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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 cm 3 or 0.000700 m 3

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Bottle rocket calculations

Bottle Rocket Calculations

Example using 40 psi and 700 ml of water


Select air pressure and water volume
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 0.000700 m3

    • ρ = density of water = 998 kg/m3 which is a constant

    • Mass = 998 kg/m3 x 0.0007 m3 = 0.6986 kg


Calculate average water mass flow rate
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 = 0.0105 m

    • A = π (0.0105)2 = 0.0003462 m2

    • Cd is given at 0.98


Water mass flow rate cont
(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 = (40 + 26) / 2 = 33psi

    • Convert psi to N/m2

    • 14.7 psi = 101,353.56 N/m2, so 33 psi =

    • 101,353.56 x 33 / 14.7 = 227528.4 N/m2

  • Note: N = kg m/s2


Water mass flow rate cont1
(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 = 0.0003462 m2 x 0.98 x (√2 x 998 kg/m3 x 227528.4 N/m2) = 7.2302 kg/s


Water exit velocity thrust
Water Exit Velocity & Thrust

  • Water exit velocity V = ṁ / ρA = 7.2302 kg/s / (998 kg/m3)(.0003462 m2) = 20.926 m/s

  • Rocket thrust ft = ṁ x V = 7.2302 kg/s x 20.926 m/s = 151.3 kg m/s2 or 151.3 N


Net force on rocket
Net Force on Rocket

  • Net force f = ft – fd – (mave x g)

  • F = 151.3 kg m/s2 – 0 – ((0.3 + 0.7 kg)/2) x (9.8 m/s2) = 146.4 kg m/s2 or 146.4 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
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) / ((0.3 + 0.7 kg)/2) = 292.8 m/s2


Range
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 / 7.2302 kg/s = 0.097 s


Range cont
(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

  • 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 = 69.96 m


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