A turbojet engine is a heat engine that turns fast-moving air and fuel into thrust for flight. Its basic sequence is often remembered as suck, squeeze, bang, blow: intake, compression, combustion, and exhaust. This matters because turbojets helped make high-speed aviation possible by producing a powerful jet of exhaust instead of relying on a propeller.
The same physics ideas appear in many jet engines, rockets, and gas turbines.
Key Facts
- Turbojet stages: intake, compressor, combustor, turbine, nozzle.
- Thrust comes from accelerating air backward, so the engine is pushed forward by Newton's third law.
- The compressor raises air pressure before fuel is added and burned.
- The turbine extracts energy from hot exhaust gases to spin the compressor on the same shaft.
- Thrust can be estimated by F = mass flow rate × change in velocity, or F = ṁ(v_exit - v_inlet).
- Combustion adds thermal energy, and the nozzle converts much of that energy into high-speed exhaust.
Vocabulary
- Intake
- The intake is the front opening and duct that guides incoming air smoothly into the engine.
- Compressor
- The compressor is a rotating set of blades that squeezes incoming air to higher pressure.
- Combustor
- The combustor is the chamber where fuel mixes with compressed air and burns to create hot expanding gas.
- Turbine
- The turbine is a set of blades turned by hot exhaust gases, and it powers the compressor through a shaft.
- Nozzle
- The nozzle is the narrowing exit that accelerates exhaust gases into a fast jet to produce thrust.
Common Mistakes to Avoid
- Thinking the turbine directly pushes the aircraft forward, because the turbine mainly extracts energy to spin the compressor while the exhaust jet creates most of the thrust.
- Putting combustion before compression, because fuel burns much more effectively when the air has first been squeezed to high pressure.
- Assuming thrust comes from air pushing on the front intake, because thrust is mainly caused by accelerating a mass of air and gas backward through the engine.
- Forgetting the compressor and turbine share a shaft, because the turbine must mechanically drive the compressor for the engine to keep running.
Practice Questions
- 1 A turbojet takes in 40 kg of air per second at 120 m/s and exhausts gas at 620 m/s. Estimate the thrust using F = ṁ(v_exit - v_inlet).
- 2 A compressor increases air pressure from 100 kPa to 800 kPa. What is the compressor pressure ratio?
- 3 Explain why a turbojet needs both a compressor and a turbine, and describe how energy moves through these parts during operation.