The Evolution of the World‘s Largest Aircraft Engines

The same cannot be said about large engines. Over the course of their time in development, around 10 or 20 years less than that of small engines, big powerplants have swapped technologies five or six times before settling, for the most part, on a formula that delivers ungodly amounts of power with unbelievable levels of reliability. 

But it wasn't always that way. 

In the beginning of powered flight, designers quickly settled, with a bit of palavering, on gas-reciprocating piston engines (with air cooling for them soon winning favor as well). For more pep, aviation followed a new technology, the rotary engine, which was limited, complicated, hard to operate and unreliable. Other than that! 

So it wasn't long before a new kid showed up, one that had the ability to grow big and strong, something rotaries would never be able to do. That engine was, of course, the radial engine, and the advances it allowed were revolutionary. 

The cylinders of a radial (always an odd number for the sake of four-stroke firing order) are fixed, and cooling is provided by direct airflow into the engine while the plane is in flight, along with a recirculating oil system. It was so much simpler, more capable and reliable than rotary design that, in retrospect, it is no surprise the rotary didn't fade into the sunset as much as it just went away. 

Early radial engines included the Anzani three-cylinder model, which powered Louis Blériot's English Channel-crossing Blériot XI, and the nine-cylinder Salmson radial engine, both of which debuted before WWI. But rotaries seized the day, and it wasn't until around the end of the conflict that radials' advantages were fully understood. 

While radial engine developments in Germany and England predated it, the Pratt & Whitney Wasp engine was the most influential early radial engine, first run in 1925. P&W would go on to produce more than 35,000 of these 600-hp, 1,344-cubic-inch, nine-cylinder, single-row engines, which were used in dozens of different aircraft models. 

The power output was orders of magnitude greater than the most powerful rotaries, and these radial engines had far better power-to-weight ratios as well. Operating these engines was easy compared to rotaries---power was infinitely adjustable with a radial---and radials were far more reliable than rotaries, too.  

Unlike rotaries, they were also scalable, to a great degree, anyway. Multi-row radials produced by Pratt & Whitney, including the Double Wasp, which powered the Republic P-47, the Vought F4U Corsair, Grumman Hellcat and many others, were 2,400-hp-class engines. Competitor Wright produced its Cyclone series, which ranged up to the twin-row, 18-cylinder Duplex-Cyclone, which could produce as much as 3,700 hp. 

Radial engines, popular in both military and private transportation aircraft, were the predominant engine type leading up to and during WWII, as well as for a time afterward---the late-war B-29 Superfortress was powered by a quartet of Duplex-Cyclone engines. But the writing was on the wall for radial engines even before the end of the war, as the advantages of inline engines highlighted some of the issues with radials, and turbojets were just over the horizon. 

With the ever-rising bar of expectations for military aircraft, engines had to get more powerful, more reliable and sleeker, too. A couple of inline and V-configuration engines got some momentum in the 1910s, including the Liberty V-12, which powered some WWI aircraft late in the war, as well as ground vehicles and boats. A powerful engine, the V-12 version could produce 400 hp, far greater output than the rotary engines it supplanted. The downside was that much of that additional power was for naught, lost on the draggy and inefficient aircraft it was fitted into. 

But as aircraft throughout the 1920s and early '30s got more aerodynamically efficient, V-configuration engines came into their own. By 1933, British firm Rolls-Royce flew the first prototype of what would go on to become the most legendary engine of the coming global conflict, a supercharged (in most applications) V-12 engine that produced a sound that some call the most beautiful engine sound ever produced---I am of that opinion, too. That early prototype was buggy, had serious cooling problems, and didn't deliver near the hoped-for power output of 1,000 horsepower. But it had promise, and the company continued to develop it, working through the engine's issues, like cooling, which designers largely overcame with a conventional liquid-cooling system. 

By 1936, the V-12 was producing better than 1,000 hp and began going into the production of military aircraft. The Supermarine Spitfire prototype had used an earlier prototype of the Merlin engine; it would go on to use production versions of the Merlin. The model 45 produced 1,470 hp, more than 10 times the power of the most widely used fighter plane engines of WWI, which had unfolded just 20 years earlier. 

Arguably, the ultimate expression of the Merlin-powered fighter is the North American P-51D Mustang, which was outfitted with a version of the Rolls-Royce Merlin built by Packard in the United States, which could turn out 1,490 horsepower under normal operations and 1,790 in emergency (war emergency power) mode. With the big Merlin, the P-51 helped turn the tide of the war with its great range, which allowed it to accompany B-17 bombers deep into enemy territory, and great speed, making it more than a match for the fighters of the Luftwaffe, with the exception of the Me-262---but more on that plane in a bit. 

Just as the limitations of the rotary engine opened the door late in World War I to new powerplant technologies, so, too, did the limitations of piston aero engines give way to the development and wide adoption of jet engines, principally the turbojet at first. 

In what ways did piston engines fall short? Let us count the ways! They were heavy, mechanically complex (in multirow radials, incredibly so), relatively unreliable and, lastly and most importantly, they needed a propeller. What is wrong with propellers? One big thing is they can only spin so fast, and the bigger you make them, the faster their tips spin, and once the tips go supersonic, the aerodynamic penalties are staggering. By the end of the 1930s, big-engine piston fighters were knocking on the speed of sound, and it became clear that the biggest thing holding them back was the propeller---well, along with the limited amount of horsepower you could get out of a piston engine without making it prohibitively heavy and complex. None of it would work, and in an age when speed meant power and survival, there had to be a better way. 

That way was the turbojet engine, first developed independently by British engineer Frank Whittle and German designer Hans von Ohain. Compared to even a low-powered piston engine, a jet engine is mechanically simple, and the concept---that the propulsion would be provided by direct thrust and not the use of a secondary device, the propeller---freed aircraft designs of the need for a propeller. It was clear that the speed of sound would fall before long. 

Even by the end of the war, the end was nigh for piston-engine-powered fighter aircraft. The Luftwaffe even fielded a remarkably effective jet fighter, the Messerschmitt Me-262, a twin-jet fighter that was produced at the end of the war in limited numbers, which was in part because Allied bombers went to great lengths to target the plants where the jet was being produced. 

It's hard to pin the mantle on any one turbojet engine as the one, but the General Electric J47 is a good place to start. Developed from previous-generation GE engines, the J47 made its mark, powering some of the most iconic early jet fighters, including the North American F-86, as well as groundbreaking post-war bombers such as the Boeing B-47 Stratojet and the Convair B-36 Peacemaker. General Electric churned out more than 36,000 J47s in just over eight years. Developing about 6,000 pounds of thrust, the J47 was able to power sufficiently sleek jets to low supersonic speeds. Before long, turbojet engines were increasingly powerful. A decade after the J47 made its first flight, GE's YJ93 turbojet was capable of producing up to 30,000 pounds of thrust with afterburner, powering the North American XB-70 to up to Mach 3.1. 

Turbojet engines would revolutionize commercial air travel as well, helping manufacturers create airliners, like the groundbreaking Boeing 707, that could hurdle continents by flying at high subsonic speeds while carrying previously unimaginably large payloads. 

Okay, here's an engine you might never have heard of, but it was the first popular engine of a type that has arguably had more use/impact than all of the others combined: the turbofan. 

When Boeing came out with its 777, which started flying with the airlines in 1995, its defining feature was its twin-engine design. It was long thought (and regulated as such) that twin-engine aircraft didn't have enough reserve power to fly long legs were one of its engines to fail on a long overwater segment. But the 777 helped flip such thinking on its head, and today it's the four-engine airliners, like the 777's stablemate, the legendary 747, that are endangered species. And the way that the 777 did its magic was by utilizing the largest engines in the world. 

With fuel efficiency being king today, twins have become the de facto long-haul platforms of the majority of major airlines that fly intercontinental routes. 

And the biggest is yet to come. Slated to power the in-development Boeing 777X, the General Electric GE9X is a behemoth of an engine and a miserly one, at that. It is the largest jet engine in the world, and the most powerful. How big? At 134 inches, its fan's diameter is greater than that of the 737's fuselage. Generating an eye-popping 134,000 pounds of thrust, it is more powerful than the rockets that brought America's first astronauts into orbit. And while it produces 6,400 pounds of thrust more than the previously recognized most powerful turbofan engine, the 777's GE-90-115B, it is 10% more fuel-efficient. 

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