SpudFiles

Your dad might be talking about a blowgun, they have a sort of trigger on them, but you would really want to make a piston valve for that to have a real use which probably isnt the best idea for a first cannon. I say go with the ball valve, my first was a spring loaded ball valve (theres several of them in the showcase section) and contrary to what everyone says they do have a fair amount of power behind them if they have a decent spring to open it. Or another option would be to try a valveless, as far as I know I am the only one to do this on a large cannon though and is tricky to find ammo that seals perfectly.

thats weird guys but mine were like that too but i have figured out that a basic combustion is way less power full then some of my pneumatics also the chance of something blowing up are way more higher on a pneumatic

Well a ball valve seems fine to me, however, perhaps the sprinkler would do you better for the larger scale. My first cannon was also pneumatic with ball valve maybe it could help you

http://www.spudfiles.com/forums/my-firs ... 15309.html



btw, that really sucks you guys can't access PVC easily in the UK:/

we can get pvc up to i think 35mm quite cheap but anything bigger than that starts to get really expensive. My next project needs 4inch pvc but it costs about £60 for a 6m length but you cant get it much shorter than that.

60 for a 18' lenth seems reasonable actually, just not in my currency though.

KRN wrote:btw, that really sucks you guys can't access PVC easily in the UK:/

I get along without...


That little beast can present a good challenge, energy wise, to cannons many times it's size. And in terms of velocity, I've measured results of over 1000 fps.
You can't do that with PVC, because it can't safely take the pressures involved.

And that brings me neatly on to the next point - aside from 300 psi, the reason the cannon can manage that performance is the piston valve.
Although you can certainly work a cannon around a ball valve - I've done it myself back in my newbie days - the valve is the heart of the cannon.

For best performance, the valve needs to be able to do the business. A ball valve has limits on it's flow, and it's opening times can be measured in hundreds of milliseconds - far longer than it takes the projectile to leave the barrel.

A beefier valve can manage more flow, and open much faster - both good things. Basically, a good valve allows more air to get behind the projectile, and really give it some ooomph.

@Trollhameran: Try here - http://www.trssupplies.co.uk/product_in ... cts_id=258
The size you'll want for roughly 4" is the 110mm size - about £30 after VAT for the 5m length, so not quite as heavy on the pocket.

However, don't try mixing imperial and metric, that's just going to be ugly. Do it all in metric.

dudeman508 wrote:my parents wont let me make cumbustions either

Well, then mabye you should just close the door



That part about pneumatics being safer than combustions is a complete myth, combustions produce less pressure than a pneumatic for less time.

Also, "alot of power" is a relative term- compared to a toy air gun, a rusty ball valve actuated by a retarted, cross-eyed sloth will seem pretty godly. The best part about pneumatics is that you can change the valve easily, so if you don't end up liking the ball valve, you can easily swap in a sprinkler valve.

TurboSuper wrote:so if you don't end up liking the ball valve, you can easily swap in a sprinkler valve.

Although, not of course, if you've solvent welded it together.

As for whether combustion or pneumatic is safer - that depends on what question is being asked.

Like Rag has stated, unless you have a lot of money or a special supplier that can get you large bore PVC, it might be best to go copper.

You could make a high velocity paintball 'sniper' with just a few feet of 15 or 20mm copper pipe as a chamber, a 3/4" QEV (valve), a blow gun for the 'trigger' (pilot valve) and a few feet of what ever size copper is needed for paintballs or even marbles maybe.

Then you can slowly make it better, add breechloading, make a design that incorporates a stock and handle, maybe chuck a 'safety' on the cannon somewhere and you could even add scopes, lasers, bipods and homemade muzzle breaks or suppressors.

TurboSuper wrote:Also, "alot of power" is a relative term- compared to a toy air gun, a rusty ball valve actuated by a retarted, cross-eyed sloth will seem pretty godly.

Now you certainly won't find any of those around here...

dudeman508 wrote:my parents wont let me make cumbustions either

Dudeman, sorry to hear about your parents limitations on combustions. Don't hold it against them but they are simply uninformed and trying to keep you from "blowing yourself up"...something parents typically just do by reflex.

If you're interested and think it would help, I would be glad to talk directly to your mom or dad by email or even phone to help assuage their fears. Just let me know via PM. I'm a parent too.

KRN wrote:btw, that really sucks you guys can't access PVC easily in the UK:/

It's easy enough but it's not the same when you can't pick bits up and fiddle with them before buying anything.

TurboSuper wrote:
That part about pneumatics being safer than combustions is a complete myth, combustions produce less pressure than a pneumatic for less time.

Combustions produce 1000's of PSI IIRC. The only thing that stops it from exploding is the fact that it is instantaneous.

Coodude26 wrote:

TurboSuper wrote:
That part about pneumatics being safer than combustions is a complete myth, combustions produce less pressure than a pneumatic for less time.

Combustions produce 1000's of PSI IIRC. The only thing that stops it from exploding is the fact that it is instantaneous.

Try 40 PSI in a hairspray and 60-80 PSI in a metered combustion.

Coodude26 wrote:Combustions produce 1000's of PSI IIRC. The only thing that stops it from exploding is the fact that it is instantaneous.

No......

Even under perfect lab conditions, propane combustion is limited to around 120 psi.
In the real world, peak pressure is likely to be 60-90 psi depending on chamber shape and ignition details. The average pressure on the projectile is likely to be of the order of 25-40 psi.

Don't start out with a combustion, they are much more complex and likely to frustrate you on your first attempt. For a combustion, you need: chamber, barrel, fuel metering system, ignition, mixing, and venting. Compare to a pneumatic which needs: chamber, valve, barrel.

And propane combustion doesn't produce thousands of psi if it starts at atmospheric pressure... Just figure out how hot it gets, and you can get a rough idea of the pressure, because as both reactants and both products are gases, there's no overall volume change.

I'd say for a basic starter gun, something like this: HPHH. Easy to throw together in an hour or so, and can shoot your basic tater a good 200m or so at normal operating pressure of a few hundred psi. Even at high bike pump pressures, not something you'd want to get hit by.