# What are the mass flow rate and exhaust velocity for a CF6 or GE90 turbofan?

For a typical turbofan jet engine (two examples given in the title), what is the exhaust velocity and mass flow rate of air at sea level and cruising altitude (~ FL350)?

Also, does the specific impulse vary at different altitudes, since the density of air decreases with increases in altitude?

Looking specifically at the GE90, we see that it has take-off SFC (specific fuel consumption) of 7.9 mg/N-s. Using the formula $I_{sp} = 1/(g_o·\text{SFC})$ given in paragraph 4 of the Wikipedia article Specific Impulse we get a specific impulse of 12,285 s. If we use the cruise SFC of 15.6 mg/N-s, we get a specific impulse of 6,536 s. Is this an effect of the lower density of air, or simply of the lower fuel required in the cruise? I don't know.
• Specific impulse is lower at altitude, which means more fuel is required for the same thrust. This would likely be effect of the higher airspeed as you need more power for the same $\Delta v$. The lower temperature increases the thermodynamic efficiency, but not enough to offset the reduction due to the inlet velocity. The total consumption is only lower in cruise because you need less power to maintain speed in the less dense air. Mar 29, 2017 at 18:51