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How a rocket engine turns propellant into push

Two liquids are pumped into a chamber and burned. The hot gas thrown out of the nozzle pushes back on the engine. Open the throttle and follow the flow from the tanks to the flame.

Cutaway of a liquid bipropellant rocket engine Fuel and oxidizer tanks feed two pumps through throttle valves into an injector. The propellants burn in the combustion chamber and the gas accelerates through the throat and nozzle. The gas thrown down pushes the engine up with an equal and opposite force. SEA LEVEL: AIR ROCKET STAGE · CUTAWAY FUEL what gets burned OXIDIZER carried on board THROAT REACTION gas pushes engine up 0 kN ACTION engine pushes gas down 0 kN 12345678
Where is the rocket?
Fuel flow0kg/sof 28.6 kg/s at full
Oxidizer flow0kg/s2.5× the fuel, by mass
Chamber pressure0barmodel: grows with flow
Thrust0kNflow × exhaust speed
Tap a number on the drawing, or a part in the list, to see what it does and dim everything else.

Action and reaction

The engine pushes a stream of burning gas down and out. The gas pushes back on the engine with the same force, up. That pair is the whole trick: the rocket needs nothing to push against, only something to throw.

The throttle sets how much propellant is thrown each second. The green and red arrows always have the same length, because they are one pair of forces.

It carries its oxidizer, so it works in vacuum

A jet engine breathes air for its oxygen. A rocket stores oxidizer in its own tank, so the flame needs no atmosphere. In space nothing presses on the nozzle exit, so the plume spreads wide and thrust is a little higher.

thrust ≈ mass flow × exhaust speed. Illustrative numbers: 100 kg/s, 2.5 : 1 oxidizer to fuel, 2.8 km/s at sea level and 3.1 km/s in vacuum. Simplified model: pump drive, cooling, ignition and steering are not modelled.