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 RB211 turn Rolls-Royce from a significant player in the aero-engine industry into a global leader. Already in the early 1970s, the engine was reckoned by the company to be capable of at least 50 years of continuous development. The RB211 was officially superseded in the 1990s by the Rolls-Royce Trent family of engines, the conceptual offspring of the RB211. In 1966, American Airlines announced a requirement for a new short-medium range airliner with a focus on low-cost per-seat operations. While it was looking for a twin-engined plane, the aircraft manufacturers needed more than one customer to justify developing a new airliner.