Lunar Regolith

Research on Lunar Regolith Pneumatic Excavation and Delivery

 

Project Background
       We have to collect, excavate and convey the lunar regolith when it comes to lunar exploration, lunar resource exploitation and lunar station construction. Due to  the high vacuum, low gravity, and large temperature difference between day and night, as well as the great friction of the lunar soil on the surface of the moon, cost limitation of conveyance, and capacity limitation of transportation, conventional gravity type of excavation and blasting excavation are inappropriate. As a result, new excavation method is needed.
       In 1994, Schaefer et al. proposed a new way to collect soil sample from Mars, which involved compressed gas. Zacny applied this method to the excavation of lunar regolith, and solved the excavation problem in the complex lunar situation. The basic pneumatic excavating principle is shown in Figure 1. The compressed gas expands quickly when getting in a vacuum. The gas molecules and regolith particles collide with each other, and produce an explosion effect. The regolith particles then get a high speed, and go into the collecting device through excavating pipe with the gas molecules.

 

 

Figure 1. Basic pneumatic excavating principle

 

Significance of this Research
       With the new boom of lunar exploration, China has developed the “Chang E” Lunar Exploration Project. By now, the task of the second phase has been successfully completed. The main task of the third phase is to fetch samples from the moon.
       Pneumatic excavating method makes good use of the high vacuum condition, and is able to handle the great friction of the lunar soil on the moon with simple devices and a few mechanical components, and is high reliable. It’s significant to the lunar sample collection, the construction of lunar station and the national space exploration.

 

 

Figure 2. Imaginary photo of lunar station

 

Research Contents
       Pneumatic excavation and conveyance of lunar regolith involve flow field, gravitational field on the moon as well as temperature field. Such a multi-field coupling problem is complex, and many factors will have impacts on the efficiency of the excavation. At present, researchers are working on pneumatic experiments and numerical simulation, as shown in Figure 3 below.

 


Figure 3. Technology Roadmap

 

Research Achievements to Date
       Because of the complex condition on the surface of the moon, we mainly focus on the mechanism of the pneumatic excavating method, the simulation of the surface on the moon and the affecting factors of the excavation efficiency. The main achievements include:
1. Early in our research, we designed a set of vacuum pneumatic excavating test apparatus, and analyzed factors like the form of gas injection, burial depth of excavating pipes, gas pressure and vacuum degree through orthogonal test.

 

Figure 4. Test apparatus

 

2. Based on aerodynamics, we analyzed the process of vacuum jetting. We also simulated the gas flow field in high-vacuum condition using FLUENT, a fluid dynamics analysis software. We can see from Figure 5 that the compressed gas (N2) can reach the speed of 896m/s when expanding to the vacuum. With the minimum speed of the compressed gas at 425m/s, the regolith particles will get a high kinetic energy when colliding with the molecules of the compressed gas, which demonstrates the feasibility and high-efficiency of the pneumatic excavating method.

 

(a) Distribution of gas flow rate

(b) Streamlines

 Figure 5. Flow field analysis


3. We tried to work out the influence of the burial depth on the pneumatic excavating method. The result shows that the efficiency of gas weight will increase with the burial depth, just as Figure 6 shows. If the burial depth is shallow, the conveying speed will be high, but the efficiency of the gas weight will not be high.

 

(a) 8-cm-diameter excavating pipe

(b) 10-cm-diameter excavating pipe

Figure 6. Relation between the efficiency of gas weight and burial depth of excavating pipe

 

4. We studied the impact of the compressed gas pressure on the pneumatic excavating method by excavating expriments. As Figure 7 shows, the efficiency of gas weight increases with the gas pressure, and is closely related to the diameter of the excavating pipe.

 

(a) 8-cm-diameter excavating pipe

(b) 10-cm-diameter excavating pipe

Figure 7. Relation between the efficiency of gas weight and compressed gas pressure


5. We studied the impact of the excavating pipe diameter on the pneumatic excavating method by pneumatic excavating expriments. As Figure 8 shows, when the pressure of the compressed gas is low and the diameter of the excavating pipe is 8cm, or when the pressure of the compressed gas is high and the diameter of the excavating pipe is 10cm, the efficiency of gas weight is relatively high.

 

 

(a) 156KPa gas pressure

(b) 261KPa gas pressure

Figure 8. Relation between the efficiency of gas weight and diameter of excavating pipe

 

6.    We studied on the coupling relationship among the burial depth of the excavating pipe, the compressed gas pressure and the efficiency of gas weight for comprehensive consideration, as shown in Figure 9.
 

 

(a) 8-cm-diameter excavating pipe

 

(b) 10-cm-diameter excavating pipe

 

(c) 12-cm-diameter excavating pipe

   Figure 9. Coupling relation of multi-factor

   
7. On the basis of laboratory experiments, we used two-fluid model to numerically simulate pneumatic excavation using FLUENT, and we discovered the trend in which the volume fraction of lunar regolith changes with the space position, as shown in Figure 10.

 

 

t=0s

t=0.1s

t=0.2s

 

t=0.3s    t=0.4s    t=0.5s    t=0.6s

 

t=0.7s     t=0.8s     t=0.9s    t=1.0s
Figure 10. Volume fraction of solid particles

                                 

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