SpudFiles

New member here and loving this resource and community! I'm building an air cannon and had a few specific questions I couldn't quite find answers for by searching around. Also just looking for any general advice on my design.

I want to build an air cannon that will launch a gopro like camera as high and as far as possible. The camera will be inside a custom-designed enclosure that will fit snugly in the barrel. The expected weight for the projectile is 200-250 grams. I was planning to use a 19-gallon steel air tank from McMaster and a Goyen dust collector valve in an over/under configuration. The barrel has to be 2.5" or 3" to accommodate the size of the camera and I was thinking of using CLEAR schedule 80 PVC. I'll likely fab up a simple rolling frame out of square tubing. Ok here are my questions!

1) Any general thoughts or concerns to the overall design? Probably charge it to 100psi max.
2) The air tank I'm looking at has a single 2" NPT port and two 1.5" NPT ports. My initial thought was to just use the single 2" pipe with a 2" dust collector valve, BUT would it be better to connect both the 1.5" ports together via a 3" tee (and some step downs) and connect that to a 3" dust collector valve?
3) Am I crazy to consider an 8ft long barrel? I've been messing around with GGDT and it seems like the longer the barrel the more muzzle velocity I'll get.
4) Considering all this, what kind of distance should I expect with a 250g projectile if I've got the barrel pointed up at a 45 degree angle?

Thanks in advance for any help.

1 depends on duster valve and wether you use 3 or 2 and 1/2 but make sure it is good pipe but I can not determine exact psi because you did not give me the exact parts
2 it is good to have long barrel for max velocity so 8 foot barrel is not crazy and make sure you have right adapters to get it
range depends on psi but at 100 psi I think 400 to 600 yard range but depends on psi barrel length and projectile shape

1) Any general thoughts or concerns to the overall design? Probably charge it to 100psi max.

Always remember that PVC is prone to cracking if mistreated. In other words, while a PVC barrel can be done, the larger it is the more cognizant you have to be about its treatment. Be nice to it lest it grenade on your. That's the redneck in me answering. The engineer in me will tell you NOT to use PVC and to instead use copper tubing or similar for such applications.

2) The air tank I'm looking at has a single 2" NPT port and two 1.5" NPT ports. My initial thought was to just use the single 2" pipe with a 2" dust collector valve, BUT would it be better to connect both the 1.5" ports together via a 3" tee (and some step downs) and connect that to a 3" dust collector valve?

I'm too lazy to do the math at the moment, but I strongly suspect you should just go with the 2" port and valve. Unless you're pushing Mach, you're not likely to see a huge performance difference. So, uh, what were your muzzle velocity predictions?

3) Am I crazy to consider an 8ft long barrel? I've been messing around with GGDT and it seems like the longer the barrel the more muzzle velocity I'll get.

While making a barrel TOO long can actually decrease performance, its rare that anyone ever has issues with such. In other words, I suspect an 8' barrel will be just fine.

4) Considering all this, what kind of distance should I expect with a 250g projectile if I've got the barrel pointed up at a 45 degree angle?

You already said you're using GGDT. In the tools menu there's a crude external ballistic's calculator. Use it.

As D_Hall says, PVC is a bit sketchy*, and (as with all pipes), pressure rating goes down as diameter goes up. Schedule 80 is better than 40, but it doesn't mean twice the pressure, you get maybe a third as much again.

With a longer barrel, you're going to be exposing the pipe to more pressure, and for longer. intuition says that this is going to be worse for the pipe than a shorter barrel, even at the same pressures, and this is probably the case, but I'm not entirely convinced this is 100% true, I can come up with equally plausible theories why a shorter barrel might fail sooner.

It's the ageing aspects of PVC that scare me the most. It probably maybe won't fail on the first firing, or the second. But every shot is reducing the safety margin, at the same time it gives the user more perceived confidence in its actual safety. Not every PVC launcher explodes. But I won't touch the damn stuff, except for what t's designed for - drainage. I've done a lot of really sketchy stuff over the years, but there's no medals** for making stuff more dangerous than it needs to be.

YMMV, of course.

* If nothing else, when it fails, it can fail catastrophically, with a glass-like shattering rather than tearing. It's quite brittle, and UV exposure makes this worse, and it really doesn't like continual shock loading, which is pretty much what you're going to expose it to in a launcher. Plus the solvents used for "gluing" the pipe can degrade pressure rating.

** Even the Darwin award is not a physical thing.

agreed defiantly do not charge it over 100 psi until you know exact psi ratings