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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 they’re used for fighter jets),
but they’re 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 energy – kind 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 between – rough 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.
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