From "delivering people" to "delivering goods", the gorgeous transformation of the Tianzhou spacecraft

In May 2023, the Tianzhou-6 cargo spacecraft was successfully launched. Tianzhou-6 has been improved on the original design, with its volume increased by 20% and its carrying capacity increased from 6.9 tons to 7.4 tons, a full 1,000 kilograms.

Today we are going to talk about what a cargo spacecraft is, what is the difference between it and a manned spacecraft, and what it means to carry an extra 1,000 kilograms of stuff.

When it comes to the Tianzhou cargo spacecraft, we have to start with Tiangong, not the current Tiangong space station, but the Tiangong-1 and Tiangong-2. If you compare them with the Tianzhou cargo spacecraft, you will find that they look very similar. In fact, if we look at the official introduction of the Tianzhou spacecraft, we will find such a sentence that the Tianzhou cargo spacecraft was developed on the basis of fully inheriting the technology of the Tiangong-1 target spacecraft and manned spacecraft. In other words, the experimental Tiangong target spacecraft that we made in order to verify the orbital rendezvous and docking technology, space station technology, etc., was not wasted after being used twice, but became the Tianzhou cargo spacecraft that we can use all the time.

Then some smart students will say that converting a space laboratory into a cargo spacecraft sounds like a bigger detour than converting a manned spacecraft into a cargo spacecraft. After all, manned spacecraft and cargo spacecraft are both spacecraft, and it sounds like they are easier to convert.

You know, in aerospace, when it comes to manned space, there are a lot of things that need to be added. Let's not talk about seats and manual control systems, which can be removed after all. There is an important thing on a manned spacecraft that a cargo spacecraft cannot use, and that is the return capsule. A manned spacecraft cannot be a one-way trip. You have to send things up and take them down. But a cargo spacecraft is not like that. What is sent up is cargo, and what is brought down is garbage. We can let the garbage burn up in the atmosphere with the spacecraft. Therefore, there is no need for the heat insulation design of the return capsule, as well as parachutes, life support systems, etc., and the reliability and safety requirements will be lower than those of a manned spacecraft.

Tiangong Space Station (Photo from Tuchong.com)

In this case, if we use Shenzhou to modify it, it will not be big enough and it will be a waste. Although it does not need to return, the environment inside the cargo spacecraft must be a living environment for people after it is launched, because astronauts have to drill into it from the space station to pick up goods. It is a small warehouse. Although people do not need to live in the warehouse, they must be able to enter the warehouse. You see, there is no need to return, and the inside must be a pressurized environment that people can enter, isn’t it similar to the Tiangong-1 back then? So the researchers built the Tianzhou cargo spacecraft based on Tiangong-1. And used it to conduct a docking experiment with the Tiangong-2 target spacecraft.

During the construction of the space station, most of the food, supplies and toys for the astronauts were delivered by the Tianzhou rocket. But did you notice that the Long March 2F rocket was used to launch Tiangong-1 and Tiangong-2? The low-Earth orbit carrying capacity of this rocket is only 8.8 tons. But if it is Tianzhou, it will weigh 13 tons if it is filled with things. The Long March 2F rocket is not enough. So when we launched the Tianzhou cargo spacecraft later, we used the new Long March 7 rocket.

In addition to delivering express packages, the Tianzhou spacecraft also has a very critical technology. We know that airplanes have aerial refueling technology, and the space station must have a space refueling technology, that is, on-orbit propellant replenishment. Why does the space station need to refuel on orbit? This is because the altitude at which the space station flies is not actually completely vacuum. There will be a little bit of air, which will slowly lower its orbit until it finally falls. So every now and then, we have to refuel the space station.

Our Tiangong space station uses ion thrusters, which are very fuel-efficient, but their thrust is very small. There is no problem in maintaining this kind of orbit, but if we need to do orbital maneuvers, such as finding a string of Starlink satellites climbing up and there is a risk of collision, we have to use traditional thrusters to push twice to quickly change the orbit. Both the fuel in the ion thrusters and the fuel in the traditional thrusters have to be sent up by Tianzhou. If the space station itself is not powerful enough, the cargo spacecraft can still push the space station like a tugboat. It can be said that after the space station is built, how long it can maintain normal operation mainly depends on the Tianzhou cargo spacecraft. So don't underestimate it.

This time, Tianzhou has more cargo, so the benefits are obvious to everyone. With more cargo carried each time, the astronauts can live better. In addition, even the original cargo capacity can allow the astronauts to live well. The cargo spacecraft is larger, so we can launch fewer times. For example, we used to launch two times a year, but now we can launch three times in two years. This will make it cheaper to operate the space station.

Is it possible for the Tianzhou spacecraft to be further upgraded in the future? It is possible, because the Long March 7 still has one or two tons of low-Earth orbit carrying capacity left, so it is possible to make it heavier and larger. This will provide better protection for our astronauts.

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

Author: Gou Sheng

Review: Gou Bingchen

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.