Air Powered Potato Cannon

I had to build a potato canon for a science project and needed one where I could get a consistent launch force. The typical hairspray cannon would not be appropriate for this purpose so I constructed this air powered launcher as a modified version of Pnewton's Petard from the excellent book "Backyard Ballistics". Typical results were a range of over 300ft using a weight, nonsealing projectile, I am sure a potato would go much farther.

Step 1: Materials

The materials needed for constructing the air cannon are as follows:

·shaping file
·22-inch length of 1½-inch diameter schedule-40 PVC pipe
·1.5-inch diameter male-threaded PVC pipe adapter
·PVC primer
·PVC cement
·Teflon pipe tape
·1½-inch diameter female-threaded PVC pipe adapter
·1½-inch diameter to 3/4-inch threaded PVC reducing bushing
·Electric drill with various sized drill bits
·1 rubber-coated, narrow-diameter, replacement-tire air valve
·2 3-inch-diameter PVC end caps
·Wrench
·2 9-inch-long pieces of 3-inch diameter Schedule-40 PVC pipe
·3-inch x 3-inch x 1½-inch PVC tee connector
·3/4-inch diameter short iron pipe nipple
·3/4-inch ball valve (It has also been suggested that a pneumatically modded sprinkler valve would allow for a faster and there for more powerful launch. I have no experience with this so for the sake of this article we will use the ball valve.)
·3-foot length of 1-inch diameter wooden dowel or broom handle
·Foot-stabilized air pump

Below is the diagram and the assembled materials.

SAFETY NOTE: I have received several comments regarding the use of Cellular Extruded Piping. In short - don't use it. It is not meant to deal with air pressure and could explode violently producing hundreds of deadly shards. I have not had this occur yet but if at all possible please use a better piping.

Comments

[nutsandbolts_64]

I still remember when I made my first one. The valve was so tight it took me 1 second to fully open it, therefore, much energy was wasted. The valve used must be airtight and smooth-opening for efficient operation. The faster the valve opens, more air can and will come out in a given amount of time. Just a reminder. And please do use ABS piping, it's safer due to its reaction to a catastrophic failure (i.e. it turns into strips and not bits and pieces similar but not exactly like a grenade, plus PVC will be hard to show up on x-ray)

[ilpug]

Ok, let me clarify something: Combustion powered guns are better built with ABS for the reason you suggested, because the action of a combustion cannon results in an almost instant shock pressurization of the system (I.E, goes boom). ABS is better suited to quick shocks or bumps, so it is superior for combustion cannons.

ABS cannot be used for air cannons though, because it simply cannot hold the pressure. PVC is designed to hold pressures up to 500PSI+, depending on the pipe diameter. The trick is to keep the pressurized air chamber in a safe spot, or pad it against bumps. ABS can, and will, explode if you try to pressurize it.

Pnewtons Petards like this one are pretty safe as far as air cannons go.

[jmlazer]

So far, ABS has worked fine for me under pressure.

[ilpug]

until it doesn't...

[nutsandbolts_64]

The trouble is that 2 years ago, most of the PVC I used wasn't pressure-rated, add to that we didn't even have ABS piping. The thing is PVC is designed to withstand water pressure. Air is a completely different fluid to work with.

Someone should seriously get a blast shield and attempt to blow up PVC and ABS pipes rated for the same pressure. That should settle the argument.

[ilpug]

I might do that at some point. Make two air chambers of equal volume, pressurize to the same pressure, and shoot them with something.

[ilpug]

I might do that at some point. Make two air chambers of equal volume, pressurize to the same pressure, and shoot them with something.

[soggymushrooms]

Hard to show up on x- ray... Way to stop people from building it

[T(M)^2]

Privyet, comrades.

The question I have for you today is as follows: I am looking to build an air cannon with a 6.35mm(or 2.5in., for all of you ingloskiis who still use the English system) bore air cannon capable of launching a .5kg.(1.1023lbs.) projectile 200ft. at a 45° angle. For whoever wants to do the math, what modifications should I make to the specifications above?

Sincerely,
ДЖ

[de0509]

Not really doing much calculations, none actually, but because you are using a quite heavy projectile, you really need a lot of force. and your caliber is big. you need the ball valve to be big as well so as to not give much resistance for the air to flow. maybe try using a big ball valve or any other valves that release pressure fast. maybe 1 inch ball valve instead of 3/4. or maybe if you can use a tee so that 2 ball valves open together. that way, you have a 1.5 inch ball valve from combining 2x0.75 inch ball valve. just make sure they open at the same time. i.e. joining the two ball valve handles together maybe?
depends on the pressure you intend to use. if low, id suggest making the chamber part longer. if high pressure, i think the original should be ok. but that is to be just enough, you may want more power right? maybe double the chamber? keep note that this also increase pumping time.

[T(M)^2]

many thanks, de0509. I shall keep your advice on my mind and in my project.
dasvaydanya.

[sjoobbani]

This works pretty well with fire extinguisher cylinders as a pressure vessel too!

[TygrFr3d]

30 PSI as a high average? If you are worried about it bursting, don't worry. I have taken a 3" chamber to 120 with no issues. And if you want good groupings, maybe have the ball valve open with a tension spring and a catch, so it will open fast, and at a consistent speed.

[Turtlepwr]

The fire and danger is half the fun

[sci]

How much safer would it be if you use schedule 40 PVC? what would be a safe air pressure?

[ilpug]

If you used schedule 40 for all the parts, and assembled them correctly, a safe pressure would be about 80 PSI. That said, you can go much higher. A good rule of thumb is to take the pressure rating of the weakest pipe in the system (usually printed on the side) and divide that by 2 to get your maximium pressure

[waycoobeens]

Where can you find that tire valve...honestly thats the only part i cannot find...the rest i just got from home depot

[ilpug]

Old bike tires.

[zgr95]

I actually found a gauge and tire valve in one i use at a plumbing supply house

[bob the builder #1]

at advanced auto or other autoparts store.

[codongolev]

a tire seriously, I got an old tire and ripped it out

[u dont know me]

I got mine at Ace hardware.(basically Ben Franklins)

[Blardo]

A friend and I decided to build one of slightly different design, which also required a tire valve. We obtained ours by cannibalizing and old, broken BMX bike's front tire.

[Hermes]

Try a car place like pep boy's. Also, you may want to check the lawn mower section at Home Depot as they may have some valves. ~Hermes

[KNEX BUILDING IS FUN]

how much does this cost on average? also wat would i need to do to make the barrel bigger, would i have to chance anthing else besides the size of the barrel? also if i use this thing with about 15 psi could i hold it whiile i fire it?

[KNEX BUILDING IS FUN]

by bigger i mean diameter

[ilpug]

Technically, you could hold this while you fired it at 100+ PSI. The design of these guns is such that it makes firing while holding a bit awkward.

[tulavatalo]

will this work with bigger pipe and bigger valve?

[Lance Mt.]

 Maybe. You'd need a bigger Air Chamber though. I'd suggest bigger than the scale your increasing it at (ie, if you double the barrel diameter, 2.5x the Air Chamber.)

[ilpug]

Note that with bigger pipe, the pressure rating decreases.

[de0509]

Hello bro. the target here is consistency right? that means something needs to be done with the trigger because opening it manually doesnt always open the valve at a constant rate. my country doesnt see those sprinkler valves, so i designed an idea to just use rubber band and some ropes. it also uses moment of force or some would say torque. check it out

http://www.instructables.com/id/Making-a-homemade-airgun-trigger-with-metal-ball-v-1/

[de0509]

EDIT: but its an idea though. have A levels coming up this month, its my final year so cant do much stuff other than study. you can also use the moment principles as an add on to the science fair booth, let people learn some actual science right?

[jamesbid123]

cool

[blazer101]

where is the cap you put the tire valve on?

[blazer101]

also, wont you need a PSI measurer

[dog digger]

All on the bike pump, air compressor etc

[jj.inc]

I always feel bad when people have to pump it up by hand, and not just hook it to an air compressor and have a fill time of 1 second +/- 0.5

[omnibot]

Don't, that's good exercise :D

[jj.inc]

Lol, at least get a foot pump, I have one of them and it works much better.

[tulavatalo]

Help i pumped upp the barrel with a projectail to over 80psi and it din't move att all