HLS FLOAT

HLS FLOAT (Human Landing System Footpad Landing Gear Optimization for Artemis Terrain) is the project I worked on as a NASA intern during the Summer of 2021. NASA's HLS program is developing updated technology for humanity's return to the Moons surface in the upcoming Artemis missions.

In previous Apollo missions, the footpads that supported the Lunar Module (LM) were made from aluminum sandwich structure that were composed of an aluminum face sheets and a honeycomb structure in between. Our team explored the use of auxetic honeycomb meta-material as the primary infrastructure that makes up a footpad. Auxetic honeycomb meta-material’s Negative Poisson Ratio characteristics enhance the footpad’s capability to absorb impact when colliding with the lunar surface and various regolith. The cells that make up the auxetic honeycomb invite the use of novel 3D printing and additive manufacturing methods due to its complex geometry and allows for experimentation on material selection that can provide further weight savings. With NASA’s current interest in landing humans back onto the moon and establishing a permanent sustainable presence via the Artemis Program, the Human Landing System (HLS) will be landing on the Lunar South Pole where there are high areas of interest for cultivating and observing volatiles such as water trapped in regolith or ice. The Lunar South Pole’s terrain differs considerably from the landing sites of the Apollo Missions in terms of ruggedness. There is evidence of larger lunar regolith in the South Pole which can pose a great threat to a lunar lander’s structural integrity and overall stability. Auxetic meta-material honeycomb structure meets the challenges of this new terrain by allowing for the footpad’s structure to absorb impact forces more efficiently as the geometry “flows” directly to the impact zone. The 3D printed manufacturability of these auxetic footpads allow for rapid prototyping and manufacture and are a novel approach to fabricating these landing gear components. There is also an interest in in-situ resource utilization of lunar regolith for additive manufacturing that can be utilized in rapid repair and replaceability of these footpad structures for lunar lander reusability and reducing overall costs in having to send part component replacements via payload on resupply missions

The Human Landing System’s footpad structure is the first primary structure part that contacts the lunar surface. It is of upmost importance to maintain the structural integrity of the footpad when contacting the lunar surface during a vertical landing as well as when the footpad collides with lunar regolith horizontally. Mission failure results when the footpad is not able to effectively distribute impact loading leading to damage of the landing gear system “upstream” as well as allowing for the lander to tip over. An auxetic meta-material honeycomb can aid in impact absorption with its unique Negative Poisson Ratio behavior within the baseplate of the footpad structure. The conventional way of manufacturing sandwich structure uses a hexagonal honeycomb core in between two metal face sheets typically made of aluminum alloy. Auxetic meta-material properties are not achieved through the material itself but through the geometric cell shapes and arrangements. The use of re-entrant walls, meaning the walls go inwards into the geometry, allows for this Negative Poisson Ratio behavior where pulling on the structure elongates it laterally, versus its conventional way of thinning out and compressing locally. This will allow for the structure to migrate toward the impact location rather than the conventional dispersion of material. This behavior was examined through Creo Simulate Finite Element Analysis where a re-entrant “bow-tie” hexagon honeycomb was compared to a conventional hexagon honeycomb. The footpad was modeled with both honeycombs inside the shell of the footpad baseplate and constrained at the top of the footpad structure restrict translational motion but allow for rotational motion to simulate a ball joint. Vertical and horizontal loading cases were considered which were determined by empirical data of Apollo Mission of lunar module terminal velocities. What was observed is that during vertical impact both footpad structures were able to distribute stresses fairly similarly but during horizontal impact conventional hexagon honeycomb structure had indented inward while auxetic “bow-tie” honeycomb migrated and twisted to the impact site. The following was an example of two-dimensional enhanced impact absorption which is beneficial if we know the general direction the impact loading will be coming from.

Since topological features cannot be predicted for most spacecraft landing scenarios, omni-directional qualities of auxetic meta-material for of the footpad were explored. revolved lattices solved the issue of creating an omni- directional structure that conformed to the circular nature of the footpad. These revolved honeycomb lattices proposed manufacturing challenges due to their closed loop structure that creates inaccessible internal voids within a completed part. Additive manufacturing was explored due to the complex lattice geometry that would be very difficult to machine via traditional methods. Chiral honeycomb lattices were ideal candidates for additive manufacturing because the structure has no overhang angles less than 45 degrees from the base, there was no need to print the piece with support structures to achieve the finished part. Since the part is a closed loop, supports inside the voids of the lattice structure would be inaccessible for removal upon print completion in-turn adding permanent non-functioning material and increasing overall footpad mass. Auxetic behavior of this honeycomb also aids in the manufacturing process by reducing internal stress during the 3D printing process. Chiral lattices have a cylindrical component at every node where the walls of the cell meet. This allows for enhanced stiffness along with a rotational property to conform to impact. These cells are comprised of centrally offset equilateral triangles with each vertex tangentially coinciding at a circle. These can be in either trigonal formation or hexagonal if stacked. Samples that were comprised of these chiral honeycomb lattices made of aluminum 6061 alloy were simulated in Creo Simulate under beam loading conditions and a 350 kN load to simulate uneven landing. They exhibited auxetic behavior and ability to distribute stress evenly. With such dense and complex geometry, 3D modeling and simulation was difficult to achieve for overall impact behavior visualization on the entire footpad structure. This limitation initiated the need for rapid prototyping via additive manufacturing to observe load impact behavior along with auxetic honeycomb structure fabrication feasibility. Mainly two types of printers could be used to manufacture the footpad which include: Selective Laser Sintering (SLS) printers and Fused Deposition Modeling (FDM) printers. SLS printers are capable of creating solid metal parts, such as aluminum, a very appealing candidate. FDM printers allow for the use composites such as carbon fiber or Kevlar infused filaments. The most feasible way to fabricate the auxetic footpad design based on current TRL levels would be to build the footpad in sections similar to that of a sandwich: a bottom shell, middle auxetic lattice core, and a top rib, spoke, ball-joint section. Utilizing NASA Langley Research Center’s ISAAC (Integrated Structural Assembly of Advanced Composites) the full-scale bottom shell of the footpad. The top section of the footpad’s geometry is much more complex, with tight corners and curvatures the method of hand laying composite plies according to a Creo model that’s fed through FiberSim or similar program in conjunction with a Laser Projection system for accurate manual layup of composite plies. All of this could be assembled with a 3D-printed auxetic honeycomb core. The core can also be divided up into interlocking sections that allow for future replaceability of portions of the footpad, so the entire structure is not lost upon technical error or excessive impact.
Auxetic meta-material honeycomb’s Negative Poisson Ratio behavior is not a property determined by the material the cells are made of but by the geometric arrangement of the lattice structures. Versus standard hexagonal honeycomb that have no internal support structure and may indent upon impact, the geometry of a re-entrant “bow-tie” hexagonal honeycomb (i.e. the walls that make up the cell point inwards) allows for the array of cells to elongate when a tensile force is applied and contract locally when there is an impact force from a collision. This Negative Poisson Ratio behavior is exhibited two-dimensionally via “bow-tie” honeycomb structures but can be further expanded three-dimensionally through a revolved chiral-hexagonal and chiral-trigonal honeycomb lattice for omni-directional improved impact absorption capabilities. Ideally, both types of patterns can be used in the baseplate infrastructure and placed in areas where we expect either pure vertical or horizontal impact loading and in areas where there will be a mix of loading conditions.
There is a growing interest of the private sector with producing lunar landing systems. Recent examples include SpaceX, Dynetics, and Blue Origin competing for a NASA contract to land humans on the moon during the Artemis Missions and Astrobotic Technology having a contract develop a lander to deliver various payloads for the Commercial Lunar Payload Services (CLPS) Program. As more landers come to and from the lunar surface, auxetic meta-material footpads could be utilized by their landing system to provide enhanced impact absorption capabilities at rough terrains. Replaceability and rapid production will play a key role in future lunar missions because of the sustainable base that will be established. If the auxetic footpad is manufactured in sections, components such as the baseplate honeycomb core could be 3D printed and replaced on the lunar surface with a “plug and play” capability which in turn could reduce overall mission cost of landers and increase overall mission

See our PowerPoint presentation below for further detail and illustration of our simulation results!