With the world’s leading cargo transport capacity, how does Tianzhou-6 “thread the needle” in space?

What is the fastest time you have ever taken from placing an order to receiving the goods? For me, receiving the goods the next day is considered fast, but the fastest time for astronauts in space is only two hours. Isn't it quite exaggerated? I was also very surprised when I heard it for the first time. It turns out that sending goods to the sky is faster than sending goods to the ground. But then I thought, they use rockets, it would be strange if it is not fast.

The record for the fastest package to space was achieved by Tianzhou-5 on November 12, 2022. It also opened the "two-hour" era of human spacecraft rendezvous and docking, which really took the urban circle to the sky. It seems easy to say, and a record is broken at every turn, but how difficult is it in fact? Let me lay out the facts for you and you will understand. First of all, the two objects that need to be docked are at an altitude of more than 400 kilometers; secondly, they are not stationary, but are flying at a speed of about 7.9 kilometers per second; then, they need to be together, or in other words, how big is the target area to be aimed at? A circle with a diameter of 80 cm. It is not an exaggeration to say that this is threading a needle in space.

The Tianzhou series of spacecraft adopts an autonomous docking method, which you can understand according to the concept of unmanned driving. It completes the docking task through the synergy of multiple systems such as laser rangefinder, optical imager, attitude control system, propulsion system, etc. Of course, during the autonomous docking process, if an abnormal situation occurs, the spacecraft can automatically interrupt the docking and wait for human intervention. But this situation has not occurred so far. These systems work together in this way-when the Tianzhou series spacecraft enters the close-range area of ​​the space station and prepares to dock, it will use the laser rangefinder and optical imager to accurately measure the distance and image the space station to obtain information such as the accurate position, attitude and motion state of the space station. The laser rangefinder uses a laser beam for ranging, and its principle is similar to measuring sound waves or radar, which can very accurately measure the distance and speed of the target.

Tianzhou-5 launch, picture from Tuchong.com

The optical imager uses cameras and lenses to take pictures and images of the space station to obtain the shape and attitude information of the space station. After obtaining the accurate position and attitude information of the space station, the Tianzhou series spacecraft will make precise attitude adjustments through the attitude control system to make its docking port face the docking port of the space station and maintain relative attitude stability. The attitude control system mainly includes the flight control system and the power control system, which can control the spacecraft's attitude, position, speed and other parameters in real time to ensure the success of the docking.

Finally, the Tianzhou series spacecraft will be fine-tuned through the propulsion system to ensure that the docking mechanism is accurately aligned with the docking port of the space station. The propulsion system can control the speed and direction of the spacecraft, allowing it to slowly approach the space station and make fine adjustments and alignments within the last few centimeters. In this way, the docking mechanism can automatically complete the docking task and lock the two together after docking to form a temporary whole. Then, the astronauts can enter and get the "home goods" from the ground. Moreover, there are quite a few hometown goods. In addition to the food and supplies for the astronauts, a large part is reserved for the space station itself to replenish its propellant.

In fact, each of China's Tianzhou cargo spacecraft has been upgraded from the previous one. For example, the Tianzhou-6, which will be launched in Wenchang, Hainan, has greatly upgraded its carrying capacity compared to last year's Tianzhou-5. Its cargo capacity has increased from 6.9 tons to 7.4 tons, making it one of the world's most powerful cargo spacecraft.

In addition to China's Tianzhou, what other space express delivery exists? Russia has the Progress spacecraft, which was originally used to transport supplies for the Salyut-6 space station. It was transformed from a manned Soyuz spacecraft, so it also inherited its three-cabin structure. The cargo cabin is located at the front, in the position of the Soyuz spacecraft orbital module, which is used to store various types of goods. When the Progress docks with the space station, the astronauts of the space station will enter the cargo cabin through the docking hatch and move out the "express parcels" delivered. After that, the waste that needs to be discarded by the space station will be transferred to the cargo cabin. During the spacecraft's return to the atmosphere, these wastes will be burned with the spacecraft. Located in the middle of the spacecraft, where the Soyuz return capsule is located, is the Progress's replenishment cabin, which stores fuel, oxidizer and water for the space station, and the Progress's cargo cabin and replenishment cabin can transport a total of about 2.3 to 2.5 tons of cargo at a time. When the Progress docks with the space station, the liquid stored in the replenishment cabin is transferred to the space station's own propulsion system through the liquid connector on the docking ring. Until now, the improved model of Progress is still serving the International Space Station.

As for the United States, after many years of vacancy after the retirement of the space shuttle, SpaceX's Dragon spacecraft finally arrived. It consists of two compartments. The conical pressurized cabin at the front of the spacecraft is used to transport general supplies. The non-pressurized cabin at the tail of the spacecraft is used to transport cargo installed outside the space station and does not need to be transported into the pressurized environment of the space station. Perhaps unlike all other cargo spacecraft, its pressurized cabin uses reverse propulsion technology, which allows for precise landing and recycling. As a result, the Dragon spacecraft can achieve two-way express delivery. On the one hand, the goods on the ground are sent to the sky, and on the other hand, the goods in the sky, such as samples obtained during experiments in space, are sent back to the ground for further analysis. The ability is indeed stronger.

In addition to the United States and Russia, the European Space Agency also has their ATV spacecraft, and the Japan Aerospace Exploration Agency also has their HTV spacecraft. In terms of cargo carrying, oil transportation, and autonomous docking, they are at roughly the same level as their international counterparts.

This article is a work supported by Science Popularization China Starry Sky Project

Author: Science Rocket Uncle

Reviewer: Liu Yong (Researcher at the National Space Science Center, Chinese Academy of Sciences)

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.