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Published byYasmin Cordray Modified about 1 year ago

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

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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 cm 3 or m 3 – ρ = density of water = 998 kg/m 3 which is a constant – Mass = 998 kg/m 3 x m 3 = kg

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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 m 2 : A = πr 2 – For diameter of ~21 cm =.021 m – Radius = d/2 =.021 m / 2 = m – A = π (0.0105) 2 = m 2 – Cd is given at 0.98

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(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/m 2, so 33 psi = – 101, x 33 / 14.7 = N/m 2 Note: N = kg m/s 2

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(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 = m 2 x 0.98 x (√2 x 998 kg/m3 x N/m 2 ) = kg/s

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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

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Net Force on Rocket Net force f = f t – f d – (m ave x g) F = kg m/s2 – 0 – (( kg)/2) x (9.8 m/s2) = kg m/s2 or N Note: M ave = 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 f d is the drag coefficient and is very low in this case

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

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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

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(Range cont.) Velocity of the bottle V rocket is a x t = (292.8 m/s 2 ) x (0.097 s) = 28.4 m/s So the range R is V 2 rocket x sin 2 Ө / 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/s 2 = 82.3 m

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

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