The flames emitted by rockets are over 3000℃, how come they don't burn themselves? They have ways to cool down

Relying on the high-temperature flame ejected from the tail, the rocket can soar into the sky, which is spectacular. But have you ever thought about this question: since the temperature of the flame ejected by the rocket is extremely high, why doesn’t the rocket itself get burned?

You may think that the rocket must have used some high-temperature resistant aerospace materials. In fact, this is not a problem that can be solved by materials, because the temperature of the flame ejected from the tail of the rocket exceeds 3000℃, while tungsten, the metal with the highest melting point in nature, has a melting point of only 3380℃, which cannot withstand the continuous burning of the rocket flame. Of course, the melting point of a synthetic substance, tantalum hafnium pentacarbide, is much higher, reaching 4215℃, but the rocket has other ways to cool it down.

The most commonly used method of rocket nozzle cooling is called "regenerative cooling". Just by hearing the name, we can feel the "economical and affordable" nature of this cooling method.

So how can regenerative cooling be economical? This starts with rocket fuel. Although all rockets are propelled by fossil fuels, the specific fuel forms are different. For example, liquid rocket fuel engines use liquid fuels, specifically liquid methane, liquid oxygen, and liquid hydrogen. Methane, oxygen, or hydrogen are all gases at room temperature. To turn them into liquids, they need to be at a very low temperature.

The temperature of liquid methane must be minus 82.6°C, the temperature of liquid oxygen is as low as minus 183°C, and the most outrageous is liquid hydrogen, whose temperature must reach minus 252.7°C.

The rocket is loaded with so much cryogenic liquid, isn't this a readily available coolant? Therefore, dense pipes are laid on the inner and outer walls of the rocket's nozzle. These pipes are usually made of nickel-chromium alloy, stainless steel or copper. The low-temperature liquid fuel does not directly enter the combustion chamber after coming out of the fuel chamber, but first runs through these pipes, so as to achieve the purpose of helping the rocket nozzle to cool down.

The low-temperature liquid fuel carries away the heat by flowing through the inner and outer walls of the rocket nozzle, and this heat is not wasted.

The cryogenic liquid that absorbs the heat is equivalent to preheating it first, and then it will bring the heat into the combustion chamber, which means that the absorbed heat will eventually come back, without any waste, which is why this cooling method is called "regenerative cooling". Regenerative cooling is not the only cooling method for rocket nozzles. There are other methods, such as "film cooling".

The main cooling material used in film cooling is still the low-temperature liquid fuel, but this time instead of letting the fuel flow in the pipe to absorb heat, the low-temperature fuel is sprayed directly onto the inner wall of the rocket nozzle.

There is a cooling belt composed of a large number of small nozzles on the inner wall of the rocket nozzle. They will spray low-temperature liquid fuel on the inner wall of the rocket nozzle, so that a layer of liquid film or low-temperature steam film will be formed on the inner wall to isolate the high-temperature gas flame ejected by the rocket from the nozzle wall, so there is no need to worry about the gas flame burning the rocket nozzle. Is there a cooling method that does not use these low-temperature fuels? Yes, that is ablation cooling.

The so-called ablation cooling is to embed a lining made of ablative material on the inner wall of the nozzle, which is equivalent to adding a layer of heat insulation to the nozzle.

These ablative materials are generally made of carbon fiber and other materials. They are called ablative materials because the lining will decompose and vaporize in the process of absorbing heat and take away the heat. When the rocket finally takes off, they will disappear. This cooling method is relatively rough, but it is cheap and simple, so it also has its own advantages. In addition to these, the cooling methods of rocket nozzles include shielding cooling, transpiration cooling, radiation cooling, etc. In short, it is because of these cooling methods that the nozzle can be protected and the rocket will not be burned by its own high-temperature flame.

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