Water Rocket Computer Model Problems

Here is the sixth Water Rocket Computer Model Problem.

Someone Right Click and Zoom In to see better or Left Click to show full screen.wrote to the water rocket group saying that they knew of a steel tube rocket, powered with steam that had a capacity of 33 litres and weighing 40kg.

The water rocket computer model does not deal with the vapourisation of water so I wondered how the rocket would perform with just water and air at ambient temperatures. This is what happened. I guessed a rocket diameter of 20cm, a pressure of 300psi and an air impulse of 4 seconds.

As always, I have included the file details so that you can reproduce it yourself to investigate the peculiar nature of the maxima displayed in the screen shot on the right.

It is a plot of Height as the output variable with Mass of water (1000g to 26000g) on the Y axis and Nozzle Diameter (10mm to 50mm) on the X axis with a post-thrust integration interval of 1ms (the picture is the same using 10ms as well).

The graph shows an area of viable flight and on the right is an area of short impulse and on the left is an area of low thrust. The maximum on the right of the viable flight area is echoed to the left and less obviously again (where the cursor is).

Water Rocket Computer Model Problem 06    
Mass of Rocket Empty 40000   g
Capacity of Pressure Vessel 33000   cm3
Rocket Diameter 20   cm
Rocket Coeff of Drag 0.56    
Nozzle Diameter 21   mm
Constant K for nozzle 0.16    
[ ] Launch Tube in use Not Used    
Duration of air impulse 4   ms
[ ] Parachute in use Not Used    
Launch Tube
Length 20   cm
External Diameter 21   mm
[X] Hollow Launch Tube Used    
Wall Thickness 2   mm
Length of Tube Empty 25   cm
Distance of Vent from End 0   cm
[ ] T-Nozzle in use Not Used    
T-Nozzle Diameter 4.75   mm
Diameter opened out flat 1.2   m
Parachute Coeff of Drag 0.9    
Deploy () Apogee.( ) Timer 4   s
Delay in opening 5   m
Mass of Water 10000   g
Pressure in Vessel 300   psi
Height 1.5   feet
Angle of Elevation 88  
Speed at Angle of Elevation 0   m/s
Temperature 10   C
Gamma of Gas in Rocket 1.402    
Density of Gas in Rocket 1.293   kg/m3
Density of Liquid in Rocket 998   kg/m3
Acceleration due to Gravity 9.81   m/s2
Atmospheric Pressure 1013   mBar
Density of Air at STP 1.293   kg/m3

The problem here is . . .
1. to find an explanation for the maxima that occur.

The variables are in the table on the left...

If you give up or you think you know what is going on, look at the answers.

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