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.

Go to...

- Beginner's Guide Home Page

*byTom
Benson
Please send suggestions/corrections to: benson@grc.nasa.gov *