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Cannon for Chemistry Class

A place to ask general spud cannon related questions.
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Cannon for Chemistry Class

Unread postAuthor: ew23 » Sun Apr 18, 2010 10:21 pm

Hi everyone I want to make a potato cannon for my chemistry project. My teacher doesn't care if it is a pneumatic or combustion cannon just as long as I can to multiple tests fairly fast. So I wanted to know the pros and cons of each type so I can figure what type I'll be building. Thanks so much. :)
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Unread postAuthor: theBOOM » Sun Apr 18, 2010 10:26 pm

For semi or automatic fire you def. go pneumatic. You probably wont get a truly "semi" working for your first build you'll probably have to reload ammo between shots but I guess that should fit your needs.
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Unread postAuthor: Marffy » Sun Apr 18, 2010 10:27 pm

how fast would you like to fire the cannon? depending on what you make they can both fire around the same speed but if you wanted a fasterish rate of fire then i would make a pneumatic cannon with a sprinkler valve or QEV. if you want it portable i would make a combustion for combustions are easy to make portable.

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Unread postAuthor: MrCrowley » Sun Apr 18, 2010 10:29 pm

What's your budget? What type of tests do you want to do?
If you just want something simple, a ball valve/sprinkler valve pneumatic would do. If you want something a little more complex and related to chemistry, an advanced (fuel metered) combustion would be good. I don't recommend building a normal "spray and pray" combustion as it would be too unreliable for tests.

Give us some more detail and we can help you narrow down what launcher would suit you best.
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Unread postAuthor: jackssmirkingrevenge » Sun Apr 18, 2010 10:41 pm

MrCrowley wrote:If you want something a little more complex and related to chemistry, an advanced (fuel metered) combustion would be good.


I agree, you could calculate the stoichiometric mixture of air and fuel and prove that it works best in practice ;)
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Unread postAuthor: Technician1002 » Sun Apr 18, 2010 11:20 pm

In rapid reloads, it is easier to get repeatability with a pnuematic. The level of venting between shots in a combustion can lead to considerable variability. On an air cannon the pressure can be easly regulated to control the power of the shots so a consistant load such as golf balls can be gently tossed a predictable distance. Below is an example. I was trying to lob a golf ball into a wheelbarrow. Other than aiming issues with it being handheld, you can clearly see the distance is very predictable on this gentle lob of the golf ball.

Will you need to have accurate power control? This is adjustable from a few feet to several hundred yards. I can toss eggs without breaking them or punch golf balls through plywood.

[youtube]http://www.youtube.com/watch?v=g496mBQox7Y[/youtube]

Same cannon with larger barrel and higher pressure.

http://www.youtube.com/watch?v=ZUTXomfdLTE&NR=1
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Unread postAuthor: jimmy101 » Mon Apr 19, 2010 7:12 pm

I assume by "multiple tests fairly fast" you mean you can collect a reasonable amount of data in a reasonable amount of time. Not that the gun is semi- or full auto.

In this time frame any type of gun would be fine. They can be fired once every minute or two. Since you probably only need a dozen of so total firings, time isn't that much of an issue.

As a chem project you need to decide what characteristic you'll be changing and how you will measure performance. Lets take the second part first; performance can be measured with a consistent ammo (maybe a BB) and SoftChrono (or just use a mic and a PC). The gun will make a "pop" when the ammo exits the barrel and another sound when it hits a target. Easy enough to extract the time of flight from the audio recording. From that, and the distance from the muzzle to the target, you can calculate the velocity. Use a short distance (a foot or so) so the measured velocity is basically the muzzle velocity.

For a low powered setup (one that might be OK at school) use a slice of potato as the ammo and a small diameter barrel, like 1/4" copper pipe. You can poke multiple "shells" out'a the slice and they should be very close to the same mass. For bonus points, measure the spud slice before and after loading the barrel and calculate (and record) the weight of each round.

For a pneumatic you can examine the affect of different chamber pressures on the muzzle velocity. (Better yet, calculate the muzzle kinetic energy, that'll remove the affects of different ammo weights.) The results will be kind of boring. The velocities will just go up as you increase the pressure.

For a combustion, study the muzzle velocity (or KE) as a function of volume percent of fuel. The results will be more interesting since there will be an optimal fuel percentage and over fueling past that point will decrease performance. You can then explain the results based on the stoichiometry of combustion.



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Unread postAuthor: ew23 » Mon Apr 19, 2010 10:04 pm

Thanks everyone for the info. For now I am just going to use a cannon that runs off hair spray even though it will be unreliable because I have to demo my concept on Friday. The good thing is that my teacher said that as the project progresses changes can be made to it. Since a lot of you recommended a pneumatic I was wondering how hard it would be to build one and the price range.

For my tests I need to have one independent variable and my teacher said it can be anything from different sized projectiles to different amounts of propellant. Then for my dependent variable it would be how far the projectile goes. I was also thinking of mounting the cannon at a certain angle so that each projectile takes off at the same angle. Do you guys have any suggestions for a good mounting angle?

Thanks again. :)
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Unread postAuthor: clemsonguy1125 » Mon Apr 19, 2010 10:17 pm

Well a ball valve will be more consistant than a spray n pray and can be built for under 20 in about 15 minute and a sprinkler valve gun can be built for about 30 and will be consistant
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Unread postAuthor: SpudBlaster15 » Mon Apr 19, 2010 10:20 pm

jimmy101 wrote:For a combustion, study the muzzle velocity (or KE) as a function of volume percent of fuel. The results will be more interesting since there will be an optimal fuel percentage and over fueling past that point will decrease performance. You can then explain the results based on the stoichiometry of combustion.


I'm liking this idea. It's simple, interesting, and anyone who has any experience with even a basic combustion spudgun can relate to the importance of accurate fueling.
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Unread postAuthor: Marffy » Mon Apr 19, 2010 11:10 pm

ew23 wrote:
For my tests I need to have one independent variable and my teacher said it can be anything from different sized projectiles to different amounts of propellant. Then for my dependent variable it would be how far the projectile goes. I was also thinking of mounting the cannon at a certain angle so that each projectile takes off at the same angle. Do you guys have any suggestions for a good mounting angle?

Thanks again. :)


I would make a wooden frame that holds the cannon at about 45 degrees but its really up to you and it doesn't really matter because you don't need it to fly the furthest possible distance.
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Unread postAuthor: jackssmirkingrevenge » Mon Apr 19, 2010 11:39 pm

You could also vary the types of propellant, then check out their chemical composition and see what works best...
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