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 of power-to-weight ratio. Despite knowing that the timetable was challenging for an engine incorporating these new features, Rolls-Royce committed to putting the RB211 into service in 1971. Lockheed felt that the new engine enabled the L-1011 to offer a distinct advantage over the otherwise similar DC-10 product. However, Douglas had requested proposals from Rolls-Royce for an engine to power its DC-10, and in October 1967 the response was a thrust version of the RB211 designated RB211-10. There followed a period of intense negotiation between airframe manufacturers Lockheed and Douglas, engine suppliers Rolls-Royce, General Electric and Pratt & Whitney, and the