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A flat tire is nuisance enough for the lucky motorist that has a spare, a jack, and a safe place to pull over. For an astronaut in a bulky spacesuit, exposed to the harsh lunar environment, it could be a mission wrecker—or worse.
That is one of the reasons that designers of tires for lunar rovers both past and future have sought to go airless, and a version now under development by Michelin, in cooperation with NASA, is no exception. Instead of an inflated tire surrounding a rigid wheel, the company designed an integrated lunar wheel with rings of flexible material extending from a hub of metal spokes, a concept based on
Michelin's terrestrial TWEEL. (The military has also explored the development of similar nonpneumatic tires.)
"Pneumatic tires are fine," says Michelin's Bart Thompson, an innovation engineer on the lunar wheel project. "But they have a single-point failure. All you have to do is have a small hole, a microscopic hole, and your structure just becomes inoperable."
But avoiding flats is just one reason for the lunar wheel's design. By eliminating rubber from the equation, the composite lunar wheel should be functional across the vast range of thermal extremes found on the moon, from the freezing nights to the searing heat of the lunar day.
And unlike the metal-weave airless tires on the Apollo moon rovers of the early 1970s, which traversed only a few dozen kilometers each during Apollos 15, 16 and 17, the new generation is designed for far more durability. Michelin is targeting a range of 10,000 kilometers, Thompson says. He adds that the ratio of load carried to wheel mass is more than three times that of the Apollo wheel, meaning the new design strikes a better balance between sturdiness and all-important launch weight.
The lunar wheel was put through its paces in August at the NASA Glenn Research Center in Cleveland on the Scarab rover, an unmanned prototype developed by Carnegie Mellon University. Scarab is a robotic rover designed as a prospecting and exploration platform, complete with a drill, that could investigate shadowed lunar craters where water may be frozen and other aspects of lunar geology. In the video below, Scarab traverses the test range encumbered by hundreds of kilograms of barbell weights, illustrating how the lunar wheel deforms under heavy loads.
Slide Show: Testing Moon Wheels
