A liquid rocket engine brings its own fuel and oxygen, pumps them into a chamber to burn, and throws the hot gas out of a widening nozzle. Follow the propellant from the tanks all the way to the thrust.
A liquid rocket engine is a machine that makes thrust by burning liquid propellant and throwing the hot gas out the back. Unlike a jet engine it carries its own oxygen, so it can work where there is no air at all. At its centre is a combustion chamber running at enormous pressure, fed by pumps, and ending in a bell-shaped nozzle.
From the outside a rocket engine is just a cluster of tanks, pipes and a nozzle. The interesting engineering — how the propellant is pumped, sprayed, burned and cooled — all happens inside, so cutting it open is the clearest way to see how the parts work together.
Fuel and oxidizer are kept in separate tanks. They are stored apart precisely because they would react if they met, so they only mix at the last moment inside the engine.
The combustion chamber runs at a pressure far higher than the tanks. Pumps driven by a small turbine take the propellant from the low-pressure tanks and force it up to chamber pressure. The tanks only have to hold liquid; the pumps do the pushing.
At the top of the chamber is a drilled injector plate. It splits the two flows into many fine jets so the fuel and oxidizer mix as thoroughly as possible, giving a clean, hot burn.
Burning makes gas thousands of degrees hot. To stop the walls melting, cold propellant is routed through a jacket of channels around the chamber before it is burned — this is regenerative cooling. It protects the metal and warms the propellant at the same time.
The hot gas passes a narrow throat and then a widening bell — the de Laval nozzle. The expansion lets the gas speed up past the speed of sound, and the fast-moving exhaust leaving the engine is what pushes the vehicle forward.
Because there is no air in space. A rocket has to bring both fuel and the oxygen to burn it, stored in separate tanks, and mix them only inside the engine.
The nozzle. Burning propellant makes very hot, high-pressure gas, and the widening de Laval nozzle lets that gas expand and accelerate to supersonic speed as it leaves — the fast exhaust is the thrust.
The combustion chamber runs at a pressure far higher than the tanks. Pumps driven by a small turbine raise the propellant to that pressure so it can be injected against it.
By regenerative cooling. Cold propellant is routed through channels in a jacket around the chamber walls before it is burned, carrying heat away and keeping the metal below its melting point.
No. A rocket works by throwing mass out the back, and by Newton's third law the engine is pushed the other way. It needs no air, which is why the same engine works in the vacuum of space.