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  • Airplane jet engines have gotten bigger and bigger over time - and it’s not just because

  • planes are getting bigger: the Airbus A350 is a smaller plane than the Boeing 747 but

  • has bigger engines. The reason? Bigger engines are simply more efficient, up to a point.

  • Modern jet engines actually consist of two propulsion systems working together: the jet

  • core, which provides the power and a small amount of thrust, and the fan, which is just

  • a big propellor driven by the jet core that provides most of the thrust.

  • Pure jet engines can be incredibly powerful (which is why theyre used for fighter jets),

  • but theyre also horribly inefficient because they shoot out their exhaust at super high

  • speeds, which equates to huge amounts of kinetic energy, since kinetic energy is proportional

  • to the speed squared, so accelerating exhaust to twice the speed takes 4 times the energy.

  • In exchange for power, they literally blast energy away as a bunch of hot air.

  • Of course, you do have to shoot some air out of the back of an engine to generate thrust,

  • but it doesn’t have to have lots of energy. Instead of accelerating a little bit of air

  • a lot, you can achieve your desired thrust by accelerating a lot of air a little bit

  • - you get the same momentum boost but save a ton of energy.

  • Essentially, if you make an engine too small, it has to accelerate the air so much as to

  • be a waste of energykind of like using a machine gun to propel your car. But if you

  • make the engine too big, then it starts to cause too much drag. An ideal engine is somewhere

  • in betweenrough estimation puts the ideal engine somewhere around 4 meters in diameter,

  • which is just slightly bigger than the current largest engines.

  • So you can expect jet engines to continue growing in size... but not forever.

  • I’d like to thank Audible.com for supporting this video. As you may have heard, Audible

  • has audiobooks of all sorts, including fiction, non-fiction and periodicals. You can try Audible

  • free for 30 days by going to audible.com/minutephysics, and if you do so I recommend the book Ender’s

  • Game by Orson Scott Card, it was one of the pivotal sci-fi novels I read as a teenager

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Airplane jet engines have gotten bigger and bigger over time - and it’s not just because

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Why Are Airplane Engines So Big?

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    Ken Lo posted on 2017/01/26
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