The research team of Harbin Institute of Technology reveals: How the five-star red flag of Tianwen-1 was unfolded on Mars

"Chinese Imprint" Photo by Xinhua News Agency

On June 11, the National Space Administration held a ceremony to unveil the first batch of scientific images of the Tianwen-1 probe landing on Mars, and released images taken by the Zhurong Mars rover, including a panoramic view of the landing site, Martian topography, "Chinese footprints" and "landing and patrolling group photos". The release of this batch of scientific images marks the complete success of my country's first Mars exploration mission.

In the two images of "Chinese Imprints" and "Group Photos of the Patrol", the bright red and square Chinese national flag is clearly visible. The locking and unfolding mechanism of the Chinese national flag independently designed and developed by Professor Leng Jinsong's team from the School of Astronautics of Harbin Institute of Technology has successfully completed the controllable dynamic unfolding of the Chinese national flag on the Tianwen-1 lander on May 15, 2021 after 202 days of Earth-Mars transfer orbit flight and 93 days of orbital exploration, flying 475 million kilometers, making my country the first country in the world to apply shape memory polymer intelligent structures to deep space exploration projects.

In the early 1990s, under the guidance and support of Academician Du Shanyi, the Institute of Composite Materials and Structures of the School of Aerospace Engineering of Harbin Institute of Technology established the research direction of intelligent materials and structures earlier in China. Under the guidance of the development concept of "combining science and engineering to serve aerospace", Professor Leng Jinsong's team has been working hard and innovating in the research fields of intelligent structural mechanics, design and aerospace applications. The team was the first in China to develop a series of shape memory polymer materials (Shape Memory Polymer, SMP) with independent intellectual property rights, such as epoxy, cyanate, and polyimide. The glass transition temperature is adjustable (37℃-322℃), which meets the requirements for use in extremely harsh environments in space.

In accordance with the overall mission requirements of the Tianwen-1 Mars exploration mission, and with the support of the China Academy of Space Technology, Professor Leng Jinsong's team successfully developed a locking and unfolding mechanism for the Chinese national flag based on shape memory polymer composite materials with independent intellectual property rights, through sophisticated mechanical theoretical analysis and ingenious intelligent structural design.

It took more than nine months for Tianwen-1 to be launched into orbit and land on Mars. The exploration was risky and difficult. The national flag locking and unfolding mechanism faced challenges such as the interplanetary space environment, the thin atmosphere of Mars, and the Martian wind. At the same time, there were difficulties such as the uncertainty of the landing environment and the inability to intervene on the ground, which put forward strict requirements on the reliability of the product. "Through repeated iterations of extreme environmental tests and verifications such as mechanics, thermal vacuum, thermal cycles, and locking and unfolding functions in the Martian environment, we innovatively designed and prepared a national flag locking and unfolding mechanism with a winding locking-unfolding mode for the landing platform. The national flag unfolding process is similar to the unfolding method of traditional Chinese calligraphy and painting, which solves the key technical problems of long-term locking and low-impact reliable unfolding in extremely harsh service environments such as low temperature and radiation." Leng Jinsong said.

According to Leng Jinsong, before landing on Mars, the national flag locking and unfolding mechanism was in a coiled storage state, and was locked for a long time through a shape memory polymer composite material structure. After landing on Mars, the shape memory composite material was heated to achieve ultra-low impact unlocking and controllable unfolding, and the five-star red flag (developed by the Lanzhou Institute of Space Technology and Physics) was released and unfolded. After unfolding, the stiffness of the shape memory polymer composite material returned to a level comparable to that of conventional composite materials, providing a high-stiffness load-bearing function. Compared with other ways of carrying the national flag, such as mounting, folding, storing and unfolding, the national flag locking and unfolding mechanism independently designed and developed has a light weight and a high storage ratio, and can be used for larger national flags. Compared with the unfolding structure based on pyrotechnics with a transient impact of more than 10,000g, the national flag locking and unfolding mechanism is almost impact-free.

The national flag locking and unfolding mechanism developed by the team achieved the controllable dynamic unfolding of the Chinese national flag on Mars for the first time. After unfolding, the five-star red flag fluttered in the wind under the shower of Martian wind, marking the Chinese probe's "Chinese identity" on Mars and paying tribute to the 100th anniversary of the founding of the Communist Party of China.

It is reported that this is the first time in the world that shape memory polymer intelligent structures have been applied in Mars exploration projects, following the successful in-orbit verification of shape memory polymer composites and their flexible solar cell systems in geosynchronous orbit environments by Harbin Institute of Technology research teams in 2016 and 2020. This indicates that my country is at the forefront of the world in the field of intelligent materials and their applications in spacecraft structures.

In the future, related technologies such as intelligent structures based on shape memory polymer composites are expected to be applied to major aerospace projects such as my country's space station, lunar exploration, manned lunar landing, Mars exploration, Jupiter exploration, asteroid exploration, ice giant exploration, and deep space exploration. It also has broad application prospects in aerospace, aviation, automobiles, high-end equipment, intelligent manufacturing, robotics, and biomedicine.