Rocket doesn't melt even at 3000 degrees Celsius, why?

Special materials 

Heat-resistant metals, such as alloys and composites, are used in rockets to survive severe temperatures. 

Insulation

Thermally conductive materials line the engines, absorbing and radiating heat. 

Fuel cooling 

Some engines circulate cold gasoline around the chamber to keep it cool. 

Short exposure 

The strong heat just quickly hits the chamber, reducing the risk of melting.

Heat distribution 

Heat spreads evenly across a vast area, avoiding concentrated hot spots. 

Regenerative cooling 

Some engines employ exhaust to cool the nozzle, creating a self-cooling loop. 

Staged combustion 

The fuel burns in stages, allowing temperatures to be controlled and overheating to be avoided. 

Constant monitoring 

Temperature sensors and systems constantly track and change temperatures as needed.