4 ms·

> This is also why I prefer pneumatic cannons to combustion Strange, since combustion based cannons tend to reach about 30 to 40 psi at most, while pneumatic e

> This is also why I prefer pneumatic cannons to combustion Strange, since combustion based cannons tend to reach about 30 to 40 psi at most, while pneumatic easily can reach 100 psi.

lazide 2y ago
Cite? 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.
- grogenaut 2y ago
  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.
  - lazide 2y ago
    Here is a nice declassified paper on using butane and propane in unconfined fuel air explosives. [https://apps.dtic.mil/sti/tr/pdf/ADA084367.pdf https://apps.dtic.mil/sti/tr/pdf/ADA084367.pdf] Quite effectively too. 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’. [https://colab.ws/articles/10.1134/s0010508211010114 https://colab.ws/articles/10.1134/s0010508211010114] There was a good research paper I found that had a quantified set of numbers, but it required paying for it. Meh. Still waiting for that cite.
    - fkyoureadthedoc 2y ago
      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.
      - lazide 2y ago
        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. [https://parts.spearsmfg.com/sourcebook/SCH40TECH_40WHTPIPE-1_T_PVC40_T.pdf https://parts.spearsmfg.com/sourcebook/SCH40TECH_40WHTPIPE-1...] 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 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 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 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 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/full14656.pdf 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.
  - bobmcnamara 2y ago
    > 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.
    - lazide 2y ago
      I just want to say, this is gloriously crazy. Kudos.
bobmcnamara 2y ago
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.
- phreeza 2y ago
  That sounds pretty terrifying!

> This is also why I prefer pneumatic cannons to combustion Strange, since combustion based cannons tend to reach about 30 to 40 psi at most, while pneumatic easily can reach 100 psi.

lazide 2y ago

Cite? 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.

grogenaut 2y ago
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.
- lazide 2y ago
  Here is a nice declassified paper on using butane and propane in unconfined fuel air explosives. [https://apps.dtic.mil/sti/tr/pdf/ADA084367.pdf https://apps.dtic.mil/sti/tr/pdf/ADA084367.pdf] Quite effectively too. 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’. [https://colab.ws/articles/10.1134/s0010508211010114 https://colab.ws/articles/10.1134/s0010508211010114] There was a good research paper I found that had a quantified set of numbers, but it required paying for it. Meh. Still waiting for that cite.
  - fkyoureadthedoc 2y ago
    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.
    - lazide 2y ago
      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. [https://parts.spearsmfg.com/sourcebook/SCH40TECH_40WHTPIPE-1_T_PVC40_T.pdf https://parts.spearsmfg.com/sourcebook/SCH40TECH_40WHTPIPE-1...] 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 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 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 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 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/full14656.pdf 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.
- bobmcnamara 2y ago
  > 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.
  - lazide 2y ago
    I just want to say, this is gloriously crazy. Kudos.

bobmcnamara 2y ago

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.

phreeza 2y ago
That sounds pretty terrifying!