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
General Electric/Rolls-Royce F136 thrust of for the Harrier's Rolls-Royce Pegasus engine. General Electric/Rolls-Royce F136 The General Electric/Rolls-Royce F136 was an advanced turbofan engine being developed by General Electric and Rolls-Royce plc for the Lockheed Martin F-35 Lightning II. The two companies stopped work on the project in December 2011 after failing to gather Pentagon support for further development. All early F-35s were to be powered by the Pratt & Whitney F135 but it was planned that engine contracts would be competitively tendered from Lot 6 onward. The engines selected would be either the F135 or an engine produced by the GE/RR Fighter Engine