This section covers the various components and systems typically found in spudguns, both pneumatic and combustion classifications.
For COMBUSTION (traditional) launchers, there are three systems. Namely, material of construction, chamber ratio along with fuel type and introduction, and the ignition system. Ammunition is of course also assumed to remain a potato, but you could get creative I suppose, via an apple or even a tennis ball for larger barrels
- Construction Materials
- Chamber/Fuel Injection
- Ignition System
A brief discussion about the various materials available, their pros and cons, and my recommendations There are many things that need to be considered when selecting the proper material for construction of a stable potato launcher. One of the most important things is its integrity: IT MUST NOT BLOW UP IN YOUR FACE!!! Another is weight, as these things are typically hand held. You could get nit picky and throw in looks, but that is why paint was invented!
Below is brief discussion about various things typically used, some not so typically used, and yet some things that you can probably only dream about.
ABS (also see topic #1 on the FAQ page)
- Technical Name: acrylonitrile-butadiene-styrene.
- Trade Name: Marbon, Cycolac, Lustran, Abson.
- Regular Applications: Sewage/Drain systems for most modern buildings within confines of foundation, highway safety devices, toys, potato launcher barrels and combustion chambers.
This material has long been that standard as far as I have known for constructing potato cannons. Strong and very light. (cellular core)
- Chemical resistance, so can stand up to the average weekender going crazy experimenting with every different thing that can be found in an aerosol can.
- Light weight, even the weakest of us has no reason to heft and bear these devices.
- Low cost, it can be obtained almost anywhere, in the correct dimensions.
- NOT PRESSURE RATED! This material has been proven to be acceptable, but standard pipe is not certified for any pressure service above those found in gravity drain applications, typically not above 10 psi. The term "cellular core" means that millions of tiny air bubbles were whipped into the plastic before it was run through the extruder. The end result is a very light weight material that in effect is nothing more than a piece of hard foam in the shape of a tube.
- If it fails, it will fail. When it goes, it will turn into many little shards of sharp plastic that will invariably find you in some way.
- Decomposition products are nasty! If you have ever taken a whiff after firing it a few times, you will get a burning in your nose. This caustic gas is not good for you!
Although an acceptable material for construction of these devices, a few cautions should be exercised. Never attempt too large a reduction from chamber to barrel in pipe sizes, and this is dependent on the chamber volume. If this is too high, a container failure is risked. Never attempt to use an oxidizer other than atmospheric air, or rupture could occur. ABS is fine for the construction of combustion spudguns, but I personally tend to stay away from it. All the chemical and heat resistance in the world isn't worth a damn if it's not pressure rated.
PVC (also see topic #1 on the FAQ page)
- Technical Name: polyvinyl chloride
- Trade Name: Pliovic, Saran, Tygon Regular
- Applications: fresh water delivery systems, electrical wire insulations, LPs, potato launcher barrels and combustion chambers.
Invented at the University of Washington Department of Chemical Engineering, this material has been so instrumental in our everyday lives. Being so inexpensive and versatile, it is used in many fashions not limited to the list above. It has even found widespread use in the potato discharging industry!
- Inexpensive, the pipe is almost free. Fittings are where the money is dropped, but well worth it
- Multitude of dimensions to chose from. Pretty much any configuration of pipe fitting can be found somewhere in PVC, but get out your checkbook! 6" sch 80 will run you right around $40 for a 10' piece.
- Pressure Rated. What you see is what you get. The rated pressure on the side of the pipe is already conservative, so you don't need to incorporate safety factors, unless you want to. Fittings are always stronger than bulk pipe, as long as the glue joints are made and allowed to cure properly and are Schedule 40.
- Limited Chemical Resistance. Aldehydes and ketones are out, as these readily dissolve the polymer, as in a glue joint, just read the label on the can: Acetone, THF, MEK. Some fuels should be totally avoided such as carb cleaner and insect repellant, don't say I didn't warn you!
- More brittle than ABS, but by not using oxygen and just burning fuel and air....should never be a problem.
- Cellular core PVC is common at many hardware/home improvement stores. Make sure you buy SCH 40 pipe with a pressure rating printed on the pipe. If the pipe is not specifically marked with a certain PSI rating, IT IS NOT PRESSURE RATED!!
PVC pipe is the most widely used material to date, it provides many ideal characteristics for use in this field. Just look out for chemical incompatibilities with certain fuels, but most will not present a problem. I can sort of feel a small war going on between the use of ABS vs. PVC, and with my experience would go with PVC if I had to chose between these two things. Reading further may provide more information about what may even be better these days...
Carbon Steel Pipe
- Technical Name: Fe (with ~0.3% C, ~1% Mn, and ~0.3% Si)
- Trade Name: Steel Regular
- Applications: Building superstructures, bridge structures, reinforcing concrete, potato launcher barrels and combustion chambers.
Been around for so long, and still finds use even here. There is one outfit that actually sells assembled units using threaded steel pipe, but I wouldn't want to pay the freight charges!
- Strong, able to easily withstand any pressure short of a completely enclosed solid fuel explosion.
- Inexpensive, at least for metal construction. Of course metal costs more than plastic, but the gains in strength and safety might be well worth it.
- Heat conduction can pose a problem over extended firing periods. Air cooling is probably sufficient.
- Rough interior of pipe is not suited for a barrel. The surface will grind away potato as it is exiting leading to blowby and possibly no exit at all.
- WEIGHT!!! This would most certainly have to be put on wheels or in the back of a pickup, if of conventional dimensions. A smaller model could be hand held.
Steel is tough but probably not a good choice for metal construction. Probably the most safety for your buck, but you need three people to hold the silly thing where you want it to go!
- Technical Name: Al (with ~0.3% Cu, ~1.0% Mn, and ~0.6% Si).
- Trade Name: Wrought heat treated structural grade aluminum.
- Regular Applications: Lightweight frames, airplane parts, potato launcher barrels and combustion chambers.
Ah yes, the crème de la crème of launcher build materials, the glorious SP9004 is made entirely of it (with G8 steel fasteners). Ease of machining for custom parts coupled with low weight and high strength make this in my mind the most suitable material for making potato launchers these days. It may cost a bit more than plastic, but the gains in safety, fuel options, oxidizer options, and internal operating pressures is well worth it!
- Easy to machine. The metal is easily cut and formed to any specifications with the correct tools. Don't get too fancy with the CAD, or the machinist will really skin you.
- Not too expensive for the benefits you can reap. Although some dimensions are expensive, they are worth it in the long run. Pipe sizes are very inexpensive.
- Readily available in almost any location. Any machine shop will stock nominal sizes that you may require, and pipe can be found at a metalworking shop. Boeing Surplus is the best way to get stuff, but only if you live near Seattle.
- High thermal conductivity. If you fire it many times without good ventilation, it can become quite warm. I get around this by immediately unloading fresh oxygen just after firing, even if I am not firing again. This blows out all the remaining hot gas.
- Fittings not easily available. Threaded pipe and fittings are special order indeed, especially larges sizes. It is probably easier to have the pipes fit together with O-ring sealed custom adapters.
I would go for aluminum in a minute, because of the advantages gained in strength, weight reduction, and versatility. It is so easy to just drill a few holes in the main beam, tap into them, and affix more devices to make it more functional and look cooler! Just watch out for extravagant machine shop bills, keep the design as simple as possible in that regard. Now a brief look at just one more material that may be better than any other mentioned here.
- Technical Name: Carbon and glass fiber impregnated epoxy composite matrix.
- Trade Name: Varies by Manufacturer of Prepreg.
- Regular Applications: Fishing poles, airplane parts, potato launcher barrels and combustion chambers.
Well, just a brief discussion about what advantages this may provide, even though it is impractical as of yet to use it in this application! Being super strong yet light weight, this would be ideal for construction of a potato launcher. One small drawback is the price it could incur. Forming a custom mold and having the entire thing made from one continuous prepreg would be astronomical. Heat resistance could pose a problem, but easily fixed with the correct cooling system.
Proper fuel introduction is pretty important, as this is a major factor in launcher performance. The fuel must be in the vapor state to burn. This is true for any fuel/air combustion. Hairspray I find only works well in warm weather, as too much condenses in cooler weather before it can be fired. That is why I have switched exclusively to propane fuel. Contrary to popular belief, it is soooo cheap, as one bottle will give at least 500 rounds from a typical launcher! At say $3 per 16 oz. bottle, that's about 0.6 cents per round; the potato costs more than that!
Chamber Ratio is important to launcher performance, but is also a safety issue as well. Typical launchers run in the 1:1 to 1.5:1 Chamber to Barrel volume ratio, with the following considerations. It is inadvisable to use a barrel longer than 4 feet, and a chamber longer than 24 inches. This is due to the dynamics of burning something rapidly, and trying to get the potato out of there before the things blows up. If you were to experiment with a potato that could NOT possibly leave the barrel, (PLEASE don't), charged the chamber with fuel, and set it off, the unit would most certainly explode. Bad. A potato is used due to its ease of loading, relative ease of sliding in the barrel, and the fact it is so light in mass, that is is outta there long before combustion is complete. Given this, don't shoot heavy projectiles as well, spuds work great. Use a pneumatic launcher to launch heavier stuff.
An effective way I use for propane injection has the bottle connected directly to the launcher with a hose. Get the slim bottle, and you can put it in your pocket. Obtain a Bernz-O-Matic torch attachment for the bottle, and cut the tube off just before it bends, ~3/4" from the valve knob. Thread this 1/8" MNPT, and use appropriate hose and tube fittings to get into the chamber. I have a 1/2" gate valve at the chamber that must be opened and closed--this acts as the check valve. Do not fire with the gate valve open. A hydraulic system check valve could be used instead, as in the SP9004.
In this configuration, only about 1/2 second of propane is required (nominal chamber size), controlled with the small torch valve knob, NOT the gate valve. To attach the 1/2" gate valve to the chamber, use a 1/2" semi-close pipe nipple, and thread it directly into the plastic of the housing. This requires you drill a hole about 3/4" diameter or so. To fill, follow these steps. Vent the chamber by removing the 4" cap and get a fresh load of air. Load potato to regular position in barrel. Replace cap. Open gate valve 2-3 turns. Crack propane valve for just a brief time (you will get good at this). Close gate valve tightly. Ready to go!!! For best results, let gas disperse for at least 5 seconds after injection for optimum performance.
Get a SGTC Propane Metering Device for exact volumetric measurement and dispensing of propane directly into your chamber every time.
Ways to provide an effective means for reliably firing your potato instrument. This condition is reached by following the most basic rules: DO SOMETHING THAT WILL START A FIRE! We all did it as small children (ok, maybe not everyone...), sometimes in the wrong places! In this case we want some source of heat, which can be obtained in various ways.
There are many ways in which to ignite a potato launcher. Here we are not talking about pneumatic devices, those use something totally different to fire! The three ways that I have gathered for firing the gas in the chamber are as follows:
- Electric BBQ Igniter
- Electronic 9v BBQ Igniter
- Flint Spark Producer Device
- High voltage stun gun
I like and use the electric igniter for all of devices and they have always worked great. Had one fail after MANY uses, but that was good too because I got to tear it apart and see how it worked. A simple method for implementing this effectively is described below. Getting it wet with liquid fuel is not a problem for its function, but wet fuel indicates another problem (see the fuels section).
Flint igniters are also used. Apparently they work fine with two exceptions I know of. If you get it wet with fuel, it sometimes will fail to function. Also, it requires a twisting motion with the fingers, which means one hand cannot be gripping the entire device very well. OK for seasoned spud launcher wielders, but I always recommend that you keep both hands on the wheel!
Another way yet is the use of a stun gun. This can be set up/installed much in the same way as a BBQ igniter but will allow the electrodes to be much farther apart thus creating a longer, hotter ignition spark.
Installation of an Electric Igniter (plastic chamber construction only)
- One replacement BBQ starter ($12)
- Two 2" long round head #6-8 wood screws ($0.20)
- Wire leads as required (junk pile)
- Black Silicone RTV (or electrical tape is fine)
2.After glue has set on chamber fittings, drill two holes slightly larger than the shank of the screw but small enough the threads will bite securely. Do this in the thick part (about 1/2" of plastic) 45 degrees apart, along the radius, preferably near the rear of the chamber. Keep them at least 1" from either end.
3.Insert the screws almost all the way (they should bite pretty good into the plastic. If they don't, get one larger screw size.
4.Take the two electrical leads, either the ones that came with the spark producing plunger, or your own custom extension wires. Strip the ends back 3/4" and tin if desired, then wind around each screw in clockwise direction. If you want to be really slick you will solder on rings of the correct size.
5.Finish tightening screws. At this point the tips should be about 3/8" apart, which is fine. The desired range is 1/8" to1/4", which can be changed by bending a screw slightly.
6.Proper insulation is now required, unless you like getting shocked REALLY BAD (I know I do!) Either dab enough RTV (1/8") all around exposed screw heads and remaining bare wire, or wind 6-8 turns of electrical tape around the heads.
7.Insulate the plunger prior to attachment. Secure wires in their appropriate sockets on the igniter, then wind tape around and around it. You should be able to hold it in you hand and actuate it now without being shocked (only way to test!)
8.Affix to launcher. I like to use a two hole 1/2" thin wall conduit strap to mount it directly to the side of the chamber, resting the top of the plunger against the edge of the reducer. Do this, and you are ready to roll!
For PNEUMATIC launchers, five things can be counted. (This section under construction).
- Valve Types
- Launcher Chassis/Layout
- Construction Materials
- Valve Actuation
Pneumatic Valve selection for your launcher.
There are 3 main types of valve that are typically used for 'backyard' pneumatic launchers/spudguns. Yes, I am aware that there are over 100 different types/all sizes of valves, most not mentioned here, but I list the major types due availability and cost issues with specialty and industrial valves.
- Ball Valve: Operated by throwing a quarter-turn handle. Very simple, but not as efficient (fast) as more automated valves.
- Sprinkler/Diaphragm Valve: Differential diaphragm operation, normally actuated with an electric solenoid but can be modified to be actuated pneumatically. Pneumatic trigger actuation will fire the valve much faster and will get rid of the battery pack and mess of wires.
- Piston Valve: Usually built into the launcher, as a homemade sliding piston controlled by a small secondary reservoir. Not as fast as a diaphragm valve but will allow much higher air-flow.
Want the best of both worlds??? The Supah-Valve combines the high speed actuation of a diaphragm valve, and the massive air dumping capacity of a piston valve, all in one package, ready to use, right out of the box. Check it out on the components page.
When constructing a pneumatic launcher...There are 2 basic configurations.
- Inline - The chamber, valve, and barrel are all in a straight line.
- Over/Under - The barrel and chamber run parallel to each other.
The main thing to consider when deciding what type of configuration to build your pneumatic launcher is over all size/length and durability. An inline launcher will tend to be quite long and be more likely to be damaged if it is accidentally dropped or knocked over. An over/under will generally be about half the length of an inline launcher and because the barrel and chamber are usually straped/clamped together in this configuration....The structural integrity of this style of launcher is much stronger.
More to come here, just got an outline going here now....
document created 7/6/2002 10:23:27 PM;
last modified 9/23/2002 4:35:18 AM