Gas Gun Design Tool
Common problems for first time users
Read, and follow, the installation instructions for GGDT before installing.
In the "Resevoir Data" section (parameters describing the pressure chamber) the default value for "Inner Diam" is 1.95 inch. This value is for a coaxial design where the barrels is within the chamber. For other designs, for example ball valves or diaphragm valves, this value should be set to zero.
Suitable parameters for various valves
(This section needs a lot more info)
A rough rule of thumb for calculating the Flow Coefficient (Cv) for various valve types;
Cv = K * D2
Where D is the seat diameter in centimetres, and K is
= 1 for a sprinkler or ball valve = 2 for a piston of QEV = 3 for a burst disc.
This formula is not 100% accurate, but it's easy to use, remember, and reasonably accurate. Using this formula we get a Cv of 3.4 for a 1/2" QEV.
As of version 4.2, in modeled the following features:
- Valve configuration and opening times. In fact, GGDT models five different types of valve: chamber sealing pilot (see piston valves), barrel sealing pilot (ie, barrel sealing diaphragm valves and piston valves, burst disk, hammer valve, and "generic." Each of these valves have different behaviors and GGDT accounts for these behaviors (more on that below).
- Pressure drop across the valve orifice.
- 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 orifice 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. (see shock heating)
However, it does not consider:
- 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.
However, these are not a major concern in most launchers, as they require very long barrels or very high pressures to notice. Typically, the GGDT outputs numbers within 5-10% of the measured value, although this is somewhat clouded by not knowing the proper input numbers.
See Hall's page on use
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