A turbojet is a jet engine that produces thrust entirely by accelerating a stream of hot exhaust gas out of a nozzle at high speed. It was the first type of jet engine to fly, powering the German Heinkel He 178 on August 27, 1939, and it remains the simplest form of gas turbine engine used in aviation. Every turbojet works on the same basic principle: air comes in the front, gets compressed, mixed with fuel and ignited, then blasted out the back faster than it entered. That difference in speed is what pushes the aircraft forward.
How a Turbojet Produces Thrust
The core idea is straightforward. Air enters the engine, and the engine adds energy to it by burning fuel. The heated, high-pressure gas then expands through a nozzle and exits at a much higher velocity than it had when it entered. Thrust is the result of that velocity change: it equals the mass of air flowing through the engine every second multiplied by the difference between exit velocity and intake velocity. More air, or a bigger speed difference, means more thrust.
This process follows a thermodynamic cycle called the Brayton cycle, which has four stages. First, incoming air is compressed to raise its pressure and temperature. Second, fuel is injected into the compressed air and ignited in a combustion chamber at roughly constant pressure. Third, the hot gas expands through a turbine, which extracts just enough energy to keep the compressor spinning. Fourth, the remaining high-energy gas accelerates through a nozzle and exits the engine, producing thrust. The entire cycle is continuous: air flows through the engine nonstop as long as fuel is being burned.
The Five Main Components
A turbojet has five major sections arranged in a straight line from front to back.
- Inlet: The front opening that captures air and slows it down before it reaches the compressor. During flight, this deceleration converts some of the aircraft’s forward speed into higher air pressure, giving the compressor a head start.
- Compressor: A series of spinning blades and stationary vanes that progressively squeeze the air into a smaller volume. Each row of blades raises the pressure a little more. The compressor is often described as the “cold section” of the engine because no fuel has been added yet.
- Combustion chamber (burner): The section where fuel is sprayed into the compressed air and ignited. Temperatures here are extreme. At the entrance to the turbine just downstream, gas temperatures can reach as high as 2,000 K (about 3,140 °F). Turbine blades survive these conditions partly because cooler air bled from the compressor is channeled over and through them, allowing the engine to operate well above the melting point of the blade metal itself.
- Turbine: One or more stages of blades that extract energy from the hot gas to drive the compressor. The turbine and compressor sit on the same shaft, so the work the turbine pulls from the exhaust flow is exactly the work the compressor needs to squeeze the incoming air.
- Nozzle: A tapered duct at the rear that accelerates the exhaust gas back down to the surrounding atmospheric pressure. This acceleration is where thrust is finally generated. The gas leaves the nozzle at high speed, and Newton’s third law does the rest.
Afterburners and Extra Thrust
Some turbojets, particularly those in military fighters, include an afterburner between the turbine and the nozzle. The afterburner injects additional fuel directly into the hot exhaust stream and ignites it, producing a significant boost in thrust. The tradeoff is efficiency: fuel burned in an afterburner doesn’t burn as cleanly or completely as fuel in the main combustion chamber. You get more thrust, but you burn far more fuel per unit of thrust gained. That’s why afterburners are used in short bursts for takeoff, supersonic acceleration, or combat maneuvering rather than for sustained cruise.
How Turbojets Differ From Turbofans
If you’ve flown on a commercial airliner in the last few decades, you’ve flown on turbofan engines, not turbojets. The key difference is the bypass ratio, which describes how much air flows around the engine core compared to how much flows through it. A turbojet has zero bypass: every bit of incoming air passes through the compressor, combustion chamber, and turbine. A turbofan, by contrast, adds a large fan at the front that pushes a substantial volume of air around the outside of the core. In high-bypass turbofans, 70% or more of the total thrust comes from this fan-driven “cold” airflow rather than from the hot exhaust.
This matters for two practical reasons. First, turbofans are more fuel-efficient at the subsonic speeds where airliners operate. Moving a large mass of air at moderate speed produces thrust more economically than moving a small mass of air at very high speed. Second, turbofans are significantly quieter. A turbojet generates all its thrust from a single high-velocity exhaust stream, which contains more energy and produces more low-frequency noise. A turbofan spreads the same thrust across a larger, slower-moving volume of air, resulting in less acoustic energy overall.
Where Turbojets Still Excel
Turbojets are best suited for high-speed flight. Their simple, streamlined design creates less aerodynamic drag than a large-diameter turbofan at supersonic speeds, and their efficiency improves as airspeed increases because the inlet does more of the compression work for free. That makes them a natural fit for supersonic military aircraft and certain missile systems. The Concorde’s engines, for example, were closer to a turbojet design with partial bypass rather than the high-bypass turbofans found on a Boeing 787.
At lower speeds, though, their disadvantages are hard to ignore. The high exhaust velocity that works well at Mach 2 is wasteful at Mach 0.8, burning more fuel for the same amount of thrust. Combined with the noise issue, this is why turbojets have largely disappeared from commercial aviation. Today you’ll find them mainly on fast military jets, older aircraft still in service, and small expendable platforms like cruise missiles and target drones where simplicity and compact size matter more than fuel economy.
Why Turbojets Are So Loud
Turbojet noise is distinctive: a deep, sustained roar that carries over long distances. The physics behind it comes down to exhaust velocity and frequency. Because all the thrust exits as a single, fast-moving jet of hot gas, the exhaust stream contains more kinetic energy than a turbofan producing the same thrust. That energy dissipates as sound. The noise also sits at lower frequencies than turbofan noise, and low-frequency sound waves travel farther and penetrate structures more easily, which is why turbojet-powered aircraft are perceived as louder even at the same decibel level. This acoustic profile was one of the strongest motivations for the aviation industry’s shift to turbofan engines starting in the 1960s.