Introduction

This was my second combustion potato cannon. It was a significant improvement in efficiency and size over the first cannon. The following content is from the original project page.

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 quite a lot due to its 6mm thick walls. The power behind this new cannon is a brilliant feeling! It quite happily fires potatoes well out of sight, I just need to remember to remove the length of dowel used to load the potato before firing…it’s not supposed to be a javelin cannon!

The one disadvantage is its size and weight which make it particularly difficult to transport places – I usually get a friend to help and we usually get to deal with the public’s bemused looks on our way to the firing range.

Materials used

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

Construction of the cannon

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 build-up 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 snugly when solvent welded to the side (see photo). The position of this whole was to position the handle as close to the centre 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 outer diameter. I applied solvent weld all around the boss and around the hole 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).

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. 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.

This next bit was probably the trickiest. 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 plastic tubing and cut square sections of ABS for sides. It was a bit of a ‘make it up as you go along’ method, but it seemed to work just fine!

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.

After leaving the cannon’s joints to fully set it was ready to go!