The nozzle is cooled from the inside
A rocket nozzle faces a punishing combination: hot combustion gas rushes through it while pressure pushes that gas hard against the walls. The wall cannot simply be made thick enough to soak up the heat indefinitely; it would still heat up, and extra mass is costly to launch. Many engines instead use regenerative cooling: they route liquid propellant through narrow passages built into the nozzle wall before sending it to the combustion chamber.
A moving heat sink
The channels put relatively cool liquid close to the hot inner surface. Heat passes through the nozzle material into the flowing propellant, which carries that energy away instead of letting the metal or alloy keep getting hotter. The liquid is continuously replaced, so the wall is cooled throughout the burn rather than relying on a one-time reserve of cold material.
The channels must be designed to move enough propellant without weakening the nozzle. Their shape and placement also matter: the gas does not heat every part of the nozzle equally, so cooling has to reach the regions under the greatest thermal stress. The warmed propellant then continues into the engine, where it is burned with the other propellant.
Cooling that also feeds the engine
This arrangement turns a potential waste stream into part of the engine’s flow: the liquid absorbs heat on its way to combustion, and the nozzle gets protection without a separate cooling system. It does not make the wall cold; it keeps heat from accumulating faster than the flowing propellant can carry it away. That is what lets a lightweight nozzle survive a sustained blast of hot gas long enough to do its job.
