Burt Rutan's White Knight and SpaceShip One, Photo Courtesy of Scaled Composites
Thermodynamics and Propulsion

11.4 Other expressions for efficiency - $ I_{sp}$ and SFC

Sometimes the overall efficiency of aircraft engines is expressed in alternative parameters: specific impulse, $ I$ , and thrust specific fuel consumption, TSFC or just SFC. Both of these parameters measure how effectively fuel is used, and have dimensions.

Specific Impulse ($ I$ or $ I_{sp}$ ):

$\displaystyle I_{sp}$ $\displaystyle = \frac{\textrm{thrust}}{\textrm{fuel weight flow rate}} \qquad \textrm{(units of seconds)}$    
  $\displaystyle = \frac{F}{\dot{m}_f g} =\frac{F}{f \dot{m}g},$    

where $ \dot{m}_f =f\dot{m}$ is the fuel mass flow rate. To find the fuel-air ratio, $ f$ , we employ a control volume around the combustor and carry out an energy balance.

Specific Fuel Consumption (SFC or TSFC):

$\displaystyle SFC = \frac{\textrm{mass flow rate of fuel}}{\textrm{thrust}} \qquad \textrm{(units of lbm/hr/lbf or kg/s/N)}$