It is expected that on April 30, the Shenzhou 17 spacecraft will return to Earth, and the Dongfeng landing site is also ready to welcome the triumphant return of the Shenzhou 17 spacecraft. Although the principle of the spacecraft's return to home seems simple, braking and decelerating, separating the module, re-entering the atmosphere, opening the parachute, activating the thrust reverser engine, and finally landing on the ground, the actual situation is far more complicated than imagined.
Because the spacecraft flies around the earth in a circular orbit about 400 kilometers above the earth's surface, flying at a speed of more than 7 kilometers per second, it needs to land accurately to the Dongfeng landing field when returning home, and the difficulty of landing is comparable to "10 rings in 10,000 miles".
How thrilling the ship's return was
Looking at the world, there are actually many countries that can send spacecraft into space, and many countries also have the ability to send probes to the moon or to Mars orbit, but so far, only three countries, China, the United States, and Russia, have the ability to independently launch manned spacecraft.
This actually shows that the technical requirements of manned spaceflight are quite high, after all, manned spacecraft carries astronauts, and it is not only necessary to send astronauts to space safely, but also to bring astronauts back safely.
One of the main reasons why other countries have not been able to carry out manned space missions is that they are not sure that astronauts will return home safely after a mission. Looking back at the history of human spaceflight, we know that the process of launching and returning to the spacecraft is very thrilling.
In particular, when the spacecraft returns to Earth from outer space, it will be exposed to high temperatures of up to 1000-2000 °C, and it is precisely because the outside temperature is so high that the spacecraft will enter the black barrier area, and the signal of the spacecraft will be blocked and lost contact for a short time. Under such high temperatures, ordinary materials are unbearable, but the return capsule of the manned spacecraft will not be burned.
When the Shenzhou-17 spacecraft is evacuated from China's space station, astronauts will wear in-cabin spacesuits and enter the spacecraft's return capsule. After evacuating from the Chinese space station, the first step of the spacecraft is to brake and decelerate, that is, to reduce the flight speed, and then separate the orbital module and the propulsion module at the right time, leaving only the spacecraft return module.
The spacecraft return capsule carrying astronauts also needs to adjust the flight attitude and enter the earth's atmosphere at a suitable angle in the right position, which is also very critical at this time, and the choice of angle is very important.
Moreover, because the aerodynamic heating effect of the spacecraft return capsule is very strong after entering the atmosphere, the external temperature will rise sharply, and the hottest part is in the direction of the spacecraft return capsule, so when re-entering the atmosphere, it is also necessary to control the flight attitude of the return capsule, so that its bottom insulation bottom faces forward, so that the insulation bottom can withstand higher temperature burning.
It looks simple, but it is dangerous if the ship's return capsule loses control of its flight attitude during the return process. Because the external temperature of the spacecraft return capsule is quite high, the heat resistance of the external materials is different, among which the insulating outsole at the bottom is the most heat-resistant, and the re-entry attitude with the insulated outsole facing down can achieve the greatest protective effect. If there is a flip during the descent, leaving the other parts facing forward, it may cause problems with the return capsule.
As the altitude of the spacecraft's return capsule continues to decrease, the speed will become slower and slower, but it will not reach the level of a safe landing, so the parachute will be opened when it is about 10 kilometers above the ground. With the help of a large parachute, the spacecraft return capsule can better slow down, and at this time, it will also throw away the thermal insulation bottom at the bottom of the spacecraft's return capsule, and when it is almost 1 meter from the ground, the thrust reverser engine at the bottom of the spacecraft return capsule will be activated, allowing the return capsule to land steadily on the ground.
Since the spacecraft will open a parachute with an area of up to 1,200 square meters during the final deceleration, such a parachute cannot be controlled, so it will drift with the wind, and it will not be able to achieve 100% accurate landing to the pre-selected landing site. If the wind is relatively small on the way back, the actual landing point deviation may not be very large, and near the pre-selected landing site, if the wind is relatively strong on the return home, the actual landing point of the spacecraft may deviate far.
China and the United States are stepping up testing the same "protective cover"
It is unavoidable that the spacecraft will be exposed to high temperatures when it returns to Earth from outer space. Because these spacecraft fly very fast, and the Earth has a dense atmosphere, when the spacecraft breaks into the atmosphere at a very fast speed, the aerodynamic heating effect will be very strong, and the temperature will naturally rise sharply.
Many countries do not have the technology to return spacecraft at high speed, so they do not have the ability to launch manned spacecraft to send astronauts into space to carry out flight missions.
Although it is impossible to avoid violent combustion when the spacecraft returns, with the advancement of technology, a new thermal insulation technology will emerge in the future, that is, inflatable heat shields. At present, China and the United States are stepping up the development of the same "heat shield" so that the spacecraft can better return home.
At present, the exterior of the return capsule of manned spacecraft is coated with a layer of ablative material and thermal insulation material, that is, a protective cover is installed on the outside of the spacecraft return capsule to resist the heat generated by the aerodynamic heating effect when the spacecraft returns home. Although the exterior of the spacecraft soared to one or two thousand degrees Celsius when it returned to Earth, the temperature inside the spacecraft capsule remained below 30 degrees Celsius.
Although such a technology can protect the spacecraft return capsule from being burned by high temperatures, there are some drawbacks to this design, one of which is the weight. As we all know, the cost of launching a spacecraft is very high, and nearly 45% of the weight of the entire spacecraft return capsule is used in these ablative materials. Moreover, these ablative materials are disposable and will fall off and take away the heat on the way back to Earth.
With the continuous development of technology, we are now developing in the direction of reusable spacecraft, and now there is an urgent need for a lighter "protective cover", and the inflatable heat shield that scientists are currently developing can achieve such an effect. Inflatable heat shields offer a number of advantages over the technology currently in use.
The first advantage is that the volume can be large or small, because the inflatable heat shield is inflated, which can be stored up when launched and reinflated to become larger when returning to Earth.
The second advantage is that the inflatable heat shield can be larger, the windward area will also be larger, and the air resistance will naturally increase, which can better decelerate.
Although the technical advantages of inflatable heat shield are very obvious, the difficulty of development is quite high, which can be called hellish difficulty.
Because the inflatable heat shield is inflated, which means that its strength is limited, when the spacecraft return capsule returns to the earth at a very fast speed, how to ensure that the inflatable heat shield is not damaged when it breaks into the earth's atmosphere at such a fast speed. Moreover, the temperature is quite high when returning to Earth, how can the material of the inflatable heat shield withstand such a high temperature of burning.
Although it is quite difficult to develop, the mainland has made some achievements in the development of inflatable heat shields, and has also carried out practical tests. As early as 2020, the Long March 5 rocket we launched carried out the first test module of the flexible inflatable cargo return module.