Space station launch success, but the Long March 5 rocket makes people sweat, the booster was burnt?

Those who are concerned about the Long March 5B launch vehicle of the Chinese space station "Tianhe" core module TV broadcast may have noticed a detail: during the flight of the rocket, the surface of the booster first peeled off the paint, then blistered and turned yellow as if it was baked by high temperature, which made people sweat, worrying that it exploded.

Some friends ask, the Long March 5 is not the legendary "ice arrow" it, it is loaded with liquid oxygen temperature is -183 ℃, liquid hydrogen is as low as -253 ℃, its surface should be cold, how can be scorched, could it be something wrong?

In the 154th second, the surface of the booster was "scorched"

Indeed, all these changes happened in a few tens of seconds. In the 76th second after the launch of the rocket, the surface of the third and fourth booster is still as clean as new, and in the next 100 seconds or so, the booster seems to be baked by fire, the paint is falling off in pieces, but also a little bit of blistering and yellowing. Until the 176th-second booster successfully separated, the heart hanging in the throat was finally put back into the belly.

The 76th second, the surface of the rocket was as clean as new

Where does the heat energy of the scorched rocket come from?

As mentioned earlier, the Long March 5 has the name "ice arrow", its core stage is propelled by two YF-77 hydrogen-oxygen engines, above the engines are liquid hydrogen and liquid oxygen storage tanks; outside the core stage, four booster rockets are fixed symmetrically, respectively by two YF-100 liquid oxygen/kerosene engine propulsion, booster rocket internal above is Liquid oxygen storage tank is above and kerosene storage tank is below. The temperature of liquid oxygen is so low that it is unlikely to be hot enough to scorch the booster shells.

The internal structure of Long March 5B is shown

In fact, inside the "tilt-head cone" at the top of the booster is a set of connection mechanisms, which is connected to the core stage of the rocket and transmits the booster power to the rocket. Due to the "offset concentration force" of over 300 tons, the designers went to great lengths to optimize the force structure at this location and did not install any electromechanical equipment that could generate heat.

In other words, the heat source that scorches the rocket is not inside the booster but is generated by the friction between the rocket and the air during the flight.

In the cold winter, we often rub our hands together, and the friction between two hands generates heat, which is understood by kindergarten children.

Rubbing hands will be warm

The nature of heat is particle motion. When a molecule or atom accelerates, it radiates more energy outward, which is heat. As the rocket moves faster and faster, some parts of it will push and compress the air in front of it, and the air molecules will hit faster and more intensively to raise the temperature of the rocket's surface. This is physically known as aerodynamic heating, or pneumatic heating for short.

Air is a fluid, and the aerodynamic heating mechanism is very complex. It has to do with the speed of the object's motion, the shape and roughness of the object's surface, the density of the air, and the ambient temperature, and a whole bunch of other conditions, and it roughly follows the following curve.

Speed of motion and surface temperature change curve

It is difficult to use a formula to calculate the surface temperature, for example, someone took the picture of the U.S. SR-71 "Blackbird" high-altitude high-speed reconnaissance aircraft and said that when it is flying at three times the speed of sound, the surface temperature of the fuselage can be as high as 300 ℃.

Blackbird's surface temperature is said to reach 300℃

"Unless you install a temperature sensor underneath the fuselage skin, you won't know exactly how hot the surface of the Blackbird is because it changes from time to time with changes in altitude, speed, and attitude. Many times the aerodynamic heating curve of a device is "blown" out, you have to put it in a supersonic wind tunnel to blow while measuring how hot it is.

The European Space Agency designed the Ariane 5 (Ariane 5) rocket through extensive wind tunnel experiments and came up with the following relationship between the surface air flow field, pressure, and temperature when the rocket was flying at Mach 0.9.

Ariane 5 aerodynamic heating experiment

The Ariane 5 rocket is more similar to the Long March 5 rocket booster in terms of aerodynamic shape, with the difference that the former has only two booster rockets while the latter has four. During the high-speed flight, the airflow fields of both face a similar situation at the booster head cone: this location generates compression waves due to the squeezed air, and thus aerodynamic heating occurs.

The formation of compression waves and pressure release during the transonic flight will result in a "sonic boom cloud", which is a process of warming - condensation - sublimation of water vapor in moist air, which happens to occur around the time the vehicle reaches the speed of sound.

The space shuttle crosses the speed of sound when the "sonic boom cloud"

From the live screen, the Long March 5B launch vehicle crossed the speed of sound between the 60th and 70th seconds of flight, and the "sonic boom cloud" disappeared, at which time we saw a small piece of paint fall off the surface of the head cone of the third booster, which was probably stripped by the strong air friction.

Long March 5 exceeds the speed of sound in the 70th second

As the rocket flew faster and faster, the air pressure at the booster head cone rose sharply. The aerodynamic heating caused the surface temperature of the booster rocket to soar, and the air squeezing and friction generated a hot airflow of about 800℃ not only stripped the paint into large pieces but also baked the varnish of the booster column section to blistering and discoloration, making people mistakenly believe that the rocket would soon have an explosion. The animation below, accelerated by a factor of 20, shows how the rocket's surface paint changes from the start of supersonic speed to just before booster separation.

Change of booster surface paint 100 seconds after supersonic speed

Some of my friends said, why paint the rocket, it will add a lot of extra weight, wouldn't it smell good to have a stainless steel shell like Musk's starship spacecraft? Most rockets are painted white not just for aesthetic reasons, a more important reason is that white reflects most of the light spectrum and acts as insulation before launch, which is important for cryogenic liquid fuel rockets. Musk is doing experiments, saving money in the first place.

Although the Long March 5B is a new rocket, the rigorous attitude of Chinese scientists, in the design process, must have done a lot of wind tunnel experiments, the booster because of aerodynamic heating resulting in surface burning should have been in control, in the manufacture of the rocket also fully considered the response. But for the masses, watching the "scorched" big rocket fly, the mood is understandably nervous, after all, the Long March 5 is lifting our Chinese space station, which is the pride of the Chinese nation ah!