Rolls-Royce Kestrel

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Rolls-Royce Kestrel IB

The Kestrel was a 700 hp (520 kW) V-12 aircraft engine from Rolls-Royce, their first cast-block engine^[1] and the pattern for most of their future piston-engine designs. Used during the interbellum it remains somewhat obscure, although it provided excellent service on a number of British fighters of the era.

The Kestrel came about as a result of the excellent Curtiss D-12, one of the first truly successful cast-block engines. Earlier designs had used individually machined steel cylinders that were screwed onto a crankcase, whereas the cast-block design used a single block of aluminum that was machined to form cylinders. The result was both simpler to build as well as lighter, requiring only an investment in new machining equipment.

The D-12 was one of the most powerful engines of its era, and continued to swap records with other high-power engines of its era. No British company could offer anything like it, and when Fairey selected it for their Fairey Felix design the Air Ministry had enough and ordered Napier & Son and Rolls-Royce to start work on cast-block engines of their own.

Arthur Rowledge, one of Napier's chief engineers and the designer of the famous Napier Lion, was becoming fed up with management and left for Rolls. In this one move any Napier design effort ended while Rolls' got a boost. Applying every known advance since the D-12 was introduced, Rowledge designed the new engine to use supercharging at all altitudes, allowing it to outperform "naturally aspirated" engines by as much as they were willing to increase the boost pressure.

One key advance in the Kestrel was the use of a pressurized cooling system. Water boils at 100 °C at standard atmospheric pressure, but this temperature decreases with altitude. As it does so its ability to carry heat away from the engine drops, to the point where at high altitudes a gigantic radiator needs to be used to cool it again. The solution was to pressurize the entire cooling system, thereby not only preventing the drop in cooling performance with altitude, but in fact increasing the boiling point even on the ground. The Kestrel was built to maintain enough pressure to keep the boiling point at about 150 °C.

The engine first shipped in 1927 at 450 hp (340 kW), which soon improved in the IB model to 525 hp (390 kW). This model saw widespread use in the famed Hawker Hart family that dominated British air power during the early 1930s. However it was not long before line improvements increased power dramatically; the V model provided 695 hp (520 kW) at 3,000 rpm with no basic change to the design, while the XVI used in the Miles Master delivered 745 hp (560 kW). Messerschmitt also tested its first Messerschmitt Bf 109 prototypes with Kestrel in 1935.

Increased availability of higher octane fuels in the late 1930s allowed the engine to be boosted to higher power levels without suffering from ping, and the Kestrel eventually topped out at 1,050 hp (780 kW) in the XXX model of 1940.

A later development of the Kestrel was the Peregrine, a "rationalized" Kestrel. In practice development of the Peregrine was troubled and it was cancelled with few built.