Rolls-Royce Merlin

The Merlin was a 12 cylinder, 60° "V", 27 litre, liquid cooled piston aircraft engine built during World War II by Rolls-Royce and under licence in the United States by Packard. It was one of the three best aero engines in the world at that time, and powered many notable warplanes.

The Merlin name came from a bird (a small falcon also known as "pigeon hawk") rather than King Arthur's legendary magician.

History

In the early 1930s Rolls started planning for the future of its aero engine development programmes, and eventually settled on having two basic designs. The 700 horsepower (500 kW) Rolls-Royce Peregrine was an updated, supercharged development of their existing V-12, 22 L Rolls-Royce Kestrel which had been used to great success in a number of 1930's designs. Two Peregrines bolted together on a common crankshaft into an X-24 layout would create the 1,700 hp (1,300 kW) 44 L Rolls-Royce Vulture, for use in larger planes like bombers. There was also the possibility that the famous 36 L 'R' engine from the Supermarine racing planes could be developed into a 1,500 hp (1,100 kW) class engine of its own, itself a development of the Rolls-Royce Buzzard, a scaled up Kestrel.

However this plan left a large gap between 700 and 1,500 hp (500 and 1,100 kW). To fill the gap work was started on a new 1,100 hp (820 kW) class design as the PV-12 – PV for "private venture" as they received no money for work on the project. It first flew on the front of a Hawker Hart biplane in 1935, using the new evaporative cooling system then in vogue. The cooling system proved to be somewhat suspect, and when supplies of ethylene glycol (Prestone) from the US became available, the engine was switched to this system instead.

In 1936 the Air Ministry called for new fighter aircraft with airspeeds that would eventually have to be over 300 mph (480 km/h). Two designs were eventually selected for development, the Hawker Hurricane and Supermarine Spitfire. Both were designed around the PV-12 instead of the Kestrel, and were the only modern fighters on the drawing boards. The PV-12 was instantly catapulted to the top of the supply chain and became the Merlin. First widely delivered as the 1,030 hp (770 kW) Merlin II in 1938, production was ramped up quickly. The Merlin I had a 'ramp head' where the inlet valves were at a 45 degree angle to the cylinder. This was not a success and only 172 were made before the conventional flat head arrangement where the valves are parallel to the cylinder was adopted for the Merlin II.

Early Merlins were considered to be rather unreliable, but their importance was too great for this to be left alone. Rolls soon introduced a superb quality control programme to address this. The programme consisted of taking random engines right off the end of assembly line and running them continuously at full power until they broke. They were then disassembled to find out which part had failed, and that part was redesigned to be stronger. After two years of this the Merlin matured into one of the most reliable aero engines in the world, and could be run at full power for entire eight hour bombing missions without complaint.

As it turns out, the Peregrine saw use in only two aircraft, the Westland Whirlwind and the Gloster F9/37. Although the Peregrine appeared to be a satisfactory design, it was never allowed to mature as Rolls-Royce's priority was troubleshooting the Merlin. The Vulture was fitted to the Hawker Tornado and Avro Manchester, but proved unreliable due to big-end failures caused by lubrication problems. With the Merlin soon pushing into the 1,500 hp (1,100 kW) range on its own, both engines were cancelled in 1943.

By the end of its production run, over 150,000 Merlin engines were built.

Upgrades

Most of the upgrades to the Merlin were the result of ever-increasing octane ratings in the aviation fuel available from the US, and ever more efficient supercharger designs. At the start of the war the engine ran on the then-standard 87 octane aviation spirit and could supply just over 1,000 hp (750 kW) from its 27 L displacement compared to 1,100 hp (820 kW) from the 34 L Daimler-Benz DB 601.

The next major version was the XX which ran on 100 octane fuel. This allowed it to be run at higher manifold pressures, which was achieved by increasing the "boost" from the centrifugal type supercharger. The result was that the otherwise similar engine delivered 1,300 hp (970 kW). This process continued, with later versions running on ever-increasing octane ratings, delivering ever-increasing power ratings. By the end of the war the "little" engine was delivering over 1,600 hp (1,200 kW) in common versions, and could deliver 2,070 hp (1,544 kW) in the Merlin 130/131 versions used on the de Havilland Hornet.

Comparisons to Luftwaffe engines

In comparison, the Luftwaffe had no similar ability to increase octane ratings, and had to continually introduce larger and larger engines to maintain parity. The result was that their planes had considerably worse power-to-weight ratios than the Merlin-powered planes they faced, and the continual, and often complete, change in engines designs meant that shortages were commonplace. This exacerbated one of the major problems for the Luftwaffe that had plagued them from the mid-1930s until the end of the war.

Other uses for the engine

An unsupercharged version of the Merlin was also produced for use in tanks, the Rolls-Royce Meteor, which in turn led to the half-size "Meteorite".

A Spanish built version of the Messerschmitt Bf 109 G-2, the Hispano Aviacion Ha 1112 M. 1. L Buchon, was built with the Rolls-Royce Merlin 500/45 of 1,600 hp engine; with a four-bladed propeller, in Hispano Aviacion factory in Seville

Packard's legacy

The Merlin was considered to be so important to the war effort that blueprints were sent to the US for safekeeping, to be handed over in case of the UK's capitulation. When this was no longer an issue in 1943, the Packard company started production in the US as the V-1650, originally for use in US-built Spitfires. The V-1650 performed so much better than its US counterpart (the Allison V-1710) that it would eventually replace it in the P-51 Mustang, which then went on to be viewed as one of the best fighters of the war.

When the first of the Packard-built Merlins arrived in Britain, the engineers at Rolls-Royce stripped it down and were amazed to find that the production-line built Packard engine, far from being as bad as they expected it to be for component tolerance, was actually better. Up until then, RR Merlins were hand built, every face being finished off by hand and this time-consuming process placed great strain on the production cabability of the skilled workforce involved in the manufacture of these engines. The Packard engine changed many minds, although there were still some at RR who remained unconvinced of the quality of the American engine, produced as it was by a largely unskilled and semi-skilled female workforce. In the end, the engine's proformance removed any doubts about its quality and workmanship.

Although it is not commonly known, Packard greatly improved the maintainability of the engine, and their changes were also incorporated in subsequent British production.

Automobile use

Michael Wilcock of Sussex, England built the Swandean Spitfire Special, using a Merlin XXV engined acquired from a a scrap yard for fifty pounds. The engine was installed in a home-brewed chassis confected from two Daimler Dingo scout car chassis. The car was run in the Brighton Speed Trials in 1953, and was sold to James Duffy of St. Louis, Missouri in 1956. As of 2005, the vehicle is still in St. Louis, where it is undergoing restoration.

In the 1960s Paul Jamison put a Merlin engine (some say it actually was a Rover built Rolls-Royce Meteor, which was a de-tuned Merlin without superchargers and with steel components replacing some aluminium ones) into a chassis he had built himself. He did not get around to building a body, and sold the car to Epsom automatic transmission specialist John Dodds, who fitted a fibreglass body based on the shape of the Ford Capri and named the machine "The Beast". Originally it had a grille from a Rolls Royce, but after complaints from them he had to change it. According to Dodds' account he once drove by a Porsche driver on the autobahn who then called Rolls Royce asking about their "new model". The Beast was once listed in the Guinness Book of Records as the world's most powerful road car. The engine came from a Boulton Paul Balliol training aircraft which would give 1,262 hp (941 kW) at 8,500 feet (2,600 m). No supercharger was fitted to the engine in car so it "only" delivered about 850 hp (630 kW). The car used a General Motors TH400 automatic transmission. The Beast is alive and well in Marbella, Spain and is still owned by Dodds.

In the mid-1970's Jamison designed a second Merlin-powered car. This one had six wheels - two in front and four driven at the rear - and a mid-engined layout. The vehicle was featured by one of the British weekly motoring magazines (either "Autocar" or "Motor"), and is said currently to reside in a museum in Sweden.

Around 1990, Jamison began work on a third Merlin-powered car, using a genuine 1930's Rolls-Royce chassis, but this vehicle remained uncompleted at the time of his death.

Recently in Australia, Rod Hadfield, of the Castlemaine Rod Shop, used the engine in a 1955 Chevrolet BelAir Sports Coupe, which was named "Final Objective."^*

Variants

This is an incomplete list of representative Merlin variants. Engines of the same power output were typically assigned different model numbers based on supercharger or propeller gear ratios, differences in cooling system or carburetors, engine block construction, or arrangement of engine controls.

Merlin II or III - 1,040 hp (775 kW) at 3,000 rpm at 5,500 ft (1,680 m); used in Spitfire Mk.I and Hurricane Mk.I fighters.
Merlin X - 1,130 hp (840 kW) at 3,000 rpm at 5,250 ft (1,525 m); used in Halifax Mk.I, Wellington Mk.II, and Whitley Mk.V bombers.
Merlin XX - 1,480 hp (1,105 kW) at 3,000 rpm at 6,000 ft (1,830 m); used in Hurricane Mk.II and Beaufighter Mk.II fighters, Halifax Mk.II and Lancaster Mk.I bombers.
Merlin 32 - 1,645 hp (1,230 kW) at 3,000 rpm at 2,500 ft (760 m); used in Barracuda Mk.II bomber.
Merlin 45 - 1,470 hp (1,100 kW) at 3,000 rpm at 9,250 ft (2,820 m); used in Spitfire Mk.V
Merlin 46 - 1,415 hp (1,055 kW) at 3,000 rpm at 14,000 ft (4,270 m); high-altitude version used in Spitfire PR.Mk.IV and PR.Mk.VII
Merlin 61 - fitted with a new two-speed two-stage supercharger providing 1,565 hp (1,170 kW) at 3,000 rpm at 12,250 ft (3,740 m), and 1,390 hp (1,035 kW) at 3,000 rpm 23,500 ft (7,170 m); high-altitude version used in Spitfire Mk.VII, Mk.VIII, Mk.IX, and PR.Mk.XI

Specifications (Merlin 61)

References

Bridgman, L. (ed.) (1998) Jane's fighting aircraft of World War II. Crescent. ISBN 0517679647
Rubbra, AA. (1990) Rolls-Royce piston aero engines: A designer remembers. Rolls-Royce Heritage Trust. ISBN 1872922007

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