Big Stuff

[Jimmy K]

Ok so I did a little work on the cannon today.

I realized that I won't be able to muzzle load it, so I started on the external bolt mechanism. Who knows if it will work or not. I found a bunch of 4" to 4.5" O-rings and a sleeve for the 4" barrel so I should be set. Hopefully I'll get lucky.

I also got a guy to make me some pressure tanks (don't worry, he is certified in welding pressurized systems). He is making two ten gallon tanks; one for each valve. Hopefully this will let me get rid of the 90 degree turn that I would have to put in somehow.

Also, where the two 3" pipes will come in and meet the 4" barrel, I was going to use a sch 40 straight tee with these bushings. Would this combo hold up to the rush of air and the recoil? If not, what are my other options?

I also started thinking about ammo. I was thinking of using some home brew sabot rounds instead of using fruit and vegetables. I made a prototype out of an inch steel rod (about a foot to start) with a heavy metal tip, 4 small fins welded on the back, and hard Styrofoam for the casing. It seemed to fit and everything, but I'm a little worried about what would happen right after it exited the barrel. It seems as if the cartridge would end up making the round tumble after detaching from the round body. Has anyone made sabot rounds?

[dewey-1]

The mass of the foam sabot should not affect the projectile so long as it separates properly.
Even if it did slightly affect the projectile, the fins will restabilize it as it flies.

[jimmy101]

Sounds like you getting things worked out pretty well.

One comment, for a pumpkin chucker you don't want a fast valve. You really don't need it either. For more robust ammo then a fast valve helps performance, for fragile ammo (like veggies) the perfect valve would be one that opens just fast enough to maintain a constant pressure on the ammos butt. That pressure would be just below the pressure that destroys the ammo.

With a very large chamber and a very fast valve you can control the maximum pressure by varying the chamber pressure. Since the chamber is so large there is minimal pressure drop as the ammo moves through the barrel. That gives the best possible performance without vaporizing the ammo. But that requires that the chamber volume is considerably more than the barrel volume.

An elegant pumpkin chucker would use a fairly small chamber pressurized to much greater than what the ammo will withstand. The pressure would then be controlled by the opening characteristics of the valve.

[Jimmy K]

Since the chamber is so large there is minimal pressure drop as the ammo moves through the barrel. That gives the best possible performance without vaporizing the ammo. But that requires that the chamber volume is considerably more than the barrel volume.

How much more (volume) are you talking for the chamber vs. the barrel? The 20 foot barrel has a volume of about 13 gallons, and I'm doing 10 gallon chambers per valve. So that's a ratio of about 1.5:1. I still have time to change the chamber size if necessary.

[Technician1002]

That chamber ratio is a good value for use with pressure of ~20 PSI to about 100 PSI. It will perform well in the ~60 PSI range.

[Jimmy K]

The bolt is done. Works well. Got a great fit on the o rings and the metal sleeve.

Next is the manifold. From how I see it, I have these options:

http://www.grainger.com/Grainger/Test-Tee-1WKL8
http://www.grainger.com/Grainger/Two-Wa ... -Tee-1WJN8
http://www.grainger.com/Grainger/pvc-an ... 9bZ1z13960

I'm concerned with instant failure right where these two valves are dumping air. Any thoughts?

[spudtyrrant]

The bolt is done. Works well. Got a great fit on the o rings and the metal sleeve.

Next is the manifold. From how I see it, I have these options:

http://www.grainger.com/Grainger/Test-Tee-1WKL8
http://www.grainger.com/Grainger/Two-Wa ... -Tee-1WJN8
http://www.grainger.com/Grainger/pvc-an ... 9bZ1z13960

I'm concerned with instant failure right where these two valves are dumping air. Any thoughts?

try these guys, they're much cheaper. As to your concern, you don't have to worry about the PVC failing where these valves are dumping air, as long as you use sch 40 it will be more than strong enough.

[Jimmy K]

Even DWV fittings?

[linuxexorcist]

DWV fittings are generally considered unsafe for use in pressure applications, if at all possible get NSF rated fittings
the site spudtyrrant linked to carries rated tees

[Jimmy K]

FlexPVC only carries double wyes in DWV. I want to try to stay away from 90 degree bends (tees) to ensure maximum flow.

[spudtyrrant]

FlexPVC only carries double wyes in DWV. I want to try to stay away from 90 degree bends (tees) to ensure maximum flow.

90 degree bends will not really effect you flow any significant amount, the air will flow through the tees faster than your sprinkler valves.

[Jimmy K]

the air will flow through the tees faster than your sprinkler valves

Oh my. I'll go ahead and pick up that tee then.

[Technician1002]

Ok so I did a little work on the cannon today.

I also started thinking about ammo. I was thinking of using some home brew sabot rounds instead of using fruit and vegetables. I made a prototype out of an inch steel rod (about a foot to start) with a heavy metal tip, 4 small fins welded on the back, and hard Styrofoam for the casing. It seemed to fit and everything, but I'm a little worried about what would happen right after it exited the barrel. It seems as if the cartridge would end up making the round tumble after detaching from the round body. Has anyone made sabot rounds?

I tried a couple of things. I tried some foam swim noodle with poor results. It shrinks with the burst of air pressure and passed the projectile in the barrel. It may be OK for low pressure, but with an 8 Bar shot it shrinks to about 1/8th the volume. Don't use a compressible sabot.


A solid shell sabot worked much better. It provided a great seal and added little mass. This was used to see if a larger diameter bore could provide higher force and acceleration on a golf ball. The result is a 2.5 inch barrel works better than a golf ball barrel when fed with a 2 inch valve. How much better, I'm not sure as I do not have a chrony. Impact damage was used to gauge the result. Heat was used to create the golf ball pocket.



That sabot tended to dome on launch so aim was poor. A variation works well.

[Jimmy K]

You were using PVC for a barrel, right? What kind of friction was there between your sabot rounds and the barrel?

[Technician1002]

Yes, thinwall PVC. The sabots slide in the same as pop cans. Very low friction. Some of them require a small shake to bounce the golf ball on it to send it down, but it does not require a ramrod.