Shenzhou 14 is so dangerous to return to the earth? Shenzhou spacecraft needs to accurately land at Dongfeng landing ground from a place 400 kilometers away from the earth. The difficulty is comparable to "10 rings in a thousand miles". Let me talk to you about how return capsule returns to the earth.
angle control is accurate, otherwise it is impossible to enter the atmosphere and return to the earth. It is a high-risk process. The spacecraft orbits the earth at a speed of 7.8 kilometers per second. The Shenzhou spacecraft runs in orbit at a speed of up to 7.9 kilometers per second. If you want to enter the atmosphere, the first thing you need to do is braking: the brake engine starts to work, so that the orbital height of the spacecraft continues to decrease. After it is reduced to a certain height, the spacecraft adjusts its posture and enters the return attitude, and then the return capsule separates from the orbital cabin and starts to enter the atmosphere. The deceleration process of the spacecraft and the orbit entering the atmosphere are accurately calculated and require very precise accuracy.
The appearance of the return capsule looks like a "big clock" with a narrow upper and wide upper lower. Through the engine's attitude adjustment, it returns to the earth in a lift-like manner with the big bottom facing forward. The return capsule needs to establish the correct reentry attitude angle (the angle between the velocity direction and the local horizontal plane). This angle must be precisely controlled within a certain range and must enter the atmosphere at a suitable "reentry angle" at a specific height. If the reentry angle is too steep, it will cause the return capsule to enter the atmosphere too quickly, causing violent friction and burn; if the reentry angle is too flat, it will be "bombed" back to the outer space by the atmosphere like a tile drifting, and it is likely that it will never be able to return to the ground again.
Not only should the return capsule adopt a comfortable return posture, but astronaut also should "lie flat" in the seat at an angle almost parallel to the bottom. The purpose of choosing such a posture to better alleviate the impact formed during the spacecraft's deceleration process and thus bring more protection to yourself.
The sentence is easy to say, but it is very difficult to do. The spacecraft orbit design team needs to carefully design the return orbit plan and add a prediction and correction guidance method to help the return capsule calculate from the main calculation based on real-time position, aerodynamic parameters, aiming point deviation, etc. during the reentry process, to adapt to the changes in the landing point and the characteristics of the large orbit change range, and ensure that the returned "orbit" is not bad at all.
The entire return process is complicated, and no errors can be made in any link. We will see that the Shenzhou spacecraft looks like a big bell. These are based on the theory of American physicist Henry Allen. He found that the front end of the spacecraft that re-enters the atmosphere at high speed strongly compresses the air, forming an umbrella-shaped shock cone in the atmosphere ahead. The air density of the shock wave at the front edge of the shock wave has increased sharply, and it looks like a moving wall in front of the spacecraft, and the spacecraft is moving forward in the wake of the shock wave cone. Since the shock cone in direct contact with the static air ahead is the shock cone rather than the spacecraft itself, aerodynamic heating is mainly generated by compression and friction between the shock front and the static air ahead.
If the spacecraft surface and the shock wave front are maintained at a certain distance, the heat generated by aerodynamic heating will mainly conduct and dissipate in the shock wave with higher air density. Under the protection of the wide and thick boundary layer around, the spacecraft will bear much smaller heat load. Therefore, an important way to reduce the thermal load of the spacecraft is to move the shock cone forward and try to stay away from the spacecraft body. Based on this discovery, Henry Allen proposed the idea of a spacecraft that if the return capsule is shaped like a bell or conical, with a wide and round bottom, an umbrella shock wave will be generated in front of it when the air enters the atmosphere at a speed of 20 times the speed of sound. In addition, it can also ensure that the spacecraft does not roll during the reentry process, and the flat bottom facing downward reentry attitude can produce the greatest protection effect.
2000 degrees high temperature, black barrier blocks the spacecraft signal
. After entering the atmosphere, the test is not over. The spacecraft return capsule will experience the test of high temperature vibration harsh environment. According to calculations, the spacecraft returning from the universe often has 20 times the speed of sound when it descends from a high altitude to reach 60 to 70 kilometers above the ground, and the temperature outside the cabin can reach up to 2,000 degrees. The outside of the cabin must be used to use fire jacket to prevent it from burning.
even has protection from fire protection. However, because the air density is getting stronger and stronger, the return capsule rubs violently with the air, causing the temperature at the bottom to reach thousands of degrees Celsius. The return capsule is surrounded by flames, and vibration noise overload will occur in the cabin. In the six months in space, it has caused great losses to the astronaut's bodies. The high temperature and vibration harsh environment will cause great burden on the astronaut's bodies.
During the landing process, due to pneumatic heating, gas molecules close to the surface of the return capsule are decomposed and ionized, forming an plasma layer. Since plasma has the ability to absorb and reflect electromagnetic waves , the plasma layer that wraps the return capsule is actually a plasma electromagnetic wave shielding layer. Therefore, when the return capsule enters a state wrapped in plasma, the radio signals outside the capsule cannot enter the capsule, and the radio signals inside the capsule cannot be transmitted outside the capsule. For a moment, the contact inside and outside the cabin is lost, which is a black barrier phenomenon.
In this process, the ground cannot control the spacecraft through any remote control method, and the aircraft relies on the aircraft to fully automatically handle the status. The range of the black barrier depends on the shape of the reentry body, material, reentry speed, frequency and power of the transmitted signal. The black barrier causes difficulties in real-time communication and reentry measurement when the manned spacecraft reentry return, and there is currently no good solution.
parachute landing, ignite 10 milliseconds at the same time
, finally landing using the parachute. The landing accuracy of the spacecraft return capsule does not mainly depend on the parachute, because the parachute used by the spacecraft is a "uncontrolled umbrella" and will drift with the wind. The main thing that determines the accuracy of the landing point is actually the "umbrella opening point", which means that when the parachute is opened during the return process, the space position of the return capsule is . At present, we have achieved world-leading in this field because we have innovated "adaptive prediction guidance technology".
. In the system, the "Navigation" is responsible for giving the current return capsule position, speed and spatial orientation, " guidance " is responsible for providing a method to reach the "umbrella opening point", and "control" is responsible for executing.
There are many deviations and large deviations during flight. For example, the initial position, speed, attitude deviation of the return capsule, dynamics and control deviations such as aerodynamic coefficient, engine thrust, return capsule mass, and environmental deviations such as atmospheric density.
Secondly, the effective control time is short, only 350 seconds, and the dynamics are complex during the process. When we finally land, we must know that when the cabin is about 10 kilometers away from the ground, the speed of the spacecraft has dropped below 330 meters per second. At this time, the static pressure altitude controller on the return capsule automatically determines the altitude by measuring the atmospheric pressure and decelerates the parachute, gradually reducing the return capsule speed to about 7 meters per second.
However, the return capsule still has a high speed and high kinetic energy. The "head-and-hard" impact generated at this speed is very likely to cause damage to the astronaut's spine.
Therefore, the return capsule will stop urgently when it is 1 meter away from the ground. The four landing reverse thrust engines installed at the bottom of the return capsule will automatically ignite, and use extremely strong buffering to help them implement "soft landing". Although the reverse thrust engine is not big, it can generate 3 tons of upward thrust when is ignited. When the return capsule landed, the four units generated a total of 12 tons of upward thrust, offsetting most of the kinetic energy of the return capsule, thereby achieving the purpose of slowing down. also further absorbs energy through the vibration-absorbing system composed of energy-absorbing shell, vibration-absorbing material and seat buffering mechanism at the bottom of the return bilge, thereby ensuring that the astronauts are absolutely safely landing.
In order to ensure the safety of the astronaut and the equipment in the return capsule, the four landing reverse thrust engines must be ignited simultaneously within 10 milliseconds. In this way, the return capsule will be successfully completed.
Many scientists say that it is easier for spacecraft to fly than return, and the return process must be done exactly the same. As long as there is a little deviation, the price will be the life of the astronauts.
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