Following a recent piece here, a reader (signed in as “guest”) has written to ask just how rocky the road was for some early jet-engine builders. Thus, we have a complete departure from my usual subject of motorcycles, but one which still proves instructive.
We know that spark-ignition engines for motorcycles have evolved from 1885 to 2021 through every kind of difficulty. So how did it go for the gas turbine?
It was tough. Those of us who take an interest in aviation know that the German Luftwaffe committed Messerschmitt’s twin-engined Me 262 to combat late in World War II, and know also that the Allies were fortunate that Adolf Hitler initially insisted that they be used as light bombers in addition to their role as fighters. Their greatly superior speed would have allowed their pilots to enter or break off combat at will by merely nudging the throttles.
A Simple Gas Turbine Explanation
A gas turbine’s operating cycle shares important features with our familiar internal-combustion piston engines. Intake air is compressed (typically through many stages of an axial-flow rotary compressor). Then a portion of that air enters a circular burner array into which fuel is sprayed, resulting in continuous combustion. Diluted by pure air down to a tolerable temperature, this hot gas next expands through one or more turbine stages which extract from the gas the energy to spin the compressor. Hot combustion gas, greatly expanded in volume by combustion heat, is expelled from the rear of the engine at a much higher velocity than that of the entering intake. The result is thrust. If shaft power is also required, as in turboprop or turbofan engines, one or more power-turbine stages can be added to extract the necessary energy.
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Making such a completely novel powerplant operational was a nightmare. Nickel and cobalt, two of the prime constituents of heat-tolerant refractory metals, were urgently needed for armor alloys and for cutting tools. This compelled engineers to overcome the turbine-blade temperature problems less with materials and more by active blade cooling. A relatively simple nickel-chromium stainless steel called Tinidur was initially used; a second material—Cromadur—was later adopted under pressure to further reduce use of essential metals. The material was formed into sheet, which was then folded and welded into hollow airfoil shapes through which cooling air could flow. Each turbine consisted of a ring of such blades joined to the rim of a disc and attached to a shaft which drove the compressor.
25-Hour Engine Life
It’s one thing to achieve steady operation on a test bench under the control of engineers and technicians. It’s quite another to deliver reliable field performance in the hands of highly stressed combat pilots. Much work was necessary to avoid compressor stalling and surging. Service life was short, less than one-tenth of the 250 hours expected from piston engines of that time.
In Britain, during the interwar period, the idea of jet propulsion was a one-man crusade carried forward by Frank Whittle, a serving officer of the RAF. The British Air Ministry at first dismissed Whittle as having it all wrong, enlisting a respected engineer to write a refutation of the whole idea. When Whittle persisted with incredible energy, even to the point of finding private backing to construct and make a successful proof-of-concept prototype, the ministry reversed itself, taking the view that of course it had been Whittle’s duty to develop the engine, thank you very much, and told “We’ll take it from here.” Years later Whittle would be rewarded with a tip of 100,000 pounds and a ride on the supersonic Concorde airliner.
Because of the whirling turbine blades’ harsh operating conditions, combustion gas had to be cooled by diluting it with compressed air, making the engines extremely thirsty. Yet here is the essential point: Because the gas turbine had become crucial to modern air power, its development received the all-out support of major governments. Extreme fuel consumption made jet engines usable only in fighters at first, but rapid development of improved refractory metals allowed constant increases in turbine inlet temperature. This in turn made the engines more fuel efficient, because more energy could be extracted as thrust by expanding combustion gas from higher temperature and pressure. This is analogous to raising the compression ratio of a conventional piston engine.
Meanwhile, the kingpins of the piston aero-engine business were sure that full transition to jet power would take many years, so they busied themselves with developing advanced combinations of proven piston engines, adding elements such as exhaust-driven power recovery turbines, a technology based upon that of wartime turbochargers. One that reached service was Wright’s TC-18 Turbo-Compound, which powered Lockheed’s Constellations and Douglas’ DC-7. Another design, Napier’s Nomad, a large, flat-12 two-stroke diesel coupled through a variable drive to a power recovery turbine, never made it out of prototype. While the TC-18′s reduced fuel consumption did make direct flights from New York to London or Paris possible for the first time (previous trans-Atlantic hops had to refuel at Gander, Newfoundland), other such interim projects were stopped cold by the rapid progress of the gas turbine.
Keeping the Blades Cool
Governments are aviation’s “factory teams,” able to push technology with almost unlimited budgets. So here came the progress; not only did refractory metals improve rapidly, so did their design and, most important of all, their cooling. The basic idea was to sheathe each blade in a thin but continuously replaced layer of cooler air, flowing from inside the blade to emerge on its outside surface through a multitude of tiny holes (drilled by such means as EDM or laser evaporation). Blade-cooling air, like the dilution air added to reduce turbine inlet temperature, reduces efficiency by reducing the temperature of the hot gas expanding through the system. The next step, long in development, will be the use of uncooled ceramic blades and stator vanes.
At each increase of survivable turbine inlet temperature there was a corresponding reduction in fuel consumption. As those who endure dreary thermodynamics courses know, overall system efficiency depends strongly on maximizing the difference between hottest and coolest cycle temperatures. Just as in piston engines, raising the temperature and pressure of combustion did wonders for performance and fuel efficiency. At present, during operation at takeoff power, jet-engine turbine blades operate in a stream of combustion gas hotter than the melting point of their material.
There were basic mechanical problems. For a time in the 1950s, the 300-hour lifetime of main shaft ball bearings determined MTBO—mean time before overhaul. The major load on those bearings was not the weight of the rotating parts but the natural outward “fling” of the heavy, large-diameter balls themselves at high shaft speeds. A significant increase in bearing fatigue life came from developing extremely “clean” bearing steels containing few crack-nucleating internal impurities, brought about by vacuum remelting, which allowed impurities to boil away. Improvement also came from developing lighter bearing balls that were hollow, assembled by friction welding. Later, even lighter balls would be made of “stone”—ceramics such as sapphire and, eventually, silicon nitride.
Although early jet engines were lubricated by conventional aviation oils, rapid oxidation forced development of synthetic oils designed to deliver stable performance at higher temperatures. Two steps along the way have been diesters and neopentyl polyol esters.
Developing the Ducted Fan
Here’s a fundamental principle of propulsion: It is more efficient to accelerate a large mass of air rearward by a small amount than to accelerate a small amount of mass rearward by a large amount. Think of it like this: A tractor’s tires push back on the whole earth and accelerating it very little has high propulsive efficiency. Now, try using a rocket engine to do spring plowing. The result is very inefficient because the rocket’s extreme exhaust velocity (~7,000 feet per second) carries away most of the energy while tractor and plow receive almost none.
The same applies to our aircraft’s pure turbojet engines. In this first engine type, all of the thrust is a reaction to accelerating its core engine exhaust stream to very high speed. This in turn led to designs which used part of the turbine’s power to drive a ducted fan, which in reality is just a specialized propeller spinning inside a short tube. This increased propulsive efficiency at lower speeds, such as the 540–580 mph typical of most airliners, by accelerating a larger mass of air rearward, but to a lesser speed.
The ratio of an engine’s core airflow to its fan airflow is called its bypass ratio, and in recent efficient airliner engines this has risen as high as 11:1. This means that by far the greater part of the engine’s thrust comes from the ducted propeller or fan, with only a small fraction generated by the inefficient high-speed exhaust of the core turbine engine.
Intensive study of turbine-blade reliability, the combination of extreme temperature and the centrifugal force of high-speed rotation, revealed that “creep,” or the gradual increase of blade length as a result of tensile stress, was mainly mediated by events in the intergranular zones. These are the less-structured jumbles of atoms joining the metal’s many crystals. Then let’s eliminate the intergranular zones by making hot-section parts as single crystals! Methods were developed to achieve this, operating entirely in controlled conditions in vacuum. In addition, very large engines actively control the clearance between rotating blade tips and engine casing.
The disc that supports the high-pressure turbine’s many blades is one of modern manufacturing’s most highly developed pieces. In order to control grain size in the finished part, these discs are made from metal powders solidified into solid form by Hot Isostatic Pressing (HIP), which subjects parts to a pressure of 15,000 psi while holding them at elevated temperature. Many steps of closely controlled heat treatment are necessary for the final result: a disc that never breaks into three hatchet-shaped pieces that slice outward through whatever is in their way.
Optimizing Shaft Speed
In the past, designers have sought greater efficiency by allowing different parts of the engine to operate closer to their most efficient shaft speeds. Engines with two or even three concentric shafts, each bearing its own compressor and turbine stages, have been built toward this goal. An alternative approach is to give compressor stator vanes controllable pitch, as was done years ago in certain automatic car transmissions.
Even so, a significant problem remains, because the fan, which can be 10 feet across in large engines, must turn quite slowly for best efficiency in order to avoid seriously supersonic tip speeds. In contrast, the core engine is most efficient at much higher rpm. Pratt & Whitney’s solution is the GTF, or geared turbofan, in which the fan is driven by the turbine through reduction gearing. Rolls-Royce is developing a similar system. In one set of figures, such gearing reduces fuel consumption by 12 percent, and the more efficient speeds afforded to fan and core allow cost reductions from the elimination of some stages; in one such design, a reduction by 900 blades was possible.
What can justify the large R&D costs associated with such things? A Boeing 777 burns 17,000 pounds of fuel per hour in cruise: 12 percent of that pencils out to 2,040 pounds less fuel that the airline has to buy—every hour—for just one 777. The less fuel an aircraft has to carry, the more it can earn through greater payload. The much-loved 747s are now rapidly disappearing from service because of their higher cost of operation, as they burn 25,000 pounds per hour.
The combination of unending pressures from military and commercial users has driven steady refinement, so much so that present airliner engines may be “on the wing” for 25,000 hours. Compare this with the most reliable of piston engines, Bristol’s sleeve-valve Hercules, which on occasion gave 4,000 hours of operation before overhaul; a close second was P&W’s R-2800 at 3,500 hours. High-power military piston engines as on Strategic Air Command’s B-36 bomber of the 1950s, might typically achieve less than one-tenth of this because they spent so much time at higher power settings. The comparison between a touring bike’s engine and a racebike’s comes to mind.
Under discussion: the goal of fan engines as long-lived as airframes, expected to reliably operate 50,000 hours or more. A major tool in this work is the development of a continuously updated digital model of each engine based upon a “nervous system” of sensors; this is constantly updated to the manufacturer, where software monitors any developing problem. At the average 30-mph speed of a motorcycle in mixed urban and highway operation, that 50,000 hours would translate to one and a half million miles of riding.