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First Pneumatic Gun/Physics Project

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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First Pneumatic Gun/Physics Project

Unread postAuthor: Hotrod » Fri Jan 05, 2007 7:37 pm

My friends and i decided on a potato gun for a physics project but our teacher wouldn't let us use combustibles so we used air.

It looks pretty sweet and this forum really helped us make it so i thought i'd share.

The barrel is a 5' piece of 2"pvc, the chamber is a 2' section of 3" pvc, filled up with a car tire valve. We use an air compressor to fill it up to 140psi in about 15sec. We've fired it over 200 times and it never gets boring.

The valve is a simple 2" ball valve with a wooden extension added to make it easier to open.

The barrel screws off and we have a 4' long, 1.5" barrel that screws on.

For ammo we used normal medium potatoes stuffed in the barrel.

The farthest we launched a potato was 879ft at 45degrees. We did some calculations and the potato leaves the barrel at 120mph at 125psi. The acceleration out of the barrel is from 0-120mph in 1/20 of a second, or really fast.

Gun cost $52 and i had the wood for the stand.

Sorry if the quality is bad, i took them with my phone.

This in front of my friends pool
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Pointed straight up
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My friend launching
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Again in action
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The far soccer goal in the distance of this pic was the maximum range
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Drilled a 3/8in hole with a spade bit, Pressure gauge was wound with teflon tape and then screwed into the hole. Haven't had any problem at all with it.
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Shows our air compressor, and the extension of the handle.
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The potato shot 879ft left this mark in the ground
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Unread postAuthor: judgment_arms » Fri Jan 05, 2007 7:45 pm

Slick! I like the stand, how’d ya figure the velocity, math or a radar gun? People are going to yell at you for using DWV fittings. Like the modded ball valve, we get lots o’ modded sprinkler valves but vary few modded ball valves.
Welcome to the forums, friend!
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Unread postAuthor: Petrie » Fri Jan 05, 2007 7:51 pm

Whats the big deal with DWV? whats it stand for? are they weaker?

By the way very nice gun especally for first time
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Unread postAuthor: benstern » Fri Jan 05, 2007 7:55 pm

much much weaker. easily shatter with pressure. (when they shatter shards goes into your vital organs)

that cannon does not look that powerful. blame the ball valve.
switch to modded sprinkler valves for better and more consistant performance.
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Unread postAuthor: judgment_arms » Fri Jan 05, 2007 8:04 pm

DWV stands for Drain, Waste, Vent. It’s not pressure rated, the only thing it’s good for is combustions, but I personally wouldn’t use it period, except for clean outs for breach loading. Getting fraged isn’t fun…
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Unread postAuthor: Hotrod » Fri Jan 05, 2007 8:21 pm

benstern, judgement_arms, thank you for that information. I didn't know that the ball valve was DWV or i would have steered clear of it. We didn't have the time (or knowledge) at the time to modify a sprinkler valve, but we plan on making another one for fun in a few weeks so we will use a sprinkler valve instead.

As for speed we fired it straight up(@90degrees) and timed the time it took to hit the ground.

The time going up is 1/2 the total time of flight and plugged it into the equation 1/2At^2=D where D=distance a falling body drops, t=time, and A=acceleration due to gravity or 9.8m/s/s.

When you have the distance you can divide it by the time and get Average Velocity (Distance/Time=velocity). That is the AverageVelocity because it goes from the initial velocity to 0. Think about a car that goes from 0-20mph in 10sec. the average velocity over those 10sec=10mph)

The average velocity times 2=the initial velocity (or top speed)

Not that anyone cares. We didn't have a radar gun so we found another way...
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