Post by Leafy on Jan 21, 2009 20:12:13 GMT -5
This is the most simple method of determining your dump chamber volume or your operating pressure. First download the gddt program from thehalls-in-bfe.com/GGDT/
Now you have to determine so things like what type of valve you're using. Mostly you're going to end up using the generic valve. Next you need to choose if you want the dump chamber to be your variable or your pressure. If you cant change the size of your dump chamber then you have to change your pressure. Personally I like to choose a pressure near where I want my op (I usually do 200 PSI) and then play with the volume till I'm shooting 310 fps. Anyways The next thing you need to determine is you "dead volume" the "dead volume" is the volume between the seal on the valve and the ball when everything is in its firing positon (the bolt is forward). Plug this into the simulator. The next thing to do is in the chamber data section set the inner diameter to 0 since this is for weird chamber shapes that you wont in counter in paintball, for the most part. The shape of your dump chamber does not matter, so put 1.1284 in for your outer diameter so that your chamber length is the same as your chamber volume. Next enter your seat diameter of your valve, this can be estimated for those of you with odd shaped seats. And if you have multiple ports like on a spool valve count ports as number of valves.
Next you have to set the bore diameter and the barrel length. I use a .689" for the bore. for the barrel length use the effective length, as in the distance till the porting starts. I use 6" just to make sure I leave a margin of error. Leave the friction the way it is and the mass of a paintball is 3 grams approx. I usually use .686 as my projectile diameter since there seems to be little fluctuation in velocity in this simulator.
Now hit calculate.
Ok here is where you need to do some guess work. you need to determine how your dwells. The open time is the amount of time it takes the valve to fully open, on a poppet this is a very shot time. the dwell is how long the valve stays open, this is not what the dwell on the gun is set to, since on most makers there is 6 ms of travel time at a minimum.
Now you can start playing with your numbers to figure out what size dump chamber you need or what your operating pressure should be, depending on what you choose as your constant. You should be shooting for at least 310 fps if not a little higher just to add a safety margin.
Another feature in this program is the gun optimizer, i really havent had much luck with it, I always get a divide by zero error. Have fun.
Now you have to determine so things like what type of valve you're using. Mostly you're going to end up using the generic valve. Next you need to choose if you want the dump chamber to be your variable or your pressure. If you cant change the size of your dump chamber then you have to change your pressure. Personally I like to choose a pressure near where I want my op (I usually do 200 PSI) and then play with the volume till I'm shooting 310 fps. Anyways The next thing you need to determine is you "dead volume" the "dead volume" is the volume between the seal on the valve and the ball when everything is in its firing positon (the bolt is forward). Plug this into the simulator. The next thing to do is in the chamber data section set the inner diameter to 0 since this is for weird chamber shapes that you wont in counter in paintball, for the most part. The shape of your dump chamber does not matter, so put 1.1284 in for your outer diameter so that your chamber length is the same as your chamber volume. Next enter your seat diameter of your valve, this can be estimated for those of you with odd shaped seats. And if you have multiple ports like on a spool valve count ports as number of valves.
Next you have to set the bore diameter and the barrel length. I use a .689" for the bore. for the barrel length use the effective length, as in the distance till the porting starts. I use 6" just to make sure I leave a margin of error. Leave the friction the way it is and the mass of a paintball is 3 grams approx. I usually use .686 as my projectile diameter since there seems to be little fluctuation in velocity in this simulator.
Now hit calculate.
Ok here is where you need to do some guess work. you need to determine how your dwells. The open time is the amount of time it takes the valve to fully open, on a poppet this is a very shot time. the dwell is how long the valve stays open, this is not what the dwell on the gun is set to, since on most makers there is 6 ms of travel time at a minimum.
Now you can start playing with your numbers to figure out what size dump chamber you need or what your operating pressure should be, depending on what you choose as your constant. You should be shooting for at least 310 fps if not a little higher just to add a safety margin.
Another feature in this program is the gun optimizer, i really havent had much luck with it, I always get a divide by zero error. Have fun.
A bit of FYI for people interested in who wrote the program, heres some excerpts from the faq (since I can't link directly to them for some reason).
What is GGDT
GGDT is my best attempt at a simulation of pneumatic cannons. This means everything from BB and paintball guns to stomp rockets to pumpkin chuckers and anything inbetween. The idea is to plug in a bunch of known parameters about the gun in question and have GGDT tell you what's going to happen with a high degree of accuracy.
How accurate is GGDT
As with any simulation, if the input data is garbage, the output data will be garbage as well. However, I've had multiple reports from individuals who designed guns with the aid of GGDT and after all was said and done they found that GGDT matched the performance of their guns to within 5%. And that was with the older versions. The newest version of GGDT should be even more accurate but in the interest of disclosure I must admit that there's been very little data returned to me as yet.
How does GGDT work
GGDT is not a simple set of equations for which a few variables are plugged in and answers spit out. It is a full-blown one-dimensional simulation. So if you're a student who's looking for information for a science fair project or something, all I can tell you is to read up on Newtonian mechanics, thermodynamics, and compressible flow. If texts on those topics are beyond your comprehension, then so too are the inner workings of GGDT. If those same texts are within your comprehension, then you probably know everything you need to know about how GGDT works.
What phenomena does GGDT not take into account
Alas, no model is perfect and I am far from being a world renowned expert on internal gun ballistics. I wish I could claim laziness, but the simple fact is there are some topics for which I simply do not have the mental resources for. These topics were as a result, ignored....
What is GGDT
PBf Feedback:
GGDT is my best attempt at a simulation of pneumatic cannons. This means everything from BB and paintball guns to stomp rockets to pumpkin chuckers and anything inbetween. The idea is to plug in a bunch of known parameters about the gun in question and have GGDT tell you what's going to happen with a high degree of accuracy.
How accurate is GGDT
PBf Feedback:
As with any simulation, if the input data is garbage, the output data will be garbage as well. However, I've had multiple reports from individuals who designed guns with the aid of GGDT and after all was said and done they found that GGDT matched the performance of their guns to within 5%. And that was with the older versions. The newest version of GGDT should be even more accurate but in the interest of disclosure I must admit that there's been very little data returned to me as yet.
How does GGDT work
PBf Feedback:
Can you give me the equation(s) behind the curtainPBf Feedback:
GGDT is not a simple set of equations for which a few variables are plugged in and answers spit out. It is a full-blown one-dimensional simulation. So if you're a student who's looking for information for a science fair project or something, all I can tell you is to read up on Newtonian mechanics, thermodynamics, and compressible flow. If texts on those topics are beyond your comprehension, then so too are the inner workings of GGDT. If those same texts are within your comprehension, then you probably know everything you need to know about how GGDT works.
What phenomena does GGDT not take into account
PBf Feedback:
Alas, no model is perfect and I am far from being a world renowned expert on internal gun ballistics. I wish I could claim laziness, but the simple fact is there are some topics for which I simply do not have the mental resources for. These topics were as a result, ignored....