SpudFiles

I recently made a simple sprinkler valve cannon (4 foot 2in pvc pipe gas chamber to a blowgun modified sprinkler valve). I've done tests at around 100 psi and I've getting around maybe 100-150 feet. I've seen similar guns at half that pressure go 200 feet. I think it might be my barrel length because its only about 10-12 inches long. What is a good barrel length for this kind of cannon and would changing the gas chamber diameter help performance (I.E. doing would 2 feet of 4 in pvc be = to 4 feet of 2 inch or is the 4in better?)

I'd say have at least a 5 foot barrel, but thats a 9 foot cannon. You might actually get better performance with a 12'' chamber and a 2' barrel. If you can change it, get about 1.5 feet of 4'' pvc for the chamber and have a 4 foot barrel and you should be good.

You failed to mention what diameter you're barrel is and what you plan on launching out of it?

g4bandit wrote: (I.E. doing would 2 feet of 4 in pvc be = to 4 feet of 2 inch or is the 4in better?)

2 feet of 4" pipe = 301.29³"
4 feet of 2" pipe = 158.27³"

A 1:1 C:B ratio is fine for a pneumatic, although overkill (larger C:B ratios) yields better performance (velocity), so long as your launcher is ergonomic.

You could also try an over/under design, to cut down on overall length:


][

][ 8) 8) 8) ][



= 1" 90°'s
= Barrel
= Chamber
8) = Modded Sprinkler Valve

Good effort on the attempted emoticon diagram, but no (dead volume... C:B ratio... 5 90°elbows...).

More barrel length or more chamber volume will yield better results. I'd switch to an over-under design (like omniscient suggests) and get a much longer barrel. All of my (real) cannons have had at least a 3:1 C:B ratio, which is way overkill, but if you can spare the weight, length, and pumping time, why not?

saefroch wrote:Good effort on the attempted emoticon diagram, but no (dead volume... C:B ratio... 5 90°elbows...).

I suppose you also think that my attempted emoticon diagram also consists of: 18 barrel lengths, 6 chamber lengths and 3 modded sprinkler valves?

Using a valve large enough to flow the required volume of air is important too. Feeding a 2 inch barrel with a 1 inch or smaller valve will hurt performance. You may consider changing out the 90's for 2 inch. High flow in small bends limits flow.

Part of the reason for the in line design is because I use these cannons while airsofting as a rpg (It's a 2in barrel firing nerf pocket footballs). I suppose I'll just bite the bullet and make a 3 foot barrel and just chop a foot of the gas chamber. When I get some more cash together I'll go buy some 4 inch pipe and make a better gas chamber. Any idea's on were to get 4 inch pipe? My local home depot only have 4 inch in abs piping and I'm relatively sure the abs they stock isn't pressure rated. Or is it? anyways, worst case senario theres a general plumbing supply store a few miles away that stocks up to 6 in pvc but they only sell it in 20 foot lengths =/.

omniscient wrote:

saefroch wrote:Good effort on the attempted emoticon diagram, but no (dead volume... C:B ratio... 5 90°elbows...).

I suppose you also think that my attempted emoticon diagram also consists of: 18 barrel lengths, 6 chamber lengths and 3 modded sprinkler valves?

@ the op...Is there a reason that you can't just swap the chamber with the barrel, cut the cap off the now barrel, cap the now chamber and add the fill stuff?

You don't need or want max performance to shoot nerf rockets around/at people.

Max performance means I don't have to rush out as far to get within firing distance. The barrel is so short that if I switched them it would probably fire worse than it does now.

You could make an RPG-style launcher, using only the 2" pipe: <a href="http://i1083.photobucket.com/albums/j39 ... VCRPG4.jpg" target="_blank">NERF ROCKET RPG</a>

Does this design get better performance?

The above design should theoretically perform better as the air doesn't have to go through any bends (other than within the sprinkler valve itself) and there's not much dead space to speak of, though you lose a bit of compactness compared to the over/under design.