The Hercules powered a number of aircraft types, including Bristol's own Beaufighterheavy fighter design, although it was more commonly used on bombers. The Hercules also saw use in civilian designs, culminating in the 735 and 737 engines for such as the Handley Page Hastings C1 and C3 and Bristol Freighter. The design was also licensed for production in France by SNECMA.
Design and development
Shortly after the end of World War I, the Shell company, Asiatic Petroleum, commissioned Harry Ricardo to investigate problems of fuel and engines. His book was published in 1923 as “The Internal Combustion Engine”.[1] Ricardo postulated that the days of the poppet valve were numbered and that a sleeve valve alternative should be pursued.[2]
The rationale behind the single sleeve valve design was two-fold: to provide optimum intake and exhaust gas flow in a two-row radial engine, improving its volumetric efficiency and to allow higher compression ratios, thus improving its thermal efficiency. The arrangement of the cylinders in two-row radials made it very difficult to utilise four valves per cylinder, consequently all non-sleeve valve two- and four-row radials were limited to the less efficient two-valve configuration. Also, as combustion chambers of sleeve-valve engines are uncluttered by valves, especially hot exhaust valves, so being comparatively smooth they allow engines to work with lower octane number fuels using the same compression ratio. Conversely, the same octane number fuel may be utilised while employing a higher compression ratio, or supercharger pressure, thus attaining either higher economy or power output. The downside was the difficulty in maintaining sufficient cylinder and sleeve lubrication.
Manufacturing was also a major problem. Sleeve valve engines, even the mono valve Fedden had elected to use, were extremely difficult to make. Fedden had experimented with sleeve valves in an inverted V-12 as early as 1927 but did not pursue that engine any further. Reverting to nine cylinder engines, Bristol had developed a sleeve valve engine that would actually work by 1934, introducing their first sleeve-valve designs in the 750 horsepower (560 kilowatts) class Perseus and the 500hp (370kW) class Aquila that they intended to supply throughout the 1930s. Aircraft development in the era was so rapid that both engines quickly ended up at the low-power end of the military market and, in order to deliver larger engines, Bristol developed 14-cylinder versions of both. The Perseus evolved into the Hercules, and the Aquila into the Taurus.
These smooth-running engines were largely hand-built, which was incompatible with the needs of wartime production. At that time, the tolerances were simply not sufficiently accurate to ensure the mass production of reliable engines. Fedden drove his teams mercilessly, at both Bristol and its suppliers, and thousands of combinations of alloys and methods were tried before a process was discovered which used centrifugal casting to make the sleeves perfectly round. This final success arrived just before the start of the Second World War.[2]
In 1937 Bristol acquired a Northrop Model 8A-1, the export version of the A-17 attack bomber, and modified it as a testbed for the first Hercules engines.[3]
In 1939 Bristol developed a modular engine installation for the Hercules, a so-called "power-egg", allowing the complete engine and cowling to be fitted to any suitable aircraft.[4]
A total of over 57,400 Hercules engines were built.
Variants
Hercules fitted to a Vickers Varsity on display at the Newark Air MuseumBristol Hercules in Aviation Museum Kbely, PragueBristol Hercules engine. Note the absence of pushrods on the cylinders. Each cylinder has two exhaust ports on the front (short L-shaped tubes) and three intake ports on the back supplied through a single manifold.Bristol Hercules XVII engine
Hercules I (1936) – 1,150hp (860kW), single-speed supercharger, run on 87 octane fuel.[5]
Hercules II (1938) – 1,375hp (1,025kW), single-speed supercharger, run on 87 octane fuel.[5]
Hercules III (1939) – 1,400hp (1,000kW), two-speed supercharger, run on either 87 or 100 octane fuel.[6]
Hercules IV (1939) – 1,380hp (1,030kW), single-speed supercharger, run on 87 octane fuel.[6]
Hercules V (1939) – 1,380hp (1,030kW), civil prototype derived from the Hercules IV but not developed.[6]
Hercules VI (1941) – 1,615hp (1,204kW), two-speed supercharger, run on either 87 or 100 octane fuel.[6]
Hercules VIII – 1,650hp (1,230kW), very high-altitude version of the Hercules II, single-speed supercharger with an auxiliary high-altitude single-speed 'S' supercharger.[6]
Hercules X (1941) – 1,420hp (1,060kW), derived from the Hercules III.[7]
Hercules XI (1941) – 1,590hp (1,190kW), derived from the Hercules III, run on 100 octane fuel.[8]
Hercules XIV (1942) – 1,500hp (1,100kW), developed for the civil market and used by BOAC, run on 100 octane fuel.[8]
Hercules XVMT – 1,650hp (1,230kW), very high-altitude development of the Hercules II, single-speed supercharger with an auxiliary high-altitude turbo-supercharger.[8]
Hercules XVI (1942) – 1,615hp (1,204kW), two-speed supercharger, run on either 87 or 100 octane fuel.[8]
Hercules XVII (1943) – 1,615hp (1,204kW), two-speed supercharger locked in 'M' gear.[8]
Hercules XVIII – low-level development of the Hercules VI with cropped 12in (300mm) supercharger impellers.[8]
Hercules XIX (1943) – 1,725hp (1,286kW), a development of the Hercules XVII, the two-speed supercharger had cropped 12in (300mm) impellers locked in 'M' gear.[8]
Hercules 36 – a development engine derived from the Hercules VI and Hercules XVI, run on 100 octane fuel. The Hercules 38 was a further development of the Hercules 36.[8]
Hercules 100 (1944) – 1,675hp (1,249kW), the first in a new sub-series of Hercules engines designed primarily for the impending post-war civil market. The entire series was split, some versions had standard epicyclic reduction gearing and parallel versions had a new torquemeter-type reduction gearing.[9]
Hercules 101 – 1,675hp (1,249kW), developed from the Hercules 100. The Hercules 103 was the torquemeter version. The Hercules 110 was a further development of the Hercules 101.[10]
Hercules 105 – 1,675hp (1,249kW), developed from the Hercules 101 with modified supercharger gears.[10]
Hercules 106 – 1,675hp (1,249kW), developed from the Hercules 101. The Hercules 107 was the torquemeter version.[10]
Hercules 120 – 1,715hp (1,279kW), high-altitude development of the Hercules 101. The Hercules 121 was the torquemeter version. The Hercules 200 was further modified version of the Hercules 120.[10]
Hercules 130 – 1,715hp (1,279kW), development of the Hercules 100. The Hercules 134 was a development with modified mounting ring and exhaust pipes for a rear manifold.[10]
Hercules 216 – 1,675hp (1,249kW), development of the Hercules 106 with the Hercules 230 power section and single speed supercharger. Applications:[10]
Hercules 230 – 1,925hp (1,435kW), development of the Hercules 130 with the re-designed power section and modified mounting ring and exhaust pipes for a rear manifold. The Hercules 270 was a development. The Hercules 231 and Hercules 271 were the torquemeter versions.[11]
Hercules 232 – modified development of the Hercules 230 for improved performance. The Hercules 233 was the torquemeter version.[10]
Hercules 234 – modified development of the Hercules 232. The Hercules 235 was the torquemeter version. The Hercules 238 was a military version of the Hercules 734 which itself was based on the Hercules 234.[10]
Hercules 260 – modified development of the Hercules 230 to suit reversible propellers. The Hercules 261 was the torquemeter version.[12]
Hercules 264 – 1,950hp (1,450kW), a development of the Hercules 260. The Hercules 265 was the torquemeter version.[12]
Hercules 268 – a further development of the Hercules 260. The Hercules 269 was the torquemeter version.[12]
Hercules 630 – 1,675hp (1,249kW), a civil development of the Hercules 100. The Hercules 631 was the torquemeter version.[12]
Hercules 632 – 1,690hp (1,260kW), a civil-series engine developed from the Hercules 630. The Hercules 633 was the torquemeter version. The Hercules 638 and Hercules 672, along with their torquemeter versions the Hercules 639 and Hercules 673 were developments of the Hercules 632.[13]
Hercules 634 – 1,690hp (1,260kW), a civil-series engine developed from the Hercules 630 with modified mounting ring and exhaust pipes for a rear manifold. The Hercules 635 was the torquemeter version.[12]
Hercules 636 – a civil-series engine developed from the Hercules 630 with modified mounting ring and exhaust pipes for a rear manifold. The Hercules 637 was the torquemeter version. The Hercules 637-2 and Hercules 637-3 were further torquemeter developments.[12]
Hercules 730 – 2,040hp (1,520kW), a civil-series engine developed from the Hercules 230 and 630 with improved power section, the Hercules 731 was the torquemeter version.[14]
Hercules 732 – a civil-series engine developed from the Hercules 730 with modified mounting ring and exhaust pipes for a rear manifold. The Hercules 733 was the torquemeter version.[14]
Hercules 734 – 1,980hp (1,480kW), a civil-series engine developed from the Hercules 730. The Hercules 735 was the torquemeter version. The Hercules 238 was a military version of the civil Hercules 734.[13]
Hercules 736 – 2,040hp (1,520kW), a civil-series engine developed from the Hercules 730. The Hercules 737 was the torquemeter version.[14]
Hercules 738 – a civil-series engine developed from the Hercules 730.[14]
Hercules 739 – the torquemeter version of the Hercules 738.[14]
Hercules 750 – a civil-series engine developed from the Hercules 730 to suit braking propellers. The Hercules 751 was the torquemeter version.[14]
Hercules 758 – 2,080hp (1,550kW), a civil-series development of the Hercules 750, the Hercules 759 was the torquemeter version. The Hercules 790 and its torquemeter version the Hercules 790 were further developed from the Hercules 758.[15]
Hercules 760 – a civil-series engine developed from the Hercules 730.[14]
Hercules 762 – 2,080hp (1,550kW), a civil-series high-altitude development of the Hercules 730 with modified supercharger. The Hercules 763 was the torquemeter version.[14]
Hercules 772 – 1,965hp (1,465kW), a civil-series development of the Hercules 762. The Hercules 773 was the torquemeter version.[16]
The Rolls-Royce Merlin is a British liquid-cooled V-12 piston aero engine of 27-litre 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.
The Bristol Jupiter is a British nine-cylinder single-row piston radial engine that was built by the Bristol Aeroplane Company. Originally designed late in World War I and known as the Cosmos Jupiter, a lengthy series of upgrades and developments turned it into one of the finest engines of its era.
The Bristol Pegasus is a British nine-cylinder, single-row, air-cooled radial aero 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 Mercury and Jupiter engines, later variants could produce 1,000 horsepower from its capacity of 1,750 cubic inches by use of a geared supercharger.
The Bristol Mercury is a British 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 by use of a geared supercharger.
The Bristol Perseus was a British nine-cylinder, single-row, air-cooled radial aircraft engine produced by the Bristol Engine Company starting in 1932. It was the first production sleeve valve aero engine.
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The Aquila was a nine-cylinder single-row radial aircraft engine designed by the Bristol Engine Company starting in 1934. A sleeve valve engine, its basic design was developed from the Bristol Perseus. The Aquila was never used in production, but further developments led to the Bristol Hercules, Bristol Taurus, and Bristol Centaurus.
The Taurus is a British 14-cylinder two-row radial aircraft engine, produced by the Bristol Engine Company starting in 1936. The Taurus was developed by adding cylinders to the existing single-row Aquila design and transforming it into a twin-row radial engine, creating a powerplant that produced just over 1,000 horsepower with very low weight.
The Centaurus was the final development of the Bristol Engine Company's series of sleeve valve radial aircraft engines. The Centaurus is an 18-cylinder, two-row design that eventually delivered over 3,000 hp (2,200 kW). The engine was introduced into service late in the Second World War and was one of the most powerful aircraft piston engines to see service.
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The Rolls-Royce Kestrel is a 21.25 litre V-12 aircraft engine from Rolls-Royce. It was their first cast-block engine, and used as the pattern for most of their future piston-engine designs. Used during the interwar period, it was fitted to a number of British fighters and bombers of the era, including the Hawker Fury and Hawker Hart family, and the Handley Page Heyford. The Kestrel engine was also sold to international air force customers; in this role it was used to power prototypes of the German Messerschmitt Bf 109 and the Junkers Ju 87 "Stuka" dive-bomber, as the Junkers Jumo 210 engines were not ready to be fitted. Several examples of the Kestrel engine remain airworthy today.
The Rolls-Royce Eagle Mk XXII is a British 24-cylinder, sleeve valve, H-block aero engine of 46 litre displacement. It was designed and built in the early-1940s by Rolls-Royce Limited and first ran in 1944. It was liquid-cooled, of flat H configuration with two crankshafts and was capable of 3,200 horsepower at 18 psi boost.
The Rolls-Royce Crecy was a British experimental two-stroke, 90-degree, V12, liquid-cooled aero-engine of 1,593.4 cu.in capacity, featuring sleeve valves and direct petrol injection. Initially intended for a high-speed "sprint" interceptor fighter, the Crecy was later seen as an economical high-altitude long-range powerplant. Developed between 1941 and 1946, it was among the most advanced two-stroke aero-engines ever built. The engine never reached flight trials and the project was cancelled in December 1945, overtaken by the progress of jet engine development.
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The Armstrong Siddeley Tiger was a British 14-cylinder air-cooled aircraft radial engine developed by Armstrong Siddeley in the 1930s from their Jaguar engine. The engine was built in a number of different versions but performance and dimensions stayed relatively unchanged. The Tiger VIII was the first British aircraft engine to use a two-speed supercharger.
The SNECMA 14R was a 14-cylinder two-row air-cooled radial engine developed in France just prior to the start of World War II from the Gnome-Rhône 14N. The 14N radial engine was itself an improved version of the popular pre-war Gnome-Rhône 14K Mistral Major series; designed and manufactured by Gnome et Rhône, a major French aircraft engine manufacturer whose origins pre-date the First World War.
Lumsden, Alec (1994). British Piston Engines and Their Aircraft. Shrewsbury: Airlife Publishing Ltd. ISBN1-85310-294-6.
White, Graham (1995). 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: Society of Automotive Engineers. ISBN1-56091-655-9.
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