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# Bottle Rocket Calculations - PowerPoint PPT Presentation

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

Example using 40 psi and 700 ml of water

• 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

• 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

• 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

• 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 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 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

• 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

• 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

• 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 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