Editing Turbojet (section)

== Components ==

[[File:Axial compressor.gif|thumb|An animation of an axial compressor. The stationary blades are the stators.]]
[[File:Turbojet operation- centrifugal flow.png|thumb|Schematic diagram showing the operation of a centrifugal flow turbojet engine. The compressor is driven by the turbine stage and throws the air outwards, requiring it to be redirected parallel to the axis of thrust.]]
[[File:Turbojet operation- axial flow.png|thumb|Schematic diagram showing the operation of an axial flow turbojet engine. Here, the compressor is again driven by the turbine, but the air flow remains parallel to the axis of thrust.]]
[[File:Turbine Based Combined Cycle (TBCC) engine integration.svg|thumb|Schematic diagram of a Turbine Based Combined Cycle (TBCC) engine integration. 1) Air intake for low flight speeds 2) Air intake for high flight speeds 3) Turbojet 4) Statoreactor 5) Stator reactor nozzle 6) Turbojet nozzle.]]

=== Nose bullet ===
{{main|Nose bullet}}
A nose bullet is a component of a turbojet used to divert air into the intake, in front of the accessory drive and to house the starter motor.

=== Air intake ===
An intake, or tube, is needed in front of the compressor to help direct the incoming air [[Laminar flow|smoothly]] into the rotating compressor blades. Older engines had stationary vanes in front of the moving blades. These vanes also helped to direct the air onto the blades. The air flowing into a turbojet engine is always subsonic, regardless of the speed of the aircraft itself.

The intake has to supply air to the engine with an acceptably small variation in pressure (known as distortion) and having lost as little energy as possible on the way (known as pressure recovery). The ram pressure rise in the intake is the inlet's contribution to the propulsion system's [[overall pressure ratio]] and [[thermal efficiency]].

The intake gains prominence at high speeds when it generates more compression than the compressor stage. Well-known examples are the Concorde and [[Lockheed SR-71 Blackbird]] propulsion systems where the intake and engine contributions to the total compression were 63%/8%<ref>{{cite book | last1=Trubshaw | first1=Brian | last2=Edmondson | first2=Sally |title=Brian Trubshaw: Test Pilot |publisher=Sutton Publishing |year=1999 |isbn=0750918381 |at=Appendix VIIIb}}</ref> at Mach 2 and 54%/17%<ref>{{cite report|url=http://www.enginehistory.org/Convention/2013/HowInletsWork8-19-13.pdf |title=How Supersonic Inlets Work: Details of the Geometry and Operation of the SR-71 Mixed Compression Inlet |access-date=16 May 2016 |url-status=dead |archive-url=https://web.archive.org/web/20160509025601/http://www.enginehistory.org/Convention/2013/HowInletsWork8-19-13.pdf |archive-date=9 May 2016  |author=J. Thomas Anderson |at= Fig.26 |publisher=Lockheed Martin Skunk Works |date=August 19, 2013}}</ref> at Mach 3+. 
Intakes have ranged from "zero-length"<ref>{{cite journal | last=Sóbester | first=András | title=Tradeoffs in Jet Inlet Design: A Historical Perspective | journal=Journal of Aircraft | volume=44 | issue=3 | date=2007 | issn=0021-8669 | doi=10.2514/1.26830 | doi-access=free | pages=705–717 | url=https://eprints.soton.ac.uk/46202/1/AIAA-26830-529.pdf }}</ref> on the [[Pratt & Whitney TF33]] [[turbofan]] installation in the [[Lockheed C-141 Starlifter]], to the twin {{convert|65|ft}} long, intakes on the [[North American XB-70 Valkyrie]], each feeding three engines with an intake airflow of about {{convert|800|lb/s|kg/s}}.

=== Compressor ===
The turbine rotates the compressor at high speed, adding energy to the airflow while squeezing (compressing) it into a smaller space.  Compressing the air increases its [[pressure]] and temperature. The smaller the compressor, the faster it turns. The (large) [[General Electric GE90|GE90-115B]] fan rotates at about 2,500 RPM, while a small helicopter engine compressor rotates around 50,000 RPM.

Turbojets supply [[bleed air]] from the compressor to the aircraft for the operation of various sub-systems. Examples include the [[environmental control system]], [[De-icing|anti-icing]], and fuel tank pressurization. The engine itself needs air at various pressures and flow rates to keep it running. This air comes from the compressor, and without it, the turbines would overheat, the lubricating oil would leak from the bearing cavities, the rotor thrust bearings would skid or be overloaded, and ice would form on the nose cone. The air from the compressor, called secondary air, is used for turbine cooling, bearing cavity sealing, anti-icing, and ensuring that the rotor axial load on its thrust bearing will not wear it out prematurely. Supplying bleed air to the aircraft decreases the efficiency of the engine because it has been compressed, but then does not contribute to producing thrust.

Compressor types used in turbojets were typically axial or centrifugal. Early turbojet compressors had low pressure ratios up to about 5:1. Aerodynamic improvements including splitting the compressor into two separately rotating parts, incorporating variable blade angles for entry guide vanes and stators, and bleeding air from the compressor enabled later turbojets to have overall pressure ratios of 15:1 or more. After leaving the compressor, the air enters the combustion chamber.

=== Combustion chamber ===
The burning process in the [[combustor]] is significantly different from that in a [[piston engine]]. In a piston engine, the burning gases are confined to a small volume, and as the fuel burns, the pressure increases. In a turbojet, the air and fuel mixture burn in the combustor and pass through to the turbine in a continuous flowing process with no pressure build-up. Instead, a small pressure loss occurs in the combustor.

The fuel-air mixture can only burn in slow-moving air, so an area of reverse flow is maintained by the fuel nozzles for the approximately stoichiometric burning in the primary zone. Further compressed air is introduced which completes the combustion process and reduces the temperature of the combustion products to a level which the turbine can accept. Less than 25% of the air is typically used for combustion, as an overall lean mixture is required to keep within the turbine temperature limits.

=== Turbine ===
Hot gases leaving the combustor expand through the turbine. Typical materials for turbines include [[inconel]] and [[Nimonic]].<ref>{{Cite web|url=http://www.flightglobal.com/pdfarchive/view/1960/1960%20-%201525.html|title=1960 &#124; Flight &#124; Archive}}</ref> The hottest turbine vanes and blades in an engine have internal cooling passages. Air from the compressor is passed through these to keep the metal temperature within limits. The remaining stages do not need cooling.

In the first stage, the turbine is largely an impulse turbine (similar to a [[pelton wheel]]) and rotates because of the impact of the hot gas stream. Later stages are convergent ducts that accelerate the gas. Energy is transferred into the shaft through momentum exchange in the opposite way to energy transfer in the compressor. The power developed by the turbine drives the compressor and accessories, like fuel, oil, and hydraulic pumps that are driven by the accessory gearbox.

=== Nozzle ===
{{main|Propelling nozzle}}

After the turbine, the gases expand through the exhaust nozzle producing a high velocity jet. In a convergent nozzle, the ducting narrows progressively to a throat. The nozzle pressure ratio on a turbojet is high enough at higher thrust settings to cause the nozzle to choke.

If, however, a convergent-divergent [[de Laval nozzle]] is fitted, the divergent (increasing flow area) section allows the gases to reach supersonic velocity within the divergent section. Additional thrust is generated by the higher resulting exhaust velocity.

===Thrust augmentation===
Thrust was most commonly increased in turbojets with [[Water injection (engine)|water/methanol injection]] or [[Afterburner|afterburning]]. Some engines used both methods.

Liquid injection was tested on the [[Power Jets W.1]] in 1941 initially using [[ammonia]] before changing to water and then water-methanol.  A system to trial the technique in the [[Gloster E.28/39]] was devised but never fitted.<ref>{{Cite web|url=https://www.flightglobal.com/pdfarchive/view/1947/1947%20-%201359.html|title=1947 &#124; 1359 &#124; Flight Archive}}</ref>

==== Afterburner ====
{{main|Afterburner}}

An afterburner or "reheat jetpipe" is a combustion chamber added to reheat the turbine exhaust gases. The fuel consumption is very high, typically four times that of the main engine. Afterburners are used almost exclusively on [[supersonic aircraft]], most being military aircraft. Two supersonic airliners, Concorde and the [[Tu-144]], also used afterburners as does [[Scaled Composites White Knight]], a carrier aircraft for the experimental [[Scaled Composites SpaceShipOne|SpaceShipOne]] [[suborbital]] spacecraft, and [[Boom XB-1]], an experimental supersonic aircraft.

Reheat was flight-trialled in 1944 on the [[Power Jets W.2|W.2/700]] engines in a [[Gloster Meteor|Gloster Meteor I]].<ref>{{cite book |title=World Encyclopedia of Aero Engines |edition=5th |author= [[Bill Gunston]] |publisher=Sutton Publishing |year= 2006 |page=160}}</ref>