What are the merits and demerits of adding a gearbox to larger jet engines like the Trent 900. If its about torque, why not use a magnetic gearbox? What are the demerits of using magnetic gearboxes in a geared turbofan engine?
One major benefit is that gearing slows down the fan tips below supersonic, giving a major reduction in noise and drag. The newer massive turbofan jets are ever improving in efficiency, and, with gearing, are taking a page out of the turboprop book to do it.
The point of the gear box is not about torque but bypass ratio. With modern turbofans part of the means by which efficiency is achieved is to send as little of the air through the engine core as possible. This leads to dramatically different rotation rates required to achieve the proper blade tip speeds in the low pressure compressor and at the fan. Adding a gear box allows both the fan and low pressure compressor to operate at optimal speeds.