Hall Consulting's
Gas Gun Design Tool


Main Window Input

The main window of GGDT is horizontally divided in two regions. The upper portion contains all user gun-specific imulation input. The lower portion contains all simulation output. Obviously, this page is concerned with the upper portion.

Comments
This is nothing more than a place for the user to put any special comments about the gun design. The name of the gun? The revision number? The telephone number to that cute girl across the street? It simply doesn't matter.

Reservoir -> Gas
A pull-down box that defines what gas is being used to charge the gun's reservoir. The choices are obvious at run time but in the name of completeness I'll list them here as well: air, carbon dioxide, hydrogen, helium, nitrogen, and steam.

Reservoir -> Temperature
The temperature at which the reservoir is maintained (note that it is also assumed that this is the ambient temperature in the gun's environment).

Reservoir -> Pressure
The (guage) pressure to which the reservoir is charged.

Reservoir -> Outer Diameter
GGDT assumes that the reservoir is a right cylinder with a longitudinal void in it. The outer diameter input corresponds to the outer diameter of the reservoir cylinder. Note that we are talking about the outer diameter of the air volume, not the outer diameter of the piece of pipe you might use to actually make such a reservoir; it is instead the inner diameter of that pipe (which defines the outer diameter of the reservoir volume). Clear as mud?

Reservoir -> Inner Diameter
As just discussed, GGDT assumes the reservoir to be a right cylinder with a longitudinal void. This void is included to allow for easy modeling of co-axial designs. If you don't know what a co-axial design is, you probably don't have one and should use 0.0 as your inner diameter. If you are running a co-axial design, the inner diameter corresponds to the outer diameter of the gun barrel (In keeping with the nomenclature previously discussed, the "inner" in inner diameter is referencing the inner diameter of the volume of gas contained by the reservoir.).

Reservoir -> Length
The length of the right cylinder that defines the reservoir volume.

Reservoir -> Volume
The volume of the right cylinder that defines the reservoir.


Valve -> Type
A pull-down box that defines what type of valve is being utilized. The choices are obvious at run time but in the name of completeness I'll list them here as well: chamber sealing pilot, barrel sealing pilot, burst disc, and generic. If you've further questions as to the attributes of these valve types, I suggest a visit to the Spudtech Forums. Somebody there will be able to answer your question (and if it shows up enough, I'll probably add it to the FAQ).

Valve -> # of Valves
For various reasons, some gun designs use multiple valves in parallel. GGDT allows for the modeling of such designs but it should be noted that it assumes that all valves are identical in construction. So... How many valves are there in this design?

Valve -> Flow Coefficient
Typically, valve performance is specified by a valve manufacturer in one of two ways: (1) by Cv or (2) by Kv. GGDT allows for either of these methods and a third; efficiency. Which method is used is defined in the configuration dialog box. For those who may not be familiar with these nomenclatures, a few words are in order. A valve that has a Cv of 1.0 will allow one gallon of water to flow through it per minute with a one psi pressure drop. If I remember correctly, a valve that has a KV of one will allow one liter of water flow through it per minute with a one bar pressure drop. In general, these two parameters will be listed in the manufacturer's literature. If it is not, I suggest using efficiency (30-50% would be typical values).

Valve -> Seat Diameter
The valve's seal seat diameter. In layman terms? If you've got a "1 inch valve" odds are pretty darn good that you're gonna want to enter 1 inch here.

Valve -> Open/Close Time
(Not shown in screen shot) GGDT contains one valve class that is a bit of a "catch all" valve. It's the valve to use when any of the other valves just won't do a good job for the valve in question. That said, this variable defines how quickly the generic valve opens and closes.

Valve -> Dwell Time
(Not shown in screen shot) The second variable concerned solely with the generic valve type, dwell time defines how long the valve in question will remain fully open.

Valve -> Dead Volume
With the notable exception of co-axial barrel sealed designs, virtually all other gun designs possess "wasted volume" downstream of the valve seal but upstream of the barrel breech. This volume is defined here.


Valve -> Piston Diameter
While it is true that not all pilot-actuated valves are piston valves (an example of one that is not would be the diaphragm-based sprinkler valves we're all familiar with), GGDT does consider all pilot-actuated valves to be piston valves. Obviously, this variable defines the diameter of the piston. If you still don't know what I'm talking about, once again I'll suggest a trip over to the Spudtech Forums.

Valve -> Piston Mass
The mass of the afore-mentioned piston.

Valve -> Vent Diameter
All pilot-actuated valves require a vent to bleed the pressure from the upper chamber (thereby triggering the valve). The vent diameter ultimately defines the flow rate through the vent (and thus greatly affects the speed at which the valve opens).

Valve -> Pilot Volume
All pilot-actuated valves possess an upper chamber that is obviously integral to their operation. Yup, you guessed it. This variable defines the volume of that upper chamber.

Valve -> Hamr Force
(Not shown in screen shot) This value represents the average force exerted by the spring in a hammer valve.

Valve -> Hamr Dist
(Not shown in screen shot) This value represents the distance traveled by the hammer/striker in a hammer valve.

Valve -> Hamr Mass
(Not shown in screen shot) This value represents the mass of the hammer/striker in a hammer valve.

Barrel -> Bore
The inner diameter of the gun's barrel. Duh. Actually, I shouldn't say that. If you are attempting to model something akin to an Airburst Rocket you'll want to use the outer diameter, but now I'm getting into some pretty advanced stuff that's probably gonna confuse a lot of you so perhaps I should have just stopped at the first sentence?

Barrel -> Length
The length of the gun's barrel. Duh. Actually, I shouldn't say that. If you are attempting to model something akin to an Airburst Rocket you'll want to use twice the length, but now I'm getting into some advanced stuff and.... Well, you know.


Projectile -> Friction
Most projectiles will experience some friction in the barrel. As the reader may imagine, this variable defines frictional forces in the simulation. However, it should be noted that this value is NOT the coefficient of friction that many with physics training may expect to see. Such data can be difficult or impossible to obtain. Instead, GGDT uses a more practical approach to friction; that of equivelent pressure. Place your projectile in your barrel and blow begin to pressurize the breech. At what pressure does the projectile first begin to move at? Whatever that pressure is, put it in the blank! (Note: Human lungs can produce about 2 psi. If you can use the barrel like a blowgun and get the projectile to move....)

Projectile -> Mass
I've nothing cute to say here. 'Tis nothing more exotic than the projectile mass.

Projectile -> Diameter
Again, nothing fancy. This defines the projectile's outer diameter.

Projectile -> Init Position
Not all guns operate with the projectile flush with the barrel breech. Some guns require that the projectile be at a non-zero position in the barrel. Use your imagination and do what you think is right.


Compute Performance
When you've got all your input data to your satisfaction, push this button and GGDT will do it's thing! Be patient though. GGDT takes a fair amount of CPU cycles. It may take a few seconds for it to return with it's answer.


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