One of the British technical achievements which proved most important for the country’s war effort was the Rolls-Royce Merlin engine. It not only powered the Spitfire, but also the Hurricane, Lancaster, Mosquito and the Mustang, together with some variants of the Halifax, Beaufighter and Wellington. The vast majority of Spitfires produced were fitted with various variants of the Merlin. Even today the Merlin is one of the most celebrated piston aero engines of all time.

This article attempts to describe the origins of its development.

It Began With a Kestrel

The design of the Merlin owns much to the company’s most successful aero engine of the early 1930s, the Rolls-Royce Kestrel. The Kestrel was a successful inline V-12 engine which provided excellent service on a number of RAF aircraft of the period. Most notably, it was selected to power  the entire family of highly successful and acclaimed family of Hawker biplanes, the Hart, Audax, Demon, Hardy, Hind and Osprey two-seaters and Fury and Nimrod fighters, the aircraft which had a prominent role during the RAF’s inter-war period.

The Kestrel was Rolls-Royce’s first cast-block engine. Earlier aero engine designs had used individually machined steel cylinders that were screwed onto a crankcase, whereas the cast-block design used a single block of aluminium that was machined to form cylinders. The result was both simpler to build as well as lighter, requiring only an investment in new machining equipment. 

The idea of a cast-block came about through excellent American Curtiss D-12, one of the first truly successful cast-block engines. The D-12 was one of the most powerful engines of its era, and continued to swap records with other high-power engines of its era. No British company could offer anything like it, and when Fairey selected it for their Fairey Felix design the Air Ministry had enough and ordered Napier & Son and Rolls-Royce to start work on cast-block engines of their own.

The most gifted engineer at Napier and the designer of the famous Napier Lion, Arthur Rowledge, had become disillusioned by Napier’s management and Rolls-Royce quickly seized the opportunity to offer him a job and a position in charge of the Kestrel development.  Applying every known advance since the D-12 was introduced, Rowledge designed the new engine to use supercharging at all altitudes, allowing it to outperform “naturally aspirated” engines by as much as they were willing to increase the boost pressure.

The third key advance in the Kestrel was the use of a pressurized cooling system. Water boils at 100 °C at standard atmospheric pressure, but this temperature decreases with altitude. As it does so its ability to carry heat away from the engine drops, to the point where at high altitudes a gigantic radiator needs to be used to cool it again. The solution was to pressurize the entire cooling system, thereby not only preventing the drop in cooling performance with altitude, but in fact increasing the boiling point even on the ground. The Kestrel was built to maintain enough pressure to keep the boiling point at about 150 °C.

The Kestrel was first produced in 1927 at 450 hp, which soon improved in the IB model to 525 hp. This model saw widespread use in the famed Hawker Hart family that dominated British air power during the early 1930s. However it was not long before line improvements increased power dramatically; the V model provided 695 hp at 3,000 rpm with no basic change to the design, while the XVI used in the Miles Master delivered 745 hp. In 1935, Messerschmitt also tested its Messerschmitt Bf 109 V1 prototype monoplane fighter with Kestrel engine.

Increased availability of higher octane aviation fuels in the late 1930s allowed the engine to be boosted to even higher power levels without suffering from ping, and the Kestrel eventually topped out at 1,050 hp (780 kW) in the Kestrel XXX model of 1940.

The Peregrine

In the early 1930s, Rolls-Royce started planning for the future of its aero engine development programmes and eventually settled on pursuing two main designs. The 700-horsepower Rolls-Royce Peregrine was to be an updated, supercharged development of the existing 22 litre Rolls-Royce Kestrel. Two Peregrines bolted together on a common crankshaft into an X-24 layout would create the 1,700 hp 44 litre Rolls-Royce Vulture for use in larger and heavier aircraft such as bombers.

There was also the possibility that the famous 36-litre ‘R’ engine from the Supermarine racing planes, itself a development of the Rolls-Royce Buzzard, could be developed into a 1,500 hp-class engine,  a sort of scaled-up Kestrel. This line design materialized only much later, leading the Rolls-Royce Griffon.

Initial development of the Peregrine promised to be very successful. However, the Peregrine-Vulture plan left a large gap between 700 and 1,500 horsepower, and Henry Royce saw the need for a more powerful 12-cylinder engine that would utilise the technology developed from the Kestrel production and could be fitted to single-engine aircraft.

The company management authorised the project to develop on a new 1,100 hp-class design under the designation PV-12. PV stood for “private venture” as the company received no government money for  development of such engine.

PV-12 becomes the Merlin

The PV-12 was initially designed to use the new water/steam evaporative cooling system then in vogue, similar to one previously (and unsuccessfully) used with the Rolls-Royce Goshawk. The new engine was first run on 15 October 1933 and first flown in April 1935 powering the Hawker Hart biplane. Later on the old the company-owned Hawker Horsley became testbed aircraft for the second prototype engine. In 1936, Rolls-Royce decided to invest in a more flexible flying testbed and the choice fell on Heinkel He 70, high-performance German courier aircraft which also had an enclosed passenger cabin from which several engineers could monitor the engine in flight. One example of He 70G powered with Kestrel V was purchased by Rolls-Royce and subsequently used in the Merlin tests during 1936-1937.

Never resolved problems with the evaporative system manifested themselves with all clarity during trials. The system was notoriously failing during taxiing, when there was no cooling airflow around the condensers and thus the steam could not be cooled quickly enough. Similar problems occurred during climbs at high power, this time for the surplus of heat produced by the engine which exceeded the maximum capacity of the cooling system.

Fortunately, in mid-1930s Prestone in the United States has perfected the use of ethylene glycol as a new and cooling medium, much more efficient than water. As glycol became available, the PV-12 was changed to the conventional liquid cooling system instead.

In 1936, the Air Ministry revised their requirements for new fighter aircraft. One of the most fundamental changes was the requirements for airspeed over 300 mph (480 km/h). This would clearly require an engine with more power than the planned 700 hp of the Peregrine, and having learned about the performance of the new Rolls-Royce engine, the government became instantly interested and decided to fund its subsequent development. It is then that the engine received the name Merlin. The name came from the bird – a small falcon also known as “pigeon hawk” – rather than King Arthur’s legendary magician. However, in the film The First of the Few, Sir Henry Royce refers to King Arthur’s Merlin rather than the bird, but his could be due to some propaganda purposes of the day.

Further modifications to the prototype were designated by capital letters. Thus the Merlin B became the first to use ethylene glycol cooling system; further modifications included changes to the cylinder head and cylinder block castings well as the the cooling system.

Merlin C was sufficiently developed to power the prototypes of new metal stressed-skin monoplanes for the RAF. On 6 November 1935 the prototype Hawker Hurricane (as yet unnamed), took off to its maiden flight,  powered by Merlin C producing 900hp and driving a Watts fixed-pitch two-bladed wooden propeller. This was followed by the Fairey Battle which first flew on 10 March 1936, and the Supermarine Spitfire prototype K5054 on 5 march 1936. 

Altogether, Rolls-Royce built and tested at least 33 prototype engines, ending with the production-ready Merlin F. This engine could develop 1,035 hp and was, among other trials, fitted to the Spitfire prototype.

Merlin Enters Production

The first production version Merlin I was assigned for Fairey Battle production. Only 175 had been built and these were considered to be rather unreliable. As a result, Rolls-Royce introduced an ambitious reliability-improvement programme to fix the problems. This consisted of taking random engines from the end of assembly line and running them continuously at full power until they failed. Each was then dismantled to find out which part had failed, and that part was redesigned to be stronger. After two years of this programme the Merlin had matured into one of the most reliable aero engines in the world, and could sustain eight-hour combat missions with no problems.

A particular problem with Merlin I was its ‘ramp head’ where the inlet valves were at a 45-degree angle to the cylinder. This solution was not a success and was replaced with Kestrel-style conventional flat head arrangement wherein the valves are parallel to the cylinder. This modification was designated Merlin II.

The Merlin II was introduced in 1938 and after that the production was quickly stepped up. Development of a constant-speed propeller lead to the Merlin III, which was the Merlin II adapted for the use of such propeller and provided with a constant-speed unit.

Merlin II and Merlin III were produced in great numbers, over 9700 in total. These engines powered just about every new RAF aircraft of the period: Hawker Hurricane Mk. I, Supermarine Spitfire Mk. I, Boulton-Paul Defiant Mk. I, Fairey Battle Mk. I, Hawker Henley.

The Only Alternative

As it turned out, the Peregrine saw use in only two aircraft, the Westland Whirlwind and the Gloster F9/37 prototype. Although the Peregrine appeared to be a satisfactory design, it was never allowed to fully mature; Rolls-Royce’s top priority was then troubleshooting the Merlin. More importantly, the Peregrine did not have the development potential as the Merlin, and the excellent Whirlwind fighter that was powered by a pair of Peregrines was only produced in small quantities.

The Vulture was developed, but proved unreliable owing to excessive problems with lubrication. Consequently, it had a very troubled time in development and two aircraft programmes based on the Vulture, the Avro Manchester bomber and the Hawker Tornado fighter had to be cancelled.

With the Merlin pushing into the 1,500 hp range, the Peregrine and Vulture were both cancelled in 1943.

It was supplanted in service by the Rolls-Royce Griffon which was a development of the R engine. The Griffon only became available in quantity during the last two years of the war.

Britain’s second manufacturer of in-line aero engines, Napier, was pursuing the development of 24-cylinder H-pattern engines, of indisputable potential but far greater complexity. Napier Dagger of the 1,000 hp-class was the first engine with this layout and had problems with cooling, maintenance, manufacturing and weight. These problems weren’t solved during the Dagger’s lifetime. The Dagger powered the Hawker Hector and Handley Page Hereford, both produced in strictly limited numbers and never used operationally.

The 2,000 hp Napier Sabre became available in any numbers during 1942, but the unresolved problems of its predecessor plus unreliable valves dogged its service introduction on the Hawker Typhoon during entire 1943.

Thus it was the Merlin that had to meet all Britain’s in-line aero-engine needs for the early war years.