Is it true that a spacecraft entering the atmosphere heats up from friction with the air

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(ORDO NEWS) — There are such commonplace judgments, which, on the one hand, are not entirely far from the truth, but on the other hand, incorrectly reflect the essence of the phenomenon.

Yes, when entering the dense layers of the atmosphere, the skin of the spacecraft begins to heat up, so much so that if there were no thermal insulation, it would inevitably collapse. How often are meteoroids destroyed, for example, before reaching the Earth‘s surface.

At the same time, even in the popular science literature, one has to meet the assertion that the whole thing is in friction against air. But this is no longer true. The culprit is aerodynamic heating, but what is it?

Why do meteorites burn up in the atmosphere?

Of course, the main reason for heating is the meeting of an object rushing at supersonic speed with a sufficiently dense gaseous medium.

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Entering the dense layers of the Mars Rover lander as interpreted by the artist
When talking about meteoroids, it is important to note the difference in terminology. A meteorite is a body that has fallen on the surface of a large celestial object. But the burnt body is called a meteor. So the question of the combustion of meteorites in the atmosphere is not entirely correct.

Meteor bodies enter the atmosphere of our planet at a speed of 11.2 to 72 km/s. An important process during the fall of meteoroids to the earth is ablation. This is the “blowing” of a part of the substance from the surface of the body. This is due to the flow of hot gas.

What is aerodynamic heating?

Friction against the air, of course, occurs, and in this case some amount of heat is released, however, another physical process, called aerodynamic heating, heats up the skin of the descent vehicle and makes fireballs flying towards the earth burn and explode.

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Shuttle with heating zones
If the device flies at a speed that is three times the speed of sound (about 1 km / s), then the air near its surface is heated to 400 K (126 ° C), when entering the atmosphere of our planet with the first cosmic speed (about 8 km / s ) the temperature is already much higher – 8000 K (7727 ° C), and with the second cosmic velocity it is even about 11000 K, which equals 10727 ° C. From the areas of gas with an increased temperature, heat is transferred to the ship and aerodynamic heating occurs.

As is known, a shock wave is formed in front of a body moving in a gas at supersonic speed – a thin transitional region in which there is a sharp, abrupt increase in the density, pressure and velocity of matter.

Naturally, when the gas pressure increases, it heats up – a sharp increase in pressure leads to a rapid increase in temperature.

The second factor – this is actually aerodynamic heating – is the deceleration of gas molecules in a thin layer adjacent directly to the surface of a moving object – the energy of the chaotic movement of molecules increases, and the temperature rises again.

And already hot gas heats up the body itself rushing at supersonic speed, and heat is transferred both with the help of heat conduction and with the help of radiation.

True, the radiation of gas molecules begins to play a significant role at very high velocities, for example, on the 2nd space. So the reason is not limited to the friction in the atmosphere alone.

How does aerodynamic heating affect aircraft?

The problem of aerodynamic heating is faced not only by spacecraft designers, but also by developers of supersonic aircraft – those that never leave the atmosphere.

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Concorde Aerodynamic heating of the aircraft occurs in conjunction with the heating of its fuel system. So, the fuel can be heated up to 150 °C and above. Heating continues until an equilibrium is formed between the influx of heat from friction and its removal to the environment.

It is known that the designers of the world‘s first supersonic passenger aircraft – Concorde and Tu-144 – were forced to abandon the idea of ​​making their aircraft fly at a speed of Mach 3 (they had to be content with “modest” 2.3). The reason is aerodynamic heating.

At such a speed, he would heat up the skins of the liners to temperatures that could already affect the strength of aluminum structures. Replacing aluminum with titanium or special steel (as in military projects) was impossible for economic reasons.


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