The world of warfare was revolutionized by the development of black powder, fireworks, cannons, and the like. You don’t need any of that chemical nonsense to just have fun, though, as this compressed air cannon from [OtisLiu153] demonstrates.
The build uses PVC pipes for both the barrel and the air tank. In the case of the latter, avoiding over-pressurization is key to avoiding injury, though some will say you should simply never build a PVC pipe pressure vessel at all. In this case, [OtisLiu153] strictly recommends 150 psi as a limit, which is nicely within the 280 PSI rating of the 2″ Schedule 40 PVC being used. Though, as they note, the connections in the design aren’t necessarily up to the same rating.
Off-the-shelf couplings are used to piece everything together, with the twin-reservoir design also acting as a useful shoulder mount. Charging the cannon is done via a Schrader valve, as you might find on a bike’s inner tube, and firing is achieved via a ball valve.
Of course, if you build such an air cannon yourself, just be careful with your aim. Video after the break.
Thanks for digging up the pumpkin 🎃 cannon story!
I guess generically, it is an “air cannon”. But more commonly referred as a Potato or Spud Gun! Lots of videos on this topic showing the fun and also trouble that can happen for those that like a bit of mischief. Why not suggest Schedule 80 pipe due to thicker walls? Good call on the air sprinkler valve… quickly releasing the air is a key design requirement. Have fun and stay safe!
Wondering at least 2 things, what the muzzle energy is and what category of gun it fits into (if any).
In the US air rifles are essentially unregulated in that there are no license requirements to own or use one (hunting rules are different and highly variable). Some cities have specific ordinances though about BB guns, sling shots etc. You do have to be 18 to buy one though.
As to the muzzle energy- this is fun. No apologies for mixed units.
Assuming a 2″ diameter barrel that is a 1″ radius which is, rounding, 2.5 cm. Area of that would be pi*2.5^2 ~~20cm^2. Assuming same height of the projectile as the diameter, that gives a volume of about 100cm^3, or 100 mL. If you guesss that a potato is 75% water, that would be a ~75 g projectile.
Using the SWAG method to determine muzzle velocity:
~950-1000 fps is speed of sound and no way is this thing firing supersonic.
A cruddy BB gun fires a tiny projectile at ~700 fps (~6 ft-lb energy)
A wristrocket slingshot about 150-200fps (realllly roughly 10 ft-lb energy depending on ammo type).
So let’s say 400 fps for our potato hunk. Astonishingly, that works out to 400 FPE! That is quite a bit! About as much as a 9mm pistol round.
Of course energy is exponential to velocity so at
200 fps –> 100 FPE
300 fps –> 230 FPE
450 fps –> 520 FPE
I love you people that just throw Metric and Imperial systems together.
THANKS! We’re like bilinguals. Although I personally hate the STUPID imperial system, in the US we use both metric and imperial although the general public uses the latter nearly 100%. For those in technical fields, metric is used. Unfortunately, sometimes there is confusion:
Mars Climate Orbiter
https://en.wikipedia.org/wiki/Mars_Climate_Orbiter
The spacecraft encountered Mars on a trajectory that brought it too close to the planet, and it was either destroyed in the atmosphere or escaped the planet’s vicinity and entered an orbit around the Sun. An investigation attributed the failure to a measurement mismatch between two software systems: SI units (metric) by NASA and US customary units by spacecraft builder Lockheed Martin.
OOPS!
Something else that has around that amount of energy is a man jogging. (roughly 200lb, 8mph). Jogging into someone, apart from the nonzero possibility of knocking them over into a pit of snakes, is a lot less of a problem than shooting them.
Lower PSI
Okay, so perhaps he carries a dowel rod while jogging. It still doesn’t have the same result as a supersonic projectile of equal energy and starting cross section. It can be tough to get high energy and low cross section without high velocity of course, but within that constraint and without being too specific…
There’s rough lines you can draw for speed of projectiles – usually modern stuff is broken into low/medium/high, but you can have lower than low if you include somewhat heavy projectiles like arrows or crossbow bolts or maybe some of the things powered by compressed air (though compressed air can reach higher than this). The slowest aren’t that much different from sticking a pointy thing into somewhere. Subsonic projectiles can still bounce or turn sideways and such, but at low enough speeds, many shapes and materials won’t deform that much or cause much fragmenting in a soft target material. Mostly you’d expect them to just go in until they run out of speed or run out of target. Mildly supersonic projectiles, or well designed slightly subsonic ones, can definitely have significantly more effect than just making a hole the similar to their starting diameter. For instance, they can expend energy making their cross section larger, but they also can just shove material out of the way fast enough that the material that was shoved will itself propagate damage. Then there’s significantly supersonic projectiles, a popular example being 5.7. Admittedly you’re not going to expect equal pressure in general for this realm of projectile. Also, while these can be convenient for making their way through things in the way of the target, that is something you can do just with sufficient surplus energy and a hard enough pointy thing. But they also do more of the “shoving damage” as I’ve apparently renamed it, even without splitting apart or anything. Things act more like fluids at high enough speeds. The relative hardness of projectile and target will of course matter. Impacts to spacecraft are even higher speed; a paint fleck can vaporize things. Extremely high impact speeds can resemble explosions, in how most materials react. In terms of temperatures too, from all the dissipated energy.
It doesn’t really work like that though. I can very easily fire a 9mm pistol with one hand whereas I certainly could not stop a man jogging into me with one hand out.
That’s my point! The result of all these equal-energy collisions is so widely different that it is not a good basis on which to evaluate their danger.
In New Jersey they are considered firearms and if capable of firing projectiles greater than 60 caliber are also destructive devices.
f. “Firearm” means any handgun, rifle, shotgun, machine gun, automatic or semi-automatic rifle, or any gun, device or instrument in the nature of a weapon from which may be fired or ejected any solid projectable ball, slug, pellet, missile or bullet, or any gas, vapor or other noxious thing, by means of a cartridge or shell or by the action of an explosive or the igniting of flammable or explosive substances. It shall also include, without limitation, any firearm which is in the nature of an air gun, spring gun or pistol or other weapon of a similar nature in which the propelling force is a spring, elastic band, carbon dioxide, compressed or other gas or vapor, air or compressed air, or is ignited by compressed air, and ejecting a bullet or missile smaller than three-eighths of an inch in diameter, with sufficient force to injure a person.
c. “Destructive device” means any device, instrument or object designed to explode or produce uncontrolled combustion, including:
(1) any explosive or incendiary bomb, mine or grenade;
(2) any rocket having a propellant charge of more than four ounces or any missile having an explosive or incendiary charge of more than one-quarter of an ounce;
(3) any weapon capable of firing a projectile of a caliber greater than 60 caliber, except a shotgun or shotgun ammunition generally recognized as suitable for sporting purposes;
Wow. I learned that NJ and Rhode Island consider BB guns “firearms.” That also includes paintball guns. Definitely do not get caught with a potato gun in NJ! From my non-lawyer read, you would be guilty of at least illegal possession of a firearm and also a destructive device (2.5 cm caliber is biiiiggg) plus probably a bunch of other stuff.
The NJ laws are so broad and the phrase “with sufficient force to injure a person” so open to interpretation that a drinking straw could be considered a firearm if used to blow spit wads.
Ok. Odd that for air rifles they include a minimum size of 3/8″ for the projectile. There are big bore .50 hunting air rifles that pack a whopping 600-900FPE. Since that is above the minimum size and below the max size of 60cal, it might be a loophole. Not that I’d ever want to try. Or go to Mew New Jersey for any reason anyway (oh snap)
PVC fails in interesting and unfortunate ways when pressurized. You are basically cozying up to a fragmentation grenade. Have fun.
The impact resistance of PVC plastics – without special additives – drops sharply below room temperature. It turns brittle. What works fine on one summer evening may explode in your face come fall.
It’s a bit hard to find relevant graphs now that search engine optimization has ruined the game, but here’s one:
https://www.researchgate.net/figure/Ductile-and-brittle-area-for-PVC-and-PET-polymers_fig11_282602035
It’s a bit hard to tell by the graph, but in general you lose about 90% of the impact resistance of PVC when you go from 23 C to 0 C, and it gets worse if you have any scratches or dents in the plastic already because those act as stress risers. You also have to remember that discharging the pressure suddenly causes a strong temperature drop, so you can go well below ambient temperature in use. The first shot might be okay, but when you go to re-pressurize from the tank, it blows up.
2″ Schedule 40 PVC pipe has a minimum burst pressure of 890 PSI at 23 C and a maximum operating pressure of 166 PSI. If you de-rate that by 90% to account for outdoors temperature down to freezing and/or the effect of scratches and dents, you get 89 PSI burst pressure. Much less than the 150 PSI suggested here.
Correction: 2″ Schedule 40 PVC fittings have this rating.
Point being, an engineer won’t sleep at night with a safety factor below about 1.5, because you know there will be circumstances that compromise that factor further down no matter what you do. If you have foreseeable conditions with a nominal safety factor below that, you simply refuse to do it.
If you want to go further, look up:
https://en.wikipedia.org/wiki/Stress_intensity_factor
https://en.wikipedia.org/wiki/Stress_concentration
Generally speaking, seemingly small amounts of “surface roughness” have a significant contribution. It’s not very difficult to lose 20-50% of your impact or fatigue resistance to these effects, which is why it’s generally wise to assume a safety factor of at least 1.5 for a minimum – and plan for the inevitable breakdown. For any serious work involving things that the public or the private consumer may use, it’s not uncommon to need a safety factor up to 5-10 by law.
As a plumber I was taught never to put air in pvc. Air escapes explosively while a liquid doesnt, which is what the psi rating is for — a liquid
Easy fix is to use ABS or PEX instead for compressed air, both will split and tear instead of shattering.
There is also some blue coated aluminum pipe specifically made for compressed air lines, one brand name is AIR NET by Atlas Copco, but there is another brand sold at northern tool for shop air systems.
Copper pipe is also considered a good alternative, usually available in larger sizes for commercial buildings under the DWV marking (drain, waste and vent) and if you’re not afraid to drop some coin, thick walled ACR is also available up to 2in.
Thanks
I prefer the old potato cannon, 3 vegetable cans taped together and a tonic can at the bottom with a hole at it’s base.
a little gas in the tonic can , load a potato or a tennis ball and hold a match to the hole …… good times in Island Falls Maine in 1976 😁
Yeah, the Anarchist Cookbook version. Sometimes it works, sometimes it doesn’t.
The good point is that the reason it fails is because of being fuel rich, so it’s very difficult to get it blow up in your face. People who build spud guns using electric ignition systems and camping gas etc. can dial in the right fuel mixture that goes off a little too well, and then it goes all over the place.
The trick to getting it to shoot right was jest a teaspoon of gas and shake the thing in a circular motion while holding it upright ….. didn’t know that when i made my first one and lets just say there was some singeing :)
I can tell you from my own experiments, it won’t light up unless it’s slightly rich, and it won’t go fast unless it’s slightly lean.
Needless to say, I figured out a way to ignite the fuel charge lean with a squirt of butane next to the spark gap. On the first try with that setup, the whole thing went to shreds.
We always used a flammable propellant spray can. Typically if we put up paint can marker nozzle on it, with a small blast back hole through the rear, we could quick shot spray in the chamber, and stick a camper lighter inside. Worked like a charm, and I never remember having a single one fail. We use those things out in the cold, in the heat Etc.
Broke a hell of a lot of camp lighters though.
I blew up my PVC potato cannon when I tried oxyacetylene in pursuit of moar power than butane. Most of it was wrapped in duct tape, and that’s probably what saved my eyesight (prescription glasses help too) but I had to pick a few shards of PVC out of my legs.
I’m in no position to give advice, but please be careful out there. Safety glasses!
>[OtisLiu153] strictly recommends 150 psi as a limit, which is nicely within the 280 PSI rating of the 2″ Schedule 40 PVC being used.
Please be more careful in your reporting:
>Also, although schedule 40 2″ PVC is rated for up to 280 PSI, the valve connections and PVC joints DRASTICALLY lower the pressure rating.
The actual rating for the fittings is 166 PSI, which is dangerously close to the 150 PSI recommended in the article, leaving almost no safety margin for actual wear and tear. These are parts that were never meant to be handled after installation, so the ratings do not even apply to the use case.
This uses a modified sprinkler/irrigation valve to fire. That’s not a ball valve.
Ball valves are terrible design for these. Quick exhaust is only option
I came for the safety natzees and am leaving satisfied.
Thank you all. I reapplied my helmet, pads and mouth-guard. Now I’m about to get back into my prepper shelter. Wish me luck.