John G's Purple Pounder Spudzooka Rev 1.1 Lawyer stuff Build the device described in this file at your own risk. You assume all liability if you hurt yourself or someone else. Introduction Welcome to my spudzooka page. On this page I present "Purple Pounder", a spud howitzer that perfects the use of MAPP gas and air for propellant. This spud howitzer fuels itself, guarantees a perfect air/fuel mixture every shot and does not need any sort of forced ventilation between shots. Simply open the end, ram a spud, fuel and fire. Firing is initiated by a battery powered sparker. Other features include a folded combustion chamber, perfect balance on the shoulder and easy spud loading. Principle of operation The key to making the spud gun fire reliably and powerfully each and every time is ensuring that the air/fuel ratio is always perfect. The only way to guarantee a repeatable mixture is to mix the air and fuel before introduction to the combustion chamber. An ordinary propane torch is perfect for this purpose. It is designed to mix the optimum amount of air with the fuel. On the spudzooka, the torch is modified slightly to make a lean-of-optimum mix. This is necessary because the mix is richened by the flow resistance of the piping and fitting. The gas is conducted via vinyl tubing to the combustion chamber. The combustion chamber is gassed AFTER loading the spud. This is necessary both from a safety prospective and so as not to disturb the mixture. If the spud is in the chamber, blocking the barrel, there is no exit for the air as the A/F mix is conducted in. Therefore a vent is necessary. At the other end of the combustion chamber a ball valve is installed that when opened, vents the chamber to air. MAPP Fuel has a characteristic odor so one can simply sniff the vent to roughly judge when the chamber is filled with fuel/air. This is a critical design feature. No more random, un-metered squirt of flammable liquid. No guessing how much. Simply open the propane torch valve and let it run until MAPP flows out the vent. Close the vent and fire. MAPP gas is used instead of propane because it is much more powerful and because it is more tolerant of small mixture errors. Propane will work but the spudzooka will not be as powerful or reliable. Ignition is via a battery powered gas grill igniter. This igniter fires about 4 sparks per second and is powered by a single AAA battery. This is MUCH more reliable than lantern sparkers or piezo grill lighters. The spark plug is an oil burner plug. The electrodes extend into the chamber at least an inch. Another critical design element is the gas inlet and the spark plug being located next to each other. This ensures a perfect mixture around the plug even if it isn't absolutely perfect elsewhere in the chamber. This perfect mix ignites easily and delivers maximum power. Materials The major components of the Purple Pounder are as follows: 4 ft, 4" cellfoam PVC pipe 1 ea 4" right elbow 1 ea 4" female cleanout port 1 ea 4" x 2" PVC reducer 4 ft 2" solid PVC pipe 1 ea Lowe's UPC# 47362 84683 Battery powered gas grill igniter. (photo igniter_1 & 2) 1 ea ordinary propane torch 1 ea 1/2" ball valve 1 ea 1/2 X 3/8" bushing 2 ea 3/8" short nipple 1 ea 3/8" street nipple 1 ea 3/8 vinyl tubing, approx 18" 1 ea oil burner spark plug, extended nose 1 length high voltage wire. Everything listed above should be available from Lowe's or Home Depot. The battery powered grill igniter is new this year. It uses a single AAA battery and fires about 4 sparks per second. Spark length is about half an inch. Construction Photo_1 shows the the overall layout. Components are labeled. The igniter is attached to the pipe with epoxy cement. Masking tape is used to build a dam around the igniter and then thin epoxy is poured in. This is a very secure mount and yet it can be removed with a sharp knife if ever needed. The PVC tubing is assembled using conventional primer and cement. Use a liberal amount of cement and allow the joints to dry overnight for maximum strength before use. Before assembling the barrel to the rest of the gun, use a grinder to sharpen the muzzle end so that it will cut a perfect spud plug as the spud is rammed in. The outside of the tube should be ground to an angle so that the ground area is about 3/8" wide. Then a die grinder or round file is used to cut a very shallow taper inside the tube. This should be very slight. The inside taper ensures the spud slug is compressed as it is rammed in. Photo_2 shows the details of the vent valve and the igniter. The vent valve is a crucial component. It allows the combustion chamber to vent after the spud is seated and while the MAPP gas/air is filling the chamber. Without this the chamber could not be filled. The vent valve is attached by drilling a hole in the PVC just large enough to receive the end of the 3/8" close nipple. The nipple is threaded into the pipe, cutting threads as they go. The nipple is removed, coated with epoxy and threaded back in. Extra epoxy is filleted around the nipple to provide more strength. It is important to use the epoxy, as the system generates sufficient pressure to blow the nipple out of the plastic. Photo_3 shows some detail of the gas system and the spark plug. The street elbow and spark plug are attached using the same epoxy system described above. Note the screwed-together joint. This is addressed below. Photo_4 shows another view of the gas system. Notice again the screwed- together joint. This joint is screwed together with 8 sheet metal screws. It is NOT glued. This serves several purposes. First, it allows the gun to be disassembled for transport. Two, it serves as a safety release that should blow apart before the tube bursts if overpressure occures. I have not tested this theory so rely on it at your own risk. On the elbow socket where the 4" pipe will seat a bead of RTV silicone is applied so that the pipe will butt into it when the pipe is inserted. This provides an air-tight seal but has very little adhesive property. Photo_5 is a shot down the barrel. One can see the knife edge ground on the muzzle of the barrel. Also note the slight curvature of the barrel. This curve counteracts everyone's tendency to aim low. It was formed by propping the gun by its base and the muzzle while the barrel was heated to approx 190 degrees F using a heat gun to blow hot air through the barrel. By the time the barrel is that hot, it will have sagged a bit under its own weight. Photo_6 is a closeup shot of the explosive gas meter. This is a safety meter used to detect explosive atmospheres in closed spaces. It provides protection to workers by telling them if the area they are about to enter has an explosive gas in it. For the spudzooka, it is used to sniff the vent and gives a readout of when the gas mix is optimally explosive. This allows one to achieve full power without wasting gas. This meter is not necessary for successful operation of the spudzooka but if one is available, it optimizes performance. Operation Operation is simple. Just follow these steps. 1. Remove the cleanout plug to vent the combustion chamber. 2. Select a suitable spud. It should be a bit larger in diameter than the pipe. The longer the spud, the more power, as the extra mass improves the spud's sectional density. For less than max power, the spud can be cut into halves or thirds before ramming. 3. Ram the spud down the barrel. The spud is positioned lengthwise on the muzzle and is pounded in with the heal of the hand. Once the slug is completely in the barrel, use a wooden ramrod to push the spud down to the far end of the barrel. An old broom handle makes a good ramrod. Cut it to length so that when it is fully in the barrel, the spud is positioned at the mouth of the reducer. 4. Replace the cleanout cap. 5. Open the vent valve. 6. Open the propane torch valve. 7. Sniff the vent using either your nose or the explosive meter until the MAPP gas is detected at the vent. 8. Turn the gas off. Immediately close the vent valve. 9. Shoulder the spudzooka, aim and press the igniter button. 10. Immediately after firing, open the cleanout plug to vent the combustion chamber. Good venting is not absolutely necessary, as the filling procedure introduces fresh air but a little venting will provide maximum power. FAQ Q: How powerful is this gun? A: Very. It will shoot a spud out of sight. I have caved in the door of a junk car with a full power shot. One shot fired at the optimum angle traveled over 1/4 mile! Use caution, as the spud could cause serious injury if it hit someone. Q: Can I make this thing more powerful if I use oxygen instead of air? A: Yes. However, you may also blow up the spudzooka. I have read of people using oxygen with spudzookas using much smaller combustion chambers. I have not tried oxygen in this unit and do not plan to. I suggest you do not either. Q: Can I use propane? A: Yes, but it is more finiky about the A/F mix and it is not as powerful. A tank of MAPP will last a LONG time - probably hundreds of shots. The cost difference per shot is insignificant. --------------------- Update 05/10/01 Chronograph Results Today I set up my Oehler model 35P printing bullet chronograph to do some more detailed study. Photos chrono_1.jpg through chrono_3.jpg show the setup. This chrono measures the speed of a bullet or spud by measuring the time of flight of the projectile over shadow detectors. These detectors pick up the shadow of the bullet as it flys past. This chrono uses 3 screens so that two measurements can be made and cross-checked. The chrono prints out the speed measured by screens 1 & 2 and by 2 & 3. A regular bullet will register a speed across each set of screens that agree within about 5 fps. The spudzooka presents some problems. Because the combustion process is relatively long compared to gunpowder in a firearm and because the spud offers less than perfect sealing, there is a gaseous discharge that preceeds the spud. This stuff prematurely trips the near screen. Normally one would simply back away from the chrono a few feet and fire. The problem with the spudzooka is the projectile is so large and the aim so poor that it is impossible to reliably put a spud bullet through the screens from any distance. I blew the chrono down several times trying! My solution is to ignore the front screen reading and record only the second one. Another problem is EMI. The Oehler 35P is the top of the line chrono but it seems to be VERY sensitive to EMI. As in when the spudzooka igniter is activated. My partial solution was to play with wire orientation and readout box vs the spud gun. I still ended up wasting a lot of tuber in this experiment! I shot a total of 6 shots across the screens that produced a valid reading. The fastest was 312 fps and the slowest was 288. Not a bad spread. I had earlier shot a couple of rounds across the chrono and read nearly 400 fps. I'm not sure I know the reason for the difference but I have some theories. The spud bullets today are probably a bit heavier than my test projectiles used earlier. And after several hundred shots, the barrel is eroding near the throat just like a high powered rifle barrel does. There is a LOT of drag when pushing the spud through the last few inches of barrel. Weather conditions may have an effect too - it was much cooler on the previous test day. Regardless of the causes of the variation, today's results are still notable. Firing a 1/2 lb spud at BB gun velocities is impressive. ---------------------- More questions? Email me at johngd @ bellsouth.net (remove the spaces) John De Armond. 4/25/01