A Mass Optimisation Study of the Lunar Zebro Chassis

Abstract

A prototype lunar rover is in development by students of the TU Delft since 2017. It is a nanorover based on the terrestrial ZeBRo design, now named the Lunar Zebro. The Lunar Zebro is a prototype design as a proof of concept for nanorover capabilities. With a chassis of 200 by 140 by 60 millimeters, fitting on a sheet of A5 paper, the Lunar Zebro is intended to be the smallest and lightest autonomous rover on the Moon to date. The objective is to traverse a distance of 200 meters during a lunar day, surviving the harsh environment and strong solar radiation. Due to the limited time budget there is little refinement in the structural design. The resulting functional but heavy design leaves many opportunities for optimization.
While there are many examples of successful planetary rover missions, little is published concerning the design of the structures. This report contains further analysis of the design of satellite structures. The various structure types and design requirements highlight the importance of thermal transport and resistance to mechanical launch loads. Compared to the deployed planetary rovers, the Lunar Zebro is unique in many ways. The small size facilitates production of the current monolithic chassis which is ideal in its thermal conduction and environmental sealing properties. However with a constant plate thickness and no reinforcing substructure, the structure is not an efficient loadbearing design. Due to the many requirements and unique mission profile of the Lunar Zebro, there is no clear method by which to optimise the structure.
To better understand the current structure and reduce the mass, a case study is performed with Finite Element Methods. After validating a modelling approach for thin plate reinforcement, a simplified chassis structure is generated. Maintaining the essential configuration of the chassis and connected components, the response to the static launch load of 10G is analysed. Several methods for rib placement design are tested while reducing the plate thickness. Buckling behaviour and CNC production limitations are accounted for in this approach. To minimally affect the other design requirements, the stiffness of the structure is maintained. While the placement of ribs is sensitive to the vicinity of connected components, equally stiff designs can be obtained with reinforcement grids. Reducing the plate thickness by 66.6%, a mass reduction in the order of 50% can be achieved without sacrificing stiffness. However, local adjustments are required to prevent plastic deformation in high stress areas.
From there the analysis and design of reinforcement grids is investigated further. Grids are often seen in aerospace applications due to the convenient geometries for CNC production, light weight and predicable orthotropic or isotropic behaviour. A smeared stiffness approach is investigated that relates the rib and plate interaction to composite plate theory. Applying this analysis method provides beneficial insight in the parameters and related stiffness behaviour of a grid reinforced plate. By modelling three common grid sections on a hypothetical plate design scenario with varying boundary conditions, the important criteria for the selection and design of a reinforcement grid are provided.