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Punkin' Chunkin' 2010...

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Unread postAuthor: jeepkahn » Thu Jul 30, 2009 4:38 pm

That's one of the challenges is variations in punkin "stability", one my grenade at 140psi, one may take 500+....

I paid close attention to gauges on chambers when watching videos from the contest.....

Another thing to think about, when dealing with a fleshy veggie, what does more damage, a stream of 100psi air hitting a small portion of the punkin, or hitting the whole thing with 300psi??? a butterfly valve may actually be causing a skinning effect basically peeling the punkin...

Punkins are a lot stronger on the stem end than the blossom end(almost all veggies and fruits are) so that could come into play as well...

No matter what.......

IT"S GOING TO BE FUN!!!! :twisted:
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Unread postAuthor: Technician1002 » Thu Jul 30, 2009 5:40 pm

jeepkahn wrote:That's one of the challenges is variations in punkin "stability", one my grenade at 140psi, one may take 500+....

I paid close attention to gauges on chambers when watching videos from the contest.....

Another thing to think about, when dealing with a fleshy veggie, what does more damage, a stream of 100psi air hitting a small portion of the punkin, or hitting the whole thing with 300psi??? a butterfly valve may actually be causing a skinning effect basically peeling the punkin...

Punkins are a lot stronger on the stem end than the blossom end(almost all veggies and fruits are) so that could come into play as well...

No matter what.......

IT"S GOING TO BE FUN!!!! :twisted:


I know the variability.. 80 to 100 PSI is the varibility range for apples.
One of my muzzle blast pics is highlighted by an apple failing in the barrel.

For the flow issue from a butterfly valve, some chunkers use dead space to buffer this. May be worth a shot..

Apples show the same failure points. Loading flower to the chamber results in more apple pie for what it is worth.. Good observation.

I am in full agreement on IT'S GOING TO BE FUN!!!!
:twisted:
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Attachments
PUMPKIN_1_004a.jpg
It's nice when you can be this close to a pumpkin cannon.

Photo credit.. Photo from Roberts High School.
3 frames.jpg
Mist from failing apple exits first.
3 frames.jpg (24.04 KiB) Viewed 562 times
Apple exits.jpg
The bulk of the apple exits later. Many of the smaller pieces are way ahead of the bulk of the apple.
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Unread postAuthor: jackssmirkingrevenge » Thu Jul 30, 2009 7:51 pm

I'm not sure if this would be considered legal, what if the compressed air drove a piston which in turn physically pushed the pumpkin, in the same way a nailgun works?.

Technically it's not a sabot because you're effectivly building a pneumatic catapult.
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Unread postAuthor: POLAND_SPUD » Thu Jul 30, 2009 8:31 pm

I don't think it would be considered legal... but if it was then one could use a piston attached via a wire to the rest of the gun...

it wouldn't be technically a sabot as it wouldn't leave the barrel... I am referring to the idea that has recently been discussed I am sure you remember that thread, don't you?


but as it wasn't an easy task on a smaller scale then it's going to be even harder on a pumpkin launcher
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Unread postAuthor: rp181 » Thu Jul 30, 2009 10:15 pm

Do you supply your own pumpkins? You could grow a pumpkin in a enclosed space, so it takes on that shape. It was done in a box, for a cube pumpkin =)
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Unread postAuthor: jmeyer1022 » Thu Jul 30, 2009 10:23 pm

Never knew you cannot go above what the piece is rated. Just make sure everything is rated to 5000 PSI, get a good valve, good C:B ratio, and a scuba tank. Maybe try to induce some backspin too, not sure if that'll work the same as it does on golf balls, but who knows =)

If only pumpkins wouldn't be destroyed by the blast of 5k PSI, sigh.
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Unread postAuthor: D_Hall » Thu Jul 30, 2009 11:14 pm

jmeyer1022 wrote:Never knew you cannot go above what the piece is rated. Just make sure everything is rated to 5000 PSI,

LOL... Have you priced 6" pipe in the 5000 psi range lately?
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Unread postAuthor: Technician1002 » Thu Jul 30, 2009 11:17 pm

D_Hall wrote:
jmeyer1022 wrote:Never knew you cannot go above what the piece is rated. Just make sure everything is rated to 5000 PSI,

LOL... Have you priced 6" pipe in the 5000 psi range lately?


Try 10 inch. :shock:
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Unread postAuthor: D_Hall » Thu Jul 30, 2009 11:24 pm

Well, yeah, but I *have* priced 6" recently.
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Unread postAuthor: rcman50166 » Fri Jul 31, 2009 12:58 am

My ASME team might consider registering as a side project. It seems the contest is essentially longest barrel and highest pressure (to an extent) wins. It's all an issue of money, time, and space to build. Is there any evidence of a pumpkin gun using anything else than a butterfly/ball valve? It would seem like it would have the leading edge to victory.
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Unread postAuthor: jeepkahn » Fri Jul 31, 2009 8:55 am

Funny enough, because the barrel is consider an "exhaust stack" the barrel does not have to be pressure rated to the same standards as pressure vessels... Pumpkins have to be "natural state" no shaping by growth, I am calling today to find out if you supply your own or if they have a patch where you pick from... If you bring your own It'll be a good thing, because with my wifes farming background there are ways to produce harder/thicker rinded pumpkins through use of certain fertilizers, water depravation at certain stages of growth, and a few other "methods" :twisted: ...

And depending on barrel length/sag backspin will prolly be hard not to get...
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Unread postAuthor: inonickname » Fri Jul 31, 2009 9:34 am

Butterfly and ball valves are acceptable in punkin chunkin due to the massive barrel. The huge flow you get, along with them being easy to get (and rated) makes them ideal, especially when actuated by a ram or similar.

Though there would be no issue with maximum power, burst disk or perhaps a QDV..

You could use multiple smaller pressure tanks, welded to a barrel section with the disc or QDV... Or if you can get an LPG tank from a service station..make a co-ax :twisted:

I'm sure you can figure something evil..I'm looking at the potential of a QDV with 1000+ Psi at the moment..
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Unread postAuthor: Ragnarok » Fri Jul 31, 2009 9:37 am

I don't know whether it's standard practise to use trajectory modelling in chunkin', but I dare say that throwing some computing power at figuring ideal pumpkin weight (within the set limits of course) and launch angle wouldn't hurt your chances - in a slightly more serious calculator than the GGDT one, of course.

I'm not exactly sure what the drag coefficient of a pumpkin is though.
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Unread postAuthor: jackssmirkingrevenge » Fri Jul 31, 2009 11:07 am

Ragnarok wrote:I'm not exactly sure what the drag coefficient of a pumpkin is though.


Given the uneven nature of pumpkins and the unpredictable nature of their flight, I think this is next to impossible.

In terms of design, it seems you need to find the maximum pressure/flow at which you can start accelerating the pumpkin and then build the longest possible barrel from there. I'm sure that premature fragmentation is mostly a result of excess acceleration, not velocity.
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Unread postAuthor: jeepkahn » Fri Jul 31, 2009 11:34 am

Ragnarok wrote:I don't know whether it's standard practise to use trajectory modelling in chunkin', but I dare say that throwing some computing power at figuring ideal pumpkin weight (within the set limits of course) and launch angle wouldn't hurt your chances - in a slightly more serious calculator than the GGDT one, of course.

I'm not exactly sure what the drag coefficient of a pumpkin is though.


prollyin the .5 range of Cd maybe lower given the less than smooth texture...

I guess you've got a year to finish your's , huh... given the volume of the pilot, maybe port the pilot into the barrel behind the punkin to act as a cushion for the chamber volume... To get the ball rolling so to speak...
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