Specific gas constant; how?

[SpaceCrazy5]

I've been researching a lot about rocketry, specifically rocket engines. I decided to give designing one a go, but I'm slightly stuck; how am I supposed to calculate the Specific Gas Constant of the combustion chamber gas?

It's mostly confusion to do with the molecular mass, and how that relates in terms of the oxidizer/fuel ratio, and other factors.

Sorry if I've posted this in the wrong place, I'm just confused!

 

[Thunder Chicken]

Well, first thing to know is that the combustion gases are not ideal gases, so use of a gas constant and constant specific heats is a gross simplification. It can be used as a crude and simple approximation, but I wouldn't expect anything more from it.

 

[Linguofreak]

I've been researching a lot about rocketry, specifically rocket engines. I decided to give designing one a go, but I'm slightly stuck; how am I supposed to calculate the Specific Gas Constant of the combustion chamber gas?

It's mostly confusion to do with the molecular mass, and how that relates in terms of the oxidizer/fuel ratio, and other factors.

Sorry if I've posted this in the wrong place, I'm just confused!

There may be a few people on the forum who can help, but in general this is a fairly deep subject. In general, the specific gas constant depends mostly on the structure of the molecule: all monatomic gasses tend to have about the same specific gas constant, all diatonic gasses share (more or less) a different gas constant, and so forth. The problem in a combustion chamber is that the chemical composition of the gas changes rapidly as the fuel burns.

 

[n72.75]

Would this be helpful:

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.452.5828&rep=rep1&type=pdf

Using an ideal gas is probably the easiest assumption.

The easiest way to go about it would be to just assume that the working gas is air and that the ratio of specific heats is constant and = 1.4.

https://www.grc.nasa.gov/WWW/k-12/airplane/nozzled.html

Anything else is going to be very partial differential equation heavy, as everything becomes a function of everything else.