Construction The Ignition System One of the most tedious parts of the gun to construct is the ignition system. After trying piezoelectric (mechanical) BBQ sparkers (which failed as often as they worked) I settled on an electronic igniter designed for a gas heater. The biggest challenge was making it fit inside a 2" pipe but that turned out to be fairly easy. The spark is 100% reliable and that's all that really matters. You can find an electronic igniter at local propane retailer. Look at their gas heaters and one of them will almost certainly have an electric igniter rather than the cheaper mechanical type. The one I got had one lead from the bottom and one on the top that was intended to ground to the heater frame itself. I soldered a piece of 18 gauge 40Kv wire to that lead which then gave me two wires to attach to the ends of the business end of the igniter. It is extremely important to use high voltage wire. It's going to cost you two or three bucks a foot but it's well worth it. While building the BFSG-4000 I tried using regular 300 volt "hookup" wire and quickly found that when the leads got within a quarter inch or so of one another the spark would jump straight through the insulation. You can order some 18 gauge 40Kv wire from Surplus Sales Of Nebraska. They carry a bunch of useful items besides the wire as well. I used a similar design on this igniter in the BFSG-4000 but wanted to make it completely self-contained. Using the same method I installed the ignotor unit inside the 2" pipe. I got a 2" connector (connects two pieces of 2" PVC pipe), drilled a hole in the middle of that, notched two short pieces of 2" PVC to fit around the button stem of the igniter, positioned the igniter in the connector and glued the two pieces of PVC pipe on either side of the igniter. As you can see in the image I needed to mount a piece of 1/2" (Outside Diameter) CPVC inside the 2" pipe. I took a 1 1/2" to 1/2" adapter and used epoxy to glue it inside a piece of 2" pipe. I glued a 3" piece of 1/2" CPVC pipe and a fitting in the adapter. The copper spark rod was constructed with a 4 1/2" piece of copper patch pipe (usually used to join two 1/2" copper pipes), three copper rings (usually used to crimp over 1/2" copper pipe to join pieces) and a 1/2" copper cap. I glued the copper patch pipe on a second piece of 1/2" CPVC. I then used a 1/8" thick nylon washer that I cut in half to space the rings evenly on the 1/2" pipe above the copper patch pipe (see the close up image). The cap was left unglued so that it could be removed to change out the spark rod. I drilled a small hole in the cap and placed a short brass bolt through it. One of the leads from the igniter was fed through the pipe and a ring terminal was soldered to the end and bent at a 90 degree angle. This allowed me to bolt the ring terminal to the cap. The other lead was run through a hole drilled in the adapter and was also ended with a ring terminal. This was then bolted to a strap wrapped around the lower part of the spark rod (again, see the image). This configuration leaves two 1/8" gaps between the three rings. This gap seems to work quite well with a gas heater igniter.

The Main Gun Body The gun body consists of several PVC pipe fittings connected with short pieces of 3" and 2" PVC pipe: 3" Cleanout with Plug 3" x 2" Tee 3" Connector 3" x 2" Tee 3" to 2" size reducer 2" Female connector 2" Male connector 2" 90 Degree Elbow 2" 90 Degree Elbow 2" Male connector

To the right you can see one of the two brackets used to hold the combustion chamber, barrel and gas tank in place. I made them out of 3/4" plywood and lined the inside with felt to keep from scratching the paint. I wanted to hold the barrel in place without putting stress on it and to allow it to be breach loaded and removed if necessary. To accomplish this I cut two 1/2" slices of 2" steel pipe, painted them black and lined them with felt. I then strapped them down to the brackets and ran the barrel through them. The straps are just like the ones used on the BFSG-4000, two hose clamps run through black rubber hose. The TA-3100 is currently sporting a four foot long 1 1/2" clear rifled barrel custom made by Joel over at The Spudgun Technology Center. This is positively the coolest barrel money can buy! I used screw type fittings on the elbows so that the elbow can be moved to the side when changing barrels or breach loading. This way you can unscrew the barrel move it forward to breach load or move the elbow to the side and slide the barrel out to the rear. Otherwise you would have to remove the straps completely to get the barrel off. This works well in conjunction with the steel sleeves that hold the barrel in place. You can see the BSA Laser sight mounted on top of the elbow just behind barrel. This placement was chosen because it let me sight the laser in by simply looking through the mount and placing the red dot just above the top center of the barrel end. So far this has proven to be quite accurate.   If you've never done any PVC work get someone at the hardware store to give you a quick rundown and get you the right surface prep and glue. I used Schedule 40 PVC for my gun. Schedule 80 is harder, thicker and stronger but when it bursts it makes much nastier shards than Schedule 40. This design is double thick except at the front of the combustion chamber so it's very unlikely you'll blow it up unless you start injecting exotic fuels like Acetylene and pure Oxygen into it.

The Fan System In order to force in clean air and to mix the fuel with the air I needed to have a fan inside the chamber. I ordered a 2 3/4" fan and removed it from the frame. I then hot glued the fan into the back of the 3" cleanout plug (see image above). Then I wired two 9 volt batteries in series to produce 18 volts and put them inside a 2" cleanout plug. I drilled a hole in the side and mounted a pressure switch to control the fan. I mounted the battery container to the first 3" x 2" Tee. To seal the batteries off from the combustion chamber I placed test plugs at the top and bottom of the piece of 2" PVC used to attach the battery container to the Tee. I filled the space between them with foam (like "Great Stuff") with the wire running through the test caps. The connector for the fan is inside the combustion chamber so that the fan can be replaced if it should fail.

The Direct Injection Fuel System The direct injection system is the heart of this gun. It works essentially by using a known pressure and volume to measure the volume of fuel needed for optimum combustion. First I took a standard propane torch head and cut off the neck leaving an inch or so. Then I took a 1/8" NPT thread die and threaded the end of the torch head. I then screwed on a 1/8" to 3/8" adapter. Next was a 3/8" nipple which connected the torch head to the regulator. For those unfamiliar with regulators all they do is take pressure on one side and regulate (thus the name) the pressure on the other side. You can set the pressure to whatever you need. These are often used to regulate the pressure in the lines for pneumatic tools (air wrenches, etc.). The regulator used on this gun is a Norgren R73G-3AK-RFG which will take up to 300 psi on one side and regulate it in the range of 5 psi to 60 psi on the other. I got this regulator on EBay for less than $30.00 including shipping. The rest of the brass and steel cost less than $40.00. Not cheap but considering the quality of the parts not too bad either. On the other side of the regulator I screwed in a 3/8" MPT x 3/8" Flare adapter. I had a hose fabricated with 3/8" flare fittings on each end to attach the regulator to the measuring chamber. The measuring chamber consists of another 3/8" MPT x 3/8" Flare adapter, 3/8" ball valve, a 3/8" to 1/2" adapter, a 8" long piece of 1/2" steel pipe (oddly enough it has a .625" inside diameter), another 3/8" to 1/2" adapter, another 3/8" ball valve, a 3/8" nipple, a 3/8" street elbow and lastly another 3/8" nipple screwed into the PCV combustion chamber. To install the nipple into the PVC I drilled a 9/16" hole and used a 3/8" thread tap to thread the hole. Then I applied a very small amount of epoxy to the threads, put a cap on the nipple (to alow me to use a wrench to turn it into the hole) and screwed it in so that it just broke the inside surface (about 1/2" deep). To use the system you set the pressure via the regulator and open the first ball valve. This pressurizes the steel pipe between the two ball valves. You then close the first ball valve which traps a specific volume of gas at a specific pressure between the ball valves. When you open the second ball valve a specific volume of gas squirts into the combustion chamber. The SpudTool V1.0 can be used to calculate the proper size and pressure for your direct injection system. I used it and my injector produced exactly what the calculations said it should. It may sound complicated but it's really not. Figure it this way; if you know there is three cubic inches of space between the ball valves then at one normal atmosphere (about 14.5 pounds per square inch or psi) you have three cubic inches of gas in there. So, at two atmospheres (29 psi, the guage would only read 14.5 psi since it shows the difference between the inside and outside of the pipe) there would be twice as much gas or about six cubic inches. So, if you open a ball valve and let the extra gas out so that it goes back to one atmosphere then, you guessed it, about half, three cubic inches of gas will squirt out. Adjust the pressure to get the exact amount of gas you need and you get a perfect burn.

Useful Links Burnt Latke is a great source for ideas and test data to help you optimize your spudzooka. The Spudgun Technology Center has some useful information and VERY cool parts like Clear Rifled Barrels! AdvancedSpuds.com has some good ideas for several aspects of spudgunning.

 

Dedication This gun was painted as close to purple (my wife's favorite color) as Krylon Fusion colors get (Hyacinth) in honor of my lovely wife who puts up with me making explosive devices in our living room at all hours of the day and night. Thank you Sweety.