All you engineers....

The last thing you want to do is have a tight fit, but that's in regard to compressed air cannons. Don't know hwo it differs with these things.

Although we did build a compressed air cannon to fire golf balls in our 4th year to around the 150yard mark. Managed to replicate spin too so teh ball nigh on stopped dead on the green. Turned out to be one hell of a project.
 
tres said:
Any hints or tips? For anything - design, fuel mixture/amount etc etc greatly recieved! I start mine on Thursday, video will be done
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Treat the cannon as a control volume. Dampen the vibrations critically by using a high constant spring. This will reduce the recoil effect due to the negative inertial forces. It should give you a longer range if the trejectory is correct.

If you can, add spiral grooves on the inside of the cannon. This will give your object that you're projecting a mass moment of inertia which'll make it travel much longer and reduce drag. Read up some of Bernoulli's work to see how it'll work.

Good luck :)
 
Far too many recommendations in my direction for me to feel comfortable about this :D

The furthest I've got a tennis ball is 450 metres. 8 cans, bottom one needs to have a moulded bottom like this one or it splits and you burn your leg:

cannon2.jpg


Bottom can has a small hole about this size but no smaller:

cannon3.jpg


Leave one end on every other can, cut half round and fold it over like so:

cannon4.jpg


Gaffer tape them together in a spiral so they look like this:

cannon5.jpg


cannon6.jpg


cannon7.jpg


cannon8.jpg


Then gaffer about 3-4ft of black plastic drainpipe into the end for your barrel. You'll have to tape round a few times to make it a snug fit, then gaff it to the top can to make it airtight.

Use Zippo fuel, just a short 1-second squirt in the hole and a short squirt in the top making sure it goes all the way down. Wave it at arms length 25-30 times to make sure it all evaporates, drop the tennis ball in and light the hole with a Zippo lighter, holding it hard into your hip. It will scare the **** out of you and don't be tempted to use a Clipper lighter :)
 
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I leave my cannon forums to come here and I find them here too.

Amateurs use taped cans, get on UKSGC or Spudfiles and as well as being told to use the search button several times you'll also get pointed to designs that would suit you.

I could help you make a design that your competition would be incapable of beating but as a rule of thumb, money = power.

What's your budget for this. If you're not up to spending much then might as well go for a cannon which is referred to as a "spray and pray" due to the air/fuel mixture being rather unreliable.

Enthusiast level cannons use fairly precisely metered doses of propane gas, computer fans inside the chamber to mix the fuel and air and a proper ignition system. The pressure generated from those is up to about 90psi, increasing barrel and chamber size generally increases performance as long as one is not in extreme.

Pneumatic cannons can use any pressure possible. I've got one which I made for 300psi air. If you're a machinist then unregulated CO2 or Scuba tank pressures are possible too. However they are more... technical than combustion based cannons.

The most powerful end of the spectrum are hybrid cannons. If you understand that in a normal flammable gas-air combustion cannon everything is mixed at atmospheric pressure then you'll also see that if you use compressed air and compressed flammable gas in the same space, more energy will be released. That's the basis of a hybrid cannon.

I've gone on a bit. I wrote a lot of the wikipedia article titled "Spud Gun" if you want to know more. Or visit the above forums.
 
Using multiple tins is surely a weak link! Unless you seriously spend time and weld them together so they are all air tight.
 
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just go the whole hog get a can of deodrant like lynx or something as the fuel stick the whole can inside the actual can.

make a fire and stick the bazzoka in the centre once the can of deodrant goes critical there will be a huge "BOOOOOOOOOOOM really loud , shrapnel will fly everywhere and your tennis ball will probably smash into the side of the moon.

can vouch for the boom btw :P when bomfires werent illegal as a kid we used to throw doedrant cans etc into the fire for the boom was pretty epic
 
WillyNelson said:
Can you explain why there are 'baffles' in the combustion chamber and why they're arranged in a spiral? Thanks
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I've made them with a completely empty tube and one just baffle at the top, assuming the baffle was there to stop the tennis ball dropping all the way to the bottom and it didn't work well. With an identical length tube the one with the baffles will go KABOOM when you light it, with a bright flash, and the empty tube will go floomph with a lot of slow burning flame out the top. From my limited knowledge of fluid dynamics and thermodynamics I know if you compress a mixture it will burn a lot quicker and I think this is what the baffles do. They trap the mixture and it gets compressed from below causing it to burn quicker. Faster burn means faster energy release and more energy is put into the tennis ball.

It's just my experimenting that came up with the spiral shape and it makes it easier to squirt fuel right down the tube than when they are opposite each other. I made one with an empty tube, but the baffle was one end left on with about a 35mm hole cut out the middle. It worked well but was really difficult to cut out, whereas cutting half an end open with a tin opener and folding it over is really easy.
 
I have yet to make one of these, although I am thinking about it as an excellent fireworks night project. Would be interested to find out if you could get the burn to light a fuse of a firework inserted into the tennisball...

Also I am sure I remember from somewhere that the perfect barrel length is roughly 45 times the diameter of the projectile, though not sure if that was for rifled or smooth bore.
 
Jonny69 said:
That may be so, but you don't get much more bang for your buck and you can buy everything in Tesco.

Personally, I think simplicity is the key here.
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This entirely depends on what you are doing.

Enthusiasts use pipe: PVC, ABS, copper, aluminium, brass, steel or anything similar.

It's smoother, has proper connections and looks better.

I'm using the word enthusiast in the same sense as computer enthusiast. You don't have to have an i7 instead of a C2D but you want one for the extra performance.

I'll spec a quick and simple cannon:

First of all the main components (from Kiowa)

tmp1.jpg


That is a 1m (or less) 90mm pvc pipe as the chamber, 2m of 75mm pvc as the barrel, connected with a reducer and filled from the other end through a screw plug.

The pipe was chosen to match the OD of a tennis ball.

I missed this from the picture but you also need PVC solvent which essentially temporarily melts the surfaces it is applied to, allows them to combine with each other and when the solvent evaporates it allows the plastic to reharden, welded together: Clicky


Secondly you need an ignition system

Any kind of spark system will work. Piezo based lighter such as these generate a few tens of KV at very low energy which allows a spark a couple of centimetres long:
Clicky. Easily modded by taking apart the long nozzle and rewiring the wires to terminals screwed or bolted into the ignition chamber. That particular model contains lighter fluid and makes a flame which the trigger is pressed, disassemble the lighter and discard the lighter fluid container inside it.


Finally you want a decent propellant

Contrary to popular use Hairspray/Deodorant sucks. It contains a whole load of crap which burnt or otherwise coats the inside of the chamber and your ignition terminals with a sticky residue.

Ideally you want propane or butane or similar however they don't come in aerosol form. Starter fluid in spray cans is quite effective without residueClicky.


Approximately £100 all in before working out how to connect the ignition to the chamber.


Tins are well and good for demonstration of the basic principles but you need pipe for performance and power, it's not cheap but its much better.
 
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£100 would have built nearly everything in my garage including 2 chopped bikes, industrial pogo stick, cannon, pulsejet scooter and a sledge though :eek:
 
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