Taking examples from nature: hydropneumatic spuds

A place for general potato gun questions and discussions.
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Cthulhu
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Sun Feb 02, 2020 7:16 pm

So my college education and personal interests have started making me get into mycology, and I read about a type of mold called pilobolus which fires it's sporangium at astounding rates of velocity by pressurizing water into a vesicle until the connection between the vesicle and the sporangium pops and sends the little packet flying up to 6 feet away (pretty far for a microscopic fungus!)

It kind of made me think of a reverse water rocket, where rather than the projectile using water as a medium to produce thrust, a high pressure beam of water is used to propel a projectile. I know that this works for pilobolus because it's so small and the viscosity of water is different at it's scale, but I'm curious as to what everyone thinks about a spud gun using this concept! Could perhaps a pressure washer be fitted with a latex tube with a marble shoved in the end work in a similar way? How could I try and predict the performance of what is essentially a hydraulic potato cannon?
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mrfoo
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Mon Feb 03, 2020 3:08 am

Viscosity is going to be the killer in this. You'd need something far less viscous - liquid butane or isopropanol are probably the most easily accessible low-viscosity fluids, not to mention that they are *ahem* flammable. The other option would be to vapourise a quantity of fluid, in the same way that inkjet printers work (or, if you're fixated on nature, it's the way the bombardier beetle shoots jets of boiling piss at its agressors), but, again, scale is probably against you.

Pressure washers, however, are very interesting, given that they push out around 100 bar. Water over gas compressor, anyone?
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Cthulhu
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Mon Feb 03, 2020 4:51 pm

I've never been good at physics but wouldn't you want a more viscous liquid so that it can impart more momentum to the projectile? Something like a syrupy liquid under high pressure could impart a lot of force
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Tue Feb 04, 2020 12:09 pm

Nah.

The big issue is compressibility. Water, in particular, can pretty much be considered incompressible at realisable pressures. So we put some water behind a projectile, somehow holding that in place, and raise the pressure of the water to 10, 20, 100, 400 bars, it pretty much doesn't matter. There will be a force on the projectile, to be sure, but within microseconds of us releasing the projectile and it starting to accelerate, there will be zero force exerted on it by the water, which will now be at ambient pressure. Worse, as the projectile moves down the barrel, it will be trying to pull a vacuum. It will behave in exactly the same way as a brake piston - it will move a very small distance, then stop. It's why we do hydrostatic testing of pressure vessels - as soon as there's a failure, the pressure is gone.

So the pressure needs to come from something else. Either from a force exerted by the (elastic) walls of the container, or from some external pump. Or, much like in a "water rocket", some compressed air, but in that case why not just use the compressed air itself (in a water rocket, the water is reaction mass, not "fuel" as such).
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