Scientific soil sampling instruments for lunar or planetary applications typically consist of an excavation device that acquires the sample and an analysis device to determine its content. For transporting the soil sample from one device to another, one of the most prominent solutions is to guide it through a system of feed hoppers and tubes. This transport process mainly depends on the flow characteristics of the regolith, which are influenced by environmental factors such as reduced gravity or the lack of an atmosphere. Reduced flowability or clogging could impact the functionality and reliability of the entire instrument. To prevent such problems, the flow of lunar regolith under the special lunar or planetary environmental conditions needs to be understood.

To study the effect of gravity on the flow characteristics of regolith, a series of experiments was conducted on a partial gravity parabolic flight with 0.38 g Martian and 0.16 g lunar gravity. In the experiments representative hopper geometries were filled with lunar regolith simulants (JSC-1A and NU-LHT-2M, sample mass < 50 g, grain size < 2 mm), and the flow under reduced gravity was observed. Additionally, the hoppers were evacuated (ambient pressures between 10^-3 and 10⁰ kPa) to prevent gas inclusions in the porous simulants as well as overpressurization in the receiving volumes of the containers that would inhibit the flow. 21 different hoppers were designed similar to an hourglass, with two different funnel geometries on each side. With a thickness of 25 mm they represent quasi-2-dimensional hoppers. The inclination angles of the hoppers varied from 55 deg to 75 deg in 5 deg steps, both in symmetrical and asymmetrical configuration. In addition, the outlet sizes of the hoppers were varied, offering the three outlet widths of 8 mm, 13 mm, and 18 mm. The material flow was initiated by turning the hopper upside-down. During each reduced gravity phase (26 s for Mars and 33 s for Moon parabolas), the flow was repeated 2-9 times. Together with the reference measurements under 1 g, approximately 1000 measurements for all possible hopper configurations were obtained.

The results of the experiments suggest a direct proportional relation between gravity and flow rate, as well as outlet width and flow rate. It was observed that higher funnel inclination angles tend to lead to higher flow rates, although there were some exceptions to this rule for the highest inclination angles. Asymmetrical hoppers tended to produce well repeatable flow rates and reduced clogging. Arching effects that interrupted the flow and in some cases even lead to full blocking occurred randomly and mainly under lunar gravity. At increased ambient pressure (above approximately 0.3 kPa) the release of trapped gas was observed when pouring the sample through the hopper. Under reduced gravity, a significant increase of sample volume was noticed, indicating the lower gravity-dependent compaction.