The "four-step" process of adjusting attitude, descending and parachuting to land analyzes the return process of Shenzhou XII!

Produced by: Science Popularization China

Produced by: Trantor Space

Producer: Computer Network Information Center, Chinese Academy of Sciences

On September 17, 2021, the return capsule of the Shenzhou XII manned spacecraft successfully landed at the Dongfeng landing site. After completing a three-month in-orbit mission, the three-person space mission team returned home safely.

The process of returning the capsule to Earth is full of variables and dangers, especially before passing through the atmosphere. At this time, the capsule needs to complete multiple command operations and cut into the atmosphere at the right angle to return safely.

The heat-insulating roof of Shenzhou XII is the most heat-resistant part (Photo source: CCTV.com)

When passing through the atmosphere, the temperature of the cabin's thermal insulation top can reach 2200K to 2600K, which can interfere with communications during this period. The "space trip trio" really went through many hardships and dangers before returning home.

After previous practical verification, the technology of the Shenzhou series spacecraft has become very mature. The successful landing of Shenzhou XII is another verification of the return technology of the Shenzhou spacecraft, and also demonstrates the reliability and completeness of the support status of the Dongfeng landing site.

"Four steps" - the return process of Shenzhou XII!

Shenzhou XII assembly display (Photo source: National Space Administration)

Before officially "returning home", the return process of Shenzhou XII had undergone more than ten actual combat drills, and the relevant technology had become quite mature.

The complete return process can be summarized as "four steps".

The first stage is for Shenzhou XII to leave the space station and adjust its attitude.

First, Shenzhou XII turned 90 degrees counterclockwise, the orbital module originally in the front was adjusted to the left, and the propulsion module was moved to the right, turning into a horizontal flight. The purpose of this step was to separate from the orbital module, and Shenzhou XII changed from a three-module combination to a two-module combination.

Then, it rotates 90 degrees counterclockwise, with the propulsion module in front and the return module in the back, which is the opposite of the orientation when it left the space station. This step is the second attitude adjustment of Shenzhou XII, so that the propulsion module faces forward, and after adjusting the pitch angle, the engine is started to reduce the speed by engine braking.

Subsequently, Shenzhou XII entered the second stage of return, and the dual-cabin combination of the return capsule and the propulsion capsule performed an unpowered free descent.

The dual cabins begin to descend from an altitude of 393 kilometers, and at an altitude of 140 kilometers, the propulsion cabin will separate from the return cabin. At this time, the return cabin must choose the correct re-entry attitude angle so that it can just cut into the Earth's atmosphere.

The re-entry attitude angle is very important. If the angle is too large, the speed of the spacecraft will not be easy to control. If the angle is too small, it will be bounced back by the dense atmosphere, just like skipping stones on water.

The third stage is re-entry into the atmosphere.

At this time, the return capsule is at an altitude of about 100 kilometers and enters the Earth's atmosphere at a speed of about 7 kilometers per second. At this time, the return capsule is rubbing violently with the atmosphere, and it looks like a meteor from the ground.

After experiencing high temperatures, the ablative paint burns off and leaves black marks (Image source: CCTV screenshot)

The temperature of the insulation top at the front end of the return capsule is 2200K to 2600K, which is comparable to the temperature of the combustion chamber in an aircraft engine.

The return capsule resists high temperatures by ablating materials. It needs to control its attitude to keep the heat in line with the heat protection design. At this time, the return capsule will also be wrapped by the plasma layer, forming a black barrier. The black barrier will disappear only when the altitude drops to 40 kilometers. Before that, the communication between the ground and the return capsule is interrupted.

The fourth stage is parachute deceleration and landing.

At an altitude of about 10 kilometers from the ground, the return capsule opened the guide parachute, and after the guide parachute was fully inflated, it continued to pull out the deceleration parachute.

The support helicopter had already discovered the location of the return capsule while it was still landing (Image source: CCTV screenshot)

At an altitude of 8 kilometers, the deceleration parachute separates from the return capsule, and then the main parachute is released. Then it slowly descends. At an altitude of 5 kilometers from the ground, the return capsule discards the insulation bottom, so that the reverse thrust engines can be exposed. Before touching the ground, the four reverse thrust engines are started at an altitude of 1 meter above the ground for the final deceleration.

Finally, the spacecraft touches the ground at a speed of 2 meters per second. After landing, if the wind is too strong to cause the main parachute to drag the return capsule, the astronauts can manually cut off the main parachute.

The return capsule of Shenzhou XII landed on the ground (Image source: CCTV screenshot)

Dynamic return plus measurement and control guarantee are the highlights of the two major technical applications

In order to fully ensure the safety of the astronauts, the entire return process has been tested more than ten times. In addition, the return of Shenzhou XII also applied two highlights of black technology.

The first highlight technology is to replace the previous timed and fixed-point return method with dynamically adaptive return technology.

The advantage of this is that the landing point can be predicted based on the real-time trajectory of the spacecraft, so that the return trajectory can be precisely controlled and the landing point position can be accurately grasped. This technology is in a leading position among its peers.

According to the video footage returned by the return capsule, we can see that the ground support team's helicopter had already arrived nearby when the return capsule had not yet touched the ground, which shows that our judgment of the return capsule's trajectory and landing point was very accurate.

At the same time, the dynamically adaptive return technology also shortens the return time, so that the return capsule no longer needs to return "at a certain time point", but can return as long as the conditions are met. This will shorten the waiting time for astronauts to return to the ground, which will be of great help to the subsequent normalized operation of the space station.

The second technical highlight is the space-based measurement and control support, which utilizes three Tianlian relay satellites for space-based measurement and control.

From the departure of Shenzhou XII from orbit to the separation of the propulsion module, from the altitude of 393 kilometers to 140 kilometers, all were supported by the Tianlian satellite for measurement and control. The advantage of this is that the return process reduces the dependence on ground observation stations.

As a new measurement and control method, space-based measurement and control is also the industry leader, which means that the entire return process is monitored by the Tianlian satellite. The "monitoring" that is so far away also demonstrates the technical background of aerospace technology.

Regular operation of the space station

Shenzhou XII has returned successfully, and soon Tianzhou III and Shenzhou XIII will take over and "soar into the sky", and in the future, there will be more space missions.

There is a principle here. The Shenzhou spacecraft adopts a rolling backup launch mode.

When Shenzhou XIII was ready to launch, Shenzhou XIV had already completed all pre-launch preparations;

Only after Shenzhou XIII has confirmed its safe docking with the space station will Shenzhou XIV switch to backup mode, truly being “prepared for any eventuality”.

The rolling backup mode is designed to fully ensure the safety of astronauts, and it will also be the rotation mode for the normal operation of the Tiangong space station. Next, the astronauts of Shenzhou 13 will stay in orbit for 6 months, and the six-month space journey will also be the standard configuration for future space station operations.

From Shenzhou V to Shenzhou XIII; the flight time increased from 21 hours and 28 minutes to 6 months... Astronauts can stay in space for longer and longer periods of time, and it will become easier and easier to travel to space.

We believe that China's space program will continue to make breakthroughs and its steps in exploring the universe will go further and more steadily.