Wang Xiang is the commander-in-chief of the space station system of China's manned space project
Schematic diagram of the Tiangong space station.
Invited Answerer
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Initial conditions for docking Under what circumstances can I dock?
The end of the rendezvous is the beginning of the docking. At this point, the lateral position and speed of the spacecraft relative to the space station, the triaxial attitude and angular velocity are as close as possible to zero, and only the axial flight direction maintains the pre-designed approach speed. The project begins with the state of these parameters as a condition for docking. This condition is the rendezvous control target for the flight control system and the initial range to be adapted to for the docking system. From the overall perspective of the system, the higher the accuracy of the rendezvous endpoint, the better, and the tolerance range of the docking mechanism is larger and better, which is also the interface where the system design indicators need to leave a margin when allocating.
At this moment, the rendezvous system completes the "handover", and the baton of the rendezvous and docking task is handed over to the docking system.
By the end of the rendezvous flight, the two spacecraft had achieved "1+1". The next docking will enable the two to achieve "=1" in the cabin structure, which will become the basis for the "=1" of cabin resources such as motion control, energy, information, and the environment.
In-class knowledge
How many steps does it take to dock from a single spacecraft to a combination?
As the physical process of completing the mechanical connection of two aircraft and forming a rigid assembly, the docking mainly includes three steps
01 Contact, acceptance and geometric position correction
The orbit correction made in the rendezvous flight to eliminate errors was mentioned earlier. When the rendezvous flight is completed, the position, relative speed, relative attitude, and angular velocity of the spacecraft and the space station are consistent, that is, corrected. But the bias remains. Therefore, after the docking mechanisms of the two aircraft come into contact with each other, the first thing is to eliminate the initial deviation, let the mechanical devices of both sides accept each other, and correct the position relationship between each other to achieve complete "alignment". This action is similar to the straightening action of first aligning the screw hole when twisting the screw.
Houses built on earth often use the traditional tenon and tenon structure of our country. Careful observation can be found that the head of the mortise is slightly thinner, and the entrance of the ten is slightly wider, and the contact surface structure of the space docking is similar to that of the more sophisticated tenon, through special geometric guiding features, so that the docking mechanism of the two spacecraft is closer and more aligned, so that the tight seam, you have me, I have you. This form of acceptance and correction includes rod-cone combination, ring-cone combination, and outer narrow inner width guide lobe combination, our common screw head and screw hole edge is a pair of cone combinations, and the guide valve is like two fingers interpolated with each other.
After the position correction, in order to make the relative relationship between the two spacecraft no longer change, the capture agency will "grab" the other at this time, so that they are no longer detached from each other.
02 Buffer and consume collision energy
High-speed-flying massive spacecraft, even at smaller speeds, the impact energy is considerable. At least one of the spacecraft and the space station needs to be equipped with buffers and energy-dissipating devices to slow down impact overload and dissipate or absorb impact energy.
Spring damping and hydraulic servo mechanism are buffer forms that have evolved with the development of docking technology from beginning to end, and the research of electromagnetic damping devices has also emerged in recent years. Adaptive electromagnetic device can capture and buffer energy consumption work in one, the more prominent advantage is that it adds active control link, can achieve low impact capture, and through the adjustment and control of electromagnetic parameters to adapt to a wider range of docking aircraft quality and docking initial conditions.
In actual engineering, the buffer damping system is only installed on the docking mechanism of the spacecraft, which is called the "active docking mechanism". The space station installs a "passive docking mechanism" without a buffer system. The advantage of this is that there is no complex mechanism on the side of the space station, which is conducive to long-term flight; although the mechanism on the side of the spacecraft is complex, it is not difficult to design and maintain in orbit due to the short working life.
03 Mechanical connection
After the collision energy of the two spacecraft is buffered and absorbed, the two docking end faces are pulled closer, closer together, and then rigidly connected into one by a mechanical lock system. In addition to ensuring sufficient connection stiffness and carrying capacity, for manned spacecraft, it is also necessary to achieve a seal between the two spacecraft to ensure that personnel can pass through the docking channel of the two spacecraft. Similar to the configuration principles of the buffer system, the rubber sealing ring is usually configured on one side of the spacecraft and the metal sealing surface on the side of the space station.
The environment of the cabin after the docking is connected, and it has undergone an interesting development process. The first generation of docking mechanisms of manned spacecraft aimed at breaking through rendezvous and docking technology, and did not consider sealing the connection of the module segment. In other words, the docking mechanism is "solid" and fixed. On January 16, 1969, after the Soviet Union's Soyuz-4 and Soy-5 spacecraft successfully carried out the first manned rendezvous and docking of mankind, the astronauts arrived at the "next room" by exiting the capsule. Later, the second generation of rod cone docking mechanism was designed in the form of flip and disassemble after docking. Later, there was a peripheral docking mechanism - the mechanism was arranged according to the ring, the hatch could be opened in the middle, and the docking channel was formed after the active and passive docking mechanism was docked, which could build a sealed cabin environment that directly connected the two aircraft.
At this point, the structure of the two spacecraft is solidly connected to form a combination, the circuit and liquid circuit can be connected, the manned environment is connected, and the physical foundation of "1 + 1 = 1" has been fully possessed.
At the same time, as a means of transport between heaven and earth and a non-permanent docking vehicle, the spacecraft needs to be reliably separated after the mission. Therefore, the butt lock system can be locked and unlocked, and it must be a mechanism that can be reversed. In order to ensure the reliability of separation, some docking mechanisms are equipped with fireworks on the locking system so that the connection parts are "exploded" in the event of a failure.
Typically, the spring mechanism provides the power to separate, which gives the two aircraft a certain initial separation speed. The design point of the spring mechanism is to ensure that the stable separation force can be guaranteed after long-term compression, and supplemented by a guide mechanism, so that the relative angular velocity of the two aircraft is small enough to be safely separated in the form of translational motion.
Extracurricular reading
The proposal and application of allogeneic isomorphism Why does the docking mechanism not grow into a model?
The spacecraft and the space station are docked, and the mechanical docking devices on the two spacecraft are different, one active and one passive. In the 1970s, researchers at docking institutions proposed a design concept: allogeneic isomorphism. The word corresponding to Therogynous is of Latin origin, originally meaning hermaphrodite, and is still a zoological and botanical term.
The core of "allogeneic isomorphism" is that the docking mechanism at both ends of the active and passive is exactly the same, and any two aircraft can be docked with each other as the main passive; if fully realized, the orbiting aircraft can be arbitrarily docked with each other, at least it can greatly facilitate mutual rescue.
The perfect idea of allogeneic isomorphism has not been fully realized in the world's aerospace engineering, but it has been well applied locally in terms of acceptance of docking mechanisms and guiding correction devices. The Soviet docking agency mentioned in the previous section was named APAS (Androgynous Peripheral Attachment System), which can be translated as "hermaphrodite/allogeneic peripheral docking system". Soviet designers made the geometric features of the tapered orientation into an anti-symmetrical petal-like structure, with any pair of "flowers" facing each other, and their petals could be interspersed with each other. The first generation of heterogeneous isomorphic docking mechanism APAS-75 was applied to the ASTP-75 Alliance-Apollo docking project, and the United States and the Soviet Union made the same valgus guide flap according to the agreed size specifications, and configured with their respective development of buffer damping devices. The spacecraft of the two sides are mainly passive to each other, and have successfully achieved two "space handshakes".
This design effectively unifies the main structure design of the active/passive docking mechanism, which is accepted by the developers of various countries. The Soviet/Russian docking mechanism was upgraded to APAS-89 and APAS-95, which were active and passive on the buffer device, but the guide structure remained homogeneous and still in service with the International Space Station. The newly developed adaptive electromagnetic docking mechanism in Europe also uses a similar guide valve. China's docking mechanism also belongs to the allogeneic isomorphic peripheral docking mechanism of the guide valve inverted.
The Soviet Union/Russia and the United States have long tried to standardize and unify the standards of docking mechanisms, and after several rounds of discussions with the participating countries of the International Space Station, they have developed docking interface standards. But in fact, this standard is not mandatory for all countries, and for technical and non-technical reasons, even Russia and the United States themselves have not complied with the standard. Coupled with the long development and use cycle of docking mechanisms, according to incomplete statistics, there are 4 kinds of docking and berthing systems that are incompatible with each other on the International Space Station alone, including 3 pairs of APAS-89s of the United States, more than 16 pairs of CMB and 13 "rod-cone" systems containing two incompatible variants of the Russian side. A more realistic approach than solving the consistency of the docking interface is to use whose docking mechanism is used to dock whose cabin. For example, if the ATV cargo spacecraft developed by ESA wants to dock with the Russian module, it directly purchases and installs a Russian-made docking mechanism.
The "unity of the world" of the docking agency is ideal, and the ideal situation is that there is no need for the docking agency at all. When assembling the hatch on the ground, the docking accuracy can be ensured by the tooling equipment, and the screws can be directly screwed, while the docking mechanism must be used in the sky to make up for the lack of assembly accuracy caused by the space rendezvous deviation. After the future rendezvous control accuracy is high enough, the docking mechanism can be directly evolved into an automatic assembly mechanism to achieve more efficient assembly of space facilities.
Docking dynamics related questions How to ensure that the spacecraft does not overturn the space station?
As mentioned earlier, the docking will produce impact energy. In addition to the buffering and energy-consuming devices on the spacecraft, there are several designs in the space station project that are related to this problem.
First, the buffer damping system configured on the active docking mechanism isolates the two aircraft themselves during the docking impact, and the actual effect is equivalent to hitting the target with the equivalent dynamic characteristics of the system (rather than the characteristics of the entire aircraft). Therefore, through the design of the dynamic parameters of this system, different docking targets and various initial conditions of docking can be adapted.
Second, in order not to interfere with the buffer damping process, the two spacecraft must stop the attitude control after docking, and the combination is in a state of free drift. At this time, the buffer system no longer has energy input, and only needs to consume the energy of the docking impact.
Third, one of the more difficult problems in docking dynamics is docking under eccentric conditions, at which time the docking mechanism needs to have the ability to withstand a large eccentric flip load and absorb the input energy in this direction. In the cooperation project between the US Space Shuttle and the Soviet Mir space station, the shuttle interface was set on the back, far from the center of mass, coupled with the huge mass of the aircraft, the existing docking agencies at that time could not complete the docking under this condition. To this end, the Soviet Union specially developed the APAS-89 docking mechanism, the first use of the guide flap inversion layout to expand the size of the main structure, improve the bearing capacity, and in the buffer system in series of electromagnetic dampers; the United States also modified the control scheme, after the docking contact with the shuttle head and tail translation engine with the jet pulse, to partially offset the flip torque. With the technical cooperation of both sides, the space shuttle and Mir have successfully docked many times.
Eccentric conditions are common in radial docking. In the radial docking of China's Shenzhou 13 spacecraft, the free drift deflection angle of the space station assembly during the attitude stop control is much greater than the drift angle of the previous axial docking, which is also the reason.
Robotic arm as another docking option Why is there still an advantage to the traditional docking method?
In the early space activities, the measurement orbit, aircraft autonomous measurement and control capabilities are relatively weak, in order to achieve the system goals, try to use mature mechanical technology to expand the tolerance capacity of the docking mechanism, so the docking mechanism at that time is similar to the rod-cone design, the initial deviation of the docking can be as wide as 30cm. With the development of technology and the enhancement of the measurement rail and control capabilities, the scope of the initial conditions of the docking is reduced, and the docking mechanism can be made more exquisite, reducing the tolerance and guiding structure, and reducing the volume and weight. The precise rendezvous impact energy is reduced, so that the buffer energy absorption device can also be simplified. As a result, the technology and application of weak impact docking mechanism and post-capture docking of mechanical arms have been developed.
The plan of docking after the mechanical arm is captured is actually to set the hovering point near the target at the end of the spacecraft rendezvous, and the approaching speed of the initial conditions of the docking is also controlled to zero. The scheme gives full play to the advantages of high-precision motion control of the aircraft and the functional performance of the robotic arm, and greatly reduces the requirements of the tolerance and cushioning capacity of the docking mechanism. As a universal tool, the robotic arm can serve all visiting aircraft, and the docking mechanism of the visitor can be simplified and lightweight. Another unique advantage of this scheme is that the robotic arm can transfer the spacecraft or visit the module to the docking interface in any direction after capturing the spacecraft or visiting the module, which makes the cabin assembly and construction more flexible and has a broader expansion space.
Traditional rendezvous and docking still have advantages in terms of safety: the docking process can be evacuated at any time, and the spacecraft can be emergency separated at any time during the flight of the assembly, and only one side of the spacecraft can perform the abort docking or evacuation action. If the robot arm is used to assist the docking, the abnormal in the transfer process cannot be separated immediately, and the emergency evacuation process is much more complicated and slower. SpaceX makes reasonable use of two docking methods: the Cargo Dragon spacecraft meets and hovers and is captured by the mechanical arm and docked, and the Manned Dragon spacecraft directly rendezvous and docks.
With the advancement of technology, rendezvous and docking have developed more branch technologies with their own strengths to adapt to and meet more subdivided application needs, ensuring space tasks from routine world round-trip to the construction of complex space facilities.
Zhiduo D
Rendezvous docking "1 + 1 = 1" is a philosophical proposition
From the moment of the space station's coordinated orbit adjustment before the launch of the spacecraft, the rendezvous and docking aimed at the final docking began. In this process, the rendezvous flight gradually eliminates the deviation between the rocket launch and the orbital entry, as well as the deviation introduced by the orbital measurement and each orbital maneuver, and creates the initial conditions for docking at the end of the rendezvous; the docking process continues to eliminate the relative position, speed and attitude deviation of the two aircraft at the time of contact, buffer and consume the impact energy, and finally complete the physical connection, laying the foundation for the fusion of the "1 + 1 = 1" combination. From this we can see that --
Rendezvous and docking is a complex system that extends the distribution on spatial elements and dynamically develops on time coordinates, carrying systematic, systematic and related systematic scientific thinking.
Rendezvous and docking is a set of engineering design that achieves overall optimality through control-centered technology, and runs through the scientific method of system engineering to solve multi-factor, multi-constraint, multi-objective, multi-stage, and multi-variable problems.
Rendezvous and docking is an activity to build large-scale space facilities based on the laws of orbital science and space technology, which embodies the scientific practice of system philosophy, knowledge and practice, and interaction between body (structure) and function.
The Chinese space station, which shoulders the mission of exploring the above multi-dimensional dimensions, is rushing towards its scientific, technological and engineering goals, and extending our understanding of the world.
Co-ordinator: Zhou Quan Text: Xinhua News Agency