A propelling nozzle is a
nozzle
A nozzle is a device designed to control the direction or characteristics of a fluid flow (specially to increase velocity) as it exits (or enters) an enclosed chamber or pipe.
A nozzle is often a pipe or tube of varying cross sectional area, a ...
that converts the internal energy of a working gas into propulsive force; it is the nozzle, which forms a jet, that separates a
gas turbine
A gas turbine, also called a combustion turbine, is a type of continuous flow internal combustion engine. The main parts common to all gas turbine engines form the power-producing part (known as the gas generator or core) and are, in the directi ...
, or
gas generator
A gas generator is a device for generating gas. A gas generator may create gas by a chemical reaction or from a solid or liquid source, when storing a pressurized gas is undesirable or impractical.
The term often refers to a device that uses a ...
, from a
jet engine
A jet engine is a type of reaction engine discharging a fast-moving jet of heated gas (usually air) that generates thrust by jet propulsion. While this broad definition can include rocket, Pump-jet, water jet, and hybrid propulsion, the term ...
.
Propelling nozzles accelerate the available gas to
subsonic,
transonic
Transonic (or transsonic) flow is air flowing around an object at a speed that generates regions of both subsonic and supersonic airflow around that object. The exact range of speeds depends on the object's critical Mach number, but transonic ...
, or supersonic velocities depending on the power setting of the engine, their internal shape and the pressures at entry to, and exit from, the nozzle. The internal shape may be convergent or
convergent-divergent (C-D). C-D nozzles can accelerate the jet to supersonic velocities within the divergent section, whereas a convergent nozzle cannot accelerate the jet beyond sonic speed.
Propelling nozzles may have a fixed geometry, or they may have variable geometry to give different exit areas to control the operation of the engine when equipped with an
afterburner
An afterburner (or reheat in British English) is an additional combustion component used on some jet engines, mostly those on military supersonic aircraft. Its purpose is to increase thrust, usually for supersonic flight, takeoff, and comba ...
or a reheat system. When afterburning engines are equipped with a C-D nozzle the throat area is variable. Nozzles for supersonic flight speeds, at which high nozzle pressure ratios are generated, also have variable area divergent sections.
Turbofan
The turbofan or fanjet is a type of airbreathing jet engine that is widely used in aircraft engine, aircraft propulsion. The word "turbofan" is a portmanteau of "turbine" and "fan": the ''turbo'' portion refers to a gas turbine engine which ac ...
engines may have an additional and separate propelling nozzle which further accelerates the bypass air.
Propelling nozzles also act as downstream restrictors, the consequences of which constitute an important aspect of engine design.
["Jet Propulsion" Nicholas Cumpsty, , p144]
Principles of operation
* A nozzle operates according to the
Venturi effect
The Venturi effect is the reduction in fluid pressure that results when a fluid flows through a constricted section (or choke) of a pipe. The Venturi effect is named after its discoverer, the 18th century Italian physicist, Giovanni Battista V ...
to bring the exhaust gasses to ambient pressure, while forming them into a propulsive jet; if the pressure upstream of the nozzle is high enough, the flow will reach reach sonic speed (
choke). The role of the nozzle in back-pressuring the engine is explained
below
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* Bottom (BDSM), the partner in a BDSM who takes the passive, receiving, or obedient role, to that of the top or ...
.
* The energy to accelerate the stream comes from the temperature and pressure of the gas. The gas
expands adiabatically with low losses and hence
high efficiency. The gas accelerates to a final exit velocity which depends on the pressure and temperature at entry to the nozzle, the ambient pressure it exhausts to (unless the flow is
choked), and the efficiency of the expansion. The efficiency is a measure of the losses due to friction, non-axial divergence as well as leakage in C-D nozzles.
* Airbreathing engines create forward thrust on the airframe by imparting a net rearward momentum to the exhaust gas. If thrust exceeds the resistance incurred by the aircraft moving through the air, it will accelerate. The jet may or may not be
fully expanded.
* On some engines that are equipped with an afterburner the nozzle area is varied during non-afterburning or dry thrust conditions. Typically the nozzle is fully open for starting and at idle. It may then close down as the thrust lever is advanced reaching its minimum area before or at the Military or maximum dry thrust setting. Two examples of this control are the General Electric
J-79
The General Electric J79 is an axial-flow compressor, axial-flow turbojet engine built for use in a variety of fighter aircraft, fighter and bomber aircraft and a supersonic cruise missile. The J79 was produced by GE Aviation, General Electric ...
and the
Tumansky
Sergei Konstantinovich Tumansky (russian: Серге́й Константинович Туманский; – 9 September 1973) was a designer of Soviet aircraft engines and the chief designer in the Tumansky Design Bureau, OKB-300. He worked i ...
RD-33
The Klimov RD-33 is a turbofan jet engine for a lightweight fighter jet that is the primary engine for the Mikoyan MiG-29 and CAC/PAC JF-17 Thunder. It was developed in OKB-117 led by S. P. Izotov (now OAO Klimov) from 1968 with production star ...
in the
MIG-29
The Mikoyan MiG-29 (russian: Микоян МиГ-29; NATO reporting name: Fulcrum) is a twin-engine fighter aircraft designed in the Soviet Union. Developed by the Mikoyan design bureau as an air superiority fighter during the 1970s, the Mi ...
. Reasons for varying the nozzle area are explained in section:
Nozzle area control during dry operation.
Principal geometries
Convergent nozzle
Convergent nozzles are used on many jet engines. If the nozzle pressure ratio is above the critical value (about 1.8:1) a convergent nozzle will
choke, resulting in some of the expansion to atmospheric pressure taking place downstream of the throat (i.e., smallest flow area), in the jet wake. Although jet momentum still produces much of the gross thrust, the imbalance between the throat static pressure and atmospheric pressure still generates some (pressure) thrust.
Divergent nozzle
The supersonic speed of the air flowing into a scramjet allows the use of a simple diverging nozzle
Convergent-divergent (C-D) nozzle
Engines capable of supersonic flight have
convergent-divergent exhaust duct features to generate supersonic flow. Rocket engines — the extreme case — owe their distinctive shape to the very high area ratios of their nozzles.
When the pressure ratio across a convergent nozzle exceeds a critical value, the flow
chokes
Choking, also known as foreign body airway obstruction (FBAO), is a phenomenon that occurs when breathing is impeded by a blockage inside of the respiratory tract. An obstruction that prevents oxygen from entering the lungs results in oxygen de ...
, and thus the pressure of the exhaust exiting the engine exceeds the pressure of the surrounding air and cannot decrease via the conventional
Venturi effect
The Venturi effect is the reduction in fluid pressure that results when a fluid flows through a constricted section (or choke) of a pipe. The Venturi effect is named after its discoverer, the 18th century Italian physicist, Giovanni Battista V ...
. This reduces the thrust producing efficiency of the nozzle by causing much of the expansion to take place downstream of the nozzle itself. Consequently, rocket engines and jet engines for supersonic flight incorporate a C-D nozzle which permits further expansion against the inside of the nozzle. However, unlike the
''fixed'' convergent-divergent nozzle used on a conventional rocket motor, those on turbojet engines must have heavy and expensive variable geometry to cope with the great variation in nozzle pressure ratio that occurs with speeds from subsonic to over Mach3.
Nonetheless,
low area ratio nozzles have subsonic applications.
Types of nozzle
Fixed-area nozzle
Non-
afterburning
An afterburner (or reheat in British English) is an additional combustion component used on some jet engines, mostly those on military supersonic aircraft. Its purpose is to increase thrust, usually for supersonic flight, takeoff, and combat ...
subsonic engines have
nozzle
A nozzle is a device designed to control the direction or characteristics of a fluid flow (specially to increase velocity) as it exits (or enters) an enclosed chamber or pipe.
A nozzle is often a pipe or tube of varying cross sectional area, a ...
s of a fixed size because the changes in engine performance with altitude and subsonic flight speeds are acceptable with a fixed nozzle.
This is not the case at supersonic speeds as described for
Concorde
The Aérospatiale/BAC Concorde () is a retired Franco-British supersonic airliner jointly developed and manufactured by Sud Aviation (later Aérospatiale) and the British Aircraft Corporation (BAC).
Studies started in 1954, and France an ...
below
Below may refer to:
*Earth
*Ground (disambiguation)
*Soil
*Floor
*Bottom (disambiguation)
Bottom may refer to:
Anatomy and sex
* Bottom (BDSM), the partner in a BDSM who takes the passive, receiving, or obedient role, to that of the top or ...
.
With low area ratio
At the other extreme, some high
bypass ratio
The bypass ratio (BPR) of a turbofan engine is the ratio between the mass flow rate of the bypass stream to the mass flow rate entering the core. A 10:1 bypass ratio, for example, means that 10 kg of air passes through the bypass duct for ev ...
civil
turbofan
The turbofan or fanjet is a type of airbreathing jet engine that is widely used in aircraft engine, aircraft propulsion. The word "turbofan" is a portmanteau of "turbine" and "fan": the ''turbo'' portion refers to a gas turbine engine which ac ...
s control the fan working line by using a convergent-divergent nozzle with an extremely low (less than 1.01) area ratio on the bypass (or mixed exhaust) stream. At low airspeeds, such a setup causes the nozzle to act as if it had variable geometry by preventing it from choking and allowing it to accelerate and decelerate exhaust gas approaching the throat and divergent section, respectively. Consequently, the nozzle exit area controls the fan match, which, being larger than the throat, pulls the fan working line slightly away from surge. At higher flight speeds, the ram rise in the intake chokes the throat and causes the nozzle's area to dictate the fan match; the nozzle, being smaller than the exit, causes the throat to push the fan working line slightly toward surge. This is not a problem, however, for a fan's surge margin is much greater at high flight speeds.
In rockets (with high area ratio)
Rocket motor
A rocket engine uses stored rocket propellants as the reaction mass for forming a high-speed propulsive jet of fluid, usually high-temperature gas. Rocket engines are reaction engines, producing thrust by ejecting mass rearward, in accordance ...
s also employ convergent-divergent nozzles, but these are usually of fixed geometry, to minimize weight. Because of the high pressure ratios associated with rocket flight, rocket motor convergent-divergent nozzles have a much greater area ratio (exit/throat) than those fitted to jet engines.
Variable-area for afterburning
The afterburners on combat aircraft require a bigger nozzle to prevent adversely affecting the operation of the engine. The variable area iris nozzle consists of a series of moving, overlapping petals with a nearly circular nozzle cross-section and is convergent to control the operation of the engine. If the aircraft is to fly at supersonic speeds, the afterburner nozzle may be followed by a separate divergent nozzle in an ejector nozzle configuration, as below, or the divergent geometry may be incorporated with the afterburner nozzle in the variable geometry convergent-divergent nozzle configuration, as below.
Early afterburners were either on or off and used a 2-position clamshell, or eyelid, nozzle which gave only one area available for afterburning use.
Ejector
Ejector refers to the pumping action of the very hot, high speed, engine exhaust entraining (ejecting) a surrounding airflow which, together with the internal geometry of the secondary, or diverging, nozzle controls the expansion of the engine exhaust. At subsonic speeds, the airflow constricts the exhaust to a convergent shape. When afterburning is selected and the aircraft speeds up, the two nozzles dilate, which allows the exhaust to form a convergent-divergent shape, speeding the exhaust gasses past Mach1. More complex engine installations use a tertiary airflow to reduce exit area at low speeds. Advantages of the ejector nozzle are relative simplicity and reliability in cases where the secondary nozzle flaps are positioned by pressure forces. The ejector nozzle is also able to use air which has been ingested by the intake but which is not required by the engine. The amount of this air varies significantly across the flight envelope and ejector nozzles are well suited to matching the airflow between the intake system and engine. Efficient use of this air in the nozzle was a prime requirement for aircraft that had to cruise efficiently at high supersonic speeds for prolonged periods, hence its use in the
SR-71
The Lockheed SR-71 "Blackbird" is a Range (aeronautics), long-range, high-altitude, Mach number, Mach 3+ military strategy, strategic reconnaissance aircraft developed and manufactured by the American aerospace company Lockheed Corporati ...
,
Concorde
The Aérospatiale/BAC Concorde () is a retired Franco-British supersonic airliner jointly developed and manufactured by Sud Aviation (later Aérospatiale) and the British Aircraft Corporation (BAC).
Studies started in 1954, and France an ...
and
XB-70 Valkyrie
The North American Aviation XB-70 Valkyrie was the prototype version of the planned B-70 nuclear-armed, deep-penetration supersonic strategic bomber for the United States Air Force Strategic Air Command. Designed in the late 1950s by North Ame ...
.
A simple example of ejector nozzle is the fixed geometry cylindrical shroud surrounding the afterburning nozzle on the J85 installation in the
T-38 Talon
The Northrop T-38 Talon is a two-seat, twinjet supersonic jet trainer. It was the world's first, and the most produced, supersonic trainer. The T-38 remains in service in several air forces.
The United States Air Force (USAF) operates the most ...
.
["J85 Rejuvenation Through Technology Insertion" Brisken, Howell, Ewing, G.E.Aircraft Engines, Cincinnati, Ohio, OH45215, USA] More complex were the arrangements used for the
J58 (
SR-71
The Lockheed SR-71 "Blackbird" is a Range (aeronautics), long-range, high-altitude, Mach number, Mach 3+ military strategy, strategic reconnaissance aircraft developed and manufactured by the American aerospace company Lockheed Corporati ...
) and
TF-30
The Pratt & Whitney TF30 (company designation JTF10A) is a military low-bypass turbofan engine originally designed by Pratt & Whitney for the subsonic F6D Missileer fleet defense fighter, but this project was cancelled. It was later adapted with ...
(
F-111
The General Dynamics F-111 Aardvark is a retired supersonic, medium-range, multirole combat aircraft. Production variants of the F-111 had roles that included ground attack (e.g. interdiction), strategic bombing (including nuclear weapons ca ...
) installations. They both used tertiary blow-in doors (open at lower speeds) and free-floating overlapping flaps for a final nozzle. Both the blow-in doors and the final nozzle flaps are positioned by a balance of internal pressure from the engine exhaust and external pressure from the aircraft flowfield.
On early
J79
The General Electric J79 is an axial-flow turbojet engine built for use in a variety of fighter and bomber aircraft and a supersonic cruise missile. The J79 was produced by General Electric Aircraft Engines in the United States, and under lice ...
installations (
F-104
The Lockheed F-104 Starfighter is an American single-engine, supersonic air superiority fighter which was extensively deployed as a fighter-bomber during the Cold War. Created as a day fighter by Lockheed as one of the "Century Series" of fi ...
,
F-4
The McDonnell Douglas F-4 Phantom II is an American tandem two-seat, twin-engine, all-weather, long-range supersonic jet interceptor and fighter-bomber originally developed by McDonnell Aircraft for the United States Navy.Swanborough and Bo ...
,
A-5 Vigilante
The North American A-5 Vigilante was an American carrier-based supersonic bomber designed and built by North American Aviation (NAA) for the United States Navy. Prior to 1962 unification of Navy and Air Force designations, it was designated t ...
), actuation of the secondary nozzle was mechanically linked to the afterburner nozzle. Later installations had the final nozzle mechanically actuated separately from the afterburner nozzle. This gave improved efficiency (better match of primary/secondary exit area with high Mach number requirement) at Mach2 (
B-58 Hustler
The Convair B-58 Hustler, designed and produced by American aircraft manufacturer Convair, was the first operational bomber capable of Mach 2 flight.
The B-58 was developed during the 1950s for the United States Air Force (USAF) Strategic Air ...
) and Mach3 (XB-70).
Variable-geometry convergent-divergent
Turbofan installations which do not require a secondary airflow to be pumped by the engine exhaust use the variable geometry C-D nozzle. These engines don't require the external cooling air needed by turbojets (hot afterburner casing).
The divergent nozzle may be an integral part of the afterburner nozzle petal, an angled extension after the throat. The petals travel along curved tracks and the axial translation and simultaneous rotation increases the throat area for afterburning, while the trailing portion becomes a divergence with bigger exit area for more complete expansion at higher speeds. An example is the
TF-30
The Pratt & Whitney TF30 (company designation JTF10A) is a military low-bypass turbofan engine originally designed by Pratt & Whitney for the subsonic F6D Missileer fleet defense fighter, but this project was cancelled. It was later adapted with ...
(
F-14
The Grumman F-14 Tomcat is an American carrier-capable supersonic, twin-engine, two-seat, twin-tail, variable-sweep wing fighter aircraft. The Tomcat was developed for the United States Navy's Naval Fighter Experimental (VFX) program after the ...
).
The primary and secondary petals may be hinged together and actuated by the same mechanism to provide afterburner control and high nozzle pressure ratio expansion as on the
EJ200
The Eurojet EJ200 is a military low-bypass turbofan used as the powerplant of the Eurofighter Typhoon. The engine is largely based on the Rolls-Royce XG-40 technology demonstrator, which was developed in the 1980s. The EJ200 is built by the E ...
(
Eurofighter
The Eurofighter Typhoon is a European multinational twin-engine, canard delta wing, multirole fighter. The Typhoon was designed originally as an air-superiority fighter and is manufactured by a consortium of Airbus, BAE Systems and Leo ...
). Other examples are found on the
F-15
The McDonnell Douglas F-15 Eagle is an American twin-engine, all-weather tactical fighter aircraft designed by McDonnell Douglas (now part of Boeing). Following reviews of proposals, the United States Air Force selected McDonnell Douglas's ...
,
F-16
The General Dynamics F-16 Fighting Falcon is a single-engine Multirole combat aircraft, multirole fighter aircraft originally developed by General Dynamics for the United States Air Force (USAF). Designed as an air superiority day fighter, it ...
,
B-1B
The Rockwell B-1 Lancer is a supersonic variable-sweep wing, heavy bomber used by the United States Air Force. It is commonly called the "Bone" (from "B-One"). It is one of three strategic bombers serving in the U.S. Air Force fleet along with ...
.
Additional features
Thrust-vectoring
Nozzles for vectored thrust include fixed geometry
Bristol Siddeley Pegasus
The Rolls-Royce Pegasus, formerly the Bristol Siddeley Pegasus, is a British turbofan engine originally designed by Bristol Siddeley. It was manufactured by Rolls-Royce plc. The engine is not only able to power a jet aircraft forward, but also ...
and variable geometry
F119 (
F-22
The Lockheed Martin F-22 Raptor is an American single-seat, twin-engine, all-weather stealth tactical fighter aircraft developed for the United States Air Force (USAF). As the result of the USAF's Advanced Tactical Fighter (ATF) program, th ...
).
Thrust-reversing
The thrust reversers on some engines are incorporated into the nozzle itself and are known as target thrust reversers. The nozzle opens up in two halves which come together to redirect the exhaust partially forward. Since the nozzle area has an influence on the operation of the engine (see
below
Below may refer to:
*Earth
*Ground (disambiguation)
*Soil
*Floor
*Bottom (disambiguation)
Bottom may refer to:
Anatomy and sex
* Bottom (BDSM), the partner in a BDSM who takes the passive, receiving, or obedient role, to that of the top or ...
), the deployed thrust reverser has to be spaced the correct distance from the jetpipe to prevent changes in engine operating limits. Examples of target thrust reversers are found on the Fokker 100, Gulfstream IV and Dassault F7X.
Noise-reducing
Jet noise may be reduced by adding features to the exit of the nozzle which increase the surface area of the cylindrical jet. Commercial turbojets and early by-pass engines typically split the jet into multiple lobes. Modern high by-pass turbofans have triangular serrations, called chevrons, which protrude slightly into the propelling jet.
Further topics
The other purpose of the propelling nozzle
The nozzle, by virtue of setting the back-pressure, acts as a downstream restrictor to the compressor, and thus determines what goes into the front of the engine. It shares this function with the other downstream restrictor, the turbine nozzle. The areas of both the propelling nozzle and turbine nozzle set the mass flow through the engine and the maximum pressure. While both these areas are fixed in many engines (i.e. those with a simple fixed propelling nozzle), others, most notably those with afterburning, have a variable area propelling nozzle. This area variation is necessary to contain the disturbing effect on the engine of the high combustion temperatures in the jet pipe, though the area may also be varied during non-afterburning operation to alter the pumping performance of the compressor at lower thrust settings.
For example, if the propelling nozzle were to be removed to convert a
turbojet
The turbojet is an airbreathing jet engine which is typically used in aircraft. It consists of a gas turbine with a propelling nozzle. The gas turbine has an air inlet which includes inlet guide vanes, a compressor, a combustion chamber, and ...
into a
turboshaft
A turboshaft engine is a form of gas turbine that is optimized to produce shaftpower rather than jet thrust. In concept, turboshaft engines are very similar to turbojets, with additional turbine expansion to extract heat energy from the exhaust ...
, the role played by the nozzle area is now taken by the area of the power turbine nozzle guide vanes or stators.
Reasons for C-D nozzle over-expansion and examples
Overexpansion occurs when the exit area is too big relative to the size of the afterburner, or primary, nozzle. This occurred under certain conditions on the J85 installation in the T-38. The secondary or final nozzle was a fixed geometry sized for the maximum afterburner case. At non-afterburner thrust settings the exit area was too big for the closed engine nozzle giving over-expansion. Free-floating doors were added to the ejector allowing secondary air to control the primary jet expansion.
Reasons for C-D nozzle under-expansion and examples
For complete expansion to ambient pressure, and hence maximum nozzle thrust or efficiency, the required area ratio increases with flight Mach number. If the divergence is too short giving too small an exit area the exhaust will not expand to ambient pressure in the nozzle and there will be lost thrust potential With increasing Mach number there may come a point where the nozzle exit area is as big as the engine nacelle diameter or aircraft afterbody diameter. Beyond this point the nozzle diameter becomes the biggest diameter and starts to incur increasing drag. Nozzles are thus limited to the installation size and the loss in thrust incurred is a trade off with other considerations such as lower drag, less weight.
Examples are the
F-16
The General Dynamics F-16 Fighting Falcon is a single-engine Multirole combat aircraft, multirole fighter aircraft originally developed by General Dynamics for the United States Air Force (USAF). Designed as an air superiority day fighter, it ...
at Mach2.0 and the
XB-70
The North American Aviation XB-70 Valkyrie was the prototype version of the planned B-70 nuclear-armed, deep-penetration supersonic strategic bomber for the United States Air Force Strategic Air Command. Designed in the late 1950s by North Ame ...
at Mach3.0.
Another consideration may relate to the required nozzle cooling flow. The divergent flaps or petals have to be isolated from the afterburner flame temperature, which may be of the order of , by a layer of cooling air. A longer divergence means more area to be cooled. The thrust loss from incomplete expansion is traded against the benefits of less cooling flow. This applied to the TF-30 nozzle in the F-14A where the ideal area ratio at Mach2.4 was limited to a lower value.
What is adding a divergent section worth in real terms?
A divergent section gives added exhaust velocity and hence thrust at supersonic flight speeds.
The effect of adding a divergent section was demonstrated with Pratt & Whitney's first C-D nozzle. The convergent nozzle was replaced with a C-D nozzle on the same engine
J57 in the same aircraft
F-101
The McDonnell F-101 Voodoo is a supersonic jet fighter which served the United States Air Force (USAF) and the Royal Canadian Air Force (RCAF).
Initially designed by McDonnell Aircraft Corporation as a long-range bomber escort (known as a '' ...
.
The increased thrust from the C-D nozzle ( at sea-level take-off) on this engine raised the speed from Mach1.6 to almost 2.0 enabling the Air Force to set a world's speed record of which was just below Mach2 for the temperature on that day. The true worth of the C-D nozzle was not realised on the F-101 as the intake was not modified for the higher speeds attainable.
["Test Pilot" edited by Harry Schmidt, "Mach 2 Books" Shelton CT 06484]
Another example was the replacement of a convergent with a C-D nozzle on the YF-106/P&W
J75 when it would not quite reach Mach2. Together with the introduction of the C-D nozzle, the inlet was redesigned. The USAF subsequently set a world's speed record with the
F-106
The Convair F-106 Delta Dart was the primary all-weather interceptor aircraft of the United States Air Force from the 1960s through to the 1980s. Designed as the so-called "Ultimate Interceptor", it proved to be the last specialist interceptor i ...
of 1526 mph (Mach2.43).
Nozzle area control during dry operation
![Jumo 004 im Technikmuseum Hugo Junkers Dessau 2010-08-06 Detail 03](https://upload.wikimedia.org/wikipedia/commons/6/65/Jumo_004_im_Technikmuseum_Hugo_Junkers_Dessau_2010-08-06_Detail_03.jpg)
Some very early jet engines that were not equipped with an afterburner, such as the
BMW 003
The BMW 003 (full RLM designation 109-003) is an early axial turbojet engine produced by BMW AG in Germany during World War II. The 003 and the Junkers Jumo 004 were the only German turbojet engines to reach production during World War II.
...
and the
Jumo 004
The Junkers Jumo 004 was the world's first production turbojet engine in operational use, and the first successful axial compressor turbojet engine. Some 8,000 units were manufactured by Junkers in Germany late in World War II, powering the Mess ...
(which had a design ), had a variable area nozzle formed by a translating plug known as a ''Zwiebel''
ild onionfrom its shape. The Jumo 004 had a large area for starting to prevent overheating the turbine and a smaller area for take-off and flight to give higher exhaust velocity and thrust. The 004's ''Zwiebel'' possessed a range of forward/reverse travel to vary the exhaust nozzle area, driven by an electric motor-driven mechanism within the body's divergent area just behind the turbine.
Afterburner-equipped engines may also open the nozzle for starting and at idle. The idle thrust is reduced which lowers taxi speeds and brake wear. This feature on the
J75 engine in the
F-106
The Convair F-106 Delta Dart was the primary all-weather interceptor aircraft of the United States Air Force from the 1960s through to the 1980s. Designed as the so-called "Ultimate Interceptor", it proved to be the last specialist interceptor i ...
was called 'Idle Thrust Control' and reduced idle thrust by 40%. On aircraft carriers, lower idle thrust reduces the hazards from jet blast.
In some applications, such as the
J79
The General Electric J79 is an axial-flow turbojet engine built for use in a variety of fighter and bomber aircraft and a supersonic cruise missile. The J79 was produced by General Electric Aircraft Engines in the United States, and under lice ...
installation in various aircraft, during fast throttle advances, the nozzle area may be prevented from closing beyond a certain point to allow a more rapid increase in RPM and hence faster time to maximum thrust.
In the case of a 2-spool turbojet, such as the
Olympus 593 in
Concorde
The Aérospatiale/BAC Concorde () is a retired Franco-British supersonic airliner jointly developed and manufactured by Sud Aviation (later Aérospatiale) and the British Aircraft Corporation (BAC).
Studies started in 1954, and France an ...
, the nozzle area may be varied to enable simultaneous achievement of maximum low-pressure compressor speed and maximum turbine entry temperature over the wide range of engine entry temperatures which occurs with flight speeds up to Mach2.
On some augmented turbofans the fan operating line is controlled with nozzle area during both dry and wet operation to trade excess surge margin for more thrust.
Nozzle area control during wet operation
The nozzle area is increased during afterburner operation to limit the upstream effects on the engine. To run a turbofan to give maximum airflow (thrust), the nozzle area may be controlled to keep the fan operating line in its optimum position. For a turbojet to give maximum thrust, the area may be controlled to keep the turbine exhaust temperature at its limit.
What happens if the nozzle doesn't open when the afterburner is selected?
In early afterburner installations, the pilot had to check the nozzle position indicator after selecting afterburner. If the nozzle did not open for some reason, and the pilot did not react by cancelling the afterburner selection, typical controls of that period (e.g. the
J47
The General Electric J47 turbojet (GE company designation TG-190) was developed by General Electric from its earlier J35. It first flew in May 1948. The J47 was the first axial-flow turbojet approved for commercial use in the United States. It ...
in the F-86L), could cause the turbine blades to overheat and fail.
Other applications
![Avgasrekylator saxofon](https://upload.wikimedia.org/wikipedia/commons/3/38/Avgasrekylator_saxofon.png)
Certain aircraft, like the German Bf-109 and the Macchi C.202/205 were fitted with "ejector-type exhausts". These exhausts converted some of the waste energy of the (internal combustion) engines exhaust-flow into a small amount of forward thrust by accelerating the hot gasses in a rearward direction to a speed greater than that of the aircraft. All exhaust configurations do this to some extent if the exhaust gasses are discharged in a rearward direction.
A particular thrust-producing exhaust device was patented by
Rolls-Royce Limited
Rolls-Royce was a British luxury car and later an aero-engine manufacturing business established in 1904 in Manchester by the partnership of Charles Rolls and Henry Royce. Building on Royce's good reputation established with his cranes, they ...
in 1937.
On the 1944
de Havilland Hornet
The de Havilland DH.103 Hornet, developed by de Havilland, was a fighter aircraft driven by two piston engines. It further exploited the wooden construction techniques that had been pioneered by the de Havilland Mosquito. Development of the ...
's
Rolls-Royce Merlin 130/131 engines the thrust from the multi-ejector exhausts were equivalent to an extra 70bhp per-engine at full-throttle height.
See also
*
De Laval nozzle
A de Laval nozzle (or convergent-divergent nozzle, CD nozzle or con-di nozzle) is a tube which is pinched in the middle, making a carefully balanced, asymmetric hourglass shape. It is used to accelerate a compressible fluid to supersonic speeds ...
*
Jet engine performance
The behavior of a jet engine and its effect both on the aircraft and the environment is categorized into different engineering areas or disciplines. The understanding of how a particular fuel flow produces a definite amount of thrust at a partic ...
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Rocket engine nozzles
A rocket engine nozzle is a propelling nozzle (usually of the de Laval type) used in a rocket engine to expand and accelerate combustion products to high supersonic velocities.
Simply: propellants pressurized by either pumps or high pressure ul ...
References
{{Aircraft gas turbine engine components
Nozzles
Jet engines