What do you guys think of this?

[Brad1895]

So a while back I asked about a my bigger cannon plan. Since then I made something a bit overkill, but that's another can of worms. The idea I have now is simple, 10' 2inch barrel, 2in union, and a 24"x4" chamber. The only component I need to actually buy is the chamber portion because I can swap the barrel of my current gun. The plan is to use Co2 as a power source, regulated of course. After some GGDT work, I got some scary results. At only 15PSI in the chamber, GGDT predicted a 30g, .29CoD projectile to go 300'. That can't be right. If it is, well god help those down range with catching a nerf football. I did the math, and my flow coefficient is %70. Is that right?

[Technician1002]

Your flow coefficient will depend heavily on your valve. How does the flow go through? The rate of flow will not exceed the capacity of your valve.

[Brad1895]

As I said, it's a 2 inch union burst disk valve. So the math should look like this right?
CoF= 3*25.8
The other think is that I'll need to find a way to "trigger" the burst disk. I would use a small secondary chamber filled with more pressure than the main 4" chamber. Using boyle's law, I can solve for the pressure difference right? I'd preffer to use one union, and keep the assembly as simple as possible.

[DYI]

Simple and high performance tend to be enemies. If you can accept a small delay (on the order of tens to hundreds of milliseconds, depending on flow rate), you could simply open a ball valve that rapidly fills the chamber until the burst disc ruptures. This could also be automatically controlled to avoid unnecessary waste of gas.

You could use an internal or pass-through mechanical rupturing system, probably pneumatic powered, similar to Gort's QCTBDC. In this case, some sort of blade is driven into the diaphragm from the inside with minimal flow obstruction. This'll fire pretty much exactly when you tell it to, but requires a bit of creativity to implement.

Pressure-differential triggered burst disk setups are easier to build, but they're finicky and require loading two discs. I have quite a bit of experience with these setups, and they do not "keep the assembly as simple as possible", by any stretch of the phrase.

The range figures you quote don't seem unrealistic to me. What do you think the problem is with them? GGDT tends to be very close to reality in this performance regime.

[Brad1895]

It just seems unreal the amount of power a home built cannon can have. My original thought with a fire mechanism was to have the main 4" chamber, and then a smaller 1 1/2" chamber connected with a ball valve. Once the pressure is close to rupture of the disk, I can close the ball valve, and continue to fill the smaller chamber. When it's ready to fire, open the ball valve. The other(more viable) option is to have a large ball valve just before the union. Reach desired pressure, then open the valve to burst the disk. And I misspoke about wanting it to be simple. What I should have said was that it needs to be made with few parts. I have plans to use this as artillery for airsoft/paintball games. By the way, are burst disk louder than a piston valve? It makes little difference to me, but I'm just curious as to what a burst disk sounds like. The videos seem to get their audio blown out though.

[DYI]

I recommend against a ball valve directly behind the union, as it may fire when the valve is not fully open. This will reduce performance, and could introduce large shot-to-shot variability. Your other idea is better. Any burst disk system is not ideal for use in games. Something using a cam lock fitting might be your best bet, if you can get it to seal. If you're willing to hire out some machining, it wouldn't be too hard to design a pneumatically operated custom disk holder with integral sealing faces that would be just as fast as a conventional bolt-action rifle. Otherwise, you may want to investigate your piston valve options, as performance will be nearly as good with much greater usability.

I've never actually built a piston valve myself, but burst disk valves tend to be louder and sharper sounding.

[Brad1895]

I have built a piston valve, although it's not that great... Anyways, I do have a piston valve already made, and it can be swapped gun to gun. I just need to use a slightly smaller chamber. 4.5 gallons of air doesn't make for fast filling. I do however already have a union at my disposal.

[Technician1002]

As I said, it's a 2 inch union burst disk valve. So the math should look like this right?
CoF= 3*25.8
The other think is that I'll need to find a way to "trigger" the burst disk. I would use a small secondary chamber filled with more pressure than the main 4" chamber. Using boyle's law, I can solve for the pressure difference right? I'd preffer to use one union, and keep the assembly as simple as possible.

A common way to trigger a dual disk burst disk is to fill the chamber and space between disks to about 3/4 the burst pressure and seal the space between disks. Then fill the chamber to 1.5 times the single disk burst pressure. This places about 3/4 burst pressure on each disk. To fire, vent the pressure between the disks. This overloads the first disk blowing it, then cascading through the second. Only a small trigger valve is needed to vent the small space between disks.

[Brad1895]

Not a bad idea for what I'd need. Here's my thought process. My current cannon has a 76" by 4" chamber, and a 12" by 2" barrel. I'm using a 2" piston valve piloted by a 1 inch sprinkler valve, and my god, this thing has some kick. What I'm thinking of doing is cutting down the chamber significantly, probably down to about 18" or so. Then use the cut off piece in a mortar of sorts. Split the section left over in two for two pieces of 29" 4" pvc, one for the barrel, and one for the chamber. Place a 2" union in between, and one softball mortar ready to go. The trigger method will be a 1/4" piece of conduit pipe resting on the burst disk. When a softball is dropped in the barrel, it pushes the conduit through the burst disk. Two guns from one.

[velocity3x]

The plan is to use Co2 as a power source, regulated of course.

Only a suggestion....but if you intend to keep your hands and fingers in their original, factory configuration, you might want to think again about using CO2 with plastic components.

[Brad1895]

Probably a good idea. If we can keep the chamber volume down, I have no complaints.