Finding downstream pressure in an air cannon solenoid

[Staffy]

I have an air cannon, which launches a ping pong ball. The setup for this includes the reservoir, solenoid and barrel. I have to find a function for the range of the launcher. I am currently stuck at working out the downstream pressure for the solenoid. At the moment I have the mass flow rate through the solenoid and the gauge pressure for the reservoir. I was wondering if there was a way of using these values to calculate the downstream pressure acting on the ball?

Any help would be greatly appreciated.

 

[FredGarvin]

I would look at it more from the standpoint of what force does the mass flow impart on the ball, not what the pressure is. To start with a very rough calculation, calculate the force produced by your mass flow if it were to impinge on a flat plate. That will be your maximum possible propulsive force. You know that you'll never get better than that.

 

[Staffy]

Thanks for your help Fred. I used the equation: F = mass_flowrate * gas_velocity to work out the force on the ball. This seems to give reasonable answers.
Nathan

 

[FredGarvin]

Don't forget to add a small fudge factor in there as well because there will be some leakage past the ball.

Also, don't forget that if you have an appreciable delta P across the valve, chances are the flow will be choked.