How do jet engines start?

If you've ever watched a jet engine start, you've probably noticed that the blades begin to rotate slowly before spinning up to full speed -- but why? Listen in as Marshall Brain breaks down the science behind jet engines in this episode.

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Episode Transcript

Speaker 1 (00:00):
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Welcome to brain Stuff from how stuff works dot com

(00:22):
where smart Happens. Hi Am Marshall Brain with today's question,
how do they start jet engines? On airplanes? A jet
engine has two sets of veins that are connected together
by a shaft. The hot gases produced by the burning

(00:44):
fuel drive one set of veins in exactly the same
way that the wind turns a windmill. These veins are
at the back of the engine and the hot exhaust
casts are blowing against them to spin them. Those veins
then connect to a shell aft which goes towards the
front of the engine to spin the turbans compressor veins,

(01:05):
so when the exhaust veins are spinning, the compressor veins
are also spinning and compressing the air that comes into
the engine. The starting process normally uses an electric motor
to spin that main turbine shaft. The motor has bolted
to the outside of the engine, and it uses gears
and another shaft to connect to that main shaft. The

(01:28):
electric motor spins the main shaft until there's enough air
blowing through the compressor and the combustion chamber to light
the engine. Fuel starts flowing in an igniter similar to
a spark plug ignites the fuel. Then fuel flow is
increased to spin the engine up to its normal operating speed.
If you've ever been at an airport and watched a

(01:50):
big engine start up on a jet, you know that
the blades start rotating slowly. You can see them spinning
at the front of the engine. The electric star motor
does that. Then you can hear a pop and see
smoke come out the back of the engine as the
fuel first starts to ignite. Then the engine spins up
and starts producing thrust. On smaller turbine engines, especially home

(02:14):
built models, another way to start the engine is to
simply blow air through the air intake with a hair
dryer or a leaf blower. This technique has the same
effect of getting air moving through the combustion chamber, but
it doesn't require the complexity or weight of an attached
starter motor. Besides the starter shaft, most big jet engines

(02:37):
include another output shaft for driving things like electrical generators,
air conditioning compressors, and so on needed to operate the
plane and keep it comfortable. This shaft can connect to
the main turbine shaft at the same point the starter does,
or somewhere else on the shaft. Do you have any
ideas or suggestions for this podcast? If so, please send

(03:01):
me an email at podcast at how stuff works dot com.
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