To be attractive to airlines an engine needs to be as efficient as possible, minimizing fuel consumption and the amount of maintenance it requires. High fuel efficiency requires high compression ratios and engine temperatures, which in turn require extremely efficient compressors, components that are both incredibly strong and incredibly lightweight, and materials that can withstand extreme temperatures. And a commercial jet engine must successfully operate hour after hour, day after day, for tens of thousands of hours before being overhauled.
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Only a handful of companies produce them: GE (both independently and via CFM, its partnership with France’s Safran), Pratt and Whitney, and Rolls-Royce.1 Developing a new engine is a multi-billion dollar undertaking. Pratt and Whitney spent an estimated $10 billion (in ~2016 dollars) to develop its geared turbofan and CFM almost certainly spent billions developing its LEAP series of engines. (As with leading edge fabs and commercial aircraft, the technical and economic difficulty of building a commercial jet makes it one area of technology where China still lags. China is working on an engine for its C919, but hasn’t yet succeeded.)
It’s not that building a working commercial jet engine itself is so difficult. It’s that a new engine project is always pushing the boundaries of technological possibility, venturing into new domains — greater power, higher temperatures, higher pressures, new materials — where behaviors are less well understood. Building the understanding required to push jet engine capabilities forward takes time, effort, and expense.
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Air is taken into the front of the engine, then run through a compressor, increasing the air’s pressure. This compressed air flows into a combustion chamber, where it’s mixed with fuel and ignited, producing a stream of hot exhaust gas. This exhaust gas then drives a turbine, which extracts energy from the hot exhaust as it expands, converting it into mechanical energy in the form of the rotating turbine. This mechanical energy is then used to drive the compressor at the front of the turbine.
In a gas turbine power plant, all the useful work is done by the mechanical energy of the rotating turbine. Some mechanical energy drives the compressor, while the remaining energy drives an electric generator. In a jet engine, the energy is used differently: some energy drives the compressor via the turbine, but instead of using the remaining energy to generate electricity, a jet engine uses it to create thrust through hot exhaust gases, pushing the aircraft forward the same way an inflated balloon propels when air rushes out of it.
Building a functional jet engine requires several key supporting technologies. One such technology is the compressor. In a Brayton cycle engine, roughly 50% of the energy extracted from the hot exhaust gas must be used to drive the compressor (this fraction is known as the back work ratio). Because the back work ratio is so large (a steam turbines has a back work ratio closer to 1%), any losses from compressor inefficiencies are proportionally very large as well. This means that a functional jet engine needs turbines and compressors that transfer as much energy as possible without losses. Whittle was successful partly because he built a compressor that ran at 80% efficiency, far better than existing compressors. Many contemporaries believed Whittle would be lucky to get 65% efficiency — jet engine designer Stanley Hooker noted that he “never built a more efficient compressor than Whittle”.
Another important advance was in turbine materials. The fuel in a jet engine burns at thousands of degrees, and the turbine needs to be both strong and heat-resistant to withstand the rotational forces and temperatures. Whittle’s first engine used turbine blades of stainless steel, but these failed frequently and it was realized that stainless steel wasn’t good enough for a production engine. The first production engines used turbine blades made of Nimonic, a nickel-based “superalloy” with much higher temperature resistance. As we’ll see, the need to drive engine temperatures higher and higher has pushed for the development of increasingly elaborate temperature resistant materials and cooling systems.
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And while piston engines could be made from comparatively thick and sturdy castings and forgings, much of a jet engine was made from thin sheets of exotic alloys carefully bent into shape, which required novel and complex manufacturing techniques.
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During the Korean War, an Air Force report noted that jet engine failures were the leading cause of major accidents: in 1951 alone there were 149 such failures, destroying 95 aircraft and killing 25 pilots. Engines were so unreliable that they made Air Force recruitment difficult: pilots “were no longer eager to join the Air Force if they had to learn to fly jets.”
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The demands of commercial service would continue to push jet engine performance higher and higher: Higher compression ratios and temperatures to minimize fuel consumption, and longer times between overhauls. This meant continually pushing the technology forward. For instance, early jet engines were made mostly from steel and aluminum, but by the 1960s they were being fashioned mostly from titanium and “superalloys” like Inconel. Turbine blades, already difficult to fabricate in the 1950s, got even more complex, with elaborate internal structures to allow cooling air to flow through the turbine blade.
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On a turbojet, the hot exhaust exits the engine at a high speed, but jet engines are at their most efficient when the exhaust stream is as slow as possible. Air moved by the fan around the sides of the engine will be much slower than the hot exhaust from the combustion chamber, improving engine fuel efficiency. This slower air also makes much less noise — an important factor, since people were getting fed up with the noise from jets. A large fan also makes it easier to increase engine thrust, making it possible to power larger, heavier aircraft.
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By the 1970s, more than 30 years after the first jet-powered aircraft flew, it was still incredibly difficult and expensive to bring a new jet engine into service. Development costs were approaching a billion dollars: Rolls-Royce spent $874 million (close to $7 billion in 2025 dollars) to bring its RB211 into service, and delays and cost overages on the program bankrupted the company, forcing the British government to nationalize it.
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The difficulty is building an engine that meets its various performance targets — thrust, fuel consumption, maintenance costs, and so on. There’s no point in designing a new engine if it doesn’t significantly improve on the state of the art, and that means engine development projects are constantly pushing technological boundaries: higher compression ratios, hotter temperatures, lighter weight, larger fans, and so on. An engine that isn’t an improvement over what’s already on the market won’t be competitive, and engine performance targets will often be contractual obligations with the aircraft manufacturers buying them.
Making these improvements requires constantly driving engine technology forward. Turbine blades, for instance, have been forced to get ever more advanced to withstand rising exhaust temperatures: modern turbine blades have elaborate internal cooling structures, are made from high-temperature superalloys like Inconel or titanium aluminide, and are often made from a single crystal to eliminate defects at material grain boundaries. And while the carbon fiber fan blades on the RB211 were unsuccessful, manufacturers didn’t give up, and such blades are used on the CFM LEAP engine.
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A jet engine must direct and control an enormous amount of heat energy — a modern large jet engine will generate power on the order of 100 megawatts — and it must do so using as little mass as possible. A 1930s Ford V8 car engine weighed around 7 pounds for every horsepower it generated. A WWII aircraft piston engine weighed around 1 to 2 pounds per horsepower. The 50s-era J57 jet engine weighed closer to 0.1 to 0.2 pounds per horsepower it generated.
A commercial jet engine must operate for thousands of hours a year, year after year, before needing an overhaul, demanding high durability and high fatigue resistance. It must burn fuel at temperatures in the neighborhood of 3000°F or more, nearly double the melting point of the turbine materials used within them. Turbines and compressors must spin at more than 10,000 revolutions per minute, while simultaneously minimizing air leakage between stages to maximize performance and efficiency.
A commercial jet engine must operate across a huge range of atmospheric conditions – high temperatures, low temperatures, both sea level and high-altitude air pressures, different wind conditions, and so on. It must withstand rain, ice, hail, and bird strikes. It must be able to successfully contain a fan or turbine blade breaking off.