dead space

[PVC Arsenal 17]

I've run into a flaw in my most recent pneumatic nerf gun configuration.

There will be, no matter much I move stuff around, 3" of dead space between the outlet of the valve and the dart. The diameter of the space is 0.572", making for a dead volume of about .771 in^3.

Since I can't get GGDT to run on my computer for whatever reason (like I'd be able to figure it out anyway...) I'm going to post as many facts about the parts used and if at all possible, someone please work out the numbers for me and tell me if my design will suffer any significant decrease in performance compared to if there was no dead space. It would be very much appreciated.

Known valve specs:
Exhaust Time: 100 cubic inches in 2 seconds
Flow Rate: 20.2 scfm at 125 psi
QE11-M-44

Barrel specs:
12" x 0.528" (length can be reduced if necessary)

Operation pressure:
100 psi

I will try to find out the weight of the nerf dart, but I do know that it's 2" long. Unfortunately this is all I know.

All help is very much appreciated.[/url]

[btrettel]

Without running any numbers I'll tell you it's not likely to decrease performance significantly. In fact, I'm willing to bet it's around the optimal amount. Sorry if I had confused you with our correspondence. I think as a rule don't have too much but don't worry if it's not a particularly large amount (like 3 cubic inches in a Nerf gun). I was not originally aware of the precise amount of dead space so I assumed it was too much without thinking.

1.1 grams is a good guestimate for a Nerf dart... my darts weigh around that much. I intend to manipulate their weight once I figure out the drag coefficient as to minimize requisite kinetic energy to get a certain range.

[shamelessplug]You might want to try my simulation BAGS if you can't use GGDT, especially if you want to learn a bit more about the inner workings of a basic pneumatic. As I intend to use BAGS primarily with Nerf it should be adequate for you.[/shamelessplug]

[Technician1002]

I've run into a flaw in my most recent pneumatic nerf gun configuration.

There will be, no matter much I move stuff around, 3" of dead space between the outlet of the valve and the dart. The diameter of the space is 0.572", making for a dead volume of about .771 in^3.

Since I can't get GGDT to run on my computer for whatever reason (like I'd be able to figure it out anyway...) I'm going to post as many facts about the parts used and if at all possible, someone please work out the numbers for me and tell me if my design will suffer any significant decrease in performance compared to if there was no dead space. It would be very much appreciated.

Known valve specs:
Exhaust Time: 100 cubic inches in 2 seconds
Flow Rate: 20.2 scfm at 125 psi
QE11-M-44

Barrel specs:
12" x 0.528" (length can be reduced if necessary)

Operation pressure:
100 psi

I will try to find out the weight of the nerf dart, but I do know that it's 2" long. Unfortunately this is all I know.

All help is very much appreciated.[/url]

With a 12 inch long barrel and 2 seconds to discharge 100 cu in of air, that thing is severely restricted someplace. Do you mean 2 ms? 100 cu in of air is roughly twice the volume of my marshmallow cannon. It dumps fast enough to toss rolled up t shirts 200 feet from a 36 inch long tube.

[PVC Arsenal 17]

The actual chamber volume is not 100 cu in. I just copied that off the valve's specification sheet. My guess, and I have not worked it out yet, but the chamber volume shouldn't be less than 2 cu in.

@btrettel- Thanks for the info. I will try out BAGS right away.

[Technician1002]

The actual chamber volume is not 100 cu in. I just copied that off the valve's specification sheet. My guess, and I have not worked it out yet, but the chamber volume shouldn't be less than 2 cu in.

@btrettel- Thanks for the info. I will try out BAGS right away.

I just saw the 2 seconds to vent 100 cu in, at 100 PSI and thought, slower than a tire valve.

[jackssmirkingrevenge]

Some limited testing I had done here.

The 16 inch barrel is made up with a ten inch barrel and a 6 inch extension, so I took the opportunity to measure shots for 10 inch effective barrel length, starting from zero and 6 inches away from the piston respectively. I got 225, 227 and 221 fps for a normal 10 inch barrel, and 198, 190 and 196 fps for the 16 inch barrel with the projectile 10 inches away from the muzzle.

An almost 13% decrease in velocity for the extra dead space, meaning 26% less muzzle energy. A very limited test, granted, but food for thought nonetheless

[PVC Arsenal 17]

The actual chamber volume is not 100 cu in. I just copied that off the valve's specification sheet. My guess, and I have not worked it out yet, but the chamber volume shouldn't be less than 2 cu in.

@btrettel- Thanks for the info. I will try out BAGS right away.

I just saw the 2 seconds to vent 100 cu in, at 100 PSI and thought, slower than a tire valve.

Yeah the valve I'm using is a super small 1/8" QEV.

[POLAND_SPUD]

a super small 1/8" QEV

it's not a full ported valve, is it ?
1/8" is still quite big... if it's not fully ported then it might be a 5mm version

[PVC Arsenal 17]

It is a Pneumadyne QE11-M-44. I will be replacing it with a Clippard JEV-F2F2 eventually anyway.