The one difference that matters
A turbojet sends every kilogram of air through its core and accelerates it to a very high speed. A turbofan uses part of the turbine's work to drive a large fan, and most of the air that fan moves never goes near the combustor. It leaves through the bypass duct, a little faster than it arrived, alongside the smaller core jet.
The bypass ratio is the mass of bypass air per kilogram of core air. A turbojet is a bypass ratio of 0.
Side by side
| Turbojet | Low-bypass turbofan | High-bypass turbofan | |
|---|---|---|---|
| Bypass ratio | 0 | below about 1 | about 5 to 12 |
| Jet speed | very high | high | moderate |
| Fuel use at subsonic speed | highest | in between | lowest |
| Noise | loudest | loud | quietest |
| Size for the same thrust | slimmest | slim | large diameter |
| Typical use | early jets, missiles, drones, model jets | fighters, often with an afterburner | airliners and transports |
Why the fan saves fuel
Thrust is mass flow times the change in speed. To make a given thrust, you can speed up a little air a lot or a lot of air a little. The energy that stays in the jet behind the aircraft grows with the square of the jet's speed, so the small, fast jet of a turbojet wastes far more of its fuel's energy as a roaring wake. Moving a lot of air gently, as a fan does, wastes less: its propulsive efficiency is higher. The jet thrust calculator shows the numbers.
What the game's dyno shows
On the same standard test (sea level, standard day, fuel measured at 70 % power):
| Engine | Thrust | Afterburner | Fuel at 70 % | Spool-up | Mass |
|---|---|---|---|---|---|
| Training turbojetFree | 55.6 kN | 72.2 kN | 25.0 mg/N·s | 15.3 s | 1,577 kg |
| Fighter turbofanPro | 67.5 kN | 87.3 kN | 22.4 mg/N·s | 10.8 s | 1,663 kg |
| Airliner turbofanPro | 76.7 kN | – | 11.2 mg/N·s | 8.2 s | 1,417 kg |
The Airliner turbofan burns 11.2 mg/N·s, less than half the 25.0 mg/N·s of the Training turbojet, and the low-bypass Fighter turbofan sits in between at 22.4 mg/N·s. Run both in the game and listen: the turbofan's slower jet is also the quieter one.
So why are turbojets still around?
A fan only helps when the aircraft is flying slower than a fast jet would. At high supersonic speed a slim, fast jet is what you need, and big fans add drag and weight. Turbojets are also simple and cheap, which suits missiles, target drones and model jets. Fighters split the difference with a low bypass ratio and an afterburner; above that speed range, ramjets and scramjets take over.