TSFC Calculator

The specific fuel consumption (SFC) is a standard measure of the fuel burn rate of jet engines. It indicates how much mass of fuel is burned per unit time to maintain a given thrust. The lower the value, the more efficient the engine. This calculator will provide the SFC in your chosen units when you input the fuel flow and thrust. If one of the three fields is left blank, it will be calculated from the other values.

What is TSFC?

TSFC is the ratio of fuel mass flow to thrust. In SI base units it has a unit of kilograms per newton-second, which is an inconvenient decimal number. So engineers usually convert this into other units. There are two main conventions: in US aviation they use a unit where the fuel mass per time is divided by the thrust force in pounds. This cancels out mass and force, leaving a reciprocal number per time. In metric it is converted to grams per kilonewton-second. This equates to milligrams per newton-second and allows excellent engine results to be kept within a convenient range of about ten.

This value is directly related to the efficiency of the propulsion system. The less kinetic energy that the engine expels in the form of a fast jet stream, the lower the TSFC. This principle explains why high-bypass turbofan engines dominate most civil aviation. High-bypass turbofan engines expel a large volume of air at moderate speed rather than just a small amount of air at very high speed, resulting in significantly less energy loss.

To use this calculator:

Enter the fuel mass flow and engine thrust, and get the TSFC in your desired unit. This calculator also allows conversion. If the field for TSFC is left blank, then the TSFC can be calculated from the fuel flow and thrust. If the field for Thrust is left blank, then the Thrust can be calculated from the known TSFC and fuel flow. If the Fuel Flow is left blank, then the required amount of fuel for a given target TSFC will be determined. By activating the Mission layer and entering the flight time and fuel price, the tool calculates total fuel consumption and cost. The comparison field allows the entered values to be compared with typical engine values.

Understanding The Formula

The relationships are simple.

In this case, m-dot-f is the mass flow rate of fuel and F is the thrust. Let's take a modern turbofan engine as an example. This engine burns about 0.60 kilograms of fuel per second while producing a thrust of 40 kilo-newtons.

Converted this means: 15 grams per kilonewton-second, 0.53 pounds per pound-force-hour or 54 kilograms per kilonewton-hour. These four representations describe the same engine. At a constant fuel flow rate, for each unit of time that it maintains its thrust, the engine consumes 0.60 times 3,600, which is 2,160 kilograms of propellant. The following table shows the symbols and example values for this example.

TSFC reference values by engine type

All published TSFC values are for specific operating points, usually the flight condition at engine design conditions. So the performance of the same engine can be lower when running at maximum takeoff power or in certain phases during climb. Under this assumption the following ranges apply to the flight condition.

Reasons why high bypass turbofan engines are superior:

The propulsive efficiency increases as the exhaust speed decreases and approaches flight speed. Turbojet engines accelerate all of the airflow to extremely high speeds, which creates concentrated thrust but also wastes energy in the exhaust. Turbofan engines direct most of the air around the core of the engine with a large fan and only accelerate this ambient or bypass air by a small amount. In modern turbofan engines with a high bypass ratio of nearly nine, propulsive efficiency can reach as much as 85% whereas it may be only 60% for a turbojet. This is why fuel consumption per unit thrust in modern airliners is about one-third less than core-engines of the same thrust class from the 1970s.

Applications - fuel costs and engine selection:

For long-haul flights the engines are mainly chosen for their specific fuel consumption (TSFC) as fuel is the largest single operating cost item for airlines. Even a few percent improvement can add up to millions saved per year on a large fleet. On the other hand, a gradual increase in TSFC of older engines is an early warning sign that maintenance costs may be balanced by a visit to the workshop. Combat aircraft accept higher TSFC in exchange for the higher thrust density required for supersonic flight. Cargo airlines try to use the most efficient core engines they can fit into their planes.

Hints for understanding:

The lower the value, the more efficient it is. When comparing engines make sure that fuel flow and thrust are at the same operating points. Cruise TSFC is not the same as takeoff TSFC. Be careful when converting metric to English units that you match grams per kilonewton-second exactly with milligrams per newton-second, and note that values near 15 in these units will be roughly equivalent to 0.53 pounds per pound-force-hour.