Why does steam pressure drop mainly in the nozzles of an ideal impulse turbine?

In an ideal impulse turbine, the steam pressure drops exclusively within the stationary nozzles to convert the steam's thermal energy into kinetic energy.1

This process is fundamental to the design of impulse blading. As the steam expands through the nozzles, its pressure decreases while its velocity and kinetic energy increase. This high-velocity steam is then directed onto the moving blades (or buckets), where the kinetic energy is extracted to perform work.1

In an idealized impulse stage, there is no pressure drop or further acceleration as the steam passes through the moving blades. Because the pressure remains constant across the rotor, the blades do not experience the pressure-driven forces found in reaction staging, which simplifies the mechanical requirements for the rotor and reduces the need for heavy-duty thrust bearings. By concentrating the pressure drop in the stationary nozzles, the turbine efficiently transforms the enthalpy of the steam into the rotational energy required to drive the turbine.12