RASSOR drum challenge

grabcad

The design submitted for the RASSOR challenge is intended to be constructed from 3D printed titanium for the drum body and flat carbon fiber for the end plates for ease of construction in low gravity environments found on the Moon. Due to the Moon's low gravity, I provided a full 180° rotation before the scoop flutes end, as well as a small angled lip to prevent previously gathered regolith from spilling out as the scoops begin to engage on the next rotation. A 270° or even 360° rotation for each flute was considered, but it was decided that this likely wouldn’t increase capacity enough to justify the increased weight. Instead, I believe continuing to rotate the drum until the rover reaches the dump site would help contain the excavated regolith without adding weight. With no provisions for a drive system, this provides an interior volume of approximately 43 liters and a weight of 4.1 kg at the design limits of 450 mm diameter and 360 mm in length. When running a stress simulation of the assembly, it was assumed that a full load of regolith would be present in the drum and the full weight of that load would be exerted on the carbon fiber end plate at the end of the drum (this simulation was run without gravity forces accounted for due to the inability to alter the gravitational constant for the simulation). With a density of about 1.5 g per cm3 for lunar regolith and a 360 mm lever arm, this works out to a force of about 23.2 kgf or 228 N. By using fixed constraints on the face of one of the end plates, the simulation calculated a minimum safety factor of 15, indicating the possibility of reducing mass without affecting performance if needed or to simply reduce launch and landing costs. A proof of concept model has also been made, in this case 3D printed out of PLA with a 100 mm outside diameter, an overall length of 99 mm and an internal volume of about a little over 430 cc. When testing the original design, which had a scoop throat height of about 6 mm, it was found that the diatomaceous earth used as an easily available lunar regolith substitute clogged without being properly funneled into the interior volume. To try to fix this, a second proof of concept was made with the same outside dimensions and an increased scoop throat height of about 11 mm and a reduced interior volume of around 320 cc, which resulted in about 340 grams being transferred during a second test using playground sand as a better lunar regolith simulant. At a density of 1.6 grams/cc, this works out to 212.5 cc, or about 66% of the interior volume filled, despite some spillage when transferring to the scale. With a better drive system to keep the drum turning during transit, this might be improved to over 70%. In a full-sized model, a drum with this fill ratio could be expected to carry close to or more than 30 liters of regolith. Based on the second test, a possible improvement would be to increase the throat height of the full-size drum to prevent clogging in a similar fashion to the proof of concept model. This was done by decreasing the radius of the circle used to construct the outside edge of the scoop flutes. As in the proof of concept model, increasing the throat height would lead to increased weight, however given the roughly 900 grams left in the weight allowance specified, this should be easily possible without exceeding the 5 kg limit.