Calculating Hydrogen-Oxygen Ratios at the Space Center

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Discussion Overview

The discussion revolves around the calculation of hydrogen-oxygen ratios in rocket fuel stages at a space center, focusing on the implications of these ratios for thrust and performance. Participants explore the chemistry of the combustion reaction, the engineering considerations of fuel mixtures, and the effects of varying the fuel-to-oxidizer ratios.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant calculated a mass ratio of oxygen to hydrogen of 4:1 based on ideal reaction chemistry, but noted a discrepancy in the actual ratios observed in different rocket stages.
  • Another participant corrected the stoichiometric ratio to 8:1 and mentioned that both stages used a rich fuel mixture, discussing the implications of running rich on thrust.
  • A participant questioned the differences between running fuel-rich versus oxidizer-rich mixtures, considering factors like cost, storage temperature, and reaction rate.
  • Another response indicated that a lean mixture could act as a quench, suggesting that the reaction dynamics differ significantly between fuel-rich and oxidizer-rich conditions.

Areas of Agreement / Disagreement

Participants express differing views on the appropriate mass ratios and the implications of fuel mixture strategies. There is no consensus on the optimal approach to fuel and oxidizer ratios, and the discussion remains unresolved regarding the trade-offs involved.

Contextual Notes

Participants reference specific calculations and assumptions about the combustion process, but there are unresolved aspects regarding the engineering implications of tank geometry and the effects of varying fuel mixtures on performance.

flatmaster
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I went to visit the space center in my new home of Huntsville AL the other day. I figured i'd do a quick calculation from tank capacity as read from the plaque for the second and third liquid hytrogen stages. Assuming an ideal reaction, there should be a mass ratio of oxyogen to hydrogen of 4:1. This calculation comes from the straight foreward chemistry of the reaction and the respective molar masses of hydrogen and oxygen.

The third stage was close to this ratio. However, the second stage was closer to 5:1. Could a difference this big be atribuable to the engineering of a particular geometry of the tank to fit within the structure?
 
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Also, I was sure to read the fuel masses rather than tank volumes.
 
The stoichiometric ratio is 8:1 (H2O=116O+21H, 16:2), not 4:1. Both the second and third stages used a very rich fuel mixture. The reason: While running rich does slows the exhaust, it does so just a little bit. You can think of it as if those extra hydrogen ions in the exhaust stream as hitching a free ride. The vehicle gets a corresponding free ride the other direction.
 
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Ok. I see my mistake now. I understand how running fuel rich would provide extra mass for some more momentum transfer rearwards, resulting in more thrust. However, what would be the difference between running fuel rich rather than oxidizer rich? Expense of LOX? storage temperature? Reaction rate? Can I assume that the extra reaction force from a heavier oxygen would be the same as from the lighter hydrogen?
 
flatmaster said:
Can I assume that the extra reaction force from a heavier oxygen would be the same as from the lighter hydrogen?
No. A lean mixture would essentially act as a quench.

A *very* old paper: http://digidoc.larc.nasa.gov/report/tn/19/NACA-TN-4219.PDF
 
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