Rockets can fly at high speeds because, after burning fuel, they are ejected at high speeds while also experiencing an equal and opposite reaction force, propelling the rocket forward. The nozzles of solid-fuel rockets are exposed to high-temperature airflows of 2000-3000°C. Conventional metals cannot be used in these conditions, but graphite possesses unique properties that adapt to these conditions:
It has high temperature resistance. As mentioned earlier, carbon does not melt under normal atmospheric pressure. When the temperature rises to (3620±10)K, solid carbon directly sublimates into a gas. It also has very high-temperature mechanical strength. While the mechanical strength of graphite products is not high at room temperature, it increases with increasing operating temperature. Between 2000 and 2500°C, the mechanical strength is approximately double that at room temperature, making its specific strength higher than any other material. It only exhibits some plasticity above 2500°C, undergoing some creep under load.
Graphite has excellent thermal stability. Graphite has a high thermal conductivity, higher than that of metal materials such as stainless steel, lead, and ferrosilicon. This ensures that the graphite nozzle material can quickly transfer some of the heat from the hot air flow. Graphite also has a low linear expansion coefficient, resulting in excellent thermal shock resistance, a necessary property for nozzle materials.
Graphite material does not oxidize at room temperature, but it begins to oxidize above 450℃, which is a disadvantage of graphite material. However, after oxidation, graphite material directly turns into gas and escapes. At high temperature (3620±10)K, it directly sublimates, which requires absorbing a large amount of sublimation heat, and will take away some of the heat, which is also beneficial to reducing the temperature at the nozzle.