Author Topic: NASA - Perseverance, Mars 2020 Rover : Discussion  (Read 553239 times)

Offline savuporo

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Re: NASA - Perseverance, Mars 2020 Rover : Discussion
« Reply #80 on: 05/23/2016 09:48 pm »

What, exactly, in the electronics requires heating?  Is it just capacitors?  Sensors calibrated for certain temp range?  Most electronic components do very well down into the cryogenic temperature range.

How is JWST handling this?  I thought nearly all of its components were running at very cold temps.


Normal temperature for military grade electronics ( aka 'COTS' ) is -55C to 125C at best. First issues would be batteries, capacitors, but semiconductors will start to not behave at low temperatures as well. Unless they are specially designed low-temperature parts like 500-nano BiCMOS

Here is a good read on this: link

« Last Edit: 05/23/2016 09:55 pm by savuporo »
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Offline a_langwich

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Re: NASA - Perseverance, Mars 2020 Rover : Discussion
« Reply #81 on: 05/23/2016 09:55 pm »
What about the RTG? 20 years after manufacture that should still be producing 72% of its original power if it ages in the same way that the Viking ones did. Heat output should be 85% of new.

Are you using actual power levels for the MMRTG being built for Mars 2020?

Remember that the last Pu-238 manufactured in the U.S. was made in 1988.


Yes, but every atom of Pu-238 has as much potential energy as any atom of Pu-238 has ever had.  The half-life says which atoms of Pu-238 decay to something else, but if it's still Pu-238, it's got as much energy as it ever had. 

I think I made that more confusing, sorry.

So if 20% of your sample has decayed, then you have 80% Pu-238, and 20% U-238 (the decay product), and you can separate out the U-238 and get a smaller amount of pure, hasn't-yet-decayed Pu-238. 

That's the re-blending process that's done, I believe.

Offline Robotbeat

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Re: NASA - Perseverance, Mars 2020 Rover : Discussion
« Reply #82 on: 05/24/2016 04:42 am »

What, exactly, in the electronics requires heating?  Is it just capacitors?  Sensors calibrated for certain temp range?  Most electronic components do very well down into the cryogenic temperature range.

How is JWST handling this?  I thought nearly all of its components were running at very cold temps.


Normal temperature for military grade electronics ( aka 'COTS' ) is -55C to 125C at best. First issues would be batteries, capacitors, but semiconductors will start to not behave at low temperatures as well. Unless they are specially designed low-temperature parts like 500-nano BiCMOS

Here is a good read on this: link
Mismatches in the PCB coefficients of thermal expansion can cause solder joints to strain and fail.
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Offline ccdengr

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Re: NASA - Perseverance, Mars 2020 Rover : Discussion
« Reply #83 on: 05/24/2016 05:38 am »
Mismatches in the PCB coefficients of thermal expansion can cause solder joints to strain and fail.
True, but most of the external electronics in MSL had to be qualified to survive 3x mission life worth of thermal cycles without any heating (an exception was Chemcam in its heated box at the top of the remote sensing mast.)

Most of the external heating requirements come from wet-lubricated mechanisms.
See http://llis.nasa.gov/lesson/11501

Offline MATTBLAK

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Re: NASA - Perseverance, Mars 2020 Rover : Discussion
« Reply #84 on: 05/24/2016 06:08 am »
I sure hope they toughen up the wheels of the next rover - Curiosity's have taken a hell of a hammering.
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Offline Don2

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Re: NASA - Perseverance, Mars 2020 Rover : Discussion
« Reply #85 on: 05/24/2016 07:33 pm »

What, exactly, in the electronics requires heating?  Is it just capacitors?  Sensors calibrated for certain temp range?  Most electronic components do very well down into the cryogenic temperature range.

How is JWST handling this?  I thought nearly all of its components were running at very cold temps.

There are several materials issues with temperature. Some metals and polymers can become very brittle when temperatures fall below a certain point. This is the case for normal steel, and also things like rubber. Low temperature embrittlement has been the cause of numerous engineering disasters, including Challenger. Metals like aluminum are not sensitive to this, so this is not the problem with the wheels.

Liquids like battery electrolytes and lubricants can freeze. I think batteries really don't like being frozen. I also remember that NASA tried hard to develop wheel motors that could operate at cryogenic temperatures. This would avoid the need to spend energy heating the wheels every time the rover moves. That didn't work out.

Finally there is a big issue with the temperature change between night and day. A 100C change means substantial thermal expansion and contraction. That means cyclical stresses in parts. Cyclical stresses mean fatigue cracks, and cracks mean failure. I think the connections on circuit boards are vulnerable to this.

How is JWST coping with these issues? Expensively! At least the JWST temperatures will be stable, which avoids the thermal expansion problems. I read that developing actuators suitable for the temperatures was costly. I also know that one instrument had to be dropped, because it couldn't be adapted to low temperature service. Finally, the JWST mirrors are made of beryllium rather than silicon carbide. Beryllium is only used on cryogenic mirrors due to cost.

Offline Helodriver

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Re: NASA - Perseverance, Mars 2020 Rover : Discussion
« Reply #86 on: 05/24/2016 07:43 pm »
New stronger wheel design confirmed by Deputy JPL director today at Space Tech Expo. Thicker metal, curved tread pattern for less mechanical stress.

Offline Don2

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Re: NASA - Perseverance, Mars 2020 Rover : Discussion
« Reply #87 on: 05/24/2016 07:48 pm »
True, but most of the external electronics in MSL had to be qualified to survive 3x mission life worth of thermal cycles without any heating (an exception was Chemcam in its heated box at the top of the remote sensing mast.)

Most of the external heating requirements come from wet-lubricated mechanisms.
See http://llis.nasa.gov/lesson/11501

So that means external electronics in things like instruments should be good for 6 earth years. Although Opportunity's would have been qualified for 9 months of life and yet some (but not all) of the instruments are still going.

I still think that a productive life of 10-15 years is a fair expectation for the 2020 rover.

Offline savuporo

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Re: NASA - Perseverance, Mars 2020 Rover : Discussion
« Reply #88 on: 05/24/2016 07:52 pm »
Preliminary Surface Thermal Design  of the Mars 2020 Rover
https://ttu-ir.tdl.org/ttu-ir/bitstream/handle/2346/64407/ICES_2015_submission_134.pdf

Also notes
Quote
There are many heritage hardware elements from MSL that will be re-used on M2020.  Some of these are exact copies  of  MSL  designs;  others  are  MSL  build-to-print  designs  with  slight  modifications.  The  M2020  mobility   system, which has 6 drive motors and 4 steer motors, will be essentially the same as MSL. The MSL wheels, which  sustained significant damage during traverses over sharp rock s inside Gale Crater, will be strengthened (at the cost of increased mass) for M2020.

I think it might end up as simple as adding material.
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Offline Don2

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Re: NASA - Perseverance, Mars 2020 Rover : Discussion
« Reply #89 on: 05/24/2016 08:36 pm »
I sure hope they toughen up the wheels of the next rover - Curiosity's have taken a hell of a hammering.

Even on Curiosity I don't think the wheel damage will limit the life to less than 10 years. The wheels aren't even half trashed. And the 2020 mission will have stronger ones for sure.

I wonder if the wheel damage is an example of something called stress corrosion cracking. If certain chemicals are present in the environment, it can greatly accelerate the rate at which fatigue cracks grow. I doubt anybody has ever studied the effect of perchlorate on aluminum, but I know that chloride ions do attack aluminum. Both chloride ions and perchlorates contain chlorine.

Offline redliox

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Re: NASA - Perseverance, Mars 2020 Rover : Discussion
« Reply #90 on: 05/24/2016 09:18 pm »
I wonder if the wheel damage is an example of something called stress corrosion cracking. If certain chemicals are present in the environment, it can greatly accelerate the rate at which fatigue cracks grow. I doubt anybody has ever studied the effect of perchlorate on aluminum, but I know that chloride ions do attack aluminum. Both chloride ions and perchlorates contain chlorine.

Interesting point regarding corrosive cracking, although I think it's more to do with sharp pointy rocks stabbing aluminum wheels.  There's a reason aerospace and automotive engineers are two separate fields of expertise.  I'm sure they'll improve '2020's wheels by improving on Curiosity's.
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Offline Blackstar

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Re: NASA - Perseverance, Mars 2020 Rover : Discussion
« Reply #91 on: 05/25/2016 01:06 am »
New stronger wheel design confirmed by Deputy JPL director today at Space Tech Expo. Thicker metal, curved tread pattern for less mechanical stress.

If you go higher in the thread you should find one or more JPL presentations posted by me that include a schematic of the rover that points out the stuff that is the same and the stuff that is different--and new wheels were called out.

The surprising thing to me is that apparently they have to alter the driving electronics and software to deal with the new wheels. But that makes sense when you think about it--you drive your car differently if you have racing tires vs. snow tires, so changing the mass of the wheels would naturally require changing the software that turns them and steers them.

Also, it would not surprise me if JPL has already modeled further damage to the Curiosity wheels so they know what to do if it happens. It would make sense to at least do this on a computer, and maybe they've even put some very damaged wheels on their engineering mockup to see how it handles.

Offline JH

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Re: NASA - Perseverance, Mars 2020 Rover : Discussion
« Reply #92 on: 05/25/2016 05:39 am »
Also, it would not surprise me if JPL has already modeled further damage to the Curiosity wheels so they know what to do if it happens. It would make sense to at least do this on a computer, and maybe they've even put some very damaged wheels on their engineering mockup to see how it handles.

They've been doing testing in the Mars yard at JPL pretty much constantly ever since they found out that the wheels were degrading faster than expected (a few years now). They drive around a test version of MSL on different surfaces and they also have a half-set of wheels on an arm that drives around in a circle over surfaces that they periodically switch out (they have the wheels weighted down to simulate the load on Mars). I've seen a number of wheels that they've tested completely to destruction. Amusingly, the only part of the wheel treads that survived alright were the sections with "JPL" stamped into them in Morse code.

Oh, I was able to find some pictures I took one time.

Offline Blackstar

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Re: NASA - Perseverance, Mars 2020 Rover : Discussion
« Reply #93 on: 05/25/2016 01:01 pm »
I can believe that they tested wheels to destruction. I'm wondering if they have tested pre-damaged wheels specifically to figure out controllability issues. In other words, have they been trying to figure out how they can drive with very damaged wheels?

Of course, these are two sides of a coin, but the objectives are different. Driving to destruction is intended to figure out how long the wheels could last, not how best to drive with damaged wheels.

Offline Nomadd

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Re: NASA - Perseverance, Mars 2020 Rover : Discussion
« Reply #94 on: 05/25/2016 02:10 pm »
Amusingly, the only part of the wheel treads that survived alright were the sections with "JPL" stamped into them in Morse code.

So, why can't they just stamp JPL over the whole wheel to solve the problem?
Those who danced were thought to be quite insane by those who couldn't hear the music.

Offline JH

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Re: NASA - Perseverance, Mars 2020 Rover : Discussion
« Reply #95 on: 05/25/2016 03:08 pm »
I've seen them driving the test rig up and down a hill in the yard (which I don't seem to have a picture of), but I am not sure if that was just general drive performance characterization or if it was using damaged wheels. I would be surprised if they haven't done tests on damaged wheels, though.

Offline NovaSilisko

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Re: NASA - Perseverance, Mars 2020 Rover : Discussion
« Reply #96 on: 05/26/2016 10:46 pm »
I've seen them driving the test rig up and down a hill in the yard (which I don't seem to have a picture of), but I am not sure if that was just general drive performance characterization or if it was using damaged wheels. I would be surprised if they haven't done tests on damaged wheels, though.

I seem to recall seeing photos of the test rig with Curiosity-style wheels that had been run absolutely bare, close to nothing remaining of the wheel past its "hubcap" and front rim edge.

Offline Dalhousie

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Re: NASA - Perseverance, Mars 2020 Rover : Discussion
« Reply #97 on: 05/27/2016 12:32 am »
Potential biosignatures identified?
Spirit observed some mineral fomations that looked like some seen here on Earth. Still not sure if the ones here on Earth were made by life though.
'Cauliflower' Silica Formations on Mars: Evidence of Ancient Life?
http://www.americaspace.com/?p=91183

Geyserite are generally associated with microbial activity.  See:

https://hal-insu.archives-ouvertes.fr/insu-01163128/document

http://www.mdpi.com/2078-1547/5/2/430

Steve Ruff's presentation on possible geyserite at the 2nd 2020 rover landing site workshop resulted in Gusev crater being added to the list

http://marsnext.jpl.nasa.gov/workshops/2015_08/16_Ruff_M2020_2nd_LSW_presentation_v3.pdf

Here are some photos of geyserites from NZ

Apologies in advance for any lack of civility - it's unintended

Offline redliox

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Re: NASA - Perseverance, Mars 2020 Rover : Discussion
« Reply #98 on: 05/27/2016 09:58 am »
Geyserite are generally associated with microbial activity.  See:

https://hal-insu.archives-ouvertes.fr/insu-01163128/document

http://www.mdpi.com/2078-1547/5/2/430

Steve Ruff's presentation on possible geyserite at the 2nd 2020 rover landing site workshop resulted in Gusev crater being added to the list

http://marsnext.jpl.nasa.gov/workshops/2015_08/16_Ruff_M2020_2nd_LSW_presentation_v3.pdf

Here are some photos of geyserites from NZ

This definitely boosts my support for Gusev Crater all the more, both for the rover and human visits.  I read a few of the papers advocating a return, with hydrothermal sites being a prime reason.  The pancam pictures in the Ruff presentation do bear a resemblance to the Chilean analog site.
"Let the trails lead where they may, I will follow."
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Offline Blackstar

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Re: NASA - Perseverance, Mars 2020 Rover : Discussion
« Reply #99 on: 06/06/2016 09:54 pm »
My coworker listened in to the NAC Planetary Protection Subcommittee meeting last week. He told me a number of things about it, including that ESA has developed some very impressive processes for sterilizing their ExoMars rover instruments.

He also said that there was a presentation about the 2020 helicopter. Apparently still being developed. The plan is to set it down on the ground and move away before launching it. It will only have enough power for three flights and a single instrument--essentially a GoPro type camera. Nobody wants it anywhere near the rover.

The presentations are not on line yet, but they will end up there at some point.

 

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