Zhang Mengying, a lecturer at the School of Aerospace Science, National University of Defense Technology, will tell you about-
Parachute System: The "Flower of Peace" for Spacecraft Landing
On the morning of April 16, a red and white parachute slowly unfolded like a flower over the Dongfeng Landing Field.
With the landing of the return capsule, the three astronauts returned safely, and the Shenzhou 13 manned space mission was a complete success.
In the vast Dongfeng landing field, the parachute is the most obvious guide.
As long as its figure is captured, the ground search and rescue personnel are like a reassurance pill.
The parachute is a device that uses air resistance to achieve aerodynamic deceleration, and plays a pivotal role in the return of manned spaceflight and the detection of terrestrial objects.
Today, let's take a closer look at the parachute system.
manned spaceship parachute—
Guardian astronauts return home for 'last ten kilometers'
Let's look back to April 16 and review the return process of the Shenzhou 13 spacecraft.
The Shenzhou 13 spacecraft returned to Earth for the first time in fast return mode after 5 laps in orbit.
Compared with the return of the Shenzhou 12 spacecraft, this mode shortens the return time from more than a day to a few hours, greatly improving the comfort of the astronauts and improving the efficiency of mission completion.
After experiencing the thermal barrier and black barrier, the last step for the recovery and landing of the manned spacecraft was handed over to the Shenzhou spacecraft parachute system.
At an altitude of 10 kilometers from the ground, the speed of the spacecraft dropped from the first cosmic speed to about 200 m/s, and the multi-stage parachute opening procedure was started.
First, the static pressure level controller on the return cabin measures the atmospheric pressure to determine the height of the parachute, the parachute cover automatically pops up, and the guide parachute and the deceleration parachute are pulled out in turn.
The parachute slowed the re-entry capsule to about 180 m/s.
Subsequently, the speed and altitude of the spacecraft's return cabin were further reduced, the main parachute was ejected from the parachute cabin, and the speed dropped to about 40 m/s after two stages of inflation and deceleration.
At this time, the main umbrella of 1200 square meters was fully unfolded, and the heat-proof outsole was thrown away.
During the descent, the return capsule decelerates to about 7-8 m/s to ensure that the astronauts descend smoothly at low speed.
After layers of parachuting to decelerate, in order to avoid the "hard collision" between the return cabin and the ground in the landing buffer section, when the height is about 1 meter above the ground, the return cabin precisely controls 4 landing thrust reversers, and by means of reverse thrust "braking", the The speed is reduced to about 2 m/s to achieve a precise soft landing to reduce the impact on the astronauts.
The landing is not everything, the main umbrella may be blown up by the wind, and the towed return cabin rolls on the ground, and can roll seven or eight kilometers under the wind.
For this reason, at the moment of landing, the cutter on the main parachute will cut the parachute rope, let the main parachute fall off, and ensure that the return capsule will not be dragged away.
In fact, in order to ensure the safety of astronauts, since Shenzhou 9, the spacecraft recovery and landing system has added the astronaut manual parachute release mode on the basis of the programmed parachute release mode, which effectively avoids the threat to the astronauts from the landing site environment.
Extraterrestrial parachutes --
The messenger who escorted Zhurong's falling fire
Some people ask whether the spacecraft parachute system used in the return to Earth mission is suitable for various space missions?
Let us take the Tianwen-1 Mars exploration mission as an example to explore this question.
Mars, the red planet where Tianwen-1 landed, is known as the "probe cemetery".
Of the more than 40 Mars exploration missions in the world, only 9 have successfully passed through the Martian atmosphere and landed smoothly.
The reason why China has become the second country in the world to successfully achieve a soft landing on Mars and carry out Mars exploration is due to the Mars parachute.
On May 15, 2021, after a 296-day space journey, China's first Mars probe, Tianwen-1, carried the Zhurong rover and landed steadily at the preferred landing spot - the Utopia Plain in the northern hemisphere of Mars.
After about 5 minutes of aerodynamic deceleration, the detector speed dropped to about Mach 2.
At this time, the Mars parachute "comes on stage", and its task is to decelerate Tianwen-1 from the supersonic speed of Mach 2 to 95 m/s.
The Martian atmosphere is thin, at an altitude of 10 kilometers, the density of the atmosphere is only 1/100 of the density of the Earth's atmosphere.
Therefore, in the landing stage, the Mars parachute escorting Tianwen-1 is quite different from the parachute used for the Earth return mission in terms of parachute opening conditions, working state and structural design.
Due to the low density of the Martian atmosphere, after the aerodynamic deceleration of the probe, the speed is still faster than the speed of sound, but the dynamic pressure is only about 500 Pa.
When the parachute is opened under supersonic conditions, a bow-shaped shock wave field will appear in front of the parachute canopy.
Although the shock wave is not enough to complicate the parachute flow environment, there is a detector in front of the parachute.
The detector not only generates a shock wave itself, but also forms a thin and long wake behind it.
It is conceivable that when the wake of the detector and the shock wave in front of the parachute "narrowly meet", a highly complex unsteady flow field will form around the parachute, causing the parachute to vibrate violently and "breath" like a jellyfish, resulting in a series of serious " Surge" phenomenon.
The low density of the atmosphere presents the challenge: a larger canopy area is required to achieve the same aerodynamic drag, with a corresponding increase in system weight and volume.
At the same time, it will reduce the stability of the parachute and increase the swing angle.
And low dynamic pressure means low aerodynamic resistance, which further increases the difficulty of the umbrella to restore stability from the "breathing" and swing state.
In such a state, opening the umbrella can be described as "stunning step by step".
Therefore, in the structural design of the Mars parachute, in order to adapt to the problem of significant stability interference under the condition of low density and low dynamic pressure, it is necessary to improve the strength of the parachute, the ability of the local structure to resist vibration and other complex mechanical environments, and pay attention to improving the anti-damage expansion of the parachute. Ability.
The parachute used by Tianwen-1 is a zigzag disc-slotted umbrella: an annular band is added around the disc of the flat circular umbrella, with a slit in the middle.
At the same time, adding triangular banners to the bottom part can meet the system stability under the condition of low density and low dynamic pressure, and enhance the bearing capacity of the local structure.
In terms of local reinforcement materials, in order to cope with the Martian environment, the Tianwen-1 parachute uses a new aramid material.
In addition, in order to improve the connection strength, the umbrella rope also has a built-in space, and a new plug-in process is used to directly plug the material bodies together to avoid the loss of strength caused by sewing.
The future of parachutes --
Large group umbrella technology can be expected in the future
In order to decelerate better, the development of a larger single umbrella is not the first choice.
On May 8, 2020, three giant red and white umbrella flowers hung with a conical blunt cabin and landed slowly over the Dongfeng Landing Field.
This is the return capsule of a new generation of manned spacecraft test ship.
The new-generation manned spacecraft weighs twice as much as the Shenzhou spacecraft and can carry up to seven astronauts back to Earth.
This requires a parachute deceleration system adapted to the new mission.
The recovery of a new generation of manned spacecraft test ships adopts the large-scale parachute technology.
The group parachute consists of three single parachutes with the same size as the parachute area of the Shenzhou spacecraft, which can ensure that the "big guy" of the new generation of manned spacecraft can effectively and safely decelerate in the "last ten kilometers" of going home.
In addition to the group umbrella technology, there are more research points that need to be paid attention to for different mission requirements.
For example, the simulation study of parachutes on the earth.
Different planets have different atmospheric environments, and traditional low-altitude airdrop experiments cannot effectively verify whether the planetary parachute system is qualified.
In order to know whether the parachute can work reliably as expected in different planetary environments, it is necessary to reasonably select key parameters, simulate the actual conditions of the parachute in the planetary atmosphere, and pass multiple ground verifications to accurately answer this question.
Another example is the research on the material and strength of parachutes.
Researchers must track the development of high-performance flexible fabric materials, and the improvement of material properties will lead to changes in design solutions.
In the future, parachutes will still be the preferred deceleration device for spacecraft landing.
According to the plan, China's manned space project will complete six launch missions in 2022, including the Tianzhou-4 and Tianzhou-5 cargo spaceships, the Shenzhou-14 and Shenzhou-15 manned spaceships, and the Wentian experimental module Hemeng. The launch of the experimental module.
By then, China's space station will be fully completed, and China's manned spaceflight project can be described as "a good show."
Let us look forward to the red and white spaceship parachutes blooming high above the Dongfeng landing site again to escort the astronauts home safely.
Zhang Zhaoxing and Gu Ying