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(SFC is proportional to speed, not inversely proportional. (Error introduced in 2007!) Replaced Mach numbers with mph (since the conversion depends on temperature) and calculated energy imparted per weight of fuel for Concorde engine.)
TSFC may also be thought of as fuel consumption (grams/second) per unit of thrust (kilonewtons, or kN). It is thus thrust-specific, meaning that the fuel consumption is divided by the thrust.
TSFC or SFC for [[reaction engine|thrust engine]]s (e.g. [[turbojet]]s, [[turbofan]]s, [[ramjet]]s, [[rocket engine]]s, etc.) is the mass of [[fuel]] needed to provide the net thrust for a given period e.g. lb/(h·lbf) (pounds of fuel per hour-pound of thrust) or g/(s·kN) (grams of fuel per second-kilonewton). Mass of fuel is used rather than volume (gallons or litres) for the fuel measure since it is independent of temperature.<ref>[http://selair.selkirk.bc.ca/Training/Aerodynamics/range_prop.htm Specific Fuel Consumption<!-- Bot generated title -->]
Specific fuel consumption of air-breathing jet engines at their maximum efficiency
This figure is inversely proportional to [[specific impulse]].
SFC varies with throttle setting,altitude and climate. For jet engines, flight speed also has a significant effect upon SFC; SFC is roughly proportional to air speed (actually exhaust velocity), but speed along the ground is also proportional to air speed. Since work done is force times distance, mechanical power is force times speed. Thus, although the nominal SFC is a useful measure of fuel efficiency, it should be divided by speed to get a way to compare engines that fly at different speeds.
For example, [[Concorde]] cruised at