# Bottle Rocket Calculations

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Bottle Rocket Calculations
Example using 40 psi and 700 ml of water

Select air pressure and water volume
Select air pressure i.e. 40 psi Select water volume i.e. 700 ml Find mass of water Mass = density x volume Vol = 700 ml = 700 cm3 or m3 ρ = density of water = 998 kg/m3 which is a constant Mass = 998 kg/m3 x m3 = kg

Calculate average water mass flow rate
Average mass flow rate, ṁ, of water out of nozzle: ṁ = A X cd X √(2ρΔP) Find A of nozzle in m2: A = πr2 For diameter of ~21 cm = .021 m Radius = d/2 = .021 m / 2 = m A = π (0.0105)2 = m2 Cd is given at 0.98

(water mass flow rate cont.)
Find average pressure acting on the water ΔP = (Pi + Pf) / 2 or (Pi (1+Vi/Vf)) / 2, since PiVi = PfVf, so Pf = (PiVi)/Vf Pi = 40 psi Vi of air = 2 Liter – 0.7 L = 1.3 L Vf = 2 L Pf = 40 (1.3) / 2 = 26 ΔP = ( ) / 2 = 33psi Convert psi to N/m2 14.7 psi = 101, N/m2, so 33 psi = 101, x 33 / 14.7 = N/m2 Note: N = kg m/s2

(water mass flow rate cont.)
Back to calculating average mass flow rate, ṁ, of water out of nozzle: ṁ = A x cd x √(2ρΔP) ṁ = A x cd x √(2ρΔP) ṁ = m2 x 0.98 x (√2 x 998 kg/m3 x N/m2) = kg/s

Water Exit Velocity & Thrust
Water exit velocity V = ṁ / ρA = kg/s / (998 kg/m3)( m2) = m/s Rocket thrust ft = ṁ x V = kg/s x m/s = kg m/s2 or N

Net Force on Rocket Net force f = ft – fd – (mave x g)
F = kg m/s2 – 0 – (( kg)/2) x (9.8 m/s2) = kg m/s2 or N Note: Mave = mass of empty rocket + mass of water selected Mass of rocket was weighted at 300 g, or 0.3 kg Water selected was 0.7 kg fd is the drag coefficient and is very low in this case

Rocket Acceleration The rocket acceleration is a result of the net force acting on the mass f = mave x a, so a = f/ mave A = (146.4 N) / (( kg)/2) = m/s2

Range To find the range you need to find the amount of time it takes for the water to exit and the velocity of the rocket The time to expel the water is the mass of the water divide by the mass flow rate: t = m H2O/ ṁ = 0.7 kg / kg/s = s

Velocity of the bottle Vrocket is a x t =
(Range cont.) Velocity of the bottle Vrocket is a x t = (292.8 m/s2) x (0.097 s) = 28.4 m/s So the range R is V2rocket x sin 2Ө / g Ө is the launch angle which is 45° in this case. R = (28.4 m/s)2 x sin 2(45) / 9.8 m/s2 = 82.3 m

Final Range Final range is affected by drag factor
Drag factor, Dc, for bottle shape is low, i.e. 0.15 Drag force D = 1- Dc = 1 – 0.15 = 0.85 Final range Rf = R x D = 82.3 m x 0.85 = m