The Shenzhou-9 spacecraft, which completed its docking mission with Tiangong-1 in space, brought three astronauts back to Earth safely. This is the fourth successful manned space mission that China has carried out so far, but do you really know how the spacecraft returned to the ground safely from an altitude of hundreds of kilometers? Take Shenzhou 9 as an example
Manned spacecraft
Shenzhou 9
Braking flight and free taxi stage: Choose the right speed Angle "Shenzhou 9" spacecraft after separation braking deceleration into the return orbit after docking with "Tiangong-1", "Shenzhou 9" spacecraft followed the space station to the orbit 350 kilometers above the ground, its operating speed is about 7.692 kilometers per second. After receiving the instruction from the ground to decide to return, the spacecraft astronauts first started the separation step to separate the Shenzhou 9 spacecraft from Tiangong 1 and become an independent aircraft. At this time, Shenzhou 9 is still in the orbit of Tiangong-1, and its speed is unchanged. After that, the astronauts need to adjust the flight parameters of the spacecraft and start the brake rocket in the opposite direction of the spacecraft to reduce the flight speed of the spacecraft. After the speed of the spacecraft decreases, the inertial centrifugal force of its flight decreases, and it is gradually pulled to the ground by the earth's gravity, and the spacecraft will leave the original flight orbit, enter the free taxi stage, and gradually transition to the orbit into the atmosphere. When the altitude dropped to 140 km above the ground, the propellant module and the re-entry module separated, the propellant module burned up as it passed through the atmosphere, and the re-entry module continued its descent. The "Angle of re-entry" into the atmosphere is crucial. The free glide phase is not without power, but it is not without speed. The deceleration process and the trajectory of the spacecraft's entry into the atmosphere are precisely calculated, and the main technical requirement is to obtain the right "reentry Angle" to enter the atmosphere at a specific altitude. The "reentry Angle" of the spacecraft, which is the Angle between the direction of flight and the local level when it enters the atmosphere, is the key to whether the spacecraft can return to the ground safely. Under normal circumstances, this Angle should not exceed 3°. If the reentry Angle is too large, the spacecraft will enter the atmosphere too fast, and the spacecraft itself will be burned and fall to the ground like a meteor. If the re-entry Angle is too small, the spacecraft will be "bounced" back into space and cannot return to the ground, and because the spacecraft often has very little fuel, it will be unable to complete the next re-entry orbit adjustment and fly into space. In 1965, the first spacewalk of the former Soviet Union astronaut Leonov aboard the "Rise 2" spacecraft in the return voyage, because almost missed the best reentry Angle, and the astronauts were scared out of a cold sweat, thanks to timely adjustment in place, to avoid terrible consequences.
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Landing stage: Mainly designed to protect the safety of astronauts, a variety of buffer means to ensure that the spacecraft landing at a low speed at a height of about 40 kilometers from the ground, the spacecraft has been basically out of the "black barrier area". By about 10 kilometers above the ground, the speed of the spacecraft has dropped to less than 330 meters per second, equivalent to the speed of sound. At this time, the static pressure altitude controller on the return capsule determines the altitude by measuring the atmospheric pressure, automatically opens the canopy of the parachute, first brings out the guiding parachute, and then pulls out the decelerating parachute. At this time, the speed of the re-entry module is about 180 meters/second, and the astronauts will be subjected to a large impact force. Through the action of the retarding parachute, the speed of the re-entry capsule was reduced to about 80 m/s. After 16 seconds of operation, the parachute is separated from the re-entry module and the main parachute is pulled out. At this time, the descent speed of the re-entry capsule was gradually reduced from 80 m/s to 40 m/s, and then reduced to 8 m/s to 10 m/s.