GGDT for modeling BBMG?

[spencergs]

Hi all,

I've been playing around with the fantastic GGDT software. Can it be used to model a BBMG? I figured valve open time should be zero.

Also another question about GGDT...would it be wise to set the valve seat diameter to the smallest diameter orifice in my air pathway? Ie I am using a valve with a 15mm seat but it is supplied from an HPA tank with 4mm ID tubing. Should I set the valve seat diameter to 4mm?

[jackssmirkingrevenge]

I've been playing around with the fantastic GGDT software. Can it be used to model a BBMG? I figured valve open time should be zero.

It's a little difficult because of the constant flow, with a significant volume the air can expand into before entering the barrel. It's also a dynamic volume that increases as the BBs empty out. I had done some crude tests with a pressure gauge mounted inside the BB chamber of a BBMG, starting with a pressure of 110 psi in the air chamber the BB chamber pressure never rose above 20 psi.

Also another question about GGDT...would it be wise to set the valve seat diameter to the smallest diameter orifice in my air pathway? Ie I am using a valve with a 15mm seat but it is supplied from an HPA tank with 4mm ID tubing. Should I set the valve seat diameter to 4mm?

Certainly the tubing would be the choke point. What does your design look like? Detailed diagrams would be helpful.

[spencergs]

I've been playing around with the fantastic GGDT software. Can it be used to model a BBMG? I figured valve open time should be zero.

It's a little difficult because of the constant flow, with a significant volume the air can expand into before entering the barrel. It's also a dynamic volume that increases as the BBs empty out. I had done some crude tests with a pressure gauge mounted inside the BB chamber of a BBMG, starting with a pressure of 110 psi in the air chamber the BB chamber pressure never rose above 20 psi.

Any idea if the muzzle velocity is accurate with any given dead volume? That BB chamber pressure does sound pretty similar to what GGDT was predicting (as max barrel pressure).

Also another question about GGDT...would it be wise to set the valve seat diameter to the smallest diameter orifice in my air pathway? Ie I am using a valve with a 15mm seat but it is supplied from an HPA tank with 4mm ID tubing. Should I set the valve seat diameter to 4mm?

Certainly the tubing would be the choke point. What does your design look like? Detailed diagrams would be helpful.

I don't have any diagrams, but let's see how well I can describe it. I'm building airsoft weapons to fit onto a robotics platform. One is a smoke grenade launcher (35mm 15g projectile), and one is a BBMG. Both need to be safe on an airsoft field. Both need to be fed from the same regulated air source.

I have a 3000psi (48ci) air tank, feeding an adjustable regulator that can output 0-160psi. 6mm flexible pneumatic tubing connects the regulator to a Y splitter, to feed two solenoids (one for each 'device'). Each solenoid has a 1/2" internal orifice, 4.8cv and 100psi rating. I also have a smaller solenoid that I could use instead with 2mm internal orifice if I need to reduce the airflow to the BBMG.

The grenade launcher in its current form is a 1-1/4" sch 80 nipple 4" long and a threaded endcap, tapped with a 1/8" npt fitting for the flexible airline. I can make the barrel a bit longer if needed. Safe velocity for the launcher is no faster than someone could throw it...say about 60 feet.

The BBMG is also a 1-1/4" sch 80 nipple 4" long with threaded endcaps on both sides. This needs to be a compact unit for where it's being mounted. Low velocity is okay since targets will be fairly close. I actually want this to be fairly inaccurate so that there's less chance of hurting someone with the high ROF up close, and requiring less "aiming" with the robotics platform. The barrel is an airsoft pistol barrel about 4" long, with nearly the entire barrel centered inside the bb chamber. The vast majority of the time, it will be fired up at an angle.

I am planning to make a vortex block for this as my experiments with a couple cloud configurations have not been successful (although I'm thinking a lighter projectile would help...currently using .28g bbs, and I could go as low as .15g bbs). I tried the air inlet through the back of the rear endcap, which could not effectively move the mass of bbs. I tried the air inlet through the bottom side of the rear endcap (blowing up vertically), which works for a few rounds and then jams. These experiments were using my smaller 2mm solenoid at 50-100 psi. I also have the hopup rubber for the pistol barrel that fits over the breech end and can act as a detent if needed and some longer sections of brakeline if I need a longer barrel.

[jimmy101]

Another challenge is that the chamber pressure probably oscillates a fair amount (pressure goes up when a bb is actually in the barrel, down when the barrel is empty).

One approach might be to start with ROF and muzzle velocities for a BBMG some has already built, use that to calibrate GGDT, then model the variations you are interested in.

http://www.inpharmix.com/jps/Jims_chrono.html
scroll down to the section on the BBMG for some data. The basic gun was a vortex with a 3 foot length of 1/4" OD (3/16" ID) stainless steel tubing for the barrel and the compressor was at 120 PSI.

[spencergs]

I think I just figured out my BBMG design. I tried putting the barrel along the bottom of the bb chamber, with the air inlet through the back of the endcap, inline with the barrel...about 3/4" inch between air inlet and barrel. With the detent, it works beautifully.

I nearly packed it full of .28g bbs and ran it at 100psi with my small solenoid. I kept the gun completely horizontal and stationary and got a consistent 70-80 RPS until it was less than half full and then the ROF started to drop off a bit. When I got near the end (about 25% left) the ROF dropped off to 40ish RPS. I think with my intended use, it's going to work well just as it is. I think I could add a plunger if I need to keep the ROF and feeding more consistent. FPS was 117-131 throughout. Which might be okay for my use...if not, I'll switch to the larger solenoid.

[jackssmirkingrevenge]

Pictures/videos of your endeavors are always welcome here

[farcticox1]

I made a vortex block, it works quite well, holds 2000+ BBs, needs a lot of air volume though

[youtube][/youtube]

[spencergs]

Looks like lots of interesting data there Jimmy. I'll look at that in more detail alongside my data from the one I built.

Farcticox1 that thing is cool!

JSR, here ya go

[youtube][/youtube]


Anyone know how long it takes to lose the new user posting restrictions? Waiting 2-3 days for my posts to be approved and published is a bit of a downer...

[spencergs]

Finished my project! Haven't been able to test it outside yet, but it at least fits my standards for safety on an airsoft field. The 1/2" solenoid is massive in relation to my project, so likely will keep the smaller solenoid on the BBMG. Running at 90psi.

[youtube][/youtube]

[youtube][/youtube]

[jackssmirkingrevenge]

JSR, here ya go

Sweet! You need a better backstop, individual energy might be low but lots of BBs hitting the same spot will quickly chew through stuff!

Anyone know how long it takes to lose the new user posting restrictions? Waiting 2-3 days for my posts to be approved and published is a bit of a downer...

With 5 posts you should be out of it now

Finished my project! Haven't been able to test it outside yet, but it at least fits my standards for safety on an airsoft field.

Nice! I was wondering what all those Digi-key boxes were doing lying around

You can definitely get away with a smaller valve, I used a servo-operated ball valve on my sentry project:

[youtube][/youtube]



You'll have proportional flow control that will be able to control power to some extent, for example open it less for short range to reduce the risk of injury.

Using a ball valve instead also means that it could also be connected directly to the rear of the BB chamber along with the controlling servo, reducing the amount of dead space in the air line and making it more efficient - if "efficiency" is indeed a concept that can be spoken of with this sort of BBMG

[spencergs]

Re: backstop
This probably concludes my indoor testing. I broke my ankle a few weeks ago (primary reason for building the airsoft land drone) and I can't get this thing downstairs to bring it outside lol.

Re: 5 posts
Oh good. I was thinking it might be faster to mail you guys letters.

You have a thread for that turret? I might look into a computer vision turret for my next project. I have never seen a ball valve turn so effortlessly...did you modify or lubricate it?

[jackssmirkingrevenge]

Here is the sentry project thread.

I had got it to sort-of work but definitely something I want to revisit in the future, especially as I had made it before I had access to machine tools and epoxy was the order of the day.

Good luck with your recovery!

Note that ball valves get easier to open as they get smaller, this is the model I was using:



I didn't need to lubricate or modify it.

If you search for a "1/8" ball valve" on eBay you'll find plenty of choice for cheap, I would really recommend it instead of the solenoid valve.

[hectmarr]

It serves to drive away felines like cats, friends of wife, curious neighbors? I like the installation with the motorized valve!

[jackssmirkingrevenge]

friends of wife

Eres un fenomeno Hector!