[fancyketchup]

> 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.