SpudFiles

It's more or less official, my neighbor Rick and I are gonna enter the 2010 contest... We hope to have a design settled on by October, and the build completed by February(in time for my birthday), Registration deadline is June 2010...

Idea's and Sponsors are welcome, and If it's ok with PCGUY, we'll be flying a SF banner and it'll prolly be a Spud Related Name on the side of it...

There prolly won't be a lot of details of the build made until closer to the registration deadline(to prevent ideas from being stolen by the competition)...

Some constraints to be remembered when tossing ideas out:
1. must used compressed air(no other gases allowed).

2. no sabots/wadding/etc allowed.

3. build must meet astm and osha standards.

4. no combustion.

5. punkin must exit the barrel in one piece.

6. punkin must weigh between 8-10lbs.


I will say this, it will be a piston valved launcher and vacuum assist will prolly be used(more to maintain punkin integrity than for performance boost)...

Is there a limit on barrel length?

jeepkahn wrote:I will say this, it will be a piston valved launcher and vacuum assist will prolly be used(more to maintain punkin integrity than for performance boost)...

If you don't think the pumpkin will withstand the leading air in the barrel, what makes you think it will survive impact with and flight through the atmosphere?

Piston valves are pretty quick, but I'm almost positive they use modified butterfly valves in this competition. If you could design a reusable burst disk
where there is a split in the middle and the flaps blow out to the side but close back up after the shot. Piston valve will do fine I'm sure, just throwing out suggestions. Also I believe these can only go to 300 PSI. If your pumpkin is going to be 8-10 lbs. Let's consider it's going to be a big barrel
1.) Piston needs to be big
2.) Making something that's strong but light.

D_Hall wrote:

jeepkahn wrote:I will say this, it will be a piston valved launcher and vacuum assist will prolly be used(more to maintain punkin integrity than for performance boost)...

If you don't think the pumpkin will withstand the leading air in the barrel, what makes you think it will survive impact with and flight through the atmosphere?

evacuating the barrel will allow us to use more pressure than without evacuating the barrel(and according to ggdt it will add about 75fps vs no vacuum)... And on a large surface area object like a punkin, a vacuumed barrel will add 1020 lbs of force (that won't degrade with chamber/barrel pressure dropping) to the punkin...

theoretically , using ONLY vacuum on a 20ft barrel, the muzzle velocity would exceed 300fps, granted performance gain will be minimal, that type of suction force will greatly reduce the "percieved weight" of the punkin...

jeepkahn wrote:evacuating the barrel will allow us to use more pressure than without evacuating the barrel

Huh? You lost me here.

theoretically , using ONLY vacuum on a 20ft barrel, the muzzle velocity would exceed 300fps, granted performance gain will be minimal, that type of suction force will greatly reduce the "percieved weight" of the punkin...

Lesson 1: There's no such thing as "suction force". What normal people refer to as "suction" is really nothing more than air PUSHING on the other side of the object. In a vacuum gun, 100% of the propulsive force comes from the air entering the barrel on the breech side. There is 0 force on the vacuum side.

1. must used compressed air(no other gases allowed).

2. no sabots/wadding/etc allowed.

3. build must meet astm and osha standards.

4. no combustion.

5. punkin must exit the barrel in one piece.

6. punkin must weigh between 8-10lbs.

Rules sound a little gay.
except the fact the weight is same as a baby ,But that's just deranged.
Hope you Chunk a funkin' punkin' an win though!!!
WoohOo
How will you use "no wadding" and get the funkin chunkin punkin to exit in one piece and go far I wonder??? 8)

D_Hall wrote:

jeepkahn wrote:evacuating the barrel will allow us to use more pressure than without evacuating the barrel

Huh? You lost me here.

theoretically , using ONLY vacuum on a 20ft barrel, the muzzle velocity would exceed 300fps, granted performance gain will be minimal, that type of suction force will greatly reduce the "percieved weight" of the punkin...

Lesson 1: There's no such thing as "suction force". What normal people refer to as "suction" is really nothing more than air PUSHING on the other side of the object. In a vacuum gun, 100% of the propulsive force comes from the air entering the barrel on the breech side. There is 0 force on the vacuum side.

I understand the no such thing as suction(I meant to use quotations around that to denote the idea rather than the actuality)... as far as being able to increase pressure,because a punkin is a "sealed capsule" evacuating the barrel creates a pressure differance between the inside of the punkin and the outside, and theoretically enable it to withstand more pressure from the chamber side before crushing... but I'll have to test to make sure that it won't explode from the vacuum... Ever seen a marshmallow in a vacuum chamber???

the no wadding thing is going to be tough. can you cut the pumpkin so that it fits snug in the barrel as long as it is still 8-10 lbs?

better yet if you know what you are using for a barrel you can grow the pumpkin in a piece of that pipe and it should grow so that it is a nice fit in the barrel without cutting or anything. they do this with watermelons except in a box to make shipping easier.

There prolly won't be a lot of details of the build made until closer to the registration deadline(to prevent ideas from being stolen by the competition)...

that's a shame you know... it's good to have some outsiders that can criticise your design... I suggest using PMs


can you use frozen pumpkins ?
is there really a pressure limit ??

I agree that a burst disks seem like a good idea here... you might need to find a way to lower it's efficiency for a few milliseconds to avoid turning the pumpkin into a salad but I am sure you're aware of it..

good luck

jmeyer1022 wrote:Piston valves are pretty quick, but I'm almost positive they use modified butterfly valves in this competition. If you could design a reusable burst disk
where there is a split in the middle and the flaps blow out to the side but close back up after the shot. Piston valve will do fine I'm sure, just throwing out suggestions. Also I believe these can only go to 300 PSI. If your pumpkin is going to be 8-10 lbs. Let's consider it's going to be a big barrel
1.) Piston needs to be big
2.) Making something that's strong but light.

I missed this post earlier...

Yes, butterflys are the norm, but if the punkin can handle a faster valve then piston is the way to go... We are playing with the idea of using a "collapsing toolie", where the sealing face can fold after opening to reduce flow restriction... With the materials we have available even a 5-10lb piston will not be a problem(we have access to really big springs and really big metal for the chamber)...

The barrel will be 10" more than likely, length is to be decided(thats why I've been interested in terminal velocities in pipe and such) we hope to be able to use a "relatively" short barrel 20-24ft...

There's no pressure limit, BUT, you cannot exceed the rated pressure of any component....


http://www.punkinchunkin.com/rules.htm

so it looks like my idea of growing a pumpkin inside a piece of pipe the same as you barrel is ok.

I have been fascinated with this sport for years.

My 2 cents worth may be worth something. Due to the competition, many contestants don't reveal secrets.. They have too much invested... I don't blame them. However with digging.. the secrets are buried here and there..


Here are the secrets without revealing sources..

is there really a pressure limit ??

Yes.. The weakest link in most of these is the pumpkin. The limit appears to be about 30-35 PSI before pie in the sky.. Projectile must leave the barrel intact. The white pumpkins are stronger. Avoid ripe.. get green as possible.

Butterfly valves are common because huge forces break pistons.. There is little penalty in using a slightly slower valve at 35 PSI and a 10 LB projectile.. Model it in GGDT.

As for a vacuum.. Remember the pumpkin is hollow. It takes compression forces better than internal pressure.. Plan on adding a EQ port into the pumpkin to prevent pie in the barrel.

Low pressure and very high chamber volume is the norm for a long easy acceleration to high speed. Barrel friction on a 10 inch pipe is low.

Good luck. Would love to see the results.. and forget 50 PSI and above solutions. They make pie.

When you get close to one,, you find out..

The pumpkins used are a specially bred variety that are almost all rind...

The average pressure for the cannons last year was 250psi, some pushed 300 without PITS results...

using known parameters for some of last years competitors ggdt gives way low expected performance in the ballistics tool...

But even without GGDT it requires 1100fps+ with an 8-10lb sphere to attain over 3500ft, the record is 4483ft and that gun uses one of the shorter barrels in the contest....

jeepkahn wrote:The pumpkins used are a specially bred variety that are almost all rind...

The average pressure for the cannons last year was 250psi, some pushed 300 without PITS results...

using known parameters for some of last years competitors ggdt gives way low expected performance in the ballistics tool...

But even without GGDT it requires 1100fps+ with an 8-10lb sphere to attain over 3500ft, the record is 4483ft and that gun uses one of the shorter barrels in the contest....

It seems my data is outdated.. I last found info about 4 years ago.. I believe one of the competitors to the Q36 pumpkin modulator had some info online..

The high pressure you mention is often the compressor trailer pressure.. Do you have a reference to the new high pressure stuff?



Trying to find info most often brings up something like this..

Chunkin Up was fabricated circa 1997 and has gone through many modifications since then. The exact specifications are top secret.

Edit.. looked up pressure for pie in the sky.. Better results.. People hide what works but often tell what fails..
Open a valve. Watch the pumpkin soar.

At 140 psi, Nietubicz' pumpkin bursts inside the tube, emerging from the 65-foot barrel of his air cannon looking like the old Atari game "Asteroids."