How does a jet engine work?

Your question of the month in January (2017)
Adrian (10) from Zürich

Passenger aircraft are some of the biggest and heaviest aircraft around – giants that weigh tons and can only take off from the ground with the help of incredibly powerful jet engines. If the pilot turns the jet engines to full power, the aircraft accelerates, achieves 250 km/h in just a few seconds and takes off...but how exactly does it work?

Thanks to lots of metal blades attached to a large rotating ring, the fan, (1), air is sucked into the inside of the jet engine and heavily compressed (2), thereby increasing the pressure and temperature. From there, the air enters the combustion chamber (3), into which kerosene (the fuel) is injected and ignited. The hot combustion gases rush backwards out of the combustion chamber at great speed (around 1,000 km/h!) and set the turbine (4) behind it in motion. This motion is transferred forward via a shaft and drives both the compressor (2) and the thrust-generating fan (1).

A small proportion of the combustion gases are propelled backward out of the jet engine and also create thrust. On modern jet engines, most of the thrust is generated by the fan (1). This so-called “cold thrust” (blue arrow on the outside) makes up 80% of the jet engine’s power. Only approx. 20% is produced as “hot thrust” from the turbine at the rear.

Make your own jet propulsion system:


Thread some thread through a straw and suspend it e.g. across a corridor. Now blow up a large, long balloon. Hold the opening closed but don't knot it and ask someone to attach the straw to the balloon using sticky tape. Once the balloon is stuck to the straw on the thread, let go of the opening. The compressed air will be pushed out of the balloon and will blow your straw along the thread.