Thrust

Stand on a skateboard and hurl a heavy ball backwards: you roll forwards. That — scaled up until the "ball" is a tonne of air every second — is a jet engine. Everything else in this course (the compressor, the fire, the spinning) exists only to do one thing faster and harder: throw air backwards.

Newton, cashing out

Thrust is momentum bookkeeping. Take in air moving at flight speed, spit it out faster, and the force you feel is the rate of momentum change:

Mass flow $\dot{m}$ (kilograms of air per second) times how much you speed it up. Both knobs work: throw more air, or throw it faster. A large airliner engine at take-off moves over a tonne of air per second and produces around 400 kN — the weight of forty family cars, from a machine you could park a van inside.

The equation already contains the course's deepest design argument. Big-and-slow versus small-and-fast is a real choice — and it turns out nature charges very differently for the two. Kinetic energy grows with velocity squared while momentum grows only linearly, so hurling a little air very fast wastes energy as hot, fast exhaust. Gently accelerating a lot of air is fundamentally more efficient than violently accelerating a little — remember that sentence; it resurfaces in the bypass lesson as the reason airliner engines are fat.

No push-back required

The stubborn myth: "the jet pushes against the air behind it". It doesn't — a rocket works in vacuum, and a jet's thrust is generated inside the engine: pressure acting on the machinery itself. The momentum equation is the honest summary of millions of molecular collisions against compressor blades, casings and nozzle walls, all netting out to a forward shove on the metal. Throw mass one way, the thrower moves the other. The atmosphere behind is not a wall; it is merely where the thrown air ends up.

The propeller's problem

Propellers already do "throw air backwards", beautifully — a big disc gently accelerating a huge mass flow, exactly what efficiency wants. But propellers hit a wall: as aircraft speed climbs past ~700 km/h, the blade tips (already moving fast from spinning) go supersonic, shocks form (the previous course's final lesson), and efficiency collapses in noise and drag. The 1930s question — how do you throw air backwards at 900 km/h? — is what the next nine lessons answer. The machine that solved it first ran in 1937, and its story is on this site: