turbojet

A turbojet is a gas turbine engine that produces thrust by compressing incoming air, mixing it with fuel, igniting the mixture, and expelling the hot exhaust at high velocity. The core components are arranged in a straight line: inlet, compressor, combustion chamber, turbine, and nozzle. Unlike turbofan engines, which bypass much of the incoming air around the core, a turbojet sends all air through the compression and combustion process, making it thermally efficient at transonic and supersonic speeds but noisy and fuel-hungry at subsonic cruise.

The compressor and turbine are mechanically linked on a single shaft. As hot exhaust gases expand through the turbine, they spin the rotor, which in turn drives the compressor to draw in and pressurize fresh air. This self-sustaining cycle begins at engine start, when external power or air velocity spins the shaft to initiate compression and combustion. The pressure ratio across the compressor, typically ranging from 4:1 to 8:1 in military turbojets, directly affects engine efficiency and thrust output.

Why turbojets dominated early jet aviation

Turbojets powered the first combat jets because their simple architecture allowed high shaft speeds and high exhaust temperatures without the mechanical stress that would damage turbofan stages. The Rolls-Royce Welland and General Electric J47, used in aircraft from the 1940s through 1960s, were pure turbojets. They excel above Mach 0.8 because the ram effect of incoming air at speed reduces the workload on the compressor, improving efficiency. Conversely, at takeoff and low-speed flight, turbojets are inefficient because the compressor must do nearly all the work of pressurizing air, consuming enormous fuel.

Maintenance issues in turbojets center on compressor blade erosion from sand ingestion, hot-section degradation from thermal cycling, and bearing wear. The turbine inlet temperature, sometimes exceeding 1000 degrees Celsius in high-performance engines, limits service life and demands strict inspection intervals. Bleed air taps from the compressor are common, routing pressurized air to aircraft systems like pressurization and anti-ice, which reduces available thrust and must be accounted for in performance calculations.

Turbojets have been largely replaced in commercial service by turbofans, which separate the core exhaust into a smaller, hotter jet stream and a larger, cooler bypassed airflow. This arrangement improves fuel economy dramatically at subsonic speeds. Military and research supersonic aircraft still use turbojets or hybrid designs because the high thermodynamic efficiency at high Mach numbers and the ability to manage inlet shock conditions remain valuable, despite the penalty in noise and fuel burn.