The Bristol Jupiter was a British nine-cylinder single-row piston radial engine built by Bristol Aeroplane Company. Originally designed late in World War I, a lengthy series of upgrades and developments turned it into one of the finest engines of its era. It was widely used on many aircraft designs though the 1920s and 30s. It was also built under license in France (as the Gnome-Rhône Jupiter), Poland (as the PZL Bristol Jupiter), Italy (as the Alfa Romeo 126-RC35) and in the Soviet Union (as the M-22). Thousands of Jupiters of all versions were produced.
The Jupiter was fairly standard in design, but featured 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, a change was made to a forged head due to the rejection rate of the castings.
In 1925, Fedden started designing a replacement for 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 best known for powering the Handley Page HP.42 Hannibal airliners, which flew the London-Paris route in the 1920s. Other civilian uses included the de Havilland Giant Moth and Hercules, the Junkers G 31 (which would evolve into the famous Ju-52), and the huge Dornier Do X flying boat which used no less than twelve engines.
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 saw widespread use in licensed versions, with fourteen countries eventually producing the engine. In France, Gnome-Rhone produced a version used in several local civilian designs, as well as achieving some export success. Siemens-Halske took out a license in Germany and produced several versions of increasing power, eventually resulting in the Bramo 323 Fafnir, which saw use in wartime models. In Japan, the Jupiter was license-built from 1924 by Nakajima, forming the basis of their own subsequent radial aero-engine design, the Kotobuki. 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.
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