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construction questions

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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construction questions

Unread postAuthor: Grignacke » Sun Jun 20, 2010 3:35 pm

Hello, I'm very new to spud cannons and their paraphernalia.
I was given a very simple schematic of a starting spud gun, with a 4" by 36" chamber and a 2" by 36" barrel, with a lantern striker for ignition, a 4" to 2" reducer between the chamber and barrel, and Aqua Net Hairspray propellant.
I perused the wiki for information, and learned about CB ratios; according to the wiki, the optimal ratio is between .6 and .8, and, using some math, I figured out the optimal chamber length for me is 5.4". Is this correct?

Second off, what are some methods for mounting a fan onto the chamber? would it go in the end and be exposed? Pictures provided would be fantastic. :D

Finally, as the hair spray method doesn't provide accurate results per launch, how would a fuel injector work, and with what kind of fuel?

Sorry if there are too many questions, and thank you for your time! =)
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Unread postAuthor: jor2daje » Sun Jun 20, 2010 3:44 pm

5.4" seems a little on the small side to me but it could work.

the fan can either be mounted with screw going through the chamber, or you glue or screw it in from the inside. On one of my combustions a made a little mount out of pvc and solvent welded it to the inside.

placement of the fan has a little debate, but somewhere about 1/3 of the chamber distance from the back has worked quite well for me. You want it to be able to draw in fresh air and mix the fuel.

Metered fuel is easy to do, Im pretty sure its in the wiki also, mapp and propane gas both work very well. All you do is get a torch nozzle and modify it so you can hook it up to a regulator, then that feeds into a specific lengh of tube with valves on both ends. You open one valve to let propane fill the tube, close it, then open the other to let that "metered" amount of propane into your chamber.
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Unread postAuthor: Grignacke » Sun Jun 20, 2010 3:57 pm

So the end of the chamber is open to the air, and the fan circulates / sucks in air? And I assume the chamber is sealed when firing is commenced, right?
(Also, thank you for the fast response :D)
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Unread postAuthor: jackssmirkingrevenge » Sun Jun 20, 2010 4:04 pm

As suggested above, have a look through the Image first, I'm sure many of your questions will be answered.

Also, on a tangent, congratulations on being the first member in the history of this forum to use the word "paraphernalia" :D
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Unread postAuthor: Grignacke » Sun Jun 20, 2010 4:30 pm

To the wiki I go.
Also, jackssmirkingrevenge, your profile picture and name are amazing. You should win a medal for the amount of win they are.

Thank you very much to all!
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Unread postAuthor: jhalek90 » Sun Jun 20, 2010 5:48 pm

Good luck on your cannon!

Its nice to see a new face on the forum, and especially one that understands research!

On a side note, you will quickly learn that EVERYTHING about JSR is win.
He is one of the most crafty people on here... simply look at a few of his topic for evidence. /

WELCOME TO SPUDFILES!
:shock:
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Unread postAuthor: jackssmirkingrevenge » Sun Jun 20, 2010 10:18 pm

jhalek90 wrote:On a side note, you will quickly learn that EVERYTHING about JSR is win.


Not *everything*, Exhibit A...

:roll: :wink:
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Unread postAuthor: Technician1002 » Sun Jun 20, 2010 10:50 pm

Those who fail to learn from mistakes of others are condemmed to make them for themselves.

Learning from JSR's mistakes is a good thing.
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Unread postAuthor: sgort87 » Mon Jun 21, 2010 8:42 am

Back to the original post, I'll answer your questions since the wiki isn't always clear to the beginner.

Good thought on the C:B ratio, but that is a formula for a fixed chamber volume to find the optimum barrel volume. For a fixed barrel volume, normally the bigger the chamber, the better (until a certain point, which is pretty damn big). Also, the .7:1 ratio is for propane fuel. 1.5:1 is said to be best for hair sprays and stuff like that.

The fan goes inside the chamber to create turbulence and ventilation, and normally blows toward the barrel. During fueling and shooting, the fan is sealed in by the launcher's chamber cap. When it's time to vent, the cap is removed and the fan draws new air through. It normally has the corners notched off it and holds itself in the chamber, but if it's a smaller fan, there have been methods of holding the fan using clips and/or screws.

There's a lot to say about your third question, so I'm going to instead tell you to read up on metered propane.
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