Shenzhou-17 is already on the way back, although China's manned space technology has been very mature, but in fact, returning to the earth from space is still full of great risks, and the difficulty is comparable to "10 rings in 10,000 miles".
What is the 5-lap fast return scheme
The Shenzhou-17 manned spacecraft will adopt a 5-lap rapid return scheme, which means that after completing the mission, the spacecraft will return to Earth after flying in space for about 5 laps, going through stages such as braking deorbit, free taxiing, re-entering the atmosphere, and parachute landing. Before the Shenzhou 13 spacecraft, the Shenzhou spacecraft adopted the "nominal ballistic" return method, which had extremely strict requirements for the time and location of the deorbit. The window is only a few minutes or even seconds. Therefore, manned spacecraft such as Shenzhou 10 and Shenzhou 11, even if they are separated from the space station equipment, still have to continue to orbit in their original orbits, slowly looking for or waiting for the arrival of the return window. Generally speaking, this process requires a manned spacecraft to circle about 15 times in low-earth orbit, which takes nearly a day. In order to see the timing and positioning, accurately ignite the fire derailment and enter the return window. Such a "timed and fixed-point" return method can no longer meet the needs of astronauts in the space station era for frequent round-trips and emergency returns.
The space vehicle flies in orbit around the earth, the earth's gravitational force Fg wants to pull the aircraft to the ground, and the centrifugal force Fv generated by the spacecraft flying around the earth's orbit wants to keep the aircraft away from the earth, and the magnitude of the centrifugal force is related to the speed v of the flight, and the greater the flight speed, the greater the centrifugal force. It is really like a lot of water droplets on an umbrella, turn the umbrella quickly, the water droplets will splash out along the tangent direction of the umbrella rotation, the faster the umbrella turns, the faster the water droplets splash out, this is the centrifugal force of the water droplets to overcome the effect of the adhesion between the water droplets and the umbrella. Spacecraft S flying in orbit at a certain altitude of H0 is affected by both the gravitational force of the earth Fg and the centrifugal force Fv.
If these two forces are equally large, the spacecraft will remain in orbit at its original altitude and operate at a constant velocity of V0. If the speed of flight accelerates and the centrifugal force increases beyond the gravitational pull of the earth, the spacecraft will fly away from the earth, and vice versa, the spacecraft will move closer to the earth, gradually decrease the altitude of the orbit, and finally enter the atmosphere and fall to the earth (as shown in Figure 2). It can be seen that in order to return the spacecraft flying along the orbit to Earth, it is necessary to "slow down" the spacecraft. In order to accurately land the aircraft at the predetermined location, it is also necessary to grasp the "timing" and "size" of the deceleration
Therefore, the spacecraft needs a time to return to the earth, when the Shenzhou spacecraft returns, the earth under its feet continues to rotate like an automatic slide boat, and the spacecraft needs to wait for the right time in orbit before it can lower the orbit and jump down, and the deceleration process of the spacecraft and the orbit into the atmosphere are accurately calculated and required to be very accurate.
The shape of the re-entry capsule is like a "big bell" with a narrow top and a wide bottom, and it returns to the earth in a lift-up return mode with the big bottom facing forward through the attitude adjustment of the engine, and the re-entry capsule should establish a correct re-entry attitude angle (the angle between the direction of velocity and the local horizontal plane). This angle must be precisely controlled within a certain range, and it must enter the atmosphere at a specific altitude with a suitable "reentry angle". If the re-entry angle is too steep, it will cause the re-entry capsule to enter the atmosphere too fast and burn up due to severe friction; if the re-entry angle is too gentle, it will be "catapulted" back into outer space by the atmosphere like a drifting tile, and it is likely that it will never be able to return to the ground.
Not only should the return capsule adopt a comfortable return position, but the astronauts should also "lie flat" in the seat at an angle almost parallel to the outsole. The purpose of choosing such a position to return is to better cushion the impact formed during the deceleration of the spacecraft, so as to bring more protection to yourself.
The spacecraft orbit design team needs to carefully design the return orbit scheme, and add a prediction correction guidance method to help the return capsule calculate and maintain the reentry trajectory according to the real-time position, aerodynamic parameters, aiming point deviation, etc. during the re-entry process, so as to adapt to the change of the landing point and the characteristics of a wide range of orbit changes, and ensure that the return "orbit" is not bad at all.
Shenzhou 17 returned to a thrilling scene
After entering the atmosphere, the test is not over, we can see that in the CCTV news, it is specifically mentioned that Shenzhou 17 left the black barrier area, which is the most thrilling scene of the spacecraft's return.
The spacecraft's return capsule will be tested by high temperatures, vibrations, and harsh environments. 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 60~70 kilometers above the ground, and the temperature outside the capsule can reach up to 2000 degrees.
Even with the protection of fire shelters. However, because the air density is getting bigger and bigger, the return capsule rubs violently with the air, so that the bottom temperature is as high as thousands of degrees Celsius, and the return capsule is surrounded by flames, and there will be vibration noise overload in the capsule, which has caused great wear and tear to the astronaut's body for 6 months in space, and the harsh environment of high temperature vibration will cause great burden to the astronaut's body.
During landing, gas molecules close to the surface of the re-entry capsule are broken down and ionized due to pneumatic heating, forming a 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 the state of being wrapped by plasma, the radio signal outside the cabin cannot enter the cabin, and the radio signal in the cabin cannot be transmitted outside the cabin.
In this process, the ground cannot control the spacecraft by any remote control method, and the aircraft is relied on to process the status fully automatically.
The range of the black barrier depends on the shape, material, reentry speed, and frequency and power of the transmitted signal. The black barrier makes it difficult for real-time communication and reentry measurement of manned spacecraft during re-entry and return, and there is no good solution at present.
The whole return process is complex, and there can be no mistakes in any link. Any slight mistake may cause the re-entry capsule to deviate from its predetermined landing point by several hundred to several thousand kilometers and land in an area or sea surface where people are rarely inhabited, and the astronauts will lose their precious life support, and it may also make the spacecraft fly like a loose horse into the vast space and become a meteor falling between the stars, and may eventually crash into a certain body.
summary
In addition, the Dongfeng landing field is located in the Badain Jaran Desert and the Gobi region, where the climatic conditions are extremely harsh. In April, the Dongfeng landing field, cold air is frequent, wind and sand are frequent, which brings great challenges to the landing of the spacecraft, the return capsule is greatly affected by the high-altitude wind when the parachute is opened, and it is easy to drag and roll after landing, and the weather conditions such as high-altitude wind and ground wind at the landing site are higher when the manned spacecraft returns to landing. In order to cope with the extreme weather after the spacecraft landing, the Dongfeng landing site has been prepared for precipitation, strong winds, sand and dust, high temperatures, low temperatures and other conditions. The meteorological office of the Jiuquan Satellite Launch Center has strengthened the density of consultations for several days, and has been continuously monitored and analyzed.
It can be said that the mainland has made all preparations for the return of Shenzhou-14, and let us welcome the Shenzhou-17 astronauts home.