December 1942: Merlin powers Spitfire fighter to new heights

2 min read

The iconic status of the Second World War Spitfire fighter was powered by successive improvements by Rolls-Royce to its Merlin engine

There are few aircraft that are as iconic as the Spitfire. Often contrasted to the more workmanlike qualities of its RAF partner, the Hurricane, the fighter plane played an important role in RAF Fighter Command during all theatres of the Second World War. However, the Spitfire wasn’t a fixed entity, being subject to a number of key improvements throughout the conflict.

It was under this aegis that The Engineer was invited by Rolls-Royce Ltd in December 1942 to inspect an example of the firm’s new Merlin 61 supercharged aero-engine, which was being fitted by the RAF to an improved Spitfire then operating with Fighter Command.

The Engineer reconnaissance team explained: “By using a double-stage supercharger with a water-cooled passage between the first and second stages of the supercharger and a cooler between the supercharger outlet and the induction pipe to the rear cylinder, it is found possible with the new engine to develop double the power output as compared to that of the Merlin III, the first engine to be fitted to the Spitfire fighters.”


The article was able to position such a development as part of a continuous wartime process. It recalled that at the beginning of the war and during the Battle of Britain, every RAF first-line fighter was fitted with the previously mentioned Merlin III engine, and, striking a more patriotic note, “the complete defeat of the Luftwaffe in August and September 1940 definitely established the technical superiority of British machines. The superiority was not obtained by chance, but every move of the enemy had been anticipated and a definite counter-move worked out”.

The Engineer showed how these technical innovations had impacted on air tactics: “Early in the war, German aircraft resorted to low-flying tactics, and in order to counter this, Rolls-Royce immediately increased the sea-level power of the Merlin engine by 40 per cent by raising the supercharger pressure. This move so improved the performance of the Spitfire at low altitude that German aircraft were forced to fly higher.”

Merlin 61 engine
A side view of the Merlin 61 engine.

German ME.109s were therefore moving higher and higher into the sub-stratosphere; an arena where the Luftwaffe was deemed to have an advantage because its aircraft were smaller and lighter. Such a move led to further engine iterations, with the Merlin 45 and Merlin XX being introduced into the Spitfire and Hurricane, respectively, to improve their performance at height.

Indeed, this article makes for fascinating reading, showing how technical competition with the Luftwaffe and changes in air tactics were thoroughly intertwined in driving forward engine developments. The Engineer had previously reported on 14 August 1942 on the advent of the new German Focke-Wulf 190 with its 1600HP air-cooled supercharged engine. However, The Engineer was able to strike a reassuring note (to its British readers at least), stating that the new Focke-Wulf had “caused some uninformed persons to believe that the Germans had stolen a march on us in the high-performance fighter class of aircraft, but, as the enemy fighter losses continually show, the improved Spitfire with its new Merlin 61 engine was there to surpass it”.

Other notable developments on the Merlin engine include aeronautical engineer Beatrice Shilling’s correction of a serious defect on the Rolls-Royce engines used in the Hurricanes and Spitfire aircraft. To prevent the Merlin engines from cutting out during diving, Shilling designed a flow-restricting device that reduced fuel deprivation to the engine. She was later awarded an OBE for her efforts during the war.

Read more about the technologies that helped the Allies win the Second World War