Charging — the compression of the intake air to increase mass flow-rate and oxygen available for combustion — of some form was a requirement for high-altitude flight and as the power of engines improved there was no reason not to use it all the time.

[2] Four Kestrel/Peregrine cylinder banks attached to one crankcase and driving a common crankshaft would produce the contemporary Rolls-Royce Vulture, a 1,700-horsepower (1,300 kW) X-24 which would be used for bombers.

[6] The Air Ministry requirement for the F9/37, a cannon-armed fighter (the Hurricane and Spitfire were armed with machine guns only at this point), was curtailed and there was no further progress with the design.

[7] In August 1940 Ernest Hives, head of the Rolls-Royce aero engine division, wrote to Air Chief Marshal Wilfrid Freeman expressing his wish to stop work on the Peregrine, Vulture and another engine development project, the Rolls-Royce Exe, to concentrate efforts on the Merlin and Griffon but Freeman disagreed and stated that Peregrine production should continue.

[8] While reliability problems were not uncommon for new Rolls-Royce engines of the era, the company's testing department was told to spend all of their time on developing the more powerful Merlin to maturity.