Why do engineers deliberately run most chemical rocket engines fuel-rich instead of stoichiometrically balanced?
Answer
Lighter exhaust molecules ($ ext{CO}$ and $ ext{H}_2$) resulting from the fuel-rich burn yield a larger efficiency gain via higher exhaust velocity.
Running fuel-rich results in lighter combustion products like carbon monoxide and diatomic hydrogen. The efficiency gain from having this lighter exhaust stream outweighs the penalty of ejecting some unburned fuel.
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