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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: jackssmirkingrevenge » Sat Aug 01, 2009 4:00 pm

Why heat the air when you can just use higher pressure?
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Unread postAuthor: jonnyboy » Sat Aug 01, 2009 4:06 pm

jackssmirkingrevenge wrote:Why heat the air when you can just use higher pressure?


I think it's because heating a gas raises it speed of sound so higher velocity's can be achieved. Without higher pressure, all of the stuff as to stay within the safety limits so lesser rated pipe is cheaper.

I read the rules why don't you just use a propane heater?
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Unread postAuthor: Ragnarok » Sat Aug 01, 2009 5:50 pm

jeepkahn wrote:I did, BTW, link to the rules on an earlier post... :wink:

I am aware. I did look and didn't see anything, but I assumed you had a greater degree of familiarity with them than I did, so for that reason, I deferred to your knowledge.

jackssmirkingrevenge wrote:Why heat the air when you can just use higher pressure?

It is actually a perfectly valid suggestion.

Assuming that either pressure or acceleration exceeding certain levels are responsible for "pie", then a heated gas can keep acceleration up later in the barrel, without raising pressure or acceleration beyond those "pie" limits earlier in the barrel.
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Unread postAuthor: KineticAmbitions » Sun Aug 02, 2009 1:50 pm

A 20 foot long barrel is not going to be successful here - the ranges of the competitive launchers suggest trans-sonic muzzle speeds. Reaching those speeds in a 20 foot long barrel would require accelerations which would surely smash the pumpkin. Hence, the extremely long barrels used by the serious competitors.

I'm not trying to cook the pumpkin, just trying to raise internal sos...


In the scenario I gave, where the gas temperature is 520K, the SOS in the chamber would be 460m/s. If you could raise the temperature in the chamber to the boiling point of water, the internal SOS would be about 390m/s, or 1270fps. With proper design of the other parts, correct firing angle, and a good punkin, this would likely be sufficient for competitive results. Higher temperatures are, of course, better, unless there is enough blowby for the heated gases to erode your projectile.

Basically, the reason that you need to heat the air is that for competitive results, your projectile will be trans-sonic, or even supersonic when it leaves the barrel of the cannon. The maximum ranges obtained by the winners seem to show that quite clearly.

Also, I would suggest an 8" instead of 10" barrel, for the simple reason that if you have a projectile which is at the weight limit of 10 pounds, an 8" punkin presents less surface area for air resistance than does a 10" one. This would depend on how dense you can grow your projectiles ( :lol: ), but the smallest diameter barrel you can get away with while maintaining maximum projectile mass should be optimum.
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Unread postAuthor: ramses » Mon Aug 03, 2009 9:59 am

KineticAmbitions wrote:Also, I would suggest an 8" instead of 10" barrel, for the simple reason that if you have a projectile which is at the weight limit of 10 pounds, an 8" punkin presents less surface area for air resistance than does a 10" one. This would depend on how dense you can grow your projectiles ( :lol: ), but the smallest diameter barrel you can get away with while maintaining maximum projectile mass should be optimum.


but it would also reduce the force on the projectile at a given pressure. I would still go with 8", and use the extra money you saved to add to the length of the barrel.

I would like to point out that air compressors tend to heat the air they compress enough to merit a "warning, hot surface" label on the tank.
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Unread postAuthor: jeepkahn » Tue Aug 04, 2009 8:48 am

We may go with a longer barrel(30-36ft, but as I said in an earlier post, the World record was set with a relatively short barrel compared to most of the competitors... And 8" will prolly be the barrel size....

Another interesting thing I noticed when reviewing videos this weekend, is that they seem to be firing at more than optimal angles, 45+ degrees, which may be because there are serious air currents at 600+ft..., or maybe they subscribe to the fallacy that 45 degrees is the best angle...
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Unread postAuthor: Technician1002 » Tue Aug 04, 2009 8:56 am

jeepkahn wrote:Another interesting thing I noticed when reviewing videos this weekend, is that they seem to be firing at more than optimal angles, 45+ degrees, which may be because there are serious air currents at 600+ft..., or maybe they subscribe to the fallacy that 45 degrees is the best angle...


This is most likely an illusion. Many shots from behind the cannons make the barrel angle appear higher than it really is. Keep looking. Find photos taken directly down the line of fire. They are way under 45 degrees.

Shots from behind and with telephoto to try to catch just the pumpkin and muzzle blast will appear to be nearly straight up.
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Side view of air cannons.
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Unread postAuthor: Ragnarok » Wed Aug 05, 2009 4:16 am

jeepkahn wrote:... which may be because there are serious air currents at 600+ft...

Although possible, that's only of use if the currents are in the right direction.

However, air does reduce in density with altitude, reducing drag somewhat. This can actually make ideal launch angles in excess of 45 degrees exist, however, it only really occurs at very high altitudes.

Mostly it just nudges up the ideal launch angle a tiny fraction over what it is if modelled with air of fixed density.
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Unread postAuthor: DinerKid » Wed Aug 05, 2009 6:57 am

i know you said that you get three minutes to fill and fire. can you fill from like a scuba tank or do you need to use a compressor?
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Unread postAuthor: jackssmirkingrevenge » Wed Aug 05, 2009 7:07 am

Technician1002 wrote:Find photos taken directly down the line of fire. They are way under 45 degrees.


Fork in hell, they have the bleedin' Paris Gun in the lineup!
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Unread postAuthor: inonickname » Wed Aug 05, 2009 7:14 am

jackssmirkingrevenge wrote:
Technician1002 wrote:Find photos taken directly down the line of fire. They are way under 45 degrees.


Fork in hell, they have the bleedin' Paris Gun in the lineup!


The gun was capable of hurling a 94 kilogram (210 lb) shell to a range of 130 kilometres (81 miles) and a maximum altitude of 40 kilometres (25 miles) — the greatest height reached by a human-made projectile until the first successful V-2 flight test in October 1942.

At the start of its 170-second trajectory, each shell from the Paris Gun reached a speed of 1,600 metres per second (5,200 ft/s).


Dear god, it sounds like something Larda would build.

It's accuracy seems similar to other German things.. (think V1), with the accuracy being say..almost likely to hit a city..
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Unread postAuthor: jackssmirkingrevenge » Wed Aug 05, 2009 7:35 am

inonickname wrote:It's accuracy seems similar to other German things.. (think V1), with the accuracy being say..almost likely to hit a city..


Don't knock it, the thing was firing into the stratosphere for the first time, I think that they managed to target Paris reliably 130 kilometres away is a pretty good achievement, at a time when most people didn't own cars and the aviation industry was a series of wood-and-fabric kites.

These days we tend to take technological marvels like the EX-171 forgranted but you have to look at weapons like the Paris Gun in the context of the time it was made.

Considering its guidance system and the brilliant simplicity of its construction and powerplant, the V1 was also an excellent weapon - if it wasn't the Allies would not have deditacted so many resources to its destruction. The same goes for the V2.
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Unread postAuthor: inonickname » Wed Aug 05, 2009 7:54 am

The V1 was no poor weapon- but it had nothing on the V2. The V1 used a pulsejet for propulsion, and the reed valves would often fail before the bomb reached it's location. I can't recall the figures- but the amount that failed en route is staggering. The V2 was a much more effective weapon.

Though that they can hit a target from that far- and with such altitudes that the coriolis effect had to be compensated for is impressive.

Though satellite, laser and so on guided missiles can hardly be used for comparison..
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Unread postAuthor: Ragnarok » Wed Aug 05, 2009 6:35 pm

jackssmirkingrevenge wrote:I think that they managed to target Paris reliably 130 kilometres away is a pretty good achievement, at a time when most people didn't own cars and the aviation industry was a series of wood-and-fabric kites.

Ballistically, it's impressive, even today - punting anything 75 miles in unpowered flight is no small feat!
As far as practicality and efficiency, not so much. They killed an average of less than one person with each shell (admittedly, they also wounded an average of two).

Actually, I'm glad you reminded me about this though, because I was looking into it recently. Although clear figures on the Paris gun are a little hard to find, most can be estimated pretty well - so I was fiddling about with modelling it in the range calculator, because it's a wonderful mix of complicated factors: Coriolis, Earth curvature, very high velocities, atmospheric thinning, rifling, exceptionally long hang times...

So, all in all, a good test of simulator accuracy, and great fun for a total geek like me.
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Unread postAuthor: cdheller » Wed Aug 05, 2009 8:17 pm

soo ,anything about freezing the punkin in the rules
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