How does a jet engine produce thrust?

An airliner accelerates along a runway even though its engines have no wheels pushing on the ground. Their moving partner is the air. Each second, the engines give a stream of gas more backward momentum, and that interaction gives the aircraft a forward force. This force is thrust. Understanding it means following both the gas through the engine and the energy that makes the gas accelerate.

In a simple turbojet core, incoming air first meets a compressor: rows of rotating blades work with stationary rows to raise the air's pressure. Compressing air takes mechanical work. Fuel is then sprayed into a combustion chamber, where it reacts with oxygen in the compressed air. Combustion adds energy and greatly raises the gas temperature. It is a continuing flow process, rather than the repeated sealed-cylinder explosions people may picture from a car engine.

The hot gas next expands through a turbine. Turbine blades extract some of the gas's energy as shaft work. A shaft connects the turbine back to the compressor, so the outgoing stream helps power the preparation of incoming air. This connection is a crucial part of the engine's self-sustaining operation once running. The turbine does not take all the available energy: the flow must still have enough to produce a useful exhaust jet.

At the nozzle, the remaining pressure and thermal energy help accelerate the exhaust. An engine's thrust comes from forces distributed through its interaction with the gas, not from exhaust having to push against a runway or a wall. A convenient way to calculate the overall force is to compare momentum entering and leaving the engine. Momentum is mass times velocity, so both the quantity of gas and the change in its speed matter.

For a steady flow with exit pressure matching the surrounding air, and with added fuel mass small enough to neglect, the approximate thrust magnitude is F = ṁ(Ve − V0). Here ṁ is air mass per second, while Ve and V0 are rearward exit and inlet speeds relative to the engine. If 20 kilograms pass through each second and the speed increases from 100 to 300 meters per second, thrust is 20 × 200 = 4,000 newtons. The gas gains rearward momentum; the engine experiences forward thrust. The speeds are stipulated example values, not a prediction from fuel flow alone.

A real calculation also includes fuel in the exhaust mass and a pressure force if the exit pressure differs from the surroundings. Most modern airliners use turbofans, whose turbines also drive a large fan. Much of the incoming air bypasses the combustion core, but the fan still accelerates it backward. This reveals two ways to obtain thrust: greatly accelerate a smaller mass flow, or give a larger mass flow a gentler speed change. The large circular engine opening on an airliner is a clue to the second strategy.