Bristol Mercury

The Bristol Mercury is a nine-cylinder, air-cooled, single-row, piston radial engine. Designed by Roy Fedden of the Bristol Aeroplane Company it was used to power both civil and military aircraft of the 1930s and 1940s. Developed from the earlier Jupiter engine, later variants could produce 800 horsepower (600 kW) from its capacity of 1,500 cubic inches (25 L) by use of a geared supercharger.

Preserved Bristol Mercury VII on display at the Royal Air Force Museum London
Type Piston aero engine
Manufacturer Bristol Aeroplane Company
Designed by Roy Fedden
First run 1925
Major applications Bristol Blenheim
Gloster Gladiator
Fokker D.XXI
Number built 20,700
Developed from Bristol Jupiter
Developed into Bristol Pegasus

Almost 21,000 engines were produced, with a number also being built in Europe under licence. At least three Bristol Mercuries remain airworthy in 2010, with other preserved examples on public display in aviation museums.

Design and development

The Mercury was developed by the Bristol Aeroplane Company in 1925 as their Bristol Jupiter was reaching the end of its lifespan. Although the Mercury initially failed to attract much interest, the Air Ministry eventually funded three prototypes and it became another winner for the designer Roy Fedden.

With the widespread introduction of superchargers to the aviation industry in order to improve altitude performance, Fedden felt it was reasonable to use a small amount of boost at all times in order to improve performance of an otherwise smaller engine. Instead of designing an entirely new block, the existing Jupiter parts were re-used with the stroke reduced by one inch (25 mm). The smaller capacity engine was then boosted back to Jupiter power levels, while running at higher rpm and thus requiring a reduction gear for the propeller. The same techniques were applied to the original Jupiter-sized engine to produce the Pegasus.

The Mercury's smaller size was aimed at fighter use and it powered the Gloster Gauntlet and its successor, the Gloster Gladiator. It was intended that the larger Pegasus would be for bombers but as the power ratings of both engines rose, the Mercury found itself being used in almost all roles. Perhaps its most famous use was in a twin-engine light bomber, the Bristol Blenheim.

In 1938 Roy Fedden pressed the Air Ministry to import supplies of 100 octane aviation spirit (gasoline) from the USA. This new fuel would allow aero engines to run at higher compression ratios and supercharger boost pressure than the existing 87-octane fuel, thus increasing the power. The Mercury XV was one of the first British aero engines to be type-tested and cleared to use the 100-octane fuel in 1939. This engine was capable of running with a boost pressure of +9 lbs/ and was first used in the Blenheim Mk IV.[1]

The Mercury was also the first British aero engine to be approved for use with variable-pitch propellers.

The Bristol company and its shadow factories produced 20,700 examples of the engine.[2] Outside the United Kingdom, Mercury was licence-built in Poland and used in their PZL P.11 fighters. It was also built by NOHAB in Sweden and used in the Swedish Gloster Gladiator fighters and in the Saab 17 dive-bomber. In Italy, it was built by Alfa Romeo as the Mercurius. In Czechoslovakia it was built by Walter Engines. In Finland, it was built by Tampella and mainly used on Bristol Blenheim bombers.



Mercury I
(1926) 808 hp, direct drive. Schneider Trophy racing engine.
Mercury II
(1928) 420 hp, compression ratio 5.3:1.
Mercury IIA
(1928) 440 hp
Mercury III
(1929) 485 hp, compression ratio 4.8:1, 0.5:1 reduction gear.
Mercury IIIA
Minor modification of Mercury III.
Mercury IV
(1929) 485 hp, 0.656:1 reduction gear.
Mercury IVA
(1931) 510 hp.
Mercury IVS.2
(1932) 510 hp.
Mercury (Short stroke)
Unsuccessful experimental short stroke (5.0 in) version, 390 hp.
Mercury V
546 hp (became the Pegasus IS.2)
Mercury VIS
(1933) 605 hp, see specifications section.
Mercury VISP
(1931) 605 hp, 'P' for Persia.
Mercury VIS.2
(1933) 605 hp.
Mercury VIA
(1928) 575 hp (became the Pegasus IU.2)
Mercury VIIA
560 hp (became the Pegasus IM.2)
Mercury VIII
(1935) 825 hp, compression ratio 6.25:1, lightened engine.
Mercury VIIIA
Mercury VIII fitted with gun synchronisation gear for the Gloster Gladiator
Mercury VIIIA
535 hp, second use of VIIIA designation, (became the Pegasus IU.2P)
Mercury IX
(1935) 825 hp, lightened engine.
Mercury X
(1937) 820 hp.
Mercury XI
(1937) 820 hp.
Mercury XII

(1937) 820 hp

Mercury XV
(1938) 825 hp, developed from Mercury VIII. Converted to run on 100 Octane fuel (previously 87 Octane).
Mercury XVI
830 hp.
Mercury XX
(1940) 810 hp
Mercury 25
(1941) 825 hp. Mercury XV with crankshaft modifications.
Mercury 26
825 hp. As Mercury 25 with modified carburettor.
Mercury 30
(1941) 810 hp, Mercury XX with crankshaft modifications.
Mercury 31
(1945) 810 hp, Mercury 30 with carburettor modifications and fixed pitch propeller for Hamilcar X.




A Bristol Mercury (Mk. 20) powered Westland Lysander (G-AZWT) remains airworthy in 2017 at the Shuttleworth Collection, Old Warden, United Kingdom and is flown at home air displays throughout the summer months.[5]

A second Bristol Mercury (Mk. 30) is airworthy as of 2017 at The Shuttleworth Collection, Old Warden powering the collection's Gloster Gladiator (G-AMRK) and is flown at home air displays throughout the summer months.[6]

The Canadian Warplane Heritage Museum has a Lysander IIIA in flying condition as does the Vintage Wings of Canada.[7][8]

Engines on display

Specifications (Mercury VI-S)

Data from Lumsden[10]

General characteristics

  • Type: Nine-cylinder single-row supercharged air-cooled radial engine
  • Bore: 5.75 in (146 mm)
  • Stroke: 6.5 in (165 mm)
  • Displacement: 1,519 in³ (24.9 L)
  • Length: 47 in (1,194 mm)
  • Diameter: 51.5 in (1,307 mm)
  • Dry weight: 966 lb (438 kg)



See also

Related development

Comparable engines

Related lists



  1. Warner 2005, pp. 100, 112, 135.
  2. Bridgman (Jane's) 1998, p. 270.
  3. List from Lumsden 2003, pp. 104–108
  4. List from Lumsden, the Mercury may not be the main powerplant for these types
  5. The Shuttleworth Collection - Lysander Retrieved: 07 February 2017
  6. The Shuttleworth Collection - Gladiator Retrieved: 07 February 2017
  7. Canadian Warplane Heritage Museum - Westland Lysander Retrieved: 22 September 2010
  8. Vintage Wings - Westland Lysander Retrieved: 22 September 2010
  9. RAF Museum - Bristol Mercury Retrieved: 4 August 2009
  10. Lumsden 2003, p.105.


  • Bridgman, L, (ed.) (1998) Jane's Fighting Aircraft of World War II. Crescent. ISBN 0-517-67964-7
  • Gunston, Bill. World Encyclopedia of Aero Engines: From the Pioneers to the Present Day. 5th edition, Stroud, UK: Sutton, 2006.ISBN 0-7509-4479-X
  • Lumsden, Alec. British Piston Engines and Their Aircraft. Marlborough, Wiltshire: Airlife Publishing, 2003. ISBN 1-85310-294-6.
  • Warner, G. The Bristol Blenheim: A Complete History. London: Crécy Publishing, 2nd edition 2005. ISBN 0-85979-101-7.
  • White, Graham. Allied Aircraft Piston Engines of World War II: History and Development of Frontline Aircraft Piston Engines Produced by Great Britain and the United States During World War II. Warrendale, Pennsylvania: SAE International, 1995. ISBN 1-56091-655-9

Further reading

  • Gunston, Bill. Development of Piston Aero Engines. Cambridge, England. Patrick Stephens Limited, 2006. ISBN 0-7509-4478-1
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