Why Jet Engines Are So Big

Propulsive efficiency

An interactive lesson with 6 steps, about 5 minutes: you predict, drag and solve, with instant feedback on every answer. It runs in your browser and needs JavaScript.

What you’ll learn

• Thrust = mass flow × speed increase.
• Propulsive efficiency $\eta = 2/(1 + v_j/v)$: best when the exhaust is only a little faster than the plane.
• Big fans move lots of air gently, which is why modern jet engines are huge.

Steps

1. Push a lot of air gently
2. A fast, thin jet
3. Design the fan
4. The limit
5. Wasted energy
6. Same thrust, more air