What is a Turbojet?

A turbojet is an air-breathing gas turbine engine that produces forward thrust by drawing in air, squeezing it mechanically with a compressor, igniting it with fuel, and expelling the high-velocity exhaust out of a rear nozzle.

Unlike ramjets and scramjets—which require high forward speed to compress incoming air—a turbojet uses powered, rotating blades to generate internal air pressure. This allows a turbojet-powered aircraft to start from a complete standstill (Mach 0) on a runway.

How a Turbojet Works

1. Air Intake: Atmospheric air enters the engine through the front inlet.
2. Compression: A series of rotating and stationary blades (axial or centrifugal compressors) mechanically compress the air, increasing its pressure and temperature.
3. Combustion: The compressed air enters the combustion chamber, where fuel is continuously injected and ignited, producing intense heat and rapidly expanding gases.
4. Turbine: The expanding gas passes through a turbine, forcing it to spin. The turbine extracts a portion of the exhaust’s energy to turn the drive shaft that powers the front compressor.
5. Exhaust Nozzle: The remaining hot, pressurized gas accelerates out through a converging nozzle at the rear at high velocity, generating thrust according to Newton’s third law.

Key Capabilities & Limitations

• Self-Starting Capability: Because it generates compression mechanically, it produces static thrust at zero flight velocity.
• Speed Range: Performs best from subsonic speeds up to low supersonic speeds (around Mach 2 to 2.5).
• Upper Mach Limits: At speeds approaching Mach 3, incoming ram air temperature increases dramatically, risking thermal damage to the internal turbine blades and making the mechanical compressor an aerodynamic hindrance.
• Fuel Efficiency: Modern commercial aircraft generally use turbofans (turbojets with a large front fan that bypasses additional air around the core engine) rather than pure turbojets, as turbofans offer much greater fuel efficiency at subsonic cruising speeds.

What is the difference between a turbojet and a turbofan engine?

The primary difference between a turbojet and a turbofan lies in how they handle incoming air to generate thrust. While a turbojet directs 100% of the incoming air through the engine’s core for burning fuel, a turbofan uses a large front fan to push a significant portion of the air around the core, bypassing the combustion process entirely.

How They Work

• Turbojet: All incoming air enters the compressor, mixes with fuel in the combustion chamber, spins the turbine, and blasts out the back as a high-velocity exhaust jet. It generates thrust by taking a relatively small volume of air and accelerating it to extreme speeds.
• Turbofan: The engine has a turbojet at its center (the core), but the turbine also powers a massive fan at the front of the engine. While some air enters the core to keep the engine running, the vast majority of the air caught by the fan flows completely around the outside of the core. It generates thrust by taking a very large volume of air and accelerating it to a moderate speed.

Key Differences Compared

Feature	Turbojet	Turbofan
Airflow (Bypass Ratio)	Zero bypass. Every drop of air passes through the combustion chamber.	High bypass. In modern airliners, 80% to 90% of the air bypasses the core.
Primary Source of Thrust	The hot, high-velocity exhaust jet leaving the rear nozzle.	The massive front fan acting much like a multi-bladed propeller.
Fuel Efficiency	Highly inefficient at standard subsonic speeds.	Vastly more fuel-efficient at subsonic cruising speeds.
Noise Level	Extremely loud. The high-velocity hot exhaust tearing into the static cold atmosphere creates violent sound waves.	Significantly quieter. The slower, cooler bypass air wraps around the hot exhaust, acting as a muffler.
Primary Applications	Early jet fighters, supersonic interceptors, and the Concorde.	Modern commercial airliners, cargo planes, and modern fighter jets.