# What's the actual oxidizer/fuel ratio in the combustion chamber of the Vulcain?

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?