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Homework Help: Jet Engine Thrust question help

  1. Aug 19, 2009 #1
    1. The problem statement, all variables and given/known data
    A jet engine is being tested in the laboratory and it is found to consume 20 kg of air per second while using 0.2 kg of fuel per second. Given that the exit velocity of the gases is 500 m s-1, calculate the thrust generated by the engine.

    2. Relevant equations
    F = M(V-U) where F is thrust, M is mass and V and U are final and initial velocities.

    3. The attempt at a solution
    not sure what to be doing here as i don't know what's going in where. what goes in for U the initial velocity?
  2. jcsd
  3. Aug 19, 2009 #2

    Shooting Star

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    Hi, welcome to PF.
    Can this equation be correct? The left hand side has the dimension of force while the right hand side has the dimension of momentum.

    Let's do it in a very simple way.

    In one second, how much mass is exiting the rocket? That is given.

    So, what is the impulse on this mass?

    We know that Impulse = Force X time.

    What low should be used now?
    Last edited: Aug 19, 2009
  4. Aug 19, 2009 #3
    Hi Shooting Star!
    Thanks for the reply!

    how do we know how much mass is exiting the rocket?
    we know how much fuel and air it consumes and the exit veolcity of the gases but not the mass exiting do we?
  5. Aug 19, 2009 #4

    Shooting Star

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    In this case , "consumes" means that the fuel and the air mixes and combustion takes place and the end product exits as a hot gas , producing thrust on the docket . That is given in the very first sentence of the problem . Do you understand now ?
  6. Aug 19, 2009 #5
    ok i get what you mean there! total mass is 20.2Kg

    so why do we need the impulse? and as far as the thrust goes how do we now find what this would be. I don't need you to solve for me, just let me kno what's happening with it.
  7. Aug 19, 2009 #6

    can anyone help on this?
    much appreciated!
  8. Aug 19, 2009 #7

    Shooting Star

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    The fuel and air mixture exit the rocket after burning and therefore produces a reaction force on the rocket. The impulse on the existing gas has been given.The impulse is the change in momentum and the time is one second. Use the equation I have given you. The force on the exiting gas must be equal to the thrust on the rocket, according to Newton's third law of motion.
  9. Aug 19, 2009 #8


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    I think we're making this problem more difficult than it needs to be; and it seems that way because of a simple mistake. You are trying to use Conservation of Momentum for a fluid, but you forgot time rate.
    F \neq m(V_2 - V_1) \\
    F = \dot{m}(V_2-V_1)
    As shooting star mentioned, ALWAYS check your units. If you're not getting Newtons, then you're not calculating force. Also, there may be a pressure component that should be included, if not, then you haev mass flow rate and initial and final velocities. Plug and chug.
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