Editing Hawker Siddeley Harrier (section)

===Engine===
{{main|Rolls-Royce Pegasus}}
[[File:Aircraft engine RR Pegasus cut-out RH.jpg|thumb|Rolls-Royce Pegasus engine on display, sections have been cut out to provide an internal view|alt=Aircraft engine, partially uncovered as an exhibit]]

The Pegasus turbofan jet engine, developed in tandem with the P.1127 then the Harrier, was designed specifically for V/STOL manoeuvring. [[Bristol Siddeley]] developed it from their earlier conventional [[Bristol Siddeley Orpheus|Orpheus]] turbofan engine as the core with [[Bristol Olympus|Olympus]] compressor blades for the fan. The engine's thrust is directed through the four rotatable nozzles.<ref>Gunston, W. T. [http://www.flightglobal.com/pdfarchive/view/1960/1960%20-%201299.html "Bristol Siddeley's Fans".] ''Flight'', 12 August 1960, pp. 210–211. Retrieved 31 July 2011.</ref> The engine is equipped for [[Water injection (engine)|water injection]] to increase thrust and takeoff performance in hot and high altitude conditions; in normal V/STOL operations the system would be used in landing vertically with a heavy weapons load.<ref name= 'flight pegasus 190'/> The water injection function had originally been added following the input of US Air Force Colonel Bill Chapman, who worked for the Mutual Weapons Development Team.<ref name= Jefford_27>Jefford 2006, p. 27.</ref> Water injection was necessary in order to generate maximum thrust, if only for a limited time, and was typically used during landing, especially in high ambient temperatures.<ref name= Nordeen_118/>

The aircraft was initially powered by the Pegasus 6 engine which was replaced by the more powerful Pegasus 11 during the Harrier GR.1 to GR.3 upgrade process.<ref name= 'flight pegasus 189'/> The primary focus throughout the engine's development was on achieving high performance with as little weight as possible,<ref name= 'flight pegasus 189'>Gunston 1977, p. 189.</ref> tempered by the amount of funding that was available.<ref name= Jefford_39/> Following the Harrier's entry to service the focus switched to improving reliability and extending engine life;<ref name= 'flight pegasus 190'>Gunston 1977, p. 190.</ref> a formal joint US–UK Pegasus Support Program operated for many years and spent a £3-million annual budget to develop engine improvements.<ref name= 'flight pegasus 189'/> Several variants have been released; the latest is the Pegasus 11–61 ([[Mark (designation)|Mk]] 107), which provides {{convert|23800|lb(f)|kN}} of thrust, more than any previous engine.<ref name="RRpeg408">[http://www.rolls-royce.com/Images/pegasus_tcm92-6700.pdf "Pegasus – Power for the Harrier."] {{webarchive|url=https://web.archive.org/web/20110715200732/http://www.rolls-royce.com/Images/pegasus_tcm92-6700.pdf |date=15 July 2011 }} ''Rolls-Royce'', 2004. Retrieved 17 April 2010.</ref>