Crashed Prototype Firebrand DD810 11 October 1942
The Sabre engine epitomized the pinnacle of piston engine technology not only in the 1930s, when it was designed, but even by contemporary standards. It was an extraordinary piece of engineering, notable for its complexity, technological sophistication, and specific power, though it was also plagued with challenges. Many of the Sabre’s difficulties can be attributed to the wartime urgency in England to deploy cutting-edge technology quickly, often before ironing out all design and manufacturing issues.
Designed by Frank Halford in the mid-1930s, the Sabre engine continued the H-configuration with dual crankshafts, deviating from the Rapier and Dagger in several key areas. Unlike its predecessors, the Sabre featured horizontally oriented cylinders instead of vertical ones, which created a more compact engine suitable for tight cowling. Given the limitations of air cooling for high-performance inline engines evident in the Dagger, the Sabre utilized liquid cooling. This approach not only improved heat management but also allowed for tighter cylinder spacing, making the engine even more compact.
One of the major issues with the Sabre was its use of sleeve valve technology, which aimed to enhance volumetric efficiency compared to poppet valves. The engine’s four banks of six cylinders, with a bore of 5.0 inches and a stroke of 4.75 inches, resulted in a displacement of 2238 cubic inches, nearly identical to that of the Rolls-Royce Griffon. The two-piece, vertically split cast aluminum crankcase supported the two six-throw crankshafts, which rotated in the same direction. While counter-rotating crankshafts might have improved primary balance, the design was dictated by the propeller reduction gear, which contributed to the Sabre’s remarkably smooth operation.
Early Sabre engines included twelve first-order counterweights per crankshaft, which were later removed from the Sabre V onward to reduce weight. The crankshafts were supported by seven lead bronze bearings each. The Sabre V and subsequent models featured thin-wall strip bearings from Yandervell, who later designed the Yanwall Formula One race cars.
The propeller reduction gearing showcased Napier’s innovative approach. It employed a complex “back gear” system with four pinions to drive the propeller. To address the challenge of equal tooth loading in high-speed, high-torque applications, Napier designed a system where each crankshaft drove a pair of straight-cut, first-stage pinion gears. These were coupled with second-stage pinions featuring a helix angle, which created end thrust and helped balance tooth loading. This ingenious system, supported by a centrally pivoted balance beam and preloaded volute springs, ensured balanced power transmission and minimized reduction gear issues.
The sleeve drive mechanism was similarly innovative. It used two hollow longitudinal shafts driven from the front to operate the sleeves of the upper and lower cylinder banks. Each shaft was supported by 14 plain bearings and coupled with a flange. The sleeve drive incorporated six skew gears driving bronze wheels with ball bearings, providing efficient and reliable operation. The sleeves themselves were chrome-molybdenum steel nitrided forgings, with a design that included helically cut slots to manage oil consumption and wear.
For supercharging, the Sabre featured a sophisticated setup with a double-entry, two-sided impeller, fed by a four-barrel SU carburetor. The supercharger drive was managed by long, torsionally flexible quill shafts running coaxially with the sleeve drive shafts, using cone clutches to handle two-speed operation. The cooling system was robust, with a flow rate of 367 gallons per minute and an oil flow rate of 41 gallons per minute.
Starting the Sabre engine involved a Coffman cartridge starter, which, despite its benefits of reduced weight and smaller battery requirements, presented challenges. The starter could cause severe acceleration that might damage the engine if not properly managed. Various schemes were developed to address these issues, including a priming system with a mixture of fuel and lubricating oil.
Development of the Sabre began in 1935, initially based on a diesel engine study. The engine passed its initial acceptance tests in January 1938, with power output rapidly increasing from 1350 to 2050 horsepower. By June 1940, it had successfully completed its type test, making it the world’s first 2000-hp production engine. Despite a planned production rate of 1000 engines per year, the Sabre faced numerous difficulties, including a disastrous record attempt flight in June 1940 and problems with contra-rotating propellers and three-speed supercharging, which were never fully pursued or implemented.