as the title would suggest, does anyone know a calculation or solution to finding the current mixture ratio used on the latest block 5 configuration?

This would be easier if so many people did not post a different result to the same question.


Kerosene tank capacity
- First stage  : 123,500 kg
- Second stage : 32,300 kg

Liquid oxygen tank capacity
- First stage  : 287,400 kg
- Second stage : 75,200 kg

I have found

Kerosene : LOX = 3 : 7 (which gives 0.428571) similar but different to below


123,500 / 287,400 = 0.429714

Are either of these answers even remotely in the range of current usage possibility? 0.428571 on one side and 0.429714 on the other.

  • 1
    $\begingroup$ Where are your data coming from? $\endgroup$ Commented Sep 7, 2018 at 23:29
  • 1
    $\begingroup$ Also, what are you actually trying to figure out, at a high level, with your recent questions? $\endgroup$ Commented Sep 7, 2018 at 23:35
  • $\begingroup$ If you're comparing the ratio to what would be expected by the chemical reactions, keep in mind that the Merlin engines in Falcon 9's are designed to run fuel-rich (i.e. there is a slight excess of fuel). $\endgroup$
    – DrSheldon
    Commented Sep 8, 2018 at 4:32
  • $\begingroup$ Just casually looking through rocket calculation, Russell Borogove. $\endgroup$ Commented Sep 8, 2018 at 7:33

1 Answer 1


You're on the right track to finding the ratio; unless a particular ratio is published by the engine vendor, the best you can do is divide total propellant mass consumption as you're doing here.

By convention, mixture ratios are given as mass of oxidizer to mass of fuel (rather than fuel to oxidizer). You should divide 287400 by 123500 and retain no more than 4 significant figures: 2.327:1.

Ratios near 2.6:1 are typical of kerosene/LOX but different designs will use different ratios, so that figure is plausible.

It's not appropriate to take a ratio of numbers that have been arbitrarily rounded off and extrapolate 6 significant figures from the result, so there's no support for your 0.428571 value, and thus no conflict with the 0.4297 figure. Those figures differ only by a quarter of a percent anyway.


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