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Added reference link for the final turbofan image
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G. Putnam
G. Putnam
  • 131
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Several points limit the top speed of the jet engine

  • Efficiency equation you're probably referring to is: $\eta_p = \frac{2}{1 + \frac{v_9}{v_0}}$
    • This never does stop increasing
    • Yet the growth is logarithmic with speed ~0.4*ln(v0)
    • Trying to fly 2x your speed only gains 1.33x efficiency
    • This type of efficiency only measures how much of the engine's fuel burn effort is applied to the airframe, rather than just moving air.
  • This efficiency matters, but it also needs Specific Impulse
  • Finally, all these considerations also ignore drag
    • The airplane is flying through air
    • The air has density and inertia.
    • To put the airplane where the air used to be you have to do something with the air
    • Generally means diverting or deflecting the air, or piping it through the plane
      • Busemann Biplanes are interesting variation on the last idea, although they don't produce lift.
      • Drag is generally thought of as two terms (Lift Drag and Parasitic Drag)
      • Lift Drag, drag because you're redirecting airflow to create lift, conveniently goes down with speed.
      • However, Parasitic Drag, drag because the airframe shape is diverting / deflecting air and the air has a friction effect on the skin, effectively rises as $V^2$
      • You put them together, and you get a drag curve that looks like this: DragCurves
      • This is one of the main reasons that most airplanes have a cruising speed.
  • Ideally, what you would want (if it was not highly complex) is an engine that acts like a turboprop, then a turbofan, then a ramjet, and then a scramjet.
    • Move a lot of air slowly at low speeds, and then move less air with a much higher exit velocity at high speeds.

Several points limit the top speed of the jet engine

  • Efficiency equation you're probably referring to is: $\eta_p = \frac{2}{1 + \frac{v_9}{v_0}}$
    • This never does stop increasing
    • Yet the growth is logarithmic with speed ~0.4*ln(v0)
    • Trying to fly 2x your speed only gains 1.33x efficiency
    • This type of efficiency only measures how much of the engine's fuel burn effort is applied to the airframe, rather than just moving air.
  • This efficiency matters, but it also needs Specific Impulse
    • Like others have noted, its not just an issue of fuel burn work efficiency
    • The engine is translating a certain amount of the fuel burn into work on the airframe.
    • How much of that work then gets applied as acceleration?
    • From Wikipedia: Specific impulse (usually abbreviated Isp) is a measure of how efficiently a reaction mass engine (a rocket using propellant or a jet engine using fuel) creates thrust.
    • Specific Impulse pretty much uniformly goes down with speed. SpecificImpulseChartWikipedia
    • Together, you end up with curves like the below for turbofans TurboFan
  • Finally, all these considerations also ignore drag
    • The airplane is flying through air
    • The air has density and inertia.
    • To put the airplane where the air used to be you have to do something with the air
    • Generally means diverting or deflecting the air, or piping it through the plane
      • Busemann Biplanes are interesting variation on the last idea, although they don't produce lift.
      • Drag is generally thought of as two terms (Lift Drag and Parasitic Drag)
      • Lift Drag, drag because you're redirecting airflow to create lift, conveniently goes down with speed.
      • However, Parasitic Drag, drag because the airframe shape is diverting / deflecting air and the air has a friction effect on the skin, effectively rises as $V^2$
      • You put them together, and you get a drag curve that looks like this: DragCurves
      • This is one of the main reasons that most airplanes have a cruising speed.
  • Ideally, what you would want (if it was not highly complex) is an engine that acts like a turboprop, then a turbofan, then a ramjet, and then a scramjet.
    • Move a lot of air slowly at low speeds, and then move less air with a much higher exit velocity at high speeds.

Several points limit the top speed of the jet engine

  • Efficiency equation you're probably referring to is: $\eta_p = \frac{2}{1 + \frac{v_9}{v_0}}$
    • This never does stop increasing
    • Yet the growth is logarithmic with speed ~0.4*ln(v0)
    • Trying to fly 2x your speed only gains 1.33x efficiency
    • This type of efficiency only measures how much of the engine's fuel burn effort is applied to the airframe, rather than just moving air.
  • This efficiency matters, but it also needs Specific Impulse
  • Finally, all these considerations also ignore drag
    • The airplane is flying through air
    • The air has density and inertia.
    • To put the airplane where the air used to be you have to do something with the air
    • Generally means diverting or deflecting the air, or piping it through the plane
      • Busemann Biplanes are interesting variation on the last idea, although they don't produce lift.
      • Drag is generally thought of as two terms (Lift Drag and Parasitic Drag)
      • Lift Drag, drag because you're redirecting airflow to create lift, conveniently goes down with speed.
      • However, Parasitic Drag, drag because the airframe shape is diverting / deflecting air and the air has a friction effect on the skin, effectively rises as $V^2$
      • You put them together, and you get a drag curve that looks like this: DragCurves
      • This is one of the main reasons that most airplanes have a cruising speed.
  • Ideally, what you would want (if it was not highly complex) is an engine that acts like a turboprop, then a turbofan, then a ramjet, and then a scramjet.
    • Move a lot of air slowly at low speeds, and then move less air with a much higher exit velocity at high speeds.
Source Link
G. Putnam
G. Putnam
  • 131
  • 3

Several points limit the top speed of the jet engine

  • Efficiency equation you're probably referring to is: $\eta_p = \frac{2}{1 + \frac{v_9}{v_0}}$
    • This never does stop increasing
    • Yet the growth is logarithmic with speed ~0.4*ln(v0)
    • Trying to fly 2x your speed only gains 1.33x efficiency
    • This type of efficiency only measures how much of the engine's fuel burn effort is applied to the airframe, rather than just moving air.
  • This efficiency matters, but it also needs Specific Impulse
    • Like others have noted, its not just an issue of fuel burn work efficiency
    • The engine is translating a certain amount of the fuel burn into work on the airframe.
    • How much of that work then gets applied as acceleration?
    • From Wikipedia: Specific impulse (usually abbreviated Isp) is a measure of how efficiently a reaction mass engine (a rocket using propellant or a jet engine using fuel) creates thrust.
    • Specific Impulse pretty much uniformly goes down with speed. SpecificImpulseChartWikipedia
    • Together, you end up with curves like the below for turbofans TurboFan
  • Finally, all these considerations also ignore drag
    • The airplane is flying through air
    • The air has density and inertia.
    • To put the airplane where the air used to be you have to do something with the air
    • Generally means diverting or deflecting the air, or piping it through the plane
      • Busemann Biplanes are interesting variation on the last idea, although they don't produce lift.
      • Drag is generally thought of as two terms (Lift Drag and Parasitic Drag)
      • Lift Drag, drag because you're redirecting airflow to create lift, conveniently goes down with speed.
      • However, Parasitic Drag, drag because the airframe shape is diverting / deflecting air and the air has a friction effect on the skin, effectively rises as $V^2$
      • You put them together, and you get a drag curve that looks like this: DragCurves
      • This is one of the main reasons that most airplanes have a cruising speed.
  • Ideally, what you would want (if it was not highly complex) is an engine that acts like a turboprop, then a turbofan, then a ramjet, and then a scramjet.
    • Move a lot of air slowly at low speeds, and then move less air with a much higher exit velocity at high speeds.