Waxing eloquent on Pumpkin Chunking, Potato Cannons, and Spud Guns :)
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So you want to know about 'tater cannons and punkin chunkers, as I like to call them? Cool! Let's talk.
There are two basic types of your normal tater cannon and spud gun or punkin chunker, the combustion type, and the pneaumatic type. The combustion type use a combustible propellant to fire a projectile, much like a real firearm or cannon, whereas the pneumatic ones simply use compressed air. In spud guns, both types are used pretty extensively. In punkin chunkers, pneaumatics are used almost exclusively.
So let's talk punkin chunkers for a minute. Combustion based firing of a projectile that weighs eight to twelve pounds (a pumpkin) can be exceedingly dangerous because of the pressures involved. Pneumaitc punkin chunkers are a little bit safer to use and operate. Thus, almost every single punkin chunkin competition that I am aware of allows only pneumatic chunkers to enter. Not to say that there aren't any combustion ones out there, they just aren't seen much because they aren't allowed at competitions and punkin shootouts.
Your BASIC punkin chunker consists of an old 100 gallon propane tank still in good shape, with several feet of 10 or 12 inch steel well pipe casing welded to one end, coming straight out of the center of the end of the tank. Within a foot or two of the tank, there is a strong airtight butterfly valve, then another ten to thirty feet of well pipe casing that makes up the barrel of the cannon. The butterfly valve has a strongly welded catch to hold it closed, with a very strong spring (like a car hood spring) to snap it open when the butterfly valve catch is released. And they make sure this whole contraption is anchored down to something substantial like a semitruck bed or a goodsized flatbed trailer, or concrete pylons in the ground.
To operate this little beast, ram a pumpkin (and wadding) down the barrel almost to the butterfly valve, close the butterfly valve and secure, and pump up the tank to anywhere from 10 psi to 40~50 psi (most guys use big commercial air compressors to do this). When you are ready to fire, make sure the field downrange is clear, blow/honk your horn to warn everyone that a shot is about to be fired (very very important and can get you kicked out of some competitions if not done), then spring your buterfly valve release.
What can you expect from a firing like this? You can expect an eight to ten POUND pumpkin to fire (loudly) and if launched at 30 degrees (claimed to be just about the ideal firing angle), to travel downrange well over a quarter mile. These are NOT toys, and should be respected. Before building and shooting your own, go to a competition (google it to find the one nearest you), and talk to someone who has done it before (I have not built one myself, but have only gone to competitions and was even on the pit crew once for the current unofficial world record holder) to get an idea of the pressures and dangers involved. When you suddenly release this much volume of air pressure, it can be VERY dangerous if not respected, and have heard of several gun ruptures when designs were flawed or adequate reinforcement was not used in key areas of pressure buildup in the guns.
The burning question of the day for several years, was can a pumpkin travel faster than the speed of sound? Most definitely yes! The pit crew I was on at one competition I attended, had a newly upgraded cannon with which they were trying for the world record distance shoot. He unofficially shot over a mile!! And from a few hundred yards away either uprange of downrange, two distinctly different bangs were heard with each firing - one was the actual firing of the gun, the second was the pumkin breaking the sound barrier. Needless to say, numerous pumpkins also did not survive that transistion and were reduced to pumkin pie in midair *chuckle*. But let me tell you, there's nothing quite like firing one of these big boys. The air shakes. The ground shakes. Your insides shake. We were laughing like little kids after each shot it was so much fun...
Now let's talk spud guns
Spud guns can also be a lot of fun to shoot. Pneaumatic ones typically do not produce quite the report that combustion guns do though, and in some ways are instrinsically safer than combustion ones to use and operate. But they are tied to an air source, whether it be an extension cord for an air compressor, or a tank of compressed air, or something like that. Combustion cannons don't have this problem. Pneaumatics can also be pretty finicky unless you get the parts built "Just Right". Likewise with combustion cannons, there are lots of variables that can affect performance and relaibility. Bottom line though, if you want a cannon that will shoot every single time once you get it built and tuned right, go with pneaumatics. They will shoot pretty much in any humidity, temperature, or most other other environmental variable extremes (don't let them freeze or operate in freezing weather though, frozen condensation inside the gun can be deadly). Conversely, Combustion cannons rely on the proper fuel-air mixture, and environmental variables make this mixture ratio change all over the place. But if you think about it, your car is also combustion based, and it has been designed to be reliable in any weather, so these variables CAN be accounted for.
I will talk more about combustion cannons here though, simply because I prefer them and have a lot more experience with them than with pneaumatics. And my guns, once I got them figured out, also shoot pretty reliably.
So your basic combustion 'tater cannon is just a long PVC pipe "barrel" glued atop a wider diameter and typically shorter length of PVC pipe "chamber". The other end of the chamber screws on or off, and a source of ignition extends into the chamber somewhere. A potato is rammed down the barrel almost to the chamber, fuel is sprayed into the chamber, the end is quickly screwed on, the ignition source is ignited, and bang, you have combustion and a potato launch.
What about special design considerations (and there are a few)? Let's talk shop.
I once did a design study with a class of high school chemistry students building potato guns. We found that we typically seemed to like and get best results from cannons where the barrel length was at least three to four times longer than the chamber length, and where the chamber diameter was no less than three to four times larger than the barrel diameter. If the barrel is too short, then you have lots of wasted potential potato acceleration, lots of extra bang, and lots of extra (and dangerous) flame coming out the end of the barrel. Another way to look at it is if the chamber is too long in relation to the barrel, the shockwave and flame front that proceeds through the chamber will not have even had the time to reach the other end of the chamber and complete its burn before the potato has already exited the barrel. And looking at this, you can see why shorter wider chambers also work better than long slender ones - they develop their full pressure much faster and you get a far better shot. Additionally, shorter and wider chambers also tend to ignite more reliably due to better mixing of the fuel-air mixture.
You have several different options here, the objective being to get a spark into the barrel. I've heard of some guys actually using fuses (which I heartily DON'T recommend), whereas most people using something mechanical or electric. Best method that I know of in the mechanical realm is to get one of those twist strikers that you can get for BBQs, and mount it so the striker part is inside your chamber. If you have your fuel-air mixture even close to being right, this will ignite it and you'll get a shot. A second option that I know is sometimes used, is to take apart one of those click lighters that have the 6 inch extension on them to light fireplaces with. You take the two wires and run them to a spark plug mounted into the chamber, or to two screws screwed through the chamber walls to form a spark gap of about a sixteenth of an inch inside. These are pretty unreliable after a little time as the little piezo element in the clicker is just not made for the heavy duty use 'tater cannons put it through. The last main option is to buy one of those heavy duty red-button BBQ ignitor clickers. This gives the best reliability for the money although these too wear out after a lot of use, and the wires can corrode at some of the different connection points after time (I solder all my connections and recommend you do the same, then insulate them well - nothing quite like a little high voltge shock to surprise you and mess up your shot when firing). Take the two wires from the clicker and run them to a spark plug (OK) or two screws screwed into the chamber that form a sixteenth inch spark gap inside the chamber (safer). Alternately, I know of some guys who use full blown electronic ignition that gives a nice fat spark every time to produce reliable shooting. But that goes beyond simple and cheap 'tater gunning so we won't discuss it in depth here.
Most guys (and girls - I know some pretty good/nice spud gunnerettes too :-) ), put their ignition source at one end of the chamber or the other. But let's think about this.
Ignition at the bottom of the chamber will produce a forward traveling shockwave that pushes the unburnt fuel-air mixture before it as it travels, and can cause lots of pretty, dangerous, and completely useless flame exiting the gun following the potato. These guns also resonate as the shockwave travels back and forth inside the chamber and barrel, which produces the characteristic whump, whump, whump sound that a lot of the big guns of this type make. On the good side though, these have the most reliable ignition (assuming that when you spray your hairspray into the end of your chamber, you spray so it hits/coats the spark gap).
Ignition at the midpoint of the barrel produces a dual shockwave inside the chamber that builds pressure twice as fast as the previous method did due to there being two shockwaves expanding out from the center of the chamber. Produces a much better shot and wastes a lot less fuel in propelling the potato. But this design also has some inherent potential flaws that have to taken into account. Because the pressure builds twice as fast, you also end up with almost double the pressure for that split second just before the potato exits the barrel. Make sure ALL your glue fittings are good and strong (DON'T be stingy with the glue in building any of these things!). And because you have entered the chamber through a sidewall instead of through one of the stronger ends through the extra plastic of the fitting, you have inherently weakened your chamber. Make sure you are using THICK plastic for you chamber pipe or even split your chamber in two and reconnect them through a straight-thru connector fitting through which you can then put your ignition source (the thicker plastic of the fitting replacing the weakened chamber wall situation).
Ignition at the top of chamber produces a different result than either of the other two options. It produces a downward traveling shockwave that compresses the unburnt fuel-air mixture ahead of it. This actually increases the burn rate as it goes, and produces a much faster pressure buildup and higher overall pressure than ignition from the bottom will. The problem comes in where the ignition shockwave is actually traveling in the opposite direction than that of the potato so you lose that potential acceleration. Likewise, it also produces a much higher pressure spike on the screwed in end of the chamber and can cause a blowout if it is not not screwed in tightly. You also get a lot of that resonant whumping after the shot as the shockwave travels back and forth in the chamber and barrel. But total pressure developed can be higher than with the first option, you have less visible flameshow, and it will produce a pretty good shot.
They all work. Just use the one that meets your needs. I personally don't have a huge preference.
Multiple ignition points. BAD BAD idea unless you are using a hefty electronic high voltage generator with power to burn. Otherwise, your spark from the clicker will fire whichever spark gap (and ONLY that one) happens to have the lowest breakdown potential at that instant in time. Thus, your shooting will be unreliable and accuracy attrocious (because you might get a different burn style with each shot, depending on which spark gap actually fired). Additionally, having the extra spark gaps will actually lower the intensity of the spark gap that does finally fire. Just a bad idea overall unless you put some money into a power supply that has enough juice to actually fire all your spark gaps (and then its a great thing).
I like areasol AquaNet hairspray. Nice and cheap. But don't let it sit in your gun unfired for long. It tends to gum things up (it's HAIRSPRAY, remember?). WD-40 also works, as does straight propane. I don't like propane or even WD-40 much, because they don't ignite quite as easily as areasol AquaNet hairspray seems to (though WD-40 isn't too bad - but be careful with WD-40 for some of the liquid dangers it poses, that I mention shortly). Some guys swear by carbeurator cleaner, and I've had decent luck with it as well, and it doesn't have the gumming problem that hairspray does. It just costs more. Liquid propellants are a BAD BAD idea. They leak all over the place, and are a HUGE fire hazard. Gasoline and alcohol fall into this category. Just don't go there. They weaken/corrode the plastic, build huge pressures inside that weakened plastic upon ignition (that can cause deadly bursting), and if leaking, flame from a shot can ignite them and cause serious fires. Just don't go there. And don't EVER, EVER, EVER, EVER use acetelyne and Oxygen from an oxyacetylene torch!!!! It will most likely explode your gun, put plastic shrapnel INTO you, and very likely kill you. It's just DUMB DUMB DUMB (I saw what it did once when some free oxyacetelyne vapors from a machine shop ignited - we'll just say it scared the pee outta me - and I was over 40 feet away from the blast). So Don't. Just use one of the propellants I have already mentioned as being popular.
In my spare time, I've actually been experimenting with hydrogen and oxygen fuel mixtures for improving gas mileage, but for a spud gun propellant, this is just a bad idea too. It's simply TOO explosive. Of course, the hydrogen is hard to contain anyway, but just don't try this combination. It might just work too well, and hurt you in the process.
Potatos are the ammunition of choice. They are heavy, cheap, and fly pretty good. But just about any fruit or vegetable will work. Apples work very well too, as do oranges, just beware that their juices contain sugar and tend to make things very sticky and gummy after awhile and can retard the effectiveness of your gunning. Eggs work good too if your barrell is sized right (pointed end of egg pointed towards chamber when inside barrel). Just be aware that a lot of eggs don't make it out of the barrel in one peice. Farm eggs work better than store eggs though, because they have thicker shells. Exploded eggs also tend to slick-up the barrel and make for even better gunning, and smell like breakfast at the same time (fried eggs). And they just dry up and flake off the inside of the barrel after the gun has sat idle for a few days and thus clean up nice too. Tennis balls in the right sized barrel work really good, though they tend to look a bit singed after a shot, and racquet balls also work. And of course, if you put a little wadding in above and below them, you can shoot whole handfulls of paintballs for a deadly shotgun effect too *chuckle*.
How much propellant?
That all depends. Trail and error will be your best guide. Start with a one second spray and see if it fires. If not, then clear the barrel, open the breech, blow fresh air into the chamber (good lungs or a little handheld fan both work well - just rememnber that your exhaled breath only contains about 16% Oxygen versus 21% in the atmosphere - you'll get better ignition and shooting if you use a fan), and try again with a two second spray, and so on and so forth. Once you start getting ignition, experiment some more. At some point, you will find the most reliable spray time that produces the best shot. Remember, spraying too long will work just as poorly as not long enough, as the spray will displace the oxygen in the chamber needed for combustion and produce a poor shot, if one at all. What seems to work best for me is roughly one second of hairspray for every 145 cubic inches of chamber volume when at room temperature (Volume = 3.14 x radius(inches) squared x length of chamber(inches)). Of course this value will probably be different if you are using a different propellant like WD-40 or propane or something. Keep notes that you can go back to and look for your best shot patterns in.
I built a propane gun once. Just mounted a torch fitting from a little propane torch onto the gun into the chamber, screwed on a little propane tank, and used the torch valve to meter the flow of propane into the gun. Though it DID work, it was not near as reliable as hairspray (for me anyway), and produced less pressure and therefore distance on the shot. In English? Less bang fo the buck. I abandoned that design fairly quickly. Tried butane too (like from cigarette lighters), but that's hard to get in quantity, and I never got it to work reliably. Not that it can't be done, because there are nail guns out there used in construction all the time that use butane, and use it reliably (it could also potentially be dangerous, as I know butane builds pressure pretty fast). I just never got it to work right for me. I always just seemed to keep coming back to good 'ole areasol AquaNet hairspray.
For colder weather gunning, I made sure the gun was room temperature when I started, and then used a little handheld hair dryer/hot-air blower to fill the chamber each time with warm air. I also kept the can of hairspray in my pocket to keep it warm also. Gunning that day wasn't too bad really. Cold air really just seems to kill reliable ignition.
Where all have I gunned? Deserts, forests, lakes, you name it. At almost everthing. Big plywood targets, bottles, cans, etc. Daytime, nighttime, whenever. I also like to get mine out and fire it with just wadding and no ammunition on New Years eve and the 4th of July. At dusk, you can see your target still, but the flame out the end of the barrel makes for a fun effect. Lakes are good if you string a line of different colored milk cartons out into the water with an anchor at the far end, that you can measure off distance with - especially if you are gunning in a distance competition. Somebody with a set of binoculars can tell pretty easy where the 'tater hit in relation to the milk cartons. Some of the 'taters also produce some pretty nice plumes. And hey, you're feeding the aquatic life at the same time. Football fields work with the smaller guns, especially if shooting at a target of some sort like an old car or something, but beware with the bigger ones - a football field is just plain too short if you have an accidental firing that puts a 'tater up into the sky. A half pound potato traveling at a couple hundred feet per second can KILL a person. It will hit with more force than some handgun bullets hit. Don't take that risk. So, moral of the story, is don't be shooting towards civilization, and beware of your backdrop. And it's always a good idea to yell "fire in the hole!" before each shot too, so people know a launch is about to occur and can look up and get out of the way in case of a bad launch, especially if doing 'hang time' competitions where the guns are pointed nearly straight up into the sky (I don't suggest this type of competition unless strict observance of all safety rules is followed, all spectaters are at least 50ft away, and the guns themselves are NEVER pointed upwards at more than 80 degrees down range. Failure to follow these rules could result in someone's death and manslaughter charges being filed against you. Don't go there.)
Rules of Thumb
Don't walk around with a loaded cannon.
Treat your launcher just like you would a loaded gun.
Be sure your backdrop is safe, sturdy, and not likely to fail and let a loose spud hurt something beyond it.
Don't point at anything you don't plan to shoot.
Keep the barrel pointed in a safe direction at all times (downrange usually).
Be sure everyone is out of your field of fire before arming, aiming and shooting (everyone behind you).
DO NOT fire combustion based guns ANYWHERE in or near highly flammable items or areas. DO NOT shoot them indoors, in a tinder dry forest, or a dry grass desert!!!!! If a house fire, range fire, or forest fire result from your actions, the government is likely to sue YOU to recover the costs of fighting the fire (it's been done). Flames can come several feet out the end of the barrel when firing some models (BTW - very pretty at night). By the same token, don't get your face or any body part ANYWHERE near the end of the barrel when firing. Serious burns can result.
Use only quality parts and good glue to assemble these guns.
Make sure all parts are clean, DRY, and room temperature before attempting to assemble or glue, and don't be stingy with the glue either.
Let all glue joints cure for at least 24 hours (or as directed on the can of glue) after assembly before shooting.
Assemble in a well ventilated area. Glue vapors can explode and/or make you sick (personal experience there).
Never get your face near a loaded or misfired gun breech while opening the breech (singed ALL the hair off my face once when I broke this rule and the gun went off).
Use safety goggles when shooting, and possibly even hearing protection.
Yell "fire in the hole!" before each shot, so people know a launch is about to occur and can look up and get out of the way in case of a bad launch.
When shooting the Paintballers in live simulations, MAKE SURE to wear approved face protection and heavy clothing and follow ALL rules normally associated with safe Paint Balling.
Use only under adult supervision.
Just plain use good common sense when using these guns.
Disclaimer And just as a car salesman is not responsible for damage you incur while driving a car you bought from him, I am not responsible for any damages or 'bad' things that may happen from your use/misuse of this information, these kit plans or from spud guns. I will only say that myself and close friends have built lots of different models, I have tried to give you the best information I have from these experiences, and other than a few minor "Did I just do that?" and 'egg-in-the-face' happenstances, we have had nothing but lots of good safe fun using them. Know your stuff, be safe, and have fun!