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Can Compression cause Thrust?

  1. Oct 7, 2008 #1
    What if someone took a jet engine and took out the combustion and turbine mechanism. Then that person found an outside source to power the compression mechanism. Can the compression mechanism alone produce significant thrust? That is, with an outside power that does not add heat into the system (such as an electric device that just powers the compression rings). Also the engine is static.

    Thank you for any input,
  2. jcsd
  3. Oct 7, 2008 #2


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    I believe that what you're describing is just basically a ducted-fan engine.
  4. Oct 7, 2008 #3
    Well a ducted fan is precisely what I am trying to avoid, mostly because the amount of thrust needed would require a massive fan. What I am talking about, however, is a series of a compression rings. That is, can I achieve thrust simply by compressing and compressing air and then releasing it through a nozzle? (compression rings are significantly smaller, by my research)
  5. Oct 7, 2008 #4
    Wohoho! I totally see what you are talking about! They are indeed massive regardless
  6. Oct 7, 2008 #5


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    how much thrust are we talking about?
  7. Oct 7, 2008 #6
    Does an equation exist to determine how much thrust is produced from pressure? And does one exist to determine how much pressure is produced through each stage of a compression chamber?

    Also, is there a recommended book that perhaps goes into the details of propulsion via compression and not combustion?

    You are the man btw, Danger. I don't know why I wasn't able to find anything about ducted fan engines before, Google has failed me!
  8. Oct 7, 2008 #7
    To minger,
    Enough thrust to lift a human vertically into the air.
  9. Oct 7, 2008 #8


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    Yea, a ducted fan can work easily.
  10. Oct 7, 2008 #9
    if you look at the thrust distribution over a jet engine, a lot of forward thrust is created at the compressor. The turbine and compressor spools actually pull apart from each other creating tension in the power shaft between the two.

    Also read about the supersonic inlets (SR-71). Thrust is generated by the inlet itself. As air moves through shockwaves at the inlet, its pressure increases. There is a higher pressure on the inside of the inlet cone than there is on the outside. It is essentially a compressor at supersonic speeds, which also transmits forward thrust on the structure.
  11. Oct 7, 2008 #10
    Does an inlet really come into play at subsonic speeds, besides just providing air into the rest of the engine?
  12. Oct 7, 2008 #11
    at subsonic speeds the purpose is simply to provide air to move smoothly without much frictional losses. It allows the engine to pull in more air than its area captures (like during take off) while reducing frictional losses. subsonic inlet design is vastly different than supersonic design, and designing an inlet for both (which is usually the case for supersonic jets) is a balancing act.
  13. Oct 8, 2008 #12
    thrust is achieved only due to the expansion of compressed fluid in a jet engine. Heat is added to the fluid so that a turbine can extract "more" work than that required by the compressor & auxiliary units to operate.

    If no heat is added, ideally compressor turns only itself, producing no thrust. Turbine produces just enough for the compressor.

    Else it is a ducted fan(as already pointed), if the compressed fluid is directly discharged into atmosphere.
  14. Oct 9, 2008 #13
    wel talking about the book and the equation... i think most of them would have studied it: BERNAULLI's theorem.. and it jit can be found in any fluid mechanics book..
  15. Oct 9, 2008 #14
    But if the compression is powered by an outside source, then there is no need for a turbine. So only the compressor is in the nacelle, well a compressor and a fan (to draw more air in), both powered externally. Does the compressed air then turn into thrust?
  16. Oct 9, 2008 #15


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    Sure, if you have an outside source of power which to power the compressor and fan then there's no need.
  17. Oct 9, 2008 #16


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    A resounding yes. However, you may not see the thrust numbers you may think you will compared to the power you need to put in. When we test compressors on their own, one of the major components of the design is what we call a balance piston. The purpose of the balance piston is to counter some percentage of the thrust load created by the compressor. Despite certain statements, the turbine does not fully counteract the compressor thrust. That would be a perfectly balanced system and would be next to impossible to have over the entire run range of the engine. As a matter of fact, the bearing(s) responsible for thrust retention need to have some level of load on them for them to operate properly.
  18. Oct 10, 2008 #17
    Can you explain what the purpose of counteracting the compressor thrust is? Or is that just another way to saying that the turbine "uses" the thrust generated by the compressor?
  19. Oct 10, 2008 #18
    dude, what is the problem in understanding that turbine is acting as the EXTERNAL SOURCE that you have been talking about, for so long.

    Compressor needs mechanical work to operate, turbine provides it with mechanical work. The reason gas is heated is that more amount of work needs to be extracted than necessary for the compressor(friction, auxiliary units etc).
  20. Oct 14, 2008 #19
    Like you said, a turbine needs gas to be heated in order to provide power to the compressor. I am trying to find out an alternative that DOES NOT require gas, hence does not require a turbine.

    From my research, I have only found ducted fans that use fans. Would adding a compressor still classify the engine as a ducted fan? I ask because the purpose of a ducted fan is to produce high air velocity, and a compressor would produce low air low velocity.
    But also Force = Pressure * Area

    So what is the difference between producing more force and having higher air velocity?
  21. Oct 15, 2008 #20
    I am having trouble in understanding what you actually want to prove.

    Anyways lets see, a jet engine also has a ducted fan, which accounts for almost 80% of total thrust the engine produces. Now where do you get energy to turn this fan?? yup, one employs a turbine. To operate a turbine, one needs compressed gas(as the turbine exhausts into atmosphere, thats why compressed). Secondly, why gas is heated?? So that, it not only extracts work for the compressor, but also for the fan & all other accessories(air conditioning unit, control surface actuator, actually each & every need of an aircraft ).

    You ll better understand if you see brayton cycle(standard cycle for gas power units) on T-s diagram. If the turbine is directly fed the fluid from the compressor, it ll only be able to turn the compressor alone & that too, ideally.

    Try to apply conservation of energy law.

    Would the compressor not require an energy source?? And suppose the energy source is anything but a turbine, what is the compressor doing??, feeding a ducted fan compressed air??

    A jet engine accelerates a small small amount of air to a very high velocity, while a propeller accelerates a large amount of air to a moderate velocity. Therefore to obtain some specified value of thrust, either mass flow rate is increased or acceleration is increased(Newton's second law).
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