What is the combustion efficiency of liquid fuel rockets?

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SUMMARY

The combustion efficiency of liquid fuel rockets is often misunderstood, with reported values ranging from 95% to below 50%. However, the key metric for rocket performance is specific impulse (sp I), which is optimized at an oxidiser to fuel (O:F) ratio lower than the stoichiometric ratio for complete combustion. This design choice intentionally allows for the presence of combustible products in the exhaust plume, enhancing sp I rather than indicating inefficiency. Rocket engineers prioritize metrics such as impulse density, specific impulse, and thrust per engine diameter over traditional combustion efficiency.

PREREQUISITES
  • Understanding of specific impulse (sp I) in rocket propulsion
  • Knowledge of oxidiser to fuel (O:F) ratios
  • Familiarity with impulse density concepts
  • Basic principles of thrust generation in rocket engines
NEXT STEPS
  • Research the specific impulse (sp I) calculations for various liquid fuel rockets
  • Study the effects of oxidiser to fuel (O:F) ratios on rocket performance
  • Explore impulse density and its implications for rocket design
  • Investigate thrust per engine diameter and its impact on engine sizing
USEFUL FOR

Aerospace engineers, rocket propulsion specialists, and students of rocket science will benefit from this discussion, particularly those focused on optimizing liquid fuel rocket performance.

Cato
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How efficiently is the fuel burned in a typical liquid fuel rocket engine? I've heard numbers ranging from 95% to below 50%.
 
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This is not a direct answer to your question. However, it indicates that combustion efficiency is not the right question.

https://www.raeng.org.uk/publications/other/22-rocket-engine-bssc said:
As such, we find that an optimum sp I is achieved at a value of O:F (oxidiser to fuel ratio) that is significantly lower than the stoichiometric ratio (please refer to Reference 7) for complete combustion. This leads to the presence of combustible products in the plume. These combustibles are here by design to enhance sp I , i.e. they should not be regarded as unburnt fuel. sp I is the main driver of hybrid rocket design rather than combustion efficiency.
 
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As anorlunda indicates, combustion efficiency really isn't the correct figure of merit. Instead, rocket engineers look at things like impulse density (how much momentum change can you get from a given volume of fuel), specific impulse (how much momentum change can you get from a given mass of fuel), and thrust per engine diameter (different fuels will require different sizes of engines to produce the same thrust).
 

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