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Thrust calculations

  1. Nov 21, 2009 #1

    Edi

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    I am usually bad with calculations, so please help me out.
    It's not a homework, merely my interest...

    So, lets say I have a tube with some air pressure in it and I open up one end of the tube so the pressurised gas exits it in one direction... (basically a rocket)
    Pretty simple, huh? :D

    But what I want to know is how to calculate the speed of the gas exiting, the thrust, the reaction force etc.
    And while we are at it, what if the pressure in the tube is lower than outside, what speed will the outside gas enter the tube, how fast it will fill depending on the size, etc?

    Please forgive me asking for such relatively simple calculations. ;)
     
  2. jcsd
  3. Nov 21, 2009 #2
    The force can roughly be approximated to just be the pressure force which is simply

    [tex]\frac{P_{gage}}{Area} = Force[/tex] where the gage pressure is the difference of the pressures in the tube and of the atmosphere.

    Once you know that F = [tex]\beta \rho A V_{avg}^2[/tex]

    Where rho is the density, a is area, v is velocity and beta is some correction factor (probably fairly small to the point where it could be negligible)

    Same equations apply regardless of if the pressure difference is negative or positive.
     
  4. Nov 21, 2009 #3

    russ_watters

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    Staff: Mentor

    You can use bernoulli's equation for this: assume all of your static pressure is converted to velocity pressure.
     
  5. Nov 21, 2009 #4

    rcgldr

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    Homework Helper

    One issue is that the pressure at the open end of the tube would not be ambient due to momentum of the affected air, both inside and outside the tube. Note that rockets generrate more thrust in space than they do in the atmosphere because the momentum of the air reduces the effective exhaust velocity component of thrust.
     
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