How difficult is it for Chang’e to return from”digging”? See the story of China’s first retractable satellite

By yqqlm yqqlm

Source:Science and Technology Daily

Lu Binghong Science and Technology Daily reporter Fu Yifei

November 30, 2020, Chang’e-5 probe successfully separated the lander ascender assembly from the orbiter returner assembly in the orbit around the moon. Next, Chang’e 5 will land on the moon and take samples. If all goes well, it will return to Earth with lunar samples in more than half a month.

This is one of the most complicated aerospace missions in our country so far. The subsequent difficulties are not only the automatic sampling and packaging of the lunar surface, the take-off of the lunar surface, and the rendezvous and docking of the lunar orbit, but also the return of the lunar soil to the earth is also a major problem.

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A schematic diagram of the separation between the lander and ascender assembly and the orbiter and returner assembly.

The return of human spacecraft from space was initially achieved through returnable satellites. The successful recovery of my country’s first retractable satellite 45 years ago became an important milestone in China’s space history.

Let us review the story of the year.

In August 1960, U.S. President Eisenhower proudly announced at a regular press conference that the United States had successfully recovered an American flag that was brought back into space by the Discovery 13 satellite.

However, the”Discoverer” is not a scientific research project on the space environment as described by Eisenhower, but a top secret spy satellite program. These satellites were not launched to send the American flag to space, but to take and send back photos of the Soviet Union.

Since then, returnable satellites have become the commanding heights of the US-Soviet competition in the high-tech field, and have become the main means of contending for space supremacy, conducting strategic reconnaissance, and acquiring various intelligence.

In order to break the superpower’s monopoly on this technology, when my country was developing the Dongfanghong-1 satellite, it began to tackle key problems such as the returnable satellite and its required attitude control.

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The Long March 2 rocket launched my country’s first returnable satellite.

After five years of research and development, at 11:30 on November 26, 1975, the Long March 2 carrier rocket was ignited at the Jiuquan Satellite Launch Center, sending my country’s first returnable satellite into its intended orbit.

According to the plan, after the satellite has been in space for 3 days, it will be controlled by the Xi’an Satellite Measurement and Control Center to return to the ground and implement recovery.

From the 12″predecessors” before the”Discoverer 13″ in the United States, it can be seen that it is not easy for the satellite to complete a trip that has gone and returned.

Not only is it required that the satellite must maintain a high-precision operating attitude during operation and return, but all the instruments on the satellite must work accurately and in accordance with the procedures, and it is also required that the ground maintains high-precision measurement and control and is safe and reliable Of recycling. A slight mistake can cause serious consequences.

For Chinese astronauts who have never had satellite recovery experience before and have to start from scratch, the challenges are extremely severe.

When the satellite was launched into space, preparations for recovery had already begun.

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A mobile monitoring and control station performing missions in the depths of the Badain Jaran Desert.

In the depths of the Badain Jaran Desert in Inner Mongolia, the active monitoring and control unit of the Xi’an Center responsible for issuing instructions to separate the satellite instrument module and recovery module has been waiting for four months. They went deep into the desert, only to spare a few minutes for satellite recovery.

In Suining, Sichuan, the planned satellite recovery area, the Xi’an Central Survey and Recovery Department has been carefully preparing for several months, and has surveyed all the plants and trees in this area clearly.

In order to ensure the success of satellite recovery, even the Ministry of Posts and Telecommunications has participated. The state mobilized 103 lines as dedicated communication lines, and 27 provinces, municipalities and autonomous regions participated in the communication guarantee work.

But the accident still happened.

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In the activity measurement and control cabin, scientific and technical personnel are receiving telemetry data.

The satellite was in orbit for only a few laps, and telemetry data showed that the gas source curve on the satellite dropped abruptly.

The gas supply bottle on the satellite is specially carried to guarantee the satellite operation for 3 days, and the attitude control of the ground to the satellite also depends on the pressure of this gas supply. If calculated according to the pressure consumption curve, the satellite will run out of pressure in less than 3 days. More seriously, the satellite’s attitude cannot be adjusted, which means it cannot return to the ground.

For a time, there was a panic in the measurement and control hall of the Xi’an Satellite Measurement and Control Center, located deep in the Qinling Mountains.

“Do you want to recycle in advance?”

“If the satellite should be reclaimed in advance but not reclaimed in time, the problem may be serious if the satellite falls abroad!”

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Qi Siyu is monitoring satellite telemetry data.

Everyone was talking, and Qi Siyu, the leader of the orbit calculation team, was silent and nervously calculating.

Qian Xuesen arrived and asked Qi Siyu as soon as he came:”Tell me, how many days can the satellite last?”

“You can persist on the satellite and don’t need to recycle it in advance.” Qi Siyu said .

The hall is quiet.

It’s easy to slap your chest, but this is not the time to be a hero. Science emphasizes rigor and reliability, which must be supported by sufficient reasons.

Qian Xuesen stared at Qi Siyu and asked:”Do you dare to guarantee when you can come back? Why?” A few laps of descent has a decreasing trend. We think this is a phenomenon that occurs when the satellite adjusts its attitude. This situation can be controlled by commands to keep the air pressure maintained.”

Sure enough, first When the sky passed, everything was normal for the satellite; the next day passed, the satellite was still safe.

But when the satellite was about to return, a new danger appeared-

Just as the Xi’an Center was preparing to issue a satellite opening command, the two control computers actually began to”fight”, one The judgment is to issue a command, and the other judgment must cut off the command.

At this time, the satellite speed is close to 7 kilometers per second. If the command time differs by 5 seconds, the landing point will deviate by 40 kilometers; if the speed direction angle of the return point deviates by 1 degree, the landing point voyage will deviate 300 kilometers.

It is only 15 seconds before the satellite flies out of the sky over China. If the control command is too late to issue, the satellite return module may fall into other countries, with disastrous consequences.

This is a lesson learned. On April 13, 1959, when the United States recovered the”Discoverer 2″ satellite, due to the premature ejection order, the film in the return cabin fell into Norwegian territory, causing an international uproar.

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Qi Siyu is studying the data.

At the critical moment, Qi Siyu grabbed a stack of calculation results and rushed.

He didn’t have time to walk around the door, so he broke the window and broke into the command hall, shouting:”Fat!”

At noon on November 29, 1975, a coal mine in Yingpan Commune, Liuzhi, Guizhou Above, four miners were sitting at the wellhead chatting. Suddenly, a big round fellow fell from the sky, swept away the crown of a big pine tree, and then crashed to the ground.

The news of”the alien from outside” was reported quickly, and the survey and recovery department rushed to the satellite landing site.

Everyone found at the scene that due to the high temperature damage to the returnable satellite skirt, the 650 kg return capsule was not opened during the reentry section, and many parts of the satellite were damaged. Threw it out. But what is surprising is that due to the buffer of the big tree, the return cabin was not broken, and all the films were intact!

In Beijing, Marshal Ye Jianying looked at the photos taken by the satellite and happily said to everyone:“It doesn’t matter if it is more than 400 kilometers away. The first time we recovered a satellite, it was a victory to land on the land of China. !”

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Return to the cabin to land.

As a result, China became the third country after the United States and the Soviet Union to successfully recover satellites and created a miracle of successful satellite recovery.

In the following decades, the Xi’an Satellite Measurement and Control Center successfully implemented 23 returnable satellites, 11 Shenzhou spacecraft, and the Chang’e-5 reentry and return flight tester. The recovery success rate was maintained. 100%. The error of the landing point was reduced from the initial 400 kilometers to 10 kilometers, 2 kilometers, 1 kilometers… When the Shenzhou VII mission, the predicted result of the return capsule landing point was only 374 meters away from the actual measurement.

However, the recovery of Chang’e-5 is different from the previous mission. This is the first time that a Chinese spacecraft has returned to Earth with the lunar soil at high speed.

When a low-Earth orbit spacecraft reenters and returns to the atmosphere, its speed is usually the first cosmic speed of about 7.9 kilometers per second. The return speed of Chang’e-5 is close to the second cosmic speed of 11.2 kilometers per second.

For this reason, researchers have proposed a semi-ballistic jumping reentry and return technical solution, which is like a water drift on the surface of the atmosphere. The returner first enters the atmosphere at high speed, and then”jumps” with the help of the lift provided by the atmosphere.”Get up, re-enter the atmosphere and return to the ground at the first cosmic speed.

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In 2008, the Xi’an Satellite Measurement and Control Center successfully implemented the recovery mission of the Shenzhou-7 return capsule.

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In 2013, the Shenzhou 10 recovery site.

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In 2016, the Xi’an Satellite Measurement and Control Center successfully recovered the Practice No. 10 satellite.

In 2014, my country launched the Chang’e-5 re-entry and return flight tester, which simulated the whole process of Chang’e-5’s flight to the moon, circling the moon, and return, and successfully verified the jumping reentry and return technology. , Making our country the third country in the world to successfully return spacecraft from lunar orbit to the ground after the United States and the Soviet Union.

The Chang’e-5 re-entry and return program inherits the design of the previous flight tester, and the mission re-entry voyage is basically the same as the flight tester, but the weight of the sample container containing the lunar soil is uncertain. It may affect the quality characteristics of the returner, which puts forward higher requirements on the robustness of the returnr-guided navigation control system (the control system maintains certain performance characteristics under certain parameter perturbation).

The task is difficult, but we are confident.

Let us work with the astronauts and look forward to its return.

Source:Science and Technology Daily The picture in the article is provided by the author