Water Rocket Computer Model Problems

Here is the fourth Water Rocket Computer Model Problem.

The screen shot below is from the 3D optimisation of the default 2 litre water rocket.

Again 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 Flight Time as the output variable with Pressure in Vessel (18psi to 131psi) on the Y axis and Mass of Water (212g to 1487g) on the X axis with a post-thrust integration interval of 10ms and a model time limit of 1 second.

The graph shows a number of lines that run all of the way across the display.

 Variables Water Rocket Computer Model Problem 04 Rocket Mass of Rocket Empty 120 g Capacity of Pressure Vessel 2050 cm3 Rocket Diameter 9.5 cm Rocket Coeff of Drag 0.56 Nozzle Diameter 21.5 mm Constant K for nozzle 0.16 [ ] Launch Tube in use Not Used Duration of air impulse 50 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 Parachute Diameter opened out flat 1.2 m Parachute Coeff of Drag 0.9 Deploy () Apogee.( ) Timer 4 s Delay in opening 5 m Initial Mass of Water 850 g Pressure in Vessel 75 psi Height 1.5 feet Angle of Elevation 88 ° Speed at Angle of Elevation 0 m/s Temperature 10 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 curiously shaped lines.

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.