The Rolls-Royce Kestrel (internal type F) is a 21.25 litre (1,295 in³) V-12 aircraft engine from Rolls-Royce. It was their first cast-block engine, and used as the pattern for most of their future piston-engine designs. Used during the interwar period, it was fitted to a number of British fighters and bombers of the era, including the Hawker Fury and Hawker Hart family, and the Handley Page Heyford. The Kestrel engine was also sold to international air force customers, in this role it used to power prototypes of the German Messerschmitt Bf 109 and the Junkers Ju 87 "Stuka" dive-bomber, as the Junkers Jumo 210 engines were not ready to be fitted. Several examples of the Kestrel engine remain airworthy today.
Design and development
Origin
The Kestrel engine was engineering in response to the Curtiss D-12 engine, one of the first truly successful cast-block designs. Earlier engine designs had used individually-machined steel cylinders that were bolted onto a crankcase, whereas the cast-block design used a single block of aluminium that was machined to form the cylinders. The resulting design was lighter and significantly stronger than the separate-cylinder type, and proved simple to convert existing production facilities as it required only an investment in new machining equipment.
The Curtiss D-12 was one of the most powerful engines of its era, and continued to exchange records with other contemporary high-power engines such as the Napier Lion. At the time, none of the British aero engine manufacturers could offer an engine which offered a similar power rating which was also as light and compact as the D-12.
Arthur Rowledge, Chief Designer at Napier and the designer of the Napier Lion engine, joined Rolls-Royce in 1921 to take up the roll as "Chief Assistant to Mr F. H. Royce". , Rowledge and his team designed the new engine to use supercharging at all altitudes, allowing it to outperform naturally aspirated engines which suffered higher performance losses as altitudes increased due to the loss in inlet air pressure.
The prototype Kestrel engine was first run in 1926, and one first flew in 1927, with a power rating of 450hp (335kW). The engine was normally aspirated in its initial form.
Improvements
The engine was first produced in 1927 at 450 hp (335 kW), which was soon improved in the I-B version to 525 hp (390 kW) by increasing the compression ratio to 7:1. The I-B variant saw widespread use in the Hawker Hart family of aircraft, a mainstay of British air power during the early 1930s.
Development continued and the V model introduced the centrifugal supercharger, increasing power to 695hp (520kW).
Increased availability of higher octane aviation fuels in the late 1930s allowed the engine to be boosted to higher power levels without suffering from detonation. The mark-XVI engine used in the Miles Master M.9 prototype delivered 745hp (500kW), and the XXX variant of 1940 saw service at 720hp (537kW).
Cooling system
One key advance in the Kestrel was the use of a pressurised cooling system. Water boils at 100°C at standard atmospheric pressure, but this pressure decreases as altitude increases, and therefore the boiling point of water decreases with altitude. The amount of heat rejected by an air-to-air cooling system is a function of the maximum coolant temperature and volume, so the resulting decrease in cooling capacity became a limiting factor for aero engine power in this period, as the coolant has to be kept below boiling point.
The solution was to pressurise the engine's entire cooling system, thereby raising the temperature at which the coolant would boil: not only does this help mitigate the decrease in cooling performance with altitude, but allows a smaller cooling system to be used in the aircraft for the same heat load. The Kestrel was built to maintain coolant pressure to keep the boiling point at about 150°C.
Variants
The Kestrel was produced in 40 distinct variants which can be divided into three main groups: normally aspirated, medium supercharged, and fully supercharged. One variant, the Kestrel VIII, was configured as a 'pusher engine' for the Short Singapore flying boat. Apart from supercharging, the variant differences centred on varying compression ratios and propeller reduction gearing.