This is my latest combustion cannon, and I have tried to make it as efficient as possible with a 0.8:1 barrel to chamber ratio. It measures 3.65 meters long and weighs a heck of a lot due to its 6mm thick walls. The power behind this new cannon is amazing it will fire anything you can fit down the barrel way out of sight, potatoes even get obliterated when exiting the barrel. The one disadvantage is loosing the huge bang of the combustion because all the energy is used into moving the projectile! It can fire a potato out of sight and golf balls with wadding go even further. I even accidentally left the ram rod in the barrel and fired - no problems the pole flew across two fields and the recoil on the cannon flew itself backwards along the ground a couple of meters, fantastic fun.

The other disadvantage is its size and weight make it particularly difficult to transport places - I usually get a friend to help!

All piping/plumbing components are made from PVC.

• 110mm Ø, 80mm long pipe, 3.2mm thick
• 110mm Ø, 65mm long pipe, 3.2mm thick
• Two 50mm Ø, 3000mm long pipe, 2mm thick
• 110mm Ø, access plug
• Two 110mm Ø, double socket
• 110mm Ø, to 50mm Ø, reducer
• 110mm Ø, strap boss
• 50mm Ø, to 32mm Ø, reducer
• 32mm Ø, access plug
• 32mm Ø, double socket
• 32mm Ø, long tail bend
• 32mm Ø, 40mm long pipe
• 50mm Ø, 20mm long pipe
• Two 300mm lengths of 2mm thick insulated copper wire
• Piezoelectric sparker from a lighter
• Sheet of PVC 80mm x 80mm x 2mm
• Two: 25mm lengths of 2mm Ø thick insulated copper wire (mains wire)
• Solvent weld cement

Note: Method for preparing surfaces that need to be solvent welded are as follows:
Prepare the two surfaces that are to be joined by lightly sanding them to roughen the surfaces and clean with methelated sprits or some other cleaner, then apply the solvent weld generously to one face of the material and then promptly push them together firmly.

To make the combustion chamber section, I took the 110mm Ø, 65mm long pipe and marked out a straight line on which I would cut along. This was to produce a sleeve to fit on the outside of the 110mm Ø, 80mm long pipe as the chamber needed to be thicker to withstand the large pressure buildup from the combustions when firing.
Getting the sleeve onto the other pipe was quite tricky - I used pieces of wood to pry the sleeve open. I then applied a small amount of solvent weld to the outside of the other tube and passed the inner pipe through it. I then removed the wood to close the sleeve over the inner pipe, making sure it was in the centre.


I then cut rings from a length of 110mm Ø pipe; and cut a plate to fit in the 110mm Ø reducer (see photo). This is simply to add strength to the reducer as it needs to be thicker and stronger around the 'corners' of the join in the reducer.




I solvent welded two 110mm Ø double sockets to each end of the combustion chamber section.
I then solvent welded the 110mm Ø access plug and 110mm Ø reducer into these double sockets.







Using a hole saw, I cut a 50mm hole in the front, top of the combustion chamber to allow the igniter to enter the chamber and allow the strap boss to fit snuggly when solvent welded to the side (see photo). The position of this whole was to position the handle as close to the center pivot of the cannon as to compensate for the weight of the 3 meter barrel! This will then make holding the cannon easier.




This next bit was another tricky bit. The strap boss now doesn't fit perfectly around the chamber as I have added the sleeve and increased its diameter. I applied solvent weld all round the boss and whole on the chamber, and QUICKLY held it in position with a couple of g-cramps to hold the strap boss flush with the side of the chamber - it was very important that this was a good, strong fixing! I also used a bent screw to fasten the strap boss shut shown in the photo.



To make the handle I solvent welded the 50mm Ø to 32mm Ø, reducer; the 32mm Ø, 40mm long pipe (female to female connector); the 32mm Ø, long tail bend and the 32mm Ø, double socket (male to male connector), as shown in the photo left.






I cut a segment about 20mm wide in the 50mm Ø, 20mm long pipe as shown in the top right of the photo. This needs to fit inside the reducer and leave enough room to let the two copper wires to fit snugly between the gap (refer to photo on right).
With the sheet of ABS I made a blanking plate by cutting out a 50mm Ø circle. I cut a notch out the side to allow the copper wires to pass through snugly as well (see photo on right). The blanking plate is used to stop the combusting fuel pressure from blowing up in to the handle section when the cannon is fired.



The copper wires were passed through the handle and the blanking plate and ring were then solvent welded into place. The gap between the end of the wires is important for successful ignition of the propellant; set the gap between the two wires to about 5mm and change accordingly when cannon is complete.






To attach a wire to the metal base of the sparker, I made a 'cup' to fit around the end of it, out of thin metal and soldered a wire to this. You must not solder to the base/end of the sparker as this causes it to fail and stop sparking!!
Using the rest of the ABS sheet I cut and made a casing around the sparker. I used a short section of oval tubing (which is used on the end of the fabric for blinds) and cut square bits of ABS for sides. Its a bit of a "make it up as you go along" method, but it seems to work just fine! Just look at my photos below...




I then cut a square hole in the access plug to allow the sparker to fit through it; as shown.
I solvent welded the sparker holder to the access plug and held it in place for about 5 minutes to allow the weld to set.







I soldered the wires from the sparker to the two copper wires in the handle, and then separately wrapped the exposed parts of the wire with insulting tape.








This handle was solvent welded onto the chamber of the cannon, making sure that the ignition wires where positioned correctly within the chamber.











This is yet another tricky bit, which involves a lot of brute force and a wall! I cut a straight line down one of the 3 meter 50mmØ pipes using a steady bench, clamp and a jig saw to make a sleeve for the barrel. Then I inserted the end of the uncut 3 meter pipe into the sleeve. Using the wall I 'carefully' banged the pipe into the sleeve (making sure I didn't fracture the piping). the sleeve should be short of being flush with the inner pipe to allow the barrel to fit into the reducer (see photo). I also cut the other of the barrel so it was flush. The barrel was then solvent welded into the reducer, with the help of a friend. 

This is just a small guideline to operating the cannon - to get best results you will need to practice and develop you own methods of operation.



• Unscrew the access plug to allow air to freely pass through the cannon before loading a potato/projectile.
• Take a large potato and firmly press it down on the end of the barrel to cut a potato projectile.
• Using a large dowel push the potato down the barrel.
• Now you add the propellant - spray about a seconds worth of hair spray (or some other propellant) into the combustion chamber. You will need to perfect the correct amounts by trial and error.
• Quickly screw the access plug back on to the chamber.
• Ready aim FIRE!