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Chamber:Barrel and GGDT?Two questions:
How important is chamber to barrel ratio in pneumatics? We're working on a new gun, in total the chamber is about 312 inches of 3" PVC. We were thinking of going with a 10 foot barrel (120) inches of 2" PVC (and perhaps down the road, 1.5"). Is this alright? Is the barrel too long or too short? My other question is about GGDT. When I put in my chamber information (312" of 3" diameter), it tells me my volume is like 2200 in^3, but by my math (pi*3^2*312), I get more like 8800 in^3. Am I doing something wrong, or is there something wrong with the program? Thanks for the help!
Hm lol are you sure you mean a 312 INCH chamber?!?!?!
Im pretty sure that is wrong... unless your building a giant pneumatic ... and the barrel seems kinda long but it depends on what the real size of your chamber is... GGDT is right... dude just imagine 312 inches of chamber... 12 inches = 1 foot so you have a 35 foot chamber? I dont think so Check your math bro BTW  any1 up for a chat in TEH SPUD CHATz? Edit: anyway I forgot to post any actuall "help" to get your volume the most accurate way possible is get a measuring cup and fill your chamber with water ... or another option is to go to the wiki and under the "calculators" section there is a bunch of calculation sites which can get you a more or less accurate volume of your launcher... but you should really figuire out the right digets before putting them in
Last edited by theBOOM on Fri Jul 24, 2009 4:41 pm, edited 2 times in total.
actually 12 inches = 1 foot so it would be more like 26 feet and he might have meant 312 cubic inches
I ♥ ♣'in baby seals
You're using the diameter not the radius in your equation. It should be (pi*1.5^2*312)
The radius formula for the area of a circle is nonsense. I have no idea why they teach people that as it confuses them more than it helps. Real people use the following formula for area of a circle:
A = (pi * d^2) / 4 It's quiet easy to derive this from the radius based formula, yes, but most people don't bother.
All spud gun related projects are currently on hold.
Because there are some pretty interesting mathematical properties for circles... Area = pi * r^2 Circumference = 2 * pi * r Those who've had Calculus will notice that the circumference is the derrivative of the area. In other words, dArea/dr = Circumference. This is a very interesting feature and one that breaks down if you teach diameters. Area = pi/4 * d^2 dArea = pi/2 * d != Circumference. Now, given the elegance of the natual world and how so many things are interrelated in mathematics, why would you want to teach the method that breaks down due to a algebraic substitution intended to appease lesser minds?
I did in fact mean 312inches; I have a total of 26 feet of chamber space (of course, it's not linear).
That's just a real quick (top down view) drawing of what our chamber looks like. There's four vertical chambers, 5' long of 3", plus the crossbeams comes out to about another 68 feet, so I'm taking the conservative side of things and saying about 26' total of chamber. The red dot is where it comes up vertically to a (swappable) 45/22.5 degree elbow for the barrel to attach. Thanks for catching my math error, I don't know why I completely blanked and use d instead of r. So, now that we've verified the chamber volume (approx 2200 cubic inches), does anyone have any notes about my barrel length? 10' of 2"? Thanks for the help!
I'm a real person and I always use R in the circumference, area and volume calc's. The key is to be consistent (and to remember the correct formula ). As D_Hall said, the equations using R instead of D have the advantage that they fall directly out'a Calculas. Indeed the volume of a sphere, (4/3)Pi(R^3) falls out that way as well.
You're Barrel is WAAAAAAAY too short.... that chamber volume would be oversized on a 50ft long 2" barrel...(being general here)....
Why such a massive chamber??? unless you're attempting to make a 2" hammer valve for multiple shots... One thing that peope forget is frictional losses in pipe, once you get to a certain length the projectile will not go ANY faster regardless of pressure because a gas can only flow a certain speed through a certain pipe, whether it be from valve restrictions or barrel size, there's always a given point of diminishing or non existent returns...
I've personally seen it when we were designing the t shirt launcher. Below is a time graph of an in barrel position over time. The blips are created as the shirt passed coils every foot on the barrel. The shot is a 10 foot barrel. The first few feet is omitted to expand the last few feet to check the increase in speed the last few feet. Each dotted square of the grid on the horizontal is 2.5 ms. Using that the FPS for each foot of travel can be measured. Measure it yourself on screen. The time for the last 3 feet is the same to travel each foot. We reached a terminal velocity in the barrel. Due to these results, we cut off about 3.5 feet of the barrel.
Tech,
Any particular reason why the last coil had reversed polarity? A simple oversight or did you want to "mark" it?
i would upsize the barrel to at least 3", 2" with that sized chamber is just impractical i would go with a 15 foot long 3" barrel and a 3" porting piston valve if it was me(but its not obviously ) or massively downsize your chamber to fit that barrel whichever you prefer
I ♥ ♣'in baby seals
I marked it. Often we used just too high a sweep speed. It let us know if the low amplitude created a missed trigger event on the first coil or we didn't capture the last event. On some shots the initial speed was too low on the first coil to reliably trigger the scope and it would trigger on the second coil instead. Note we predicted the scope time to very closely hit the end of sweep. Expanding it helps take measurements. A quick glance at the trace verifies the last trace shown is the end of the barrel.
I agree. In testing even with a 2 inch valve on ours, we got better launch speed with the 3 inch barrel. The recoil needs to be watched on larger diameter barrels. Heavy projectiles can provide quite a kick. I got bruised by launching a Gateraid. My current project is a 3 inch porting.. I do intend to waste a 2 inch barrel on it with a venturi entrance for an attempt at SOS. GGDT predicts well over SOS on a light projectile at 200 PSI. Flow and gas acceleration is one of the reasons for a short fat chamber on mine. Goal with a sabot is a supersonic golf ball. Maybe it can't be done. I'm going to try. @ SpudJoe.. The graph is of the speed of a T shirt in 10 feet of 2.5 inch pipe. The tank is 700 cubic inches. A 2,000 cubic inch chamber on a 2 inch pipe will run into the flow restriction of a 2 inch pipe. The big chamber is a waste on the 2 inch barrel. You may want to model it more in GGDT using other size chambers. My 3 inch is going smaller in the chamber and higher pressure than my 2 inch.
Note that I'll be one of the first to admit that GGDT gets a bit iffy at supersonic velocities. I've done my best while trying to keep the sim generic and it's numbers are better than nothing but... Well, I wouldn't hang my hat on them.
@ D_Hall and jimmy: I was rather over the top in that post. I should have phrased it differently: "I have no idea why they push one formula when other more useful ones exist."
This formula I find much more useful. It doesn't appease lesser minds; lesser minds would never realize you could do it this way. Jimmy's right too. Anything you are comfortable with is acceptable as long as you're consistent.
If won't break down if you do the differentiation properly with the chain rule. You still differentiate with respect to the radius. dA/dr = (dA/dd) * (dd/dr) = circumference A(d) = (pi / 4) * d ^ 2 > dA/dd = (pi / 2) * d d(r) = 2 * r > dd/dr = 2 So... dA/dr = (pi / 2) * d * 2 = pi * d = circumference
Last edited by btrettel on Sat Jul 25, 2009 4:45 pm, edited 2 times in total.
All spud gun related projects are currently on hold.
 
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