I'm working on a simple exercise on the Vulcain, but I'm having a hard time figuring out what the real O/F ratio is, in the combustion chamber. I need the mixture ratio in the chamber to calculate the flame temperature and the gas properties.

On Wikipedia it states that it's 6.2, but links no sources to that number; The book "Rocket Propulsion Elements" by Sutton reports an O/F of 5.35; In "Rocket and Spacecraft Propulsion" by Turner it's 5.3; Old notes from a class I had a few years ago state O/F = 6.2.

Now, I am not only looking for the correct number, but I am wondering as well if these numbers refer to the Chamber mixture ratio, or the ratio of mass between the tanks. Please remember that the Vulcain uses a Gas Generator, which inevitably has a different O/F than the main combustion chamber: the global O/F of the tanks would be different than the one in the chambers.

What do you think? Could it be that maybe I find two different values 5.3 and 6.2 because they are referring one to the tanks and the other to the main combustion chamber?

My hypothesis is that the ratio is 6.2 in the chamber and 5.3 in the tanks, because the Gas Generator requires (I think) a fuel-rich mixture, but I would love to have some literature to back this up.

I also have a side question, not sure if I should add it here but it is still regarding the O/F. I calculated that the maximum Isp is given for a much lower O/F, about 4. Why do they use a higher O/F then? It is just due to inaccuracies in my calculations?


1 Answer 1


This May 2001 paper gives

  • Overall mixture ratio = 5.3
  • Gas Generator mixture ratio = 0.9
  • Combustion Chamber mixture ratio = 6.3
  • $\begingroup$ Thank you very much, looks like I had the right idea.. I have been looking for a paper like this for days, did you just find it or did you know it existed? Otherwise I need to re-evaluate my google skills $\endgroup$
    – Iverie
    Commented Nov 8, 2018 at 17:55
  • $\begingroup$ To me these numbers don't exactly add up; using the flowrates given in the same paper I got a combustion chamber ratio of 5.8. But there are probably cooling flows or such that aren't accounted for in these simple numbers. $\endgroup$ Commented Nov 8, 2018 at 17:56
  • $\begingroup$ I googled for "technical papers on vulcain rocket engine" - putting the 'technical papers' in cuts out a lot of junk results. $\endgroup$ Commented Nov 8, 2018 at 17:58
  • $\begingroup$ Thanks for the tip. You are right about the flow rate not adding up though. But the gas generator mixture ratio looks right.. The gas generator flow should be double what it's stated $\endgroup$
    – Iverie
    Commented Nov 8, 2018 at 18:01
  • $\begingroup$ Very nice! I inserted twice the GG flow and now the ratios come out right. $\endgroup$ Commented Nov 8, 2018 at 18:01

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