need some help with c:b ratios

[trae08]

ok i think i posted a thread like this before but couldn't find it. so my problem is this. i have a 5 ft piece of sdr 21 "gold ball barrell" sleeved in sch 80. i want to make a nice gun with this and i looked on the spud wiki about c:b ratios and it didnt have nothing about pneumatics that i found. just said there vary depending on valve and pressure and stuff.

so if i have this 5 ft piece of golf ball barrell, a 1 1/2 inch sprinkler valve and a constant pressure of 100 psi how long should my chamber be? oh yea and it'll be a 4 inch by xxx chamber

thanks for your help.

and ive tried using the ggdt but im just confused by it.

[hi]

pneumatics can be off more than a combustion.

i would say about 1 1/2 feet would be plenty of power with a 4'' chamber.

[boilingleadbath]

With pneumatics, there is no chamber size that will maximize performance, given a certain barrel size.

Instead, the muzzle velocity increases as you increase the chamber size, all the way up to infinite chamber volume.

Obviously, an infinitely large chamber is impractical... so one has to comprise.

The GGDT is a good tool to help you decide how big your chamber should be, because it educates you.
(and no moping if you are running a mac - just find a friend that doesn't.)

[trae08]

im not running a mac it just always says that my barrell is unnecessarily long

[Modderxtrordanare]

im not running a mac it just always says that my barrell is unnecessarily long

Then shorten it?

A screenshot would help us.

[jimmy101]

With pneumatics, there is no chamber size that will maximize performance, given a certain barrel size.

Instead, the muzzle velocity increases as you increase the chamber size, all the way up to infinite chamber volume.

Obviously, an infinitely large chamber is impractical... so one has to comprise.

The GGDT is a good tool to help you decide how big your chamber should be, because it educates you.
(and no moping if you are running a mac - just find a friend that doesn't.)

To put it more precisely (and accurately) as the chamber volume is increased the muzzle velocity increases asymptotically to a maximum muzzle velocity. The muzzle velocity will not increase forever. A 5000 cubic foot chamber versus a 50 cubic foot chamber on a sensibly sized barrel (say 2"x6') will perform the same.

GGDT is your friend. The difference in length between a barrel that gives 99% of the theoretical maximum velocity for a given chamber is much longer than the barrel that'll give you 90% of the maximum. Typically, the difference in barrel length between 99% and 90% is about 2.5x

[trae08]

ok heres a screenshot.




edit: thats just what i am planning on maing. and i plan on shooting golfballs.

[DYI]

What pipe has an inner diameter of 4.3"?
If it is a coaxial design, why is the pipe running through the chamber a different size than the barrel? They are usually a single piece. If you are making a coaxial piston (which I assume you are from the given stats), the valve type should be barrel sealing, not generic.

If possible, decrease the size of pipe running through the chamber to 2 inch, so that you have more chamber volume, or increase the diameter (or length) of the outer pipe.

Your design is confusing, and I don't really understand why you are running a 4" OD pipe through a 4.3" ID chamber. For your given chamber size you need a shorter or thinner barrel.

[Pilgrimman]

I think your problem is this: you put 4.30 and 4 for the Outer/Inner chamber measurments. That inner diameter thing is designed for coaxial guns. For yours, you should leave inner blank, and enter 4 inches for outer diameter. Hope I've helped!

[trae08]

my bad i dunno what to put into this thing or what my pipe id and stuff is. its a over and under design. the chamber is 4 inch sch 40 pvc the barrell is sdr 21 golf ball barrell. and the projectile is a golf ball. the valve is a 1 1/2 inch sprinkler.

if you could help me out with what numbers i need and where to put them thats be great.. or even how to find out what numbers i need.

[DYI]

Well if it's not coaxial, why do you have a pipe of 4" outer diameter running through the chamber? Why do you have any pipe at all running through the chamber?

A quick check of the pipe info page shows that the ID of schedule 40 4" PVC pipe is 3.996 inches.

You can get the numbers for sprinkler valves off of D_Hall's website. They are barrel sealers (although someone here is bound to disagree)

[Pilgrimman]

For outer diameter, put 4 inches. I don't know the numbers for the 1 1/2" valve, but the flow % is about 23%. I don't know the I.D. of SDR-21, but I'd guess about 1.8 inches. You should know your barrel and chamber lengths, so it's obvious where you enter them in the program. The valve is Chamber-sealing, and the seat is 1 1/2". The friction is what pressure causes the projectile to move from it's initial position. It should be less than 1 psi for a golfball. You can find the mass of the ball with a google search, and the diameter of the ball is only slightly less than the I.D. of SDR-21. That's all I can say for the program. Hope I've helped again!

[trae08]

the numbers and placement of them look better?

[boilingleadbath]

Trae, you have a couple hundred posts. Why can't you read the help manual for the GGDT?

#1) Don't model a sprinkler valve as a generic valve. Model it as a sprinkler valve - ie, a chamber sealing piston valve.

#2) The inside diameter of 1.5" sdr 21 is not 1.8", a golfball doesn't weigh 5 grams, and the diameter of a golfball isn't 1.7".

Now, admittedly, modeling a sprinkler valve can be difficult, largely due to the lack of availability of good data. However, a good guess can be achieved if one scales the values of a 1" sprinkler valve appropriately. (1" sprinkler values can be found on D-hall's website)

The size of the pipe can be found in many places, such as harvel.com's website.

The size of a golfball is easily googled, as is it's mass.
************

Yeah, I could have just spoon fed you all these numbers from memory, but I didn't want to.

[trae08]

ok thanks Blb that helped out alot. and thanks everyone else to. i think ive got it figured out now.


edit: whats init position mean? on the projectile side.