MMX Locomotion Subsystem: mechanics for extraterrestrial low gravity drive

The advent of exploring low-gravity environments gives the opportunity to land rovers on celestial bodies without any landing platform and perform manipulative tasks under mostly unknown conditions. In addition to common loads, for example vibration, operation and thermal loads, the rover will face also impact loads during touchdown. This circumstance re-quires additional mechanisms to protect exposed parts, like the legs and wheels of a rover. Previous research attaches the wheels to the rover body or the landing platform through cup-cone interfaces at the wheel hub, which leads to unfavorable force distribution at the wheel rim in certain load cases, especially if the wheel represents the first point of contact during touchdown. This paper gives a detailed description in the mechanical design and testing of the locomotion subsystem (LSS) of the Martian Moons eXploration (MMX) rover. As the rover will fall to the moon Phobos unprotected and without any landing platform, the exposed locomotion subsystem has a high probability of being the initial contact point at touchdown. Besides the driv-etrains and thermal hardware, a novel hold down and release mechanism (HDRM) will be introduced as an integral part of the locomotion subsystem. The HDRM is realized using three support structures at the wheel rim and one fixation in the wheel axis. In this way, the exposed locomotion subsystem will be stabilized in described load cases, since each support structure forms a closed kinematic loop with the wheel and the central fixation in stowed configuration. This approach leads to vibration and impact resistant behavior.

1. Introduction

The sample return mission, Martian Moons eXploration (MMX), of the Japan Aerospace Agency (JAXA) carries a rover on its way, which will be deployed as a scout on Phobos to perform several scientific tasks during the mission. The deployment aims to ballistically descend the MMX rover on the surface of Phobos and start with its scientific objectives after uprighting with the MMX Rover's Locomotion Sub-system (LSS). The latter is a contribution by the Robotics and Mechatronics Center (RMC) of the German Aerospace Center (DLR) and objective of this paper. The subsystem is an integral part of the MMX rover and provides the link to interact physically with the surface of Phobos in milli-g environment. Not only is it a technical demonstration for driving at low gravity, in addition it serves as a sunpointing device for the solar arrays and supports multiple scientific objectives: e.g. the positioning of the internal scientific payload relative to the surface of Phobos or the direct interaction of the surface regolith to give informations on its characteristics.