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2 questions

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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2 questions

Unread postAuthor: theepicmool » Sat Nov 22, 2008 11:14 am

1. The wiki article for fittings says you can tell if they are pressure rated by how they look etc. To what extent (in PSI) does this mean they are rated?

2. What is the best size for the corner/pipe between the chamber and the barrel (keep in mind that the valve by the barrel is 1")
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Unread postAuthor: FishBoy » Sat Nov 22, 2008 11:19 am

1. depends on the size of the pipe

2. well what size is your chamber?
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Unread postAuthor: theepicmool » Sat Nov 22, 2008 11:22 am

Chamber=6ft 4"
Barrel=6ft 2"
The size of the pipe in question 1 is dependent on the answer of question 2 (with the exception of couplers).
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Unread postAuthor: Ragnarok » Sat Nov 22, 2008 12:11 pm

A quick one from me: What ever happened to descriptive topic titles!
Vague titles severely cripple your chances of getting help!

That rant over... There is still inadequate information to answer question #1.

For number #2, if you have a 1" valve, then 1" pipe would be adequate, but as you're already presumably buying a big stick of 2" pipe for the barrel, then use some of the 2" to save having to buy a separate length of 1" pipe. It will also be a bit sturdier and look less imbalanced.

Also, there is no need to have such a ridiculously large chamber for that barrel. You could halve the length of that chamber (or drop the diameter to 3" instead) and still get almost exactly the same performance for half the work/time filling it up!

Personally, considering the likely flow restriction at your valve, I'd cut the chamber back to 4 or 5 feet of 3" pipe, which would massively reduce the air needed to fill it up, drop the weight and bulkiness, but at the cost of almost no power.
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Unread postAuthor: theepicmool » Sat Nov 22, 2008 12:39 pm

My bad on the title. In that case the fittings would be 2 1" elbows and the couplings to bring the 1" up to the proper 4" (or 3") and 2" pipe sizes.
The reason for the large chamber is because of spudtechs recommended 4:1 ratio (keep in mind my valve will be modded). If i got 10ft of 3" i would be cut back to a 1.5:1 ratio.
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Unread postAuthor: Gippeto » Sat Nov 22, 2008 12:52 pm

Forget what "spudtech" tells you.

Download ggdt and use it. 4:1 is bloody ridiculous!

Anything over 1:1 has seriously diminishing returns.(Most of my stuff is closer to .8 to 1)

I modeled a generic launcher using your barrel, and some different chambers.

Going from 1:1 to 4:1 does yield an extra 20 fps, but at the cost of pumping a HELL of a lot more air.

Really, without a chrony, you're not going to notice the difference, except that it will be a lot easier to fill. :D
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Unread postAuthor: SEAKING9006 » Sat Nov 22, 2008 1:05 pm

And on top of that, if you use a burst disc you will get even better performance.
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Unread postAuthor: theepicmool » Sat Nov 22, 2008 1:19 pm

sweet. What did ggdt say for distance? (its erroring on my PC) Sweet savings on couplers etc.. too with smaller pipe :)
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Unread postAuthor: Ragnarok » Sat Nov 22, 2008 1:25 pm

Note: Wrote this post over some time, so it might be a tad out of date.

theepicmool wrote:The reason for the large chamber is because of spudtechs recommended 4:1 ratio.

Completely unnecessary excess. Anything over 2:1 is highly wasteful.
Spudtech's information pages haven't been updated in years, and you won't find an active veteran on this forum who would tell you 4:1 was worth it over 2:1. The 4:1 school of thought is long since dead.

keep in mind my valve will be modded

That doesn't change anything, because it's not the opening time that's the matter, it's the choked flow of the valve.. A 1" Sprinkler valve has a Cv (flow coefficent) of about 6.
Compare this to what your 2" barrel is capable of... which is of the order of 100.

This flow choke of 6% Valve:Barrel flow means that any chamber size increases over about 2:1 can't come into play, meaning you're adding all that extra air for no muzzle energy gain. Essentially, you halve your efficiency for absolutely no reason.

For an example, when I made HEAL - a decent enough example of a flow efficent cannon, which has a barrel of just 0.8" diameter, I used a valve with a flow of ~11.2, compared to the barrel's flow of ~16 - and even with that huge 70% V:B flow ratio, I still only used 1.5:1 C:B, because more would have given me almost nothing in the way of an increase. (Also, it would have made the cannon look funny to give it more chamber)

If i got 10ft of 3" i would be cut back to a 1.5:1 ratio.

4ft of 3" pipe would give a 1.5:1 ratio, which is actually about where the power gains from increasing the chamber size become increasingly insignificant.

Doing a quick model of your cannon in GGDT, assuming a 100 psi pressure and a 150g projectile, a 4ft 3" chamber will generate about... 237 ft-lbs of muzzle energy.
A 6ft 4" chamber gives you 247 ft-lbs of muzzle energy.

That's a pitiful 4% increase in muzzle energy, which will be utterly unnoticeable except with a shooting chronograph (and even then, spotting it within natural variation would be hard), for a 166% increase in chamber size and fill time - and a bulkier, less practical cannon.
That my friend, is what we call in the business, an utterly pointless exercise.

It also points out the critical factor of valve flow. You're generating an efficiency of about 8% there.
To skip back to HEAL - although it uses 300 psi (but due to it's small chamber, uses a mere third of the air relative to what a 100 psi 4' long 3" chamber would), it generates 300 ft-lbs in a cannon a fraction of the size you're talking about, and efficiencies of about 40%. (Not quite worthy of the High Efficency Air Launcher acronym, but not bad by any means)

It's worth nothing that if you can find a larger valve, you'll get a much more powerful cannon. With the 3" Dia 4' long chamber, with a 2" sprinkler valve instead (and thus 4 times the air flow), you get 700 ft-lbs, almost three times the muzzle energy from no changes but a large valve.
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Unread postAuthor: theepicmool » Sat Nov 22, 2008 2:20 pm

Thanks for your responce. Are 2" sprinkler valves available anywhere? Would a ball valve be a better idea? I'm pretty convinced of the 4ft 3" pipe but that 4% efficiency rating puts me off the project altogether. Would this even rival a combustion cannon in terms of fire power?
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Unread postAuthor: Sticky_Tape » Sat Nov 22, 2008 2:25 pm

2'' sprinklers can be pretty costly and ballvalves have low power a 1'' sprinkler should be good. Maybe you should mod the sprinkler pneumatically with one of those sweet 2.79 blowguns ehh ehh.
http://www.spudfiles.com/forums/viewtopic.php?t=305
EDIT: David Vaini I think that it would be pretty hard to find it in his area I wouild just stick with the 1''.
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Last edited by Sticky_Tape on Sat Nov 22, 2008 2:32 pm, edited 1 time in total.
You can tell how awesome a cannon is by the pressure used.
http://www.spudfiles.com/forums/high-pr ... 12803.html
xnt rnm ne z ahtbg
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Unread postAuthor: Davidvaini » Sat Nov 22, 2008 2:28 pm

mmm depends on your area.. 2 sprinkler valves can be purchased in my area for less than $15 each.

People have moded two sprinkler valves to use one trigger.
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Unread postAuthor: Ragnarok » Sat Nov 22, 2008 2:38 pm

No, a ball valve would give you an efficiency even lower than 8%.
(@Sticky_Tape: You've not been reading have you? We're already talking about a modded valve. The problem is valve flow, not valve opening time.)

2" sprinklers can be found in certain areas, or on Ebay, but they will cost you more than a 1" valve - and they are also quite bulky, but they provide some decent power. Learning how to find tricky to source parts is a useful skill in spudding.

However, I would really stress that 1" sprinkler valves aren't much good on a cannon of this size (well, actually, they're not great anywhere!) - even two 1" valves would be fairly poor, as they'd only yield half the flow of a 2" sprinkler, and then it takes a whole load of extra plumbing to rig it all together.
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Unread postAuthor: theepicmool » Sat Nov 22, 2008 2:47 pm

What is the alternative? Would you suggest a bust disk? Even with a 1" valve do you think i would still get better performance then a typical combustion one?
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Unread postAuthor: SEAKING9006 » Sat Nov 22, 2008 2:59 pm

Burst discs (and even better, TRIGGERED burst discs) are absolutely the fastest and most efficient valve out there. There is next to no opening time, and there is no awkward plumbing to restrict airflow. However, using them can be tricky, and reloading them is a pain.
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