Rolls-Royce Kestrel


The Rolls-Royce Kestrel is a 21.25 litre V-12 aircraft engine from Rolls-Royce. It was their first cast-block engine, 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 was 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. Several examples of the Kestrel engine remain airworthy today.

Design and development

Origin

Earlier in-line engine designs were generally built on top of a cast aluminum crankcase, with individually-machined steel cylinders, bolted on top. Given the forces involved, the connection of the cylinders to the crankcase had to be robust, adding weight, and as a whole the engine was reliant on the structure of the crankcase to hold it together.
In 1923, the Curtiss CR won the 1923 Schneider Trophy. The CR was powered by the recently introduced Curtiss D-12 engine, which replaced the individual cylinders with a cast aluminum block. This gave the engine much greater strength, allowing the rest of the engine to be greatly simplified, making it much lighter overall, as well as easier to assemble as the two parts simply bolted together in a single operation. It was also easy to convert existing assembly lines to the new system as the cast blocks were already being produced for the crankcase, and all that was required was new machines to mill the blocks to the accuracy needed for the pistons.
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 which was also as light and compact as the D-12. The D-12 was licensed by Fairey and introduced to as the Felix.
Arthur Rowledge, Chief Designer at Napier and the designer of the Napier Lion engine, joined Rolls-Royce in 1921 to take up the role as "Chief Assistant to Mr F. H. Royce". Rowledge built a team to introduce a new engine using the cast block, but set the goal to surpass the D-12. This would be accomplished using supercharging at all altitudes, allowing it to outperform naturally aspirated engines. Previously, supercharging was primarily used for high-altitude designs to offset the loss of ambient air pressure as the aircraft climbed, and thereby maintain power. But with the new construction technique, the engine would be so strong that it could be supercharged at all altitudes without overstressing the cylinders, allowing a smaller engine to operate as if it was larger, and thus improving its power-to-weight ratio.
The prototype Kestrel engine was first run in 1926, and one first flew in 1927, with a power rating of 450hp. The engine was normally aspirated in its initial form.

Improvements

The engine was first produced in 1927 at, which was soon improved in the I-B version to 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.
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, and the XXX variant of 1940 saw service at.

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 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

In early Kestrel variants, unsupercharged engines were available in two compression ratios, 'A' engines had a compression ratio of 6:1, and 'B' engines a high compression ratio of 7:1. The Kestrel was designed to be fitted with a gear-driven supercharger, in early Kestrel variants 'MS' engines were moderately supercharged and 'S' engines were fully supercharged. A number of Kestrel variants were produced by rebuilding or modifying earlier Marks.
  • Kestrel I, F.X – maximum continuous power, direct-drive left-hand tractor drive, compression ratio 6:1.
  • Kestrel IA, F.XIA – maximum continuous power, geared right-hand tractor drive, compression ratio 6:1. Production of new-built engines totalled 18.
  • Kestrel IB, F.XIB – maximum power at 2,700 rpm for 5 minutes using 77 octane fuel, maximum continuous power. Compression ratio 7:1. Production of new-built engines totalled 580.
  • Kestrel IB3, F.XIB3 – maximum continuous power, compression ratio 7:1. Production of new-built engines totalled 92.
  • Kestrel IB4, F.XIB4 – maximum continuous power, compression ratio 7:1. Production of new-built engines totalled 5.
  • Kestrel IB5, F.XIB5 – maximum continuous power, compression ratio 7:1. Production of new-built engines totalled 34.
  • Kestrel IMS, F.XIMS – maximum continuous power, medium supercharged, compression ratio 6:1. Production of new-built engines totalled 1.
  • Kestrel IS, F.XIS – maximum continuous power, fully supercharged, compression ratio 6:1. Production of new-built engines totalled 9.
  • Kestrel IIA, F.XIIA – maximum continuous power, compression ratio 6:1. Production of new-built engines totalled 31.
  • Kestrel IIB, F.XIIB – maximum continuous power, compression ratio 7:1. Production of new-built engines totalled 20.
  • Kestrel IIMS, F.XIIMS – maximum power at 2,700 rpm for 5 minutes using 77 octane fuel, maximum continuous power. Tractor drive, medium superchanged, compression ratio 5.5:1. Production of new-built engines totalled 82, some were later converted to pusher drive for the Short Singapore.
  • Kestrel IIMS.2, F.XIIMS.2 – maximum continuous power, medium superchanged, compression ratio 6:1. Production of new-built engines totalled 64.
  • Kestrel IIMS.5, F.XIIMS.5 – maximum power at 2,700 rpm using 77 octane fuel, maximum continuous power, medium superchanged, compression ratio 6:1. Production of new-built engines totalled 5.
  • Kestrel IIMS.6, F.XIIMS.6 – maximum continuous power, medium superchanged, compression ratio 6:1. Production of new-built engines totalled 16.
  • Kestrel IIS, F.XIIS – maximum power with boost at 2,750 rpm for 5 minutes using 77 octane fuel, maximum continuous power. Fully supercharged, compression ratio 6:1. Normally tractor drive, although versions fitted to the Gloster TC.33 were pusher drive.
  • Kestrel IIIB, F.XIVB – maximum continuous power, compression ratio 7:1.
  • Kestrel IIIMS, F.XIVMS – maximum continuous power, medium superchanged, compression ratio 5.5:1. Normally tractor drive, although versions fitted to the Short Singapore III were pusher drive.
  • Kestrel IIIMS.2, F.XIVMS.2 – maximum continuous power, medium superchanged, compression ratio 6:1. Production of new-built engines totalled 20.
  • Kestrel IIIMS.4, F.XIVMS.4 – maximum continuous power, medium superchanged, compression ratio 6:1. Production of new-built engines totalled 16.
  • Kestrel IIIMS.6, F.XIVMS.6 – maximum power with boost at 2,700 rpm for take off or 1 minute using 77 octane fuel, maximum continuous power. Medium superchanged, compression ratio 6:1. Production of new-built engines totalled 16.
  • Kestrel IIIS, F.XIVS – maximum power with boost at 2,700 rpm for take off or 1 minute using 77 octane fuel, maximum continuous power. Fully superchanged, compression ratio 6:1. Production of new-built engines totalled 71.
  • Kestrel IIIS.3, F.XIVS.3 – maximum continuous power, fully superchanged, compression ratio 6:1. Production of new-built engines totalled 48.
  • Kestrel IV – maximum continuous power, fully superchanged, compression ratio 6:1. Single engine built, it became the prototype Rolls-Royce Goshawk.
  • Kestrel V, V Spl. – maximum power with boost at 2,900 rpm using 87 octane fuel, maximum continuous power. Fully superchanged, compression ratio 6:1. Production of new-built engines totalled 1,178.
  • Kestrel VDR – maximum power with boost at 2,350 rpm using 77 octane fuel, maximum continuous power. Fully superchanged, compression ratio 6:1.
  • Kestrel VI – maximum power with boost at 2,700 rpm for take off or 1 minute using 87 octane fuel, maximum continuous power. Fully superchanged, compression ratio 6:1. Production of new-built engines totalled 258.
  • Kestrel VII – maximum continuous power, medium superchanged, compression ratio 6:1.
  • Kestrel VIII – maximum continuous power, medium superchanged pusher drive, compression ratio 6:1. Production of new-built engines totalled 133.
  • Kestrel IX – maximum continuous power, medium superchanged tractor drive, compression ratio 6:1. Production of new-built engines totalled 136.
  • Kestrel X – maximum power at 2,900 rpm for 5 minutes using 87 octane fuel, maximum continuous power. Compression ratio 7:1. Production of new-built engines totalled 1,161.
  • Kestrel XDR – maximum continuous power, compression ratio 7:1.
  • Kestrel XI – maximum continuous power, compression ratio 7:1. Production of new-built engines totalled 55.
  • Kestrel XII – maximum continuous power, compression ratio 7:1.
  • Kestrel XIV – maximum continuous power, fully supercharged, compression ratio 6:1.
  • Kestrel XV – maximum continuous power, fully supercharged, compression ratio 6:1.
  • Kestrel XVI, XVI Spl. – maximum continuous power, fully supercharged, compression ratio 6:1. Production of new-built engines totalled 95.
  • Kestrel XVI – maximum continuous power, fully supercharged, compression ratio 6:1.
  • Kestrel XVI – maximum power with boost at 3,000 rpm for 5 minutes using 87 octane fuel, maximum continuous power. Fully supercharged, compression ratio 6:1. Converted from a Mk V for the Miles Kestrel.
  • Kestrel XXX – maximum power with boost at 2,750 rpm for take off or one minute using 87 octane fuel. Fully supercharged, compression ratio 6.2:1. Final Kestrel variant used in the Miles Master, they were manufactured by rebuilding older Kestrels, usually Mk Vs and Mk XVIs, with updated components.