High- & Low-Bypass Turbofan Engine

Airbreathing gas-turbine jet engine using a core gas generator to drive a ducted fan that accelerates a large bypass airstream.

Specifications & Metrics

type: Aerospace & Rocket Propulsion
working principle: Brayton-cycle core turbine extracts shaft power to drive a multi-blade ducted fan, maximizing Froude propulsive efficiency η_f = 2 / (1 + V_jet/V_aircraft)
operating regime: Subsonic cruise Mach 0.75–0.90 (high-bypass BPR 5:1–12:1); supersonic combat (low-bypass BPR 0.2:1–1:1)
efficiency or isp: Effective specific impulse ~3,000–6,000 s (30–75% of net thrust produced by cold bypass air)
originator: Frank Whittle (1936 UK patent 471,368); Rolls-Royce Conway (first production turbofan, 1950s)

First patented by Frank Whittle in 1936 and put into commercial service with the Rolls-Royce Conway in 1960, a turbofan divides incoming air into a core stream that passes through the compressor, combustor, and turbines and a cold stream that bypasses the core.
Because kinetic energy wasted in the exhaust wake scales with the square of jet velocity (½m·v²) while thrust scales linearly with momentum (m·v), accelerating a large mass of bypass air to a lower speed yields far higher Froude propulsive efficiency and lower jet noise at subsonic airliner speeds (500–1,000 km/h).
Modern commercial high-bypass turbofans (such as the GE9X, Rolls-Royce Trent XWB, and Pratt & Whitney epicyclic geared turbofan) operate at bypass ratios of 9:1 to 12:1, generating over 70% of their net thrust from the single-stage front fan.