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Bristol Jupiter

The Jupiter was a 9 cylinder one-row radial aircraft engine designed during WWI by Roy Fedden of Cosmos Engineering. During the rapid downscaling of military spending after the war, Cosmos went bankrupt in 1920, and was eventually purchased by Bristol Aeroplane Company on the strengths of the Jupiter design. Over a number of years the engine matured into one of the most reliable on the market, and although production started as early as 1918, it didn't stop until 1930.

The Jupiter was fairly standard in design, but feature four valves per cylinder, which was uncommon at the time. The cylinders were machined from steel forgings, and the cast heads were later replaced with aluminium alloy following studies by the RAE. In 1927 they changed to a forged head due to the rejection rate of their castings.

As early as 1925 Roy Fedden had started putting some effort into eventually replacing the Jupiter. Using a shorter stroke to increase the RPM, and including a supercharger for added power, resulted in the Bristol Mercury of 1927. Applying the same techniques to the original Jupiter-sized engine in 1927 resulted in the Bristol Pegasus. Neither would fully replace the Jupiter for a few years.

The Jupiter is most famous for powering the Handley Page HP42 Hannibal airliner, which flew the London-Paris route in the 1920s. Other civilian uses included the De Havilland DH.61 Giant Moth and DH.66 Hercules, the Junkers G31 (which would evolve into the famous Junkers Ju 52), and the huge Dornier Do.X flying boat, which used no less than ten.

Military uses were less common, but included the parent company's Bristol Bulldog, as well as the Gloster Gamecock and Boulton-Paul Sidestrand. It was also found in prototypes around the world, from Japan to Sweden.

The Jupiter also saw widespread use in licensed versions, with no less than fourteen countries eventually producing the engine. In France the Gnome-Rhone company produced a version that was used in several local civilian designs, as well as had some export success. The most produced version was in the Soviet Union, where their M-22 version powered the famous Polikarpov I-16, which was built in the thousands.


Bore / Stroke / Displacement: 5.75" x 7.5", 1753 cu in (28.7 litre)
Compression ratio: 5:1
HP: 435 hp at 1575 RPM (max continuous), 580 hp at 1950 RPM (take-off)
Weight: 730 lbs