Thermal design and flight validation of the chang’e-4 relay satellite

The Chang’e-4 relay satellite operates in the HALO orbit at the Earth-Moon Lagrange L2 point, which can provide relay communication to the lander and the rover operating on the lunar far side to maintain their contacts with the earth and is a key link of the Chang’e-4 exploration mission. Unlike most LEO (Low Earth Orbit) satellites, the outer space environment of the Chang’e-4 relay satellite is much harsher. The Chang’e-4 relay satellite must suffer from a torrid environment that faces direct sunlight or direct exposure to 4K deep cold space in a shadow zone which may last longer than 6 hours. The Chang’e-4 relay satellite must ensure the operation and temperature control of instruments and equipment during the shadow zone with a limited battery level. This proposes high demands on the TCD (Thermal Control Design). The entire satellite resources were reasonably used for integrated TCD with limited battery level and limited weight. According to the working mode of the Chang’e-4 relay satellite and its torrid environment, its TCD was completed successfully, and on-orbit test data conformed to the expected results and theoretical calculation. TCD of the Chang’e-4 relay satellite has worked normally since the day that the Chang’e-4 relay satellite was launched into space.


Introduction
The Chang'e-4 relay satellite was launched successfully on May 21, 2018, and is now operating at the Earth-Moon Lagrange L2 point which is almost 460 thousand kilometers away from the earth [1] .It is now providing reliable long-distance data transmission between the TT&C stations on the earth and Chang'e-4 lunar probes, including the lander and the rover, which were launched on December 8, 2018, and landed as planned on the far side of the moon on January 3, 2019 [2] .
The external heat flux of spacecraft is the main heat source received by spacecraft in space and is one of the main factors affecting spacecraft temperature.For LEO satellites, the external heat flux is not only from the sun but also from the earth (infrared and reflection), and it is in a good environment in orbit [3] .
The Chang'e-4 relay satellite faced challenges such as a novel mission orbit, large changes in external heat flux, and a long shadow zone.TCD was required to maintain the equipment within the appropriate temperature range throughout all mission phases, despite the limited resources of energy, weight, and strict temperature requirements for the equipment in the face of large variations in heat flux and multiple operational modes.This study analyzes the characteristics of TCD of the Chang'e-4 relay satellite based on the space environment during its operation.We propose a TCD scheme and analyze the results of the on-orbit test.

Mission features
Due to the Chang'e-4 relay satellite's unusual orbit, operation mode, and project restrictions， Thermal Control Design (TCD) had to consider the following features and requirements: The configuration of the Chang'e-4 relay satellite is shown in Figure 1, and the large parabolic antenna is installed on the +Z side.The size of the antenna is much larger than that of the satellite, which significantly affects the thermal status of the satellite.The Chang'e-4 relay satellite is far from both the earth and the moon in orbit, with only solar heat flux.The external heat flux of the Chang'e-4 relay satellite varies dramatically, and it completes a cycle of external heat flux changes in about 29 days.Except for the ±Y surface, the temperature control of the equipment outside the satellite in orbit is affected by two extreme states: direct sunlight or direct exposure to the cold space [4][5][6] .
The Chang'e-4 relay satellites will be affected by shadows caused by the moon or the earth, with the longest shadow zone lasting 6 hours or more.The satellite is directly exposed to the deep cold space at 4K.How to maintain the temperature of the equipment under the limited power resources during the long shadow zone and achieve a smooth transition is a challenge.

Cooling surface design
For the Chang'e-4 relay satellite, all operational modes are long-term working states.The heat consumption is more than 700 W during the relay communication mode, while it is only 150 W during the long shadow zone (including compensation heating power).TCD must be able to adapt to the impact of changes in equipment operational modes of the Chang'e-4 relay satellite.

Thermal control design scheme
Because the large antenna is equipped on the +Z surface of the Chang'e-4 relay satellite, the +Z direction does not select the AC cooling surface.The +Y and -Y directions have no sunlight exposure and are selected as the main cooling surfaces.Except for cooling surfaces and field of view requirements, all other surfaces are covered with MLI (multi-layer insulation) to minimize the external heat flux changes on the satellite.
To ensure that equipment is in the appropriate temperature range, it is necessary to accurately calculate the cooling surface area.The calculation results are then incorporated into the simulation calculation for verification, and the final confirmation of the cooling surface area is made.In estimation, the surface with MLI absorbs and radiates negligible heat, receiving external heat flow through the cooling surface and radiating heat into space.According to the above condition, the heat Q 1 absorbed by the cooling surface is: where Q内 is internal heat, q 1 is the solar heat flux, is the solar absorption rate of the cooling surface, ε is the emissivity of the cooling surface, and A is the area of the cooling surface.
The heat Q 2 radiated by the cooling surface is: where σ is the Stefan Boltzmann constant.
In the thermal balance, there is: We perform generation selection analysis on the estimated results in simulation calculation, and the final determined cooling surface is shown in Table 1 and Figure 2

Thermal control design of the large parabolic antenna
The composition of the parabolic antenna is shown in Figure 3.The large parabolic antenna has no internal heat source, which is exposed to space and has a complex structure.The antenna is a vital payload for achieving the relay communication mission of the Chang'e-4 relay satellite, and the success of its thermal control design ensures the success of the mission.
The thermal environment of the antenna is very harsh, with temperatures ranging up to 300℃.The thermal expansion coefficients of various components that make up the antenna are different, which can cause thermal deformation of the large antenna in orbit, leading to a decrease in antenna gain and affecting antenna performance.TCD is to ensure that all components of the antenna are always within the required temperature index range and that thermal deformation does not affect the normal operation of the antenna system [7] .
The calculation of the antenna performance changes caused by thermal loads is an important part of the antenna design process.Due to the large volume and complex configuration of the antenna, it is very difficult to achieve ground thermal test verification, especially the antenna thermal deformation.This work can only be obtained through analysis and calculation.The design calculation of the antenna in orbit thermal deformation is as follows: A multi-level temperature control loop design is adopted to cope with the long-term ultra-low temperature environment during the shadow period in orbit.The purpose of analyzing the thermal deformation of the antenna in orbit is to combine the TCD of the antenna design and consider them together.The calculation results are used to guide the material selection of the antenna.This ensures that the performance of the antenna can still meet the design requirements under the action of thermal loads in orbit.
Due to the complexity of the antenna thermal analysis calculation model, it is difficult to obtain the thermal performance parameters of various materials.Therefore, the key thermophysical parameters related to the antenna need to be obtained through ground experiments, which requires the design of specialized experiments to obtain the key parameters.The thermal analysis calculation of the antenna is through the temperature field distribution and its changes in orbit with the external heat flow.This is a process of repeated iterations such as antenna design, thermal analysis calculation, and thermal deformation analysis [8][9] .
After completing the antenna design and thermal design, the temperature field and thermal deformation cloud map of the maximum thermal deformation of the antenna in orbit are iteratively calculated, as shown in Figure 4 [10] .

Multi-stage temperature control circuit design
Due to the limited energy and control resources of the Chang'e-4 relay satellite, it is impossible to increase the number and power of heating circuits indefinitely to cope with the complex thermal environments.To ensure that the Chang'e-4 relay satellite safely navigates through the long shadow zone, the active TCD is optimized, and different temperature control strategies are adopted according to different thermal environments.The multi-stage temperature control circuit design is used, and the selection of temperature control points and heating power are both obtained through multiple iterations of thermal analysis calculations.
Multi-stage heat control circuit design is used, and the active temperature control mode of the satellite is set to three modes: normal mode, pre-shadow mode, and shadow mode, which is shown in Table 2.
Considering the limited energy, a special design is carried out for the weak points of heat leakage during the shadow: (1) During the shadow, the thermal control focuses on ensuring the safety of the battery, propulsion system, and platform equipment that need to remain operation, while the temperature of other equipment can be maintained at the storage temperature; (2) We focus on the difference in the power between the main and backup heating circuits of the equipment, as the backup circuit has a higher power.The active temperature control is implemented by using the main circuit in the normal mode and pre-shadow mode, and the backup heating circuit is switched to the shadow mode to ensure the temperature of the equipment that needs to be focused on during the shadow zone.

Design of shadow crossing strategy
During the transit of the Chang'e-4 relay satellite shadow, how to ensure energy and temperature safety under limited energy conditions has been specially designed for thermal control.The strategy is designed as follows: Before entering the shadow zone, about 14 hours in advance, the heating circuit is switched from normal mode to pre-shadow mode for heating.At the same time, the high heat-consuming equipment on the satellite is turned on to raise the temperature of the Chang'e-4 relay satellite.
After the satellite enters the shadow zone, the heating circuit on the satellite is switched to shadow mode, mainly to ensure the temperature level of critical equipment such as the storage tank, pipeline, and battery.
Approximately 30 minutes after the satellite exits the shadow zone, the heating circuit is switched to normal mode, and some compensating heaters begin to work.The temperature of the equipment on the satellite gradually rises, and the equipment can be turned on only when its temperature satisfies the working temperature.

On-orbit test
The Chang'e-4 relay satellite was launched on May 21, 2018, and has been in orbit for more than five years.The temperature of the Chang'e-4 relay satellite's equipment in orbit meets the requirements.The specific analysis of the in-orbit performance of the TCD is as follows: For the LEO satellite, the degradation of the thermal control coating outside the satellite will cause the equipment temperature to gradually increase.However, for the Chang'e-4 relay satellite, the degradation of the thermal control coating is slower, combined with the fact that the main cooling surface has no sunlight exposure.The Chang'e-4 relay satellite has been working normally for more than five years.The maximum temperature of the equipment of the Chang'e-4 relay satellite in orbit only increased by less than 3℃.For example, the two platform equipment, the central control unit, and the momentum wheel, had the highest temperature during the stage of the Chang'e-4 landing on the moon when the Chang'e-4 relay satellite was under full load operation.Later, they entered a stable working state, and the temperature curve changes of the two equipment in orbit are shown in Figure 5.
The lowest temperature of the Chang'e-4 relay satellite appears about 20 minutes after the end of the shadow zone because of the influence of the overall heat capacity of the satellite.After the Chang'e-4 relay satellite enters the shadow zone, the temperature of the equipment begins to rapidly decrease, especially for the equipment installed on the main cooling surface, such as the payload B, which reaches -32.8℃.The typical temperature curve of the Chang'e-4 relay satellite's equipment during the 6-hour shadow zone is shown in Figure 6.
The highest temperature of the Chang'e-4 relay satellite occurs when the satellite is in full load working mode, and the temperature of the equipment was relatively high, with some close to the upper limit of its working temperature.The equipment with the highest temperature is the FOG, and the highest temperature of its mounting surface is 50.0℃, which has reached its upper limit of the working temperature of 50℃.The in-orbit temperature of the satellite is approximately 5℃ higher than that of the thermal analysis.Especially when the angle between the sunlight and the satellite's +Z-axis is between 150° and 160°, it is the highest temperature condition of the satellite.The main reasons are as follows: 1) Due to the presence of a large parabolic antenna, the size of the antenna is too large, and the antenna has a significant thermal impact on the satellite.
2) The metal mesh surface that makes up the antenna has a large area ratio, and the metal mesh is woven from gold-plated molybdenum wire with a dense weave.In the thermal analysis calculation, the impact of the metal mesh on various thermal properties parameters (infrared emissivity, solar absorption, transmittance, etc.) under direct sunlight only considers the state when sunlight directly strikes the metal mesh surface, but does not consider the actual situation of sunlight incidence and reflection on the metal mesh surface.During the in-orbit period, sunlight enters at different angles, and some metal mesh surfaces with gold-plated molybdenum wire form a surface similar to gold-plating, with very little sunlight transmission, reflecting the satellite and causing an increase in external heat flow received by the satellite surface.Especially when the angle between sunlight and the satellite's +Z-axis is between 150° and 160°, the reflected external heat flow from the mesh surface of the antenna is the largest.According to the temperature analysis, the thermal analysis model has been modified.In this state, part of the metal mesh surface of the antenna needs to be treated as having no transmission of sunlight, and only about one-sixth of the area can be analyzed under direct sunlight.
3) Considering the limited resources of the Chang'e-4 relay satellite, the high-temperature working conditions of the mode and the low-temperature conditions during the shadow passage need to be considered in the thermal control design.The temperature target values for the equipment under low and high-temperature conditions are close to the upper and lower limits of their temperature.During the in-orbit period, due to the reflection of the large parabolic antenna, the temperature of the satellite is higher than the expected value, which requires the thermal analysis model to be modified to accurately predict the temperature changes of the satellite.Overall, the TCD of the Chang'e-4 relay satellite has been proven to be effective during its long-term orbit, ensuring the stable operation of the satellite and providing vital support for the Chang'e-4 lunar exploration mission.

Conclusions
The Chang'e-4 relay satellite has been working normally for more than five years.Reliability measures and targeted design performance have been confirmed on orbit.The paper focuses on the harsh environment of China's first lunar relay satellite, combined with the characteristics and difficulties, and completes the design of the thermal control subsystem.It proposes a multi-stage heat control circuit design to solve the problem of the long shadow of the Chang'e-4 relay satellite.This technology can be applied as a reference for similar satellites.
For satellites with large antennas installed, it is necessary to consider the thermal impact of the antenna's reflection of sunlight on the satellite and conduct sensitivity thermal analysis to ensure that thermal analysis calculation fully covers all operating conditions in orbit.
The Chang'e-7 relay satellite is a relay satellite operating in a large elliptical orbit on the moon and will be launched in 2024.The successful application of TCD on the Chang'e-4 relay satellite provides an important reference for TCD of the Chang'e-7 relay satellite.

Figure 2 .
Figure 2. Cooling surface and thermal analysis model of satellite.

Figure 3 .
Figure 3. Configuration of the parabolic antenna.

Figure 4 .
Figure 4. Temperature cloud map and thermal deformation cloud map of the parabolic antenna.

Figure 5 .
Figure 5. Temperature curve of typical equipment in orbit of the Chang'e-4 relay satellite.The in-orbit temperature of the satellite is approximately 5℃ higher than that of the thermal analysis.Especially when the angle between the sunlight and the satellite's +Z-axis is between 150° and 160°, it is the highest temperature condition of the satellite.The main reasons are as follows:1) Due to the presence of a large parabolic antenna, the size of the antenna is too large, and the antenna has a significant thermal impact on the satellite.2)The metal mesh surface that makes up the antenna has a large area ratio, and the metal mesh is woven from gold-plated molybdenum wire with a dense weave.In the thermal analysis calculation, the impact of the metal mesh on various thermal properties parameters (infrared emissivity, solar

Figure 6 .
Figure 6.Temperature curve of typical equipment of the Chang'e-4 relay satellite during shadow.

Table 1 .
. The cooling surface of Chang'e-4 relay satellite.

Table 2 .
Multi-stage temperature control circuit.