Assume that pressue and velocity are constant over the entire control surface except at the exhaust area Ae. Thus,
Using continuity, , or .
Net momentum outflow from the control volume is Thus, , or Define a fuel-air ratio Thus,
Note: Thrust comes from: a) Increase in momentum of the air + fuel (momentum thrust) b) Pressure at the exit plane being higher than the outside pressure (pressure thrust). When the exit velocity is subsonic, Pe = P1 because a pressure difference cannot be maintained across a subsonic jet. Thus, for subsonic jet exhausts, For engines with bypass flow (some flow going through a fan or propeller, but not going through the burner), the thrust equation is where is the bypass ratio