By WANG Xiaoxia & FU Yifei

Imagine trying to thread a needle at a distance of over 16,000 km. That's what scientists compare the technique of space rendezvous and docking to, due to its large spatial distance and high degree of accuracy.

After three decades of arduous effort in manned spaceflight and deep space exploration, China has realized the vision of independently completing space rendezvous and docking and become one of the leading countries in terms of this technology, ensuring the implementation of major future missions such as manned spaceflight and lunar exploration.

Starting from scratch

In the 1980s, the National High-tech R&D Program, namely the 863 Program,  was approved, laying the foundation for China's manned space program. Experts at the 502 Institute of the China Academy of Space Technology (CAST), affiliated to China Aerospace Science and Technology Corporation (CASC), recognized the strategic significance of developing space rendezvous and docking technology.

Starting from scratch, Wu Hongxin, renowned expert in control theory and control engineering and academician at the Chinese Academy of Sciences (CAS), led the researchers to develop the full-coefficient adaptive control theory in their humble laboratory.

In 1999, Wu's student, CASC scientist Xie Yongchun took over the baton, and prioritized the application of engineering theory as the top mission of her team.

She led the team to begin the derivation from basic dynamic equations, calculate from circular orbits to elliptical orbits, and integrate the previous achievements in orbital dynamics and relative motion dynamics, successfully solving the fundamental problems of long-distance guidance.

Xie applied the "golden section coefficient" theory from her doctoral dissertation to the design of Shenzhou-8 spacecraft control parameters. In November 2011, Shenzhou-8 successfully completed its first automatic docking with Tiangong-1 in less than 44 hours after its launch, marking a major advancement for China's space program.

Fast-tracking development

In the following two years, China successively launched Shenzhou-9 and Shenzhou-10 spacecraft, which conducted rendezvous and docking with Tiangong-1, using both automatic and manual operations, comprehensively verifying the homegrown technology.

However, during the same period, Russia achieved rendezvous and docking of cargo spacecraft and manned spacecraft with the International Space Station within six hours.

"We cannot rest on our laurels, and must keep innovating and breaking through," said Xie. Scientists optimized the complex calculation process, and moved the process from ground telemetry and remote control to the spacecraft for autonomous execution, so as to realize autonomous rendezvous rapidly and significantly improve the efficiency of long-distance guidance.

To meet the high-frequency rendezvous and docking requirements during the construction and operation phases of the space station, the team launched a research and development program on the "initial all-phase" technology.

After more than 3,000 tests and analysis of more than 200 extreme conditions, the spacecraft is enabled to precisely complete the docking within the stipulated time, regardless of the phase difference with the space station when entering orbit, said Xie.

In September 2017, the Tianzhou-1 cargo spacecraft conducted the in-orbit autonomous rapid rendezvous and docking with Tiangong-2 space lab in 6.5 hours after the launch, marking another leap forward for China in this technology.

China's benchmark

In 2020, when China's 6.5-hour plan had demonstrated a high level of maturity and stability, Xie put forward a bold challenge: "Under the current engineering conditions, the theoretical limit is two hours. Can we achieve it?"

The team decided to remove the "seek" step, which is used in the short-distance stage to eliminate long-distance guidance deviation. This alone can save about 40 minutes. They overcame nearly a hundred technical challenges, optimized the core design 127 times, and were finally able to design a 2 hour rendezvous and docking plan.

In November 2022, the Tianzhou-5 spacecraft completed the rendezvous and docking with the Chinese space station in just 1 hour and 57 minutes after launch, setting a new world record.

Although this plan is faster, it is not economic, said the CASC's Chen Changqing, explaining that this plan has high requirements for rocket performance, and greatly increases the fuel consumption.

Therefore, the scientists no longer blindly pursued speed but have chosen to redesign a 3 hour plan based on the previous plans. According to Chen, this plan takes into account both efficiency and reliability, and has been implemented in the Tianzhou-7, Tianzhou-8 and Tianzhou-9 missions and will continue to be used in the future.

So far, all 37 rendezvous and docking missions carried out by China in Earth orbit and Lunar orbit have been completed. The country has independently developed this technology and set its benchmark for the rest of the world.