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May 20, 2017 1 Comment
I once called tensioned wire wheels “the most ingenious contrivance in all of human engineering.”
Extraordinary claims must be based on extraordinary evidence (Carl Sagan) and tensioned wheels have it in spades. Think, more than two billion nearly identical wheels see daily use on our planet. There is no widely used structure with such astounding strength to weight, and they conquered gravel on the moon!
For space nerds, all the details of this vehicle are here. Lunokhod 1 was the first robot rover to move freely on an astronomical surface beyond earth, landing on the moon Nov 17, 1970. Over the next 322 days of operation it rolled 10K, shot over 20,000 images and performed soil analyses. It was active nearly 4X longer than planned.
Eight tensioned wire wheels were driven by dedicated electric motors. Four flanges, one broad rim with three spoke hole circles.
A French model maker and member of the French Space Authority, Vincent Meens, built an award winning model in 2005. He shares details not otherwise available. Here is his drawing of the wheels of Lunokhod 2, a similar vehicle that landed Jan 15, 1973.
Thanks to Vincent’s study, we know Lunokhod wheels have 48 spokes with external nipples. The wheelchair-style hubs are large flange (140mm dia). Each carries four flanges whose spokes connect to nipples in three hole tracks.
The outer flange has eight spokes reaching the outer nipple track. These are x1 lacing, four pairs. The center two flanges each have sixteen spokes. Both send eight spokes to a hole track to the left and eight to the right. The inner flange is like outer, sending its 8 spokes to the innermost nipple track.
The largest spacing of flanges is between the two center, making the spoke bracing for the center 16 nipples the most effective. The outer flange has essentially no bracing angle. As greatest lateral forces to wheelchair wheels are from the outside, this seems strange except to make the wheel as narrow as possible. Of course!
The vehicle weighs 756 kilograms (1,667 lb) and is 2.2m (7’) long and 1.6m (5’) wide. Each wheel is the diameter of a 20” BMX wheel with a 1.9” tire. Is anyone surprised BMX wheels were chosen to tackle gravel on the moon?
It would be great to know more. What are the materials? Predating aerospace carbon fiber, all is likely metal, probably much titanium. Spokes look like 12 or 13G with J-bend elbows. Hard to see nipples but they appear external with square drive. All is amazingly familiar.
So, tensioned wire wheels first imagined by Sir James Cayley for flying are now sitting on the moon. Those for the 2020 Mars rover don’t appear lighter or stronger than Lunokhod’s from nearly 40 years ago. They look designed to flex, an odd feature as the levers on which they move are already jointed to follow terrain at very low speed.
Would be great to have the Mars Mission team explain this design choice. A few of us might argue that tensioned spokes are lighter and better proven. But after all, this rover will not have much competition on the surface. It’s a race to Mars but once landed, more like a pub crawl!
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