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Rolls-Royce Merlin

The Rolls-Royce Merlin is a British liquid-cooled V-12 piston aero engine of 27-litre (1,650 cu in) capacity. Rolls-Royce designed the engine and first ran it in 1933 as a private venture. Initially known as the PV-12, it was later called Merlin following the company convention of naming its four-stroke piston aero engines after birds of prey. The engine benefitted from the racing experiences of precursor engines in the 1930s.

After several modifications, the first production variants of the PV-12 were completed in 1936. The first operational aircraft to enter service using the Merlin were the Fairey Battle, Hawker Hurricane and Supermarine Spitfire. The Merlin remains most closely associated with the Spitfire and Hurricane, although the majority of the production run was for the four-engined Avro Lancaster heavy bomber.

The Merlin continued to benefit from a series of rapidly-applied developments, derived from experiences in use since 1936. These markedly improved the engine's performance and durability. Starting at 1,000 hp for the first production models, most late war versions produced just under 1,800 hp, and the very latest version as used in the de Havilland Hornet over 2,000 hp.

One of the most successful aircraft engines of the World War II era, some 50 versions of the Merlin were built by Rolls-Royce in Derby, Crewe and Glasgow, as well as by Ford of Britain at their Trafford Park factory, near Manchester. A de-rated version was also the basis of the Rolls-Royce/Rover Meteor tank engine. Post-war, the Merlin was largely superseded by the Rolls-Royce Griffon for military use, with most Merlin variants being designed and built for airliners and military transport aircraft.

The Packard V-1650 was a version of the Merlin built in the United States. Production ceased in 1950 after a total of almost 150,000 engines had been delivered. Merlin engines remain in Royal Air Force service today with the Battle of Britain Memorial Flight, and power many restored aircraft in private ownership worldwide.

Origin
In the early 1930s, Rolls-Royce started planning its future aero-engine development programme and realised there was a need for an engine larger than their 21-litre (1,296 cu in) Kestrel, which was being used with great success in a number of 1930s aircraft. Consequently, work was started on a new 1100 hp-class design known as the PV-12, with PV standing for Private Venture, 12-cylinder, as the company received no government funding for work on the project. The PV-12 was first run on 15 October 1933 and first flew in a Hawker Hart biplane (serial number K3036) on 21 February 1935. The engine was originally designed to use the evaporative cooling system then in vogue. This proved unreliable and when ethylene glycol from the U.S. became available, the engine was adapted to use a conventional liquid-cooling system. The Hart was subsequently delivered to Rolls-Royce where, as a Merlin testbed, it completed over 100 hours of flying with the Merlin C and E engines.

In 1935, the Air Ministry issued a specification, F10/35, for new fighter aircraft with a minimum airspeed of 310 mph. Fortunately, two designs had been developed: the Supermarine Spitfire and the Hawker Hurricane; the latter designed in response to another specification, F36/34. Both were designed around the PV-12 instead of the Kestrel, and were the only contemporary British fighters to have been so developed. Production contracts for both aircraft were placed in 1936, and development of the PV-12 was given top priority as well as government funding. Following the company convention of naming its piston aero engines after birds of prey, Rolls-Royce named the engine the Merlin after a small, Northern Hemisphere falcon (Falco columbarius).

Two more Rolls-Royce engines developed just prior to the war were added to the company's range. The 885 hp Rolls-Royce Peregrine was an updated, supercharged development of their V-12 Kestrel design, while the 1700 hp 42-litre (2,560 cu in) Rolls-Royce Vulture used four Kestrel-sized cylinder blocks fitted to a single crankcase and driving a common crankshaft, forming an X-24 layout. This was to be used in larger aircraft such as the Avro Manchester.

Although the Peregrine appeared to be a satisfactory design, it was never allowed to mature since Rolls-Royce's priority was refining the Merlin. As a result, the Peregrine saw use in only two aircraft: the Westland Whirlwind fighter and one of the Gloster F.9/37 prototypes. The Vulture was fitted to the Avro Manchester bomber, but proved unreliable in service and the planned fighter using it – the Hawker Tornado – was cancelled as a result. With the Merlin itself soon pushing into the 1500 hp range, the Peregrine and Vulture were both cancelled in 1943, and by mid-1943 the Merlin was supplemented in service by the larger Griffon. The Griffon incorporated several design improvements and ultimately superseded the Merlin.

Development
Initially the new engine was plagued with problems such as failure of the accessory gear trains and coolant jackets. Several different construction methods were tried before the basic design of the Merlin was set. Early production Merlins were unreliable: common problems were cylinder head cracking, coolant leaks, and excessive wear to the camshafts and crankshaft main bearings.

Early engines
The prototype, developmental, and early production engine types were the:
 * PV-12
 * The initial design using an evaporative cooling system. Two built, passed bench type testing in July 1934, generating 740 hp at 12000 ft equivalent. First flown 21 February 1935.

 * Merlin B
 * Two built, ethylene glycol liquid cooling system introduced. "Ramp" cylinder heads (inlet valves were at a 45-degree angle to the cylinder). Passed Type Testing February 1935, generating 950 hp at 11000 ft equivalent.

 * Merlin C
 * Development of Merlin B; crankcase and cylinder blocks became three separate castings with bolt-on cylinder heads. First flight in Hawker Horsley 21 December 1935, 950 hp at 11000 ft.

 * Merlin E
 * Similar to C with minor design changes. Passed 50-hour civil test in December 1935 generating a constant 955 hp and a maximum rating of 1,045 hp. Failed military 100-hour test in March 1936. Powered the Supermarine Spitfire prototype.

 * Merlin F (Merlin I)
 * Similar to C and E. First flight in Horsley 16 July 1936. This became the first production engine, and was designated as the Merlin I. The Merlin continued with the "ramp" head, but this was not a success and only 172 were made. The Fairey Battle I was the first production aircraft to be powered by the Merlin I and first flew on 10 March 1936.

 * Merlin G (Merlin II)
 * Replaced "ramp" cylinder heads with parallel pattern heads (valve stems parallel to the cylinder bore axis) scaled up from the Kestrel engine. 400-hour flight endurance tests carried out at RAE July 1937; acceptance test 22 September 1937. It was first widely delivered as the 1,030 hp Merlin II in 1938, and production was quickly stepped up for Fairey Battle II.

 * Merlin III
 * Merlin II with standardised de Havilland/Rotol SBAC propeller shaft, and dual accessory-drive. 1,030 hp at 3,000 rpm at 10,250 ft at +6.5 lb boost. Formed basis for the Rolls-Royce/Rover Meteor tank engine

 * "Racing" Merlin
 * Racing engine for 1937/38 "Speed Spitfire" world speed record attempt. Merlin III with strengthened pistons, connecting rods, and gudgeon-pins, running on increased octane fuel, developed 2,160 hp at 3,200 rpm and +27 lb boost, a power/weight ratio of 0.621 lb per horsepower. Completed 15-hour endurance run at 1,800 hp, 3,200 rpm at +22 lb boost.

 * Merlin IV
 * Merlin with pressure-water cooling for Armstrong Whitworth Whitley IV.

 * Merlin V
 * Merlin for Fairey Battle V.

 * Merlin VIII
 * Medium-supercharged Merlin developed for Fairey Fulmar I, rated 1,010 hp at 2,850 rpm at 6,750 ft, 1,080 hp at 3,000 rpm for take-off using 100-octane fuel.

 * Merlin X
 * First Merlin with two-speed supercharger, 1,145 hp in low gear at 5,250 ft, 1,010 hp in high gear at 17,750 ft. First of Rolls-Royce unitised "Power Plant" installation designs for this engine in 1937 and used in Handley Page Halifax I, Vickers Wellington II, and Armstrong Whitworth Whitley V and VII.

 * Merlin XII
 * Merlin fitted with 0.477:1 reduction gear installed in some Spitfire IIs with three-bladed Rotol constant-speed propeller. Rated at 1,150 hp at 3,000 rpm at 14,000 ft.

 * Merlin XX
 * Merlin X with Stanley Hooker re-designed supercharger incorporating re-designed inlet and improved guide vanes on impeller with revised blower gear ratios; 8:15:1 for low gear, 9:49:1 for high gear. New larger SU twin choke updraught carburettor. Engine interchangeable with Merlin X. Rated at 1,240 hp at 2,850 rpm in low gear at 10,000 ft and +9 lb boost; 1,175 hp at 2,850 rpm in high gear at 17,500 ft at +9 lb boost. Revised Rolls-Royce unitised "Power Plant" installation design. Engine used in Bristol Beaufighter II, Boulton Paul Defiant II, Handley Page Halifax II and V, Hawker Hurricane II and IV, and Avro Lancaster I and III. First Merlin produced by Packard Motor Car Company as V-1650-1 and designated by Rolls-Royce as Merlin 28.

Production engines
The Merlin II and III series were the first main production versions of the engine. The Merlin III was the first version to incorporate a "universal" propeller shaft, allowing either de Havilland or Rotol manufactured propellers to be used.

The first major version to incorporate changes brought about through experience in operational service was the XX, which was designed to run on 100-octane fuel. This fuel allowed higher manifold pressures, which were achieved by increasing the boost from the centrifugal supercharger. The Merlin XX also utilised the two-speed superchargers designed by Rolls-Royce, resulting in increased power at higher altitudes than previous versions. Another improvement, introduced with the Merlin X, was the use of a 70%–30% water-glycol coolant mix rather than the 100% glycol of the earlier versions. This substantially improved engine life and reliability, removed the fire hazard of the flammable ethylene glycol, and reduced the oil leaks that had been a problem with the early Merlin I, II and III series.

The process of improvement continued, with later versions running on higher octane ratings, delivering more power. Fundamental design changes were also made to all key components, again increasing the engine's life and reliability. By the end of the war the "little" engine was delivering over 1,600 hp in common versions, and as much as 2,030 hp in the Merlin 130/131 versions specifically designed for the de Havilland Hornet. Ultimately, during tests conducted by Rolls-Royce at Derby, an RM.17.SM (the high altitude version of the Merlin 100-Series) achieved 2,640 hp at 36 lb boost (103"Hg) on 150-octane fuel with water injection.

With the end of the war, work on improving Merlin power output was halted and the development effort was concentrated on civil derivatives of the Merlin. Development of what became the "Transport Merlin" (TML) commenced with the Merlin 102 (the first Merlin to complete the new civil type-test requirements) and was aimed at improving reliability and service overhaul periods for airline operators using airliner and transport aircraft such as the Avro Lancastrian, Avro York (Merlin 500-series), Avro Tudor II & IV (Merlin 621), Tudor IVB & V (Merlin 623), TCA Canadair North Star (Merlin 724) and BOAC Argonaut (Merlin 724-IC). By 1951 the time between overhauls (TBO) was typically 650–800 hours depending on use. By then single-stage engines had accumulated 2,615,000 engine hours in civil operation, and two-stage engines 1,169,000.

In addition, an exhaust system to reduce noise levels to below those from ejector exhausts was devised for the North Star/Argonaut. This "cross-over" system took the exhaust flow from the inboard bank of cylinders up-and-over the engine before discharging the exhaust stream on the outboard side of the UPP nacelle. As a result, sound levels were reduced by between 5 and 8 decibels. The modified exhaust also conferred an increase in horsepower over the unmodified system of 38 hp, resulting in a 5 knot improvement in true air speed. Still-air range of the aircraft was also improved by around 4 per cent. The modified engine was designated the "TMO" and the modified exhaust system was supplied as kit that could be installed on existing engines either by the operator or by Rolls-Royce.

Power ratings for the civil Merlin 600, 620, and 621-series was 1160 hp continuous cruising at 23500 ft, and 1725 hp for take-off. Merlins 622–626 were rated at 1420 hp continuous cruising at 18700 ft, and 1760 hp for take-off. Engines were available with single-stage, two-speed supercharging (500-series), two-stage, two-speed supercharging (600-series), and with full intercooling, or with half intercooling/charge heating, charge heating being employed for cold area use such as in Canada. Civil Merlin engines in airline service flew 7,818,000 air miles in 1946, 17,455,000 in 1947, and 24,850,000 miles in 1948.

Basic component overview (Merlin 61)
From Jane's:
 * Cylinders
 * Twelve cylinders consisting of high-carbon steel liners set in two, two-piece cylinder blocks of cast "R.R.50" aluminium alloy having separate heads and skirts. Wet liners, ie. coolant in direct contact with external face of liners. Cylinder heads fitted with cast-iron inlet valve guides, phosphor bronze exhaust valve guides, and renewable "Silchrome" steel-alloy valve seats. Two diametrically opposed spark plugs protrude into each combustion chamber.

 * Pistons
 * Machined from "R.R.59" alloy forgings. Fully floating hollow gudgeon pins of hardened nickel-chrome steel. Three compression and one oil-control ring above the gudgeon pin, and one oil-control ring below.

 * Connecting rods
 * H-section machined nickel-steel forgings, each pair consisting of a plain and a forked rod. The forked rod carries a nickel-steel bearing block which accommodates steel-backed lead-bronze-alloy bearing shells. The "small-end" of each rod houses a floating phosphor bronze bush.

 * Crankshaft
 * One-piece, machined from a nitrogen-hardened nickel-chrome molybdenum steel forging. Statically and dynamically balanced. Seven main bearings and six throws.

 * Crankcase
 * Two aluminium-alloy castings joined together on the horizontal centreline. The upper portion bears the wheelcase, supercharger and accessories; and carries the cylinder blocks, crankshaft main bearings (split mild-steel shells lined with lead bronze alloy), and part of the housing for the airscrew reduction gear. The lower half forms an oil sump and carries the oil pumps and filters.

 * Wheelcase
 * Aluminium casting fitted to rear of crankcase. Houses drives to the camshafts, magnetos, coolant and oil pumps, supercharger, hand and electric starters, and the electric generator.

 * Valve gear
 * Two inlet and two exhaust poppet valves of "K.E.965" steel per cylinder. Both the inlet and exhaust valves have hardened "stellited" ends; while the exhaust valves also have sodium-cooled stems, and heads protected with a "Brightray" (nickel-chromium) coating. Each valve is kept closed by a pair of concentric coil-springs. A single, seven-bearing camshaft, located on the top of each cylinder head operates 24 individual steel rockers; 12 pivoting from a rocker shaft on the inner, intake side of the head to actuate the exhaust valves, the others pivoting from a shaft on the exhaust side of the head to actuate the inlet valves.

Technical improvements
Most of the Merlin's technical improvements resulted from more efficient superchargers, designed by Stanley Hooker, and the introduction of aviation fuel with increased octane ratings. Numerous detail changes were made internally and externally to the engine to withstand increased power ratings and to incorporate advances in engineering practices.

Ejector exhausts
The Merlin consumed an enormous volume of air at full power (equivalent to the volume of a single-decker bus per minute), and with the exhaust gases exiting at 1,300 mph it was realised that useful thrust could be gained simply by angling the gases backwards instead of venting sideways.

During tests, 70 pounds-force (310 N; 32 kgf) thrust at 300 mph, or roughly 70 hp was obtained, which increased the level maximum speed of the Spitfire by 10 mph to 360 mph. The first versions of the ejector exhausts featured round outlets, while subsequent versions of the system used "fishtail" style outlets, which marginally increased thrust and reduced exhaust glare for night flying.

In September 1937 the Spitfire prototype, K5054, was fitted with ejector type exhausts. Later marks of the Spitfire used a variation of this exhaust system fitted with forward-facing intake ducts to distribute hot air out to the wing-mounted guns to prevent freezing and stoppages at high altitudes, replacing an earlier system that used heated air from the engine coolant radiator. The latter system had become ineffective due to improvements to the Merlin itself which allowed higher operating altitudes where air temperatures are lower. Ejector exhausts were also fitted to other Merlin-powered aircraft.

Supercharger
Central to the success of the Merlin was the supercharger. A.C. Lovesey, an engineer who was a key figure in the design of the Merlin, delivered a lecture on the development of the Merlin in 1946; in this extract he explained the importance of the supercharger: "The impression still prevails that the static capacity known as the swept volume is the basis of comparison of the possible power output for different types of engine, but this is not the case because the output of the engine depends solely on the mass of air it can be made to consume efficiently, and in this respect the supercharger plays the most important role ... the engine has to be capable of dealing with the greater mass flows with respect to cooling, freedom from detonation and capable of withstanding high gas and inertia loads ... During the course of research and development on superchargers it became apparent to us that any further increase in the altitude performance of the Merlin engine necessitated the employment of a two-stage supercharger."

As the Merlin evolved so too did the supercharger; the latter fitting into three broad categories:
 * 1) Single-stage, single-speed gearbox: Merlin I to III, XII, 30, 40, and 50 series (1937–1942).
 * 2) Single-stage, two-speed gearbox: experimental Merlin X (1938), production Merlin XX (1940–1945).
 * 3) Two-stage, two-speed gearbox with intercooler: mainly Merlin 60, 70, and 80 series (1942–1946).

The Merlin supercharger was originally designed to allow the engine to generate maximum power at an altitude of about 16000 ft. In 1938 Stanley Hooker, an Oxford graduate in applied mathematics, explained "... I soon became very familiar with the construction of the Merlin supercharger and carburettor ... Since the supercharger was at the rear of the engine it had come in for pretty severe design treatment, and the air intake duct to the impeller looked very squashed ..." Tests conducted by Hooker showed the original intake design was inefficient, limiting the performance of the supercharger. Hooker subsequently designed a new air intake duct with improved flow characteristics, which increased maximum power at a higher altitude of over 19000 ft; and also improved the design of both the impeller, and the diffuser which controlled the airflow to it. These modifications led to the development of the single-stage Merlin XX and 45 series.

A significant advance in supercharger design was the incorporation in 1938 of a two-speed drive (designed by the French company Farman) to the impeller of the Merlin X. The later Merlin XX incorporated the two-speed drive as well as several improvements that enabled the production rate of Merlins to be increased. The low-ratio gear, which operated from takeoff to an altitude of 10000 ft, drove the impeller at 21,597 rpm and developed 1,240 hp at that height; while the high gear's (25,148 rpm) power rating was 1,175 hp at 18000 ft. These figures were achieved at 2,850 rpm engine speed using +9 pounds per square inch (1.66 atm) (48") boost.

In 1940, after receiving a request in March of that year from the Ministry of Aircraft Production for a high-rated (40000 ft) Merlin for use as an alternative engine to the turbocharged Hercules VIII used in the prototype high-altitude Vickers Wellington V bomber, Rolls-Royce started experiments on the design of a two-stage supercharger and an engine fitted with this was bench-tested in April 1941, eventually becoming the Merlin 60. The basic design used a modified Vulture supercharger for the first stage while a Merlin 46 supercharger was used for the second. A liquid-cooled intercooler on top of the supercharger casing was used to prevent the compressed air/fuel mixture from becoming too hot. Also considered was an exhaust-driven turbocharger, but although a lower fuel consumption was an advantage, the added weight and the need to add extra ducting for the exhaust flow and waste-gates meant that this option was rejected in favour of the two-stage supercharger. Fitted with the two-stage two-speed supercharger, the Merlin 60 series gained 300 hp at 30000 ft over the Merlin 45 series, at which altitude a Spitfire IX was nearly 70 mph faster than a Spitfire V.

The two-stage Merlin family was extended in 1943 with the Merlin 66, which had its supercharger geared for increased power ratings at low altitudes, and the Merlin 70 series that were designed to deliver increased power at high altitudes.

While the design of the two-stage supercharger forged ahead, Rolls-Royce also continued to develop the single-stage supercharger, resulting in 1942 in the development of a smaller "cropped" impeller for the Merlin 45M and 55M; both of these engines developed greater power at low altitudes. In squadron service the LF.V variant of the Spitfire fitted with these engines became known as the "clipped, clapped, and cropped Spitty" to indicate the shortened wingspan, the less-than-perfect condition of the used airframes, and the cropped supercharger impeller.

Carburettor developments
The use of carburettors was calculated to give a higher specific power output, due to the lower temperature, hence greater density, of the fuel/air mixture compared to injected systems. However, the Merlin's float controlled carburettor meant that if Spitfires or Hurricanes were to pitch nose down into a steep dive, negative g-force (g) produced temporary fuel starvation causing the engine to cut-out momentarily. By comparison, the contemporary Bf 109E, which had direct fuel injection, could "bunt" straight into a high-power dive to escape attack. RAF fighter pilots soon learned to avoid this with a "half-roll" of their aircraft before diving in pursuit. A restrictor in the fuel supply line together with a diaphragm fitted in the float chamber, jocularly nicknamed "Miss Shilling's orifice", after its inventor, went some way towards curing fuel starvation in a dive by containing fuel under negative G; however, at less than maximum power a fuel-rich mixture still resulted. Another improvement was made by moving the fuel outlet from the bottom of the S.U. carburettor to exactly halfway up the side, which allowed the fuel to flow equally well under negative or positive g.

Further improvements were introduced throughout the Merlin range: 1943 saw the introduction of a Bendix-Stromberg pressure carburettor that injected fuel at 5 pounds per square inch (34 kPa; 0.34 bar) through a nozzle directly into the supercharger, and was fitted to Merlin 66, 70, 76, 77 and 85 variants. The final development, which was fitted to the 100-series Merlins, was an S.U. injection carburettor that injected fuel into the supercharger using a fuel pump driven as a function of crankshaft speed and engine pressures.

Improved fuels
At the start of the war, the Merlin I, II and III ran on the then standard 87-octane aviation spirit and could generate just over 1,000 hp from its 27-litre (1,650-cu in) displacement: the maximum boost pressure at which the engine could be run using 87-octane fuel was +6 pounds per square inch (141 kPa; 1.44 atm). However, as early as 1938, at the 16th Paris Air Show, Rolls-Royce displayed two versions of the Merlin rated to use 100-octane fuel. The Merlin R.M.2M was capable of 1,265 hp at 7,870 ft, 1,285 hp at 9,180 ft and 1,320 hp on take-off; while a Merlin X with a two-speed supercharger in high gear generated 1,150 hp at 15,400 ft and 1,160 hp at 16,730 ft.

From late 1939, 100-octane fuel became available from the U.S., West Indies, Persia, and, in smaller quantities, domestically, consequently, "... in the first half of 1940 the RAF transferred all Hurricane and Spitfire squadrons to 100 octane fuel." Small modifications were made to Merlin II and III series engines, allowing an increased (emergency) boost pressure of +12 pounds per square inch (183 kPa; 1.85 atm). At this power setting these engines were able to produce 1,310 hp at 9000 ft while running at 3,000 revolutions per minute. Increased boost could be used indefinitely as there was no mechanical time limit mechanism, but pilots were advised not to use increased boost for more than a maximum of five minutes, and it was considered a "definite overload condition on the engine"; if the pilot resorted to emergency boost he had to report this on landing, when it was noted in the engine log book, while the engineering officer was required to examine the engine and reset the throttle gate. Later versions of the Merlin ran only on 100-octane fuel, and the five-minute combat limitation was raised to +18 pounds per square inch (224 kPa; 2.3 atm).

In late 1943 trials were run of a new "100/150" grade (150-octane) fuel, recognised by its bright-green colour and "awful smell". Initial tests were conducted using 6.5 cc of tetraethyllead (T.E.L.) for every one imperial gallon of 100-octane fuel (or 1.43 cc/L or 0.18 U.S. fl oz/U.S. gal), but this mixture resulted in a build-up of lead in the combustion chambers, causing excessive fouling of the spark plugs. Better results were achieved by adding 2.5% mono methyl aniline (M.M.A.) to 100-octane fuel. The new fuel allowed the five-minute boost rating of the Merlin 66 to be raised to +25 pounds per square inch (272 kPa; 2.7 atm). With this boost rating the Merlin 66 generated 2,000 hp at sea level and 1,860 hp at 10500 ft.

Starting in March 1944, the Merlin 66-powered Spitfire IXs of two Air Defence of Great Britain (ADGB) squadrons were cleared to use the new fuel for operational trials, and it was put to good use in the summer of 1944 when it enabled Spitfire L.F. Mk. IXs to intercept V-1 flying bombs coming in at low altitudes. 100/150 grade fuel was also used by Mosquito night fighters of the ADGB to intercept V-1s. In early February 1945, Spitfires of the Second Tactical Air Force (2TAF) also began using 100/150 grade fuel. This fuel was also offered to the USAAF where it was designated "PPF 44-1" and informally known as "Pep".

Production
Production of the Rolls-Royce Merlin was driven by the forethought and determination of Ernest Hives, who at times was enraged by the apparent complacency and lack of urgency encountered in his frequent correspondence with the Air Ministry, the Ministry of Aircraft Production and local authority officials. Hives was an advocate of shadow factories, and, sensing the imminent outbreak of war, pressed ahead with plans to produce the Merlin in sufficient numbers for the rapidly expanding Royal Air Force. Despite the importance of uninterrupted production, several factories were affected by industrial action. By the end of its production run in 1950, 168,176 Merlin engines had been built; over 112,000 in Britain and more than 55,000 under licence in the U.S.

Derby
The existing Rolls-Royce facilities at Osmaston, Derby were not suitable for mass engine production although the floor space had been increased by some 25% between 1935 and 1939; Hives planned to build the first two or three hundred engines there until engineering teething troubles had been resolved. To fund this expansion, the Air Ministry had provided a total of £1,927,000 by December 1939. Having a workforce that consisted mainly of design engineers and highly skilled men, the Derby factory carried out the majority of development work on the Merlin, with flight testing carried out at nearby RAF Hucknall. All the Merlin-engined aircraft taking part in the Battle of Britain had their engines assembled in the Derby factory. Total Merlin production at Derby was 32,377. The original factory closed in March 2008, but the company maintains a presence in Derby.

Crewe
To meet the increasing demand for Merlin engines, Rolls-Royce started building work on a new factory at Crewe in May 1938, with engines leaving the factory in 1939. The Crewe factory had convenient road and rail links to their existing facilities at Derby. Production at Crewe was originally planned to use unskilled labour and sub-contractors with which Hives felt there would be no particular difficulty, but the number of required sub-contracted parts such as crankshafts, camshafts and cylinder liners eventually fell short and the factory was expanded to manufacture these parts "in house".

Initially the local authority promised to build 1,000 new houses to accommodate the workforce by the end of 1938, but by February 1939 it had only awarded a contract for 100. Hives was incensed by this complacency and threatened to move the whole operation, but timely intervention by the Air Ministry improved the situation. In 1940 a strike took place when women replaced men on capstan lathes, the workers' union insisting this was a skilled labour job; however, the men returned to work after 10 days.

Total Merlin production at Crewe was 26,065.

The factory was used postwar for the production of Rolls-Royce and Bentley motor cars and military fighting vehicle power plants. In 1998 Volkswagen AG bought the Bentley marque and the factory. Today it is known as Bentley Crewe.

Glasgow
Hives further recommended that a factory be built near Glasgow to take advantage of the abundant local work force and the supply of steel and forgings from Scottish manufacturers. In September 1939, the Air Ministry allocated £4,500,000 for a new Shadow factory. This government-funded and -operated factory was built at Hillington starting in June 1939 with workers moving into the premises in October, one month after the outbreak of war. The factory was fully occupied by September 1940. A housing crisis also occurred at Glasgow, where Hives again asked the Air Ministry to step in.

With 16,000 employees, the Glasgow factory was one of the largest industrial operations in Scotland. Unlike the Derby and Crewe plants, which relied significantly on external subcontractors, it produced almost all the Merlin's components itself. Hillingdon required "a great deal of attention from Hives" from when it was producing its first complete engine; it had the highest proportion of unskilled workers in any Rolls-Royce-managed factory”. Engines began to leave the production line in November 1940, and by June 1941 monthly output had reached 200, increasing to more than 400 per month by March 1942. In total 23,675 engines were produced. Worker absenteeism became a problem after some months due to the physical and mental effects of wartime conditions such as the frequent occupation of air-raid shelters. It was agreed to cut the punishing working hours slightly to 82 hours a week, with one half-Sunday per month awarded as holiday. Record production is reported to have been 100 engines in one day.

Immediately after the war the site repaired and overhauled Merlin and Griffon engines, and continued to manufacture spare parts. Finally, following the production of the Rolls-Royce Avon turbojet and others, the factory was closed in 2005.

Manchester
The Ford Motor Company was asked to produce Merlins at Trafford Park, Stretford, near Manchester, and building work on a new factory was started in May 1940 on a 118 acre site. Built with two distinct sections to minimise potential bomb damage, it was completed in May 1941 and bombed in the same month. At first, the factory had difficulty in attracting suitable labour, and large numbers of women, youths and untrained men had to be taken on. Despite this, the first Merlin engine came off the production line one month later and it was building the engine at a rate of 200 per week by 1943, at which point the joint factories were producing 18,000 Merlins per year. In his autobiography Not much of an Engineer, Sir Stanley Hooker states: "... once the great Ford factory at Manchester started production, Merlins came out like shelling peas ...".

Some 17,316 people worked at the Trafford Park plant, including 7,260 women and two resident doctors and nurses. Merlin production started to run down in August 1945, and finally ceased on 23 March 1946.

Total Merlin production at Trafford Park was 30,428.

Detroit
As the Merlin was considered to be so important to the war effort, negotiations were started to establish an alternative production line outside the UK. Rolls-Royce staff visited North American automobile manufacturers to select one to build the Merlin in the U.S. or Canada. Henry Ford rescinded an initial offer to build the engine in the U.S. in July 1940, and the Packard Motor Car Company was selected to take on the $130,000,000 Merlin order (equivalent to $ in dollars). Agreement was reached in September 1940, and the first Packard-built engine, a Merlin XX, designated the V-1650-1, ran in August 1941. Total Merlin production by Packard was 55,523.

Six development engines were also made by Continental Motors, Inc.

Variants
This is a list of representative Merlin variants, describing some of the mechanical changes made during development of the Merlin. Engines of the same power output were typically assigned different model numbers based on supercharger or propeller gear ratios, differences in cooling system or carburettors, engine block construction, or arrangement of engine controls. Power ratings quoted are usually maximum "military" power. All but the Merlin 131 and 134 engines were "right-hand tractor", i.e. the propeller rotated clockwise when viewed from the rear. In addition to the mark numbers, Merlin engines were allocated experimental numbers by the Ministry of Supply (MoS) – e.g.: RM 8SM for the Merlin 61 and some variants – while under development; these numbers are noted where possible. Merlin engines used in Spitfires, apart from the Merlin 61, used a propeller reduction ratio of .477:1. Merlins used in bombers and other fighters used a ratio of .42:1.

Data from Bridgman (Jane's) unless otherwise noted:

 * Merlin II (RM 1S)
 * 1,030 hp at 3,000 rpm at 5500 ft using + 6 psi boost (41 kPa gauge; or an absolute pressure of 144 kPa or 1.41 atm); used 100% glycol coolant. First production Merlin II delivered 10 August 1937. Merlin II used in the Boulton Paul Defiant, Hawker Hurricane Mk.I, Supermarine Spitfire Mk.I fighters, and Fairey Battle light bomber.

 * Merlin III (RM 1S)
 * Merlin III fitted with "universal" propeller shaft able to mount either de Havilland or Rotol propellers. From late 1939, using 100-octane fuel and +12 psi boost (83 kPa gauge; or an absolute pressure of 184 kPa or 1.82 atm), the Merlin III developed 1,310 hp at 3,000 rpm at 9000 ft; using 87-octane fuel the power ratings were the same as the Merlin II. Used in the Defiant, Hurricane Mk.I, Spitfire Mk.I fighters, and Battle light bomber. First production Merlin III delivered 1 July 1938.

 * Merlin X (RM 1SM)
 * 1,130 hp at 3,000 rpm at 5250 ft; maximum boost pressure +10 psi; this was the first production Merlin to use a two-speed supercharger; Used in Halifax Mk.I, Wellington Mk.II, and Whitley Mk.V bombers. First production Merlin X, 5 December 1938.

 * Merlin XII (RM 3S)
 * 1,150 hp; fitted with Coffman engine starter; first version to use 70/30% water/glycol coolant rather than 100% glycol. Reinforced construction, able to use constant boost pressure of up to +12 psi using 100-octane fuel; Used in Spitfire Mk.II. First production Merlin XII, 2 September 1939.

 * Merlin XX (RM 3SM)
 * 1,480 hp at 3,000 rpm at 6000 ft; two-speed supercharger; boost pressure of up to +14 psi; Used in Hurricane Mk.II, Beaufighter Mk.II, Halifax Mk.II and Lancaster Mk.I bombers, and in the Spitfire Mk.III prototypes (N3297 & W3237). First production Merlin XX, 4 July 1940.

 * Merlin 32 (RM 5M)
 * 1,645 hp at 3,000 rpm at 2500 ft; a "low altitude" version of Merlin with cropped supercharger impellers for increased power at lower altitudes and a maximum boost pressure of +18 psi; fitted with Coffman engine starter; used mainly in Fleet Air Arm aircraft, mainly the Fairey Barracuda Mk.II torpedo bomber and Supermarine Seafire F. Mk.IIc fighters. Also Hurricane Mk.V and Spitfire P.R. Mk.XIII. First production Merlin 32, 17 June 1942.

 * Merlin 45 (RM 5S)
 * 1,515 hp at 3,000 rpm at 11000 ft; used in Spitfire Mk.V, PR.Mk.IV and PR.Mk.VII, Seafire Ib and IIc. Maximum boost pressure of +16 psi. First production Merlin 45, 13 January 1941.

 * Merlin 47 (RM 6S)
 * 1,415 hp at 3,000 rpm at 14000 ft; high-altitude version used in Spitfire H.F.Mk.VI. Adapted with a Marshall compressor (often called a "blower") to pressurise the cockpit. First production Merlin 47, 2 December 1941.

 * Merlin 50.M (RM 5S)
 * 1,585 hp (1,182 kW) at 3,000 rpm at 3800 ft; low-altitude version with supercharger impeller "cropped" to 9.5 in in diameter. Permitted boost was +18 psi (125 kPa gauge; or an absolute pressure of 225 kPa or 2.2 atm) instead of +16 psi (110 kPa gauge; or an absolute pressure of 210 kPa or 2.08 atm) on a normal Merlin 50 engine. Merlin 50 series was first to use the Bendix-Stromberg "negative-g" carburettor.

 * Merlin 61 (RM 8SM)
 * 1,565 hp at 3,000 rpm at 12250 ft, 1,390 hp at 3,000 rpm at 23500 ft; fitted with a new two-speed two-stage supercharger providing increased power at medium to high altitudes; +15 psi boost; used in Spitfire F Mk.IX, and P.R Mk.XI. First British production variant to incorporate two-piece cylinder blocks designed by Rolls-Royce for the Packard Merlin. Reduction gear ratio .42:1, with gears for pressurisation pump. First production Merlin 61, 2 March 1942.

 * Merlin 63 & 63A
 * 1,710 hp at 3,000 rpm at 8500 ft, 1,505 hp at 3,000 rpm at 21000 ft; strengthened two-speed two-stage development of Merlin 61; +18 psi boost; Reduction gear ratio .477:1; Merlin 63A did not have extra gears for pressurisation and incorporated a strengthened supercharger drive quill shaft. Used in Spitfire F Mk.VIII and F. Mk. IX.

 * Merlin 66 (RM 10SM)
 * 1,720 hp at 5790 ft using +18 psi boost (124 kPa gauge; or an absolute pressure of 225 kPa or 2.2 atm); low-altitude version of Merlin 63A. Fitted with a Bendix-Stromberg anti-g carburettor; intercooler used a separate header tank. Used in Spitfire L.F. Mk.VIII and L.F. Mk.IX.

 * Merlin 76/77 (RM 16SM)
 * 1,233 hp at 35000 ft; Fitted with a two-speed, two-stage supercharger and a Bendix-Stromberg carburettor. Dedicated "high altitude" version used in the Westland Welkin high-altitude fighter and some later Spitfire and de Havilland Mosquito variants. The odd-numbered mark drove a Marshall Roots-type blower for cockpit pressurising.

 * Merlin 130/131
 * 2,060 hp; redesigned "slimline" versions for the de Havilland Hornet. Engine design modified to decrease frontal area to a minimum and was the first Merlin series to use down-draught induction systems. Coolant pump moved from the bottom of the engine to the starboard side. Two-speed, two-stage supercharger and S.U. injection carburettor. Corliss throttle. Maximum boost was 25 psi gauge; or an absolute pressure of 270 kPa or 2.7 atm). On the Hornet the Merlin 130 was fitted in the port nacelle: the Merlin 131, fitted in the starboard nacelle, was converted to a "reverse" or left-hand tractor engine using an additional idler gear in the reduction gear casing.

 * Merlin 133/134
 * 2,030 hp; derated for use at low altitude 130/131 variants used in Sea Hornet F. Mk. 20, N.F. Mk. 21 and P.R. Mk. 22. Maximum boost was lowered to +18 psi gauge (230 kPa or 2.2 atm absolute).

 * Merlin 266 (RM 10SM)
 * The prefix "2" indicates engines built by Packard, otherwise as Merlin 66, optimised for low-altitude operation. Fitted to the Spitfire Mk.XVI.

 * Merlin 620
 * 1,175 hp continuous cruising using 2,650 rpm at +9 psi boost (62 kPa gauge; or an absolute pressure of 165 kPa or 1.6 atm); capable of emergency rating of 1,795 hp at 3,000 rpm using +20 psi boost (138 kPa gauge; or an absolute pressure of 241 kPa or 2.4 atm); civilian engine developed from Merlin 102; two-stage supercharger optimised for medium altitudes, and used an S.U. injection carburettor. "Universal Power Plant" (UPP) standardised annular radiator installation development of that used on Lancaster VI and Avro Lincoln. The Merlin 620–621 series was designed to operate in the severe climatic conditions encountered on Canadian and long-range North Atlantic air routes. Used in Avro Tudor, Avro York, and the Canadair North Star.

Applications
In chronological order, the first operational aircraft powered by the Merlin to enter service were the Fairey Battle, Hawker Hurricane, and Supermarine Spitfire. Although the engine is most closely associated with the Spitfire, the four-engined Avro Lancaster was the most numerous application, followed by the twin-engined de Havilland Mosquito.

List from Lumsden 2003

 * Armstrong Whitworth Whitley
 * Avro Athena
 * Avro Lancaster
 * Avro Lancastrian
 * Avro Lincoln
 * Avro Manchester III
 * Avro Tudor
 * Avro York
 * Boulton Paul Balliol and Sea Balliol
 * Boulton Paul Defiant
 * Bristol Beaufighter II
 * CAC CA-18 Mark 23 Mustang
 * Canadair North Star
 * CASA 2.111B and D
 * Cierva Air Horse
 * de Havilland Mosquito
 * de Havilland Hornet
 * Fairey Barracuda
 * Fairey Battle
 * Fairey Fulmar
 * Fairey P.4/34
 * Fiat G.59
 * Handley Page Halifax
 * Handley Page Halton
 * Hawker Hart (Test bed)
 * Hawker Henley
 * Hawker Horsley (Test bed)
 * Hawker Hotspur
 * Hawker Hurricane and Sea Hurricane
 * Hispano Aviación HA-1112
 * I.Ae. 30 Ñancú
 * Miles M.20
 * North American Mustang Mk X
 * Renard R.38
 * Short Sturgeon
 * Supermarine Type 322
 * Supermarine Seafire
 * Supermarine Spitfire
 * Tsunami Racer
 * Vickers F.7/41
 * Vickers Wellington Mk II and Mk VI
 * Vickers Windsor
 * Westland Welkin

Postwar
At the end of World War II, new versions of the Merlin (the 600- and 700-series) were designed and produced for use in commercial airliners such as the Avro Tudor, military transport aircraft such as the Avro York, and the Canadair North Star which performed in both roles. These engines were basically military specification with some minor changes to suit the different operating environment.

A Spanish-built version of the Messerschmitt Bf 109 G-2, the 1954 Hispano Aviación HA-1112-M1L Buchon, was built in Hispano's factory in Seville with the Rolls-Royce Merlin 500/45 engine of 1600 hp – a fitting powerplant for the last-produced version of the famous Messerschmitt fighter, as the Bf 109 V1 prototype aircraft had been powered by the Rolls-Royce Kestrel V-12 engine in 1935.

The CASA 2.111 was another Spanish-built version of a German aircraft, the Heinkel He 111, that was adapted to use the Merlin after the supply of Junkers Jumo 211F-2 engines ran out at the end of the war. A similar situation existed with the Fiat G.59 when available stocks of the Italian licence-built version of the Daimler-Benz DB 605 engine ran short.

The Australian built Avro Lincoln from A73-51 used Australian built Commonwealth Aircraft Corporation Merlin 102s. A total of 108 CAC Merlins were built by the time production ended.

Alternative applications
A non-supercharged version of the Merlin using a larger proportion of steel and iron components was produced for use in tanks. This engine, the Rolls-Royce Meteor, in turn led to the smaller Rolls-Royce Meteorite. In 1943, further Meteor development was handed over to Rover, in exchange for Rover's gas turbine interests.

In 1938, Rolls-Royce started work on modifying some Merlins which were later to be used in British MTBs, MGBs, and RAF Air-Sea Rescue Launches. For these the superchargers were modified single-stage units and the engine was re-engineered for use in a marine environment. Some 70 engines were converted before priority was given to producing aero engines.

Experiments were carried out by the Irish Army involving replacing the Bedford engine of a Churchill tank with a Rolls-Royce Merlin engine salvaged from an Irish Air Corps Seafire aircraft. The experiment was not a success, although the reasons are not recorded.

Surviving engines
One of the most successful of the World War II era aircraft engines, the Merlin continues to be used in many restored World War II vintage aircraft all over the world. The Royal Air Force Battle of Britain Memorial Flight is a notable current operator of the Merlin. In England the Shuttleworth Collection owns and operates a Merlin-powered Hawker Sea Hurricane IB and a Supermarine Spitfire VC – Both can be seen flying at home displays throughout the summer months.

Engines on display
Preserved examples of the Rolls-Royce Merlin are on display at the following museums:
 * Atlantic Canada Aviation Museum

 * Aviation Heritage Museum (Western Australia)

 * City of Norwich Aviation Museum in Horsham St Faith, Norfolk.

 * Montrose Air Station Heritage Centre

 * Norwegian Museum of Science and Technology, Oslo, Norway

 * Polish Aviation Museum, Kraków (Cracow), Poland

 * Rolls-Royce Heritage Centre, Derby – several versions, including displayed superchargers, reduction gears and other components

 * Royal Air Force Museum, Cosford & London

 * Science Museum (London)

 * Shuttleworth Collection

 * Smithsonian Air and Space Museum, Washington, DC

 * Wings Museum, West Sussex, England

title: Rolls-Royce Merlin


text:
Air Ministry

The Air Ministry was a department of the Government of the United Kingdom with the responsibility of managing the affairs of the Royal Air Force, that existed from 1918 to 1964. It was under the political authority of the Secretary of State for Air.

The Air Committee
On 13 April 1912, less than two weeks after the creation of the Royal Flying Corps (which initially consisted of both a naval and a military wing), an Air Committee was established to act as an intermediary between the Admiralty and the War Office in matters relating to aviation. The new Air Committee was composed of representatives of the two war ministries, and although it could make recommendations, it lacked executive authority. The recommendations of the Air Committee had to be ratified by the Admiralty Board and the Imperial General Staff and, in consequence, the Committee was not particularly effective. The increasing separation of army and naval aviation from 1912 to 1914 only exacerbated the Air Committee's ineffectiveness and the Committee did not meet after the outbreak of the First World War.

The Joint War Air Committee
By 1916 the lack of co-ordination of the Army's Royal Flying Corps and the Navy's Royal Naval Air Service had led to serious problems, not only in the procurement of aircraft engines, but also in the air defence of Great Britain. It was the supply problems to which an attempt at rectification was first made. The War Committee meeting on 15 February 1916 decided immediately to establish a standing joint naval and military committee to co-ordinate both the design and the supply of materiel for the two air services. This committee was titled the Joint War Air Committee, and its chairman was Lord Derby. It was also at the meeting on 15 February that Lord Curzon proposed the creation of an Air Ministry. As with the pre-war Air Committee, the Joint War Air Committee lacked any executive powers and therefore was not effective. After only eight sittings, Lord Derby resigned from the Committee, stating that "It appears to me quite impossible to bring the two wings closer together ... unless and until the whole system of the Air Service is changed and they are amalgamated into one service."

Membership
The Joint War Air Committee was composed as follows:
 * Chairman – Lord Derby
 * Director of Air Services (Admiralty) – Rear Admiral C L Vaughn Lee
 * Superintendent of Aircraft Design (Admiralty) – Commodore M F Sueter
 * Squadron Commander W Briggs
 * Director of Military Aeronautics (War Office) – Major-General Sir David Henderson
 * Lieutenant-Colonel E L Ellington

Advisory Members were also appointed as required.

The first Air Board
The next attempt to establish effective co-ordination between the two air services was the creation of an Air Board. The first Air Board came into being on 15 May 1916 with Lord Curzon as its chairman. The inclusion of Curzon, a Cabinet Minister, and other political figures was intended to give the Air Board greater status than the Joint War Air Committee. In October 1916 the Air Board published its first report which was highly critical of the arrangements within the British air services. The report noted that although the Army authorities were ready and willing to provide information and take part in meetings, the Navy were often absent from Board meetings and frequently refused to provide information on naval aviation.

The second Air Board
In January 1917 the Prime Minister David Lloyd George replaced the chairman Lord Curzon with Lord Cowdray. Godfrey Paine, who served in the newly created post of Fifth Sea Lord and Director of Naval Aviation, sat on the board and this high level representation from the Navy helped to improve matters. Additionally, as responsibility for the design of aircraft had been moved out of single service hands and given to the Ministry of Munitions, some of the problems of inter-service competition were avoided.

The Air Board initially met in the Hotel Cecil on the Strand, familiarly known as the "Hotel Bolo". This was a humorous reference to Bolo Pasha (shot for treason in 1918 by the French government) whose attempts to undermine the French war effort with German-funded newspaper propaganda were likened to the unhelpful rivalry between the RFC and the RNAS.

Establishment of the Air Ministry
Despite attempts at reorganization of the Air Board, the earlier problems failed to be completely resolved. In addition, the growing number of German air raids against Great Britain led to public disquiet and increasing demands for something to be done. As a result, Lloyd George, the British Prime Minister, established a committee composed of himself and General Jan Smuts, which was tasked with investigating the problems with the British air defences and organizational difficulties which had beset the Air Board.

Towards the end of the First World War, on 17 August 1917, General Smuts presented a report to the War Council on the future of air power. Because of its potential for the 'devastation of enemy lands and the destruction of industrial and populous centres on a vast scale', he recommended a new air service be formed that would be on a level with the Army and Royal Navy. The new air service was to receive direction from a new ministry and on 29 November 1917 the Air Force Bill received Royal Assent and the Air Ministry was formed just over a month later on 2 January 1918. Lord Rothermere was appointed the first Air Minister. On 3 January, the Air Council was constituted as follows:

 * Lord Rothermere, Air Minister and President
 * Lieutenant-General Sir David Henderson, Additional Member and Vice-President
 * Major-General Sir Hugh Trenchard, Chief of the Air Staff
 * Major-General (formerly Rear-Admiral) Mark Kerr, Deputy Chief of the Air Staff
 * Major-General (formerly Commodore) Godfrey Paine, Master General of Personnel
 * Major-General Sefton Brancker, Controller-General of Equipment
 * Sir William Weir, Director-General of Aircraft Production in the Ministry of Munitions
 * Sir John Hunter, Administrator of Works and Buildings
 * Major J L Baird Permanent Under-Secretary

The Air Ministry continued to meet in the Hotel Cecil on the Strand. Later, in 1919, it moved to Adastral House on Kingsway. The creation of the Air Ministry resulted in the disestablishment of the Army Council's post of Director-General of Military Aeronautics.

1918–1921
In 1919 the RAF and the Air Ministry came under immense political and inter service pressure for their very existence, particularly in a climate of significantly reduced military expenditure. The battle was kickstarted by the resignation in December 1918 of William Weir the President of the Air Council (the governing body of the Royal Air Force), who wished to return to his commercial activities.

This led the Prime Minister, Lloyd George, to create a Secretary of State for Air, but not as a Cabinet position, and on 9 January 1919 offered Winston Churchill the two posts of Secretary of State for War, which was a Cabinet position, and Secretary of State for Air both of which he accepted.

This combination under one person by was criticised in both the press and Parliament. However, Churchill re-iterated that the continued "integrity, the unity, the independence of the Royal Air Force will be sedulously and carefully maintained". During 1919 it was also decided that civil aviation was to be brought into the Air Ministry rather than being dealt with by either the Board of Trade or the Foreign Office.

The Army and the War Office had largely agreed to the continued existence of the RAF due, in part, to the enthusiasm for the air service by the Army's political leader Winston Churchill. However, one of the main difficulties for the RAF and Air Ministry in 1919 was the opposition by the Royal Navy to losing their own air service and subsequent lobbying that personnel for naval air purposes afloat be naval officers and ratings – this would have led to a recreation of the now disbanded Royal Naval Air Service. This negotiation led to the creation of RAF Coastal Area the predecessor of RAF Coastal Command to deal with its relationship with the Navy. Throughout 1919 there were discussions between Sir Hugh Trenchard Chief of the Air Staff and Sir Rosslyn Wemyss First Sea Lord as to the nature of the relationship between the Air Force and Air Ministry and the Navy and the Admiralty.

In 1919 the Air Ministry formally took control of supply, design and inspection of all aircraft (aeroplanes and airships) from the Ministry of Munitions. This helped put the existence of Air Ministry on a firmer footing.

Throughout 1919 Churchill persistently supported an independent air force. He presented the White Paper, largely written by Sir Hugh Trenchard, on the future of the RAF on 12 December 1919. It was this White Paper that was to be the effective charter for the RAF and Air Ministry in subsequent years.

1921–1927
In February 1921 Lloyd George appointed Churchill to the Colonial Office and appointed his Chief Whip, Frederick Guest as Secretary of State for Air on 1 April. During his eighteen months in office he played "a minor part in the desperate struggle to maintain the air force's institutional independence in the face of hostile attacks from the War Office and the Admiralty". More importantly in the long term he was also responsible for the appointment of Sir Sefton Brancker to develop civil aviation.

With the fall of Lloyd George Sir Samuel Hoare became the Secretary of State for Air in October 1922 under Bonar Law. On Law's death Stanley Baldwin became Prime Minister and gave the position Cabinet status in May 1923, and Hoare remained in the post until January 1924, when a Labour government took power. Lord Thomson was made Secretary of State for Air. A supporter of airships, Thomson was responsible for the Imperial Airship Scheme, which involved the construction of R101 at the Royal Airship Works at Cardington.

After the fall of the MacDonald government in November 1924 Hoare returned to the Air Ministry. He was interested in developing air links to the Empire and Dominion countries, particularly India and South Africa. He negotiated a subsidy from the Treasury for Imperial Airways to start a service from Cairo to India. Hoare, with his wife Lady Maud, flew on the inaugural 13-day flight to Delhi, leaving Croydon on 26 December 1926 and arriving on 8 January 1927. The air route to Cape Town, after much negotiation, was finalised in 1929, before he left office, but only commenced in 1932.

1927–1929
His time at the Air Ministry was marked by several important developments that were to confirm the status of the Royal Air Force as a separate entity, play a part in the growth of civil aviation and to develop the awareness of the public about aviation.

An early priority for Sir Hugh Trenchard, Chief of the Air Staff 1919–1930, was to establish the officer cadet training college at Cranwell as a permanent establishment. It was Hoare's job to negotiate with the Treasury for the necessary funds. After much resistance Hoare managed to include a provision for permanent buildings in his estimates for 1929. The foundation stone of the Royal Air Force College Cranwell was laid in 1929 and formally opened in 1934.

Trenchard had conceived the idea of a university air officer training corps, a sort of Territorial Army for the R.A.F. Hoare and particularly his well connected Parliamentary Private Secretary the academic Sir Geoffrey Butler, then created University Air Squadrons, at Cambridge University then at Oxford University in October 1925, without, however the militarism of the Officer Training Corps and in close collaboration with scientific and engineering work of the Universities.

The Air Ministry was also responsible for civil aviation. Early on Hoare set up the Civil Air Transport Subsidies Committee under the Chairmanship of Sir Hubert Hambling to look at the system of subsidies to competing air lines. They reported in February 1923, favouring a single commercial company to run Britain's air routes. In March 1924 Imperial Airways was created from a merger of the four largest airlines.

The third aspect of Hoare's time at the Air Ministry (after the R.A.F. and civil airlines) was to make public opinion sympathetic to air power and air travel. His much publicised flight to India in 1926-7 was part of this. He also realised the importance of the Schneider Trophy and was instrumental in making sure that the R.A.F was involved. Britain's winning entries in 1927, 1929 and 1931 were flown by R.A.F. pilots and the teams partially subsidised by the Air Ministry.

Aircraft production
The Air Ministry issued specifications for aircraft that British aircraft companies would supply prototypes to. These were then assessed, if ordered the Ministry assigned the aircraft name. (see List of Air Ministry specifications).

The ordering procedure used I.T.P. (Intention to Proceed) contract papers; these specified a maximum fixed price, which could (after investigation) be less. But when Lord Nuffield got the I.T.P. contract papers for a Wolseley radial aero engine, which would have required re-orientation of their offices with an army of chartered accountants, he decided to deal only with the War Office and the Admiralty, not the Air Ministry. So the aero engine project was abandoned in 1936, see Airspeed. Nevil Shute Norway wrote that the loss of such a technically advanced engine was a great loss to Britain as well as Airspeed, and blamed the over-cautious high civil servants of the Air Ministry. When he had asked Lord Nuffield to retain the engine, Nuffield said: I tell you, Norway ... I sent that I.T.P. thing back to them, and I told them they could put it where the monkey put the nuts!

In later years the actual production of aircraft was the responsibility of the Ministry of Aircraft Production (1940–46), the Ministry of Supply (1946–59), the Ministry of Aviation (1959–67) and finally the Ministry of Technology (1967–70).

Research and development
In the 1920s and early 1930s research and development was more than 20% of the Air Ministry’s total expenditure on aircraft and equipment, making it the largest research and development spending institution in Britain, until it was outstripped by private industry in the later 1930s.

Weather forecasting
The Air Ministry was responsible for weather forecasting over the UK, from 1919 it being the government department responsible for the Meteorological Office.

As a result of the need for weather information for aviation, the Meteorological Office located many of its observation and data collection points on RAF stations.

World War II technology
In the 1930s, the Air Ministry commissioned a scientific study of propagating electromagnetic energy which concluded that a death ray was impractical but detection of aircraft appeared feasible. Robert Watson-Watt demonstrated a working prototype and patented the device in 1935 (British Patent GB593017). The device served as the base for the Chain Home network of radars to defend Great Britain.

By April 1944, the ministry's air Intelligence branch had succeeded in its intelligence efforts regarding "the beams, the Bruneval Raid, the Gibraltar barrage, radar, Window, heavy water, and the German nightfighters" (R.V. Jones). Other World War II technology and warfare efforts included the branch's V-1 and V-2 Intelligence activities.

Abolition
In 1964 the Air Ministry merged with the Admiralty and the War Office to form the Ministry of Defence.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Air Ministry