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ROVER WHEELS: Monitoring changes over time, NOTE: Read back through the thread to avoid repeating misconceptions
fredk
post May 18 2013, 03:10 PM
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QUOTE (djellison @ May 17 2013, 09:36 PM) *
Once of the test-bed rovers at JPL had flight like wheels whilst dealing with 3x the effective weight of a real rover. The wheels were punctured, dent ridden, ripped, torn, dinged, bashed, smashed, crunched. You could put your finger thru the holes in places - you could see clean thru them.

And they still worked absolutely fine.

Here's a pic showing the VSTB rover with that damage. Worth a thousand words.
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djellison
post May 18 2013, 04:04 PM
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Wonder who took those. ph34r.gif

And the wheels got a lot worse than that before they were replaced. A LOT worse.

Worst one at that point looks to be the middle right - rips and tears all over it
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jmknapp
post May 18 2013, 08:24 PM
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Good thing Mars surface gravity is only 0.38g.


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testguru
post May 18 2013, 08:57 PM
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Given the level of damage observed so far on the rover wheels and the small distance traveled, if you linearly extrapolate future damage vs distance traveled, how far can the rover drive before a wheel fails? I know no one can answer that but I would assume modeling is being done to try to answer that question??

I would also assume the rover drives will be planned to avoid sharp rocks whenever possible?
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Jimbo1955
post May 18 2013, 09:27 PM
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QUOTE (djellison @ May 17 2013, 04:36 PM) *
It is. That 'hole' is a dent, the stress of the dent has popped the anodized coating off the inside of the wheel and we have bare aluminum reflecting the sun. That dent isn't in shadow. Those that are can reflect the bright martian sky off them. Just because something is in shadow - it doesn't mean it can't 'see the sky'.

Look at it - it's not even the same color as the terrain behind it.

And even if it IS punctured - it doesn't matter.

Once of the test-bed rovers at JPL had flight like wheels whilst dealing with 3x the effective weight of a real rover. The wheels were punctured, dent ridden, ripped, torn, dinged, bashed, smashed, crunched. You could put your finger thru the holes in places - you could see clean thru them.

And they still worked absolutely fine.

That testbed now has tougher wheels simply to deal with terrestrial gravity. The lightweight scarecrow rover has flightlike wheels.

I'm not sure how long it's going to take until saying 'the wheels are fine' before it gets boring. Infact, I think it might already have passed.


I certainly don't know the coating or type of specific anodizing used, but anodizing in general is not a coating that can be "popped" off. Anodizing involves an etching process process to create a sponge-like texture into the metal. Dyes or other other additives are infused into the spongy porous surface of the metal. Could the anodized metal have a coating applied over it? Sure. I would rather suspect that a gouge in the metal which removed the anodized metal to expose the un-anodized surface.

-J
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Explorer1
post May 18 2013, 09:32 PM
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Good thing there's no shortage of soft surfaces on the way to Mt. Sharp. The dune fields are looking inviting, ironically (obviously we haven't seen them from the surface yet so pure speculation on what they're like).
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mcaplinger
post May 19 2013, 01:00 AM
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QUOTE (Jimbo1955 @ May 18 2013, 02:27 PM) *
I certainly don't know the coating or type of specific anodizing used, but anodizing in general is not a coating that can be "popped" off.

Anodizing is an electrochemical process whereby the top layer of aluminum is converted to alumina (AlO2) -- http://en.wikipedia.org/wiki/Anodizing
There are several types and they tend to produce different thicknesses of alumina. In my experience, you can get a thin layer to pop off if you work at it, but I suspect that these dents are just specular highlights off the intact anodizing, since the black anodizing is not very matte.


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mcaplinger
post May 19 2013, 01:08 AM
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QUOTE (testguru @ May 18 2013, 01:57 PM) *
Given the level of damage observed so far on the rover wheels and the small distance traveled, if you linearly extrapolate future damage vs distance traveled, how far can the rover drive before a wheel fails?

I would also assume the rover drives will be planned to avoid sharp rocks whenever possible?

It's not really a linear process. The tire is one solid piece of aluminum. It would have to be ripped all the way across so it started unpeeling from the flexures before driving would be impeded. Like the viewgraphs I linked to earlier said, localized rupture in the tire was expected.

We're talking about sharp pebbles, I don't think driving around them would be feasible.


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mcaplinger
post May 19 2013, 01:10 AM
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Doug has seen the damaged VSTB wheels in person and I haven't, so he's in a better position than me to know if the anodizing is actually broken off or not.


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djellison
post May 19 2013, 01:15 AM
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QUOTE (testguru @ May 18 2013, 01:57 PM) *
Given the level of damage observed so far on the rover wheels and the small distance traveled, if you linearly extrapolate future damage vs distance traveled, how far can the rover drive before a wheel fails?


Further than the rover will ever drive. Period.
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Bill Harris
post May 19 2013, 01:46 AM
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The likely failure point will not be on the wheel tread surface itself, but at the annulus on the inner surface where the wheel spokes attach to the wheel.

--Bill


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mcaplinger
post May 19 2013, 02:18 AM
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QUOTE (Bill Harris @ May 18 2013, 06:46 PM) *
The likely failure point will not be on the wheel tread surface itself, but at the annulus on the inner surface where the wheel spokes attach to the wheel.

I'm not following your reasoning; the entire tire is one piece of aluminum, beefed up where the flexures attach.

Maybe the slide I referenced will be useful.
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serpens
post May 19 2013, 04:42 AM
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Cool design. Wheels are NOT a likely failure mode. How many ways need this be said before the subject is dropped?
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djellison
post May 19 2013, 05:31 AM
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QUOTE (serpens @ May 18 2013, 08:42 PM) *
How many ways need this be said before the subject is dropped?


Seemingly we're not done yet. Insert heavy to industrial strength 'sigh' here.
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stevesliva
post May 20 2013, 02:12 AM
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QUOTE (djellison @ May 19 2013, 12:31 AM) *
Seemingly we're not done yet. Insert heavy to industrial strength 'sigh' here.


Is that the sound of dust being wiped off Opportunity's solar panels?
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