How do spacecraft fly along their orbits? How do two spacecraft find each other? What are the things about space station rendezvous and docking?

The rendezvous and docking is when two spacecraft meet at the same time, at the same position in orbit, at the same speed and attitude, and are structurally connected as a whole. This is a key technology for building China's space station, a prerequisite for achieving "1+1=1", and one of the most complex technologies for spacecraft in orbit. It is divided into two stages: rendezvous and docking, which is called Rendezvous and Docking (RVD) in English.

Rendezvous comes from French. A foreign colleague told me that they also use rendezvous in daily conversation to express meeting someone at a certain place, but it must be to a relatively far place, at least another city or the other end of a city; meeting in the next room is not considered a rendezvous. From this point of view, rendezvous and docking means that spacecraft meet thousands of miles away and then connect and assemble together.

When the two spacecraft merge into one, the rendezvous and docking is complete. And the curtain of all this has already been opened before the rocket is launched. In terms of space, the elements involved in rendezvous and docking far exceed the docking spacecraft themselves; in terms of time, it is a process constructed by multiple dynamic steps in sequence.

The Tiangong is about to be completed. The result comes from the beginning, and the starting point leads to the end. The new instrument is promising. A small move can have a huge impact, and the whole plan will be a good one. The rendezvous and docking has made a powerful annotation for the system engineering of aerospace.

▲Animation of the construction of China's space station. Source: CCTV

Previous article: Intersection

01

Orbital laws lay the foundation for rendezvous

Why do spacecraft fly like this?

Spacecraft fly along orbits, which are regular. The orbital laws of celestial bodies are the basis for designing rendezvous and docking.

Orbital rule 1: The lower the orbit, the faster the angular velocity. The space station flies in an orbit at an altitude of about 400km, orbiting the earth once in 1.5 hours; the synchronous orbit satellite is at an altitude of 36,000km and orbits the earth once a day; the moon is at an altitude of 380,000km and orbits the earth once a month. So, as long as the spacecraft's orbit is kept lower than the space station, the spacecraft will "naturally" catch up with the space station at a faster angular velocity. During the tracking process, the spacecraft gradually raises its orbit, and its relative speed with the space station gradually decreases. When the orbital altitude of the spacecraft and the space station is the same, the relative speed between the two is zero, and docking can be achieved.

Rendezvous and docking is often likened to "threading a needle over a thousand miles". In fact, the distance is not proportional to the difficulty. Longer tracking distance does not necessarily consume more fuel. The key lies in accurately controlling the altitude difference during the flight and the timing of the spacecraft's gradual orbit raising. It is necessary to accurately determine the orbits of the two aircraft, know the relative position and speed of the two aircraft in real time, and accurately calculate and execute orbit control. These are the difficulties.

Orbital Law 2: Spacecraft on circular orbits perform approximately uniform circular motion. Uniform circular motion is not only beneficial for ground tracking and observation, but also combined with Orbital Law 1, it can be seen that when two spacecraft fly in circular orbits at the same altitude, their relative speed will continue to remain zero. This allows us to set orbital parking points for rendezvous and docking.

Orbital law three: orbit change maneuvers within the same orbital plane save energy compared to changing the orbital plane. The spacecraft flies at a high speed of about 7km/s in the orbit. Since the speed has directionality (i.e., the vector characteristic of speed), if its direction is to be changed to a limited extent, a speed increment of the same order of magnitude as the current speed is required to achieve it. However, under the law of universal gravitation, the orbital radius is inversely proportional to the square of the speed. If the original direction remains unchanged, a relatively small speed increment can achieve a significant altitude change within the same orbital plane. Taking a 400km orbit as an example, if the inclination is to be changed by 30°, the required speed increment is about 4km/s; while within the same orbital plane, only a speed increment of about 0.3km/s is needed to increase the orbit from 400km to 1000km. In order to make full use of this law, when planning rendezvous and docking, the spacecraft and the space station should be allowed to fly within the same orbital plane as much as possible throughout the entire process from takeoff to docking.

Orbital Rule 4: Different orbital planes mean that aircraft with intersecting orbits cannot obtain the same speed when they meet. Also because of the vector characteristics of speed, at the intersection of the orbits, two aircraft can reach the same position at the same time, but at this time their speed directions are different, and the relative speed cannot remain zero. Not only that, if we only observe the lateral relative speed perpendicular to the orbital plane, the relative speed at this intersection point happens to be the largest in the entire orbital cycle. If the relative speed of the two must be zero at this moment, it will consume a lot of energy to change the speed direction of one of them. In order to dock, the speed change process must be completed in a very short time, which is equivalent to steadily reducing the gradually increasing approach speed of the intersecting orbits. The control difficulty is relatively large, and once the control is not good, they will "naturally" collide. Therefore, if there is a deviation in the orbital plane of the two aircraft, it is necessary to try to correct one of them (usually the spacecraft) so that the two will eventually meet in the same orbital plane to create good initial conditions for docking.

▲ Illustration of the relative speed relationship between two spacecraft whose orbital planes intersect.

02

The starting point of the rendezvous journey: rocket launch

Why does spacecraft launch require a “zero window”?

Before launch, the rocket and the spacecraft inside the rocket remain on the surface of the earth. We can imagine that the earth is rotating with them. From the moment of takeoff, the rocket and the spacecraft no longer move with the earth, and they break away from the direct constraints of the earth's surface and fly independently into space. Therefore, the takeoff moment is the starting point for the spacecraft to enter the flight orbit. The accuracy of this moment determines whether the rocket is carried by the earth and deviates from the expected initial conditions.

Rockets have the ability to correct deviations. However, the deviation in the take-off time causes the deviation of the orbital plane, and the energy required for correction is large. Therefore, when planning the rendezvous and docking mission, the theoretical launch time should be designed through the precise measurement and prediction of the space station orbit in advance, and then the rocket should be coordinated with the ground to take off at the theoretical time as much as possible. This is the origin of the "zero width time window" (also called "point window" or "zero window") for spacecraft launch. After take-off, the rocket's control system will further correct the residual deviation during flight to ensure the accuracy of the orbital entry point.

▲On October 16, 2021, the Long March 2F carrier rocket was launched, sending the Shenzhou 13 manned spacecraft into space. Source: Xinhua News Agency

03

Rendezvous Step 2: Orbit Entry and Tracking

Why does the space station adjust its orbit before rendezvous?

The entry point is to send the spacecraft to a specific point that is on the same orbital plane as the space station and behind and below it. The subsequent spacecraft will gradually raise the orbit according to the planned orbit change strategy and catch up with the space station within the scheduled time. Therefore, the entry point is the design of the relative relationship (altitude difference and position difference) between the two spacecraft. Different relative relationships require different orbit change strategies for tracking, and a specific relative relationship can also have different tracking strategies - for the same tracking distance, the greater the proportion of flight time in a lower orbit, the faster the tracking and the shorter the total rendezvous time.

Since the two spacecraft are in a relative relationship, the space station can make corresponding adjustments in coordination with the rendezvous. The range of the rocket's orbital entry point is limited, so the most common coordination measure for the space station is to adjust the spacecraft's orbital angular velocity by raising and lowering the altitude before the launch, so that the relative positions of the two are exactly in a suitable range when the spacecraft enters orbit, which is conducive to the subsequent tracking flight of the spacecraft. If the space station does not adjust, when the spacecraft enters orbit, the space station may be anywhere from 0° to 360° in front of it. Of course, the two spacecraft are far apart, and the spacecraft can fly in a low orbit for a longer period of time. As long as it continues to be lower than the space station, it can always catch up.

Both options have their pros and cons. If the space station is adjusted, it is conducive to the spacecraft to rendezvous with a relatively fixed orbit change strategy, and the flight time is relatively fixed, which is more conducive to the consistency of flight procedures and space-ground coordination; if the space station is not adjusted, the spacecraft can be launched every day (as long as the launch time is guaranteed to be on the same orbital plane), and there are fewer constraints on the implementation of the mission, but the rendezvous time is uncertain, and it may be 1 to 5 days. Therefore, manned spacecraft usually adopt the former option, and the space station cooperates appropriately to ensure that the rendezvous time is not too long and is certain; while cargo spacecraft do not have strong constraints on the rendezvous time, and mostly use the latter option.

▲Sketch of radial rendezvous and docking of Shenzhou 13. Source: Academy of Space Technology

04

The third step of rendezvous: long-distance tracking and close-range approach

How do two aircraft find each other?

From far to near, the spacecraft tracks the space station.

When the distance between them is far, the project measures the orbits of the spacecraft and the space station respectively, determines their respective orbits independently, and formulates the orbit change strategy based on this. The real-time orbit can be measured and predicted by the ground station, or obtained through the satellite navigation data on the aircraft. The application of Beidou global navigation makes accurate and real-time orbit determination possible.

When the distance is close enough, the two aircraft can "echo" each other, and the relative position and speed between the two can be obtained through the measurement equipment installed on the spacecraft (radar, optical measurement equipment, etc.) and the corresponding cooperative targets configured on the space station (transponders, optical targets, etc.). At this time, there is no need to rely on the absolute data measured on the ground, but to perform orbit change calculations based on the relative orbital relationship. The reason for this choice is that the closer the distance, the higher the accuracy of the relative measurement; after the relative relationship of the orbit is linearly simplified, it can greatly reduce the amount of calculation while ensuring accuracy, and can be calculated autonomously in real time on orbit through the spacecraft's control computer, which improves the real-time nature of the disposal.

The last 100 to 200 meters of the rendezvous section is called the translational approach phase. At this time, although the two spacecraft are still flying independently according to their own orbital laws, the deviation between the orbits is already very small, and it no longer consumes too much energy to directly adjust the spacecraft to a similar straight-line flight action based on the relative relationship. Therefore, it is possible and necessary to perform 6-degree-of-freedom control in 3 directions and 3-axis attitudes in this interval to ensure that the spacecraft and the space station are consistent in not only position and relative speed at the moment of docking contact, but also in relative attitude and angular velocity. Only when the two are aligned can the rendezvous and docking enter the next stage, which is the mechanical assembly process of "docking".

▲Sketch of the close approach of Shenzhou 8 and Tiangong 1. Source: Xinhua News Agency

05

Bias Correction and Constraints

What is so difficult about orbital control?

From the launch of the rocket into orbit to the tracking and approach of the two spacecraft, every step is orderly. In actual flight, errors may occur at every step. Therefore, the flight orbit control planning needs to reserve the opportunity for orbit correction, perform real-time calculations based on the actual deviation, and decide whether to implement the correction. The measurement and calculation errors at all stages will be converted into errors in the orbit control parameters, and will be superimposed with the deviation in the execution of the orbit change, which will be reflected in the flight status after orbit control.

Therefore, when the spacecraft enters orbit, the engineer plans subsequent orbit changes based on the measured orbit to eliminate orbital deviations. After each orbit control, the orbit is re-measured, and the subsequent orbit change strategies and parameters are updated based on the current status to eliminate new deviations caused by the previous orbit change while completing the existing tracking mission.

“You cannot step into the same river twice.” This quote from an ancient Greek philosopher expresses the movement and change of all things in the universe. In this sense, every stage of a space mission, represented by rendezvous and docking, faces a completely new task.

In addition to planning according to the above principles to ensure the final docking accuracy, orbit control must also consume less fuel. Therefore, the change of orbit height is implemented at the apogee and perigee as much as possible, using Hohmann transfer to achieve energy optimization; the change of orbit plane is implemented at the intersection of the orbits as much as possible, saving fuel through the most efficient control.

There are two types of constraints that have a greater impact on the implementation of the orbit control process. One is technical conditions, such as the lack of orbit determination capabilities in the early days of aerospace engineering. The other is artificial safety measures, such as the rendezvous and docking process must be carried out within the arc visible to the measurement and control, so as to facilitate timely handling of faults and ensure safety. Constraints vary depending on the conditions and capabilities of the mission implementation, and are also lifted with technological progress and the improvement of autonomous control reliability.

In summary, spacecraft rendezvous is a typical multi-objective planning problem under constraints.

▲Shenzhou 8-Tiangong 1 rendezvous and docking orbit control diagram. Source: Xinhua News Agency

06

Rendezvous requires a stop

Why does the spacecraft stop and go?

The space station flies in a circular orbit. During the tracking process, if the spacecraft changes its orbit to reach a circular orbit at the same orbital height behind the space station, the relative distance and speed of the two spacecraft remain unchanged, and the spacecraft is "parked" relative to the space station. Such parking is guaranteed by the orbital law, that is, passive safety: as long as no action is taken, there is no risk of collision.

It is necessary to set a parking point during the rendezvous and docking flight, which is mainly used in the following operations or scenarios:

(1) Switching relative measurement sensors. It is difficult to track a spacecraft from hundreds of kilometers to docking with one set of equipment. Therefore, a safe parking point with a constant relative distance from the space station is the best place to switch equipment at different measurement distances. In other words, stop and change equipment.

(2) Fault handling. Typical faults such as sensor faults can be handled at the parking point. In fact, some rendezvous plans use the parking point as a point for full system status inspection, and only release the aircraft after confirming that everything is normal. In other words, stop and check.

(3) Docking time adjustment. If there is an error in orbit control execution, the flight time will also deviate from the expected time. Setting a parking point can "eat up" the previous flight time error to ensure that the subsequent steps are executed according to the scheduled time plan. In other words, stop to correct the deviation. For rendezvous plans with time constraints such as docking stage measurement and control visibility, this adjustment capability is very important.

(4) Solve the problem of optical sensors being disturbed by sunlight. In layman's terms, when the sunlight is dazzling, wait at the anchor point and leave after the sun has turned away.

The parking point can be set behind the space station or in front of it. To continue approaching the space station from the rear parking point, it is necessary to slightly lower the orbit, raise the orbit and park after catching up. To approach from the front, it is necessary to first raise the orbit, wait for the space station to get close, then lower the orbit and park, and repeat this process in the forward and reverse directions until entering the translation approach stage.

▲The parking point setting for the manual rendezvous and docking between Shenzhou 9 and Tiangong 1. Source: CCTV

07

Radial rendezvous has advantages and disadvantages

Why doesn't the spacecraft dock with the space station from the side?

In addition to using the parking points to approach the space station from the front and rear directions until the final docking, the spacecraft can also approach the space station from below the space station and upward along the radius of the earth until docking. On October 16, 2021, the Shenzhou 13 manned spacecraft was successfully launched and completed my country's first radial rendezvous and docking.

The two spacecraft in radial rendezvous remain in the same orbital plane, which is still ideal in terms of energy consumption and final docking conditions. Radial rendezvous allows the space station to increase its ability to receive visiting spacecraft without changing its flight attitude. At the same time, thanks to the clean space background, the spacecraft has good conditions for upward observation of the space station during radial rendezvous.

The difficulty of radial rendezvous is also caused by orbital laws. Because the spacecraft is always lower than the space station, it cannot use the orbital angular velocity characteristics to achieve passive parking. If parking is required, it must use fuel for continuous control. In addition, during radial rendezvous, the spacecraft is in an "upright" posture with its head pointing to the sky and its tail pointing to the ground. The layout of equipment such as earth sensors and measurement and control antennas that adapt to the normal posture of flying parallel to the ground needs to be specially designed or adjusted.

The forward, backward and radial directions within the same orbital plane are the common ports for the space station to receive visiting spacecraft, and are also the docking directions of the Tianzhou-2, Tianzhou-3 and Shenzhou-13 currently in orbit. In the fourth orbital law described in the first section of this article, the reason why lateral docking is usually not performed directly has been explained. In lateral rendezvous and docking, the two spacecraft are in different orbital planes, and the relative speed is the highest at the intersection of the two orbital planes. If the rendezvous and docking is implemented, the control is difficult and the safety is poor. Therefore, if the cabin needs to be finally connected to the lateral docking port, it is generally docked forward, backward or radially, and then "moved" to the side with the assistance of a robotic arm or a transfer mechanism.

▲Schematic diagram of spacecraft docked at the forward, rearward and radial ports of the Tianhe core module. Source: 36kr

08

Automatic and manual modes coexist

Why is manual rendezvous required under high-precision automatic control conditions?

There are two modes of intersection: automatic and manual.

The entire rendezvous flight is based on orbital calculations. Only when the relative motion of the spacecraft meets the astronauts' direct observation, posture feeling and control habits during the translational approach phase can the human-in-the-loop, that is, human participation in the control process, be realized. In fact, in order to ensure safety, even at this stage, the project will use an automatic control system to maintain the basic attitude of the spacecraft, and the astronauts only need to perform translational control and attitude adjustment on this basis.

However, a major advantage of the manual, or human-controlled, rendezvous mode is good control accuracy, which comes from the precise stereoscopic vision of the human eye and the fine control ability of the human brain and fingers. After training, astronauts can achieve extremely high observation and control accuracy. In the early days of rendezvous and docking technology verification, due to the technical level of measuring sensors, control computers and other equipment at the time, automatic control was not as accurate as human control. When the Soviet Union was testing a new docking mechanism, it used human control to complete the final rendezvous and docking operation in order to obtain better control accuracy.

The precision of modern automatic control is high enough and stable, but human control is still retained as a redundant means. This is because machines cannot replace the ability of people to deal with emergencies on site. If an abnormality occurs when two spacecraft are very close, the real-time intervention of the ground is not as good as that of the astronauts on site, and the astronauts can make comprehensive judgments and disposal of the situation, which is more conducive to ensuring safety. It is based on this advantage that the Soyuz T-13 astronauts achieved rendezvous and docking with the out-of-control Salyut-7 space station by manual operation, and then repaired and restored the space station. At that time, Salyut-7 was in a state of completely uncontrolled free drift (fortunately the angular velocity was not large). The Soyuz T-13 first circled around it and observed it, and then followed it while aiming at the docking port and approached and docked. For non-cooperative and uncontrolled targets such as Salyut-7, since its state is not known in advance, the final approach and docking cannot be designed and optimized using the orbital laws mentioned above. Only by judging and formulating solutions based on on-site observations can people overcome difficulties and successfully implement them.

▲Salyut 7 photographed by the Russian Soyuz T13 before docking. Source: arstechnica

09

From two days to 6.5 hours

How is rapid rendezvous achieved?

On June 17, 2021, the Shenzhou XII manned spacecraft formed a combination with the Tianhe core module, and the entire rendezvous and docking time was shortened to 6.5 hours from the usual two days required by my country's manned spacecraft in the past.

A fast rendezvous process means completing the required rendezvous and orbit change at a few orbital feature points within as few flight circles as possible. Therefore, a small number of planned orbit changes and a short interval between orbital controls can effectively shorten the rendezvous time. This in turn puts forward requirements for other conditions:

(1) The rocket has high orbital insertion accuracy. Since the amount of adjustment and correction required is small, there is no need to plan too many orbit control times.

(2) Real-time and accurate orbit determination. This condition has been achieved with the support of the BeiDou global navigation system.

(3) Real-time orbit control planning and accurate calculation. Under the premise that Beidou can provide real-time and accurate orbit determination, either the computing power of the spacecraft's onboard computer is high enough to independently plan and control the orbit; or the ground has ample time to inject orbit control parameters, and the injection time does not constitute a constraint.

(4) The orbit control accuracy is high enough so that no new deviation terms are generated, and the deviation is small enough not to exceed the planned adjustment capability.

Therefore, the realization of rapid rendezvous is the result of the overall capability improvement and coordinated guarantee of the large system composed of the ground, launch vehicle, aircraft, navigation and relay satellites, etc.

▲Shenzhou XII prepares to dock with the Tianhe core module. Source: CCTV

Next: Docking

01

Docking initial conditions

Under what circumstances can docking be achieved?

The end point of the rendezvous is the starting point of the docking. At this point, the lateral position and speed, three-axis attitude and angular velocity of the spacecraft relative to the space station are as close to zero as possible, and only the axial flight direction maintains the pre-designed approach speed. The project uses the state of these parameters as the condition for the start of docking. This condition is the rendezvous control target for the flight control system and the initial range to be adapted for the docking system. From the overall perspective of the system, the higher the accuracy of the rendezvous end point, the better, and the larger the tolerance range of the docking mechanism, the better. This is also the interface where the system design indicators need to leave a margin when allocating.

At this moment, the rendezvous system completed the "handover" and the baton of the rendezvous and docking mission was handed over to the docking system.

At the end of the rendezvous flight, the two spacecraft have achieved "1+1". The subsequent docking will make the two realize "=1" in the cabin structure, becoming the basis for the "=1" of cabin resources such as motion control, energy, information, and environment.

▲Shenzhou 10 rendezvoused and docked with Tiangong 1. Source: CCTV

02

From single spacecraft to complex

How many steps are needed for docking?

As a physical process of completing the mechanical connection between two aircraft and forming a rigid combination, docking mainly includes three steps.

(1) Contact, acceptance and geometric position correction.

The previous article mentioned the orbital corrections made to eliminate errors during the rendezvous flight. 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, they are aligned. But the deviation still exists. Therefore, after the docking mechanisms of the two spacecraft come into contact with each other, the first thing to do is to eliminate the initial deviation, allow the mechanical devices of both parties to accept each other, and correct the relative position relationship to achieve complete "alignment". This action is similar to the straightening action of aligning the screw hole when tightening a screw.

Traditional Chinese mortise and tenon joints are often used in building houses on Earth. A careful observation reveals that the head of the tenon is slightly thinner, while the entrance of the mortise is slightly wider. The contact surface structure of space docking is similar to a more precise mortise and tenon joint. Through special geometric guiding features, the two spacecraft docking mechanisms are closer and more aligned, so that they fit together perfectly. This form of acceptance and correction includes a rod-cone combination, a ring-cone combination, and a guide flap combination with a narrow outer surface and a wide inner surface. The common screw head and screw hole edge we see are a pair of cone surface combinations, and the guide flaps are like two hands with fingers spread apart and inserted into each other.

After the position correction, in order to prevent the relative relationship between the two spacecraft from changing, the capture mechanism will "grab" each other at this time so that they will not separate from each other.

▲Russia's rod-cone docking mechanism. Source: ESA

(2) Buffer and consume collision energy.

When high-speed, massive spacecraft come into contact with each other at a relatively low speed, the impact energy is considerable. At least one of the spacecraft and the space station needs to be equipped with a buffer and energy dissipation device to reduce the impact overload and dissipate or absorb the impact energy.

Spring damping and hydraulic servo mechanisms are buffering forms that have evolved with the development of docking technology. Research on electromagnetic damping devices has also emerged in recent years. Adaptive electromagnetic devices can combine the work of capture and buffering energy consumption. The more prominent advantage is that due to the addition of active control links, low-impact capture can be achieved, and the adjustment and control of electromagnetic parameters can adapt to a wider range of docking aircraft masses and initial docking 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 is equipped with a "passive docking mechanism" without a buffer system. The advantage of this is that there is no complex mechanism on the space station side, which is conducive to long-term flight; although the mechanism on the spacecraft side is complex, it is not difficult to design and maintain on orbit due to its short service life.

▲The buffer damping system on the Shenzhou 8 spacecraft. Source: Xinhua News Agency

(3) Mechanical connection.

After the collision energy of the two spacecraft is buffered and absorbed, the two docking end faces are pulled closer and closer, and then rigidly connected into one through a mechanical lock system. In addition to ensuring sufficient connection stiffness and load-bearing capacity, for manned spacecraft, it is also necessary to achieve sealing between the two spacecraft to ensure that personnel can travel through the docking channel of the two spacecraft. Similar to the configuration principle of the buffer system, a rubber sealing ring is usually configured on one side of the spacecraft, and a metal sealing surface is configured on the side of the space station.

The connection of cabin environment after docking has gone through an interesting development process. The first generation of docking mechanisms for manned spacecraft aimed at breaking through the rendezvous and docking technology, and did not consider the connection of sealed cabins. In other words, the docking mechanism is "solid" and fixed. On January 16, 1969, after the Soviet Union's Soyuz-4 and Soyuz-5 spacecraft successfully carried out the first manned rendezvous and docking, the astronauts reached the "next room" by exiting the cabin. The later second-generation rod-cone docking mechanism was designed to be flippable and disassembled after docking. Later, a peripheral docking mechanism appeared - the mechanism was arranged in a ring, and a hatch could be opened in the middle. After the active and passive docking mechanisms were docked, a docking channel was formed, which could build a sealed cabin environment directly connecting the two spacecraft.

▲Artistic picture of the docking of Soyuz IV and V. Source: Russianspacenews

At this point, the two spacecraft structures are firmly connected to form a combination, the electrical circuits and fluid paths are connected, and the manned environment is connected. The physical basis of "1+1=1" has been fully met.

At the same time, as a means of transportation between the earth and the sky and a non-permanently docked aircraft, the spacecraft needs to be reliably separated after the mission is completed. Therefore, the docking lock system can be locked and unlocked, and must be a mechanism that can move in reverse. In order to ensure the reliability of separation, some docking mechanisms are equipped with pyrotechnics on the lock system so that the connection part can be "exploded" in the event of a failure.

Usually, the spring mechanism provides the power for separation, which enables the two aircraft to have a certain initial separation speed. The key point of the spring mechanism design is to ensure that the separation force can be maintained after long-term compression, and with the assistance of the guide mechanism, the relative angular velocity of the two aircraft is small enough to separate safely in the form of translation.

▲The cargo Dragon spacecraft leaves the International Space Station. Source: NASA

03

Docking dynamics related issues

How to ensure that the spacecraft does not overturn the space station?

As mentioned earlier, docking will generate impact energy. In addition to the buffer and energy dissipation devices on the spacecraft, there are several other designs related to this issue in the space station project.

First, the buffer damping system configured on the active docking mechanism isolates the two aircraft themselves during the docking collision process, which actually has the effect of hitting the target with the equivalent dynamic characteristics of this system (rather than the characteristics of the entire aircraft). Therefore, by designing the dynamic parameters of this system, it can adapt to different docking targets and various docking initial conditions.

Second, in order not to interfere with the buffering and damping process, both spacecraft must stop attitude control after docking, and the combination is in a free drift state. At this time, the buffer system no longer has energy input, and only needs to consume the docking impact energy.

Third, a more difficult problem in docking dynamics is docking under eccentric conditions, which requires the docking mechanism to be able to withstand large eccentric rollover loads and absorb the energy input in this direction. In the cooperative project of docking between the US space shuttle and the Soviet Mir space station, the docking port of the space shuttle was set on the back, far away from the center of mass. In addition, the huge mass of the aircraft made it impossible for the existing docking mechanism to complete the docking under this condition. For this reason, the Soviet Union specially developed the APAS-89 docking mechanism, which for the first time adopted the inward-turning layout of the guide flaps to expand the size of the main structure and improve the load-bearing capacity, and connected electromagnetic dampers in series in the buffer system; the United States also modified the control scheme, and after docking contact, the translation engines at the head and tail of the space shuttle were used to cooperate to execute jet pulses to partially offset the rollover torque. With the technical cooperation of both sides, the space shuttle and Mir successfully docked many times.

Eccentric working conditions are common in radial docking. This is also the reason why the free drift deflection angle of the space station assembly during the attitude control period during the radial docking of my country's Shenzhou XIII spacecraft was much larger than the drift angle of previous axial dockings.

▲APAS-89 docking mechanism on the Mir space station and Mir-space shuttle docking. Source: NASA

04

The proposal and application of heteromorphism

Why don’t docking mechanisms look the same?

When a spacecraft and a space station dock, the mechanical docking devices on the two spacecraft are different, one is active and the other is passive. In the 1970s, researchers on docking mechanisms proposed a design concept: heteromorphism. The English word "androgynous" comes from Latin, which originally means hermaphrodite, and is still a term in zoology and botany.

The core of "heterogeneous bodies with the same structure" is that the docking mechanisms at the active and passive ends are exactly the same, and any two aircraft can dock with each other actively and passively; if it is fully realized, on-orbit aircraft can dock with each other at will, which can at least greatly facilitate mutual rescue.

▲Illustration of the concept of heterogeneity

The perfect concept of heteromorphic isomorphism has not been fully realized in the world's aerospace engineering, but it has been well applied locally in the reception and guidance correction devices of the docking mechanism. The Soviet docking mechanism mentioned in the previous section is named APAS (Androgynous Peripheral Attachment System), which can be translated as "hermaphroditic/heterogeneous peripheral docking system". Soviet designers made the geometric features of the conical guidance into an antisymmetric petal-like structure. When any pair of "flowers" face each other, their petals can be inserted into each other. The first generation of heteromorphic docking mechanism APAS-75 was used in the ASTP-75 Soyuz-Apollo docking project. The United States and the Soviet Union made the same outward-turned guide petals according to the agreed size specifications, and equipped them with buffer damping devices developed by each side. The spacecraft of both sides acted as active and passive to each other, and successfully achieved two "space handshakes".

This design effectively unified the main structural design of active/passive docking mechanisms and was accepted by developers from various countries. The Soviet/Russian docking mechanisms were upgraded to APAS-89 and APAS-95, which were divided into active and passive in the buffer device, but the guidance structure remained the same and is still in service on the International Space Station. The newly developed adaptive electromagnetic docking mechanism in Europe also uses similar guide flaps. my country's docking mechanism is also a heterogeneous isomorphic peripheral docking mechanism with inverted guide flaps.

▲The heterogeneous docking mechanism in the Soyuz-Apollo docking mission. Source: Mir Hardware Heritage

The Soviet Union/Russia and the United States have long tried to standardize and unify the docking mechanism standards, and have formulated docking interface standards after multiple rounds of discussions with the participating countries of the International Space Station. But in fact, this standard is not binding on all countries. Due to technical and non-technical reasons, even Russia and the United States themselves have not followed the standard. In addition, the development and use cycle of the docking mechanism is long. According to incomplete statistics, there are 4 mutually incompatible docking and berthing systems coexisting and providing services on the International Space Station alone, including 3 pairs of APAS-89 on the US side, more than 16 pairs of CMBs, and 13 "rod-cone" systems on the Russian side, including two incompatible modifications. A more realistic approach than solving the consistency of the docking interface is to use the docking mechanism of the docking cabin of the docking cabin. For example, the ATV cargo spacecraft developed by ESA is to dock with the Russian cabin, so it directly purchases and installs the Russian docking mechanism.

The "universal harmony" of docking mechanisms is ideal, and the more ideal situation is that docking mechanisms are not needed at all. When assembling cabin segments on the ground, the docking accuracy can be ensured by tooling equipment, and the screws can be directly tightened. However, in the sky, docking mechanisms must be used to compensate for the lack of assembly accuracy caused by space rendezvous deviation. In the future, when the rendezvous control accuracy is high enough, the docking mechanism can directly evolve into an automatic assembly mechanism to achieve more efficient space facility assembly.

▲The Russian-made docking mechanism on the European ATV cargo spacecraft. Source: ESA

05

Robotic arms as another docking option

Why does the traditional docking method still have advantages?

In early space activities, the orbit determination and autonomous measurement and control capabilities of the spacecraft were relatively weak. In order to achieve system goals, mature mechanical technology was used as much as possible to expand the tolerance of the docking mechanism. Therefore, the docking mechanisms at that time were similar to the rod-cone design, and the initial docking deviation could be as wide as 30cm. With the development of technology and the enhancement of orbit determination and control capabilities, the range of initial docking conditions has narrowed, and the docking mechanism can be made more sophisticated, reducing tolerances and guide structures, and reducing volume and weight. The precise rendezvous and impact energy is reduced, so the buffering and energy absorption device can also be simplified. As a result, the weak impact docking mechanism and the technology and application of docking after capture by the robotic arm have been developed.

The solution of capturing and then docking with the robotic arm actually sets the end point of the spacecraft rendezvous as a hovering point near the target, and controls the approach speed of the initial docking condition to zero. This solution gives full play to the functional performance advantages of the aircraft's high-precision motion control and the robotic arm, greatly reducing the requirements for the tolerance and buffering capacity of the docking mechanism. The robotic arm can serve as a universal tool for all visiting aircraft, and the docking mechanism of the visitors can be simplified and lightweight. Another unique advantage of this solution is that after the robotic arm captures the spacecraft or visiting module, it can transfer it to the docking port in any direction for docking, which provides more flexible options and broader expansion space for module assembly and construction.

Traditional rendezvous and docking still has advantages in terms of safety: the docking process can be evacuated at any time if an abnormality occurs, and the spacecraft can also be separated at any time during the flight of the combination, and only one spacecraft needs to terminate the docking or evacuate. If a robotic arm is used to assist in docking, an abnormality cannot be separated immediately during the transfer process, and the emergency evacuation process is much more complicated and slower. SpaceX makes reasonable use of two docking methods: the cargo Dragon spacecraft rendezvouses and hovers, and is captured by the robotic arm before docking, while the manned Dragon spacecraft rendezvouses and docks directly.

With the advancement of technology, rendezvous and docking has developed more branch technologies with their own strengths to adapt to and meet more subdivided application needs, ensuring space missions from routine round-trip between the earth and the sky to the construction of complex space facilities.

▲Schematic diagram of the Chinese space station's robotic arm grabbing the spacecraft and re-docking. Source: Science Popularization China

end:

Rendezvous and Docking in the Context of Engineering Philosophy

From the moment the space station coordinates the orbit adjustment before the launch of the spacecraft, the rendezvous and docking with the final docking as the goal begins. In this process, the rendezvous flight gradually eliminates the deviation between the rocket launch and the orbit entry, as well as the deviation introduced by the orbit measurement and each orbital maneuver, creating the initial conditions for docking at the end of the rendezvous; the docking process continues to eliminate the relative position, speed, and attitude deviations of the two spacecraft at the moment of contact, buffers and consumes the impact energy, and finally completes the physical connection, laying the foundation for the "1+1=1" combination fusion. It can be seen from this that -

Rendezvous and docking is a complex system that extends and distributes in spatial elements and develops dynamically in time coordinates. It carries the systematic scientific thinking of integrity, systematicity and correlation.

Rendezvous and docking is a set of engineering designs that achieve overall optimization through control-centric technology, and it incorporates 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 aerospace technology. It embodies the scientific practice of system philosophy of mutual growth of knowledge and action, and interaction between body (structure) and use (function).

The Chinese space station, which is responsible for the above multi-dimensional exploration mission, is moving towards its scientific, technological and engineering goals, and is also extending our understanding of the world.

Producer: Li Xiaoyun

Editor: Guo Jianwei Qi Lijun

Proofreading: Ma Yucong

Source: Xinhuanet Sike