Turbine Tech

Sir Frank Whittle's centrifugal engine concept worked, but Hans von Ohain's axial engine design carries on today
history, aviation, technology
Written By J. Mac McClellan

It’s rare that credit for an invention is officially shared between two people. But that’s what happened with the invention of the jet engine. British engineer Sir Frank Whittle and German engineer Hans von Ohain, working independently and with no knowledge of the other, both patented the design for an aircraft jet engine in the 1930s.

Over the decades, technology buffs and governments alike squabbled over who actually invented the jet engine first. Each person had a claim. Whittle received the first patent, but a von Ohain engine flew first. The dispute was settled—as much as these things can be—in 1991 when the two inventors were jointly awarded the Charles Stark Draper Prize by the U.S. National Academy of Engineering for “their independent development of the turbojet engine.”

Whittle’s pioneering centrifugal engine led directly to jet engine development by Rolls-Royce. But von Ohain’s work more quickly led to the practical jet engine design, the axial flow compressor concept that has been used by jet engine makers since the late 1940s.

Easy Concept, Difficult Reality

A jet engine is actually a powerful air pump. The engine sucks air in the front, compresses the air, adds fuel and burns it to expand the air rapidly, which forces a high velocity jet of air out the rear to propel the airplane.

The “jet” concept of propulsion is ancient, dating back at least many hundreds of years to Chinese rockets that burned a crude form of gunpowder to generate thrust. But the hard part, the riddle both Whittle and von Ohain solved, is how to make an engine that delivers sustained power, not just a short burst of rocket thrust.

The key to the jet engine is the compressor and the turbine that powers it. The higher the air pressure the compressor can deliver to the burner section, the greater the thrust an engine produces.

Both Whittle and von Ohain first experimented with a centrifugal compressor, which is actually a wheel with vanes machined into it. Centrifugal compressors fling air outward with great force. The technology was well understood in the 1930s and was used for many pumping applications, including to force air into the cylinders of radial piston engines.

But air flung outward by the centrifugal compressor needs to be captured by a series of pipes and then directed back into the engine burner section. The first jet engines were very large in diameter to accommodate the compressor wheel and the pipes around the compressor. And compressor efficiency was pretty miserable with the air being forced through so many pipes and ports on its way to the burner section.

The key to the jet engine is the compressor and the turbine that powers it. The higher the air pressure the compressor can deliver to the burner section, the greater the thrust an engine produces.

Ingenious Solution

Von Ohain, working with engineers at the German aviation firm Junkers, soon realized that an axial compressor would be much more efficient, and more compact. An axial compressor is a series of sturdy fans that force air into an ever smaller tube behind the fan. Axial compressor sections can be mounted in a series so each stage compresses the air a little more, producing very high pressure by the time the air reaches the burner section.

Another benefit of an axial compressor over the Whittle design is that air is sucked in directly through the forward engine inlet, while the centrifugal compressor draws air in from the side. The efficiency gain of the axial compressor was enormous.

It’s unclear how much direct involvement von Ohain had at Junkers, but the Jumo 004 engine the company developed was the first to fully use the axial flow design. The Jumo 004 powered the Messerschmitt Me 262, the only jet fighter to achieve operational status during World War II.

Improvements Over Time

Though the axial compressor was clearly the best design for an aircraft jet engine, and was adopted by most companies in the engine business, the real advances in jet engine power, fuel efficiency and durability came from new metal alloy technologies and aerodynamic improvements. New advanced metal alloys are able to withstand the temperatures and pressures necessary to allow efficient burning of fuel. And the complex flow of air as it was compressed, burned and forced out the tailpipe was not well understood for many years. In fact, development of computational fluid dynamics software has been a key in improving aerodynamics within jet engines, leading to a 25 percent improvement in fuel efficiency in just the past 15 years.

But the centrifugal compressor did not disappear from turbine engines. Most APUs—the little jet engine that produces the power to heat and cool the cabin when the airplane is on the ground—use a type of centrifugal compressor. And a number of propulsion engines use a centrifugal compressor located at the end of the axial flow sections to fling the air into the combustion chamber with great force.

Of course none of the jet engine pioneers could have imagined the high-bypass fan engine design that essentially doubled the fuel efficiency of jet engines when it was developed beginning in the late 1960s. After all, a fan on a jet engine is really a very high-tech and very efficient propeller, something early jet engine designers wanted to get away from. But it is the turbofan design that powers airplanes like the Gulfstream G650ER to speeds faster than 90 percent of the speed of sound and are efficient enough to fly 7,500 nautical miles/13,890 kilometers.

All in aviation owe a great debt to both Whittle and von Ohain for having the first vision of what a turbojet engine could do. First is almost never final, but creating a jet engine that ran, and then flew, is a milestone that forever changed the way we fly.

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