The Evolution of the Jet Engine, From Whittle to the Open Fan

Every jet engine under a wing today answers the same question, asked again roughly every fifteen years since the 1930s: how do you push more air, more efficiently, through the same basic machine? The history behind it is surprisingly dramatic. There are two rival inventors who never met, a fighter that arrived too late to matter, an engine saved from cancellation with two weeks to spare, and a new design being tested in the open for the decade ahead.

Two inventors, working in secret, on opposite sides of a war neither engine helped win

The jet engine has two fathers who didn’t know about each other until later. In Britain, RAF officer Frank Whittle patented the turbojet in 1930 and spent years struggling to be taken seriously. Money was scarce, and his first test engine, the Whittle Unit, didn’t run until 1937. In Germany, Hans von Ohain reached essentially the same idea independently, working out his own theory of jet propulsion in 1933 while still a doctoral student at the University of Göttingen. Neither man knew the other existed. What decided who got there first wasn’t the idea but the backing. Von Ohain teamed up with the Heinkel aircraft company, and on 27 August 1939, days before the Second World War began, the Heinkel He 178 became the first aircraft ever to fly on pure jet power. Whittle’s aircraft, the Gloster E.28/39, didn’t fly until May 1941, nearly two years later. He had spent the 1930s trying to persuade Britain to fund something that sounded, to most people, like science fiction.

The engine that actually went to war

Neither Whittle’s nor von Ohain’s first engine went into service. That honour went to a third design: the Junkers Jumo 004, developed by Junkers under Anselm Franz. It was an axial-flow turbojet, where the air flows straight through a row of compressor stages, unlike Whittle’s centrifugal-flow design, which flings the air outwards. The axial-flow layout, not Whittle’s, became the template for nearly every jet engine since. Fitted to the Messerschmitt Me 262, the Jumo 004 powered the world’s first operational jet fighter in 1944, about 100mph faster than any piston-engined fighter of the time. It came too late and in too few numbers to matter much. Production suffered badly from bombing and from chronic shortages of the heat-resistant metals its turbine blades needed. But it proved the axial-flow jet engine was the design worth building on.

From wartime secret to airline seat

Jet airline travel began in 1952, when the de Havilland Comet entered service. It was a huge leap that slashed journey times, but the programme was badly damaged by a series of fatal crashes traced to metal fatigue cracks that started at the corners of cut-outs in the fuselage skin, a lesson in structural engineering the whole industry learned from. The aircraft that really built the jet age at scale was the Boeing 707, later powered by Pratt & Whitney’s JT3D. That wasn’t a pure turbojet like the Jumo 004 or Whittle’s engines. It was an early, low-bypass turbofan: some of the incoming air was deliberately sent around the engine’s core instead of through it. It was a small first step towards the idea behind every engine generation since. Moving more air, rather than pushing a smaller amount harder, was the way to a better engine.

The idea taken seriously: high bypass, and the widebody it made possible

The real break came in the late 1960s, when Pratt & Whitney committed to a high-bypass turbofan on a scale nobody had tried in airline service: the JT9D. It was designed for an aircraft that didn’t exist yet, the Boeing 747, covered in full in this site’s own 747 story. Instead of forcing a fairly small amount of air through the core very fast, the JT9D moved a much larger amount of air around the core more gently, for the same thrust. That made it quieter and noticeably more fuel-efficient, because moving more air slowly takes less energy than moving less air quickly. The JT9D entered service on the 747 in January 1970. It went on to power the 767, the Airbus A300 and A310, and the McDonnell Douglas DC-10. It proved high bypass wasn’t just better on paper: a whole generation of widebodies was built around it. General Electric followed closely with the CF6, entering airline service in 1971 and developed from the TF39 that GE had built for the C-5 Galaxy military transport. It’s a rare case of military engine technology flowing straight into airliners.

Two weeks from cancellation: the engine that became the most common ever built

Not every important engine arrived in triumph. CFM International, a 50/50 joint venture between GE and France’s Snecma set up in 1974, ran its first CFM56 that year and then went more than five years without selling one. By March 1979 the programme was two weeks from cancellation. It wasn’t saved by a design breakthrough. Three airlines, Delta, United and Flying Tigers, chose the CFM56 to re-engine their ageing Douglas DC-8s, and within weeks the US Air Force ordered it for its KC-135 tankers. Meanwhile Boeing had spent the same years failing to sell airlines a CFM56-powered 707 to meet tougher noise rules. So the aircraft the engine was meant to rescue couldn’t rescue it, and a completely different one did. From that near-death, the CFM56 became the standard engine of the Boeing 737 Classic and NG and the Airbus A320ceo family, two of the best-selling airliners ever built. More than 20,000 have been made, and it’s still the most common jet engine in the world by a wide margin.

The next real architectural break: letting the fan think for itself

For four decades after the JT9D, every high-bypass turbofan shared one limitation. The fan and the low-pressure turbine that drives it sat on the same shaft and spun at the same speed. That forced a compromise, because a fan wants to turn slowly for efficiency while a turbine wants to spin fast for power. Pratt & Whitney’s geared turbofan, the PW1000G family, finally broke that link by putting a reduction gearbox between the two, so each can spin at its own best speed. The maker’s claims are bypass ratios up to about 12:1, up to about 16% less fuel burned and a noise footprint up to 50% smaller than the engines it replaced. It powers every Airbus A220 and is one of the two engine choices on the A320neo family.

What comes after the geared turbofan

The next big step is already being tested, and it looks stranger than anything before it. CFM’s RISE programme removes the casing around the fan altogether, leaving the blades open in what’s called an open-rotor or open-fan design. That pushes the bypass ratio, and the efficiency, further than an enclosed fan physically allows. CFM is aiming for service entry around the mid-2030s, so it won’t carry passengers for years. It’s serious enough, though, that GE’s own newsroom published a piece on why Mentour Pilot, Petter Hörnfeldt, one of the most-watched airline pilots on YouTube, has become a public partner in explaining it. It shows how seriously the makers take the job of explaining an engine that looks, at first glance, as if something has gone missing.

Sources & Further Reading

• Timeline — Sir Frank Whittle, Inventor of the Jet Engine ↗
• Pratt & Whitney JT9D — Wikipedia ↗
• CFM56 — History and Future of the Go-Anywhere Engine — Mentour Pilot ↗
• CFM International CFM56 — Wikipedia ↗
• Why the CFM RISE Program Has Mentour Pilot Telling the World — GE ↗