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B Jet propulsion in space?

  1. Jul 2, 2016 #1
    Here's a really complex & difficult question...

    Satellites orbit Earth in the upper thermosphere. Could a jet engine (not a rocket) produce enough thrust to move a small craft? The reason I'm asking is because it most certainly would produce "some" thrust in low orbit. The atmosphere extends to roughly 70 miles above the surface. Why bother with rockets at all & just use slower / safer jets?
     
  2. jcsd
  3. Jul 2, 2016 #2

    Drakkith

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    At 70 miles above the surface the density of the atmosphere is a mere fraction of that nearer to the surface where jet engines are normally used. There simply isn't enough air to support combustion, even with a compressor, so a jet engine cannot function at all.

    Also, I've changed the thread prefix from A to B. The prefixes indicate at what level you want the explanations. An A would indicate you want a graduate-level explanation, full of complicated equations and such if appropriate.
     
  4. Jul 2, 2016 #3
    I apologize for the error I'm a bit fresh here : )

    The engine doesn't need to be powered by combustion, so it could still work. An electrical (or mechanically powered) engine would serve our purposes here just fine.
     
  5. Jul 2, 2016 #4

    Drakkith

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    Alright, but then it isn't a jet engine. How will it provide thrust to a spacecraft?
     
  6. Jul 2, 2016 #5

    DaveC426913

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    What electrically-powered or mechanically-powered engine would work in the upper atmosphere?

    I can think of one type of engine that works very well there: a rocket engine.
     
  7. Jul 3, 2016 #6
    A jet works by sucking in air & propelling it out the back. In low orbit, there is still *some* atmosphere present. My thoughts were that maybe a jet could still produce a useful amount of thrust even in such sparse atmosphere.
     
  8. Jul 3, 2016 #7

    davenn

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    nowhere near enough for that ..... even low earth orbit satellites are well above the significant atmosphere

    from wiki



    cheers
    Dave
     
  9. Jul 3, 2016 #8
    Aye. This is good. I understand that 1/100,000th of normal air pressure might not be enough to produce enough thrust to move an object.
     
  10. Jul 3, 2016 #9

    DaveC426913

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    I'm not sure it's enough to sustain combustion at all.
     
  11. Jul 3, 2016 #10

    Janus

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    It's much worse than that. At an altitude of 200 km, the density has dropped to ~2.79e-10 kg/m3. The density at sea level is 1.225 kg/m3
    or better than 4,000,000,000 times denser.
     
  12. Jul 3, 2016 #11

    D H

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    Orbiting is a matter of "falling with style": thrust is rarely needed. At 70 miles (110 km), an object is not "falling with style." It is instead just falling. Assuming a spherical satellite, the satellite needs to be orbiting at more than 140 to 160 km (depending on density) to have a chance of making even one orbit about the Earth. Anything less than that and the object is not "orbiting". It is just falling.

    I suspect that thanks to boatloads of bad science in Star Trek, Star Wars, and a plethora of other TV shows and movies, the opening poster may be confused with regard to what "orbiting" truly means. A spacecraft in orbit does not need to be constantly firing its thrusters to stay in orbit. Thrusters are used rather infrequently to counteract the tiny effects drag, perturbing effects from the non-spherical nature of the Earth's gravitational field, perturbing effects from the Moon and the Sun (and to a much lesser extent, other bodies), and to keep the spacecraft pointed in the desired direction.
     
    Last edited: Jul 3, 2016
  13. Jul 3, 2016 #12

    Nidum

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    Generally with air breathing engines the higher the altitude the faster you need to fly . Put crudely you need to fly faster at higher altitudes so that engines can scoop up enough air /second to function properly and to produce required thrust .
     
    Last edited: Jul 3, 2016
  14. Jul 3, 2016 #13

    DaveC426913

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    Are you trying to tell me that, the moment the Enterprise shuts off its engines, its orbit doesn't decay in a matter of mere hours?

    cuz them's fightin' words.

    :-p
     
  15. Jul 3, 2016 #14
    Although some oxygen exists at very high altitude, gathering the amount required to combust jet fuel would need a gigantic air intake, around a kilometer in diameter, and an enormously powerful compressor stage.
    Such an engine would consequently be very large and heavy, way too heavy to be supported by lift generated from a wing it would be attached to.
     
  16. Jul 3, 2016 #15
    I feel like my question has a been answered in a way that I can chew it : ) Thank you all for your contributions!!!
    I also want to add that I was not referring to a conventional combustion powered air-breathing jet. Rather a jet with an electrical engine, or mechanical one leaving combustion moot. I'm only interested in producing a meaningful amount of thrust in low orbit without rockets... (Impossible?)
     
  17. Jul 3, 2016 #16

    davenn

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    you are still using the words jet engine
    a jet engine requires something to be sucked in one end, burnt and thrust out the other end

    your idea of a electrical or mechanical powered jet engine is therefore not workable ... impossible ... most likely ...YES

    so ask yourself
    "what is your electrical powered motor going to thrust out its "tail pipe" since there is nothing coming in the other end ? "


    Dave
     
  18. Jul 3, 2016 #17

    Chronos

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    An ion engine requires no oxygen or combustion, they work just fine in space using EM force to eject charged particles at high velocity. In fact, they are pretty much useless in the atmosphere.
     
  19. Jul 3, 2016 #18

    Drakkith

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    True, but you still have to carry your fuel on-board, correct?
     
  20. Jul 3, 2016 #19
    Following the OP idea, no it's just not feasible in the usual concept of a 'jet engine'.
    However most jet engines used in modern airliners are actually fan-jets where the fan part is doing most of the work.
    If you could make something like that without needing combustion to power it, the idea is credible.
    No presently known method can do this that I know of though, certainly not electrical - batteries are heavy..
     
  21. Jul 5, 2016 #20

    NTW

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    I remember reading (in the late 50s or mid-60s) about a proposal of an 'orbital jet'. It should work like a ramjet, its 'fuel' being the thermal energy supplied by the compression-induced recombination of ionized gases in the outer layers of the atmosphere, 70km above the surface (or something like that)...
     
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