How Scientists Designed the Soft Lunar Landing

image: The success of the Chang’e-5 mission, due in large part to its innovative GNC system, paves the way for future space exploration missions such as an asteroid landing, as well as the most dreamy dream daring of humanity: crewed space travel.
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Credit: Space: Science and Technology

The Chinese space program’s fifth lunar exploration mission, Chang’e-5, is the first of China’s lunar sample return missions, with a simple objective: to land on the moon’s surface, collect samples, and bring them back. at home. The first step would set the tone for the rest of the mission, making it crucial to achieve a “soft landing” (when the spacecraft lands intact) on rocky lunar terrain. The guidance, navigation and control (GNC) system is crucial for this task. A recent study published in Space: Science and technology by a research team led by Dr Honghua Zhang of the Beijing Institute of Control Engineering, breaks down the design of the Chang’e-5 CNG system. “We hope that the GNC methods presented in this article will find application in future lunar exploration, crewed space exploration and other missions,” Dr Zhang said.

The Chang’e-5 spacecraft consisted of an Orbiter and Return Module (ORM), which remained in lunar orbit in space, and a Lander and Ascender Module ( LAM), which landed on the lunar surface. The ascending part of the LAM then returned to the ORM with lunar samples. The ascender then separated from the ORM and performed a controlled desorbit to land on the lunar surface. While previous Chinese lunar missions, Chang’e-3 and Chang’e-4, had successful soft landings, Chang’e-5 was the first to plan a return to ORM, offering a new challenge for the overhaul of its GNC system. Chang’e-5’s LAM contained new components, including a “light surface tension tank” for the increased amount of propellant needed to revive the LAM from the lunar surface.

The LAM should be released from the ORM in a ‘horizontal’ orientation but land in a ‘vertical’ orientation. Designing the correct method of guidance that allows for this righting process, called attitude reorientation, while avoiding sloshing of the liquid propellant, which would create an imbalance of internal forces in the LAM, derailing the descent, was crucial.

The scientists used a reconfigurable three-part attitude control that included a “quaternion” partition control, a phase and gain stabilization filter, and a dual observer. The former helped control the angle and angular velocity of the LAM, allowing for rapid and stable attitude reorientation. Sloshing was addressed by the filter, which limited the pitch, yaw and roll of the LAM, improving the robustness of the control system and helping to avoid instability. Finally, the double observer obtained better control performance by estimating and compensating for disturbances such as disturbances by sloshing, misalignment of the center of mass and disturbances induced by the thruster.

The LAM used various sensors to help it navigate its descent and landing. The inclusion of multiple sensors for the same purpose has improved the redundancy of the system, which is crucial for space missions where there is no possibility of repairing a faulty sensor. But how did the Chang’e-5 detect the faulty sensors and choose others? This was achieved through a process called intelligent heterogeneous sensor data fusion, where the outputs of many sensors with similar use were compared. If the output of a sensor was found to be significantly different, the sensor was deemed to be faulty and one of the other sensors would be chosen for navigation. According to Dr. Zhang, “The data fusion methods described in our work for on-board sensors, such as inertial measurement units (like gyroscopes and accelerometers) and velocimeters, have the potential to improve future air vehicles, both on land. and cosmic. “

The Chang’e-5 spacecraft returned to Earth on December 16, taking with it the youngest lunar samples (only 1.2 billion years old!) Ever obtained by mankind. The success of the Chang’e-5 mission, due in large part to its innovative GNC system, paves the way for future space exploration missions such as the landing of an asteroid, as well as the most daring dream of humanity: traveling in space with a crew.

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Reference

Authors: Honghua Zhang1.2, Ji Li1.2, Zeguo Wang1, and Yifeng Guan1.2

Title of the original article: Chang’E-5 powered descent navigation and guidance control

Newspaper: Space: Science and technology

DO I: 10.34133 / 2021/9823609

Affiliations: 1Beijing Institute of Control Engineering, China

2Science and Technology on Space Intelligent Control Laboratory, China

About Dr Honghua Zhang

Honghua Zhang is currently a researcher at the Beijing Institute of Control Engineering. He received his bachelor’s degree from Xidian University in 1983, his master’s degree from the Beijing Institute of Control Engineering in 1985, and his doctorate. graduated from Beijing University of Aeronautics and Astronautics in 1991, in China, respectively. His research interests are in the control of flexible spacecraft, guidance, navigation and control of landers.


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