How does the space station deal with the "ice and fire"?

Since the beginning of summer this year, the highest temperatures in many parts of my country have broken historical records, and we are experiencing the hottest summer days in decades. In the face of high temperatures, we can cool down by turning on the air conditioner on Earth, but how does the Chinese space station in space maintain a comfortable temperature? Today, we will reveal how the thermal control system of the Chinese space station makes our space home "spring-like all year round."

The Chinese space station is located in a space environment about 400km from the ground. Due to the lack of protection from the atmosphere, the surface temperature of the space station can reach up to 150℃ or more under direct sunlight, and the temperature can reach -100℃ or less on the side away from the sun. In this "ice and fire" space environment, in order to ensure that astronauts can have a space station environment suitable for work and life, a thermal control system is needed to control the temperature. The thermal control system is an important system to ensure the normal operation of space station equipment and the comfort of astronauts' life in space. The thermal control system is like our air conditioning system on the ground. By rationally organizing the heat exchange process inside and outside the spacecraft, the temperature of each part of the space station is always within the range required by the mission, providing a good temperature environment for the normal operation of space station equipment and the life of astronauts.

Different from previous manned space missions, the spacecraft in the space station period need to fly in orbit for a long time, and astronauts also need to stay in orbit for a long period of time, which puts higher requirements on temperature control. In view of the characteristics and needs of the Chinese space station mission, the thermal control team has carried out a lot of technical research and made a number of technical innovations to create a comfortable "space home".

The "central air conditioning" in the space station's thermal control system - the fluid loop is the core of the space station's thermal control system . In the Tianhe core module and the Wentian laboratory module, the fluid loops are spread throughout every corner.

The fluid circuit can evenly wrap the important parts of the space station. Through the reciprocating circulation of special liquids in the pipeline, the heat generated by the equipment in the cabin and the astronauts' lives is collected and brought to the corresponding equipment and structures through the circuit, dissipating heat in overheated areas and heating up overcooled areas, thus achieving heat dissipation and heating supplement functions. At the same time, it can also accurately control the temperature of different "rooms" in the space station to maintain uniform and stable temperature. It can be said to be a tailor-made "central air conditioner".

The Wentian experimental module is my country's largest, heaviest and most test-payloaded spacecraft. Many test payloads are installed inside the cabin, and various high-heat-consumption exposure test loads are arranged outside the cabin, which requires higher thermal control.

The research and development team developed three liquid cooling systems for the Wentian Experimental Module , which collect and radiate the heat generated by various equipment or test payloads into outer space, and can support the heat dissipation of test payloads above several kilowatts. One of them is a heat dissipation circuit specifically designed to ensure the temperature of extravehicular test payloads. For the first time, four-way valve technology was developed to solve the impact of the installation status of extravehicular test payloads on the circuit system, and its weight is superior to the multi-valve control technology used in the International Space Station. In addition to creating a "central air conditioner" to regulate the temperature of the space station, it can also passively insulate spacecraft. Thermal control coating is a type of passive thermal control.

Starting from Shenzhou XII, the Shenzhou series of spacecraft have been coated with a new silver coating, which is a magical "space sunscreen" - a low-absorption-low-emission thermal control coating.

During the construction of the space station, the Shenzhou spacecraft faced the difficulty of controlling the cabin temperature under long-term and large temperature differences. When the space station assembly is flying, the Shenzhou spacecraft may be continuously blocked by other cabins, causing the spacecraft to be in an extremely low temperature environment where the sun cannot shine for a long time, with the lowest temperature even below -100℃. When the space station assembly forms certain configurations, local areas of the spacecraft will continue to be irradiated by the sun, with the highest temperature exceeding 100℃, which brings severe tests to the normal operation of the spacecraft equipment and the living environment of the astronauts.

In response to this temperature control problem, the research and development team designed and developed a low-absorption-low-emission thermal control coating. Low absorption, as the name suggests, means that the coating material itself has low solar absorption characteristics, which can effectively reduce the temperature increase caused by solar radiation. Low emission means that the coating has a low infrared emissivity, which can effectively block the radiation heat leakage from the interior of the spacecraft to the external deep cold environment, and prevent the temperature in the cabin from continuously decreasing.

The thermal control coating is like a layer of "sunscreen", which effectively ensures the operation of the spacecraft in long-term extreme high and low temperature environments and keeps the cabin in a suitable temperature range.

Another way of passive thermal control is to put on a heat-insulating "outer coat" for the spacecraft . Take Tianzhou-4 as an example. Tianzhou-4 is in orbit about 400 kilometers above the ground. It takes about 90 minutes to orbit the earth. Sometimes it is in the shadow of the earth, and sometimes it is exposed to the direct sunlight. It needs to experience such "ice and fire" 14 times a day. Therefore, the developers put on special insulation materials to resist the harsh environment and maintain a comfortable temperature inside .

The selection of fabrics for the spacecraft's "outer shell" is very particular. They are all carefully designed based on scientific principles, and fabrics with different characteristics are adapted to the different needs of the spacecraft.

The "outer coats" of the cargo cabin and propulsion cabin of Tianzhou-4 are designed in two different colors, gray and white. The cargo cabin needs to provide a relatively high temperature suitable for the astronauts' lives, so it is dressed in a gray "outer coat" to absorb more solar heat. The propulsion cabin is mainly equipment and requires a lower ambient temperature, so it is dressed in a white "outer coat" to reflect more solar heat. In addition, the interior of the Tianzhou-4 "outer coat" adopts a multi-layer insulation component design, which has a strong warming effect. It is composed of a high-reflectivity membrane and a polyester mesh supporting the membrane. It can be repeatedly superimposed to form a multiple structure. The heat inside is reflected by layers of reflective membranes, and it is difficult to seep out of the surface, which can form a high thermal resistance to prevent heat loss. Through the collaborative work of "air conditioning", "sunscreen" and "insulation clothing", the research and development team has built the Chinese space station into a comfortable "space home" to keep "four seasons like spring". The thermal control system is like a "housekeeper" who controls the temperature of the space station, ensuring the comfort of the space station in the harsh space environment and protecting the space station to travel safely in the sky.

Further reading: Why does the space station only fly 400 kilometers high?

In addition to our Tiangong space station, the International Space Station also operates at an orbital altitude of 400 kilometers. There are many satellites here, so the question is, why are the orbital altitudes of the space station and satellites all chosen to be around 400 kilometers? Aren't there concerns about collisions?

On the one hand, the 400-kilometer altitude is sufficient for the near-vacuum environment, cloud-free telescope observation, and near-zero-gravity experimental conditions that are of concern to general space experiments. On the other hand, the safety of astronauts and the space station itself is taken into consideration. There is a space region around the Earth called the "Van Allen Radiation Belt," which is roughly divided into altitudes of 1,500-5,000 kilometers and 13,000-20,000 kilometers. It will expand up and down when space weather is disturbed. There are charged particles with very high energy and density, which are very harmful to aircraft flying in it.

In particular, because the geomagnetic field itself is not symmetrical, the geomagnetic field shape above the South Atlantic (that is, near the so-called "Bermuda") causes the radiation belt here to be relatively low in altitude. When there is a disturbance, it may be only about 1,000 kilometers in altitude. If the space station flies high, it is easy to enter the radiation belt.

What if the space station flies tens of thousands of kilometers high? When the solar activity is strong, the Earth's magnetosphere facing the sun may be compressed to about 30,000 kilometers. At this time, the space station may also be bathed in high-energy particles of the solar wind. Once these two situations occur, letting the space station enter the high-energy particle area will not only pose a huge threat to the life and health of astronauts, but also damage instruments and equipment. Therefore, it is safer and more appropriate for the space station to fly at an altitude of about 400 kilometers.

Source | General Design Department of China Academy of Space Technology