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In page Rolls-Royce R:

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Although not directly related to the Spitfire, the Supermarine engineers gained valuable experience of high-speed flight with the S.5 and S.6 aircraft, their next project being the Rolls-Royce Goshawk-powered Supermarine Type 224 prototype fighter aircraft. Technological advances used in the R engine, such as sodium-cooled valves and spark plugs able to operate under high boost pressures, were incorporated into the Rolls-Royce Merlin design.[7] The author Steve Holter sums up the design of the Rolls-Royce R with these words:[8]