Turbofan

The turbofan emerged on May 27, 1943, when Daimler-Benz ran the DB 007—a jet engine with a bypass fan that pushed air around the combustion core rather than through it. The German engineers understood the physics: a turbojet accelerates a small mass of air to high velocity; a turbofan accelerates a larger mass to lower velocity. For the same thrust, the turbofan burns less fuel. The bypass ratio—air flowing around the core versus through it—determines efficiency.

The DB 007 was canceled in May 1944. Too complex, too heavy, too speculative for a regime desperate for operational weapons. The simpler turbojet won. But the physics didn't change. Momentum is mass times velocity. Kinetic energy is half mass times velocity squared. To generate thrust efficiently, maximize mass flow and minimize velocity. The turbofan was inevitable once someone did the thermodynamics.

What had to exist first? The turbojet, providing the hot gas generator at the core. Metallurgy capable of producing large-diameter fan blades that could withstand centrifugal stress at high RPM. Aerodynamic understanding of how bypass air interacts with core exhaust. And critically, demand for fuel efficiency—turbojets worked fine if you didn't care about range or operating costs.

Commercial aviation created that demand. By the 1960s, airlines needed aircraft that could cross oceans profitably. The Boeing 747 required engines delivering twice the thrust of existing turbojets while consuming one-third less fuel. Pratt & Whitney's JT9D, first run in December 1966, achieved this through high bypass ratio. For every kilogram of air passing through the combustion core, five kilograms flowed around it, propelled by the front fan. The engine certified in May 1969; the 747 entered service January 1970.

The turbofan exhibited niche construction at global scale. By making long-range widebody aviation economical, it created selection pressure for even higher bypass ratios. Early turbofans had bypass ratios around 5:1. Modern engines reach 12:1. The GE9X powering the Boeing 777X achieves 10:1 with a fan diameter of 134 inches—larger than a Boeing 737 fuselage. Each generation pushed more air slower, extracting efficiency from the same thermodynamic principle the DB 007 demonstrated in 1943.

The technology transformed aviation economics. A turbojet-powered Boeing 707 consumed roughly 13,000 pounds of fuel per hour. A turbofan-powered 747 consumed 12,000 pounds per hour while carrying three times as many passengers twice as far. The A380, powered by Rolls-Royce Trent 900 turbofans with 8.7:1 bypass ratio, seats 850 passengers on 14-hour flights. The thermodynamic advantage compounds: better fuel efficiency enables longer range, which enables larger aircraft, which demands more efficient engines.

Path dependence locked in the turbofan architecture. Once airlines invested in turbofan-equipped fleets, switching to alternatives required replacing entire aircraft. Engine manufacturers optimized turbofans through incremental improvements—better fan blades, higher compression ratios, advanced materials. The baseline architecture from the DB 007 persists: a ducted fan driven by a gas turbine core, with bypass air flowing around the combustion section.

Today, every commercial airliner uses turbofans. Military fighters still use low-bypass turbofans or pure turbojets where supersonic performance matters more than fuel economy. But subsonic aviation—where 99% of flying happens—is turbofan territory. The selection pressure for fuel efficiency at cruising speeds made the architecture dominant.

The turbofan also reveals how physics constrains solutions. Daimler-Benz in 1943 and Pratt & Whitney in 1966 converged on the same design because thermodynamics permits no alternative. Momentum must be conserved; energy efficiency demands minimizing exhaust velocity. The bypass ratio is the solution the physics dictated. The conditions created the invention; the invention reshaped the conditions; the physics determined both.