Did you know that China's space station "flipped" in space during its construction? Why did they do that?

On September 30, 2022, after about an hour of coordination between the earth and the sky, the Wentian experimental module completed its transfer, and the Chinese space station complex temporarily changed from a "one" configuration to an "L" configuration. On November 3, the Mengtian experimental module used a similar transfer procedure to make the space station form a "T" configuration. So what technical difficulties need to be overcome in the transfer of the space station module? What are the highlights of the two module transfers? Let's find out.

The Dream Sky Cabin was successfully transferred

Why does the space station have to “flip” to be assembled?

At the beginning of the launch, the Wentian experimental module docked at the interface in front of the node module of the Tianhe core module, forming a "one" configuration. On October 31, the Mengtian experimental module was launched and successfully docked at the forward port of the Tianhe core module the next day.

Rendering of the transfer process of the Wentian cabin

Some people can't help but wonder: Why don't the two experimental cabins directly dock with the side interface of the node cabin to achieve the final state in one step?

This is mainly because the space station assembly flies at the first cosmic speed. Calculations show that if the experimental module approaches the Tianhe core module directly from the side, even if there is only a small angle between the two orbits, the relative speed of the two sides will be considerable, and the risk of collision due to control errors cannot be ruled out.

The Mengtian module is docked at the forward port of the Tianhe core module

Such a collision is obviously unacceptable to researchers. Although the strength of each module of the space station was fully considered during the design and manufacturing process, the spacecraft is limited by the launch capacity of the carrier rocket and cannot be deliberately reinforced infinitely, so it naturally cannot withstand high-speed collisions. Moreover, even if the module is safe, the drastic change in attitude will also pose hidden dangers.

Therefore, the two experimental modules were initially launched in front of the Tianhe core module, located in the same plane and orbit, and gradually approached each other at a very small relative speed to carry out docking.

According to the basic principles of orbital dynamics, during the docking process, the orbital heights of the two will still change relative to each other, but as long as the speed difference is well controlled, the change in orbital height will be very slight and will not exceed the compensation capacity of the docking mechanism.

After the initial docking is completed, after a period of testing, the Tianhe team will confirm that the status of the experimental module is normal, and will use the robotic arm to assist in transferring the experimental module to the side docking port of the Tianhe core module.

After the Wentian module was turned, the space station formed an "L" configuration

Some people may think: space is in a state of weightlessness, so it should not be difficult for a robotic arm to handle objects weighing more than 20 tons.

In fact, the inertia generated by the movement of large cabin sections cannot be ignored. If the speed is not controlled well during the transfer process, it will exceed the torque range of the robotic arm and cause danger. Therefore, the transfer process of the experimental cabin is very cautious and requires the assistance of the torque gyro.

Where does the moment gyro contribute? According to the principle of conservation of angular momentum, as long as the robotic arm moves the experimental module to the side a little, the Tianhe core module will change its angle accordingly and rotate a little in the opposite direction. In this process, the orbital direction of the space station does not actually change, but the yaw angle will change greatly, which is not conducive to the work of solar panels, heat sinks, data transmission antennas and other equipment. Therefore, the moment gyro is needed to "digest" this part of the angular momentum so that the space station assembly can keep the Tianhe core module parallel to the orbital tangent.

The effect diagram of the transfer assembly of the Dream Sky Cabin

What kind of "butterfly effect" will the thin atmosphere in space have on the space station?

Theoretically, in a vacuum environment, objects of any shape can maintain their original state of motion without changing their attitude or direction. However, reality is not so ideal. The orbital altitude of the space station reaches hundreds of kilometers, and the atmosphere is already very thin, but it will still accumulate weak atmospheric resistance, gradually forming a significant impact on the attitude and orbit of the spacecraft, so low-Earth orbit spacecraft still need to be cautious in attitude control.

For example, when the Wentian experimental module is docked with the side docking port of the node module, the problem of atmospheric resistance cannot be ignored. The Wentian experimental module is 17.9 meters long, with a large column section diameter of 4.2 meters. Together with the solar wing sails, it is equivalent to forming a huge windward surface in front of the space station.

Large compartment transfer details

Compared with similar foreign spacecraft, the International Space Station has a monthly orbital altitude drop of about 2 kilometers due to atmospheric resistance and other issues, which must be compensated by the spacecraft raising the orbit. The resistance problem encountered by the Chinese space station experimental module will also lead to various consequences, such as causing a certain torque to be generated in the entire space station assembly. If the torque is not corrected, the space station assembly will be pushed to rotate and turn excessively to one side.

Taking the Wentian module's repositioning process as an example, if the right-angled vertex of the "L" eventually points stably to the orbital flight direction, and the Wentian module and the Tianhe core module are each at 45 degrees on both sides of the orbital direction, then it is not a good position for solar power generation, heat dissipation, and radio communications. In addition, the space station assembly must be prepared to welcome the Mengtian laboratory module, and the space station assembly must also return to its normal position to prepare the front docking port of the Tianhe core module.

To achieve this, the torque caused by unbalanced air resistance must be rebalanced by the combined action of torque gyros and thrusters. Generally speaking, this type of planned work is more inclined to use torque gyros, because the fuel carried on the space station is precious and limited and must be used sparingly. Although cargo spacecraft will regularly deliver supplies to the space station, the space environment is complex and dangerous, and the space station must be prepared to deal with unexpected situations at any time. If space debris, tiny celestial bodies, spacecraft, etc. are abnormally close, the space station will need to consume precious fuel and change orbit to ensure the safety of astronauts and equipment.

In early November, after the Mengtian experimental module was initially docked with the space station assembly, it completed the transfer work and finally docked with the side interface of the Tianhe core module in another direction. This was also the result of the close cooperation of the torque gyro and the robotic arm.

With the completion of the transfer of the Mengtian laboratory module on November 3, the Chinese space station complex has formed a relatively balanced state: radially to the docking interface of the Tianhe core module is the Shenzhou manned spacecraft, with the Wentian module and the Mengtian module on both sides respectively, and the Tianzhou cargo spacecraft on the rear docking interface.

When the astronauts "hand over the shift in space", the node module docking port will also welcome the Shenzhou manned spacecraft, and will also have to face the problem of unbalanced resistance. However, the "windward area" of the Shenzhou spacecraft is relatively small, and the unbalanced torque caused will be relatively mild, which can be solved by simply correcting it with a torque gyro.

What are the advantages of China’s space station relocation plan?

Foreign space stations have previously experienced module transfers, using a vertical transfer solution, and the assembly's posture will change significantly after the transfer. In fact, the Chinese space station is the first in the world to adopt a planar transfer solution, which is more effective. However, compared with the large module weighing more than 20 tons, there is only a 100-kilogram robotic arm connecting the two modules, which is a considerable risk, so the transfer process of the experimental module has been described as "carrying two elephants on a pole."

The biggest limiting factor in the entire transposition process should be inertia. Since the kinetic energy of an object is proportional to the square of its speed, if the transposition speed is too fast, the impact force caused will be very significant, which will bring huge torque to the transposition mechanism and the robotic arm, and easily cause equipment damage. Especially when the cabin needs to stop, if a braking situation occurs, inertia may put the robotic arm in danger of being scrapped. Therefore, the plane transposition requires the Tiandi team to accurately calculate and strictly control the running speed of the cabin and the robotic arm. According to public reports, scientific research units have carried out a lot of computer simulation and air flotation platform simulation work on ground facilities, solving many problems such as attitude control, relay measurement and control link shielding, and energy balance. In addition, at the beginning of last year, the Tianhe core module performed a transposition operation on the Tianzhou cargo spacecraft. Considering that the size and weight of the Tianzhou cargo spacecraft are much smaller than the Wentian experimental cabin, it is very suitable for the Tiandi team and the robotic arm to "practice".

In short, after quite complicated operations, the experimental cabin was successfully transferred twice. If we want to make a simple description, the key points are "slow" and "accurate": "slow" means to reduce the acceleration as much as possible and not exceed the bearing capacity of the robotic arm and the transfer mechanism; "accurate" means to ensure that the experimental cabin is transferred "in place in one go".

Today, the Chinese space station has formed a horizontally symmetrical "T" configuration. Aerospace professionals believe that this has three major advantages. The first is to ensure that the overall center of mass is centered, saving energy required for attitude control. Secondly, the airlocks of the two experimental modules are located at the ends of the horizontal "T". Therefore, during normal decompression or abnormal isolation, it will not affect the formation of a coherent space in other sealed modules, thereby ensuring safety. Finally, it is to ensure that the large solar cell wings at the ends of the two experimental modules can be illuminated by the sun regardless of the attitude of the space station flying, so that the daily power generation of the space station can reach nearly 1,000 degrees, which is equivalent to the electricity consumption of an ordinary family for nearly half a year, and provide 80% of the energy for the three-module assembly of the Chinese space station. While meeting the normal operation of various scientific instruments on the space station, it can also ensure the daily life of astronauts in orbit.

Schematic diagram of the "T" configuration of China's space station

Figuratively speaking, the space station seems to be flying steadily in space, but in fact it is "performing acrobatics" all the time - with the docking and departure of spacecraft and modules, the configuration, center of gravity, force conditions, orbit, etc. of the space station are constantly changing, requiring the space and ground teams to follow the rules and make precise and meticulous adjustments. This is probably why aerospace control science is extremely challenging but full of fun.