The exhaust section is located just behind the turbine section. It generally consists of a convergent exhaust duct, an exhaust cone, and an inner tail cone. The final outlet, known as the jet nozzle, allows the hot gases to exit into the atmosphere. The exhaust duct is also referred to as the exhaust nozzle, which can be designed as a variable or vectoring nozzle, or simply as the tailpipe.
In some early gas turbine engines, the tail cone could be moved rearward. This reduced the size of the exhaust opening, increased exhaust velocity, and produced greater thrust, resulting in smoother and faster acceleration of both the engine and aircraft. Later designs replaced this feature with advanced fuel control systems to manage airflow and acceleration. In modern subsonic engines such as those used on commercial airliners and business jets, the tail cone is fixed [Figure 2]. Variable-area exhaust systems that use movable parts are found mainly on supersonic aircraft.
The tailpipe is part of the airframe and connects the engine to a specific aircraft installation. In most cases, it is built as a convergent duct and is also referred to as the jet pipe or exhaust duct. Its convergent shape accelerates the gases to the speed required to produce thrust. On subsonic aircraft, these tailpipes are generally fixed in shape and do not change flow area during operation. However, some are available in both standard and nonstandard sizes and can be exchanged to restore performance if necessary.
On some older engines, small inserts or tabs were sometimes added to the tailpipe to reduce the effective open area of the exhaust nozzle and regain a small amount of lost performance. This practice is rarely used today.
Convergent exhaust ducts are capable of accelerating gases to Mach 1, which is the speed of sound. At this point, the flow is considered choked because the velocity at the nozzle opening cannot increase further, even though the engine airflow continues to rise with airspeed.
When the ratio of Pt to Ps is calculated at Mach 1, a minimum pressure ratio of 1.89 is needed to reach the choked condition. Pt refers to the pressure at the entrance of the exhaust duct, which decreases to ambient pressure (Ps) at the nozzle exit.
Where :
γ (gamma) = 1.4 (specific heat)
M = Mach number
γ , 1, 2 = Constants
During an engine ground run, if Pt represents turbine discharge pressure and Ps represents compressor inlet pressure, then an engine pressure ratio (EPR) reading above 1.89 on the cockpit gauge indicates that the exhaust nozzle has reached the choked state.
When gas exits a choked nozzle, it spreads outward radially more quickly than it accelerates in the axial direction, since the axial velocity is fixed at Mach 1. If additional fuel is supplied once Mach 1 velocity is reached, engine speed, compression, and mass airflow increase, which also raises the pressure in the tailpipe.
The increase in exhaust nozzle pressure provides only a small gain in thrust according to the thrust formula for choked nozzles. However, this condition leads to higher fuel consumption and significantly elevated engine temperatures. To achieve supersonic exhaust velocities, as required in supersonic flight, a convergent-divergent nozzle is used instead of a simple convergent nozzle.
To understand why velocity from a convergent duct cannot exceed Mach 1, it is useful to recall Bernoulli’s principle as applied to subsonic flow. When the velocity is below Mach 1, the static pressure (potential energy) is greater than the dynamic pressure (kinetic energy). The narrowing shape of the duct forces velocity to increase by converting potential energy into kinetic energy.
As shown in Figure 3A, when air at subsonic speed moves through a convergent duct, the velocity at point W increases and reaches its maximum at point X. This happens because the potential energy of the air molecules, which was previously acting outward and perpendicular to the flow, is redirected into the direction of flow and converted into kinetic energy. At the jet nozzle, the velocity remains subsonic and the pressure equals ambient.
In Figure 3C, the velocity at point Y is subsonic and continues to increase until it reaches its maximum at point Z. At this point the flow has reached Mach 1 and cannot accelerate further. Any additional energy supplied to the duct does not increase velocity but instead leaves the nozzle as pressure greater than ambient.
Figure 3D shows that when the gas reaches Mach 1, the kinetic energy and potential energy are in equilibrium. In a convergent duct, the airflow naturally reaches a point of balance where the constricting effect of the duct shape is counteracted by the outward push of potential energy.
In practice, the nozzle reaches a choked condition at an inlet-to-outlet pressure ratio of 1.89. It is important to note that the Mach number of the nozzle flow can be lower than the aircraft’s Mach number because Mach number is affected by gas temperature. At higher temperatures, the gas must move faster to reach Mach 1.
The rear portion of the duct is enlarged to allow a constant mass of gas to flow once sonic velocity is reached. This expansion increases the gas velocity as it exits the throat, allowing it to reach supersonic speeds.
Gas traveling at supersonic speeds expands outward more quickly than it accelerates rearward because axial compression of the gas releases energy in the radial direction. [Figure 6A] The convergent-divergent duct uses this principle to generate the thrust needed for supersonic flight.
The forward convergent section builds pressure as the throat area reaches a choked condition, creating backpressure. The aft divergent section allows velocity to increase to the desired Mach number, depending on the force produced by combustion. When properly shaped, a convergent-divergent duct controls gas expansion, captures released energy, and produces the thrust required for limited supersonic flight. [Figure 6B] To achieve higher supersonic speeds, the exhaust duct is augmented with a separate fuel and ignition system called an afterburner. [Figure 7]
Supersonic flow produces the familiar shock wave phenomena. In a well-designed afterburner, these shocks do not distort the airflow inside the duct. Instead, they appear as visible shock rings in the exhaust stream. [Figure 9]
Adding an afterburner to a gas turbine engine is possible because the tailpipe gases contain a significant amount of unburned oxygen. Compressor discharge air used for combustor cooling mixes with the combusted gases at the turbine and then flows downstream to the exhaust system. At the entrance of the exhaust duct, a set of afterburner fuel nozzles, called spray bars, and an ignition system are installed. When afterburner fuel mixes with the unburned oxygen and ignites, the gases are further accelerated by the added heat energy, producing additional thrust.
Along with fuel and ignition systems, a device called a flameholder is used to ensure efficient combustion. A flameholder is a tubular grid or spoke-shaped obstruction positioned downstream of the fuel nozzles. As the gases pass over it, the flameholder creates turbulence, improving fuel-air mixing and promoting complete, stable combustion in the fast-moving airstream.
In essence, the afterburner functions as a ramjet attached to the rear of a gas turbine engine. Only certain types of gas turbines use afterburning, specifically turbojets and turbofans with mixed exhausts, where the fan and core engine gases combine and exit through a single exhaust nozzle.
Older afterburners used a two-position design. In non-afterburning mode, they formed a convergent nozzle, and when afterburning was engaged, the nozzle became convergent-divergent. Modern aircraft feature afterburner nozzles that are convergent-divergent in both modes. In full afterburner mode, both the throat area and the final nozzle size expand to their maximum dimensions and flow angles. Electronic sensors continuously adjust the nozzle area to match the mass airflow, helping to manage the low thermal efficiency and high fuel consumption typical of engines operating in afterburner mode.
Aircraft equipped with afterburners can achieve up to 100 percent additional thrust when afterburning, although fuel flow may increase three to five times. Some modern aircraft with high-performance engines require only a limited thrust increase of 15 to 20 percent. In these cases, the convergent-divergent tailpipe is often called a thrust augmenter rather than an afterburner.
An interesting feature of afterburner thrust is that even a modest increase in takeoff (gross) thrust can produce a much larger increase in net thrust during flight. On the ground, when an aircraft operates in afterburner mode, gross thrust and net thrust are the same. For example, if the afterburner increases gross thrust by 25 percent on the ground, the same afterburner can boost net thrust by as much as 100 percent in flight.
This occurs because ram drag, which reduces engine thrust, does not affect afterburner thrust. Ram drag is calculated as aircraft speed multiplied by mass airflow, while net thrust is gross thrust minus ram drag.
Example of thrust values for an aircraft:
When the nozzle is directed upward, it quickly raises the aircraft’s nose for short-field takeoffs or low-speed maneuvers. When pointed downward, the nozzle lowers the nose in forward flight without changing altitude. This level of control is not possible with conventional fighter aircraft. Only V-STOL aircraft, such as the Lockheed Martin F-35 equipped with lift-fan engines, have this capability.
Exhaust Cone, Tail Cone, and Tailpipe
The exhaust cone, also called the turbine exhaust collector, collects the gases leaving the turbine and directs them into a steady, uniform stream. This task is performed together with the tail cone, sometimes referred to as the exhaust plug, and its radial support struts. The tail cone creates a diffuser-shaped passage inside the exhaust cone, which helps increase pressure and reduce turbulence behind the turbine wheel. The struts straighten the swirling airflow and guide it in an axial direction toward the nozzle. [Figure 1] |
| Figure 1. Exhaust cone, tail cone, and support struts |
In some early gas turbine engines, the tail cone could be moved rearward. This reduced the size of the exhaust opening, increased exhaust velocity, and produced greater thrust, resulting in smoother and faster acceleration of both the engine and aircraft. Later designs replaced this feature with advanced fuel control systems to manage airflow and acceleration. In modern subsonic engines such as those used on commercial airliners and business jets, the tail cone is fixed [Figure 2]. Variable-area exhaust systems that use movable parts are found mainly on supersonic aircraft.
 |
| Figure 2 |
The tailpipe is part of the airframe and connects the engine to a specific aircraft installation. In most cases, it is built as a convergent duct and is also referred to as the jet pipe or exhaust duct. Its convergent shape accelerates the gases to the speed required to produce thrust. On subsonic aircraft, these tailpipes are generally fixed in shape and do not change flow area during operation. However, some are available in both standard and nonstandard sizes and can be exchanged to restore performance if necessary.
On some older engines, small inserts or tabs were sometimes added to the tailpipe to reduce the effective open area of the exhaust nozzle and regain a small amount of lost performance. This practice is rarely used today.
Convergent exhaust ducts are capable of accelerating gases to Mach 1, which is the speed of sound. At this point, the flow is considered choked because the velocity at the nozzle opening cannot increase further, even though the engine airflow continues to rise with airspeed.
Convergent “Choked” Nozzle Theory
When gas enters a convergent duct, the narrowing shape causes it to accelerate. As more air mass moves downstream, the duct further restricts the flow. Once the airflow reaches Mach 1, the constricting effect of the duct walls balances with the axial force of the moving air. At this point the flow stabilizes at Mach 1, a condition known as choking.When the ratio of Pt to Ps is calculated at Mach 1, a minimum pressure ratio of 1.89 is needed to reach the choked condition. Pt refers to the pressure at the entrance of the exhaust duct, which decreases to ambient pressure (Ps) at the nozzle exit.
Where :
γ (gamma) = 1.4 (specific heat)
M = Mach number
γ , 1, 2 = Constants
During an engine ground run, if Pt represents turbine discharge pressure and Ps represents compressor inlet pressure, then an engine pressure ratio (EPR) reading above 1.89 on the cockpit gauge indicates that the exhaust nozzle has reached the choked state.
When gas exits a choked nozzle, it spreads outward radially more quickly than it accelerates in the axial direction, since the axial velocity is fixed at Mach 1. If additional fuel is supplied once Mach 1 velocity is reached, engine speed, compression, and mass airflow increase, which also raises the pressure in the tailpipe.
The increase in exhaust nozzle pressure provides only a small gain in thrust according to the thrust formula for choked nozzles. However, this condition leads to higher fuel consumption and significantly elevated engine temperatures. To achieve supersonic exhaust velocities, as required in supersonic flight, a convergent-divergent nozzle is used instead of a simple convergent nozzle.
To understand why velocity from a convergent duct cannot exceed Mach 1, it is useful to recall Bernoulli’s principle as applied to subsonic flow. When the velocity is below Mach 1, the static pressure (potential energy) is greater than the dynamic pressure (kinetic energy). The narrowing shape of the duct forces velocity to increase by converting potential energy into kinetic energy.
As shown in Figure 3A, when air at subsonic speed moves through a convergent duct, the velocity at point W increases and reaches its maximum at point X. This happens because the potential energy of the air molecules, which was previously acting outward and perpendicular to the flow, is redirected into the direction of flow and converted into kinetic energy. At the jet nozzle, the velocity remains subsonic and the pressure equals ambient.
 |
| Figure 3. Theory of choked nozzles |
In Figure 3B, the gas enters the duct with high potential energy (Pe) and low kinetic energy (Ke). As it moves toward the nozzle, potential energy decreases while kinetic energy increases. In this example, the flow remains subsonic with a velocity of about Mach 0.7.
In Figure 3C, the velocity at point Y is subsonic and continues to increase until it reaches its maximum at point Z. At this point the flow has reached Mach 1 and cannot accelerate further. Any additional energy supplied to the duct does not increase velocity but instead leaves the nozzle as pressure greater than ambient.
Figure 3D shows that when the gas reaches Mach 1, the kinetic energy and potential energy are in equilibrium. In a convergent duct, the airflow naturally reaches a point of balance where the constricting effect of the duct shape is counteracted by the outward push of potential energy.
In practice, the nozzle reaches a choked condition at an inlet-to-outlet pressure ratio of 1.89. It is important to note that the Mach number of the nozzle flow can be lower than the aircraft’s Mach number because Mach number is affected by gas temperature. At higher temperatures, the gas must move faster to reach Mach 1.
Convergent-Divergent Exhaust Ducts for Subsonic Aircraft
Some subsonic aircraft now use convergent-divergent nozzles. These nozzles have a fixed geometry and feature a slightly larger final opening. They are designed to maximize cruise thrust while reducing engine noise. [Figure 4] |
| Figure 4 |
Divergent Exhaust Nozzles
Rotorcraft exhaust ducts are typically divergent in shape rather than convergent. This design reduces the small amount of thrust that would otherwise be produced, which helps improve hover performance. [Figure 5A] Another helicopter exhaust design features a fluted or scalloped outer perimeter to enhance noise suppression. [Figure 5B] |
| Figure 5 |
Convergent-Divergent Nozzle Theory for Supersonic Aircraft
Supersonic aircraft use variable geometry convergent-divergent exhaust ducts. These ducts are most effective at high Mach numbers due to the high pressure ratio across the tailpipe. High supersonic inlet ram pressure produces high exhaust duct pressure, which maximizes thrust.The rear portion of the duct is enlarged to allow a constant mass of gas to flow once sonic velocity is reached. This expansion increases the gas velocity as it exits the throat, allowing it to reach supersonic speeds.
Gas traveling at supersonic speeds expands outward more quickly than it accelerates rearward because axial compression of the gas releases energy in the radial direction. [Figure 6A] The convergent-divergent duct uses this principle to generate the thrust needed for supersonic flight.
 |
| Figure 6 |
The forward convergent section builds pressure as the throat area reaches a choked condition, creating backpressure. The aft divergent section allows velocity to increase to the desired Mach number, depending on the force produced by combustion. When properly shaped, a convergent-divergent duct controls gas expansion, captures released energy, and produces the thrust required for limited supersonic flight. [Figure 6B] To achieve higher supersonic speeds, the exhaust duct is augmented with a separate fuel and ignition system called an afterburner. [Figure 7]
 |
| Figure 7. Afterburner assembly |
In a convergent-divergent exhaust system, several changes occur when the pressure ratio exceeds 1.89:1.
In the converging section:
In the converging section:
- The airflow lines converge.
- Air density increases at the throat.
- Air velocity stabilizes at Mach 1.
- Pressure at the throat is higher than ambient.
In the diverging section:
The exhaust duct of a modern supersonic aircraft is shaped so that the rear section has a slight divergence during normal, non-afterburner operation. In this configuration, many supersonic aircraft can achieve supersonic speeds. When afterburner mode is engaged, the exhaust nozzle mechanism increases the divergence of the rear section to accommodate the higher mass airflow produced by the afterburner. Essentially, the divergent section acts as a variable-area exhaust nozzle. In non-afterburner mode, the nozzle area is at its smallest, and when afterburner mode is selected, the nozzle opens to a larger area.
When afterburner fuel is added, the gases produced during combustion expand and exert outward pressure on the walls of the rear duct. The diverging shape of the duct directs these forces forward. Research shows that fully expanded supersonic flow provides the greatest thrust augmentation. In this case, the pressure along the exhaust duct returns to ambient, so no additional thrust comes from pressure at the nozzle. However, thrust still increases because the exhaust velocity rises above Mach 1 in proportion to the energy from the fuel. [Figure 8]
- Air density decreases.
- Velocity increases to supersonic speeds.
- Pressure decreases, reaching ambient at the nozzle exit.
The exhaust duct of a modern supersonic aircraft is shaped so that the rear section has a slight divergence during normal, non-afterburner operation. In this configuration, many supersonic aircraft can achieve supersonic speeds. When afterburner mode is engaged, the exhaust nozzle mechanism increases the divergence of the rear section to accommodate the higher mass airflow produced by the afterburner. Essentially, the divergent section acts as a variable-area exhaust nozzle. In non-afterburner mode, the nozzle area is at its smallest, and when afterburner mode is selected, the nozzle opens to a larger area.
When afterburner fuel is added, the gases produced during combustion expand and exert outward pressure on the walls of the rear duct. The diverging shape of the duct directs these forces forward. Research shows that fully expanded supersonic flow provides the greatest thrust augmentation. In this case, the pressure along the exhaust duct returns to ambient, so no additional thrust comes from pressure at the nozzle. However, thrust still increases because the exhaust velocity rises above Mach 1 in proportion to the energy from the fuel. [Figure 8]
 |
| Figure 8. Pressure/velocity relationship in a convergent- divergent tailpipe |
Supersonic flow produces the familiar shock wave phenomena. In a well-designed afterburner, these shocks do not distort the airflow inside the duct. Instead, they appear as visible shock rings in the exhaust stream. [Figure 9]
 |
| Figure 9. Afterburner operation showing shock rings in exhaust |
Afterburning
Afterburning is primarily used for takeoff with heavy aircraft loads, rapid climb-out, and achieving higher supersonic speeds. It provides the maximum exhaust velocity and engine thrust for a given engine frontal area but requires additional fuel.Adding an afterburner to a gas turbine engine is possible because the tailpipe gases contain a significant amount of unburned oxygen. Compressor discharge air used for combustor cooling mixes with the combusted gases at the turbine and then flows downstream to the exhaust system. At the entrance of the exhaust duct, a set of afterburner fuel nozzles, called spray bars, and an ignition system are installed. When afterburner fuel mixes with the unburned oxygen and ignites, the gases are further accelerated by the added heat energy, producing additional thrust.
Along with fuel and ignition systems, a device called a flameholder is used to ensure efficient combustion. A flameholder is a tubular grid or spoke-shaped obstruction positioned downstream of the fuel nozzles. As the gases pass over it, the flameholder creates turbulence, improving fuel-air mixing and promoting complete, stable combustion in the fast-moving airstream.
In essence, the afterburner functions as a ramjet attached to the rear of a gas turbine engine. Only certain types of gas turbines use afterburning, specifically turbojets and turbofans with mixed exhausts, where the fan and core engine gases combine and exit through a single exhaust nozzle.
Older afterburners used a two-position design. In non-afterburning mode, they formed a convergent nozzle, and when afterburning was engaged, the nozzle became convergent-divergent. Modern aircraft feature afterburner nozzles that are convergent-divergent in both modes. In full afterburner mode, both the throat area and the final nozzle size expand to their maximum dimensions and flow angles. Electronic sensors continuously adjust the nozzle area to match the mass airflow, helping to manage the low thermal efficiency and high fuel consumption typical of engines operating in afterburner mode.
Aircraft equipped with afterburners can achieve up to 100 percent additional thrust when afterburning, although fuel flow may increase three to five times. Some modern aircraft with high-performance engines require only a limited thrust increase of 15 to 20 percent. In these cases, the convergent-divergent tailpipe is often called a thrust augmenter rather than an afterburner.
An interesting feature of afterburner thrust is that even a modest increase in takeoff (gross) thrust can produce a much larger increase in net thrust during flight. On the ground, when an aircraft operates in afterburner mode, gross thrust and net thrust are the same. For example, if the afterburner increases gross thrust by 25 percent on the ground, the same afterburner can boost net thrust by as much as 100 percent in flight.
This occurs because ram drag, which reduces engine thrust, does not affect afterburner thrust. Ram drag is calculated as aircraft speed multiplied by mass airflow, while net thrust is gross thrust minus ram drag.
Example of thrust values for an aircraft:
- Gross (static) thrust without afterburner: 16,000 pounds
- Gross (static) thrust with afterburner: 20,000 pounds (25 percent increase)
- Net (in flight) thrust without afterburner: 4,000 pounds
- Net (in flight) thrust with afterburner: 8,000 pounds (100 percent increase)
Vectoring Afterburner Exhaust Nozzles
Some military aircraft are equipped with vectoring exhaust nozzles. These nozzles allow the pilot to direct the exhaust gases upward or downward, improving takeoff performance and low-speed maneuverability. They can also reverse the exhaust flow throughout the flight envelope, including during approach, assisting in braking during landing. [Figure 10A] |
| Figure 10 |
When the nozzle is directed upward, it quickly raises the aircraft’s nose for short-field takeoffs or low-speed maneuvers. When pointed downward, the nozzle lowers the nose in forward flight without changing altitude. This level of control is not possible with conventional fighter aircraft. Only V-STOL aircraft, such as the Lockheed Martin F-35 equipped with lift-fan engines, have this capability.
Variable Area Exhaust Nozzles for Subsonic Aircraft Engines
Some subsonic aircraft use variable area exhaust nozzles, which often include a thrust reverser system. [Figure 10B] These nozzles operate in a way similar to vectoring exhaust nozzles found on supersonic aircraft afterburners. The nozzle can adjust the flow area of the exhaust throat by up to 15 percent, optimizing the performance of a small turbofan engine during takeoff, climb, and cruise.