Turbine Staging — Impulse vs. Reaction Design
Two Ways to Build a Stage
Module 1.1 established that every stage is a fixed row followed by a moving row, with pressure dropping in the fixed row and velocity dropping in the moving row. That description is exactly true for an impulse stage — but real turbines also use reaction stages, where the moving row also does real thermodynamic work, not just redirection. Most modern utility turbines use both: an impulse control stage at the inlet, followed by reaction staging through the bulk of the machine.
Impulse Staging — All the Drop in the Nozzle
In a pure impulse stage, essentially 100% of the stage's pressure drop happens across the fixed nozzle row. Steam leaves the nozzle at high velocity and roughly the same pressure it will have leaving the moving row — the moving row's job is purely to redirect the steam's flow direction, extracting momentum without any further pressure change. Because of this, impulse blades ("buckets") are shaped like symmetric cups rather than airfoils; their geometry is optimized to turn flow, not accelerate it.
Since there's no meaningful pressure differential across an impulse moving row, impulse stages produce very little axial thrust — a real practical advantage, especially for a turbine's first stage, where inlet pressure and thus potential thrust forces are highest.
Compounding — Handling a Large Pressure Drop in an Impulse Stage
A single impulse stage can only absorb so much pressure drop before blade speeds become impractical. Two compounding methods extend what one stage (or one stage group) can handle. Velocity-compounding (Curtis staging) uses one nozzle row followed by two or more moving rows with a stationary redirecting row between them, splitting the nozzle's high exit velocity across multiple moving rows. Pressure-compounding (Rateau staging) instead uses several simple impulse stages in series, each with its own smaller pressure drop. Curtis staging is compact and commonly used as a single control stage; Rateau staging is generally more efficient per stage but needs more axial length to achieve the same total pressure drop.
Reaction Staging — Splitting the Drop Between Both Rows
In a reaction stage, both the fixed row and the moving row are shaped like airfoils, and both drop pressure as steam passes through — the moving row functions partly as a second nozzle. The fraction of a stage's total pressure drop that occurs in the moving row is called the degree of reaction. A "50% reaction" stage, the most common modern arrangement, splits the drop evenly between the fixed and moving rows.
Because the moving row is itself accelerating steam rather than only redirecting it, reaction blading generally achieves higher aerodynamic efficiency per stage than impulse blading. That efficiency advantage is why the bulk of stages in a modern turbine — everything downstream of the impulse control stage — are typically reaction design.
Impulse: pressure drop entirely in the fixed row; moving row only redirects flow. Reaction: pressure drop shared between fixed and moving rows (commonly ~50/50); the moving row also accelerates steam.
The Thrust Trade-Off
Reaction staging's efficiency advantage comes with a structural cost: because pressure genuinely drops across each reaction moving row, there's a real axial pressure differential acting on every reaction stage, cumulatively producing significant axial thrust along the shaft. This thrust must be resisted by a dedicated thrust bearing, and monitoring thrust bearing temperature and rotor axial position is a meaningfully bigger operational concern on turbines with substantial reaction staging than on primarily impulse machines.
Impulse and reaction blading fail differently and wear differently. Impulse buckets see high-velocity impingement erosion at the leading edge; reaction blading, doing continuous aerodynamic work like an airfoil, is more sensitive to fouling and deposit buildup changing its effective shape. Knowing which staging type you're looking at changes what degradation mode to expect.
Why Most Real Turbines Combine Both
A pure impulse turbine (like many mechanical-drive and smaller industrial machines) or a pure reaction turbine are both real design philosophies, but large utility steam turbines commonly combine them: an impulse (often Curtis) control stage absorbs the largest single pressure drop right at the inlet — where thrust-sensitivity matters least because there's only one stage doing it — and reaction staging carries the bulk of the remaining expansion at higher per-stage efficiency. This hybrid approach captures the best characteristic of each design where it matters most.