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The Rolls Royce Merlin: Powering History’s Most Famous Fighters

It was a recognizable sound heard throughout World War II, and depending on which side you were on, either a gentle purring that symbolised safety and pride or a thunderous roar that signified you might be in the wrong place at the wrong time. 

Quite simply, the Rolls Royce Merlin was one of the most important aviation engines for the allies during the Second World War, an engine that was used in no fewer than 40 different aircraft models, but will always be remembered as the power behind the iconic Lancaster bomber, the Hawker Hurricane, and of course, the Supermarine Spitfire, while it was eventually even used on North American P-51 Mustang.   

What started out as a rather mediocre engine in the run-up to war, blossomed into one of the most important and reliable engines ever created and over its 17 year production period, nearly 170,000 of them were built, only just behind the most produced engine of the war, the Pratt & Whitney R-1830.   

The fact that the allies were eventually able to overwhelm enemy air forces was down to several reasons. Huge numbers, excellent aircraft, bloody-mindedness and almost certainly, the Rolls Royce Merlin powering it all. 

Rolls Royce  

Most watching will no doubt associate the name Rolls Royce with luxury cars more than aircraft engines – and you’d certainly be at least partly right there. 

Henry Royce built his first car out of his Manchester factory in 1904 and it immediately attracted the attention of a certain Charles Rolls who owned a car dealership. The two agreed to go into business together and the first Rolls Royce car appeared in December 1904.

They developed a solid reputation over the following decade and when Britain found itself at war with Germany in 1914, the government formally asked Rolls Royce to produce aircraft engines. The first, the Rolls-Royce Eagle, was eventually included on the British heavy bomber, the Vickers Vimy, but this came quite near to the end of the conflict.

After the war, the company opened its first factory in the U.S and gradually new models began emerging that culminated with the wonderfully stylish, and more than a little Al Capone, Rolls-Royce Phantom in 1925 and the Phantom II four years later. 

Build up to War

Rolls Royce became much more active within the aircraft industry during the 1930s. Their Kestrel, a 21 litre (5.5 gallons) 700 horsepower class V-12 aircraft engine, had been in operation since 1926 and was already proving to be a huge success with several aircraft, most notably, the Hawker Fury, the Hawker Hart, and the Handley Page Heyford. 

But by the early 1930s, it was becoming clear that something more powerful would be needed to keep pace with the rapid advancement in aircraft technology seen during the period. The result was the PV-12 (private venture – 12 cylinder), an engine that would eventually reach 1,100 hp and was first used by an aircraft on 15th October 1933. Initially, the engine was designed to use an evaporative cooling system but it proved unreliable and frequently provided far too much drag on the aircraft. 

It’s at this stage that the story of the Rolls Royce Merlin could have one of two ways. The PV-12 was a solid enough engine, but its cooling system problems meant it would probably never be used widely. This iconic engine may well have never reached the pinnacle if it hadn’t been for the introduction of ethylene glycol from the U.S, which meant that the engine could be redesigned using a conventional liquid-cooling system. All of sudden, Rolls Royce potentially had a hell of an engine on their hands, but it wasn’t quite the finished article just yet.      


Between 1935 and the end of World War II, the Merlin engine produced by Rolls Royce was a continually evolving process. Even to call it a single-engine is a little inaccurate because many were slightly tinkered with to be adapted for certain aircraft or certain conditions and attitudes.

Merlin B, C and E followed the initial PV-12 engine as the maximum horsepower slowly inched up along with its overall reliability. The Merlin E was the engine in the Supermarine Spitfire prototype which took to the skies in March 1936 and the two would go on to form a fearsome duo. 

Merlin F was more widely known as Merlin I, used in the air for the first time in July 1936 and was the first full production engine built in the class. But as I said, things were always improving, which was good, because Merlin 1, and its ramp cylinder heads, was not a success and only 172 were built.

But persistence began to pay off with the next interpretation, the Merlin II. The problematic ramp cylinders had been replaced with parallel pattern heads, which saw the valve stems parallel to the cylinder bore axis, and the 1,030-horsepower engine was accepted by the Royal Aircraft Establishment on 22nd September 1937.    

From there things moved quickly, culminating with the Merlin XX, complete with a supercharger, re-designed inlets and improved guide vanes and 1,240-horsepower at 2,850 rpm in low gear at 10,000 feet and 1,175-horsepower at 2,850 rpm in high gear at 17,500 feet. This engine was used widely and could be found in the Bristol Beaufighter II, the Boulton Paul Defiant II, the Handley Page Halifax II and V, the Hawker Hurricane II and IV, and the Avro Lancaster I and III.  

World War II

As the world careened inevitably towards war, there was a general sluggishness with the Air Ministry regarding how the production of engines, and indeed the aircraft themselves, could be sped up. One man frequently dismayed by official channels was Ernest Walter Hives, or to use his infinitely fancier name, 1st Baron Hives, who by this point was head of Rolls Royce.   

One of his favourite expressions was “Work till it hurts”, which no doubt would be completely unacceptable today, but back in the late 1930s, Hives, who worked harder than anybody, was seen as a true inspiration. 

With the Air Ministry dragging its feet over production expansion, Hives took it upon himself to make the changes that he saw would soon be inevitable. He began preparing Rolls Royce for a massive surge in production by splitting facilities between engineering and production and developing shadow factories, which were additional facilities designed to shoulder the increased burden but also to act as a safety net if a single large-scale factory was attacked. The idea was picked up by the British government who set up the Shadow Scheme, which mostly saw motor vehicle companies, such as Rootes, Daimler, Standard, Rover and Austin being adapted to build aircraft.  

During the war, some 112,000 Merlin engines were built in Britain across four separate factories in Derby, Crewe, Glasgow and Manchester, all kept at a brisk working pace by the relentless Hives. 

Every one of the engines that took part in the Battle of Britain, as the nation just about kept its head above water in the face of relentless German pressure, came from the first factory in Derby, which Hives had strong-armed the government to fund a £1,927,000 (£131 million today) expansion that was completed in December 1939, leading to 32,377 engines being produced there. 

Work on a new factory in Crewe started in March 1938 and it produced its first engines in 1939. As part of the agreement, the local authority promised to build 1,000 new houses to accommodate the workforce, but by 1939, funding had only been found for 100. Once again it was Hives who stepped in and threatened to move the entire factory if the promised houses weren’t delivered. Miraculously, the money was soon found – maybe they’d lost it, who knows. In total 26,065 Merlins were built in Crewe, which did see a strike in 1940 that saw women replace men on some sections of the lines for ten days. 

In September 1939, the Air Ministry allocated £4,500,000 (£300 million today) for a new shadow factory in Glasgow. With 16,000 workers, it was one of the largest industrial complexes in Scotland and began producing its first engines in November 1940, and by June 1941, monthly output had reached 200, rising to 400 per month by March 1942. 

23,675 engines were produced in total in the Glasgow factory, where, due to absenteeism, the working week was cut slightly to a still mammoth 82 hours per week, with a single half Sunday per month for holiday. I know that sounds absolutely mental, but this was a time of war. 

The final British factory was back where it all started for Rolls Royce in Manchester. The Ford Motor Company was asked to build the Merlins and a new factory was completed in May 1941 – but bombed the very next month. Despite this, engines began coming off the production line just a month later, and by 1943, 200 per week were leaving the factory. In total, 30,428 Merlin’s were built in Manchester, which was only slightly less than at Derby which had had a considerable head start. 


Up until this point, the Rolls Royce Merlin had been a uniquely British affair, but with the engine’s importance now going through the roof, the decision was taken to find a suitable factory across the pond in the U.S that would also begin making them. 

The Ford Company rejected an offer and it eventually fell to the Packard Motor Car Company who signed a whopping development contract of $130 million (around $2.4 billion today) to build the engines, which would be called the Packard V-1650 Merlin. The first engines appeared out of the factory in August 1941 and a total of 55,523 were built in the U.S.  

The Engine

I think you get the idea, by 1942, Merlin engines were pouring out of factories at unprecedented rates, to supply an insatiable demand that unfortunately was such because so many aircraft were being lost in action. 

As I mentioned earlier, the engine itself came with 12 cylinders built from R.R.50, a high-strength, high-temperature aluminium alloy, while the pistons were machined from R.R.59 alloy forgings and its crankshaft is a single piece made from a nitrogen-hardened nickel-chrome molybdenum steel forging. The crankcase is formed of two aluminium-alloy castings, the top part with the wheel case, supercharger, the cylinder blocks, crankshaft main bearings and part of the housing for the airscrew reduction gear, while the bottom section houses the oil sump and carries the oil pumps and filters. The Merlin has two inlet and two exhaust poppet valves made of K.E.965 steel, with both valves kept closed by a pair of concentric coil-springs.

Possibly the most significant advancement the Merlin had along the way was the improvement of the supercharger, which was the brainchild of engineer Stanley Hooker. A bit of a maverick, he was given free rein by Hives to research whatever took his interest. Superchargers are mechanically driven, forced induction devices used within a piston engine to produce more power. This additional air and fuel result in a large increase in the power produced by the engines, and with the aircraft, this usually equated to high speeds and higher maximum altitudes.    

Like the Merlin itself, the supercharger was ever-changing. The earliest, and most rudimentary, was a single-stage, single-speed gearbox, the second a single-stage, two-speed gearbox and the final a two-stage, two-speed gearbox with an intercooler, a device used to cool a gas after compression. 

Hooker improved the initial design which consequently improved airflow characteristics which in turn increased its maximum power output by some 30% and gave the aircraft a higher altitude of over 5,800 metres (19,000 ft). This Merlin 45 engine went into the Spitfire Mk V, the most produced of all Spitfire variants

Not long after, the Air Ministry made a request for a turbocharged engine that could be installed in the planned high altitude Wellington VI bomber, but Hooker went his own way and designed a two-stage supercharger which was soon included on the Merlin 61 which was rushed into the new Spitfire Mk IX – allowing the allies to compete better with the rate of climb and service ceiling over the Focke-Wulf Fw 190. This gave the new engine a 300 horsepower bump at 30,000 ft (9,100 m) meaning a Spitfire IX was nearly 110 km/h (70 mph) faster than a Spitfire V at that altitude. And when you’re in a dog fight for your life, every precious second counts.   


Look, I’m not going sit here and tell you that the Merlin engines won the war, that would be silly, but they were undoubtedly instrumental to the allied victory. It’s easy to overlook the mechanical hardware that’s far less sexy than the guns, but without a thunderous and reliable engine behind you, it doesn’t matter how good the pilot, how cutting edge the aircraft and how deadly its armaments, you’re always going to struggle. They were the solid foundation that everything could be built around.   

By the end of the war, some of the Merlin engines were producing 1,800 horsepower, almost double what they started with before the war. This was an obsessive operation that sought to squeeze every possible last drop out of what an aircraft engine could theoretically be. 

The Merlin engines played a pivotal role in almost all of the most successful British aircraft during the war, but it was the marriage of the engine with the dynamic speedster, the North American P-51 Mustang, that was perhaps the icing on the cake. As the allies began their slow march across Europe, it was often the Mustang that accompanied bombers deep into enemy territory. This was one of the fastest planes of the war, while also having the highest maximum altitude and furthest range of any fighter. If the Spitfires and the Hurricanes had had their crowning moment during the Battle of Britain, it was the Mustangs that helped to destroy the Luftwaffe once and for all. 

In 1950, production of the Merlin engines finally ceased, with nearly 170,000 having been produced between the UK and the USA between 1935 and 1950. This 12 cylinder monster had certainly played its part.   

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