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GGDT Help

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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GGDT Help

Unread postAuthor: no-limit » Fri May 30, 2008 4:37 pm

This is the best I can calulate this so far. The projectile data is not exact because my rockets go on the outside. Is there something I missed to make it better. Any help would be appreciated.
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Unread postAuthor: Lentamentalisk » Fri May 30, 2008 4:55 pm

to get more accurate data, I would double the barrel length, and make the starting position your original barrel length. This is because if the rocket goes over the barrel, the entire barrel will be empty, and the first effective bit of projectile will be a barrel length down the barrel. Just before the projectile comes off of the barrel, it will have effectively twice the barrel length.
Hope that makes sense.
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Unread postAuthor: no-limit » Fri May 30, 2008 5:44 pm

That actually ends up being the same, but doubling the barrel length does change it. It seems to me that that would work better because the rocket starts to lift off as soon as air starts to enter the barrel.
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Unread postAuthor: clide » Fri May 30, 2008 5:49 pm

Looks pretty good, but I'm a little confused as to why your pilot valve (vent diameter) is bigger than your main valve (seat diameter). Seems a bit backwards. If you are going to be using a .75" barrel then you would probably be better off using a 1" sprinkler directly rather than a piston valve actuated by a 1" sprinkler valve.

Also 50% efficiency (Flow Coef) would be pretty optimistic. I'm basing this primarily on my own valves, but I would say most tee valves should be around 40%.

I'm not sure if you are doing it already or not, but if you are using a rocket over the outside of your barrel then you barrel and projectile diameter would be the OD of the pipe since that is the diameter that pressure is working on. As Lenta already said you should also account for all the barrel that starts out empty you can either do this through initial position like he mentioned or you can directly input the volume into the "Dead Volume" field.
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Unread postAuthor: no-limit » Fri May 30, 2008 5:59 pm

I have made 4 of them but I am trying to get more data before completing this one. The last one I made does not go as good as my sprinkler valve one does. Changed the 50 % to 40%. How would I figure the dead space of the barrel since there must be air in it and I would assume that therocket launches when the air hits the barrel.

Is there a better barrel size that could beat my 1" sprinkler using a piston.

Here's the new one.
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Unread postAuthor: clide » Fri May 30, 2008 6:25 pm

Looks like you added dead space and initial position. You only need to do one or the other, they are pretty much the same thing. As you can see from the results it won't make a huge impact, but you might as well have things accurate.

Is there a better barrel size that could beat my 1" sprinkler using a piston.


Well that is the beauty of GGDT. You can model your sprinkler valve gun and find out. You would probably have to take it apart to get the actual data, but I can give you a rough estimate for the specs of a 1" sprinkler.

Flow coef: 30%
Seat: 1"
Piston: 2"
Mass: 1 oz
Vent: 0.15" for an average blowgun
Pilot volume: 1 in3 if the blowgun is hooked up directly. More if you have a bunch of hose.

Now one thing to keep in mind is that if your piston isn't particularly good then GGDT's estimate may be a bit high because I believe it assumes a fairly small equalization hole. Too much blow-by around the piston can decrease performance significantly.

Edit: You also have a pretty big chamber for the size of your barrel. You could probably decrease the size without much of an impact on performance.
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Unread postAuthor: no-limit » Fri May 30, 2008 8:53 pm

Brain exploding. Is this the best a piston can do?
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Unread postAuthor: psycix » Sat May 31, 2008 6:08 am

Clide do remember that a piston valve with those specs would beat a sprinkler valve big time.
Sprinklers may open just as quick, but due to the way they are constructed the flow is the limiting factor.
I once rode that a 1" sprinkler has the same flow as a 3/4" QEV.
And a qev looks more like a piston valve then a srpinkler.
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Unread postAuthor: no-limit » Sat May 31, 2008 12:32 pm

You probably can't get a piston to those specs. ok I did the ggdt for a sprinkler and they are about the same. What gives? I thought a piston was supposed to be a lot better. I think I have wasted a lot of time trying to make a piston.
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Unread postAuthor: clide » Sat May 31, 2008 4:43 pm

no-limit wrote:ok I did the ggdt for a sprinkler and they are about the same. What gives? I thought a piston was supposed to be a lot better. I think I have wasted a lot of time trying to make a piston.


It all depends on the size. A 3/4" piston valve isn't going to be any better than a 1" sprinkler valve. However, if you have a 2" piston valve with a 2" barrel it will blow the socks off a 1" sprinkler valve with a 2" barrel.

If you don't particularly care about the configuration of the gun then anything with a 3/4" barrel and under would be a lot easier and work just as well with a 1" sprinkler valve rather than making your own piston valve.
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Unread postAuthor: no-limit » Sat May 31, 2008 6:55 pm

Now I get it.......
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