have to bungee cord it to the step ladder.

Edit: never mind I saw the video. I think I'll do that with mine. I hope you don't mind me stealing your ideas!
P.S. Did you just bolt it to the chamber? any close up pics?
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Postby BigBang » Tue Oct 31, 2006 9:59 am

<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote"><b>Originally posted by Barasackee</b>
Do you actually fire it like the first picture, with the sling? if so, I should do the same for mine so I don't have to bungee cord it to the step ladder.

Edit: never mind I saw the video. I think I'll do that with mine. I hope you don't mind me stealing your ideas!
P.S. Did you just bolt it to the chamber? any close up pics?
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
Yeah, I use the sling when firing. The cannon is kind of heavy and bulky. Feel free to use the idea if you wish. Glad to help.

Also, take a look at the <b>Inside The Chamber</b> picture. At the top of the chamber you will see part of an eye bolt sticking into the chamber. I used two eye bolts - one in front, and one in back. To install them, I drilled a pilot hole slightly undersized, and just screwed them in.

Thanks...
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