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Spudzilla Pro BMF9000

Built a combustion cannon? Then post it here! This section is for completed, finished cannons that you have built. Please include pictures and information.
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Unread postAuthor: mark.f » Mon Jul 06, 2009 6:18 pm

Budarooski wrote:Wiki says MAPP requires 4.98% Displasive Volume Percent, but then on the next page it says 4.6%, so I'm not sure what to believe. Then again, everything I read says Propane requires 4.03%, yet the second wiki page says 4.2%.


"Displasive" volume percentage is the percentage of the chamber volume you need to inject in propane if the propane injection displaces air from the chamber (out a cleanout, out a vent hole, past the projectile, etc.). The idea is that, since the propane displaces air, there is less to burn the fuel in, and the percentage must be adjusted.

The other percentage is the percentage of the chamber volume to inject if the fuel injection displaces no air (a completely sealed chamber). In reality, the chamber doesn't need to be completely sealed either, because the fan will have homogenized the fuel mixture by the time the slight pressure caused leaks.

Overall, there shouldn't be much performance difference between which percentage you use. I've always used 4.2% for propane and 4.89% for MAPP. If you are fueling a hybrid launcher, then the method and figures used start to matter, but for a basic combustion it shouldn't matter a whole lot.
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Unread postAuthor: starman » Sat Jul 11, 2009 9:17 pm

Really well done combustion Bud! Lots of attention to detail there. Only a couple of things...

Those large screw in drain plugs are a real hazard especially on higher powered advanced models. Don't be surprised when it blows out someday soon. Also the air vent...right behind your trigger hand. If you should forget to close the valve (very EASY to do) you will be french frying your hand, wrist, fingers, etc at least...and possibly more.

Had you been able to locate that vent connection behind the interior fan, the second smaller exterior fan would have been unnecessary.
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Unread postAuthor: THUNDERLORD » Sat Jul 11, 2009 11:34 pm

Very nice design.
My only constructive criticism that the tank blocks the sight view.
Other than that I'd copy it if I was building a nice combustion. 8)
Thanks.
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Unread postAuthor: Budarooski » Sun Jul 12, 2009 10:06 am

starman wrote:Those large screw in drain plugs are a real hazard especially on higher powered advanced models. Don't be surprised when it blows out someday soon.


I filled the end-cap with epoxy to hopefully make it stronger.

Also the air vent...right behind your trigger hand. If you should forget to close the valve (very EASY to do) you will be french frying your hand, wrist, fingers, etc at least...and possibly more.


Yep, I did it once and burned off the hair on my hand.

Had you been able to locate that vent connection behind the interior fan, the second smaller exterior fan would have been unnecessary.


Agreed, but I puposely didn't want to do it because I need to stand the cannon on its end to load it and I didn't want the valve there, and I was worried it would weaken the end cap.
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Unread postAuthor: Ragnarok » Sun Jul 12, 2009 10:27 am

Budarooski wrote:Wiki says MAPP requires 4.98% Displasive Volume Percent, but then on the next page it says 4.6%, so I'm not sure what to believe. Then again, everything I read says Propane requires 4.03%, yet the second wiki page says 4.2%.

It's not quite what Mark said.

You add 4.2% of the volume of the chamber in propane - which makes 4.03% of the gas in there Propane.

Imagine if I have a room with 100 people in it - all of whom are called Derek. I then put 25 people called Keith in as well. It's a 25% addition, but only 20% of the now 125 people in the room are called Keith.

Same principle with the propane and MAPP. Add the higher percentage, which will then make up the lower percentage of the entire mix.
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Does that thing kinda look like a big cat to you?
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