There are scores of blades of various sizes in a modern jet engine, whirling this way and that and performing various tasks that help push the hundreds of tons of airplane up and through the sky. But the blades of the high-pressure turbines represent the singularly truest marvel of engineering achievement—and this is primarily because the blades themselves, rotating at incredible speeds and each one of them generating during its maximum operation as much power as a Formula One racing car, operate in a stream of gases that are far hotter than the melting point of the metal from which the blades were made. What stopped these blades from melting? What kept them from disintegrating, from destroying the engine and all who were kept aloft by its power? It seems at first blush so ludicrously counterintuitive: that a piece of normally hard metal can continue to work at a temperature in which the basic laws of physics demand that it become soft, melt, and turn to liquid. How to avoid such a thing is central to the successful operation of a modern jet engine.
For, very basically, it turns out to be possible to cool the blades by performing on them mechanical work of a quite astonishing degree of precision, work which allows them to survive their torture for as many hours as the plane is in the air and the engine is operating at full throttle. The mechanical work involves, on one level, the drilling of hundreds of tiny holes in each blade, and of making inside each blade a network of tiny cooling tunnels, all of them manufactured at a size and to such minuscule tolerances as were quite unthinkable only a few years ago.
Five conjoined high-pressure turbine blades in a jet engine, fashioned from single-crystal titanium alloy and peppered with tiny holes that allow a rush of cool air to prevent them from melting in the chamber’s lethally hot atmosphere.
Photograph courtesy of Michael Pätzold/Creative Commons BY-SA-3.0 de)
Inevitably, it was commerce that provided the spur for all this work—although the jet engine makers who worked secretly for “the dark side,” creating technologies for bombers and stealth fighters and their like, made as-yet-unacknowledged contributions, too, and about which plane makers still cannot talk. The start of work on turbine blade efficiency began in the 1950s, just as soon as piston-engined aircraft began to be eased out of the world’s main skyways, and as soon as jet engines, initially developed for military use, were being redesigned in ways that made economic sense for hauling passengers and freight over long distances at high speed. Aircraft such as the Viscount, the Comet, the Tupolev Tu-104, the Convair 880, the Caravelle, the Douglas DC-8, and, from 1958, the best known of all narrow-bodied jets, the Boeing 707, began to sweep the field. The engines with which they were equipped (the De Havilland Ghost; Pratt and Whitney’s JT3C and JT3D; Rolls-Royce’s Avon, Spey, and Conway; and for the two hundred Tupolev Tu-104s that Moscow built, the little-known Mikulin AM-3 turbojet) were all of their time state-of-the-art high-precision machines.
By today’s standards, these older engines were relatively primitive, being noisy, fuel-hungry, underpowered, and inefficient. Yet all this started to change, once again in the 1970s, as more and more aircraft were needed to fly over greater and greater distances and at higher and higher speeds. To produce the necessary thrust for the big and more economical wide-bodied jetliners that growing numbers of passengers and hard-pressed airline accountants alike were demanding, and to produce that thrust quietly and efficiently and with something of a nod to the growing environmental concerns of the latter half of the century, the new jet engines had to be huge and astonishingly powerful. They had to compress their inswept air (as much as one ton of it sucked in every second) to unimaginable pressures, they had to burn their fuel at unimaginable temperatures, and they had to create an interior holocaust, a maelstrom of fire, that tested every molecule of every metal piece that whirled and careened around inside.
This is where Rolls-Royce’s internal Blade Cooling Research Group, founded in the early 1970s, plays its part in the saga. The group’s mission was simple enough: solve the problem of keeping those high-pressure turbine blades from melting, and then jet engines could be made that would give out all the power anyone might need. For the axiom of turbinology is a simple one: the hotter the engine is run, the greater the spare pressure, and the higher the jet velocity. The hotter, in other words, the faster.
At the same time, though, the hotter the engine environment, the bigger the problem for the turbine blades. For while one might suppose the first task of a turbine blade is to drive the engine’s compressor, it actually is not. That is its secondary task. Its first task is quite simply to survive.
In Whittle’s engines, and the military jets that were built immediately after his invention turned out to be a proven success (and in the civilian world’s turbojet engine of the Vickers Viscount and the pure jet engines of the Comet, which was to become the first-ever commercial jetliner), the survival of the turbine blades was not a major issue.
They were critically important components, of course. The first blades that Whittle made were of steel, which somewhat limited the performance of his early prototypes, since steel loses its structural integrity at temperatures higher than about 500 degrees Celsius. But alloys were soon found that made matters much easier, after which blades were constructed from these new metal compounds in ways that met most of the challenges of the earliest engines. They were shaped to meet and extract energy from the peculiar violent vortices of the hot gases that swirled about them. They were fixed to the disc that carried them in a way that could manage the otherwise intolerable stresses of being whirled around at hundreds of revolutions each minute. Their shape was such that they managed to extract with remarkable efficiency the power from the chemical reaction between the hot compressed air and the fuel (gasoline in Whittle’s first laboratory, kerosene later on) delivered to them. They did not run the risk of melting, though, because the temperatures at which they operated were on the order of a thousand degrees, and the special nickel-and-chromium alloy from which they were made, known as Nimonic, remained solid and secure and stiff up to 1,400 degrees Celsius. There was adequate leeway between the temperature of the gas and the melting point of the blades. That would change, though, in the 1960s and ’70s. The leeway steadily diminished, and soon it finally vanished altogether.
For, by then, the demands made on the next generation of engines required that the gas mixture roaring out from the combustion chamber be heated to around 1,600 degrees Celsius, and even the finest of the alloys then used melted at around 1,455 degrees Celsius. The metals tended to lose their strength and become soft and vulnerable to all kinds of shape changes and expansions at even lower temperatures. In fact, extended thermal pummeling of the blades at anything above 1,300 degrees Celsius was regarded by early researchers as just too difficult and risky—unless someone could come up with a means of keeping the blades cool.
A team of about a dozen Rolls-Royce engineers promptly did just that. They worked out that it should be possible, with highly precise machining and the mathematical abilities of very powerful computers, to create an ultrathin film of relatively cold air that would swaddle each blade as it whirled around inside the engine, and which would protect it, thermally, from the hellish atmosphere beyond. The layer of cold air need be less than a millimeter thick, but if it managed to maintain its own integrity as the blade spun around, then the swaddled blade would also.
But where to acquire the cold air, inside a jet engine? The source was hidden, it turns out, in plain sight. After much pondering and experimenting, it was realized that the cooler air could come directly from the huge tonnage of atmosphere being sucked in by the fan at the engine’s front. Most of that air bypasses the engine (for reasons that are beyond the scope of this chapter), but a substantial portion of it is sent through a witheringly complex maze of blades, some whirling, some bolted and static, that make up the front and relatively cool end of a jet engine and that compress the air, by as much as fifty times. The one ton of air taken each second by the fan, and which would in normal circumstances entirely fill
the space equivalent of a squash court, is squeezed to a point where it could fit into a decent-size suitcase. It is dense, and it is hot, and it is ready for high drama.
For very nearly all this compressed air is directed straight into the combustion chamber, where it mixes with sprayed kerosene, is ignited by an array of electronic matches, as it were, and explodes directly into the whirling wheel of turbine blades. These blades (more than ninety of them in a modern jet engine, and attached to the outer edge of a disc rotating at great speed) are the first port of call for the air before it passes through the rest of the turbine and, joining the bypassed cool air from the fan, gushes wildly out of the rear of the engine and pushes the plane forward.
“Nearly all” is the key. Some of this cool air, the Rolls-Royce engineers realized, could actually be diverted before it reached the combustion chamber, and could be fed into tubes in the disc onto which the blades were bolted. From there it could be directed into a branching network of channels or tunnels that had been machined into the interior of the blade itself. And now that the blade was filled with cool air—cool only by comparison; the simple act of compressing it made it quite hot, about 650 degrees Celsius, but still cooler by a thousand degrees than the post–combustion chamber fuel-air mixture. To make use of this cool air, scores of unimaginably tiny holes were then drilled into the blade surface, drilled with great precision and delicacy and in configurations that had been dictated by the computers, and drilled down through the blade alloy until each one of them reached just into the cool-air-filled tunnels—thus immediately allowing the cool air within to escape or seep or flow or thrust outward, and onto the gleaming hot surface of the blade.
If the mathematics is performed correctly—and it is here that the awesome computational power that has been available since the late 1960s comes into its own, becomes so crucially useful—and if the placing of all these pepperings of minute holes is correctly achieved, with some holes on the blade’s leading edge, some on its chubby little body, some along the trailing edge, then this cool air will form an unimaginably thin film of comforting relative frigidity, wrapping itself around the blade and coating its whirling surface like a silvery insulating jacket. It is this, then, that allows the blade to survive the blistering heat of the onrushing fuel-air mixture, which the combustors have just set alight.*
All who see such a jet engine turbine blade, and who know something of its manufacture, see in its making the most sublime of engineering poetry, much like the finest of Rolls-Royce motorcars, one might say—the Silver Ghosts of eighty years before had many of the qualities of perfection that are engineered into today’s better aircraft engines. Each of the Rolls-Royce nickel alloy blades (which weigh less than a pound, are mostly hollow but sensationally strong, can fit easily into the palm of the hand, and, as it happens, are also, for now, essentially made by hand) is cast in a top-secret factory near Rotherham, in northern England. The most proprietary and commercially sensitive aspect of the blades, aside from the complex geometry of the hundreds of tiny pinholes, is the fact that the blades are grown from, incredibly, a single crystal of metallic nickel alloy. This makes them extremely strong—which they need to be, as in their high-temperature whirlings, they are subjected to centrifugal forces equivalent to the weight of a double-decker London bus, of around eighteen tons.
There is a delicious irony here, however. For although, as one might expect, to make such a blade requires techniques displaying the very highest order of precision and computational power, they are combined with another means of manufacturing that is of the greatest antiquity. The “lost-wax method” was known to the Ancient Greeks, for whom precision was a wholly unfamiliar concept.* It is employed specifically in this case to allow the creation of the cooling tunnels within the blade; and the wax is melted out, as in Athenian days, just before the molten alloy is poured into the ceramic mold, which is now, absent the wax, busy with the network of voids for the eventual cooling air.
Creation of the blade’s single-crystal structure is encouraged at this very point in the long and cumbersome manufacturing process, and is the company’s most closely guarded secret. Very basically, the molten metal (an alloy of nickel, aluminum, chromium, tantalum, titanium, and five other rare-earth elements that Rolls-Royce coyly refuses to discuss) is poured into a mold that has at its base a little and curiously three-turned twisted tube, which resembles nothing more than the tail of P. G. Wodehouse’s Empress of Blandings, the fictional Lord Emsworth’s prize pig. This “pigtail” is attached to a plate that is cooled with water, and the whole arrangement, once it is filled with liquid metal, is slowly withdrawn from the furnace, allowing the metal, equally slowly, to solidify.
This it does, first, at the cool end of the pigtail, but because the mold here is so twisted, only the fastest-growing crystals and those with their molecules distributed with what is called a face-centered cubic arrangement, for complex reasons known only to students of the arcana of metallurgy, manage to get through. And through this magic of metallurgy, the entire blade then assembles itself from the one crystal that makes it along the pigtail, and ends up with all its molecules lined up evenly. It has become, in other words, a single crystal of metal, and thus, its eventual resistance to all the physical problems that normally plague metal pieces like this is mightily enhanced. It is very much stronger—which it needs to be, considering the enormous centrifugal forces that dominate its working life.
Having now created the single-crystal blade, it remains only to dissolve away any of the substances that remain in its core, and then to use a technique called electrical discharge machining to drill the hundreds of tiny holes down into the cooling channels. Electrical discharge machining, or EDM, as it is more generally known, employs just a wire and a spark, both of them tiny, the whole process directed by computer and inspected by humans, using powerful microscopes, as it is happening. The process is all but silent, and it is in many ways more melting than drilling.
Here, however, comes an important moment in the story, one that has crept into the narrative all too stealthily.
The making of high-pressure turbine blades has long required the absolute concentration of legions of workers, men and women with decades of experience in hand-eye coordination and a studiously learned degree of extreme manual dexterity. These “blade runners,” as it were, have for years past learned to manage, for instance, the complexities and eccentricities of the cooling-hole drilling machines—and the more complex the engines, the more holes need to be drilled into the various surfaces of a single blade: in a Trent XWB engine, there are some six hundred, arranged in bewildering geometries to ensure that the blade remains stiff, solid, and as cool as possible.
Despite the apparent fantastical complexity of a modern jet engine—here a Rolls-Royce Trent, four of which power the enormous Airbus A380 double-decker superjumbo jets—there is essentially only one moving part, a rotor that passes through the entire length of the engine, from fan to exhaust.
Yet human lives, those of the aircraft passengers and crew, are dependent on the engine’s not self-destructing in flight. The vanishingly small number of occurrences of this kind of incident is based to a large degree on the integrity of these human-made engine blades. Because there is no doubt of the blades’ immense importance, it is worth noting that their integrity owes much to the geometry of the cooling holes that are being drilled, which is measured and computed and checked by skilled human beings. No tolerance whatsoever can be accorded to any errors that might creep into the manufacturing process, for a failure in this part of a jet engine can turn into a swiftly accelerating disaster.
This stark realization, that lives depend on the perfection of these blades, brings this one industry to a critical moment, a crucial development—the first in the story, perhaps, and one that would be unimaginable to precision’s originators, to engineers such as John Wilkinson or Joseph Bramah, Henry Maudslay or Joseph Whitworth, or indeed, to Henry Royce himself. Engineering, in this one field
to start with, seems now to have reached a degree of sophistication in which the rigorous demands of modern precision have come for the first time to outstrip the abilities of humankind to meet them.
Up until this point, the processes—whether it is the making of a cylinder or a lock or a gun or a car; the boring or the milling or the grinding or the filing; the directing of the lathe or the tightening of a screw or the measuring of flatness or circularity or smoothness—invariably involve some kind of human agency. Yet now, in this one field to begin with, but in many more as the tolerances shrink still further and limits are set to which even the most well-honed human skills cannot be matched, automation has to take over. The Advanced Blade Casting Facility can perform all these tasks (from the injection of the losable wax to the growing of single-crystal alloys to the drilling of the cooling holes) with the employment of no more than a handful of skilled men and women. It can turn out one hundred thousand blades a year, all free of errors—or as far as anyone knows.
Once, the most troubling consequence of the introduction of precision machinery was the displacement of unneeded workers, who were understandably vexed. Nowadays, it is perhaps the relative paucity of human supervision in engineering fields where human lives are at stake that has steadily become a more pressing concern.
“Our people are fantastically skilled,” remarked the manager of manufacturing at the new plant, “but they’re human, and no human is going to produce the same quality of work at the end of a shift as they do at the beginning.” Precision engineering, in this industry in particular, does now appear to have reached some kind of limit, where the presence of humans, once essential to maintaining the attainment of the precise, can on occasion be more of a drawback than a boon—as the investigation into the Qantas Airways jet engine failure amply illustrates.