Rolls-Royce Merlin exhaust flow and waste-gates, meant that this option was rejected in favour of the two-stage supercharger. Fitted with the two-stage two-speed supercharger, the Merlin 60 series gained 300 horsepower (224 kW) at over the Merlin 45 series, at which altitude a Spitfire IX was nearly faster than a Spitfire V. The two-stage Merlin family was extended in 1943 with the Merlin 66 which had its supercharger geared for increased power ratings at low altitudes, and the Merlin 70 series that were designed to deliver increased power at high altitudes. While the design of the two-stage supercharger forged ahead, Rolls-Royce also
Rolls-Royce/SNECMA M45H The engine had a low turbine entry temperature and comparatively low rotational speed. The engine was designed to be uprated without drastic redesign. Three options were water injection (+10% thrust), improved HP turbine (+10% thrust), addition of a zero-stage to the LP compressor (+25% thrust). The engine was developed at the time of the Rolls-Royce bankruptcy which resulted in delays in developing the engine. A Rolls-Royce/SNECMA M45H engine is on display as part of the aero engine collection at the Royal Air Force Museum Cosford. Rolls-Royce/SNECMA M45H The Rolls-Royce/SNECMA M45H was a medium bypass ratio turbofan produced specifically for the