This paper proposes a lunar night survival method for small rovers using an MLI (Multilayer Insulation) curtain system for long-term missions. Until recently, it was difficult to install RHU (Radioisotope Heating Units) or other temperature maintenance devices on small lunar rovers to enable lunar night survival, and so such rovers could only perform short two-week missions. Thermal analysis results show that small rovers could survive during lunar nights by moving into a shelter located inside the MLI curtain of the lander without mounting temperature maintenance devices. In order to enhance the feasibility of the MLI curtain system, we also propose ideas of a double-layer MLI and a rover configuration without solar cells.
Lunar landers and rovers that perform missions exceeding a month (one lunar day) are required to survive in a temperature environment of -190°C for two weeks without any supply of heat from external heat sources such as solar heat. As lunar landers or rovers cannot survive without a heat source during this period, most lunar exploration programs employ RHU (Radioisotope Heating Units) or RTG (Radioisotope Thermoelectric Generators) [
Specification of unmanned lunar rovers.
Project title (spacecraft/rover) | Luna 17/Lunokhod 1 [ |
Luna 21/Lunokhod 2 [ |
Chang’e-3/Yutu [ |
Chang’e-4/Yutu 2 | Chandrayaan-2 [ |
---|---|---|---|---|---|
Mission date | 1970.11 ~ 1971.9 | 1973.1 ~ 1973.5 | 2013.12 ~ 2016.8 | 2019.1~ | 2019.07 (plan) ~ 2 weeks |
Lander dry mass | N/A | 1814 kg | 1200 kg | 1200 kg [ |
1471 kg |
Rover mass | 756 kg [ |
840 kg | 120 kg | 140 kg [ |
27 kg |
Landing site | 38°N 35°W | 26°N 30°E | 44°N 20°W | 46°S 178°E [ |
71°S 23°E [ |
Country | USSR | USSR | China | China | India |
Survival method of the rover during lunar nights | RHU [ |
RHU [ |
RHU [ |
RHU | N/A |
With concerns of RHU and RTG regarding radioactive contamination, several efforts are being made to find other methods for lunar night survival. Balasubramaniam et al. [
Notsu et al. [
As of the time of this writing, survival methods for lunar nights that have been successfully proven still use RHU or RTG systems. Table
The subject of this study is a small rover with a mass range of 20~30 kg. For small lander and rover programs, it is difficult to set up an RHU or RTG system in small rovers. For this reason, small rovers were not able to survive lunar nights until recently. As shown in Table
This study proposes a lunar night survival method involving an MLI curtain system that enables small rovers of 20~30 kg to perform long-term missions. The system can be summarized as a small rover without RHUs that performs mission tasks during the daytime and continues to survive in an MLI curtain shelter on the lunar lander during lunar nights. The rover can perform short missions even during lunar nights according to power margins, and it can perform missions again upon the return of the lunar day. In order to prove the feasibility of the MLI curtain system for lunar night survival, this study introduces a thermal model of a lunar lander and a rover in a lunar thermal environment and presents the analysis results.
Okishio et al. [
MLI curtain system for the survival of a rover during lunar nights.
A rover in a shelter
A rover moving outside
A rover outside
The shape of the MLI curtain proposed in this study is similar to vertical blind curtains used in a typical home. The system has the advantage of allowing the rover to pass through the MLI curtain without assistance from any special opening/closing mechanisms. As shown in Figure
In this study, the temperature of the lander during lunar nights can be maintained using RHUs and a lid. Park et al. [
From a thermal control perspective, RHUs have the disadvantage of generating heat not only during lunar nights but also during lunar daytime. Although RTGs can produce electrical power even without sunlight, such devices have large volumes and significant weights. Considering the case of the Galileo probe [
Among the studies involving the thermal modeling of lunar regolith and the prediction of lunar surface temperatures by Vasavada et al. [
Thermal model variables for lunar surface modeling.
Modeling variables | Value |
---|---|
Moon surface IR emissivity | 0.97 |
Moon albedo [ |
|
Density of fluff (kg/m3) | 1000 |
Conductivity of fluff (W/m/K) | |
Density of regolith (kg/m3) | 2000 |
Conductivity of regolith (W/m/K) | |
Specific heat of fluff and regolith (J/kg/K) | 1050 |
Imposed heat flow (W/m2) | 0.031 |
“Fluff” in Table
Thermal model of lunar regolith.
In order to verify the lunar regolith thermal model, we used the LRO lunar surface temperature measurement data from Williams et al. [
Temperature calculation results of the lunar surface at various latitudes.
Result at latitude 0°
Result at latitude 30°
Result at latitude 60°
Result at latitude 85°
Figure
With the purpose of this study being to review a lunar night survival method for a small lunar rover, the thermal model of the lunar lander and the rover was made as simple as possible.
Table
Assumed specifications of the lunar lander and the rover.
Dry mass | 300 kg |
Size (L·W·H) | |
Specific heat | 900 kg/J/K |
Conductivity | 20 W/m/K |
Heat dissipation (day/night) | 120 W/10 W |
Operating temperature range | -20°C ~ 50°C |
Mass | 25 kg |
Size (L·W·H) | |
Specific heat | 900 kg/J/K |
Heat dissipation (day/night) | 15 W/0.5 W |
Operating temperature range | -15°C ~ 45°C |
The thermal model of the lunar lander was designed as a cubic shape consisting of 125 nodes with the width, length, and height being five nodes long. The thermal conductivity of the lander model was assumed as 20 W/m/K, which is 1/8 of the thermal conductivity of general aluminum, instead of the internal radiation heat exchange in the empty space inside the lunar lander. Table
Assumed power consumption of the lunar lander.
Unit | Day (mission profile) | Night (sleep mode) |
---|---|---|
Main computer | 40 W | 5 W |
Power converter & distributer | 40 W | 3 W |
Communication transponder | 20 W | 1 W |
Battery | 0 W | 1 W |
Location & attitude sensors | 5 W | 0 W |
Lander payloads | 15 W | 0 W |
Total | 120 W | 10 W |
As the average power consumption during a lunar night was assumed as 10 W in Table
As the assumed mass of the battery of this study was already high, it was difficult to increase the capacity of the battery to consume heater power during lunar nights. Therefore, it was concluded that the supply of the heat source for lunar night survival should solely depend on the RHU. If heater power is consumed for lunar night survival, an incremental increase in battery capacity of 360 Wh is required for the consumption of 1 W of heater power, which results in a 2.5 kg mass increase per watt. Such effects make it increasingly difficult to meet lander mass requirements.
Figure
Thermal model of the regolith, the lander, and the rover.
Overall thermal model
Modeling of the lander and the rover
As illustrated in Figure
As shown in Table
In this study, the model of the MLI uses the CR-model (conduction-radiation model) introduced by Krishnaprakas et al. [
In equation (
In this study, the radiator optical properties of the lander and the rover were 0.06 for solar absorption and 0.83 for infrared emissivity, as with the example of the UV reflective-coated OSR (Optical Solar Reflector) of Qioptiq [
In this study, the lunar exploration rover performs its missions outside the lunar lander during the lunar daytime, and moves back into the lunar lander MLI curtain during the lunar nighttime to survive, as described in the previous section. In addition, the lunar lander covers the radiator with a lid, which is a panel equipped with solar cells during lunar nights, similar to Lunokhod [
Mission configuration of a lunar lander and a rover.
Configuration during a lunar day
Configuration during a lunar night
The landing position of the lunar lander was set as the latitude 0° position of the Moon, where the temperature variation of the lunar surface is the most extreme. The solar heat flux was assumed as 1366 W/m2. In Figure
Location of lunar lander during 1 lunar day.
As described in the previous section, in order for a small lunar lander to survive in the cryogenic environment of lunar nights, it is efficient to supply heat through RHUs. From a thermal control perspective, RHUs have the disadvantage of continually dissipating heat during lunar days in addition to lunar nights. Therefore, the lunar lander should radiate the heat generated by the RHUs in addition to the heat of the lunar lander itself, and the heat dissipation area should be designed accordingly.
The procedure of designing the radiator area and the capacity of the RHU of a lunar lander is as follows. First, the radiator area of the lunar lander should be designed so that the temperature of the lunar lander does not exceed the maximum acceptance temperature during lunar days under the thermal environment described in Table
In this study, the lunar rover does not equip RHUs. The lunar rover moves into a shelter surrounded by an MLI curtain between the lunar lander and the lunar surface during lunar nights. The radiator of the lunar rover should be designed to ensure that the maximum temperature of the rover does not exceed the maximum acceptance temperature during lunar days.
The RHUs were evenly distributed inside the lunar lander, and heat load of the RHUs evenly distributed 20 watts on the 125 lander model nodes. Assuming that a single RHU weighs 40 grams and possesses a heat load of 0.9 watts [
Table
Thermal design result of the lunar lander and the rover.
Design category | Value |
---|---|
Radiator area of the lander | 3600 cm2 |
Radiator area of the rover | 640 cm2 |
Heat capacity of the RHUs in the lander | 20 Watt |
Figure
Temperature contours of the lunar lander and the rover at each location.
The solar panel was deployed after sunrise and stowed before sunset. The lunar rover moved outside the lunar lander after sunrise and returned inside the MLI curtain before sunset. In Figure
(a) Temperature trend of the lunar lander during a lunar day. (b) Temperature trend of the lunar lander during the convergence period. (c) Diagram of the lunar lander thermal model.
Figure
Figure
When the lander enters lunar nighttime, the lid covers the radiator of the lander, and the heat loss is limited to a minimum. From this point, heat exchange takes place inside the lander for a certain period of time, and the temperature gradient decreases between the lander nodes. This period was termed the “convergence period,” during which the temperatures of the lander nodes converge. The temperature trend during this period is shown in Figure
The highest temperature of the lunar regolith occurs at noon, whereas the highest temperature of the lander occurs between noon and sunset. This is caused by the fact that, although the solar heat input is greatest at noon, the temperature of the lander is further increased past noon as the solar heat input is greater than heat rejection for a certain period. The time at which the temperature of the lander reaches a peak varies depending on the thermal capacity of the lander and the environmental heat exchange situation in the lander.
Figure
Average temperature trends of the lunar lander and the rover during 1 lunar day.
Figure
In Figure
The temperature results of the lunar lander, shown in Figure
In this study, the rover was able to satisfy the acceptance temperature requirement during lunar nights by simply moving into a sheltered location within the MLI curtain of the lander without mounting thermal control devices such as a lid or RHUs. The results confirm that, with assistance from an MLI curtain in the lunar lander, a small rover is able to carry out long-term missions for periods longer than a month, similar to large-sized rovers.
In order to examine the efficiency of the lid and RHUs of the lander, the temperature results of the lander without the lid and RHUs were compared with the thermal analysis results of the lander described in the previous section, as shown in Figure
Case study on the efficiency of the lid and RHUs.
The important focal point of Figure
In the absence of a lid, the temperature drops rapidly after entering the lunar night, resulting in a larger difference between the maximum and minimum temperatures. Based on this result, an active device such as a lid is necessary for small lunar landers to survive lunar nights. Park et al. [
For cases with a lid but no RHUs, the difference between the maximum and the minimum temperatures is 80 K, which is greater than the requirement of 70 K of this study. However, the temperature requirement of the lunar lander may potentially be satisfied by efficiently arranging the internal components of an actual lunar lander or by optimizing the radiator area. However, this study could not make conclusions regarding this possibility as the lunar lander was modeled as a simple cube. Instead, a detailed study on this topic is suggested as a future research subject.
The temperature of the regolith inside the MLI curtain was compared to the temperature of the external regolith to investigate the heat protection effect of the MLI curtain of the lander on the lunar regolith. The results are presented in Figure
Comparison of the temperatures of the inner regolith and the outer regolith.
In Figure
Figure
In this study, an MLI curtain was used to achieve lunar night survival for a rover. The MLI curtain provides shelter to the rover to prevent radiative heat exchange with the outside environment and to perform heat exchange with the lander. MLI curtains should also allow the rover to pass freely and possess the ability to completely block external views. In Section
Concept configuration of a double layer MLI curtain.
In this study, it was assumed that the rover performs missions by charging power through solar panels, as in the case of rovers that were launched in the past. Based on the concept of sheltering the rover within the MLI curtain during lunar nights, removing of the solar panels and SAR (Solar Array Regulator) devices from the rover and, instead, charging energy from the lander may be considered.
Rovers that convert solar energy into electrical energy are unable to perform missions during lunar nights. Conversely, rovers that perform missions by charging energy from a lander are able to perform lunar night missions, depending on the available capacity of the lander battery. In addition, rovers that are not equipped with solar panels and SAR devices weigh less, making it easier to meet mass requirements. Therefore, by increasing the capacity of the battery by a certain mass, a rover can successfully perform lunar night missions. However, without solar panels on the rover, there exists the restriction of having to return to the lander to charge the rover battery if the remaining energy of the rover battery is insufficient, even during lunar days.
A rover that employs a battery-charging method and performs missions during lunar nights have significant advantages, as it is possible to assign larger mission goals in terms of scientific inquiry. The technology that enables a small rover to recognize the position of the charging device that is equipped in the lander reflects the automatic charging technology of commercialized robot cleaners for home use, which is a technology that can be easily implemented.
This paper proposed a lunar night survival method of small rovers for long-term missions and verified the proposed idea through thermal analyses. Various types of landers and rovers that have landed on the Moon have been investigated, and long-term missions were identified as difficult tasks for small rovers. This study proposed a MLI curtain system for the night survival of small rovers. A lunar regolith thermal model was established and compared with LRO measurement results for verification. The lunar lander thermal model was modeled with a simple cubic shape, and RHUs and a lid were combined as thermal designs to ensure that the lander could survive the lunar thermal environment and meet the acceptance temperature requirements. As a result, this study derived the optimized thermal design for a lander to survive lunar days and nights. A thermal model of the MLI curtain and the rover was developed, which is a key proposal of this study. Thermal analysis showed that the proposed night survival system involving an MLI curtain enables a small lunar rover to survive lunar nights without RHUs or other temperature maintenance devices.
The lunar lander and the rover were modeled as simple cubes in this study. Therefore, it is suggested for future studies to verify the details presented in this study using detailed thermal models of lunar landers and rovers that simulate electrical boxes and internal radiation heat exchanges. With further research on rovers that are not equipped with solar panels and instead supplied with power solely by a lander battery, lunar exploration missions can be advanced further to enable wider research areas such as lunar night periods.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that they have no competing interests.
This research was financially supported by the Korean Lunar Exploration Program of the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (MSIT) (NRF-2016M1A3A9005561).
This file includes the raw data of the charts presented in the manuscript.