Calculating Water Rocket Performance: Thrust, Velocity & More

[haruspex]

Your initial V_air is V0, and it increases to V0+Vf (assuming the pressure is enough to eject all water). Therefore, the total volume is V0+Vf. I fixed this, which gives the formula V0=Vtotal-Vf with the total volume Vtotal.

No, it increases to Vf. Have you found something in the spreadsheet that says otherwise?

 

[mfb]

Oh... sorry, I got confused by the decimal places.
You are right.

 

[Alcubierre]

P(t) or P0 is the pressure of the air inside the bottle? If so, that is obtained by the amount of psi used in the experiment, correct, which in my case, will vary from 50 psi to 65 psi (but in pascals)?
And how is the volume of the air, V0 obtained?

I apologize for all the questions, I just want to be completely clear with all the steps used.

 

[mfb]

P(t) or P0 is the pressure of the air inside the bottle? If so, that is obtained by the amount of psi used in the experiment, correct, which in my case, will vary from 50 psi to 65 psi (but in pascals)?

Right

And how is the volume of the air, V0 obtained?

The volume of the bottle minus the volume of the water inside.

 

[Alcubierre]

Okay, so haruspex's spreadsheet is fine except that I need to fix the time and change the variables to the ones I need (mass, etc).

Edit: 2 L is 0.002 cubic meters and the volume of the water is 0.001 cubic meters and that gives me 0.001 cubic meters. When I change V0 to that, all the other variables because the same.

 

[Alcubierre]

Help. Anyone? This is due in a few hours and I don't know how you got the volume of air so low when I get 0.001 cubic meters and that ruins the whole spreadsheet as it must be, apparently, at least 0.0001 cubic meters to take any effect.

 

[haruspex]

Help. Anyone? This is due in a few hours and I don't know how you got the volume of air so low when I get 0.001 cubic meters and that ruins the whole spreadsheet as it must be, apparently, at least 0.0001 cubic meters to take any effect.

This is probably too late. I plugged in Vf = .002, P0 = 4*atmospheric ("=4*I2"), M = 0.5, A = 0.002 (pi*.025^2). I played around with V0 to maximise height. I got nearly 5m at V0 = 0.00113. But go just a fraction over that and you lose big time.
Not sure about P0. If your gauge says 45 psi, is that 3 atmospheres, or 3 atmospheres in addition to the background 1 atmosphere? I'm assuming it's the second.