Glossary term
Thrust-Specific Fuel Consumption
Fuel mass flow divided by thrust, used to compare propulsion fuel use and supply range models such as Breguet.
Definition
quantityThrust-specific fuel consumption is fuel mass flow divided by thrust, commonly abbreviated TSFC.
TSFC measures how much fuel mass flow is required to produce a unit of thrust at a stated operating condition. It depends on altitude, Mach number, throttle setting, engine cycle, installation losses, fuel definition, bleed extraction, deterioration and whether thrust is installed or uninstalled. In range equations, a weight-specific fuel-consumption parameter may be required, so unit conversion must be explicit.
Thrust-specific fuel consumption is fuel mass flow divided by thrust:
where \dot{m}_f is fuel mass flow and T is thrust. In SI mass-flow form, TSFC has units of:
In many aircraft references it is reported as:
TSFC is not an engine constant. It depends on altitude, Mach number, throttle setting, engine cycle, fuel heating value, installation losses, inlet recovery, bleed extraction, nozzle state, engine deterioration and whether thrust is installed, net, gross, static or in-flight.
Engineering Role
TSFC lets engineers compare how much fuel flow is required for a unit of thrust at a specified operating point. It supports cruise range, mission fuel, reserve planning, engine-deck validation, performance monitoring and degradation assessment.
For a segment with approximately constant TSFC and thrust:
The Breguet range equation often uses a weight-specific fuel consumption parameter c with units of \text{s}^{-1}. If TSFC is given as mass flow per unit thrust in SI units, the weight-specific form is:
provided the same force and mass conventions are used consistently.
Worked Example: TSFC, Unit Conversion and Breguet Input
A cruise engine-deck point reports:
| Parameter | Value |
|---|---|
| Fuel mass flow, \dot{m}_f | 0.40\ \text{kg/s} |
| Installed net thrust, T | 18.0\ \text{kN} |
| Segment duration, t | 1800\ \text{s} |
First convert thrust:
Compute TSFC:
The approximate imperial form is:
The weight-specific parameter for a Breguet-style equation is:
Fuel mass over the 1800 s segment is:
Now check the thrust basis. If an uninstalled thrust value of 20.0\ \text{kN} were used by mistake with the same fuel flow:
and:
Engineering comment: the wrong thrust basis makes the engine appear about 10\% more fuel-efficient at the reviewed point. In a range calculation, that error would directly inflate the predicted range unless corrected by installed-thrust and installation-loss evidence.
Distinction from Related Terms
TSFC is not fuel flow. Fuel flow is \dot{m}_f alone. TSFC divides fuel flow by thrust so different thrust levels and operating points can be compared.
TSFC is not thermal efficiency. Thermal efficiency measures energy conversion. TSFC is an operational fuel-use metric that also reflects propulsion cycle, installation and flight condition.
TSFC is not power-specific fuel consumption. Shaft-power or brake-specific fuel consumption divides fuel flow by power, not thrust, and is more natural for piston, turboprop, turboshaft and shaft-driven systems.
TSFC is not the Breguet range equation. It is one input to the range model. The range equation also needs speed, lift-to-drag ratio and fuel-weight ratio.
Validation and Common Mistakes
A defensible TSFC value states fuel flow measurement basis, thrust basis, installed or uninstalled condition, altitude, Mach number, throttle setting, temperature, fuel type, bleed and accessory extraction, engine deterioration, uncertainty and unit convention.
Common mistakes include:
- mixing installed fuel flow with uninstalled thrust;
- using static sea-level TSFC in cruise range calculations;
- confusing mass-specific TSFC with the weight-specific c used in some range equations;
- mixing \text{kg/(N s)}, \text{lbm/(lbf h)} and \text{s}^{-1} without conversion;
- applying one TSFC value across climb, cruise, descent and off-design throttle settings;
- ignoring fuel heating value, bleed extraction, inlet losses or engine deterioration;
- treating catalogue TSFC as validated aircraft-level performance data.