According to the official information released by the China Manned Space Engineering Office, at 6:33 Beijing time on June 4, 2023, the return capsule of the Shenzhou-15 manned spacecraft successfully landed at the Dongfeng landing site.
What's even more exciting is that the landing point of this return trip was praised as ultra-accurate, but the actual landing point coordinates are 1580 meters different from the eagle's predicted coordinates.
Is it really that difficult to predict the foothold of the return capsule?
When Shenzhou 15 returned this time, a total of four predictions were made, and the final landing point coordinates were 100°03'52" east longitude and 41°38'07" north latitude.
The first forecast is 1,580 meters away from the landing point, the second is 1,620 meters away, the third is 1,700 meters away, and the fourth is 1,110 meters away.
And such a result is already a hit to the ten rings.
There are many reasons why it is so difficult to predict the foothold of the return capsule.
First, the speed of the spacecraft in flight is too fast.
After all the space experiments have been completed, the astronauts are ready to return home. The first thing to do is to adjust the posture of the spacecraft, turn 90° counterclockwise in the relative forward direction, and adjust from longitudinal flight to lateral flight.
At the same time, the orbital module and the return capsule of the spacecraft began to separate.
After the re-entry capsule is separated, the second posture adjustment begins, and the remaining two-compartment combination, on the basis of the attitude just now, turns 90° counterclockwise again.
After two 90° counterclockwise turns, the trajectory of the re-entry capsule at this time was more like a U-turn than before it was ready to return.
At this time, the propulsion module is in the front, the return module is in the back, and the flight attitude has gradually changed from horizontal flight to an elevation angle.
The power source is the engine of the propulsion module, and the altitude of the spacecraft drops from 399 kilometers to 380 kilometers, and the spacecraft gradually enters the return orbit while decelerating.
When it can be seen that the Dongfeng landing field is located in the orbital plane of the spacecraft, the spacecraft can begin to slow down and return to Earth, and the prediction of the foothold of the return capsule also starts from this time.
The speed of the capsule remained at 7.9 kilometers per second as it passed through the 100-kilometer-thick atmosphere. Due to the gravitational pull of the Earth, the speed is also disturbed by gravitational acceleration during the actual descent, especially at the lower the altitude and the denser the air, the faster the speed of the return capsule.
The re-entry capsule can travel seven or eight kilometers in one second, and at such an ultra-high speed, it only takes 0.13 seconds to fly 1 kilometer.
Therefore, the predicted difference distance of 1580 meters is indeed the level of hitting the bull's-eye.
Second, the inevitable black barrier
Due to the rapid speed of the capsule in the process of returning to Earth, it rubbed against the air, and the re-entry capsule was surrounded by flames, and the bottom temperature reached several thousand degrees Celsius.
The "black barrier" in the process of returning has become an inevitable problem.
The re-entry capsule goes through four phases during re-entry, which are automatic deceleration, extra-atmospheric free taxi, loading into the atmosphere, and recovery and landing.
One of the harshest conditions and the most technically demanding stage is the step of loading into the atmosphere.
As the altitude of the capsule from the ground gets lower and lower, the air becomes denser and denser, and the surface of the capsule rubs against the atmosphere, which brings a huge amount of heat.
At this time, the re-entry capsule looks like a large fireball, and the radio signal of the re-entry capsule is also cut off by the flames, and this stage is called the "black barrier area" of the radio.
The black obstacle of the re-entry capsule usually appears from an altitude of 100 kilometers above the ground and disappears at an altitude of about 40 kilometers above the ground.
In order to solve this problem, Continental changed the signal transmission mode of the re-entry capsule from traditional signal cables to optical fiber transmission, which greatly reduced the number of cables transmitted between mission vehicles, which made the signal transmission rate faster.
At present, through the continuous efforts of mainland scientists, stable tracking can be achieved when the return capsule passes through the black barrier area, which is a major breakthrough in the tracking and measurement problem of the black barrier area.
Third, the parachute opening process
When the re-entry capsule loads into the atmosphere and gradually enters the air density above the higher air density, it needs to be further slowed down, otherwise the inertia caused by excessive speed will bring very serious damage to the astronaut's spine, ranging from severe disability to serious loss of life.
Therefore, how to mitigate the impact has become the key to protecting the lives of astronauts.
Since there is no way to apply the brakes, it can increase the resistance to the return capsule and achieve the purpose of deceleration.
By the time the capsule was still 10 kilometers above the ground, its speed had dropped from 7.9 kilometers per second when it first entered the atmosphere to about 200 meters per second.
The speed dropped to 2.5%, but this speed was still quite dangerous.
At this time, the re-entry capsule automatically opened the umbrella hatch, and the guidance parachute and the deceleration parachute were pulled out one after another, and the speed also dropped to about 180 meters per second.
After 10 seconds of operation, the parachute will detach from the re-entry capsule and the main parachute will be pulled out.
We can understand it as a higher-level parachute, with a main parachute canopy area of 1200 square meters, which is equivalent to the area of three standard basketball courts.
At this point, the speed of the re-entry capsule will quickly drop to 40 meters per second and become 7 meters per second.
These parachutes are uncontrolled and drift with the direction and force of the wind.
Before the spacecraft returns to the ground, scientists will calculate the drift distance and direction during the parachute landing according to the wind speed and direction predicted by the meteorological department, so as to adjust the opening point and make the spacecraft landing point more accurate.
It's easy to say, but it's hard to do.
After all, in the case of such a speed, as long as there is a deviation, it will be a large deviation, such as the initial position of the return capsule, attitude deviation, aerodynamic coefficient, return capsule mass and other factors, will cause deviation interference to the landing point of the return capsule.
Fourth, the boost when landing.
When the re-entry capsule descends to only 6 kilometers above the ground, the heat shield originally installed at the bottom of the re-entry capsule will be thrown away according to the pre-procedure, and the thrust reverser engine will reveal the true face of Lushan at this time.
When there is still 1 meter above the ground, the thrust reverser engine will ignite and start, giving the return capsule an upward force, so that the astronauts can land safely.
At this time, an air journey is truly over.
Resources:
[1] Global Network ""Ring Time Depth" Shen 15 Crew Triumph, Which "Artifact" Escort?", 2023-06-05
[2] Popular Science Guangzhou, "Shenzhou 15 Returns!The spacecraft will burn up when it returns, why won't it be launched when it is launched?", 2023-06-14
[3] China News Network, "How Important Is the Last Meter?" "Count" to see the landing of the "Shenzhou 15" return capsule", 2023-06-04