There's a long negative history of high-speed gearing. Jet engines have one big rotating unit, and that basic simplicity is good for reliability. Turboprops have a gearbox, and fragile, high-maintenance gearboxes are a long-standing problem with turboprops. Adding a gearbox without reducing reliability is a major achievement.
High-speed gearing is indeed very difficult, but jet engines already typically have two separate, concentric rotating units (spools: https://en.wikipedia.org/wiki/Turbofan#Basic_two_spool). Also, as the article mentions, turboprops (essentially jet engines connected to a propeller) already commonly use gearing. I guess you can think of it as a turboprop with a duct.
The reality is probably just that this is a solvable problem that just wasn't worth the work while there were easier efficiency gains to be had.
> High-speed gearing is indeed very difficult, but jet engines already typically have two separate, concentric rotating units [...]
Yes, everyone knows that, but the spools are not connected by a set of gears as they are in the P&W's new engine.
> Also, as the article mentions, turboprops (essentially jet engines connected to a propeller) already commonly use gearing. I guess you can think of it as a turboprop with a duct.
The new engine is ALSO unlike most installations of turboprop engines in that the fan is driven by the power spool, not a free turbine. The free turbine (in turboprops that have them), which powers the reduction gears for the propeller, runs at a much lower speed than the power turbine.
