Five years ago today, Chang'e 4 flew to the far side of the moon! How was the "auspicious date" for the mission selected?

In the early morning of December 8, 2018, accompanied by a loud roar, a beam of light tore a hole in the darkness of the Xichang Satellite Launch Center. Then, the light drew a beautiful parabola and flew into the sky.

That night, my country successfully launched the Chang'e-4 probe using the Long March 3B carrier rocket. This important historical moment was frozen at 2:23.

The Chang'e-4 probe was successfully launched by the Long March 3B carrier rocket

Just as weddings require auspicious days, the launch date of Chang'e 4 is also very particular. The launch time is determined by technical personnel after careful selection.

How are auspicious days determined within 4 times and 6 minutes over 2 days ?

Every rocket launch has a time range that is more suitable for launch. This range is determined after comprehensive analysis and calculation based on the requirements of the spacecraft itself and various external constraints (such as orbital requirements, etc.). The size of the range is also called the width of the launch window. The window width can be wide or narrow, ranging from hours or even days to tens of seconds or even zero.

Compared with ordinary launches, the selection of the lunar exploration launch window is much more complicated and difficult. "When selecting the launch window for the Chang'e-4 lunar exploration, many constraints need to be considered to ensure that the relative relationship between the spacecraft and the sun, the earth (including ground points and low-Earth orbit), and the moon (including the landing point) meets the requirements for landing on the far side of the moon," said Tan Yi, deputy director of the Long March 3A Series Model Office of the First Academy of China Aerospace Science and Technology Corporation.

First, the lunar orbit requires adjusting the perigee longitude and flight time so that the lunar orbit can fly over the landing point and create conditions for Chang'e 4 to land on the moon. Secondly, during the lunar flight, the exposure of the probe's solar panels to the sun needs to be considered. The angle between the incident direction of sunlight and the solar panels must be kept within a certain range to ensure the energy supply of the spacecraft. Finally, during the lunar flight and landing, the spacecraft is required to be able to continuously measure and control and maintain contact with the earth.

"The effects of many constraints are not the same and equal. Some constraints will determine the lunar launch window, while some constraints will determine the daily launch window. The launch window is the intersection of launch opportunities formed under the effect of these constraints. 'Chang'e 4' will land on the far side of the moon, and the various constraint relationships are more complicated, so the launch window is only a few minutes." Qin Yi said.

Schematic diagram of Chang'e 4 landing on the moon

After a lot of calculations, technicians determined four windows for the launch of Chang'e 4. Although there are four windows, they are distributed over two consecutive days, with two launch windows per day, 46 minutes apart, and the effective width of each window is 2 minutes and 1 minute respectively.

This means that the launch window for Chang'e 4 is only 6 minutes in total.

How to ensure that Chang'e-4 gains "launch freedom" and takes off on time ?

The narrow launch window of Chang'e 4 has posed an unprecedented challenge to the adaptability of the launch vehicle. In order to ensure that the rocket can be launched on time within the required launch window, the development team has improved both the orbit design and product reliability.

"On the one hand, we use orbit design technology that adapts to multi-window and narrow window constraints. On the other hand, we adopt reliability design and redundancy improvements on the main subsystems and important single machines of the rocket to ensure that the narrow window launch constraints are met." said Jin Zhiqiang, chief commander of the Long March 3B rocket.

At the same time, as a launch vehicle using liquid fuel, once the launch window is missed for some reason, it can only release the propellant and organize the launch again. In this way, the launch mission will be greatly delayed, which will bring irreparable losses to the entire project.

In order to ensure the launch of Chang'e 4 on schedule and meet the launch requirements of four windows in two days, the rocket control system adopted the "narrow window launch + multi-window switching" technology. "In short, it is to achieve the switching of different flight sequences within a day through the 'online binding' of the flight software parameters to meet the mission requirements of multi-window launch." Qin Yi said.

The flight software of the control system has been online bound, and the modified software can be uploaded directly to the onboard computer through the serial port before launch. The flight software uses multi-window state switching technology, and through the cooperation of the ground test and launch control system, it can switch between two sets of flight sequences in one day.

In addition, the overall development team of the Long March 3B rocket also proposed a technical solution of parking the rocket for 24 hours after filling with cryogenic propellant to enter the launch window the next day, which greatly improved the rocket's adaptability.