How do liquid engines optimize performance throughout the flight profile?

Answer

Adjusting the propellant mixture ratio (O/F) dynamically

Liquid engines offer unparalleled dynamic control over their output by manipulating the propellant mixture ratio ($ ext{O/F}$), which dictates the balance between fuel and oxidizer being injected. This adjustment allows engineers to tailor performance based on the specific needs of the flight phase. For instance, at low altitude, the engine can run slightly fuel-rich to maximize thrust, even if it sacrifices theoretical specific impulse, aiding in overcoming atmospheric drag. Conversely, in the vacuum of space, the engine can be leaned toward maximizing efficiency by adjusting the $ ext{O/F}$ ratio closer to optimal specific impulse conditions. Furthermore, running slightly fuel-rich has the added benefit of generating cooler combustion products, which assists significantly in keeping the sensitive engine hardware within acceptable thermal limits.

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