How Scientists Designed The Orbit Of The Chang'E 5 Mission

The criteria (design constraints) used for the search of the launch windows included minimum delta-V budget, solar elevation of 30°–50° during the lunar surface operation, and reentry flight range of 5600 km–7100 km. The actual launch day was November 24, 2020. The flight sequence for the launch window, November 24, 2020, is displayed in the figure. CREDIT Space:Science & Technology

In the early morning of November 24, 2020, the Chang'E 5 lunar probe was launched from the Wenchang Space Launch Center and successfully executed a 23-day journey of lunar sample return (LSR) mission.

In a review paper recently published in Space: Science & Technology, Dr. Zhong-Sheng Wang and his colleagues from the Beijing Institute of Spacecraft System Engineering, addresses three key orbit design technologies in the Chang'E 5 mission, including orbit design for lunar orbit rendezvous and docking (RVD), orbit design for precision lunar landing and inclination optimization, orbit design for Moon-to-Earth transfer.

First, an overview of the Chang'E 5 mission profile is presented. The Chang'E 5 spacecraft was composed of four modules: an orbiter, a lander, an ascent module, and a reentry capsule. After the spacecraft completed a 112-hour Earth-to-Moon transfer, two lunar orbit insertion (LOI) maneuvers were conducted within a one-day interval between the two LOIs, followed by the process of which the spacecraft entering a near-circular lunar orbit with an altitude of approximately 200km.

After approximately 8 hours, the lander and ascent module assembly was separated from the orbiter and reentry capsule assembly. Approximately one day after the second LOI, the lander and ascent module assembly performed two consecutive descent maneuvers (DM) in order to lower its perilune altitude to 15km. One day after the DMs, the assembly made a powered descent to land at the expected location in the Storm Ocean.

After landing process, the lunar sample collecting operation was conducted. Two days later, the ascent module was launched from the lunar surface. Within only two days, four maneuvers were conducted by the ascent module before the initial aim point was achieved, when the ascent module was flying in a 210km×210km circular orbit, 50km ahead of the orbiter along-track.

The orbiter also performed four phasing maneuvers between the separation of the two assemblies and the lunar launch, in order to ensure that it arrived at the desired orbital location at the prescribed time of the initial aim point. The far-range, close-range, and docking operations were completed in approximately 3.5 hours after the initial aim point, followed by the transfer of the lunar sample from the ascent module to the reentry capsule.

Subsequently, the ascent module was separated from the orbiter and reentry capsule assembly and eventually collides with the Moon surface, whereas the assembly continued to fly in a circular lunar orbit of altitude 210 km. After approximately 5-6 days, two trans-Earth insertion (TEI) maneuvers were conducted with a one-day interval between them, and the orbiter and reentry capsule assembly enters the Moon-to-Earth transfer orbit. After 4-5 days, the reentry capsule was separated from the orbiter, reentered the atmosphere, and shortly landed in the chosen landing field of Inner Mongolia.

Afterwards, Dr. Wang focused on three main challenges in the orbit design for the precision lunar landing, the lunar orbit rendezvous and docking and the Moon-to-Earth transfer.

For a successful descent and precision landing at the desired location on the lunar surface, appropriate values of the altitude and argument of latitude (AOI) at the descent point have to be achieved, and when the lander touch downs on the lunar surface, the intended landing site reaches the descent trajectory plane with the spin of the Moon.

In the Chang'E 5 mission design, two key elements were identified in the orbit control strategy for precision lunar landing. One element is a two-to-two maneuver for targeting the descent point conditions to ensure the correct latitude of the landing site and descent point altitude are achieved, and the other element is an orbit plane adjustment maneuver for achieving the desired longitude of the landing site.

The objective of the orbit design for the RVD phasing stage of the Chang'E 5 mission was for the prescribed AOL to be achieved by the ascent module in the circular orbit of altitude 210km at the prescribed time of the initial aim point; in addition, the orbiter was required to execute several maneuvers before the lunar ascent of the ascent module to ensure that it arrives at the prescribed AOL in the circular orbit of altitude 200 km at the same prescribed time of the initial aim point; moreover, the ascent module must be 50km ahead of the orbiter along-track at the time of the initial aim point.

To overcome tracking difficulties and satisfy the design constraints, the lunar orbit design method based on tracking analysis is employed. Furthermore, the orbit scheme is optimized in terms of the number of maneuvers, maneuver sequence, and maneuver position to achieve the minimum total delta-V, considering the tracking constraints.

After the lunar orbit RVD operation is completed, the orbiter and reentry capsule assembly is flying in a near circular lunar orbit. After approximately six days, the assembly enters the escape trajectory by performing a few TEI maneuvers and continues to fly in the Moon-to-Earth trajectory. The optimization of the TEI strategy is a classical orbit design problem in lunar exploration missions and was critical to the accomplishment of the Chang'E 5 mission.

However, the structure of the orbiter, having a much lighter mass than that in the early mission stage, would be prone to dynamic excitation when subjected to a large thrust, which would lead to poor attitude control during the TEI maneuver and high risk of orbit control failure. Therefore, a two-impulse TEI strategy was chosen to be utilized, in which case a short thrust duration was expected for each maneuver when using an engine with a small thrust magnitude.

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