5 ms·

Energy is proportional with the speed squared, momentum with the speed. You have a fixed budget of energy, initially stored as chemical energy in jet fuel. The

Energy is proportional with the speed squared, momentum with the speed. You have a fixed budget of energy, initially stored as chemical energy in jet fuel. The engines have a more or less fixed thermodynamic efficiency, so you transform the chemical energy in a fixed amount of kinetic energy of the air being pushed out of the engine. You want that air to have as much momentum as possible (Newton’s action-reaction principle: the momentum the plane gives to the air is the momentum the air gives to the plane). Now, for a given energy, you can push some mass of air with speed v or 4 times as much with speed v/2. The momentum of the air in the latter case is double the first case: 4 m v/2 = 2mv. You always want to move more air at lower speed. You achieve that with higher and higher bypass ratios. Of course, with a higher bypass ratio the engine becomes larger and heavier (and draggier), so there are limits. But if you could increase the bypass ratio without increasing the engine weight and drag, you would always do it. As a bonus, with lower air speed come lower vibrations and noise.

mlsu 4y ago
This is also why planes with large wings (think of a glider vs. a fighter jet) are more efficient; they are pushing a larger volume of air slower.
- nautilius 4y ago
  It’s a bit more complicated than that: glider wing area ~10m2, F16 ~30m2, F22 ~80m2
TylerE 4y ago
Also a limiting factor on turboprops (due to their larger diameter) run at a much lower rpm… they’re geared down quite a bit m. This is a complicated area, but in general basically: Supersonic flow is bad. At best you get a ton of noise (like the Russian Tu-95). Efficiency also goes to hell. This is also why the SR-71 had those crazy moving brake cones… they manipulated the shockwave so the air actually entering the engine was subsonic.
- adgjlsfhk1 4y ago
  Turbofans are also starting to be geared down which is kind of wild.
GistNoesis 4y ago
Funnily it seems to be the exact opposite of the rocket equation, where you want to eject the limited mass quantity you have as fast as possible. The fuel being more like a mass reserve than a chemical energy reserve. The fuel in an airplane is also exhausted but I'm not sure how much of momentum is gained due to this rocket fuel effect, in the traditional working regime.
- Gravityloss 4y ago
  Mostly! Sorry, typical Hacker News "well, actually" comment coming up... For lower stages, you can inject water into the nozzle and ISP (exhaust velocity) goes down, but thrust goes up, more than compensating for the weight of the water tanks. But you wouldn't want to do that for an upper stage, as all the rocket below that would need to carry the water tank to a higher velocity. Saturn V second stage had variable mixture ratio, using a leaner mixture with lower ISP (but better density and thrust) at lower altitude and richer mixture with higher ISP at higher altitudes. https://yarchive.net/space/launchers/saturn_variable_mixture.html https://yarchive.net/space/launchers/saturn_variable_mixture... So, ISP is clearly not everything. At low altitudes, you're looking at basically maximum thrust and thrust to weight. At some level you're also looking at volume and thus density, to minimize cost.
  - GistNoesis 4y ago
    Cool, glad to learn something :), I'm more of a AI neurosurgeon than a rocket scientist. My upper point was more about asking about the regime in which we can consider an airplane a constant mass system, and could totally neglect the mass of the consumed fuel compared to the momentum of the air moved. One of the main difference between a jet engine and a rocket engine is that the rocket engine carry its own oxygen, but I'm also not quite sure how much the momentum of the surrounding air is relevant, in helping push the plane forward. There is probably some kind of transition where either when air density get lower, or when speed get higher, than your jet engine behave more like a rocket motor than a blowing-fan (like the turbo-prop vs turbo-fan in the other comments).
    - 4y ago

mlsu 4y ago

This is also why planes with large wings (think of a glider vs. a fighter jet) are more efficient; they are pushing a larger volume of air slower.

nautilius 4y ago
It’s a bit more complicated than that: glider wing area ~10m2, F16 ~30m2, F22 ~80m2

TylerE 4y ago

Also a limiting factor on turboprops (due to their larger diameter) run at a much lower rpm… they’re geared down quite a bit m. This is a complicated area, but in general basically: Supersonic flow is bad. At best you get a ton of noise (like the Russian Tu-95). Efficiency also goes to hell. This is also why the SR-71 had those crazy moving brake cones… they manipulated the shockwave so the air actually entering the engine was subsonic.

adgjlsfhk1 4y ago
Turbofans are also starting to be geared down which is kind of wild.

GistNoesis 4y ago

Funnily it seems to be the exact opposite of the rocket equation, where you want to eject the limited mass quantity you have as fast as possible. The fuel being more like a mass reserve than a chemical energy reserve. The fuel in an airplane is also exhausted but I'm not sure how much of momentum is gained due to this rocket fuel effect, in the traditional working regime.

Gravityloss 4y ago
Mostly! Sorry, typical Hacker News "well, actually" comment coming up... For lower stages, you can inject water into the nozzle and ISP (exhaust velocity) goes down, but thrust goes up, more than compensating for the weight of the water tanks. But you wouldn't want to do that for an upper stage, as all the rocket below that would need to carry the water tank to a higher velocity. Saturn V second stage had variable mixture ratio, using a leaner mixture with lower ISP (but better density and thrust) at lower altitude and richer mixture with higher ISP at higher altitudes. https://yarchive.net/space/launchers/saturn_variable_mixture.html https://yarchive.net/space/launchers/saturn_variable_mixture... So, ISP is clearly not everything. At low altitudes, you're looking at basically maximum thrust and thrust to weight. At some level you're also looking at volume and thus density, to minimize cost.
- GistNoesis 4y ago
  Cool, glad to learn something :), I'm more of a AI neurosurgeon than a rocket scientist. My upper point was more about asking about the regime in which we can consider an airplane a constant mass system, and could totally neglect the mass of the consumed fuel compared to the momentum of the air moved. One of the main difference between a jet engine and a rocket engine is that the rocket engine carry its own oxygen, but I'm also not quite sure how much the momentum of the surrounding air is relevant, in helping push the plane forward. There is probably some kind of transition where either when air density get lower, or when speed get higher, than your jet engine behave more like a rocket motor than a blowing-fan (like the turbo-prop vs turbo-fan in the other comments).
  - 4y ago