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Rocket thrust, mass, speed given initial m and fuel consumption rate?

  1. Mar 12, 2013 #1
    A rocket, which is in deep space and initially at rest relative to an inertial reference frame, has a mass of 59.5E5 kg, of which 7.39E5 kg is fuel. The rocket engine is then fired for 300 s, during which fuel is consumed at the rate of 480 kg/s. The speed of the exhaust products relative to the rocket is 3.55 km/s.
    (a) What is the rocket's thrust? After the 300 s firing, what are (b) the mass and (c) the speed of the rocket?

    F=ma

    Found mass by doing (300s)(480kg/s)=144000kg
    Subtracted from 59.5E5-144000=5.81E6 kg

    But I have no idea where to start on a and c. Thank you!
     
  2. jcsd
  3. Mar 12, 2013 #2

    CWatters

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    A hint for question a). Think about the units in Newtons equation..

    Force = mass * acceleration

    The units are..

    = Kg * m/s^2

    rearrange that a bit.
     
  4. Mar 12, 2013 #3

    rude man

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    I would use the momentum integral: ∫Fdt = Δp.
     
  5. Mar 13, 2013 #4
    And I would use a direct formula for calculating thrust in a variable mass system: Mass rejection.... :tongue:
     
  6. Mar 13, 2013 #5

    rude man

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    Chaqu'un a son gout ...
     
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