The Two Engines That Power Modern Aviation
Walk up to almost any commercial airliner and you'll find a turbofan engine hanging beneath the wing. Walk up to a supersonic fighter jet and you'll likely find a low-bypass turbofan or — in some older designs — a turbojet. These two engine types share the same thermodynamic core but differ profoundly in how they generate thrust and where they perform best.
Understanding the difference is foundational to aerospace engineering and helps explain why engine selection is one of the most consequential design decisions in any aircraft program.
How a Turbojet Works
The turbojet is the original jet engine architecture, developed in the late 1930s and 1940s. Its operation follows the Brayton thermodynamic cycle:
- Intake: Air enters the engine inlet and is slowed to subsonic speed.
- Compression: A multi-stage axial compressor raises the air pressure significantly.
- Combustion: Fuel is injected into the compressed air and burned, raising temperature dramatically.
- Turbine: The hot, high-pressure gas expands through a turbine, extracting just enough energy to drive the compressor.
- Nozzle: Remaining high-energy gas is expelled at high velocity, generating thrust.
In a pure turbojet, all thrust comes from this hot exhaust jet. The exhaust velocity is high, making turbojets efficient at supersonic speeds but very inefficient at subsonic cruise — they burn a great deal of fuel to produce each unit of thrust.
How a Turbofan Adds Efficiency
A turbofan adds a large fan stage at the front of the engine. This fan is driven by an additional turbine stage and accelerates a large mass of air around the outside of the core — the bypass flow. This bypass air never enters the combustion chamber; it is simply accelerated and expelled, contributing to thrust without the fuel cost of combustion.
The bypass ratio (BPR) defines how much air bypasses the core compared to the air flowing through it. A BPR of 10:1 means 10 times more air flows through the fan bypass than through the combustion core.
Why High Bypass Ratios Are More Efficient
This comes down to the physics of momentum. A given amount of thrust can be generated by:
- Accelerating a small mass of air by a large velocity (turbojet approach), or
- Accelerating a large mass of air by a small velocity (high-bypass turbofan approach)
The second option uses significantly less energy for the same thrust — this is why modern high-bypass turbofans are dramatically more fuel-efficient than turbojets at subsonic cruise speeds.
Performance Comparison
| Characteristic | Turbojet | High-Bypass Turbofan |
|---|---|---|
| Bypass ratio | 0 (no bypass) | 5:1 to 12:1 or higher |
| Propulsive efficiency | Lower at subsonic speeds | Very high at subsonic cruise |
| Specific fuel consumption | Higher | Much lower |
| Engine diameter | Smaller | Larger (due to fan) |
| Best speed range | Supersonic / high altitude | Subsonic / transonic cruise |
| Noise level | Louder | Significantly quieter |
| Primary application | Supersonic aircraft, cruise missiles | Commercial airliners, transport aircraft |
The Low-Bypass Turbofan: The Military Compromise
Modern military fighter engines are almost always low-bypass turbofans with bypass ratios between 0.2:1 and 1:1. This configuration provides better fuel efficiency than a pure turbojet while still supporting the slim engine diameter needed for an aircraft designed to fly supersonically. An afterburner can be added downstream of the turbine to inject additional fuel into the exhaust stream for short bursts of maximum thrust during combat maneuvering or supersonic acceleration.
Which Engine Does the Future Belong To?
For commercial aviation, the high-bypass turbofan will continue to dominate, with ongoing efficiency improvements through higher pressure ratios, better materials, and open-rotor (unducted fan) concepts. For high-speed and hypersonic applications, turbojets and turbofans give way to ramjets and scramjets once speeds exceed roughly Mach 3–4, where rotating machinery becomes thermally and mechanically impractical. The future of propulsion lies in seamlessly bridging these regimes — from takeoff to hypersonic cruise — in a single propulsion system.