It appears the Hydra (also known as the Double Octagon) was built as a “backup” design in case the newer sleeve valve engines being designed by Roy Fedden at the same time proved unworkable. Design of the Hydra was led by Frank Owner, who built an experimental V-4 design to test a new twin overhead cam design, a first for Bristol engines which normally used pushrods and rockers. When the V-4 ran successfully, it seems they used four such engines connected to a common crankcase to produce the Hydra.

The cams were operated by rotating shafts leading from the crankshaft at the back of the engine to the top of each cylinder row. The shaft was directly geared to one of the two camshafts, using another driven shaft to transmit power to the second camshaft on the “other side” of the cylinder heads. The arrangement was somewhat complex, but no more so than a pushrod-based system. A side-effect of the use of the overhead cams was that the cylinders were “in-line”, whereas radials typically rotated the second bank of cylinders in relation to the first in order to expose them more fully to the airflow for cooling.

The Hydra had only two valves per cylinder limiting volumetric efficiency. It is generally difficult to properly arrange pushrods for four valve operation in a multi-row radial engine, some of the rods would have to exit the crankcase between the cylinders where there is little room or spare strength. This difficulty was one of the reasons that led to Fedden’s work on the sleeve valve. This is not so much of a problem on an in-line design, and is one of the reasons in-lines of the era were able to compete in performance terms with the generally much simpler radials. The use of the overhead cams on the Hydra avoided this problem as well.

Only two Hydra’s were built. First run in 1931, one was test flown on the Hawker Harrier, and suffered severe vibration at critical RPMs. In the end, Fedden was able to develop the sleeve valve into a superb series of engines, and the Hydra is almost forgotten.

Hydra
Type: 16 cylinder, twin row, air cooled radial engine.
Bore: 5.0 in (127 mm)
Stroke: 5.0 in (127 mm)
Displacement: 1,570 cu in (25.7 L)
Length: 57 in (1448 mm)
Diameter: 46.5 in (1181 mm)
Dry weight: 1,500 lb (680 kg)
Designer: Roy Fedden
Valvetrain: Double overhead camshaft
Supercharger: Medium supercharged
Fuel type: DTD 134 petrol
Cooling system: Air cooled
Reduction gear: Epicyclic geared, reduction ratio 0.424:1, left hand tractor
Power output: 850 hp at 3,000 rpm
Compression ratio: 6:1
Power-to-weight ratio: 0.56 hp/lb

The Bristol Lucifer was a British three-cylinder, air-cooled, radial engine for aircraft, first run in 1919. Built in the UK in the 1920s by the Bristol Aeroplane Company, it produced 100 horsepower (75 kW).

The Lucifer was originally a Cosmos Engineering engine, Cosmos being taken over by Bristol in 1920.

Applications:
Albatros L 69
Avro 504
Boulton Paul P.10
Bristol M.1
Bristol Primary Trainer
Handley Page Hamlet
NVI F.K.29
Parnall Peto

Lucifer 1
Type: 3-cylinder air-cooled single-row radial engine
Bore: 5.75 in (146 mm)
Stroke: 6.25 in (159 mm)
Displacement: 487 in³ (8.0 L)
Diameter: 48 in (1219 mm)
Dry weight: 324 lb (147 kg)
Valvetrain: Four valves per cylinder, poppet valve
Fuel type: Petrol
Cooling system: Air-cooled
Power output: 100 hp (75 kW)
Compression ratio: 4.8:1
Power-to-weight ratio: 0.3 hp/lb

The Phoenix was an experimental version of the Bristol Aeroplane Company’s Pegasus engine, adapted to run on the diesel cycle. Only a few were built between 1928 and 1932, although samples fitted to a Westland Wapiti held the altitude record for diesel-powered aircraft at 27,453 ft (8,368 m) from 11 May 1934 until World War II. The primary advantage of the Phoenix was better fuel efficiency at cruise, by up to 35%.

Variants:
Phoenix I: – Diesel version of the Pegasus IF, 380 hp.
Phoenix IIM: – Medium supercharged diesel version of Pegasus IM, 470 hp.

Specifications:

Phoenix I
Type: 9-cylinder air-cooled Diesel radial engine
Bore: 5.75 in (146 mm)
Stroke: 7.5 in (190 mm)
Displacement: 1,753 in³ (28.7 l)
Length: 43.75 in (1,111 mm)
Diameter: 55.25 in (1,403 mm)
Dry weight: 1,067 lb (484 kg)
Valvetrain: Overhead valve, two intake and two exhaust valves per cylinder, pushrod-actuated.
Fuel type: Diesel
Cooling system: Air-cooled
Power output: 380 hp (283 kW) at 2,000 rpm at sea level
Specific power: 0.22 hp/in³ (9.9 kW/L)
Compression ratio: 14:1
Power-to-weight ratio: 0.36 hp/lb (0.6 kW/kg)

In late 1925 and early 1926, the RAE published a series of papers by Harry Ricardo on the sleeve valve principle. The main advantages over the traditional poppet valves was better volumetric efficiency, and the ability to operate at higher rotational speeds. This allowed a smaller engine to produce the same power as a larger one, leading to better fuel efficiency and compact design, particularly in multi-row radial engines. Roy Fedden, Bristol’s primary engine designer, became interested in the concept and by 1927 he had constructed a working two-cylinder V as a testbed, with the idea of developing it into a V-12. However several problems cropped up on the design, notably that the sleeves tended to burst during the power stroke and strip their driving gears. This led to a long series of tests and materials changes and upgrades that required six years and an estimated 2 million pounds to cure; however, by 1933 the problems had been worked out, and the Perseus went on to become the first sleeve valve aero-engine in the world to be put into large quantity production.

The result was a Bristol Mercury-sized engine adapted to the sleeve valve system, the Perseus nine-cylinder, single-row, air-cooled radial, and its smaller cousin, the Bristol Aquila. The first production versions of the Perseus were rated at 580 horsepower (433 kW), the same as the same-year model Mercury, which shows that the sleeve system was being underutilised. However this was quickly uprated as improvements were introduced, and by 1936 the Perseus was delivering 810 hp (604 kW), eventually topping out at 930 hp (690 kW) in 1939, while the Perseus 100 with an increased capacity of 1,635 cu in (26.8 L) produced 1,200 hp (890 kW) at 2,700 rpm at 4,250 ft (1,296 m). This far outperformed even the most-developed versions of the Mercury.

The Perseus saw limited use in the civilian field, notably on the Short Empire flying-boats, but was more common in the then-expanding military field where it was found on the Westland Lysander, Vickers Vildebeest, Blackburn Botha, Skua and Roc bombers.

The main contribution of the Perseus is that its mechanicals were used as the basic piston and cylinder for the “twinned” versions, the tremendously successful Hercules and Centaurus. It was in these designs that the advantages of the sleeve valve were finally put to good use, and by war’s end the Centaurus was one of the most powerful engines in the world.

Applications:
Blackburn Botha
Blackburn Roc
Blackburn Skua
Bristol Bulldog
Bristol Type 148
Cunliffe-Owen Flying Wing
de Havilland Flamingo
de Havilland Hertfordshire
Gloster Goring
Hawker Hart
Saro A.33
Short Empire
Short Scylla
Vickers Vellox
Vickers Vildebeest Mk.IV
Westland Lysander Mk.II

Specifications:

Perseus XII
Type: Nine-cylinder single-row supercharged air-cooled radial engine
Bore: 5.75 in (146 mm)
Stroke: 6.5 in (165 mm)
Displacement: 1,520 in³ (24.9 L)
Length: 49 in (1,245 mm)
Diameter: 55.3 in (1,405 mm)
Dry weight: 1,025 lb (465 kg)
Valvetrain: Sleeve valve
Supercharger: Single-speed centrifugal type supercharger
Fuel system: Claudel-Hobson carburettor
Fuel type: 87 Octane petrol
Cooling system: Air-cooled
Reduction gear: 0.5:1 turning a de Havilland variable pitch propeller
Power output:
830 hp (619 kW) at 2,650 rpm for takeoff
905 hp (675 kW) at 2,750 rpm at 6,500 ft (1,980 m)
Specific power: 0.59 hp/in³ (26.75 kW/l)
Compression ratio: 6.75:1
Specific fuel consumption: 0.43 lb/(hp·h) (261 g/(kW·h))
Oil consumption: 0.28 oz/(hp·h) (11 g/(kW·h))
Power-to-weight ratio: 0.88 hp/lb (1.45 kW/kg)

Bristol had originally intended to use the Aquila and Perseus as two of its major product lines in the 1930s, but the rapid increase in size and speed of aircraft in the 1930s demanded much larger engines than either of these. The mechanicals from both of these designs were then put into two-row configurations to develop much larger engines, the Aquila becoming the Taurus, and the Perseus becoming the Hercules.

The Taurus was a 14-cylinder two-row radial sleeve valve design, resulting in an extraordinarily uncluttered exterior and very low mechanical noise. It offered high power with a relatively low weight, starting from 1,015 hp (760 kW) in the earliest versions. It was also compact, with a diameter of 46 inches (1170 mm) which made it attractive to fighter designers. Unfortunately, the engine has also been described as “notoriously troublesome”, with protracted development and a slow growth in rated power. After several years of development, power had been increased from 1,015 hp (760 kW) to only 1,130 hp (840 kW). As the most important applications of this engine were in aircraft that flew at low altitude, engine development efforts focussed on low-altitude performance.

Bristol Taurus 11 14-Cylinder sleeve-valve double bank air-cooled radial engine of 1938 provided 1,065 h.p. at 5,000ft, and was fitted to the Beaufort Mk 1 as well as the Fairey Battle and Fairey Albacore.

The first Taurus engines were delivered just before World War II began and found some use primarily in the Fairey Albacore and Bristol’s own Beaufort torpedo bomber. Starting from April 1940, it was suggested to replace the Taurus engines of the latter by the famous Pratt & Whitney R-1830 Twin Wasp, but this change was postponed to the autumn of 1941 while attempts were made to cure the reliability problems of the Taurus, and later had to be temporarily reversed because of shortages of Twin Wasp engines. The Twin Wasp was, however, strongly preferred, especially for overseas postings, because of its much greater reliability. In later models of the Taurus engine the reliability problems were mostly cured by a change in the cylinder manufacturing process, although the engine kept a poor reputation, and in the Albacore the Taurus engine was retained until the end of that aircraft’s production in 1943.

There were no other operational applications of the Taurus engine, because its initial reliability problems discouraged the development of Taurus-powered aircraft, and because later-war combat aircraft demanded more powerful engines. Its production lines were closed down in favour of the Hercules engine.

Variants

Taurus II
1940 1,060 hp.

Taurus III
935 hp.

Taurus VI
985 hp.

Taurus XII
1940 985 hp. Supercharger ratio decreased, impeller diameter increased.

Taurus XVI
1940 985 hp.

Taurus XX
Trials engine only, one built.

Applications
Bristol Type 148
Bristol Beaufort
Fairey Albacore
Fairey Battle testbed only
Gloster F.9/37

Specifications

Taurus II
Type: 14-cylinder, two-row, supercharged, air-cooled radial engine with dual ignition
Bore: 5 in (127 mm)
Stroke: 5.625 in (143 mm)
Displacement: 1,550 in³ (25.4 L)
Length: 49.2 in (1,250 mm)
Diameter: 46.25 in (1,175 mm)
Dry weight: 1,301 lb (590 kg)
Valvetrain: Sleeve valve
Supercharger: Single-speed centrifugal type supercharger
Fuel system: Claudel-Hobson carburettor
Fuel type: 87 Octane petrol
Cooling system: Air-cooled
Reduction gear: Farman epicyclic gearing, 0.444:1
Power output:
996 hp (743 kW) at 3,225 rpm for takeoff
1,050 hp (783 kW) at 3,225 rpm at 5,000 ft (1,520 m)
Specific power: 0.68 hp/in³ (30.83 kW/l)
Compression ratio: 7.2:1
Specific fuel consumption: 0.43 lb/(hp·h) (261 g/(kW·h))
Power-to-weight ratio: 0.81 hp/lb (1.33 kW/kg)

The nine-cylinder, single-row, air-cooled radial Pegasus was designed by Sir Roy Fedden as the follow-on to the Bristol Aeroplane Company’s Bristol Jupiter, following lessons learned in the Mercury effort. The Mercury was a small engine that produced about as much power as the Jupiter, through a combination of supercharging that improved the “charge”, and various changes to improve the operating RPM. Power of a piston engine can be calculated by multiplying the charge per cylinder by the number of cycles per second; the Mercury improved both and thereby produced more power for a given size. The primary advantage was a much improved power-to-weight ratio due to better volumetric efficiency.

The Pegasus was the same size, displacement and general steel/aluminium construction as the Jupiter, but other improvements allowed the maximum engine speed to be increased from 1,950 to 2,600 rpm for take-off power. This improved performance considerably from the Jupiter’s 580 hp (430 kW), to the first Pegasus II with 635 hp (474 kW), to 690 hp (515 kW) in the first production model Pegasus III, and eventually to the late-model Pegasus XXII with 1,010 hp (750 kW) thanks to the two-speed supercharger (introduced on the Pegasus XVIII) and 100-octane fuel. This gave rise to the claim “one pound per horsepower” reflecting the excellent power-to-weight ratio.

Some notable users of the Pegasus were the Fairey Swordfish, Vickers Wellington, and Short Sunderland. It was also used on the Anbo 41, Bristol Bombay, Saro London, Short Empire, Vickers Wellesley and the Westland Wallace.

Like the Jupiter before it, the Pegasus was also licensed by the PZL company in Poland. It was used on the PZL.23 Karaś and PZL.37 Łoś bombers. In Italy Alfa Romeo built both the Jupiter (126-RC35) and the Pegasus under licence, with the engine based on the Pegasus designated as the Alfa Romeo 126-RC34 with the civil version as the 126-RC10. In Czechoslovakia it was built by Walter Engines and was known as the Pegas.

Approximately 32,000 Pegasus engines were built. The Pegasus set three height records: in 1932, 1936 and 1937. It was used for the first flight over Mount Everest, and in 1938 set the world’s long-distance record.

The Pegasus was produced in many variants, early prototype engines were unsupercharged but the majority used a geared supercharger, either single-speed or two-speed. Variant differences included compression ratios, propeller reduction gear ratios and accessories.

Applications:
ANBO IV
Blackburn Baffin
Blackburn Ripon
Blackburn Shark
Boulton Paul Mailplane
Boulton Paul Overstrand
Boulton Paul Sidestrand
Bristol Bombay
Bristol Type 118
Bristol Type 120
Bristol Type 138
Douglas DC-2
Fairey TSR I
Fairey Seal
Fairey Swordfish
Fokker C.X
Fokker D.XXI-5
Gloster Goring
Handley Page H.P.43
Handley Page H.P.47
Handley Page H.P.51
Handley Page Hampden
Handley Page H.P.54 Harrow
Hawker Audax
Hawker Hart
Hawker Osprey
Junkers Ju 52
Junkers Ju 86K-4
Koolhoven FK.52
Letov Š-328
LWS-6 Żubr
PZL.23 Karaś
PZL.37 Łoś
Saro London
Savoia-Marchetti SM.95
Short Mayo Composite
Short Sandringham
Short Sunderland
Short Empire
Short Syrinx
Supermarine Stranraer
Supermarine Walrus
Vickers Type 253
Vickers Valentia
Vickers Vanox
Vickers Vellox
Vickers Vespa
Vickers Viastra X
Vickers Victoria
Vickers Vildebeest
Vickers Vincent
Vickers Virginia
Vickers Wellington
Vickers Wellesley
Westland Wallace
Westland Wapiti
Westland PV.7
Westland-Houston PV-3

Specifications:

Pegasus XVIII
Type: 9-cylinder, single-row, supercharged, air-cooled radial engine
Bore: 5.75 in (146 mm)
Stroke: 7.5 in (190 mm)
Displacement: 1,753 in³ (28.7 L)
Length: 61 in (1,549 mm)
Diameter: 55.3 in (1,405 mm)
Dry weight: 1,111 lb (504 kg)
Valvetrain: Four pushrod-actuated valves per cylinder – two intake and two sodium-filled exhaust
Supercharger: Two-speed centrifugal type supercharger
Fuel system: Claudel-Hobson carburettor
Fuel type: 100 Octane petrol
Oil system: Dry sump with one combination pressure/scavenge pump
Cooling system: Air-cooled
Reduction gear: Farman epicyclic gearing, 0.5:1
Propeller: Rotol three-blade with variable pitch
Power output:
965 hp (720 kW) at 2,475 rpm for takeoff at sea level
835 hp (623 kW) at 2,250 rpm at 8,500 ft (2,590 m), maximum continuous climb power
965 hp (720 kW) at 2,600 rpm at 13,000 ft (3,960 m), maximum power (emergency combat – 5 minutes only)
Specific power: 0.55