The Merlin engine was first run on October 15th 1933. It passed its type testing in July 1934 when on a test run it generated 790 hp and first took to the air in February 1935. To start with the Merlin was officially called the PV-12 but Rolls-Royce had a convention of naming their engines after a bird of prey and once the PV-12 received government funding for its development it became the Merlin. It was a major improvement on the Rolls-Royce Kestrel engine in terms of power. The Kestrel had been a reliable and well-received engine but Rolls-Royce realised that it needed an engine that quickly delivered more power and development was based around the Schneider Trophy winning ‘R’ engine.
The Merlin was a liquid-cooled V-12 engine and was first used in a Hawker Hart biplane in February 1935. In the same year the Air Ministry issued a directive that required a new fighter aircraft that could fly at a minimum speed of 310 mph. The two companies that best responded to this requirement were Supermarine and Hawker. Both companies developed their prototypes around the Merlin. In 1936 both companies received orders for their aircraft – the Hurricane and the Spitfire – from the Air Ministry.
Early production Merlins had numerous problems that led to their reliability being questioned. The leaked their coolants and the cylinder head frequently cracked. However, by version Merlin ‘F’ all major issues had been resolved and the engine officially became Merlin Mark I. The engine was constantly improved. In 1937, a highly modified Spitfire was fitted with a strengthened Merlin engine that in a test generated 2160 hp. This showed a Merlin’s potential and by the time World War Two started it had gained a reputation among pilots for its reliability. There are records of the Lancaster bomber losing an engine but being able to carry on flying on just three Merlin engines left at full throttle.
One of the men involved in the Merlin’s development was a mathematician named Sir Stanley Hooker. One weakness of the early Merlin engines was the lack of power generated by its supercharger, especially at lower altitudes. Hooker solved this and the new engine with his improvements was the Merlin XX. The increased extra power was to be of great importance to the Spitfire and Hurricane during the Battle of Britain where a great deal of combat was below 6000 feet – an altitude at which the previous supercharger had not worked very well. Hooker’s improvements gave the Merlin XX an extra 22 mph power boost. Later versions had an added 30% power boost. However, the supercharger of the XX still weakened at higher altitudes. To attack high flying Luftwaffe bombers, Hooker used two superchargers in a series and the new variant engine became the Merlin 61 and was fitted to the Spitfire Mark IX.
The engine was manufactured at factories in Crewe, Derby and Glasgow. During World War Two, the engine was considered so important to the war effort that an arrangement was made to make them outside of the UK and so away from any possible chance of bombing. The Packard Motor Company was given a contract to make them.
The Mark II and Mark III engines generated 1,030 hp. The Mark XII was used in Spitfires Mark II’s and generated 1,150 hp. The Mark II Hurricane used the Merlin XX and generated 1,480 hp. The Spitfire Mark V – the most widely produced variant – used the Merlin 45, which generated 1,515 hp.
The first operational versions of the Avro Lancaster bomber used four Merlin XX engines. Each engine generated 1,480 hp giving the aircraft a total power output of 5,920 hp. More Merlin engines were produced for the Lancaster than for any other aircraft in the RAF. The twin-engine de Havilland Mosquito was the second largest user of the Merlin engine. The engine was also used by the USAAF P51 Mustang, which greatly increased its power.