A turbofan is a gas-turbine engine that uses a powered fan to accelerate air around its core as well as through it. Thrust comes from both the bypass stream and the core exhaust. This arrangement combines the energy-producing machinery of a gas turbine with the ability to move a relatively large quantity of air.[1]Turbofan EngineNASA Glenn Research Center · grc.nasa.gov · Accessed 2026-09-13Open source details
Airflow and bypass ratio
After passing through the fan, some air enters the compressor and combustion system, while the remainder travels around the core. The bypass ratio compares the mass flow going around the core with the mass flow passing through it. A high ratio therefore describes the division of airflow, rather than directly stating the percentage of thrust from each stream.[1]Turbofan EngineNASA Glenn Research Center · grc.nasa.gov · Accessed 2026-09-13Open source details
Shafts and gearing
Many turbofans use separate rotating systems, or spools, for the core machinery and the fan system. The fan-driving turbine obtains energy from the hot core flow, and a shaft transmits that power forward. Additional spools allow further separation of compressor and turbine operating speeds.[1]Turbofan EngineNASA Glenn Research Center · grc.nasa.gov · Accessed 2026-09-13Open source details
A geared turbofan adds a reduction gearbox between the fan and its driving system. Pratt & Whitney's explanation describes how this permits a slower fan while the low-pressure machinery rotates faster. The engineering purpose is to reduce the compromise between components whose preferred rotational speeds differ, while retaining the same basic division between core and bypass air.[2]Pratt & Whitney presentation on geared turbofansPratt & Whitney · media.buncombenc.gov · Accessed 2026-09-13Open source details
Aircraft roles
High-bypass turbofans are widely associated with subsonic passenger transports. Lower-bypass engines are also used in military aircraft, sometimes with afterburners for additional thrust. The fan's enclosing duct and blade arrangement permit efficient operation at flight speeds above those normally associated with conventional exposed propellers.[1]Turbofan EngineNASA Glenn Research Center · grc.nasa.gov · Accessed 2026-09-13Open source details
Noise and testing
Turbofan noise is produced by several interacting sources rather than by exhaust flow alone. NASA's DGEN AeroPropulsion Research Turbofan provides a testbed for investigating propulsion noise, controls, measurement methods and installation effects. Such experiments help distinguish contributions from different parts of the engine and their behaviour when the engine is installed.[3]DGEN AeroPropulsion Research TurbofanNASA Glenn Research Center · www1.grc.nasa.gov · Accessed 2026-09-13Open source details
In 2019, NASA used simultaneous measurements near the core-nozzle exit and farther from the engine to investigate how sound sources propagate. That combination matters because a noise signal measured outside an engine can contain overlapping contributions. Understanding those contributions supports prediction methods and the evaluation of noise-reduction techniques, rather than relying solely on the total sound level of a complete engine.[3]DGEN AeroPropulsion Research TurbofanNASA Glenn Research Center · www1.grc.nasa.gov · Accessed 2026-09-13Open source details