Editing Napier Sabre (section)

===Genesis===
[[File:Napier-Heston racer.jpg|thumb|The first aircraft designed around the Sabre engine – the [[Napier-Heston Racer]] which crashed during early flight tests.]]
During the 1930s, studies showed the need for engines capable of developing one horsepower per cubic inch of displacement (about 45&nbsp;kW/[[litre]]). Such power output was needed to propel aircraft large enough to carry large fuel loads for long range flights. A typical large engine of the era, the [[Pratt & Whitney R-1830]] ''Twin Wasp'', developed about {{convert|1,200|hp|kW|abbr=on}} from 1,830 [[cubic inch]]es (30 litres), so an advance of some 50 per cent would be needed. This called for radical changes and while many companies tried to build such an engine, none succeeded.{{Citation needed|date=March 2011}}

In 1927, [[Harry Ricardo]] published a study on the concept of the [[sleeve valve]] engine. In it, he wrote that traditional [[poppet valve]] engines would be unlikely to produce much more than {{convert|1,500|hp|kW|abbr=on}}, a figure that many companies were eyeing for next generation engines. To pass this limit, the sleeve valve would have to be used, to increase [[volumetric efficiency]], as well as to decrease the engine's sensitivity to detonation, which was prevalent with the poor quality, low-octane fuels in use at the time.<ref>Engines and Enterprise: The Life and Work of Sir Harry Ricardo, John Reynolds, 1999 {{ISBN|978-0750917124}}, p.145</ref> Halford had worked for Ricardo 1919–1922 at its London office{{#tag:ref|21 Suffolk St, Westminster, London, a little cul-de-sac off Pall Mall East.|group=nb}} and Halford's 1923 office was in Ladbroke Grove, North Kensington, only a few miles from Ricardo, while Halford's 1929 office was even closer (700 yards),<ref>Engines and Enterprise: The Life and Work of Sir Harry Ricardo, John Reynolds, 1999 {{ISBN|978-0750917124}}, p.103</ref><ref>Boxkite to Jet - the remarkable career of Frank B Halford, Douglas Taylor, 1999,{{ISBN|1 872922 16 3}}, p.73</ref> and while in 1927 Ricardo started work with [[Bristol Aeroplane Company|Bristol Engines]] on a line of sleeve-valve designs,<ref>Engines and Enterprise: The Life and Work of Sir Harry Ricardo, John Reynolds, 1999 {{ISBN|978-0750917124}}, p.186</ref> Halford started work with Napier,<ref>Boxkite to Jet - the remarkable career of Frank B Halford, Douglas Taylor, 1999,{{ISBN|1 872922 16 3}}, p.81</ref> using the Dagger as the basis. The layout of the H-block, with its inherent balance and the Sabre's relatively short stroke, allowed it to run at a higher rate of rotation, to deliver more power from a smaller displacement, provided that good volumetric efficiency could be maintained (with better breathing), which sleeve valves could do.<ref>Engines and Enterprise: The Life and Work of Sir Harry Ricardo, John Reynolds, 1999 {{ISBN|978-0750917124}}, p.187</ref>

The Napier company decided first to develop a large 24 cylinder liquid–cooled engine, capable of producing at least {{convert|2,000|hp|kW|abbr=on}} in late 1935. Although the company continued with the opposed H layout of the Dagger, this new design positioned the cylinder blocks horizontally and it was to use sleeve valves.<ref name="Flight309">[F C Sheffield] 23 March 1944. "[http://www.flightglobal.com/pdfarchive/view/1944/1944%20-%200072.html 2,200 h.p. Napier Sabre ]" ''Flight'', p. 309. www.flightglobal.com. Retrieved: 9 November 2009.</ref> All of the accessories were grouped conveniently above and below the cylinder blocks, rather than being at the front and rear of the engine, as in most contemporary designs.<ref name="Flight309"/>

The Air Ministry supported the Sabre programme with a development order in 1937 for two reasons: to provide an alternative engine if the [[Rolls-Royce Vulture]] and the [[Bristol Centaurus]] failed as the next generation of high power engines and to keep Napier in the aero-engine industry.<ref>Industry and Air Power The Expansion Of British Aircraft Production 1935-1941, Sebastian Ritchie 2007, Routledge Taylor and Francis Group, {{ISBN|0-7146-4343-2}}, p.140</ref>
The first Sabre engines were ready for testing in January 1938, although they were limited to {{convert|1,350|hp|kW|abbr=on}}. By March, they were passing tests at {{convert|2,050|hp|kW|abbr=on}} and by June 1940, when the Sabre passed the [[Air Ministry]]'s 100-hour test, the first production versions were delivering {{convert|2,200|hp|kW|abbr=on}} from their 2,238 cubic inch (37 litre) displacements.<ref name="Flight309"/> By the end of the year, they were producing {{convert|2,400|hp|kW|abbr=on}}. The contemporary 1940 [[Rolls-Royce Merlin]] II was generating just over {{convert|1,000|hp|kW|abbr=on}} from a 1,647 cubic inch (27 litre) displacement.