Water Rocket Computer Model Problems

Here is the first Water Rocket Computer Model Problem.

The screen shot below is from the 3D optimisation of a 250 ml water rocket with launch tube and T nozzle.

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

It is a plot of speed (velocity along the axis of the rocket - in this case, the launch angle is 90 degrees so effectively, it is vertical velocity) as the output variable with Mass of Water (20g to 140g) on the Y axis and Mass of Rocket Empty (10g to 150g) on the X axis.

It is clear that the highest speeds will be gained from the lowest combined weight of water and rocket, as indeed they are as displayed in the bottom left of the screen shot.

 Variables Water Rocket Computer Model Problem 01 Rocket Mass of Rocket Empty 25 g Capacity of Pressure Vessel 269 cm3 Rocket Diameter 5.7 cm Rocket Coeff of Drag 0.56 Nozzle Diameter 21.75 mm Constant K for nozzle 0.16 [X] Launch Tube in use Used Duration of air impulse 1000 ms [ ] Parachute in use Not Used Launch Tube Length 8 cm External Diameter 21.5 mm [X] Hollow Launch Tube Used Wall Thickness 2 mm Length of Tube Empty 8 cm Distance of Vent from End 0 cm [X] T-Nozzle in use Used T-Nozzle Diameter 1.3 mm Parachute Diameter opened out flat 3 m Parachute Coeff of Drag 0.9 Deploy () Apogee.( ) Timer 4 s Delay in opening 2 m Initial Mass of Water 64 g Pressure in Vessel 94.3 psi Height 1.3 feet Angle of Elevation 90 ° Speed at Angle of Elevation 0 m/s Temperature 20 C Environmental 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 large central maximum; and, 2. the maximum at the top edge towards the left.

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