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Fastest Potato Velocity?

A place to ask general spud cannon related questions.
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Unread postAuthor: joannaardway » Mon Apr 16, 2007 6:41 am

You might be surprised what could happen. At high enough speeds, things that normally are squishy and soft - tend to hold toghether a lot better on impact.

You'd probably lose a fair chunk of wall.
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Unread postAuthor: jackssmirkingrevenge » Mon Apr 16, 2007 11:45 am

joannaardway wrote:Assuming 2" long potato slug (for these purposes, the exact barrel ID and mass do not matter - the area [and thus the force from the pressure] and mass are in direct proportion), then an average pressure of about 16 bar (230 psi) would be needed.


I played around with GGDT with the correct barrel specifications, impossibly huge chamber and impossibly efficient valve and even with 50,000 psi it came nowhere close to taking a 10 gram projectile to 2,000 fps - of course perhaps GGDT isn't designed for these parameters, and we're talking about a hybrid here, not a pneumatic, but I remain very skeptical of the data presented.
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Unread postAuthor: Hotwired » Mon Apr 16, 2007 12:35 pm

Did you increase the temperature?
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Unread postAuthor: lukemc » Mon Apr 16, 2007 2:03 pm

i was convinced its posible by joannas post but for an easy test just move about 30 feet back so there could be no interruption by any thing besides the potato.
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Unread postAuthor: sandman » Mon Apr 16, 2007 2:25 pm

Killjoy wrote:Why put the paper in front? to prove that it was actually the potato that exited the barrel and traveled out and was picked up by the chrony?
I already know that the potato exited the barrel in one peice because i had a 1/4" thick piece of plywood behind the chrony and the potato nailed it dead center and left a hole that was exctly the size of the potato, so i have no doubt the potato actaully exited the barrel in one peice and was picked up by the chrony.



no, the paper is to block the gas from the cannon, gas does weird things to light
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Unread postAuthor: boilingleadbath » Mon Apr 16, 2007 2:33 pm

My understanding is that a 4x hybrid generally gives you around 400 PSI, so (applying 1/(B:C ratio + 1)*400) we get about 226 PSI (final).
That's inaccurate (how inaccurate? I'm too lazy to check), but it gives you an idea... that the average pressure is probably pretty close to the average pressure you'd need to get 2700 fps out of this gun, as calculated by Joanna.

However, Joanna fails to acknowledge the rather massive amount of gasses involved in this:
80 grams of air originally in the barrel
430 grams of fuel-air mixture originally in the chamber

...and these need to be propelled too.
Now, of course, the 80 grams of air was expelled at an average speed of about 1/2 of the projectile's, so we only count half of it.
Furthermore, 60% of the former fuel-air mixture is in the chamber when the projectile exits, which we count it as 1/18th it's mass because of it's lower velocity, and the other 40% is moving at something like .7 (average) times the velocity of the projectile, so we only count 70% of that.
Add this up, and we get 170 effective grams, or so, of gas... which is not ignorable.
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Unread postAuthor: noname » Mon Apr 16, 2007 10:24 pm

As another shred of evidence, I plugged the numbers into EVBEC Live V1.5 with an 80 gram projectile, and got a velocity of about 2400 fps. As BLB said earlier, he hadn't tested hybrids, really high velocities, and barrels with long dwell times.
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