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The Merlin V-12 engine is probably the most famous piston engine to have powered aircraft during World War II. The Merlin design was originated by Rolls-Royce in Britain and then, during WW II, built under license from Rolls-Royce in the United States by the Packard Motor Car Company. In the U.S., this engine is most closely associated with the later North American P-51 Mustang fighter models, most particularly the P-51D, identified by many historians as the best all-around fighter of WWII.
Despite its legendary success as a power plant for military aircraft, the Merlin – ironically – owes its origins not to war, but to air racing. From 1913 until 1931, the most-prestigious award in European air-racing was the Schneider Trophy, which was presented to the fastest seaplane in the world, with the provision that, if any nation won the trophy for three consecutive years, the trophy would become the permanent possession of that country. In 1927, Rolls-Royce had developed a superb V-12 aircraft engine, the Kestrel. The Kestrel begat two legendary descendants: the “R,” which, in the Supermarine S.6B, won the Schneider Trophy permanently for Britain in 1931; and the Merlin, which would power many of Britain’s best aircraft of World War II, including the Hawker Hurricane, the Supermarine Spitfire, the Vickers Wellington, the Handley Page Halifax, and the Avro Lancaster. Indeed, without the Merlin, it is highly unlikely that Great Britain could have survived Nazi Germany’s onslaught during the Battle of Britain, in 1940.
During WW II, with the British under dreadful pressure, a shortage of manufacturing facilities, and the threat of bombing to those facilities that were available, the British sought fabrication of the Merlin engine outside the county. With the U.S. not yet drawn directly into the war, capacity was still available in the slowly ramping-up U.S. industry. Originally intended to be exported exclusively to Britain for installation in British fighter aircraft, the highly successful engine was eventually installed in British fighters and bombers and, in the U.S., in the highly successful Mustang.
Between the World Wars, fighter-engine development efforts in the U.S. lagged behind those in Britain but, eventually, efforts to develop an American fighter engine of 1,000 horsepower were started. The Allison Division of General Motors developed the Allison V-1710, a V-12 engine used in the Lockheed P-38 Lightning, the Bell P-39 Airacobra, the Curtiss P-40 Warhawk, and the first model of the Mustang, the P-51A. Other early development efforts included Chrysler’s hemispherical-combustion-chambered V-16 (dual V-8 engines with a flywheel between them), a Ford V-12 (eventually trimmed of four cylinders to produce a V-8 tank engine), and others. The Chrysler engine progressed to the point of flight testing on an experimental Republic P-47 Thunderbolt (XP-47H), but production facilities were not available for an unproven engine at that time, and the war ended before any thought could be given to series production.
The Merlin V-1650 engine was produced under license by the Packard Motor Car Company in Detroit, Michigan. The engine displaced 1,650 cubic inches and was configured in a 12-cylinder V of 60 degrees. The engine produced 1,029 horsepower early in the war, and was developed to the point of producing 2,000 horsepower later. Difficult to produce and maintain in the original design (the British engines were nearly handmade), significant improvements were made in the basic design by Packard to improve the Merlin’s reliability, maintainability, and ease of manufacture. Most significantly, the Merlin had reached a stage of development ahead of its rivals (particularly the Allison V-1710), in which it was configured with a two-stage supercharger and an intercooler, which were essential for high-altitude operation. (Most Allisons had only a single-stage, gear-driven supercharger; only those Allisons that powered the P-38 Lightning received the more-efficient turbo-supercharger with an intercooler.) Nearly 150,000 Merlin engines were eventually produced.
The original Mustang fighter, itself, also resulted from a British request for help. North American Aviation was requested, for the sake of speed, to build copies of the P-40 Warhawk fighter. North American was aghast at the thought of building aircraft designed by its rival, Curtiss, and insisted that North American could produce a better fighter faster. In one of aviation history’s most-famous design efforts, a prototype Mustang, the NA-73X, was constructed in just 117 days, with its first flight six weeks later, and production aircraft available within one year. This was all the more remarkable because of the use of the as-yet-unproven laminar-flow wing. The Mustang was originally constructed with the single-stage supercharged Allison V-1710 engine (P-51A), and thus was envisioned by many as a low-altitude ground-attack aircraft. The British were very much impressed with the aircraft’s performance at low altitude, but had a greater need for a fighter with high-altitude and long-range capabilities. The British tested the aircraft with a Merlin engine installed, and the Mustang’s potential was immediately recognized. The well-balanced fighter is said by many scholars to have been the best fighter of WW II, and over 15,000 were eventually built.
The venerable Merlin represents the high-water mark of the high performance piston engine for fighter aircraft, though various piston-engine developments were perused during and after the war, including the well known Rolls-Royce Griffon. Designs were produced for Merlin V-12s that were connected bottom-to-bottom at the crankcase to produce an X-24 configuration engine. Experiments with advanced, multi-banked radials were also conducted. But the handwriting was already on the wall during WW II. By the war’s end, both the British and the Germans had operational fighter aircraft powered by jet engines, and the U.S. was not far behind. The jet engine was more powerful, less costly, much easier to maintain, and more reliable than the piston engine, and the resulting fighter aircraft were much faster than their piston-powered counterparts.
However, the story of the Merlin (and its most-famous rival, the Allison V-1710) does not end with the close of WW II: anything but! For example, in 1950, the powerboat- racing world was revolutionized by the Slo-mo-shun IV and its flying, three-point-hull design. Equipped with World War II-surplus aircraft engines, the powerboats were suddenly able to go nearly forty miles per hour faster, and the sport took on a new life as popular competitions sprang to life across the country. Notably, the original Slo-mo-shun IV was powered by the less-expensive and more-reliable Allison engine, but racers of any kind are always looking for more power and greater speed, and Merlins were quickly substituted when funds and engines became available. Like aerial combat, however, hydroplane racing was eventually taken over by the jet engine; but the nostalgic and fondly remembered roar of the big V-12s can still be heard at events today as these old warriors are lovingly exhibited by enthusiasts.
The big V-12s of WW II also found their way into the newly emerging sport of drag racing, but their success was limited, at best. While the engines produced plenty of power at the top end, they were never intended to provide the torque needed by a modern dragster. Further, the lack of a suitable drive train to get the power to the wheels meant that getting the power to the track was a problem. Moreover, these engines were costly and difficult to maintain. While a part of drag racing’s history, the big aircraft piston engines were not a major player.
Surplus V-12s also found their way into the attempts to increase the absolute land speed record. A measure of success was found here, in particular with Art Arfons, who also had participated in drag racing with these big engines. In 1961, he came close to breaking the world record, reaching a top speed of 313.78 miles per hour at Bonneville Salt Flats with an Allison-powered car.
While no longer actively racing in drag races, land-speed-record attempts, or hydroplanes, Merlin engines are still much sought-after and highly prized by enthusiasts today. Most notable is the desire for replacement engines for vintage aircraft. The Merlin engine powered a virtual Who’s Who of WW II and post-WW II aircraft, with the obvious favorite in the U.S. being the venerable P-51 Mustang. These vintage aircraft are viewed and enjoyed today in museums, at fly-ins, and at air shows all over the country, and not to be missed is the chance to ride in one of these beautiful, exciting, and memorable pieces of history. Even just hearing one roar past, flat-out, on the deck is a near-religious experience, for no engine in the world sounds like a Merlin!
- During WWII, Packard built 55,523 Merlin engines under Rolls-Royce License; 1st engine delivered in 1941; Improved by Packard to increase power & reliability; Installations included USAAF’s P-51 and P-40, plus RAF’s Lancaster & Mosquito