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A jet engine question

  1. Nov 10, 2008 #1
    A rocket flying in the atmosphere and flying in the outer space. In which conditions does it have stronger thrust?
    Provided every parameter is the same, even the air drag is considered negligible (in fact it's big).
  2. jcsd
  3. Nov 10, 2008 #2


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    Someone else will have to give you a more accurate answer, but it comes down to the design of the nozzle. For exactly the same combustion chamber, fuel, etc., one type of nozzle will be more efficient at a particular altitude than another.
    If you look at the Shuttle main engines, or the old Saturn V, you'll notice that they're sort of 'bell' shaped. That works best in thicker atmosphere. The higher it goes, the more conical the exhaust should be.
    The beauty of the linear aerospike engine is that it's 'inside-out', with the atmosphere actually providing the outer walls of the nozzle. It is therefore self-regulating as to shape efficiency.
  4. Nov 10, 2008 #3


    Staff: Mentor

    I am not a rocket scientist (perhaps DH will comment), but my understanding is exactly what Danger says. All other things being the same a nozzle designed for use in the atmosphere will provide less thrust in space and a nozzle designed for use in space will provide less thrust in the atmosphere.

    Basically the goal is to get as much of the momentum of the exhaust going straight backwards as possible.
  5. Nov 10, 2008 #4


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    The thrust of a rocket is, in one way, given by:

    [tex]F = \dot{m}v+(P_2-P_3)A_2[/tex]

    [tex]F[/tex] = Thrust
    [tex]\dot{m}[/tex] = mass flow of exhaust gases
    [tex]v[/tex] = exhaust gas velocity
    [tex]P_2[/tex] = Nozzle outlet pressure
    [tex]P_3[/tex] = Ambient pressure
    [tex]A[/tex] = Exit area of the nozzle

    Looking at this, the vacuum condition gives the greatest value for thrust since that maximizes the P*A contribution to the thrust, i.e. pressure thrust. The specific impulse from a rocket will vary with altitude as well as whether the nozzle is over or under expanded.
    Last edited: Nov 10, 2008
  6. Nov 10, 2008 #5


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    If the rocket is measured with v being very small then not so much with the drag.

    Anyway, in a vacuum the rocket's effective exhaust pressure is 1atm more than at sea level.

    I imagine the question is designed to get students to realize that rockets don't have "push against anything", as is a common newbie misconception.
  7. Nov 10, 2008 #6
    OK everyone, hold on here a second... I think there's something wrong with a vacuum condition providing greater thrust. Or maybe I'm wrong; quess we'll see:

    Consider 2 rockets, one which is situated on a hardened launch pad and the other situated such that the rocket is stable but the thrust downwardly exits into a 1000 ft cavern.

    I would bet that the first rocket launches with more effective thrust.

    Thus, higher ambient pressure conditions creates a greater "back" pressure which increases "effective" thrust(complete containment excluded of course)
  8. Nov 10, 2008 #7

    Are you sure that P2 is independent of the ambient P3?
  9. Nov 10, 2008 #8
    OK, after reading Daves post, I'm wrong.
  10. Nov 10, 2008 #9
    Hmmm... maybe not. I don't know. This is getting interesting.
    Last edited: Nov 10, 2008
  11. Nov 11, 2008 #10
    Every existing rocket engine has a higher thrust in vacuum, even the ones specifically designed for first stages.

    Just in case you hope to get more thrust from a higher pressure within the diverging section of the nozzle: remember the (air) pressure is also higher outside (above) the wall of this diverging section, cancelling out any hypothetical thrust increase.

    About rockets "pushing on air": you may also thing about a rocket whose speed is already higher than their exhaust gas velocity, meaning that its exhaust gas moves away from us instead of toward us, but still accelerates the rocket. This is very common, as orbiting rocket speeds are 7km/s upwards, and exhaust speeds about 4km/s.
  12. Nov 11, 2008 #11
    I agree with Fred and Enthalpy. Every rocket will be more efficient and provide greater thrust in a vacuum regardless of the rockets nozzle design. This mainly being due to the pressure exerted from atmo on the exhaust of the nozzle as Fred stated.

    I actually had a lab in my fluid mechanics class that asked this very question and used a air powered rocket to help demonstrate it.

    One more thing, why does the title of this thread refer to a jet engine and then the actual post refer to a rocket engine?
    Last edited: Nov 11, 2008
  13. Nov 11, 2008 #12


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    Assuming that the CD nozzle has a choked throat, P3 will have no affect on the upstream P2.
  14. Nov 11, 2008 #13
    Re: A rocket engine question

    You're right. It should be rocket engine.
  15. Nov 11, 2008 #14


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    If the rocket is close enough to the ground, there is a sort of "ground effect" (don't know if they call it that with rockets), where the ground interferes with the flow of the exhaust and does start to increase the pressure and thrust. I don't know how close the nozzle would need to be to the ground for this to be noticeable, but I suspect it would have to be pretty close - perhaps less than the diameter of the nozzle.
  16. Nov 11, 2008 #15


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    Oh well, it was nice to have felt intelligent for half an hour... :redface:
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