On September 30, 2022, after about one hour of space-ground coordination, the Wentian experimental module completed the transposition, and the Chinese space station assembly temporarily changed from a "one" configuration to an "L" configuration. On November 3, the Mengtian experimental module formed a "T" configuration for the space station through a similar transposition procedure. So what are the technical difficulties that need to be overcome for the transposition of the space station module?What are the highlights of the two transpositions?Let's do it.
The Mengtian module was successfully transposed.
Why did the space station "fly" up and assemble?
At the beginning of the launch of the Wentian experimental module, it was docked at the interface directly 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 the next day it was successfully docked to the forward port of the Tianhe core module.
Renderings of the transposition process of the Q&A cabin.
Some people can't help but wonder: why are the two experimental modules not directly connected to the side interface of the node module, and the final state is reached "in one step"?
This is mainly due to the fact that the space station assembly follows the first cosmic velocity, and 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 velocity 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 to the forward port of the Tianhe core module.
Such an impact is obviously unacceptable to scientific researchers. Although the strength of each module of the space station is fully considered in the design and manufacturing process, the spacecraft is limited by the launch capacity of the launch vehicle, and it is impossible to deliberately strengthen it indefinitely, and naturally cannot withstand high-speed impacts. What's more, even if the cabin is safe, there will be many hidden dangers if the attitude changes drastically.
Therefore, both experimental modules were initially launched to the front of the Tianhe core module, in the same plane and orbit, and gradually approached with a small relative velocity value to implement docking.
According to the basic principle of orbital dynamics, during this docking process, the orbital height of the two will still change relatively, but as long as the speed difference is well controlled, the orbital height change will be very weak and will not exceed the compensation capacity of the docking mechanism.
After the preliminary docking is completed, after a period of testing, the Tiandi team confirms that the state of the experimental module is normal, and will use the robotic arm to assist and transfer the experimental module to the side interface of the Tianhe core module.
After the transposition of the Sky Module.
The space station forms an "L" configuration.
Some people may think that space is in a state of weightlessness, and it should not be difficult for the robotic arm to dispose of more than 20 tons of things.
In fact, the inertia generated by the movement of large segments cannot be ignored. If the speed is not well controlled during the indexing process, the torque range of the robot arm will be exceeded, and a dangerous situation will occur. Therefore, the transposition process of the experimental cabin is quite cautious, and it also needs to be assisted by a moment gyroscope.
What about the "contribution" of the moment gyroscope?Determined by the principle of conservation of angular momentum, as long as the robotic arm moves the experimental module to the side a little, then the angle of the Tianhe core module will change accordingly and rotate a little in the opposite direction. In this process, the orbital direction of the space station has not changed, but the yaw angle will change greatly, which is not conducive to the work of the solar wing sail plate, heat dissipation plate, data transmission antenna and other equipment, so it is necessary to rely on the moment gyroscope to "digest" this part of the angular momentum, so that the space station assembly keeps the Tianhe core module and the orbit tangent parallel state.
Renderings of the transposition assembly of the Mengtian module.
What is the "butterfly effect" of the thin atmosphere of space on the space station?
Theoretically, in a vacuum, objects of any shape can maintain their original motion without changing their attitude and direction. However, the reality is not so idealistic. 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, which will gradually have a significant impact on the attitude and orbit of the spacecraft, so the low-earth orbit spacecraft still needs to be cautious and careful about attitude control.
For example, when the Wentian experimental module is docked to the side interface of the node module, the problem of atmospheric resistance cannot be ignored. The commander of the experimental module of the Wentian 179 meters, the diameter of the large column section is up to 42 meters, plus the solar wing windsurfing board, is equivalent to forming a huge windward side directly in front of the space station.
Large segment transposition details.
Compared with similar foreign countries, the International Space Station will drop its orbital altitude by about 2 kilometers per month due to atmospheric drag and other problems, which must be compensated by the spacecraft's orbit ascension. Then the drag problem encountered by the experimental module of the Chinese space station will also lead to various consequences, such as the entire space station assembly to generate a certain torque. If the torque is not corrected, the station assembly will be pushed to rotate and over-rotate to one side.
Taking the transposition process of the Wentian module as an example, if the right-angle vertex of "L" finally points stably to the direction of orbital flight, and the Wentian module, Tian, and core modules are at an angle of 45 degrees on both sides of the orbital direction, then it is not a good position for solar power generation, heat dissipation, and radio communication. In addition, at that time, the space station assembly had to be prepared for the Mengtian experimental module, and the space station assembly had to return to its normal position, ready to face the interface directly in front of the Tianhe core module.
To do this, the torque caused by unbalanced air resistance must be rebalanced by the combination of the moment gyro and the thruster. In general, such planned work prefers the use of moment gyroscopes, since the fuel carried on the space station is precious and limited, and must be used sparingly. While cargo spacecraft regularly send supplies to the space station, the space environment is complex and treacherous, and the space station must be prepared for the unexpected. In the event of an unusually close proximity of space junk, tiny objects, spacecraft, etc., the space station will need to consume precious fuel, change orbit, and ensure the safety of astronauts and equipment.
At the beginning of November, after the Mengtian experimental module was initially docked with the space station assembly, the transposition work was completed, and finally docked to the lateral interface of the Tianhe core module in the other direction, which is also the result of the close cooperation of the moment gyroscope and the robotic arm.
With the completion of the transposition of the Mengtian experimental module on November 3, the Chinese space station assembly has formed a relatively balanced state: the radial interface of the Tianhe core module is the Shenzhou manned spacecraft, the Wentian module and the Mengtian module are on both sides, and the Tianzhou cargo spacecraft is on the rear interface.
When the astronauts are "in space", the node module interface will usher in the Shenzhou manned spacecraft, and it will also need to face the problem of unbalanced resistance. However, the "windward area" of the Shenzhou spacecraft is relatively small, and the unbalanced torque caused by it will be relatively slight, which is expected to be solved by simply correcting it with a torque gyroscope.
What are the advantages of China's space station transposition scheme?
Foreign space stations have previously experienced module transposition, using a vertical transposition scheme, and the attitude of the combination will change greatly after transposition. In fact, China's space station is the first in the world to adopt a planar transposition scheme, and the effect is better. However, compared with the large module weighing more than 20 tons, only a robotic arm of more than 100 kilograms connects the two cabins, which is not a small risk, so the transposition process of the experimental cabin is described as "a flat shoulder to pick up two elephants".
The biggest limiting factor in the entire transposition process should be inertia. Since the kinetic energy of the object is proportional to the square of the velocity, if the indexing speed is too fast, the impact force caused will be very significant, which will bring huge torque to the indexing mechanism and the robotic arm, and easily cause damage to the equipment. Especially when the cabin needs to be stopped, if there is a braking situation, the inertia may put the robotic arm in danger of being scrapped. Therefore, plane transposition requires the TNDI team to accurately calculate and strictly control the operating speed of the module and the robotic arm. According to public reports, the scientific research unit has carried out a large number of computer and air float simulation work on ground facilities, and solved many problems such as attitude control, relay tracking and control link occlusion and energy balance. In addition, at the beginning of last year, the Tianhe core module carried out a transposition operation on the Tianzhou cargo spacecraft, considering that the volume and weight of the Tianzhou cargo spacecraft are much smaller than that of the Wentian experimental module, which is very suitable for the Tiandi team and the robotic arm to "practice".
In short, after quite complex operations, the two experimental cabins were successfully transposed, if you want to make a simple description, the essentials are "slow" and "accurate": "slow" is to reduce the acceleration as much as possible, and cannot exceed the bearing capacity of the mechanical arm and the indexing mechanism;"Accurate" is to ensure that the transposition of the experimental module is "in place at one time".
Today, China's space station has formed a horizontally symmetrical "T" configuration, which aerospace professionals believe has three major benefits. The first is to ensure that the overall center of mass is centered, saving the energy required for attitude control. Secondly, the airlock cabins of the two experimental cabins are located at the end of the "T" horizontal line, so they will not affect the other sealing compartments to form a coherent space during normal pressure relief or abnormal isolation, so as to ensure safety. Finally, it is to ensure that the large solar cell wings at the end of the two experimental modules can shine on the sun no matter what attitude the space station flies, so that the daily power generation of the space station can reach nearly 1,000 kWh, which is equivalent to the electricity consumption of an ordinary family for nearly half a year, and provide 80% of the energy of the three-module assembly of the Chinese space station, which can meet the normal operation of various scientific instruments on the space station while ensuring 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 "playing acrobatics" all the time - with the docking and departure of spacecraft and modules, the configuration, center of gravity, force and orbit of the space station are constantly changing, requiring the heaven and earth team to follow the law and adjust and improve it precisely and meticulously. That's why space control science is so challenging, yet so much fun.