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).
Because it’s more efficient in terms of power vs fuel.
Smaller stages want to run at higher speeds. Having the second spool lets you use a bigger, slower, fan, which means you can push against more air, and consequently accelerate that air less to get the same force.
It helps to think about it in terms of momentum vs kinetic energy. The thrust is proportional to how much momentum the air gets, and this is Mass * Velocity. The energy you have to spend accelerating the air is 1/2 * Mass * Velocity^2 So if you double the amount of air you’re pushing against, and accelerate it half as much, you get the same force but spend half as much energy accelerating the air. If you are able to push against something with extremely high mass, like the earth, you spend energy accelerating yourself, but you don’t really spend anything significant accelerating the earth, because you change its velocity(and energy) by a negligible amount.
There’s a couple of reasons.
Firstly, the high pressure compressor and turbine perform better at higher rpm than the low pressure compressor and turbine so allowing them to run independently closer to their optimal rpm is good for efficiency. It’s still not perfect because the compressors want to turn slower than the turbines but you’ll need a gearbox to make that happen.
Secondly, inertia. Jet engines are started by getting the high pressure spool up to speed first, usually by feeding high pressure air into it from an external source like the apu. By putting the high pressure components (which are smaller and have less inertia) on their own spool, the engine becomes much easier to start because you don’t have to put energy into rotating the bigger low pressure components.
A spool is another turbine. Some engines have 3 or more to extract as much air as possible
Each turbine controls a part of the engine.. let it be a compressor or the front fan blade!
Source: here’s a fun little post from General Electric. One of the main manufacturers of engines, https://www.geaerospace.com/sites/default/files/t901-what-is-a-spool.pdf
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