why do most airplanes use two spool engines?
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Repost from another deleted post.
If the turbine absorb most of the energy, it would have nothing to shoot back to produce thrust.
Therefore the high pressure turbine only absorbs enough energy to keep the spool running, the rest is ejected backwards to create thrust, that’s your basic turbojet.
That arrangement is unbelievably loud and often inefficient at lower speeds. Most of the energy of the jet exhaust is expanded in a way that doesn’t push the engine forward and only produce noise waves and waste heat.
Therefore another low pressure turbine is placed behind the high pressure turbine exhaust to capture as much energy as possible from the jet exhaust, the energy is used to drive a large diameter low pressure duct fan (turbofan) that is much more efficient at subsonic speeds, also much quieter. Or it is used to drive a reduction gearbox then a conventional propeller (turboprop or turboshaft).
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