Cargo Ship: Please don’t just call me “the courier”

Not long ago, my country used the Long March 7 carrier rocket at the Wenchang Space Launch Center to successfully send the Tianzhou-4 cargo spacecraft into the predetermined orbit, delivering the first batch of supplies for the Tiangong Space Station this year. In fact, in addition to transporting supplies to the space station, cargo spacecraft have many other functions. So, what are the other uses of cargo spacecraft?

The Tianzhou-4 cargo spacecraft is launched into space (Photo: Tu Haichao)

Undertake certain space experiments

"Build ships for station construction, and build stations for application." The space station has the characteristics of manned control and large platform size. It is an ideal platform for astronauts to conduct space science research and applications in orbit.

Although the space station has strong support capabilities, extensive space science research and applications have brought many demands. In addition to the main body of the space station, the cargo spacecraft is also a member of the space experiment platform, which is an important supplement to the space station's space scientific research and application. In some occasions, it plays an irreplaceable role.

The cargo spacecraft developed by many countries in the world have the ability to fly independently and control their orbital attitude. They all have large-capacity pressurized cargo holds, and some spacecraft also have non-pressurized cargo holds. These cargo spacecraft include both types equipped with large solar wings and types covered with solar cells on the surface, providing a sufficient and diverse material basis for conducting space science experiments and applications.

In addition, the cost of space launches is extremely expensive. A spacecraft launched into space is more expensive than an equal weight of gold. If cargo spacecraft are only used to transport supplies, it is inevitably a bit wasteful.

The cargo spacecraft can be regarded as a short-lived mini space station, and its rational use to carry out space experiments has become a routine operation.

Cargo spacecraft have independent flight capabilities and are suitable for space experiments that need to be carried out independently, especially those that require a higher microgravity environment. For example, Japan's HTV cargo spacecraft uses close-fitting solar cells, which have less acceleration caused by external forces such as atmospheric drag and solar pressure, making it a good space microgravity experiment platform.

The Soviet Union was the first to develop and put cargo spacecraft into use. Both the Soviet Union and its successor Russia have actively explored the use of cargo spacecraft to conduct space experiments. Russia has used the Progress cargo spacecraft to study the characteristics of engine tail flame plasma. The spacecraft was also used for rapid rendezvous and docking experiments, laying the foundation for the rapid docking of subsequent Soyuz manned spacecraft.

The U.S. Cygnus cargo spacecraft has a cargo hold with a diameter of 3 meters and a volume of 27 cubic meters, which provides a good place for conducting space experiments. NASA has conducted many "arson" experiments on the Cygnus spacecraft, that is, setting fires in its spacious cargo hold to observe real-scale space fires and study the spread of flames in microgravity.

I have a share of track control

The engineering technology of manned space stations matured relatively late, and the widespread use of cargo spacecraft in space experiments was a thing of this century. The more practical and widespread application of cargo spacecraft is in the orbit control and attitude control of space stations.

As we all know, there is almost no air in the Earth's orbit, but for a space station flying at close to the first cosmic speed, the thin high-altitude atmospheric drag is a factor that cannot be ignored. Take the International Space Station as an example, its orbital altitude drops by about 100 meters every day due to air resistance. If the solar activity is intense and the density of the Earth's upper atmosphere increases, the orbital altitude will drop even more.

Even if the space station is operating in an orbit about 400 kilometers above the Earth's surface, if the orbital altitude is not regularly increased, it will soon fall into the atmosphere and burn up.

In addition, the space station often has to actively adjust the orbital phase to provide a better launch window for the docking missions of manned and cargo spacecraft. At the same time, there are a lot of space debris and multiple low-orbit giant communication satellite constellations represented by "Starlink" scattered in the Earth's orbit. In order to ensure the safety of astronauts, the space station will change its orbit from time to time to avoid space debris and satellites in orbit.

Controlling the orbital attitude of the space station is also one of the main functions of cargo spacecraft. Although the core module of the space station is equipped with an orbital control engine, the fuel needs to be replenished by cargo spacecraft. Therefore, researchers have come up with the idea of ​​using cargo spacecraft to increase the orbit.

If a cargo spacecraft docks and stays at the space station, the cargo spacecraft's engine is mainly used to raise the orbit, while the core module's engine and fuel are more for emergency use.

In addition to orbit change operations, the space station also uses attitude control engines to adjust the flight attitude. Although the daily use of large control torque gyroscopes can meet the attitude control needs of the space station, the accumulated attitude adjustments during space flight will make the gyroscope rotate faster and faster, and finally it will be unable to output torque and lose its attitude adjustment ability.

In order to revive the control torque gyro, the attitude control engine needs to be ignited to unload the gyro and reduce its speed to a normal range.

Generally speaking, gyro unloading operations can be performed by the attitude control engines of the core module, but some cargo spacecraft can also perform such operations to reduce the burden on the core module.

At present, the orbit raising and attitude control of the International Space Station are mainly completed by Russia's Progress cargo spacecraft. In 2021, when the Russian Science laboratory module docked with the International Space Station, it exerted too much force, causing the International Space Station to flip 540 degrees. In the end, the Zvezda service module and the Progress MS-17 cargo spacecraft ignited their engines to push back, allowing the International Space Station to return to the correct attitude.

In addition to the above-mentioned cargo spacecraft, the Tianzhou-4 cargo spacecraft launched by my country also has the ability to undertake the attitude and orbit control of the space station.

Tianzhou-4 cargo spacecraft (Photo provided by the Fifth Academy of China Aerospace Science and Technology Corporation)

I am the only one who can transport goods downstream

Cargo spacecraft carry a large amount of cargo and experimental payloads into space, dock with the space station, and serve the astronauts' lives and space scientific research applications. However, the space station is not a "foodie" that only takes in but does not leave. The daily life, space experiments and applications of the resident astronauts, in addition to producing a large number of experimental results and finished products, also produce a large amount of useless waste, so the garbage on the space station must be disposed of regularly.

Unlike the ground, which has a complete set of garbage disposal facilities such as trash cans, garbage trucks and garbage stations, it is difficult for the small space station to handle and reuse garbage. Although space is vast, littering waste in orbit will not only produce a large amount of space debris, but also pose a safety threat to spacecraft including the space station. At this time, cargo spacecraft are needed to help take away the waste.

The cargo spacecraft has a huge cargo hold that can accommodate a large amount of cargo when flying upward to the space station, and can also be used to store discarded and useless garbage when separating from the space station and returning to Earth.

At the end of its life, the cargo spacecraft will separate from the space station and actively deorbit to re-enter the atmosphere and burn up to avoid becoming space junk.

In fact, since the day it was born, cargo spacecraft has been responsible for the task of destroying garbage on the way down. This not only reduces the garbage in space orbit, but also avoids the accumulation of garbage in the space station, which can be said to kill two birds with one stone.

Although astronauts conduct scientific experiments on the space station, some waste is generated, but many useful experimental results are also developed. These valuable experimental results need to be brought back to the ground. Although manned spacecraft such as Russia's "Soyuz" and my country's "Shenzhou" can be used for downlink transportation, the downlink transportation capacity of the return capsules of these spacecraft is limited and cannot meet the needs of bringing a large number of experimental results back to the ground.

In recent years, with the advancement of science and technology, the Dragon cargo spacecraft developed by the US SpaceX company has the return function, which can transport a large number of experimental results on the International Space Station back to the ground. This downlink cargo capability is currently unique.

Simulation of the Tianzhou cargo spacecraft developed by my country (Photo provided by the Fifth Academy of China Aerospace Science and Technology Corporation)

However, my country also has corresponding plans in this regard. On May 15, Bai Mingsheng, chief designer of the Tianzhou cargo spacecraft, revealed in an interview with CCTV's "Face to Face" that in the future, when my country's Tiangong space station needs to transport a large amount of materials back to the ground, a returnable cargo spacecraft will also be designed.