Determining Air Cannon Efficiency

Obviously pressure and volume is the potential energy of the system..the more the better! The arrangement of the pipework is what governs how quickly this energy is released ...

If you put a 2" restrictor in there it will really spoil the airflow... conversely, If you open up and smooth the flow as much as possible it will improve matters....

However it is a matter of degree. I don't s'pose the Y T will make a huge difference...maybe a few %. If it was a water flow rather than air the effect would be much more pronounced.

The speed of opening the butterfly valve may well be more significant.... not to mention projectile design, these should be a snug fit...discardable sabot?

Its like tuning up the air intake on an engine...

Sounds like fun...tell us more!

Are you spin stabilising your projectiles? what is their length/diameter ratio?

What is your shoe size?

What is the target?

Are you a terrorist?

Del

Hi again,

All good questions. I didn't want to get too specific because it's a rather silly application. It's a pumpkin cannon. So there is little consistency with our ammunition. We have shot a bucket of golf balls, a piece of firewood (which it split) watermelons, and just plain water. Anything that is 10 inches in diameter. Probably the most fun launch was a horizontal shot from about 30 feet. I'm not good at math but a 12 pound pumpkin at 120 psi into the door of an old Chevy pickup cab is very impressive. It made a mess but didn't go all the way through the door. Smashed it beyond use though. I've tried to get information from spud gun websites but haven't had much luck. I thought someone really good at math could just figure out the best way since it's just moving air. I realize that the projectile makes a big difference as we've seen huge variations in results by changing only the size, shape, or orientation in the breech.

It's mainly for competitions and parades but it's also a fun toy at summer bbqs. I think it's a good design but I'm sure it could be better. The two tanks were cut and butt weld flanges were put on each end. From there two 90 degree elbows feed into a single tee. An air powered cylinder opens the butterfly valve pretty quickly and down the barrel it goes. All 10 inch. It also has a muzzle break on the end but all it does is disperse water vapor from all the compressed air. The breech loader and the cylinder to raise and lower the barrel are both air powered too.

From what I'm understanding now it seems like a wye instead of a tee would only be a small gain. Is the gain in velocity or is it more volume because of less turbulence? or both? I'm also wondering if the two into one design is helping or hurting. The area of two 10 inch pipes is about the same as one 14 inch pipe. So is necking it down helping the air flow? I realize since the pumpkins are very imperfect that a lot of air is wasted and the pressure really drops fast but I don't know what to do about that. I don't see any way of keeping the pressure up as the pumpkin goes down the barrel. And I can't put too much pressure to it since anything over 80 runs the risk of blowing up the pumpkin. It just comes out in a big orange spray. 135 is the most I've ever shot one and had it remain intact.

I know the whole thing sounds pretty silly but it's just something we thought would be fun. The first one we built we had pretty good luck with. We live in Oregon and a farmer near Salem bought it after we disassembled the triggering mechanism. He devised his own, repainted it and named it "Duck Hunter". It's all orange and black. Not only for Halloween but also because OSU colors are orange and black. And the Ducks are rivals. Bottom line is this one is bigger and better and we just want to make it even better yet.

Thanks much,

Keith

I would suggest the variability in ammo is the main issue.

Maybe you could build a re-useable discardable sabot, it could be designed so it doesnt fly too far. it will give you much greater recovery of your power and better consistency and probably less pumpkin damage.

Something like a cut down plastic tub or bucket would do nicely, add some streamers to act as air brakes to slow it quickly once it leaves the muzzle... have a look at the plastic wads in a shotgun shell for inspiration.

Just don't point it at me ...!

Del

Could try cutting old blankets to make patches to wrap around the projectile to fill the void. Like the used on old flint locks patches around the bullets.

You're on to something there. You might want to turn your efforts to competition. There's an official "punkin' chunkin"" competition and web site. http://www.punkinchunkin.com/main.htm

Wow!! the ultimate air gun.... more of an air howitzer!!!

Come on tell us more....

John.

Sounds like a twin barrel t-shirt blaster to me! 1000ft to 3000ft! sure can chuck em a long way though!!

My question is I have never seen a 60 gallon tank with a 10" pipe connection.

If yours does not then some type of accumulator would help.

All so my computation say the cannon barrel has a volume of 85 gallon. Addition storage would be a big plus. Since it is such a long barrel the pressure behind the projectile is decreasing rapidly. May be loosing velocity before it leaves the barrel.

May be loosing velocity before it leaves the barrel.

More likely loosing it's rate of acceleration...

E.G Still gaining velocity but not as quickly as it was when the valve was first opened (in fact I s'pose that is almost bound to be true!)

Once initial inertia and stiction is overcome the acceleration will be at a maximum, although velocity is still low....velocity increases, but the acceleration will decrease.

As per an arrow and a bow.

There are some odd misconceptions about acceleration and velocity...some people think an arrow is still accelerating after it has left the bow...!

(The Mathematical big guns may want to join in here...pun intended..maybe with some fun graphs! )

I don't know about compressed air but in the HVAC world a wye is much more effiecent then a tee. And yes it does reduce turbulance. I think you see an improvment with the wye.

Good luck and let us know how it goes.

Hi Guest, It would help if you used the same type of valve as in a pre-charged pneumatic PCP air rifle, the valve system is instantanious and can be adjusted for efficiency. I sugest that you "Google" how does a pcp air rifle work, it will give you all the answers. Another thing is that you have to tune the barrel length to the pressures involved to gain the most effective effort out of your cannon. Too short a barrel and you will waste the potential power of the compressed air, to long a barrel will cause a loss in velocity. Spencer.

With a good sabot, the system will be much more efficient. I believe the need for fine tuning, especially barrel length, will become more apparent. Right now, the system is operating somewhere between an object propelled by an explosion behind it, the barrel only restricting lateral movement, and a rifle, with diminishing force applied as the object travels the 20 feet down the barrel. If the "explosion" theory is dominant, the valve speed and "T" or "Y" questions matter a great deal. If the propulsion is mainly expanding air behind the object for 20 feet, maybe those things don't matter as much as maybe another 60 gallon tank or preheating the cooling air as it leaves the valve, etc.

I suggest you begin with a google seach under "pumpkin chunking". You will find that lots of people do this for sport and competition. Pneumatic cannons are one of the recognized classes. Last I heard, the record was something like 5000 feet. The limit seems to be approached when the pumkin implodes in the barrel from the applied pressure. A hybrid strain of pressure tolerant pumpkins has been developed to help mitigate this problem.

Simple answer . . . Yes, a Y would be better than a T. But, keep in mind that two 10" pipes going into a 10" X 10" X 10" Y or T has some problems of its own. The two 10" pipes should have an exit of 14" diameter for some distance (has the area of 2_ 10" pipes) before reducing back to 10" (if that is the diameter needed).

I have had a hand in building air guns, and we found a marked difference in performance when changing from T to Y connections with a significant length of larger pipe, but the gun was very high velocity air also.

Now, is this really for punkin' chunkin' as some seem to have speculated?

Oops! I finally read your long post that it is a punkin' chunkin' cannon . . .

Sounds like a cool cannon. What's the chance of getting a picture............maybe even in action

YouTube - Pumpkin cannon

Loads more where that came from:-

http://www.youtube.com/watch?v=e6md570TopY

http://www.youtube.com/watch?v=73vuk77JMZ0

http://www.youtube.com/watch?v=73vuk77JMZ0

Some are right scary!!

I live in Aloha and would enjoy helping if you want it.

As stated by others the tee to wye will help a little but maybe not enough to notice. The speed of the butterfly valve opening and the pressure/volume of the tanks is more important. Upping your stored volume may get you biggest gain in distance.
If you continue to smash pumpkins in the barrel slightly reduce pressure to the butterfly valve opening line. This will lower initial acceleration. If you can't do this then increase the volume or distance between the pumpkin and the valve.

Some form of sabot in the barrel below the pumpkin will seal the air flow and stop leakage around the pumpkin. See other postings on this.

I have not yet looked for this years info but there are several people who put on pumpkin shoots in the portland area.

Silv,

I'm always looking for input. I think we have a good design but I'm open to suggestions. We shoot it whenever the mood strikes. If you are in Aloha that's close. We are in Wilsonville. We are planning on shooting it every Sunday afternoon (except the 23rd) until Halloween. It's a traffic stopper so guests are always welcome to come watch. Once we had a school teacher ride by on his bike. He stopped and asked if we could take it to Boeckman Creek for a science project. Needless to say the kids loved it. What I'm really looking for is the most efficient way to move air. I just don't know the relationship between things like speed, volume, and pressure. And what difference, if any, things like turbulence and temperature make. In other words, I need a geek. We will be taking it to various pumpkin patches this season but the Sunday shoots will be at Sandelie West Nine Golf course. Somewhere around 3pm. Beer is optional.

Keith

I could use it on my golf course ...get those slow kids in front to get a move on...!

(How come young fit kids can walk round a golf course slower than a bloke with his missus in a clothes shop?)

Grumpy Del

Now I REALLY wish I was still in Hillsboro!

This sounds really cool. Something worth thinking about is to add one or more air entrance "chambers" along the barrel length, complete with storage tanks and valves. As the projectile passes the next chamber opening you have a switch trigger the valve, adding a new tank of pressurized air behind the moving pumpkin. This will maintain the rate of acceleration without the chamber pressure becoming too great for the projectile. This has been done in large cannons with great effect.

What a great idea! You could use actuated solenoid valves on time delay. Some simple check valves to prohibit back-pressuring. You have in effect multi stage launch system.

Man I really like this one! CR4 at it's finest!

cr3

I would change propellant, use propane, mixed with air and a spark igniter. More energy, especially in a good sized breech (50 gallon drum as a breech) and with the right gas pressures, you could vary the power and therefore the distance.

You would also not need to carry so much heavy stuff with you like compressors etc..

And before anyone says "Too Dangerous", they have not worked with high pressure air!!! The difference between the gas/air mix and HP air is not worth talking about!!! Both are highly dangerous!! Only pure Oxygen is worse!!

If you don't already, grow your own pumpkins. When they reach about 9" in dia. find some sort of globular device that is 10" inside dia. and place it around it.

It is possible to make a hemispherical "plug" that a heated piece of Lexan or other suitable plastic sheet can be pressed down over. Basic thermoforming. Many companies have vacuum packaging equipment that does just that but usually with a somewhat thinner sheet than you would need to contain a growing pumpkin. You would probably need a couple hundred moulded halves to get 100 pumpkins (a years supply?). Clear plastic is probably the way to go to let in sunlight and many small holes to let out moisture. Nobody likes a rotting pumpkin!

Be careful when handling a growing pumpkin. Any excessive strain to the stem or vine will kill it.

try using a circle of 1 inch foam as wadding it will break away soon after leaving the barrel and can be reused cheap to.With a longer barrel volume of air is as important as pressure. it will help keep pressure up for the whole trip down the barrel. ..... I want one. OOH OOH .... got any video ?!

We've tried various types of wadding and a sabot. We used a plastic bucket for a sabot and it shattered into a hundred pieces. It worked, but not very well and made a great big mess with just one shot. It's sort of the same problem with wadding. It works, but makes a a terrible mess. This is more a question for guys like you, why things like pumpkins can leave the barrel in one piece but things like wadding don't. I'm sure something could work and foam is certainly worth a try. I'll see if I can track some down by tomorrow. Since we typically shoot anywhere from 3 to 10 or 15 pumpkins per outing I don't want to make a great big mess. Remember we usually shoot it at a golf course and that's not something that should be spread out over the grass. Where the pumpkins land of course makes a huge mess too, however that's a third of a mile away, in a non used part of the property. In any sort of competition, no wadding or sabot is allowed because it would leave the end of the barrel and only the pumpkin (and air and water vapor) is allowed to leave the barrel. When we do it just for fun of course anything is allowed. I haven't tried it yet because it might be borderline cheating in a competition, but if there was a way to make wadding out of nothing but water that could be interesting. We have some video but haven't put it up anywhere. We might do it on the golf course website, or maybe Silv could take some if he comes out on a Sunday. Supposed to be rainy but we'll still do a few at least.

What if you froze the pumpkin's in a round form with the same diameter as the barrel while compressing them slightly to fit the form? With them being frozen they could withstand a higher chamber pressure too. Taken to an extreme, you could use a form that had a top piece, which under pressure would form a pointed shape to the pumpkin, like a bullet.

I think it has to do with the fact that the pumpkin is round (giving more surface area for the air to push against and being able to better withstand pressures from within and out.) it also has a smooth surface thats nonporous and doesn't allow the air to get inside and push it apart. if you were to poke a bunch of holes in the pumpkin and launch it it would probably do similar. A plastic bucket is fairly rigid and not intended to take the sudden shock of the cannons wonderfully destructive .... err ..... i mean interesting pop. Try coating the inside of the barrel with a teflon or better yet that foam they use for high speed water slides ... LOL ...its smooth and has bout 1/2 an inch of give fit a pumpkin in a Lil snug grease the gun with a teflon based car polish and whoosh!!!! ....... cant member what the stuffs called though ... the foam that is ... its the same stuff the use to make the headgear boxers use.

You could freeze a tub of water to not solid. So that just a 1/2 in or so of the water makes an ice shell. Slide it in and you have a frozen water wad, aqueous in the middle.

You might be able to try slush like from a sno-cone.

cr3

I have a question, are you ONLY allowed to use air?

If not, look at these links:-

http://www.youtube.com/watch?v=zbULgsPBX6Q

http://www.youtube.com/watch?v=CpAJOPzKK-M&mode=related&search=

That should give you some good ideas!!!!

Those were interesting videos but I didn't get any good ideas. We haven't tried using a combustable gas yet, I'm sure that would be very loud and possibly very expensive. Remember, the volume is about 180 gallons, the volume in those spud guns looks like about 1 gallon.

Simple lighter refuel gas or a propane cylinder and you do not need much, in fact you need more air than gas or it will only burn instead of exploding!!!

It would certainly be cheaper than running the electric for your compressor AND it will be a lot lighter to take with you, compacter too.......

I would expect several thousand shots from a 10lb gas bottle....refill cost here in Germany about $12....

Using a pressure reducer and then timing how many seconds of gas are loaded to checkout the various possibilities.....start small....

It is certainly not any more dangerous than high pressure air, which can of course be lethal.....

The films I showed you use under arm sprays.....smell nice too!!! But more expensive than bottled gas.

1. plastic bucket (ID. of bucket same as ID. of the cannon)
2. fill bucket about 1/3 with water
3. place pumpkin in bucket and allow to float
4. freeze assembly
5. remove from freezer
6. run hot water over outside of bucket
7. remove ice/pumpkin cartridge and store in freezer till needed
8. insert into cannon ice end down

When fired, the ice will temporarily seal the barrel and by the time it exits the barrel, it might be "vaporized" to some extent (low pressure-high pressure-low pressure transition). I'm not sure it will be all water vapor, if they are that strict with that requirement, but it will surely be water and some ice. If tests show some success, decrease the amount of ice until acceptable.

With a frozen pumpkin, you might be able to increase pressure since the pumpkin itself won't be directly subjected to the air pressure, being shielded by the ice, and G forces would only be of concern.

If frozen pumpkins are not allowed, separate the frozen pumpkin from the ice and stockpile the ice molds.

I need a cool drink.

I did not make it to the shoot on sunday. I just plain forgot about it. Next sunday I have to go over to the west side for a morning meeting, I hope to remember about it then. I have made a map already. I don't own a video camera so the most I could do would be to post still photos.