MERLIN Scoop Design

Last crawled date: 1 year, 10 months ago

In the design of an extraterrestrial sampling system, it is critical that the following two items are specifically addressed.

1. The geophysical properties of the regolith targeted for collection
2. The environmental properties of the planet for collection

In the case of lunar sample collection targeted by RASSOR, two problems need to be addressed that will otherwise render a successful terrestrial design insufficient and/or incapable of collection on the moon.

1. Testing with properly baked out KSC-1 Lunar Regolith reveal higher values of cohesion (exacerbated by the triboelectric charging from particle agitation) and internal friction (likely due to particle topology). Said plainly: particles will stick and clump together inside the collection drum. Additionally, with a cohesion over 1.0 kPA (KCS-1), particles will not necessarily flow like dry sand, instead behaving like fully saturated wet sand. “After the first Surveyor landing, it was said that “the lunar soil behaves like damp beach sand”, even though everyone knew full well there was no water on the Moon. “ [1]

2. Due to the reduced lunar gravitational field (compared to earth), the centripetal forces present in the drum cannot be ignored. At the design specification of 20 rpm, the force of gravity is only 1.6 times the centripetal force present in the drum. When accounting for the significant electrostatics that will be present with fine particles in zero atmosphere, the centripetal force can overwhelm gravitational forces. Particles will adhere to the outside wall of the collection drum, even at TDC of a drums rotation.

When considering the above two points, many designs that have sample storage in the same internal cavity as sample collection will not be able to reach maximum fill ratios. Once you reach a minimum level of sample collected, that sample will clump to the outside walls of the drum preventing additional sample from being collected.

In addition to the special requirements of a lunar collection system, there are general guidelines than all extraterrestrial collection devices should implement to maximize sample flow and minimize clogging.

1. Maximum particle size able to be collected must be strictly controlled in two dimensions (length & width). If a particle/sample/rock is collected that is too large, it can become permanently jammed inside the device. By controlling length and width, the only rogue sample possible can be a cigar shaped sample. If only one dimension is controlled, disks are able to enter the system easily which quickly can lead to jamming and device failure.
2. Once maximum collection size (length and width) are controlled, internal cavities must be 3x that size in cross section. This prevents three particle bridge clogging in internal passageways.
3. No converging or diverging internal cavities and/or passageways. This minimizes particle clogging/clumping that can then be packed in with a Morse taper effect.
4. No relative internal sliding surface, moving parts, or mechanisms within the sample flow. These will ALWAYS jam.

Proposed Design Solution

The design outlined in the attached PDF and part files proposes a solution that addresses the above concerns. As each batch of sample is collected, sample is immediately diverted through an Archimedes screw to a central holding chamber. The motion to this chamber is helped by both gravitational and centripetal forces. The metering effect of the screw reduces the opportunity for sample charging and clumping as well as removing it from preventing additional sample from being collected.

Sample collection and handling is done passively through geometry with no jammable sliding surfaces or mechanisms. Maximum particle size of 10x10mm is controlled at the inlet to each scoop by steel wire. Between 13-16.6 liters of regolith is held within the central holding chamber with an additional 4.6-5.8 liters held in the Archimedes screw as overflow after the central chamber has filled. Total sample captured is 19.6-22.4 liters. Design can be further optimized to increase this range.

No attempt at mass optimization or DFM has been made, however the design is straightforward and would be easily to manufacture from anodized 6061 or tiodized 6AL-4V and be less than 5kg.

[1] https://www.lpi.usra.edu/lunar/surface/carrier_lunar_soils.pdf