These sorts of failures are usually caused by people constructing the potato cannon out of thinner schedule 40 PVC (usually colored white). If you use schedule 80 PVC (which tends to be dark gray) or ABS (which deforms instead of shattering), the risk is negligible.
This is also why I prefer pneumatic cannons to combustion. Since there's not a quick spike in pressure from a combustion reaction, the maximum pressure that needs to be contained is lower (and you can measure it with a gauge as you're pumping the air tank up). Also if you make the air tank big enough, the pressure behind the projectile stays high for longer, so you tend to get higher velocities.
When I was in high school I made a potato cannon, played around with it in the woods with some friends, and then told my parents about it. My mom wasn't too happy, but my dad took it as a challenge. He built an even more powerful pneumatic version with a tripod, sights, and a solenoid trigger mechanism. The whole family loved it. I used some extra PVC pipe to make molds for cement slugs. With a little paper wadding to get a good seal, they'd go fast enough to embed themselves a couple feet into the side of a hill. It was spectacular.
Yea, Home Depot PVC doesn't belong anywhere near compressed air. I knew a guy who wanted to outfit his garage with several compressed air drops for air tools and used plain old white PVC. Big mistake which he realized when one of the fittings shattered and launched a metal quick-disconnect through the garage door and out into the street. Thankfully nobody was in the line of fire. Please don't use PVC for air under pressure.
Id love to make more potato cannons but my main issue is exactly this. I have no idea what the pressures are, what materials would work, etc. It's all folklore and bravery
Not really.. you can find standard ratings for pipes all over the place. I just ran air lines in my garage and it was fairly simple for me to get online and see that I needed type m copper based on the pressure I'm at. Wasn't cheap, but neither is aluminum pipe and now i have a nice reel in the center of the garage. There's a bit more to calculating the pressure when exploding hairspray in an enclosed vessel, but I'd be willing to bet just as easy information to find.
I worked in a PCB shop that had a network of 1 1/2" PVC pipe for compressed air. When Tees failed we would lose production for the remainder of the day. We finally replaced the entire network with type M copper.
Well there's two of many possible outcomes, which are covered under your "depending on the circumstances", which make your comment universally true, and therefore not so informative:
1. We strengthen the pressure vessel enough so that it gives way at higher pressure, making a "stronger bomb"
2. We strengthen the pressure vessel enough so that it doesn't give way, making it not a bomb at all.
It's not at all clear if the duct tape will do 1 or 2 or something else, and this first-order logic / first-impressions design doesn't smell to me like a rigorous solution.
So I'm aware that wrapping a pressure vessel in tape has the possibility of making things worse.
I thought, for safety reasons, it was worth replying to the comment suggesting to wrap "a ton of" tape around it. At the time I replied there were no other replies.
Not trying to win a debate with you. Just trying to share that it could be a bad idea. Rather than just assuming that making the shards less sharp or fragmented was the only possible outcome.
Ours in highschool was gas powered. We worked out the ratio for a good burn and found a similarly sized metal scoop. Once loaded we'd shake it until the gasoline was fully vaporized.
It was terrifyingly loud. We'd wrap stones in duct tape, spray them with oil, then shoot them through sheet metal. Put a few through some old steel chairs. Tore the cover off some golf balls.
I suspect it may have reached higher pressures than my bike tires, but I never really considered trying to measure it.
Butane and propane can easily produce > 100 psi when used in engines, and are commonly used as replacement for gasoline and other hydrocarbons in generators and fork lifts to do just that.
In a confined enclosure (potato gets stuck or the like), it’s a pipe bomb.
by engines I'm assuming you mean something with a stroking piston which is going to reduce the total volume by somewhere from 4:1 to 13:1, so that's going to increase major pis increases by itself even before explosion. You don't have compression with butane into a potato gun, just the efffects of the rapid combustion. So it wouldn't really be a fair comparison.
There is also some nice research on proper high explosive detonation of similar mixes when using proper detonators.
It requires rather extreme amount of willful ignorance to think lighting a mixture of oxygen and butane or propane inside a enclosed space is only going to be 30-40 psi maximum, instead of ‘thank god it only hit that maybe because the potato moved’.
You're asking for citations when your whole premise is based on a wet potato getting stuck in a smooth plastic pipe. And then the potato being a durable enough seal to create a bomb. That's goofy as hell.
No. I’m asking for citations when your assertion is that flammable hydrocarbons mixed with oxygen in an enclosed container and ignited can not exceed 30-40 psi, or the bursting force of said (often brittle) plastic enclosed container. Especially since garden variety Schedule 40 PVC in the sizes we’re talking about are rated (non-shock) for over 180 PSI and many into the 200+ PSI range but there are easy to find videos of them bursting in spud guns.
And as noted in the prior declassified paper, shock fronts from these mixtures can easily move in excess of 280 meters/second even unconfined in air, with the right mixes.
Goofy? Perhaps.
Life altering? Perhaps [https://youtu.be/KqstP9ics2A?si=Omtp8N7xPW91A5TU] - and yes, while many of those failures in the video are clearly due to improper glued joints (which is a big hint at the pressures involved, and way better than other kinds of failures!), many also involve the PVC shattering. Several in the first few minutes, actually.
I made a ton of potato guns as a kid, btw. Typically using propane. But I always respected the forces involved, because I wanted to not lose an eye, kill anyone, etc.
Looks like some enterprising MIT alumni + the air force academy did some internal ballistics reseaerch [https://arxiv.org/pdf/1305.0966].
Note - I highly recommend against using acetylene, which they did for some of the tests, as it will perform a proper super-sonic detonation [https://www.icheme.org/media/10611/iv-paper-08.pdf] under the right conditions and turn your potato cannon into an IED even if the projectile moves, or potentially even with no projectile at all. Per that paper - “It appears that Acetylene is unique in that it will propagate a detonation at initial pressures below those of which it is capable of sustaining deflagration”. I would have checked this paper which even more directly addresses the topic [https://link.springer.com/article/10.1007/BF02671917] (because I love this kind of thing, obviously), but $40. :(
So looks like as long as everything goes well, they are in the approximate range of 20 psi while driving the projectile, except for acetylene which hit a hair under 90 psi. They used stoichiometrically ‘perfect’ mixes.
My guess is the failures in the videos (and anecdotally) were driven by too lean fuel mixtures, easy to do in some conditions. But this paper [https://www.scientificbulletin.upb.ro/rev_docs_arhiva/full14...] seems to indicate the opposite, and also pressures >= 120 PSI. So I don’t know.
I also saw MAPP gas apparently being used in one of them too (the yellow cylinder attached to a plumbing torch), which would also be unwise.
> You don't have compression with butane into a potato gun
Maybe not yours, but we tapped ours. This held the potato near the chamber after compressing the gasoline-air mix. I have no idea how well it held, but if you didn't seat the potato into the threads it would push the broom back out. You'd hear a poor seal leaking.
This is also why I prefer pneumatic cannons to combustion. Since there's not a quick spike in pressure from a combustion reaction, the maximum pressure that needs to be contained is lower (and you can measure it with a gauge as you're pumping the air tank up). Also if you make the air tank big enough, the pressure behind the projectile stays high for longer, so you tend to get higher velocities.
When I was in high school I made a potato cannon, played around with it in the woods with some friends, and then told my parents about it. My mom wasn't too happy, but my dad took it as a challenge. He built an even more powerful pneumatic version with a tripod, sights, and a solenoid trigger mechanism. The whole family loved it. I used some extra PVC pipe to make molds for cement slugs. With a little paper wadding to get a good seal, they'd go fast enough to embed themselves a couple feet into the side of a hill. It was spectacular.