Spud Accelerators (Spud Guns, Potato Launchers)
Spud accelerators are (usually) constructed of PVC or ABS pipe and fittings and typically use a flammable gas (propane, butane) or vapor (hairspray, carb cleaner, solvents) mixed with air and ignited to propel a piece of potato or other object at high speed. They are not toys. A three-ounce potato chunk at velocities of hundreds of feet per second has potential for damage and harm, so one must be careful and responsible lest this tremendously fun activity be regulated out of existence. The following descriptions are just that; a summary of my work (play?). They are not instructions or plans.
This one uses propane, and is all schedule 40 PVC. The chamber is made from two 4" couplings cemented together with a short length of pipe such that the couplings butt together, making a heavy duty chamber. A 2" x 4" pipe adapter is cemented into one end, with a 1-1/2" female threaded adapter cemented into it. The barrel is a 40" length of 1-1/2" pipe with a male threaded adapter which screws into the female adapter. A 'rifled' pipe is available from the Spud Gun Technology Center which greatly improves accuracy, especially if the potato is carved to a suitably bullet-like shape. If Class 200 pipe is used for the barrel one can also launch golf balls :^) . The business end of the barrel is reinforced with a coupling. The rear of the chamber is a 2" x 4" threaded pipe adapter and has a 2" threaded plug installed in it. A 2" barrel assembly (not shown) can replace this plug, with a 1-1/2" plug in the other end of the chamber; this makes the launcher more versatile. The handle is a 1-1/2" x 1-1/2" x 3/4"" tee fitting sawn lengthwise along the straight side to allow a tight friction fit on the barrel, with a short length of pipe cemented into the middle of the tee and capped. Two long 6-32 brass screws are threaded into the chamber so that they almost touch in the middle of the chamber volume. This forms a spark gap to which is connected the wires of a piezoelectric barbecue igniter conveniently mounted in the handle. A 1/4" hex head bolt is threaded tangentially into the chamber and is drilled through with a 1/16" bit, then counterbored halfway through with a larger bit. This is the fuel port; the off-axis position causes a swirling action for better mixing as the fuel is injected. The chamber volume is 1.5 liters; the optimum mix of propane is 24 : 1 air : gas, so about 60cc of propane is required. Don't even think about using oxygen unless you like hospitals!
Paint job is optional; I chose a 'camo' scheme; in retrospect probably not a real good idea since it looks kinda serious, but I like it :^). Imagination rules!
To load and fire, a short piece of pipe, sharpened, is used to cut a potato slug which is dipped in water and pushed down the barrel 'muzzleloading style' with a ramrod, leaving the rear plug loose for air displacement. A modified propane torch is used to fill a 60cc plastic syringe, the tip of which is seated in the fuel port and the gas briskly injected into the chamber. The rear plug is then tightened. The launcher is now 'hot' and the barrel must be kept aimed downrange. Beware! When fired a small quantity of hot gas will escape through the gas port, so one must make sure no part of the body or clothing is close to it. Pressing the button sends the hapless potato is on its way with a boom and a kick! After firing, the rear plug is removed and the launcher aired out by blowing down the barrel a few times, then it's ready to go again. This design has proven very reliable through hundreds of firings. It cannot be 'overloaded'; too much gas and it simply will not fire.
Measured velocity of a 75 gram, 2" long spud chunk: 350 feet per second. Calculated energy: 315 foot-pounds. Fun factor: 9. Bang for the buck: 10
Pneumatic (Compressed Air) Version
This is a piston style test-bed launcher built to explore air-driven spuds. It is constructed of Schedule 40 PVC; at 100 psi chamber pressure it is equivalent to the above described propane version in performance, and is a little quieter. It is experimental and is admittedly not nicely configured for ease of use. The chamber is a 18" length of 3" pipe with a 3" female pipe adapter on the back end and a 1-1/2" x 3" pipe adapter on the front. The barrel is a 60" piece of 1-1/2" pipe which is cemented into and passes through the front adapter (the ridge which normally prevents this having been removed) and continues to within 2" of the rear of the back adapter. A brass cleanout plug is screwed into the back adapter. This connection is further secured with pipe seal, a tight, heavy wrap of electrical tape and a band clamp. This was done to eliminate leaking from distortion of the plastic threads under pressure. Think about it; at 100 psi the pressure on that 3" diameter plug is roughly 750 pounds! A hole is drilled and tapped in the cleanout plug for a 1/4" pipe nipple which connects to a cross fitting. Installed in the cross fitting is a pressure guage, a 125 psi relief/popoff valve, and a check valve. A tire fill valve is installed in the check valve. A wire was run through the pull pin on the relief/popoff valve for a 'trigger'. Inside the chamber, just behind the end of the barrel, rests a piston. The piston is turned from UHMW polyethylene, is 2" long and has a recess in the front 1" deep and about 2-1/4" wide. Its diameter is just smaller than the I.D. of the pipe, so that it will slide back and forth. A 1/8'' thick, 2" diameter neoprene rubber seal disk is attached in the recess end of the piston with a fender washer and 1/4" bolt into a blind tapped hole in the center. When all is assembled, the piston has about 3/4" of travel between the barrel end and the rear plug. The chamber is wrapped with two old pant legs cut to length and taped to the outside just in case; I never trust the psi rating of the pipe.
To load and fire is a little more work than the propane version. Air is admitted through the fill valve; the piston moves forward, sealing the end of the barrel. Air leaks past the piston into the chamber; pressurize to about 20 psi. A potato chunk is cut with a sharpened piece of pipe and dipped in water. The potato must have a groove scraped into it for air relief. The potato with groove is pushed down the barrel. The chamber is then pressurized to 80-100 psi. The launcher is now 'hot' and the barrel must be kept aimed downrange. The relief/popoff valve is the 'trigger'. Beware! As the chamber pressure increases, this valve becomes more sensitive as the pressure approaches the design pressure of the valve, and a 125 psi valve at 100 psi is a hair trigger! Firmly pulling the wire vents the small space behind the piston, which is then driven back by the chamber pressure, exposing the potato instantly to the full chamber pressure with very little flow resistance. If the clearance between the piston and the pipe is too great, too much air will be able to get past the piston and the launcher will not fire or will fire poorly; part of the fun is getting that balance right, which requires experimenting. Due to the need for an air compressor and a slower loading cycle time, the fun factor/BFB is lower, around 7.
Serious Aluminum Pneumatic Version - Under Construction
This was designed using information gained from the projects above. It is intended to 'raise the bar' for velocity and available pressures to the 200 psi region with complete safety. Hoped-for velocity: 700 fps (at this velocity, a 75 gram potato chunk would carry about the same energy as a .44 Magnum). Chamber: 4" schedule 80 aluminum. Chamber volume: about 6 liters. Barrel: 2" OD aluminum tubing 1/8" wall (for golf ball capability!), length 84". Expected fun factor: 9 Expected BFB: 6
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Copyright © 2000 by David McNamee. All Rights Reserved.
This part of Tungsten's Castlepage last revised October 12, 2000.