Build a Potato Launcher

Everyone loves to eat potatoes, but firing them out of a cannon can be just as fulfilling. Also known as a spudzooka, potato gun, potato cannon and Spud Gun, this is a fun project that demonstrates the principles of physics. Part of the thrill and enjoyment of constructing a potato gun is to experiment with the basic design to tweak performance and learn valuable engineering lessons. Get creative with yours!

Steps

Assemble the Parts

  1. Get all your parts ready for assembly. See below for part list.
  2. Cut all your pipes to the correct length using a saw.
  3. Glue the end cap holder onto the end combustion chamber pipe (Fatter, shorter one) Make sure you don't get glue into the threads.
  4. Take the 3" to 1.5" reducer and glue the 3" end to the other side of the combustion chamber.
  5. Glue the barrel to the other end of the reducer.
  6. Put two 3-inch nails into the 3" section on either side so there about {{safesubst:#invoke:convert|convert}} apart.
  7. Put small alligator clips on the leads to your BBQ lighter.
  8. Let it dry for a day, then you're ready to fire.

Optional Part Assembly

  1. Create the air chamber. Cut two lengths of 2"-diameter pipe 30" long and a third 6" long. On the end of one, glue on the 2"x1" SPIGOT x FIPT reducer bushing. To do this, glue the bushing into the coupling, then glue the coupling onto the pipe. Using the elbows, then make a U-shaped pipe, with the 6" section as the bottom of the U. Cut the bike tube off of the valve, leaving the valve and a small flap of rubber around then the valve, about 3/4" wide. Before gluing on the pipe end cap, drill a hole in the center of the cap just big enough for the bike valve. Insert the valve, cut from the tube, and tighten the valve on the cap using the nut. Then glue the cap on the open end of pipe. Where the female threads are, thread the 3" long pipe nipple. Be sure to use the Teflon tape on the male threads of the nipple!
  2. Make the barrel. Cut a length of 2" diameter pipe 36" long (remember, you can always trim this down later!), and also cut a 4" long piece of 1"-diameter pipe. Glue the remaining reducer bushing into one end of the barrel (to do this, glue the bushing into the coupling, then glue the coupling onto the pipe), then glue the 1" pipe into the bushing. Finally, glue the 1" SOCKET x MIP adapter on the end of the 1" pipe.
  3. Put Teflon tape on the open end of the air chamber's pipe nipple. Thread the air chamber into the valve. BE SURE THE AIR WILL MOVE IN THE CORRECT DIRECTION!! Air from the chamber needs to move into the IN hole of the valve, and out of the OUT hole of the valve, as indicated by arrows. Tighten down until snug, and be sure to position the valve on the chamber so there is room for the barrel.
  4. Put Teflon tape on the threads on the end of the barrel. Thread this into the OUT hole of the valve until snug.
  5. Hook up a 24v DC or AC power source to the solenoid. 3 9v batteries wired together (in series) to make 27v works perfectly. A 12v battery will work effectively, too. Applying voltage will open the valve, cutting it off will close the valve.
  6. Load the ammunition (potatoes, fruits, balls, towels, etc.)
  7. Fill the air chamber with a normal bike pump or a REGULATED air compressor using the valve on the air chamber.
  8. Apply voltage to open the valve, thus FIRING THE GUN. Have fun!

Loading

  1. Ensure a tight seal around the projectile. This prevents the combustion gasses from escaping around the projectile instead of pushing it forward and will result in inefficient shots.
  2. Cut each potato in half and put it on a board or hard ground. Use the barrel as a cookie-cutter to force a perfect circle of potato into the muzzle and force it down with a ramrod.

Firing:

  1. Open the end cap.
  2. Spray propellant (almost all hairspray will do) inside the chamber and close end cap tightly. Seven seconds of spraying is plenty.
  3. Connect the alligator clips to the nails in the chamber.
  4. Aim away from all people and click the sparker (it may take a few sparks so don't be discouraged if it doesn't work).



Tips

  • The world of plumbing can be confusing and unorganized, so do not be afraid to ask the store staff for help.
  • After cutting pipe, remove the burrs with sandpaper or a file.
  • If your local hardware store does not have the exact fittings listed, don't worry. You will probably be able to find a way to use multiple parts (i.e. reducers, couplers, etc.) to accomplish what the parts listed do.
  • The longer the barrel, the longer the force of the combustion will accelerate the projectile. Too short of a barrel will rob the gun of power. However, if the barrel is too long, the expanding gasses begin to lose pressure and friction of the projectile in the barrel begins to slow it down. Find the optimum length for your own configuration by trial and error.
  • Read the valve's directions to get maximum flow through the valve. This often involves turning a knob or a collar on the valve.
  • Muzzle-loading firearms make use of "wadding" - patches of cloth to wrap projectiles to form a good seal with the barrel. The same concept can be applied to potato guns if other misc ammunition is used which does not lend itself to the "cookie-cutter method". The possibilities are endless.
  • If you have never worked with PVC before, buy some scrap PVC and a few cheap couplers so you can practice gluing before you build your launcher. Twist each glue joint a quarter-turn as you are pushing it together.
  • Try new ammunition! Potatoes are plentiful, but if you live near a walnut tree...
  • The volume of the combustion chamber is proportional to power of the shot, but a long chamber produces an inefficient compression wave so keep the combustion chamber short and fat.
  • The Christy's glue recommended below is a great one-step glue but requires you to work fast. Plan carefully and be ready to move once you apply the glue.
  • Pipe terminology is often confusing. Here are a few abbreviations used: FIPT - Female Threads; MIP - Male Threads; SOCKET- a slip socket; SLIP - unthreaded pipe, use glue to secure in a joint; SPIGOT - male slip on a bushing, etc.; OD - outside diameter; ID - inside diameter (PVC pipe measurements is usually measured by ID, so the OD will be larger than 2")
  • Follow the directions on the glue, and glue liberally on both the pipe and the socket. Do not use glue or plumber's dope on threads; wrap male threads in a couple of layers of Teflon thread-sealing tape.
  • Too much propellant gas is as bad as not enough. If too much oxygen is displaced then ignition will not occur. Trial and error will teach the optimum amount of propellant to use in your individual design.
  • If something needs to be sealed shut, use duct tape. It starts out as tape, but after you use the cannon the adhesive turns into a very sticky glue that keeps stuff closed.
  • The initial ignition in the combustion chamber should be close to the center of the chamber cavity for the formation of an efficient compression wave to transfer the energy of the explosion.
  • A screw inserted into the barrel where it connects to the combustion chamber will prevent the ammunition from being rammed too far and falling into the chamber.
  • There are plenty of instructions on the internet, just Google "Potato Cannon instructions". There are many variations to this cannon, so have fun and experiment, but be safe and smart!

Warnings

  • LET THE GLUE DRY!! Most accidents with these guns happen because an anxious builder does not let the glue dry. Let it dry a full 24 hours before shooting.
  • Be absolutely sure to empty out all the fumes before you test the igniter.
  • When you glue the PVC pieces together, you must hold them for at least 60 seconds so the glue can start to bond. You will need to apply a lot of pressure, or the pieces will slide apart when they dry.
  • Do not exceed 100 PSI- EVER. Most valves are rated for a maximum of 125 PSI with water, and too high a pressure WILL cause joints to fail. Even with 40 or 50 PSI you will discover the potential of this gun, so play it safe.
  • Projectile launchers involve risk. Have fun!
  • Make sure that you aren't ever looking down the barrel of the cannon when it's loaded. Make sure you wait at least 24 hours from completion of the cannon to the firing of the cannon. Make sure you never point this cannon at anyone, or do something stupid like cause property damage.
  • Only use Schedule 40 PVC pipe (PVC is white, ABS pipe is black). The 'Schedule' of pipe refers to the thickness of the pipe's walls. Any thinner than Sch 40 pipe is UNSAFE and could burst under high pressure, and is more prone to breaking.

Things You'll Need

Method 1

  • Sch 40 PVC: 1" dia, 4" long
  • Sch 40 PVC: 2" dia, 8.5' long TOTAL
  • 2" x 1" reducer bushing, SPIGOT x SOCKET
  • 2" x 1" reducer bushing, SPIGOT x FIPT
  • Two- 2" PVC Couplers
  • 2" PVC Pipe Cap
  • PVC Pipe Nipple: 1" diameter, 3" long
  • Can of Christy's Red Hot Blue Glue
  • Roll of Teflon thread sealing tape (white, approx 3/8" wide)
  • 1" Solenoid-operated sprinkler valve, U-shaped (not inline), female threads
  • Bicycle tube with a valve that has a nut, normally used to tighten the valve onto the rim. Presta-valve tubes for road bikes usually have this, while Schrader-valve tubes normally do not have a tightening nut. You need this nut to seal the valve onto the air chamber.
  • Hacksaw
  • Drill & Bits
  • Sandpaper or File

Method 2

  • 4' long of 1.5" PVC pipe for the barrel
  • 4" long of 3" PVC pipe for the combustion chamber
  • A pipe reducer from 3" to 1.5"
  • A 3" threaded end cap Socket
  • A 3" end cap
  • Coleman BBQ igniter
  • PVC glue
  • Potatoes

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