This is a list of Rolls-Royce Merlin variants. Engines of a similar power output were typically assigned different model numbers based on supercharger or propeller gear ratios, differences in cooling system or carburettors, engine block construction, starting system, or arrangement of engine controls. All Merlin engines except versions 131 and 135 were "right hand tractor", i.e. the propeller rotated clockwise viewed from behind.
740 hp (552 kW) at 12,000-foot (3,700 m) equivalent
The initial design using an evaporative cooling system. Two built, passed bench Type Testing in July 1934. First flown 21 February 1935.[3]
Merlin B
950 hp (708 kW) at 11,000-foot (3,400 m) equivalent
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.[3]
Merlin C
950 hp (708 kW) at 11,000-foot (3,400 m) equivalent
Similar to C with minor design changes. Passed 50-hour civil test in December 1935. Failed military 100-hour test in March 1936.[5]
Merlin F
(Merlin I) Similar to C and E. First flight in Horsley 16 July 1936.[6] 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 was the first production aircraft to be powered by the Merlin I and first flew on 10 March 1936.[5]
Merlin G
1,030 hp (768 kW)
(Merlin II) Replaced "ramp" cylinder heads with parallel pattern heads (valves parallel to the cylinder) scaled up from the Kestrel engine. 400 Hour flight endurance tests carried out at RAE July 1937; Acceptance test 22 September 1937.[6] It was first widely delivered as the 1,030-horsepower (770 kW) Merlin II in 1938, and production was quickly ramped up.[5]
Variant of Merlin II with universal propeller shaft, able to be fitted with de Havilland or Rotol propellers.[2] From late 1939, using 100 octane fuel and + 12 psi (83 kPa) boost, the Merlin III developed 1,310 hp (977 kW) at 3,000 rpm at 9,000 ft (2,700 m). Later developing 1,440 hp (1,074 kW) at 3,000 rpm, +16 psi (110 kPa) boost at 5,500 ft (1,676 m) for the Sea Hurricane.[2][8] Using 87 Octane fuel the power ratings were the same as the Merlin II.[7] First production Merlin III delivered 1 July 1938.[6]
Merlin VIII
1,080 hp (805 kW) at 3,000 rpm
1,275 hp (951 kW) at 3,000 rpm, +9 psi (62 kPa) boost, sea level with 100 octane
1,130 hp (843 kW) at 3,000 rpm at 5,250 ft (1,600 m) with maximum boost pressure +10 psi (69 kPa); 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 delivered 5 December 1938.[6]
Merlin XII (RM 3S)
1,175 hp (876 kW) at 3,000 rpm
1,280 hp (954 kW) at 3,000 rpm, +12 psi (83 kPa) boost, 10,500 ft (3,200 m)
Coffman cartridge starter. First version of Merlin to use 30/70% glycol/water coolant with reinforced construction, able to use constant boost pressure of up to +12 psi (83 kPa) using 100 octane fuel.[8] First production Merlin XII, 2 September 1939.[6][9]
Merlin XX (RM 3SM)
1,280 hp (954 kW) at 3,000 rpm
1,490 hp (1,111 kW) at 3,000 rpm, +16 psi (110 kPa) boost, with 100 Octane fuel at 12,500 ft (3,810 m).
A "low altitude" version of Merlin with cropped supercharger impellers for increased power at lower altitudes, as per the Merlin XXX; fitted with a Coffman engine starter; used mainly in Fleet Air Arm aircraft.[8] First production Merlin 32 delivered 17 June 1942.[6]
Merlin 33
1,400 hp (1,044 kW) at 3,000 rpm
1,400 hp (1,044 kW) at 3,000 rpm, +9 psi (62 kPa) boost, 11,500 ft (3,505 m)
Mosquito XX (Canadian), Mosquito 40 (Australia)
Packard-built Merlin 23
Merlin 35
1,280 hp (954 kW) at 3,000 rpm, 54.3 inHg (1,380 mmHg) +12 psi (83 kPa) boost
1,245 hp (928 kW) at 3,000 rpm at 11,500 ft (3,500 m)
A variant of the Merlin XX fitted with single-stage, single-speed supercharger for low altitude Spitfire use. First production Merlin 45 delivered 13 January 1941.[6] First of specialised engines for Spitfire Mk V variants and early Seafires.
Merlin 45M
1,230 hp (917 kW) at 3,000 rpm
1,585 hp (1,182 kW) at 3,000 rpm, +18 psi (124 kPa) boost, 2,750 ft (838 m)
Low-altitude version with supercharger impeller "cropped" to 9.5 in (241 mm) in diameter.[14][15]Merlin 50 series was first to use the Bendix Stromberg "negative-g" carburettor.[16][17]
Merlin 50M (RM 5S)
1,230 hp (917 kW) at 3,000 rpm
1,585 hp (1,182 kW) at 3,000 rpm, +18 psi (124 kPa) boost, 2,750 ft (838 m)
Production variant of Merlin 60 fitted with a new two-speed two-stage supercharger providing increased power at medium to high altitudes.[19] First British production variant to incorporate two-piece cylinder blocks designed by Rolls-Royce for the Packard Merlin.[20] First production Merlin 61 delivered 2 March 1942.[6]
Merlin 62 (RM 6SM)
1,390 hp (1,037 kW) at 3,000 rpm
1,110 hp (828 kW) at 2,850 rpm, +9 psi (62 kPa) boost, 29,000 ft (8,839 m)
1,233 hp (919 kW) at 35,000 ft (10,668 m);[25] Fitted with a two-speed, two-stage supercharger and a Bendix Stromberg anti-g carburettor. Dedicated "high altitude" version used in the Westland Welkin high-altitude fighter, and some later Spitfire and de Havilland Mosquito variants.
Merlin 77 (RM 11SM)
1,250 hp (932 kW) at 3,000 rpm
1,655 hp (1,234 kW) at 3,000 rpm, +18 psi (124 kPa), 10,000 ft (3,048 m)
Redesigned "slimline" versions for the de Havilland Hornet. Engine 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 giving a maximum boost of +25 psi (172 kPa). On the Hornet the Merlin 130 was fitted in the starboard nacelle: the Merlin 131, fitted in the port nacelle, was converted to a "reverse" or left-hand tractor engine using an additional idler gear in the reduction gear casing.[26]
Capable of emergency rating of 1,795 hp (1,339 kW) at 3,000 rpm using +20 psi (138 kPa) ; civilian engine developed from Merlin 102; two-stage supercharger optimised for medium altitudes, and used an S.U. injection carburettor. Universal Power Plant (UPP) installation similar to that used on 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.[29]
Preserved Merlin XX at the Royal Air Force Museum LondonAudio of RR Merlin 66/266 starting.Parallel valve Merlin cylinder head
Air Ministry. Pilot's Notes for Spitfire Mark F.VII - Merlin 64 or 71 engine; Mark F.VIII-Merlin 63,66 or 70 engine. Air Publication 1565G & H -P.N. London, UK: Air Ministry, December 1943.
Bridgman, L. Jane's fighting aircraft of World War II. London: Crescent, 1998. ISBN0-517-67964-7
Harvey-Bailey, A. The Merlin in Perspective - the combat years. Derby, England: Rolls-Royce Heritage Trust, 1983. ISBN1-872922-06-6
Harvey-Bailey, Alec and Piggott, Dave The Merlin 100 Series – The Ultimate Military Development Derby, England: Rolls-Royce Heritage Trust, 1993. ISBN1-872922-04-X
Lumsden, Alec. British Piston Engines and their Aircraft. Marlborough, Wiltshire: Airlife Publishing, 2003. ISBN1-85310-294-6.
Price, Alfred. The Spitfire Story. London: Jane's Publishing Company Ltd., 1982. ISBN0-86720-624-1.
Robertson, Bruce. Spitfire: The Story of a Famous Fighter. Hemel Hempstead, Hertfordshire, UK: Model & Allied Publications Ltd., 1960. Third revised edition 1973. ISBN0-900435-11-9.
