October 2013 marks the 80th anniversary of the first run of the most produced Rolls-Royce engine type of all time; the Merlin. In 1932, Sir Henry Royce wished to develop an engine that had the reliability of his Kestrel design, combined with the power of the 1931 Schneider Trophy-winning “R” engine in the Supermarine S6B racing seaplanes. The result was called the P.V.12, a 27 litre, V-12, liquid cooled, single-stage supercharged engine (P.V. standing for Private Venture).
Royce died on 22nd April 1933 without ever seeing this engine run and without knowing it would eventually become his most produced design. Just six months after his death, the first P.V.12 engine run was conducted on 15th October 1933.
The prototypes of two other private ventures, the Hurricane and Spitfire fighter aircraft, were designed around the 890 hp (663 kW) Merlin ‘C’. The early engine design’s performance needed improvement but was sufficient to show off the potential of these new low-wing monoplane fighters. Rolls-Royce even contributed £7,500 towards the total cost of £15,000 (equivalent to about £850,000 today) of the Spitfire prototype, K5054, which first flew on 5th March 1936.
By 1937, after significant alterations to the cylinder head design, the Merlin II was capable of 1,030 hp (768 kW). All Spitfires and Hurricanes in RAF service at the outbreak of war in September 1939 were fitted with the Merlin II, with the Merlin III being fitted in ever larger numbers by the Battle of Britain in 1940.
Merlin development never ceased and more and more power was extracted from the engine using improved superchargers and fuels. In 1942 the Spitfire Mk IX was equipped with the two-speed, two-stage supercharger, 1, 280 hp (954 kW) Merlin 61. The Mk IX’s performance was improved both in speed and ceiling and immediately outstripped the opposition to gain outright air superiority. This improved Merlin was also used in other aircraft, the most famous application being the North American P-51 Mustang which became one of the most successful fighter aircraft of WW2.
The Merlin was successfully fitted in many other aircraft types, not least the Lancaster and Mosquito. After the war the civil aviation industry restarted with Merlin-powered Lancastrian and York transports and the Canadian-built DC-4M Argonaut, many of which supported the Berlin Airlift in 1948. By the end of production in 1951, a total of 168,040 Merlin engines were produced including over 55,000 built under licence as the V-1650 by Packard in the USA.