The main problem for fluid mechanics analysis in the rocket engine is that of predicting the contained fuel mass for various propellant-to-fuel flow ratios. The analysis described here predicts a dimensionless measure of fuel mass called the fuel volume fraction. This analysis uses a coaxial free-jet computer code, and eddy viscosity equations developed for this code. The calculated variation of volume fraction with flow ratios, fuel radius, and fluid density is shown to be in general agreement with previous data. The analysis and the data predict that the required fuel volume fraction of 0.20 at the flow ratio of 50 can be obtained at a density ratio of 1.0 and a radius ratio of 0.7.