Anyhow, why am I posting then? Well, I figured as I'd done something interesting(ish), I'd post it. I built a potato cannon. This was really to cheer myself up after a spot of root canal work. I'd spend 2 hours at the dentists, and had moped about the house most of the day, finally settling down to do some internet surfing about 2.30pm. I ended up in a sort of 'what the flip was I looking for again?' type moment on a page on youTube with someone firing a potato cannon. I'd been interested in this for some time, and, thinking I'd got a spare couple of hours, I searched for a UK page where someone who'd done the same thing. Nothing against the US footage/pages I'd seen, just that I knew that if I got the UK page and there was the detail on how to make one, I'd know that I wouldn't confuse the guys in the builder's merchant with some US speak widget I was trying to order. I found this site which looked like it had promise. So I ended up with this:
Which is has the essential 2 parts to it - the below, which is the breech end, is the combustion chamber and firing mechanism, and the big long bit, which is the barrel.
My propellant is this:
My original plan was to pretty well gently lob potatoes over the garden next to mine, into the next-but-one. The idea was that I'd be able to judge it well enough so that I could call my neighbour and ask him about potato rain or something. He's ex-navy and well up for some lads type laughs with this type of thing. In the event, my test firing was so ferocious that I ran straight round there to get him round and help me test it properly.
We used this field as a range:
On firing we fired test shots with increasing amounts of spray. The first went 70 yards, the second 120, the third 150 (which is that tree line you can see at the far end of the field) and then we managed to lob a few more over the trees into the next field. Over 200 yards.
The next day we had some friends come round. Clearly a toy like this was too good not to show off. Mobile phone footage was taken!
Note that we back onto farmer's fields!
The pieces of ply in the foreground are from previous firings. A piece of potato shot from this can do serious damage!
Looking at other sites, there seems to be massive over-elaboration with these things. There's compressed air and electronic igniter circuits and all sorts. While interesting to make (no doubt), the essential parts of the fun are (1) big loud bang and (2) potato propulsion over unexpectedly long distances.
If you wish to emulate exactly what I did (and if it doesn't work out well for you, it's your look out - I take no responsibility for you maiming the neighbour's dog or whatever) you will need something like the following:
1) A length of 2" bore solvent weld waste pipe (it's sold as 55mm, but do not buy it metric - it just makes things awkward as there are odd sizes that are 'metric' and these do not fit most other waste systems). This is for the barrel. I used 70cm, as it looked about right to me. I think you could go longer, and perhaps that gives more range. You could certainly go smaller bore, and this would mean (a) your potato should achieve a higher velocity and (b) you get more 'rounds' from each spud.
2) A length of 4" bore solvent weld waste pipe (it's sold as 110mm). I used about 30 cm of this, and it's for the combustion chamber. I also used a second piece to wrap round the first - I'll explain why later.
3) 2 x 4" solvent weld straight connectors. You'll find that the other bits you need do not marry up directly to the 4" pipe, their outside dia is too great.
4) A solvent weld screw in inspection lid. This is for the breech lid.
5) A solvent weld reducer to go from 4" to 2" (or whatever bore you used - if you go lower than 2" you might need to buy 2 adaptors - 1 to go from 4" to 2", and another to go from 2" to 1.5" or whatever.
6) Solvent weld cement
7) Epoxy glue
I had the waste pipe lying around at home - the rest of the above I bought from my local Travis Perkins, along with the electrical tape and gaffer tape mentioned below.
8) A piezoelectric oven igniter. These are cheap off ebay, you can see from the pics that you need the push button type.
9) 2 x 5mm bolts, 2 x nuts to suit (for the electrodes), 2 lengths of decently thick wire (something like a bit of main beam cable from a car), a good roll of insulating tape, and a roll of gaffer tape.
10) Tools - a drill, a 5mm drill bit, a spanner for the above bolts, a pair of pliers, a soldering iron (maybe) and some solder, some sandpaper, a good woodsaw or hacksaw (to cut the pipe)
11) Hairspray (I used Tesco own brand in a blue tin), Potatoes.
12) A broom handle
What you do (read the whole set of directions before starting):
A) Put the plumbing stuff together with solvent cement.
i) The screw-in inspection lid goes on to the first straight-on connector
ii) The first straight-on connector goes onto the 4" pipe
iii) The second straight-on connector goes onto the other end of the 4" pipe
iv) The reducer goes onto the second straight-on connector
v) The 2" pipe goes into the reducer.
B) Make the igniter (the cannon's trigger)
i) Take 2 40cm-ish lengths of isulated wire, strip both ends of both pieces.
ii) Attach both pieces to the piezoelectric button (it's kind of obvious, but one connector is in the bottom of the igniter, the other is on the side.). I used solder for this, but if you have the right spade connector for the bottom terminal, you could get away with not soldering things.
iii) Loop the wire from the side terminal up and away from the one at the bottom, and wind electrical tape around the piece of the igniter below the thread generously to ensure that the two terminals are well away from each other.
iv) Loop the other stripped end of each piece of wire around the threads of each bolt and tighten the nut onto the wire to ensure good contact. You should now have a button smothered in insulating tape, but with two wires hanging away from each other with a bolt hanging off each one.
v) Liberally wrap each of the nut and bolts in insulating tape, leaving the remaining thread below the nut free of tape.
C) Putting it all together:
i) Drill two 5mm holes in the first straight-on connector such that when you poke the bolts through them, the bolt tips end up about 7mm apart. You should be drilling such that the drill bit is going through the connector and the 4" pipe. The spark should jump that with no issue.
ii) Use the epoxy glue to secure the bolts in the holes.
iii) Run the wires down either side of the combustion chamber (the 4" pipe) so that they are on opposite sides. This is important, the spark will otherwise jump between the wires, meaning no spark at the electrodes (the bolts). Wrap the combustion chamber in insulating tape to hold the wires where they are.
iv) Take 15cm of 4" pipe and cut one side lengthways. Open this up and fit it around the combustion chamber and the wires. Secure this with gaffer tape (mine was pink - gaffer tape was sold in aid of breast cancer). This may well be optional, but this is exactly what I did. I did it mostly to insulate the wires from each other.
v) Use more insulating tape to attach the button to the combustion chamber - again making sure the two wires are well away from each other.
vi) Use sandpaper to chamfer the end of the barrel down so that it will cut into a potato (see the pics)
D) Do some testing:
i) You should do this test before drilling any holes probably, but you should test that the spark will jump the 7mm. Ensure the wires from the button are well away from each other, and test using an off cut of the 4" pipe. Drill two holes and then poke the bolts through and press the button. Keep away from both the wires and the electrodes. If it wont' jump, try sanding the bolt tips a bit to clean them. Ensure the electrodes are insulated from each other as much as possible. I added the silicone sealant you can see in the photos between the electrodes for this reason.
After all of this, I waited until the next day before firing. This meant I knew all the glue was set.
Firing (make sure that there is nothing you don't mind destroying anywhere in front of the muzzle line):
i) Take the inspection lid off the breech.
ii) Push a potato down on the barrel such that the barrel cuts a hole in the potato
iii) Use the broom handle to push the potato down to the bottom of the barrel - obviously avoid pushing it all the way out into the combustion chamber.
iv) Spray hairspray for a second or so into the combustion chamber - try to avoid the electrodes.
v) Screw the inspection lid back on
vi) Position the cannon firmly so that it cannot move in any direction
vii) Press the button.
viii) Be surprised!
If it doesn't work, be careful of opening the breech and pressing the button - the flame will shoot out the back of the cannon. Fortunately my neighbour was fine about losing the odd bit of hair/eyebrow when this happened.
I also used butane/propane mix from a blowtorch, and that worked well, but no real discernible advantage over hairspray in terms of propulsion.
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