Computer drawing of an afterburning turbojet engine with the equation
 for thrust. Thrust equals the exit mass flow rate times exit velocity
 minus free stream mass flow rate times velocity.

To move an airplane through the air, thrust is generated by some kind of propulsion system. Most modern fighter aircraft employ an afterburner on either a low bypass turbofan or a turbojet. On this page we will discuss some of the fundamentals of an afterburning turbojet .

In order for fighter planes to fly faster than sound (supersonic), they have to overcome a sharp rise in drag near the speed of sound. A simple way to get the necessary thrust is to add an afterburner to a core turbojet. In a basic turbojet, some of the energy of the exhaust from the burner is used to turn the turbine. The afterburner is used to put back some energy by injecting fuel directly into the hot exhaust. In the diagram, you'll notice that the nozzle of the basic turbojet has been extended and there is now a ring of flame holders, colored yellow, in the nozzle. When the afterburner is turned on, additional fuel is injected through the hoops and into the hot exhaust stream of the turbojet. The fuel burns and produces additional thrust, but it doesn't burn as efficiently as it does in the combustion section of the turbojet. You get more thrust, but you burn much more fuel. With the increased temperature of the exhaust, the flow area of the nozzle has to be increased to pass the same mass flow. (You can investigate nozzle operation with our interactive nozzle simulator.) Therefore, afterburning nozzles must be designed with variable geometry and are heavier and more complex than simple turbojet nozzles. When the afterburner is turned off, the engine performs like a basic turbojet.

The nozzle of a turbojet is usually designed to take the exhaust pressure back to free stream pressure. The thrust equation for an afterburning turbojet is then given by the general thrust equation with the pressure-area term set to zero. If the free stream conditions are denoted by a "0" subscript and the exit conditions by an "e" subscript, the thrust (F) is equal to the mass flow rate (m dot) times the velocity (V) at the exit minus the free stream mass flow rate times the velocity.

F = [m dot * V]e - [m dot * V]0

This equation contains two terms. Aerodynamicists often refer to the first term (m dot * V)e as the gross thrust since this term is largely associated with conditions in the nozzle. The second term (m dot * V)0 is called the ram drag and is usually associated with conditions in the inlet. For clarity, the engine thrust is then called the net thrust. Our thrust equation indicates that net thrust equals gross thrust minus ram drag.

Afterburners are only used on fighter planes and the supersonic airliner, Concorde. (The Concorde turns the afterburners off once it gets into cruise. Otherwise, it would run out of fuel before reaching Europe.) Afterburners offer a mechanically simple way to augment thrust and are used on both turbojets and turbofans.

You can explore the design and operation of an afterburning turbojet engine by using the interactive EngineSim Java applet. Set the Engine Type to "Jet with Afterburner" and you can vary any of the parameters which affect thrust and fuel flow.

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byTom Benson
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