Staged Combustion Cycle (rocket)
The staged combustion cycle, also called topping cycle or pre-burner cycle, is a thermodynamic cycle of bipropellant rocket engines. Some of the propellant is burned in a pre-burner and the resulting hot gas is used to power the engine's turbines and pumps. The exhausted gas is then injected into the main combustion chamber, along with the rest of the propellant, and combustion is completed.
The advantage of the staged, or "closed", combustion cycle is that all of the engine cycles' gases and heat go through the combustion chamber. An alternative design, called a gas-generator cycle, exhausts the turbopump driving gases separately from the main combustion chamber, which leads to a few percent of loss of efficiency in thrust.
Another advantage that staged combustion gives is an abundance of power which permits very high chamber pressures that allow high expansion ratio nozzles. These nozzles give better efficiencies at low altitude.
The disadvantages of this cycle include harsh turbine conditions, exotic plumbing to carry the hot gases and complicated feedback and control. In particular, running the full oxidizer stream through both a pre-combustor and main-combustor chamber (oxidizer-rich staged combustion) produces extremely corrosive gases. Thus most staged-combustion engines are fuel-rich, as in the schematic.
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