What determines spool up time?
As barit1 noted, it's not about aesthetics - a ducted fan operates distinctly differently relative to an unducted fan (aka a propeller).
It all comes down to the inlet - a unducted fan (prop) sees the air at free-stream velocity. But when you put an inlet on, the flow conditions at the fan face are largely independent of the free-stream velocity (aside from the ram affects on pressure). As a result, a turbofan at high power will see a flow velocity at the fan face something around 0.4 - 0.5 Mach, regardless of how fast the aircraft is going. So, when a turboprop is going 0.8 Mach, the blades are supersonic and become very inefficient. However the turbofan going 0.8 Mach, the fan is still seeing ~.4-.5 Mach and retains good efficiency.
That's also why variable pitch fan blades - while a potential improvement - are not the huge benefit they are on a prop.
It all comes down to the inlet - a unducted fan (prop) sees the air at free-stream velocity. But when you put an inlet on, the flow conditions at the fan face are largely independent of the free-stream velocity (aside from the ram affects on pressure). As a result, a turbofan at high power will see a flow velocity at the fan face something around 0.4 - 0.5 Mach, regardless of how fast the aircraft is going. So, when a turboprop is going 0.8 Mach, the blades are supersonic and become very inefficient. However the turbofan going 0.8 Mach, the fan is still seeing ~.4-.5 Mach and retains good efficiency.
That's also why variable pitch fan blades - while a potential improvement - are not the huge benefit they are on a prop.
