Since Shenzhou 15, China's space station has entered a long-term manned mode, and the spacecraft has been in orbit for a long time, and scientists have discovered a major flaw in the space station.
We all know that the three docking interfaces of the Tianhe core module are the forward docking interface, the backward docking interface and the radial docking interface, of which the backward docking interface is mainly used for the docking of cargo spacecraft, so under normal circumstances, the forward docking interface and the radial docking interface are usually left for the manned spacecraft.
A space rendezvous is two of these spacecraft, one of which is usually a space station, reaching both orbit and approach very close distances (e.g. internal visual contact). The orbital velocity and position vectors of the two spacecraft are then precisely matched, keeping them at a constant distance by orbital station holding. Docking or berthing between rendezvous, allowing the spacecraft to make physical contact and establish contact between them.
It docks with the core module through a forward docking interface, and the forward docking and backward docking are a stable holding point at 200 meters, even if the spacecraft engine is not turned on.
However, radial docking and docking is very difficult, and the spacecraft must rendezvous and dock vertically upwards in a vertical and radial way, from below and the combination. This technique is very difficult, because the radial interface is not on the same line as the center of mass of the space station, if it is directly docked, the larger impact energy will form a greater deflection moment, in addition, the spacecraft approaches from under the core module, and there is a deviation in the orbital altitude. Due to the deviation of the orbit height, the speed at different altitudes is not the same, and in the process of approaching the radial docking, in addition to the change of the speed position, the angle in the direction of the speed must be controlled. So the whole docking process is highly dynamic.
Because docking is in a position with asymmetrical mass, the impact inertia generated by docking will form a deflection moment on the space station assembly, which will affect its operating attitude, so the cargo spacecraft needs to fly around the transposition to the forward counter interface of the core module to carry out mass balancing of the assembly (for the radial docking task of the spacecraft), and at the same time, with the control moment gyroscope and attitude control power system of the core module, the attitude stability of the combination under the direct radial docking condition of the Shenzhou spacecraft can be realized.
Scientists found that after the Shenzhou spacecraft docked radially, it would be seriously obscured by leaning there, and the Shenzhou spacecraft power system has three power sources: solar cell sail plates, nickel-cadmium batteries, and emergency batteries. For more than 30 minutes after shading, the solar cell sail will stop working during the shadow period when there is no sunlight, and the power to maintain the normal operation of the spacecraft will come from the power stored in advance by the nickel-cadmium battery. In order to solve the problem of energy balance, after the successful rendezvous and docking, the space station needs to provide some electrical energy for the Shenzhou spacecraft.
We all know that nickel batteries have NiCd memory effect, nickel-cadmium batteries are not completely charged and discharged for a long time, and it is easy to leave traces in the battery, and then reduce the battery capacity. In addition, there will also be an unstable situation that produces continuous switching of charge and discharge states, which is manifested in irregular charge and discharge, as well as other complex working conditions such as non-charging and non-discharging.
In the past, the spacecraft did not stay in orbit for a long time, and this problem was not obvious, but now, the spacecraft needs to stay in orbit for a long time, and the space station has entered the long-term manned residence mode, and this problem is obvious.
In order to ensure the stability of the space station system, Shenzhou 18 ushered in a major upgrade, the battery was replaced with a lithium battery, the lithium-ion battery has high capacity, light weight and other excellent performance, a prominent advantage of lithium-ion battery compared with nickel-cadmium battery is that there is no memory effect, and the life advantage will also help the Shenzhou spacecraft to have a longer time to berth the space station assembly. According to the research of scientists, lithium batteries have a long cycle life, the number of cycles can reach more than 1000 times, and the service life can reach 5~10 years in the case of balanced charging and discharging, and the life span is about 2 times that of nickel-cadmium batteries.
However, because lithium batteries are less stable than nickel batteries due to high safety, high reliability, overcharge resistance, and overdischarge resistance, for example, Boeing's lithium-ion batteries used in 787 aircraft have had fire accidents due to battery overheating. Due to the particularity of aerospace, space power has strict requirements for the safety and reliability of lithium-ion batteries, especially in the field of manned spaceflight, the requirements for the reliability and safety of the power system are almost harsh, it can be said that the use of lithium batteries in space systems is a global problem.
Therefore, for a long time, the international space community has been very cautious about the use of lithium batteries, and it was not until 2013 that the International Space Station used lithium-ion batteries.
To this end, NASA has experts in the production process of lithium batteries to review the design, power supply, hazard control and other links, especially in the design process of strict supervision, to ensure that each individual battery has no design problems.
The use of lithium batteries in China's Shenzhou 18 also shows that China has overcome a series of global problems, redesigned the thermal management system for lithium-ion batteries, and the safety of long-life and large-capacity lithium-ion batteries has been widely verified.
It can be said that with the adoption of lithium batteries, this also means that Shenzhou 17 is the final battle of nickel-cadmium batteries. Therefore, the use of lithium batteries has saved considerable weight resources for the Shenzhou 18 spacecraft, and the maximum uplink and downlink capacity has been increased by nearly 300%, and the power system of the space station and spacecraft is also more stable, providing a higher guarantee for astronauts to stay in orbit.