Written by
Jason Chetwynd-Chatwin

A fine example of a Rolls-Royce Merlin XII has been preserved at the Rolls-Royce works in Derby, England.

* Alternative engine.

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Overview

The Rolls-Royce Merlin was without doubt Britain's most significant aero engine of the Second World War. It powered the majority of the Royal Air Force's most effective frontline combat aircraft for the duration of the conflict. Other uses included a range of Fleet Air Arm applications, in the Fulmar, Barracuda, and Seafire, as well derivatives of the North American Mustang and Curtiss Kittyhawk. Its qualities of low frontal area, compact size, and high power made it ideal for fighter applications; bomber crews valued its reliability. The British Cromwell tank used a land based development of the Merlin, known as the Meteor.

After a lull in engine development in the 1920's, the Derby based company designed and produced a range of liquid cooled V-12 engines, notably the Buzzard and Kestrel. Supermarine's S.6 Schneider Trophy seaplanes used the 'R' racing engine, a Buzzard development, which gave valuable experience of running with high levels of supercharge. Rolls-Royce started design work on a new engine, the PV.12, in 1933; its major design features were broadly similar to previous Rolls-Royce V-12's, especially the Kestrel. Cylinder displacement was 27 litres, a figure that stayed the same for all subsequent marks of the Merlin. Initial design studies examined the possibility of using an inverted vee layout for the engine, and a mock up was duly shown to the aircraft manufacturers. Reactions were generally unfavourable, the consensus being that it would present installation difficulties. The German experience with engines such as the DB601 suggests that such concerns were probably unfounded.

Prototypes of the PV.12 were ready for test in October of 1933, funding up to this point having been provided by Rolls-Royce as private venture money (hence the PV prefix). The Government supplied subsequent development funding. Prototype development work on the Merlin B through to F led to a number of modifications, including changes to the cylinder head and cylinder block casting, as well as the cooling system. First flight of a Merlin took place in April (possibly Feb) 1935 aboard one of Rolls-Royce's flying test beds, a Hawker Hart. Both the Hawker Horsley and Fairey Battle were also used as flight test aircraft during the course of development.

The Merlin F passed a reduced type test in November 1936, and was put into production as the Merlin I. The ill-fated Fairey Battle was the first production aircraft to use the Merlin; the first squadron (No. 63 Sqn) received this light bomber in May 1937.

The Merlin II replaced the unsatisfactory ramp type of cylinder head with a Kestrel style flat combustion chamber. Together with the similar Merlin III, it made up the majority of the early production versions, entering service with Fighter Command on both the Hurricane and Spitfire. Some 9,739 engines of both marks were built between 1937 and 1941.

The first bulk shipment of 100-octane fuel had arrived in Britain in June 1939 from the Esso refinery in Aruba. This and subsequent tanker shipments from Aruba, Curacao and the USA were stockpiled while the RAF continued to operate on 87 octane petrol. Having secured sufficient quantities of 100 octane, Fighter Command began converting its engines to this standard in March 1940, allowing boost (manifold) pressures to be raised without the risk of detonation in the cylinders. This initial increase in maximum boost from 6 lb to 9 lb delivered a useful power growth of around 130hp at the rated altitude. Subsequent increases in permitted boost pressures throughout the war saw the Merlin's maximum boost on 100-octane fuel rise to 18 lb, allowing considerable increases in power output. The introduction of 150-octane fuel in 1944 allowed further increases to 25 lb boost.

The drive for improved high altitude performance gave rise to the Merlin X, the first mark to incorporate a two-speed supercharger in place of the previous single speed unit. The similar Merlin XX gained significant performance benefits from Stanley Hooker's work on improving the aerodynamic efficiency of the supercharger. The XX series and its developments primarily saw service on the Hurricane II and Mosquito in Fighter Command, as well as being widely used on a number of Bomber Command aircraft, notably the Mosquito, Lancaster, and Merlin Halifax.

The manufacturing complexity of the Merlin XX compared with previous marks resulted in a relatively slow rate of initial production. There was, however, an acute need to improve the performance of the Spitfire, which suffered against the Bf-109 above 20,000 ft. The result was the emergence of the single-speed Merlin 45, incorporating a number of supercharger improvements similar to those of the Merlin XX, but omitting the low altitude supercharger gearing. The engine was introduced on the Spitfire V in 1941, and variants included the 45M and 46/47 series, these being low and high altitude marks respectively.

Of the remaining Merlin developments, probably the most significant wartime marks were the Merlin 60 series of two-stage, two-speed, intercooled engines. These introduced a second centrifugal supercharger operating in tandem with the first, allowing still greater levels of compression. The growing temperatures of the compressed gas resulting from such high levels of supercharge required the addition of an intercooler to cool the charge air before it reached the cylinders. Eventually capable of running at up to 25lb boost on 150 octane fuel, these engines gave much improved high altitude performance, and greater power at all altitudes. The engines transformed the Spitfire from the Mk.IX onwards (the 60 series powered Spitfire VII & VIII flew later), and the Merlin installation allowed the Mustang to realise its potential as a world-class fighter. The similar Merlin 70 series also saw service in the later marks of bomber and night fighter Mosquito; performance was enhanced across the board.

After an abortive attempt to license Merlin production to the Ford Company in America, the Packard Motor Company took on the task of establishing a Merlin production line on the other side of the Atlantic. A contract was signed in September 1940, and Packard worked closely with Rolls-Royce in the ensuing months to prepare for Merlin production. The first production engines were delivered in August/September 1941; the Merlin 28 was the engine chosen for initial production, this was roughly equivalent to the British Merlin 22A.

Single-stage engines produced by Packard included the 29, 31, 33, 38, 224 and 225, all used on Lancasters, Hurricanes, and Mosquitos; a large number went to power Canadian built aircraft of all three types. Engines for aircraft of US manufacture were given the American V-1650 designation, with a dash number to denote the mark. The equivalent to the two-stage Merlin 66 was designated the 266 (for the Spitfire XVI), and V-1650-7 (for the Mustang). Differences between Packard and British built engines included supercharger drive gearing, as well as accessories such as magnetos and carburettors. Total production by Packard of over 55,000 engines contributed greatly to Merlin production totals, although average monthly production never exceeded that of Rolls-Royce in Britain.

Although a smaller engine than many of its contemporaries in terms of displacement, Rolls-Royce was able to extract competitive powers from the Merlin through an aggressive development program and accumulated expertise in the design of efficient superchargers. In comparison with other engines, the Merlin generally ran at significantly higher levels of boost. The Merlin's principal rival, the DB601, displaced 33.9 litres for a similar power. It was quite conservatively boosted compared with the Merlin, and the inability of Germany to secure high-octane fuel in quantity limited development in this regard.

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Variants

* Packard production totalled 55,523 Merlins of all marks.

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Specifications

Note: The same mark of engine would often be run at increasing boost levels as the war progressed. Performance figures will vary with fuel grade, supercharger gearing, and engine improvements – values given are typical.

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References

Lumsden, Alec, British Piston Aero-Engines and their Aircraft, Airlife, 1994.

White, Graham, Allied Piston Engines of World War II, Airlife, 1995.

Rubbra, A.A., Rolls-Royce Piston Aero Engines – A Designer Remembers, Rolls-Royce Heritage Trust, 1990.

Harvey-Bailey, Alec, The Merlin in Perspective – The Combat Years, Rolls-Royce Heritage Trust, 1995.

Pugh, Peter, The Magic of a Name, The Rolls-Royce Story: The First 40 Years, Icon, 2000.

Lloyd, Ian, Rolls-Royce, The Merlin at War, Macmillan, 1978.

Lloyd, Ian, Rolls-Royce, The Years of Endeavour, Macmillan, 1978.

Bingham, Victor, Major Piston Aero Engines of World War II, Airlife, 1998.

Banks, Air Commodore F.R., I Kept No Diary, Airlife, 1978.

Mason, Francis K., Hawker Aircraft Since 1920, Putnam, 1991.

Taylor, H.A., Fairey Aircraft Since 1915, Putnam, 1984.

Gunston, Bill, The Development of Piston Aero Engines, Patrick Stephens Limited, 1995.

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