Hall Consulting's
Gas Gun Design Tool


Main Window Warnings/Errors

As previously discussed in the Main Window Output portion of this webpage, GGDT's output portion of the main window includes a text box in which any detected warnings and/or errors are displayed. All warnings/errors, their precise meaning, and how GGDT deals with them will be discussed below.

ERR: Pilot volume too small
In it's internal logic related to pilot valves, GGDT assumes that in fact it is dealing with a piston valve and not a diaphragm or other flavor of pilot valve. In order for a piston valve to fully open it must be free to move back a distance equal to 1/4 of the seat diameter. Thus, a valve should have a volume of at least Seat_Diameter * Piston_Diameter^2 * pi / 16. A pilot volume smaller than this will not allow the valve to open fully. True, such valves could be made in pracitce (Why would you? I have no idea!) but the author felt that pilot volume is an easy parameter to overlook during design and opted instead to trigger an error in this instance. Simulation execution is halted.

ERR: Proj mass = 0
A projectile mass equal to zero triggers this error. Clearly, no physical object in our world can have zero mass. Further, there is too much variation in projectiles for GGDT to make any simple assumptions about projectile mass. In response, GGDT simply terminates the simulation.

ERR: Proj never cleared muzzle
Simply put, the projectile lodged in the barrel. Possible causes are numerous. Too low of pressure in the reservoir. Too small of a reservoir. Excessive frictional forces. Excessive barrel leakage (too much "slop" between the barrel and the projectile). An obscenely long barrel. All are possible causes. Simulation execution is terminated to avoid an infinite loop.

ERR: Reservoir volume <= 0
This error arises whenever the volume calculated by GGDT is less than or equal to zero. Due to input restrictions there are three potential causes of this error. (1) The reservoir's listed outer diameter is smaller than it's inner diameter. (2) The reservoir's outer diameter and inner diameter are equal. (3) The reservoir's length is set to zero. In any case, GGDT halts simulation execution.

ERR: Valve seat diam >= piston diam
This error arises when valve seat diameter is greater than or equal too the piston diameter. This situation does is impossible in the physical world. In this situation, GGDT halts simulation execution.


WRN: Barrel potentially choking flow
This warning rears it's head anytime the valve's effective oriface is greater than 65% of the cross sectional area of the barrel breech. If the valve to breech interface is blunt (ie, not tapered), then a 65% flow field is about the best one can expect in the barrel. Thus, the limiting factor for massflow is not the valve, but the breech of the gun. There is nothing wrong with such a design; and in fact, such designs are excellent performers. However, this warning may be an indicator that you're paying too much for your valve (ie, buying/building a bigger valve than you need). However... Since it is possible to get higher than 65% efficiency in the barrel with the use of a tapered interface or constant cross sectional system (read: large burst disc system), I had a decision to make. Do I allow the excessively large flow knowing that in many systems it will overestimate performance or do I restrict the flow knowing that in other systems it will underestimate performance? In the end, I split the difference between those two extremes. If this warning is triggered, GGDT uses the average value between the 65% barrel breech and the stated valve oriface.

WRN: Barrel choking flow
This warning is similar in cause to the previous warning; the difference is in the matter of degree. In this case, the valve oriface area has exceeded the barrel breech area. Even a perfectly tapered breech interface will not be able to pass all the gas the valve is capable of delivering. In this case, GGDT sets the valve oriface to equal to that of the barrel breech area.

WRN: Barrel unnecessarily long
For a given sized reservoir, pressure, valve, and projectile combination, there exists an optimum length barrel that will result in the highest muzzle velocity. If a barrel is longer than this optimal length, the projectile will actually slow down inside the gun barrel. In such a situation, the gun would actually perform better if the barrel were shortened. The above warning is nothing more than a notice to the user that the projectile is decelerating as it clears the muzzle and that the gun would produce higher muzzle velocities with a shorter barrel.

WRN: Pilot volume marginal
In it's internal logic related to pilot valves, GGDT assumes that in fact it is dealing with a piston valve and not a diaphragm or other flavor of pilot valve. In order for a piston valve to fully open it must be free to move back a distance equal to 1/4 of the seat diameter. Thus, a valve should have a volume of at least Seat_Diameter * Piston_Diameter^2 * pi / 16. In practice, one would want there to be some "extra" volume to minimize oscillation of the piston. Thus, GGDT sends up a warning flag anytime there is less than 10% "extra" volume. No other action is taken by the simulation.

WRN: Piston mass = 0
In the physical world there can be no physical object with zero mass. However, rather than declare an error and shut the simulation down, the author recognizes that it may simply be that the user desires an arbitrarily lightweight piston. Thus, GGDT sets the piston mass equal to 1/100th of a gram and continues simulation execution.

WRN: Possible gas liquification
At the end of a simulation, GGDT takes a quick look at the temperature and pressure of the barrel gases. A really crude comparison to a really crude phase diagram is made and GGDT determines if there may have been some gas liquification. Mind you, this particular portion of the program isn't the most rigorous so I wouldn't panic if I saw this warning. In the same breath, however, I'd keep my mind open to the possibility that the simulation results may be bogus due to GGDT's failure to account for gas liquification during the simulation run.

WRN: Proj diam > barrel bore
At first glance, one would think that a projectile being larger than the barrel bore should induce an error. That's an impossible situation, right? Well, sort of. Having a projectile larger than the bore is normal for something like an Airburst Rocket. Still, this situation is highly unusual and as such it is brought to the attention of the user in the event that it is an accident. Also, because programming logic has some issues with this situation (but it doesn't affect results noticibly for the systems in question), the projectile diameter is set equal to the barrel diameter at simulation run-time.

WRN: Valve coef impossibly large
This error indicates that the listed valve coefficient (Cv/Kv/Eff) translates to an effective valve oriface larger the circle described by the valve seat diameter. While this situation may not be intuitively obvious if the simulation is configured to use Cv or Kv, it becomes obvious if efficiency is used as the efficiency will be greater than 100%. In the event that situation is encountered, GGDT sets the valve's effective oriface to be equal to that of the seat area.

WRN: Valve coef optimistic
This error indicates that the listed valve coefficient (Cv/Kv/Eff) translates to an effective valve oriface larger than 65% of the circle described by the valve seat diameter. While this situation may not be intuitively obvious if the simulation is configured to use Cv or Kv, it becomes obvious if efficiency is used as the efficiency will be greater than 65%. Note that 65% efficiency is approximately the maximum efficiency possible for an untapered valve inlet coming from a much larger diameter reservoir. In this event, GGDT takes no special action beyond user notification.

WRN: Valve dead volume = 0
Most valve designs include some dead volume between the seal and the barrel breech. As this is a variable that is easily overlooked at design time, GGDT brings the situation to the attention of the user. The simulation is otherwise unaffected.


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