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Rocket Engine Thrust Formula Help

  1. May 12, 2008 #1

    does anyone know how to derive the ROCKET ENGINE THRUST formula:

    T = P/R AV + (P-B)A

    i know you need to use F=ma and Momentum flow rate equations to get the first place. but what do you do after that?

    Any help would be greatly appreciated.

  2. jcsd
  3. May 12, 2008 #2


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    Science Advisor

    Explain your variables. I see you are accounting for pressure difference across the nozzle in the second term, but I want to make sure of the variables in the first term.
  4. May 19, 2008 #3
    The thrust is the sum of the momentum change (mass flow) * (exit velocity) if the nozzle is over or under expanded than you will get extra thrust from the exit area times the pressure difference (Exit Plane Pressure - Ambient Pressure) * (Exit Area)

    [tex]\dot{m} V+({V}_{exit} -{V}_{amb}){A}_{exit} [/tex]

    The mass flow is density times velocity times area, by perfect gas law

    [tex]\rho = \frac{P}{RT}[/tex]

    [tex]\frac{P}{RT}V^{2}+({V}_{exit} -{V}_{amb}){A}_{exit} [/tex]

    so it looks like there is a T missing and the velocity should be squared. CHECK YOUR UNITS!!
    Last edited: May 20, 2008
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