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Fundamentals of air propulsion?

  1. May 27, 2007 #1
    Can anyone break down the fundamentals of air propulsion? thank you. I also have a few other questions.
    Last edited: May 27, 2007
  2. jcsd
  3. May 27, 2007 #2
    What do you mean, the the equitation for a rocket engine, or what ?

    Rocket engines, jet engines and propeller engines all have different equitations.

    Please clarify what you are searching.
  4. May 27, 2007 #3
    Jet Engines.
    Last edited: May 27, 2007
  5. May 27, 2007 #4
  6. May 27, 2007 #5
    For a clean jet engine (Normally not used any more due to noice and high fuel consume.):

    The air breathing engine is only useful if the velocity of the gas from the engine, c, is greater than the airplane velocity, v. The net engine thrust is the same as if the gas were emitted with the velocity c-v. So the pushing moment is actually equal to
    S = m (c-v)

    Most jet engines today is fan engines, but actually the basic formula should be the same ..
  7. May 27, 2007 #6
    OK a little bit more accurate (Depending how you measure the outgiong air speed.)

    The low bypass turbofans have the mixed exhaust of the two air flows, running at different speeds (c1 and c2). The pushing moment of such engine is S = m1 (c1 - v) + m2 (c2 - v)

    (If you have a exust nozzle or an afterburner like on a military aircraft the two airflow will be mixed so there will be only one airspeed to calculate, and the first formula will be valid.)
    Last edited: May 27, 2007
  8. May 27, 2007 #7
    Langbein, its a pleasure to meet you. Thanks for the info. I'm sure you have seen these guys with Jet packs. Now would that be a air breathing engine? Do you know anything about Jet packs?
  9. May 27, 2007 #8
    "Do you know anything about Jet packs?"

    No, but I have been working with "real" jet engines F-16 among other.

    Do you have a link to the "Jet Pack" ?
  10. May 27, 2007 #9
  11. May 27, 2007 #10
    These looks basically like rocket engines and not jet engines.

    When I look at the formulas I forst gave you they are not really jet engine formulas but rocket engine formulas (Wikipedia is not precise.)

    S = m (c-v)

    This should allmost be the formula for a rocket engine, a bit more accurate like this:

    S = m*k(c-v)

    Where S = trust in Newton, k = some constant, c = gassflow out of the rocket engine (m/s), v= speed of aircraft/rocket, m = massflow out of the rocket engine (kg/s)

    Will have to sleap now but will look at the question tomorrow. I might find some of my old books with some more presice info.
  12. May 27, 2007 #11
    Sounds good. Do you think its possible to build a faster or more maneuver jet pack?
  13. May 27, 2007 #12


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    I didn't check out your links, but I will say that most 'jet-packs' that I've seen were peroxide rockets. They're inherently unstable, but not uncontrollable. The 'mileage', however, is pretty scary. :eek:
  14. May 27, 2007 #13


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    The jetpack is a hydrogen peroxide rocket essentially. There is not a jet engine in it. My company built and flew a version that used an air breathing jet engine.

    The basic premise of all of these types is the change in momentum of the flowing fluid creates thrust. This is why they are referred to as "reaction engines."

    To be precise, the second equation that was posted by Langbein is pretty close. The net momentum is what counts. Although, to be precise, you need to subtract the inflow momentum of the fuel. It's usually negligible though. You also have to add, what is called, the P/A contribution to thrust due to nozzle geometry. It may be zero, it may not. It all depends on the area of flight that the particular engine is operating.
  15. May 27, 2007 #14

    I've read that about Jet Packs. Are there any other type of rockets other then Peroxide Rockets?
  16. May 27, 2007 #15


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    There are tons of other types of rocket engines other than HO rockets.
  17. May 27, 2007 #16


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    Oh man... the variety is almost endless. You could use nitric acid and unsymmetrical dimethylhydrazine if you want to be the next national spokesman for Preparation H. Icing sugar and sulphur and potasium nitrate can be interesting. Then there's the ever popular kerosene/liquid oxygen combination as used in the Saturn V. Good old black powder works reasonably well, but there's an issue with throttleability...
  18. May 27, 2007 #17
    Which type would be more suitable for a man operated jet pack?
  19. May 27, 2007 #18
    Packs with the turbojet engine work on the traditional kerosene. They have higher efficiency, greater height and a duration of flight of many minutes, but they are complex in construction and very expensive. Only one working model of this pack was made; it underwent flight tests in the 1960s and at present it no longer flies. One report indicates a sighting in 1984 of an anonymous flier in Maryland.

    I found this on wikipdeia. As you were saying, they used Kerosene.
  20. May 27, 2007 #19


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    That would be the peroxide unit, which is why most of the focus has been in that area. As Fred mentioned, his company tried out a turbojet system. Others have as well. There are serious issues with all of them.

    edit: Just caught your last post which you sneaked in while I was composing this one. I was actually referring to the LOX/kerosene rocket. Most jet engines use kerosene of some grade or another (Jet A, Jet B, etc.), but they can burn diesel or several other fuels. The same J-34 turbojet that burned premium jet fuel in military fighters is now on the dragstrip sucking diesel. Some others use propane or natural gas. One of the great things about jets is their versatile diet.
    Last edited: May 27, 2007
  21. May 27, 2007 #20

    Thanks for all the info. I'm curious how possible it will be in the future to have a Jet Pack that can self sustain for longer then a few minutes and can go faster and longer distances. When I think of the future of Jet packs, the movie Rocketeer comes to mind.
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