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Planetary Rovers: A Study on Wheels

A concise discourse contrasting the various wheel types a planetary rover might be designed to use

Introduction

The wheels on a rover that is tasked with traversing difficult terrain are a matter of great deliberation and consideration for engineers designing them.

For a large part, the wheels are responsible for the motion of the rover, along with which come traction, weight, size, shock absorption, and several other factors that need to be optimized, often with tradeoffs and compromises.

This article aims at compiling the many wheel types that have been used in the rovers of the past. 


The Rover Wheel Hall of Fame


Apollo LRV

This wheel was developed by NASA and Goodyear who worked together for the Apollo missions, aimed at increasing the astronauts’ range and ease of maneuverability on the lunar surface. The wheels had large flexible wire mesh wheels with stiff inner frames which held the wire mesh in place and limited the maximum deflection. Thin tread strips made of titanium and shaped like chevrons were attached to the carcass to enhance floatation in soft lunar soil yet still allow for flexibility, covering about 50% of the wheel contact area [1]. The seamless metallic mesh is made of zinc-coated piano string wire [2], which helps in absorbing shock during traversal on undulating and rocky terrain. 

The basic rubber-pneumatic design used on Earth does not have the same utility on the moon. Lunar tires need to be designed to develop traction on sandy undulated terrain, in regions that humans have never even seen up close. Plus, the prospect of an immobilizing 'flat tire' would be devastating to the mission.

-Vivake Asnani, principal investigator at NASA

This kind of wheel is what has come to be known as non-pneumatic (pneumatic meaning air-filled). NASA and Goodyear plan to work together to develop and study mesh wheels, to be used when astronauts return to the surface of the moon.

Pathfinder: Sojourner

The pathfinder was a microrover sent to the planet Mars, whose primary mission was to determine microrover performance in the then poorly understood planetary terrain of Mars. Made (more) popular by the movie The Martian, it employed wheels measuring 130 mm in diameter[3]. The soil-wheel interface had stainless steel cleats for better traction. 

The rover was the first wheeled rover to ever rove on a different planet. The six wheels were propelled by brushed motors coupled via planetary gears (2000:1)[4]. The wheels had a relatively simple design with no additional shock absorption capabilities, unlike its successors. There were several instruments to measure the soil-wheel interactions and provide data of wheel sinkage using various sensors, which would be valuable data for future missions[5]