Hall Consulting's
Gas Gun Design Tool

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.

Can you expect similar accuracy? That depends. Is there a gun very similar to yours in the design library? If so, then yes, you should see similar accuracy. If not, then we find ourselves back at "garbage in / garbage out." Still, garbage is relative. Without much work at all you should be getting numbers that are about 10% accurate for a one-of-a-kind gun.

And as one final note on the accuracy thing... GGDT seems to have more difficulties modeling low pressure guns (<30 psi). I suspect this is due to valve springs (whose forces/effects are in the noise level when looking at a high pressure system) becoming significant factors as air pressure dwindles. Alas, few of us have the equipment to measure spring forces and such so there's not much we can do about it. I will say, however, that GGDT assumes that a valve must experience a 3 psi pressure differential in order to open.

How does GGDT work? Can you give me the equation(s) behind the curtain?
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.

Can GGDT be used to model combustion-based spud guns or hybrids?
No, but that's why I created HGDT. It's a package very similar to GGDT, but designed to model hybrid and combustion-based spud guns.

What phenomena does GGDT attempt to take into account?
Oh, geez. That's a tough one to answer. I tried to take a LOT of stuff into account and I'm likely to forget to list them all. However....

• Valve configuration and opening times. In fact, GGDT models four different types of valve: chamber sealing pilot, barrel sealing pilot, burst disc, and "generic." Each of these valves have different behaviors and GGDT accounts for these behaviors (more on that below).
• Pressure drop across the valve oriface.
• Temperature (and thus pressure) increase in the valve pilot due to work performed by gun gases on the valve piston/diaphragm.
• Gas leakage from the main valve body into the upper valve chamber (pilot).
• Performance differences due to different gases.
• Temperature effects on gas properties (and thus, performance).
• Performance limitations due to flow choking in the valve or the barrel.
• Valve effective oriface increases due to lowered valve throat Mach number.
• Temperature (and thus pressure) drop in the barrel due to work performed by the gas accelerating the projectile.
• Gas leakage around the projectile in the barrel.
• Compressibility (Mach) effects on air pressure both in front of and behind the projectile to include the creation of shocks.

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....

• Energy losses associated with turbulence or frictional forces between the gas and the gun's reservoir/barrel walls. In other words, pressure drops due to bends or rough edges in the gun's plumbing.
• Reservoir fineness ratio's effect on performance.
• Freezing or liquification of gun gases.

Will you release the source code to the public domain?
Not just, "No." "Hell no." Don't like that answer? Go write your own GGDT-like application and release it yourself.

Are you ever gonna release a version for the Macintosh?
No. GGDT is a hobby; nothing more. I have made exactly zero dollars and zero cents for all the work I've put into it. With motivation like that, I don't see the point in buying a new computer just so I can do the port and make a few folks happy. What you CAN do, however, is run a Windows emulator on your Macintosh. There's a free one called "Guest PC." Feel free to download it.

I've got an Acme Model XYZ valve. What are the design paramters for input into GGDT?
Off the top of my head? I have no idea! I'm trying to build a library of sorts (it should be pretty easy to find from the main menu), but very few people forward useful data on such stuff to me. Your best bet is to visit the Spudtech Forums and ask around over there.

Who are you? What's your day job? Why do you know all this wierd stuff?
My name is David Hall. I'm a Mechanical Engineer employed at the Naval Air Warfare Center, Weapons Division, Ordnance Test & Evaluation Branch. I started my career writing flight simulations. These days I test bombs, rockets, and guns. Combine those two assignments and voila.

OK folks, that's it! If you've got a question, ask me.

Note: If you came here via a direct link from somewhere else, odds are you'll want to click here.