How did it come from and where is it going? Uncovering the development ideas of the Tianzhou series cargo spacecraft

When the Shenzhou spacecraft goes up to the sky, astronauts can carry a few hundred kilograms of carry-on luggage at most. In 2013, after two astronauts of the Shenzhou 11 flight crew stayed in the Tiangong-2 space laboratory for a month, the supplies were basically exhausted. When China decided to build and operate its own "Tiangong" space station for a long time, a problem that followed was how to replenish the propellant on the space station? What should astronauts do if they run out of daily necessities for a long time?

Cargo spacecraft came into being. Today, five of China's Tianzhou series cargo spacecraft have set sail, traveling between heaven and earth, and building a life supply line for manned space projects. As a frequent visitor on the transportation channel between heaven and earth, where does it come from and where is it going?

Kaijian: Tianzhou achieves a leap forward in space propulsion technology

Genetic differences create different life forms. The design and development of cargo spacecraft must be completed based on its own characteristics and the needs of the space station mission. This is a process of gradual discovery, exploration, breakthrough and improvement.

In 2011, the Fifth Academy of China Aerospace Science and Technology Corporation began to carry out the project feasibility study of the cargo spacecraft, established the overall technical plan for the configuration of the cargo cabin and the propulsion cabin, and used Tianzhou-1 as my country's first experimental cargo spacecraft to verify the rationality of the spacecraft platform design itself.

Schematic diagram of the Tianzhou-1 cargo spacecraft

Dang Rong, deputy chief designer of the Tianzhou series cargo spacecraft, introduced that due to the large size of cargo spacecraft, the center of mass position will change with the different cargo loading amounts of each mission, and the loading of on-orbit waste on the cargo spacecraft will also affect the control of the center of mass. If the attitude control thrusters are installed near the center of mass and on the rear end face like other spacecraft, it will not be able to adapt well to the requirements of each mission, easily resulting in poor attitude stability and even affecting rendezvous and docking.

This is the first issue that needs to be considered when developing a cargo spacecraft. In order to ensure that the cargo spacecraft can adapt to different center of mass positions and high-precision control requirements, Tianzhou-1 has 36 engines of four types arranged on the front cone and rear end of the spacecraft.

"Transporting as much cargo as possible" is the main goal pursued by designers. Zhang Zhenhua, deputy chief designer of the Tianzhou cargo spacecraft system, said that this requires the spacecraft to increase its carrying capacity and reduce its own weight.

In terms of improving the carrying capacity, the high-efficiency load-bearing shelves in the ship adopt a beam-plate structure based on honeycomb panels and carbon fiber vertical beams, forming a large number of standard loading units with good force transmission effect, while the structure-to-cargo weight ratio is only 8%. In terms of reducing its own weight, all key electronic units of the spacecraft use domestically produced 100-volt thick-film power modules, which not only reduces the weight of power distribution equipment and cables, but also lays the foundation for 100-volt high-voltage transmission.

An important cargo to be transported by Tianzhou is propellant, which requires breakthroughs in the on-orbit propellant replenishment technology. Prior to this, only Russia and the United States had mastered the on-orbit propellant replenishment technology, and some foreign counterparts also expressed their willingness to provide technical cooperation to China, but the research and development team insisted that "space refueling technology must be mastered in our own hands" and spent 10 years to overcome a series of key technologies. Finally, in 2017, the Tianzhou-1 cargo spacecraft completed the on-orbit propellant replenishment of the Tiangong-2 space laboratory, achieving a technological leap in my country's space propulsion field.

First up: Tianzhou as a flight verification platform

Tianzhou-1 must seize the precious flight opportunity and, while fulfilling its own mission, carry out a large amount of preliminary technical verification for space station construction and manned spacecraft.

In order to verify the key components used in the space station in advance and accelerate the realization of independent control, Tianzhou-1, as a flight verification platform, made drastic use of 44 newly developed domestic core components in seven categories. Whether it is the application of 100-volt high-voltage fully distributed power distribution system and high-voltage lithium battery, or the internal Ethernet high-speed information system of the cargo spacecraft, and the separate planning of power supply and information flow for the payload, the research and development team has established a new set of electronic and information systems. "At that time, we went to all the manufacturers and did all kinds of tests." Dang Rong said.

When it came to Tianzhou-2, the development team also accomplished several major things.

First, verify the autonomous rapid rendezvous and docking technology. For cargo transportation, the timeliness requirement is not high, but considering the emergency life-saving requirements of manned spacecraft, Tianzhou-2 must take on the responsibility of a pioneer - Tianzhou-2 completed an 8-hour autonomous rendezvous and docking, and also conducted a two-hour rapid rendezvous and docking in-orbit test before leaving orbit.

Second, in order to improve the reliability of rendezvous and docking, Tianzhou-2 cooperated with astronauts to complete manual remote control experiments. Dang Rong said that unlike the previous astronauts who looked at the target aircraft on the active aircraft to control it, the astronauts remotely controlled the cargo spacecraft to dock with the space station on the space station. The cargo ship must first obtain images and then transmit them to the space station. The astronauts then control them based on the received images. This will be affected by time delays. To this end, the development team upgraded the "air-to-air communication machine" for two-way communication between spacecraft, increasing its transmission performance from kilobytes to megabytes.

Third, the ability of two-way grid-connected power supply between Tianzhou and the space station has been opened up. The space station project is a large system. When such a complex system flies in orbit, it is inevitable that the solar panels will be blocked from the sun, which will temporarily make the power supply capacity of the spacecraft unable to meet the power demand of a specific working mode. The space station can supply power to cargo spacecraft, and when the space station is short of power, Tianzhou can also "feed back to the mother ship" to provide it with power.

Fourth, the feasibility and effectiveness of using the robotic arm to operate the transfer of space station modules were tested , the space station module transfer technology and the robotic arm large-load control technology were verified, and experience was accumulated for the subsequent on-orbit assembly and construction of the space station.

At this stage, Tianzhou is a technological pioneer.

Improvement: Continuously enhancing the user experience of Tianzhou

With the successful completion of the "Tianzhou-1" and "Tianzhou-2" missions, the technical status has become increasingly stable. Starting from "Tianzhou-3", the cargo spacecraft has officially entered the batch development mode.

Designing a humane cargo spacecraft that is more ergonomically designed and can gradually enhance the user experience was the design concept that aerospace personnel most wanted to adhere to at that time.

From the cargo bags, the more than 160 bags carried by Tianzhou-2 are all beige. The three astronauts of the Shenzhou-12 manned spacecraft reported that they could not quickly identify the type of materials because the colors were the same. Because the color of the special materials used in the cargo bags is difficult to change, the development team began to "tie bows" on the cargo bags, with different colored straps representing different types of goods.

On Tianzhou-4, the cargo packages were directly equipped with colored "identity cards". There are six colors: light blue, dark blue, green, purple, light brown, and dark brown. Green represents food for astronauts, light brown represents photography equipment packages, and purple represents medical experimental materials. The developers have established a digital management system for materials throughout the mission cycle from the ground to the cloud, which can realize automatic information entry of cargo and dynamic information management of material placement. "This system also uses VR technology to provide astronauts with visual operating instructions, so that they can find cargo more conveniently." said Feng Yong, commander-in-chief of the Tianzhou series cargo spacecraft.

This design concept is reflected in all aspects: all places that astronauts can touch are obtuse angles, soft light and color are chosen as much as possible, floor tiles are made into a rough surface to avoid glare, different colors are used to create a "heaven and earth" visual experience, different frequencies and sounds are used to distinguish the types of space station alarms, and the cushioning foam in the cargo bag is divided into blocks and miniaturized to facilitate astronauts to carry more downlink supplies... Anything related to the five senses of touch, smell, taste, vision, and hearing must be taken into consideration by the developers.

During the Tianzhou-3 mission, the developers hung a floral-dressed calf doll on the cargo bag. "Astronauts may experience visual fatigue in space, and I hope this can give them a different experience," said Feng Yong.

Later, this "little flower cow" was hung on the door of astronaut Tang Hongbo's "home".

Upgrade: Tianzhou has "change" written into its genes

As more scientific experiments are carried out on the space station, new requirements are put forward for cargo spacecraft. According to the current feedback, the propellant brought to the space station by Tianzhou every time is enough, and there is even some left over. In the future, it will need to carry more cargo, which means that the Tianzhou-6 cargo spacecraft will face a structural adjustment.

Zhang Zhenhua introduced that the cargo cabin of the cargo spacecraft is divided into three sections: the front cone, the column section and the rear cone. The rear cone section is a non-sealed structure with the spacecraft's own platform electronic equipment built in. The improved spacecraft has reduced 4 of the original 8 propellant tanks, freeing up space for platform electronic equipment, freeing up space in the rear cone section and increasing cargo capacity.

The only thing that remains unchanged in the development of Tianzhou is change. Feng Yong believes that the design phase of Tianzhou has passed, and the current cargo spacecraft are put into production in batches, and their status is relatively stable, which guarantees the speed and quality of manned space projects to the greatest extent, but this does not mean that it will not make changes.

"We are always using new products, new technologies, new processes and new materials." Feng Yong said that some single-point innovations will often lead to mutations one day. Before that, we must do a full technical verification, ensure reliability, strictly control every detail, and make sure that it is neither "over-designed" nor "under-designed", and at the same time consider the cost issue. Because commercial cargo spacecraft, returnable cargo spacecraft, or other new cargo spacecraft may all appear in China.

The Tianzhou-5 cargo spacecraft and fairing assembly stand in the factory. Photo by reporter Su Dong

On November 12, 2022, the Tianzhou-5 cargo spacecraft took off. This is the first cargo spacecraft to be launched after the Chinese space station forms the basic "T" configuration. In the future, the "Tianzhou" will also achieve faster transportation speed, greater load capacity, higher transportation efficiency, and more advanced technical verification.