Rolls-Royce piston aero engines were all named after birds of prey. The immediate predecessor of the Merlin was the Kestrel, which had a capacity of 21.2 litres and 745 horsepower. In 1932, Sir Henry Royce ordered the development of a new engine, entirely funded by his Rolls-Royce company at Derby. Its reference number, PV-12, stood for “Private Venture”. It was renamed the Merlin when the Air Ministry agreed to provide finance towards it from October 1933.

The goal was to produce an engine that generated as much horsepower as its weight and air displacement made possible at high altitude. This was achieved through the use of advanced metallurgy and unusual alloys. The new engine started out with 750hp, which was to rise, with further development, to 1,030hp by the outbreak of the war in 1939. Within another three years, this had become 1,730hp.

Virtually every night during the war, the city of Derby, where Rolls-Royce had its main plant, throbbed with what became known as the “Derby hum” – the sound of aircraft engines being tested.

Spry in the sky

Flying at up to 360mph, and climbing at a rate of 2,500ft per minute, the Spitfire owed its smooth handling and extreme manoeuvrability to the Merlin engine. The prototype that completed its first test flight in 1936 was powered by the Merlin C engine, which had been superseded by the time the plane entered squadron service by the 12-cylinder, 27-litre Merlin Mk II.

Among the Merlin’s innovative features was a novel cooling system that used a pressurised mixture of water and ethylene glycol (antifreeze). A powerful two-stage supercharger powered by the belts of the crankshaft pumped a mixture of fuel and oxygen into the cylinders in concentrations sufficient to improve the engine’s power. The principle is the same as using bellows to blow air on to a fire to increase its intensity.

Churchill considered the Merlin engine such an indispensable part of Britain’s air defences that, in 1940, he secretly ordered a set of blueprints to be sent to the United States in case the UK was overrun by Nazi Germany. There, it was developed by Packard in Detroit. Henry Ford turned down the opportunity because he was convinced that Britain would lose the war anyway.

The outcome of the Battle of Britain made that far less likely. As www.fathom.com puts it: “After the war, Lord Tedder, Marshal of the Royal Air Force, who had been in charge of the development of aircraft and engines during the period of the battle, attributed the British victory to three predominant factors: the skill and bravery of the pilots, 100-octane fuel and the Rolls-Royce Merlin engine.”

Having reached the limit of its development potential, the Merlin began to be replaced from 1942 by Rolls-Royce’s Griffon engine.