On the evening of April 30, the astronaut trio returned from a "business trip" in Tiangong.
At 5:46 p.m. yesterday evening, the return capsule successfully landed, and 50 minutes later, the three astronauts successfully exited the capsule, and their mission was a complete success.
Before the Shenzhou-17 astronaut crew returned, the Shenzhou-18 crew successfully arrived at the space station, and the two groups of astronauts successfully completed the handover.
The space station has entered the normal operation mode, and in the future, such high-frequency launches and returns will also become a normal phenomenon. This also means that the mainland's space technology development is quite mature.
So, what was the process of the Shenzhou-17 astronaut crew going home, and what are the requirements for the landing area of the return capsule? We might as well take a look at them one by one.
Smooth landing after 187 days
At almost 5 o'clock in the afternoon of April 30, under the control command of the Beijing Flight Control Center, the return capsule and the orbital module of the astronaut crew were separated.
The next 50 minutes were the moment of "intimate contact" between the capsule and the atmosphere. Under the tremendous gravitational pull, the capsule will fall a distance at high speed.
At the final elevation, the re-entry capsule throws and opens the parachute. When it is almost close to the ground (this distance is 1 meter), the last high-tech device under the capsule will come into play.
This equipment is the thrust reverser engine, just listen to the name, it will give it an upward thrust again when the return capsule is about to land at the end, so as to reduce the impulse of the return capsule to the greatest extent.
Four thrust reversers were installed on the re-entry capsule, and yesterday when the re-entry capsule was 1 meter above the ground, four engines were successfully operational.
Don't think that these 4 engines are very simple to manufacture, they are specially developed by the 806 Institute of the Eighth Academy of Aerospace Science and Technology Group.
At the moment when the re-entry capsule is about to land, the falling speed is still 8 meters per second, and the thrust reverser engine needs to generate braking force exactly at a fraction of the time. The ignition time reserved for the engine is only 10 milliseconds, and all four devices need to generate thrust reversing at the same time.
It can be said that this is the last instrument and equipment that astronauts will work on and play a huge role in the process of returning home. After the thrust reverser engine is successfully running, the astronauts can be considered to have really returned home.
Shenzhou-17 was launched on October 26 last year, and three astronauts, Tang Hongbo, Tang Shengjie and Jiang Xinlin, worked and lived on the space station for 187 days. During this period, two spacewalks were also carried out.
Among the three astronauts, Tang Hongbo has carried out two missions before and after, so his time in space has reached 279 days, which is also the record of the longest flight time of a mainland astronaut in orbit.
This time, the Shenzhou 17 astronaut crew still adopted a 5-lap rapid return plan.
What is "5 Lap Fast Return"?
Since Shenzhou-13 in 2022, astronauts have gone home with a 5-lap fast return method.
The "circle" here refers to the number of circles the aircraft has circled the earth, and the rapid return refers to the small number of circles that the aircraft has circled the earth in orbit, and it only takes 5 times to start entering the atmosphere.
According to the engineers and technicians of the manned spacecraft system, after separating from the space station, the spacecraft does not go directly to the earth, but needs to fly around the earth again and find a suitable angle before it can enter the atmosphere.
Technically, this angle is hard to find. If the angle is not right, it will either cause the spacecraft to continue to orbit away from the atmosphere. Either it will enter the atmosphere at the wrong angle, causing damage to the spacecraft in excessive friction.
When Shenzhou-12 returned, the spacecraft circled the Earth 11 times in the process of circumnavigating the Earth to find an angle. It takes about 1.5 hours to go around the earth every time, so it takes a long time to go around the earth for more than a dozen times.
Reducing the number of laps can reduce the overall time of the spacecraft to orbit, thereby reducing the time it takes for astronauts to go home. Reduced to 5 laps, which is a fast return scheme.
According to the introduction of engineering and technical personnel, the reduction of the number of laps is a compression and cropping of some flight events in the whole process.
Through the overall time compression, the number of orbital orbit circles of the spacecraft can also be compressed. However, although the time in the sky is compressed, the reaction time in the area of the ground landing site is short.
In order to achieve faster reaction capabilities, the entire system of the landing site was repeatedly rehearsed. Shenzhou-13 successfully adopted this mode, and since then the fast return mode has become the standard solution.
In the fast return mode, the time required for the spacecraft to separate from the space station and successfully land is compressed to about 9 hours. In this way, the astronauts will be able to return home in a relatively faster way.
Although the overall time has been compressed, there are still many dangerous steps and complications that the spacecraft needs to go through in the process of returning to Earth.
What to experience in the "fall".
Although the spacecraft does not require a lot of extra power to return to Earth, the whole process also requires the necessary power and the right angle.
Figuratively speaking, it's like a bicycle slipping on a long, steep slope, and although you don't need to pedal it, controlling the direction and braking are crucial.
When the spacecraft is in orbit, it docks with the space station. When the astronauts want to go home, they need to use the control instructions of the ground control center to separate the spacecraft from the space station step by step.
After separation, because the spacecraft is still quite high (more than 350 kilometers) from the Earth at this moment, the spacecraft at this time will not fall towards the Earth.
Under its own power, the spacecraft can leave the orbit where the space station is located and then approach the Earth at a constant speed.
Although no driving force is required in this process, the speed of the entire flight is calculated. If the spacecraft wants to "brake", it is to use the fuel it carries to produce a counter-thrust.
Normally, how the speed changes, the astronaut has to follow the instructions from the ground. The ground staff is calculated by the altitude, speed, and range of operation of the spacecraft.
And in this process, it is the process of the spacecraft orbiting the earth. Once the angle is found, the spacecraft will gradually approach and enter the atmosphere as it continues to fall.
By this time, the exterior of the ship will be subjected to high temperature pressure. The mainland's Shenzhou series of spaceships are non-recyclable, that is, the spacecraft is used once and then no longer used. As a result, the exterior of the spacecraft is insulated by ablation.
To put it simply, it is to cool and insulate the spacecraft as a whole by self-combusting of the outer thermal insulation coating of the spacecraft.
This layer of polymer material melts and evaporates under intense friction, absorbing and taking away most of the heat in the process.
The exterior coating insulates the main heat, and the interior of the ship is also designed to insulate and cool down. During the entire descent, the astronauts felt the temperature in the capsule, which was about 30°C.
Although the high temperature can be isolated, the ionization area formed by the high temperature around the spacecraft cannot be avoided in any case, which is also known as the "black barrier".
In this process, the communication between the spacecraft and the ground will be interrupted, and the astronauts need to control the spacecraft to complete a series of actions on their own. In this case, they are like kites with broken strings, completely unaware of what will happen in the next second.
In the area of the black barrier, unknown accidents are prone to occur. A return from the Soviet era, in which the astronaut later died due to an air leak in the capsule.
Enduring the heat and loss of contact, the spacecraft continued to fall downward, and when the altitude from the ground dropped to about 40 kilometers, the astronauts were able to reconnect with the ground.
As the altitude continues to fall, the device on the re-entry module of the spacecraft will automatically determine the altitude by measuring the atmospheric pressure, so that the parachute can be opened within a specific altitude range.
Once the parachute is successfully opened, the speed of the spacecraft's descent can be reduced from about 330 meters per second to about 180 meters per second.
As the velocity continues to decrease, the main body of the ship will detach from the re-entry capsule at a certain altitude. At this time, the return capsule will open a main parachute again, and then the speed of the fall will continue to slow down, until it decreases from tens of meters per second to about 8 meters per second.
Although the speed has been greatly reduced at this moment, if it lands in such a way, the astronauts in the capsule will still be injured by the huge momentum even if they take protective measures.
So the final key node is when the thrust reverser engine is about to appear. Eventually, it can further reduce the speed of the spacecraft to less than 2 meters per second.
It was only at this moment that the astronauts really returned to Earth. The area they are in, as long as there is no miscalculation, will definitely be the Dongfeng landing field.
Why did you choose the venue here?
The area, known as the Dongfeng Landing Field, is a natural Gobi desert. It is located in the Ejina Banner of Inner Mongolia, next to the Badain Jaran Desert.
The area is open and sparsely populated, with no complex vegetation and only sparse and low bushes, so this is very conducive to the search of people on the ground and the take-off and landing of helicopters.
Before Shenzhou XII, this area was only a backup area for the spacecraft to return. Since Shenzhou 12, it has become the main site for return, and search and rescue personnel are also on regular emergency standby.
In terms of maintenance costs, it can rely on all the resources of the Jiuquan Satellite Launch Center to operate, and only one local search and rescue force needs to be deployed to meet the demand.
Moreover, the sparsely populated area will not affect the local economic construction. On the contrary, we can develop space tourism and promote the economic development of the region.
In addition to search and rescue, the work of the landing site is to ensure the health of astronauts after leaving the capsule.
From the seats used by astronauts after leaving the capsule, to the equipment such as medical insurance tents and medical vehicles, they were all developed by the landing site before.
Compared with the previous way of exiting the capsule, starting from Shenzhou-13, astronauts will enter the medical insurance carrier after exiting the capsule, shortening the exposure time in the field environment.
epilogue
The Shenzhou-17 astronaut crew returned smoothly yesterday, and the ground search and rescue personnel were very skillful in all steps, and every successful landing was an accumulation of technology and experience.
However, in the field of spacecraft return, because the situation faced is different each time, the past experience is only a kind of reference, and for the operation of the entire support system, a new foolproof plan must be formulated every time.
Each successful return is not only a safe return of astronauts, but also a sign that the development of the mainland's aerospace industry has reached a new level.
Resources:
"Shen 17 Return Capsule Landing Precise "Braking", Shanghai Aerospace Escorts Astronauts Home for the 12th Time" The Paper April 30, 2024
"Shen 17 manned spacecraft adopts 5-lap fast return scheme, landing site is ready", Beijing Youth Daily, April 30, 2024
"The whole journey only takes more than 9 hours, China's manned spacecraft adopts fast return mode to "go home" for the first time", Beijing Youth Daily, April 17, 2022