Experts explain the whole process of Shenzhou 12's return to earth
2021-09-15: [Original Article Link].
China's Shenzhou twelve spacecraft cheng zu of 3 astronauts have been "temple" space station "and" core nearly 3 months, according to the plan, they will return to the ground by the Shenzhou 12 spacecraft in mid-to-late September.
The return of the spacecraft to the ground is completed by the spacecraft system, the astronaut system and the landing field system, and the technology is more complicated. This time, the "Shenzhou 12" astronaut returned to the ground from the space station for the first time, and the manned re-entry module landed at the Dongfeng landing site for the first time, so it has some characteristics that are different from the past. We invite Pang Zhihao, the National Chief science communication expert on space exploration technology, to talk to readers about relevant scientific issues.
The Shenzhou spacecraft capsule landing
The spacecraft returned in four stages
Simply put, the return of the spacecraft to the ground is the process of the spacecraft breaking away from its original flight orbit, reentering the Earth's atmosphere along a descending orbit, decelerating by friction with the air, and safely landing on the ground. The return of the "Shenzhou" spacecraft can be divided into the following four stages.
The first stage is the braking deceleration phase. To make the spacecraft return to the ground, it is necessary to reduce the flight speed of the spacecraft, change the direction of flight, make it out of the original flight orbit, into the orbit of descending flight.
Specific process, beginning with the Shenzhou twelve spacecraft with "and" core between the docking mechanism unlock 2 spacecraft disengaged. The propulsion system is then activated to produce a separation speed. At the same time start the 2 of the spacecraft attitude and orbit control system to ensure that the ship to the predetermined requirement withdrawal.
When the spacecraft is running for the last lap in space, the ground sends a return command to the spacecraft, and the spacecraft immediately adjusts its attitude and yawks to the left (counterclockwise turn) 90 ° relative to the forward direction, and becomes a lateral flight state, this is the first time to adjust the attitude; Then the orbital module and the return module are separated at a relative speed of 1~2 m/s; Then the combination of the return capsule and the propulsion module is turned 90 ° counterclockwise to make the propulsion cabin forward, this is the second time to adjust the attitude; After reaching this braking attitude, the engine on the spacecraft's propulsion module ignites, causing the spacecraft to reduce its speed and enter the orbit of returning to the Earth.
The second stage is the free glide stage. After the combination of the return capsule and the propulsion module leaves the original orbit, it descends freely in an unpowered flight state. When the height of the combination of the return module and the propulsion module is reduced to 140 kilometers from the ground, the propulsion module and the return module are separated, the propulsion module burns when it enters the atmosphere, and the return module continues to descend, and eliminate the interference of the return cabin attitude separation caused by the separation of the two cabins, establish the correct reentry attitude angle (the angle between the speed direction and the local horizontal plane), and prepare for re-entry into the atmosphere. This angle must be precisely controlled within a certain range, generally 1.5 ° to 1.7 °, because if the re-entry attitude angle of the return capsule is too large, the return capsule will re-enter the atmosphere due to too fast, if the maximum overload exceeds the standard, the astronauts can't stand it, and the return module will even burn in the atmosphere like a meteor; If the re-entry attitude angle is too small, the return module will pass by the edge of the atmosphere and cannot return.
The third stage is the re-entry stage. The return capsule begins to re-enter the atmosphere at a distance of 100 kilometers from the ground. When the return capsule enters the atmosphere at a speed of 7.9km/s, it will produce violent friction with the atmosphere, making the return capsule into a flash of fireball, surrounding the plasma gas layer, shielding electromagnetic waves, at this time, the friction between the surface of the return capsule and the atmosphere forms a "black barrier", which temporarily loses contact with the ground until the black barrier disappears when it is about 40 kilometers away from the Earth, and the connection between the return capsule and the ground is restored.
During the re-entry process, from re-entry into the atmosphere to an altitude of 20 kilometers, the return capsule achieves return lift control by changing the roll angle of the spacecraft, make the return overload no more than 4G (acceleration unit of gravity), and can return to the landing field more accurately.
The fourth stage is the recovery landing stage. The recovery landing system started working at about 10 kilometers from the ground. It first opens the umbrella hatch cover, and then pulls the guide parachute, the parachute, the top umbrella and the main parachute in turn. Among them, the speed parachute can reduce the speed of the return cabin from 200 m/s to 60~70 m/s, and the main parachute can reduce the speed of the return capsule from 70 m/s to 5~6 m/s. In addition, the return cabin to a certain height (about 5.5 kilometers from the ground) to throw off the heat-proof undersole of the return cabin to expose the reverse thrust engine at the bottom of the return cabin. The Earth at about 1 m, 4 backstepping engine ignition to capsule approximately 3 meters per second speed of a soft landing of a touch on the, so as to ensure the astronauts of the landing security.
After the return capsule landed safely, its marking system began to work, indicating its location, so that the search and rescue system can find the target in time. This time, in order to quickly and accurately find the re-entry module, to ensure the success of the entire mission and the safety of the astronauts, the 510 Institute of the Fifth Space Academy has developed a new international rescue indicator, it integrates positioning information acquisition, data processing, and coding modulation transmission. It has high positioning accuracy and can realize the reliability and effectiveness of rescue in emergency situations.
Schematic diagram of separation of return capsule and propulsion compartment
Reentry capsules atmosphere schematic diagram
For the first time enabled deputy landing
The landing field system will play an important role in the return cabin landing process. The system is responsible for the return measurement of the spacecraft's return module, search and search, and astronaut rescue.
The choice of the landing site involves many technical and social issues, because the choice of the landing site should not only consider the location of the launch site, the inclination and height of the operating orbit, the return braking point position, the return capsule return orbit and the safety of the astronauts, it is also closely related to the latitude and longitude coverage of China, the topography and geomorphology of the mainland, meteorology, traffic, and the search and rescue capabilities of land landing on the sea.
According to the above requirements, after a large number of surveys, investigations, analyses and comparisons, China finally chose the Sipiwang Banner area in central Inner Mongolia as the main landing site. Taking into account factors such as small meteorological correlation, flat and open terrain, many opportunities for return, and full use of measurement and control equipment, China has chosen the southeast area of Jiuquan Satellite Launch Center, which is 1000km away from the main landing site, as the secondary landing site, that is, Dongfeng landing.
The return of the "Shenzhou 12" spacecraft is more difficult than ever. One is because before Shenzhou-12, manned spacecraft returned to Earth from a fixed orbit, but in space station missions, the orbital height of the space station will change with different nodes at different times, in order to save the propellant consumption of the space station, the orbit of the space station will not be adjusted for the return of the spacecraft, so the reentry point of the spacecraft is no longer fixed. For this reason, the "Shenzhou" team re-designed the return orbit adaptively, adjusted the return height of the manned spacecraft from a fixed value to a relative range, and improved the return algorithm to improve the return adaptability and reliability of the manned spacecraft.
Second, because "Shenzhou 12" is no longer landing at the main landing site of SZiwang Banner, which was commonly used in the past, but at the Dongfeng landing site. Dongfeng landing in Alxa League, Inner Mongolia Ejina Banner in the south-central part of the region. The southern part of the field is a small sand belt in the Badain Jaran Desert. The Eastern part gradually enters the Badain Jaran Desert. Its theoretical aiming point is located in the southeast of the manned space launch site. The area is about 120 kilometers wide from east to west and 190 kilometers long from north to south, from which several areas can be selected as landing areas. The area has a temperate arid desert climate, with dry and cold winters, hot summers, less rain and windy. Since the landing site is located on the desert and the Gobi, there are also mountainous or potholed terrain, so the search and rescue difficulty is higher than before. This time, the search and rescue capabilities of the Dongfeng landing site will be tested for the first time.
During and after the landing of the return capsule, the ground search and rescue work is very important. At present, China generally adopts the mode of "searching and rescuing astronauts in the air and disposing of the return cabin on the ground", which can realize "fast positioning, fast arrival and safe exit". Therefore, landing site search and rescue usually includes two parts: air search and ground search and rescue recovery.
Air search helicopters are generally equipped with 7 aircraft: 1 Command aircraft, 1 communication aircraft, 1 search and rescue aircraft, 1 medical supervision and medical support aircraft, 3 medical ambulance aircraft, they are all equipped with "Beidou" satellite navigation and positioning system, dynamic information can be transmitted back to Beijing Aerospace Flight Control Center in real time, mainly responsible for re-entry module search and astronaut rescue.
Search space formed on the overlap of the 3 levels: one is the long-range search target is found, in the re-entry capsule a blackout before with radar capsule for tracking and measurement forecast landing position; second, the medium-distance search found the target, after the return module out of the black barrier, the use of unified S-band measuring equipment to track and measure the opening point of the return module to the main umbrella, the helicopter VHF directional instrument receives beacon information and tracks the return module to the landing point; the third is a close search to find the target, centered on the forecast location of the landing site, using a helicopter orientation and a vehicle-mounted high-frequency directional instrument to search for the return module.
After the helicopter search and rescue astronauts are completed, the processing and transportation tasks of the return module will be handed over to the command and dispatch vehicle, engineering transport vehicle, astronaut transport vehicle, A ground search and recovery fleet consisting of special vehicles such as return cabin cranes and load trucks, which are responsible for the follow-up disposal and recovery of the on-site return cabins and carry out on-site related disposal according to predetermined procedures, transport the return cabin and payload to a nearby railway station and transfer it to Beijing by rail.
Main parachute carry capsule steady decline
Medical insurance is very important after landing
The search and rescue team distinguishes the types of work by the color of clothing. The red overalls are the search and rescue personnel of the landing field system, and the white overalls are the medical supervision and medical insurance personnel of the astronaut system, the blue overalls are the return cabin disposal personnel of the spacecraft system.
Generally, the return cabin disposal personnel first check the return cabin, including whether there is any residual propellant in the thrust-back engine, and use a special gamma-ray source protective cover to shield the gamma source of the gamma altimeter at the bottom of the return capsule, prevent radiation from harming rescue workers. Then, open the return cabin door, the medical supervision and medical insurance personnel enter the return module to conduct preliminary medical examination and confirmation of the astronaut's health status, and assist the astronauts in the return cabin to initially adapt to ground gravity, including physical movement, stand step by step to adapt, replenish drinking water and nutrient solution. After the astronauts get out of the cabin, they also need to carry out safe treatment of the return cabin again. First, turn off the power supply of the return cabin, take out the spare pyrotechnical backup unit, and then remove the gamma source of the altimeter and store it in a special protection container.
Due to the "Shenzhou twelve" astronauts for 3 months, the astronauts in space of the heavy workload of the compared fatigue orthostatic tolerance decreased more, therefore, the "Shenzhou 12" astronauts need to be in the cabin for a long time to adapt to the ground gravity environment, and then out of the cabin with the assistance of the staff, and all activities after the cabin are taken in a sitting position, to ensure the safety of astronauts. This is because astronauts will have problems such as osteoporosis, muscle atrophy and poor stance endurance after living and working in a long-term weightless environment. If they walk on their own, they will easily fall or even fracture. In the early days of returning to the ground, you should avoid hugging with the astronauts, and you should not throw the astronauts into the air because of excitement.
Since then, the medical supervision and medical insurance personnel assisted the astronauts in the medical supervision medical insurance helicopter to take off the spacesuit, implement sanitation and cleaning, and replace the underwear; Conduct physical examinations for the astronauts, collect samples, and collect physiological data; evaluate the health status of the astronauts, give the necessary medical supervision and medical insurance treatment; Send the astronauts to board the plane and accompany the whole process.
In the event of an astronaut's injury, the medical ambulance personnel arriving at the scene should make use of on-site medical technology and support conditions to carry out necessary emergency on-site medical treatment for the astronauts, including fracture fixation, wound or wound hemostasis and bandaging.
After returning to Beijing, the astronauts have to recover their bodies. They are generally divided into three stages: medical isolation period, medical recuperation period and recovery period.
The medical isolation period is about 14 days. The astronauts adapt to the Earth's gravity environment in the astronaut apartment, improve the cardiovascular system and support the function of sports organs, improve the standing endurance, and eliminate post-flight fatigue.
During the isolation period, the astronaut doctor shall perform medical examinations on the astronauts, including routine examinations in clinical subjects, standing endurance examinations, balance function examinations, body composition analysis, cardiovascular regulation and control functions, human body function status detection, etc. Important inspection items should be dynamically tracked, combined with physical training and recovery, and gradually increase the load in accordance with the principle of gradual and orderly progress. You can arrange some balance training, walking training, medical gymnastics swimming, and relaxation for the astronauts.
According to the results of clinical medical examination and physiological function examination, astronaut doctors should comprehensively analyze and evaluate the health status of astronauts to guide follow-up recovery measures. During this period, the astronauts were basically isolated from the outside world.
The medical recuperation period is about 20 to 30 days. The astronauts will stay in a sanatorium with good weather and good air, and gradually increase their activity while continuing to recover their health. Astronauts can be arranged for rehabilitation and recuperation to further enhance the physical fitness of the astronauts, strengthen the reserve of physiological functions, improve the ability of disease prevention and disease resistance, and restore the astronauts to their pre-flight state as soon as possible. Rehabilitation measures such as landscape treatment and physical exercise can be arranged in the nursing home.
The resumption period is about 3 months. During this period, the physiological parameters of the astronauts should be restored to their pre-flight state. After 3 months, if the astronauts are in good health, they can start normal daily training.
[The interesting knowledge]]
Astronaut "inverted seat" returns
When the re-entry module re-enters the atmosphere, the astronauts are sitting on the "inverted seat", that is, the direction of the seat of the astronauts is opposite to the flight direction of the spacecraft. This is because the return capsule has been in a "brake" deceleration state during the reentry process. The use of "inverted seats" can make the astronaut's head and upper body press on the seat back with shaped cushions, in this way, the astronauts are easy to withstand large overloads.
Source: Beijing Daily Client | Reporter Wang Hongliang
Edit: Wang Haiping
'Edit flow': Guo
Note: This is auto-translated version of an article meant for Chinese audience. A mature and nuanced reading is suggested.