How does a rocket engine heated to 3200 ° C withstand high temperatures?

In the combustion chamber of a rocket engine used in a spacecraft, the gas can be heated to about 3200 ° C. This high temperature is necessary for the engine to function properly, and various measures have been taken to withstand this high temperature, which exceeds the melting point of many materials.

Engine Cooling --Why Rocket Engines Don't Melt | Everyday Astronaut

There is an injector at the top of the rocket engine chamber, which pumps fuel and oxidizer into the chamber at high pressure. The fuel and oxidizer sent into the chamber mix and ignite and burn, producing enormous energy. However, without any ingenuity, the walls of the metal chamber will melt.

◆ 1: Heat sink
One option is to thicken the walls of the chamber. The thick wall acts as a heat sink, lowering the overall temperature before the hot gas melts the metal layer. However, weight reduction is important when making a rocket, so heavy metal walls cannot be made as thick as possible. Also, when the melting point of all metals is reached, it melts, so it is not suitable for long-term use.

For this reason, heatsinks can be used with maneuvering thrusters that only operate for a short period of time, but are not suitable for main propulsion engines that continue to run for minutes.

◆ 2: Ratio of fuel and oxidizer
Another option is to adjust the fuel to oxidant ratio to lower the exhaust temperature. A ratio where the total amount of fuel and oxidizer reacts perfectly will release as much heat as possible, which is great if you want maximum power, but if you want to reduce heat. Is not suitable.

◆ 3: Abrasive cooling
'Ablative cooling' is known as an easy and efficient cooling method. This is a method that uses the heat of vaporization, which is the same as the heat shield used by spacecraft that re-enter the atmosphere. A heat shield made of carbon composite material with a high melting point melts the layer at high temperatures and takes heat away, preventing high heat from penetrating into the spacecraft. For rocket engines, there is a layer of carbon composite inside the walls of the chamber and nozzle.

However, there are known problems such as the engine cooled by this method cannot be reused. Famously, the engine used by the Apollo program to bring astronauts back to Earth could not be tested until it was actually used on the Moon.

◆ 4: Regenerative cooling
In the case of liquid fuel rocket engines, a method called 'regenerative cooling' is used. This is to send the propellant from the inside of the wall surface of the chamber and the nozzle to the chamber via the injector, and it was a big breakthrough for the rocket engine.

One of the challenges of 'regenerative cooling' is that the pressure inside the wall needs to be higher than the pressure inside the chamber. However, high pressure inside a narrow wall can cause leaks.

◆ 5: Film cooling
As a method following 'regenerative cooling', 'film cooling' is known, in which a fluid is injected between the inside of the chamber and the nozzle and the wall to create a boundary between the hot gas and the wall. The simplest method is to increase the concentration of fuel or oxidizer around the injector, and since the chamber is rich in fuel, fuel is generally used. With this method, fuel that cannot obtain the amount of oxidizer required for the reaction flows around the outer circumference and absorbs heat by changing the phase from liquid to gas.

◆ 6: Radiative cooling
In addition, SpaceX's 'Marlin' engine and Rocket Lab's 'Rutherford' engine radiate heat into space from metal parts. This is similar to how the sun transfers heat through a vacuum. Nozzle extensions for 'Merlin' and 'Razaford' are usually made of very thin metals that can withstand high heat loads such as niobium alloys.

However, it has the disadvantage of being fragile because it is very thin. In addition, because niobium is highly reactive with oxygen, such an engine can only operate in a vacuum environment in reality, and complicated work is required during manufacturing.