Author Topic: NASA - Mars 2020 Rover  (Read 65523 times)

Offline redliox

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Re: NASA - Mars 2020 Rover
« Reply #60 on: 05/15/2016 02:04 am »
There's a Mars 2020 update from March here:

http://mepag.nasa.gov/meeting/2016-03/21_MEPAG_160303_FINAL%20v2.pdf

They are scheduled to go into Phase C (Design and Development) in April/May, so presumably they are about to do that any time now. Considering that they already have a lot of hardware in hand, they should move through this faster than MSL/Curiosity did.

Hmm...so not as much detail on how the samples will be processed although it says adaptive cache is baselined.  It's also interesting to see they're still considering the helicopter idea, more imaging in general EDL, and microphones.  Inversely, cubsats and a ring parachute are off the list; both make sense if they're trying to duplicate Curiosity as much as possible.

It appears that they've divided the landing sites into fluvial versus hydrothermal sites.  I'm tempted to lean more towards the hydrothermal group because it includes Columbia Hills i.e. Gusev Crater, which has the advantage of better study, along with prominently considered Nili Fossae.  They have a good variety to chose from, although I hope consideration for MSR will be considered.
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Offline Blackstar

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Re: NASA - Mars 2020 Rover
« Reply #61 on: 05/15/2016 02:54 am »
They have a good variety to chose from, although I hope consideration for MSR will be considered.

Sample return is going to drive landing site selection. It is the primary reason they are doing this mission.

Offline redliox

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Re: NASA - Mars 2020 Rover
« Reply #62 on: 05/15/2016 07:00 am »
They have a good variety to chose from, although I hope consideration for MSR will be considered.

Sample return is going to drive landing site selection. It is the primary reason they are doing this mission.

Would you say the launch needs of a future MAV be part of that?  If so I am assuming some leeway is being considered beyond a strictly equatorial site.
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Offline Star One

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Re: NASA - Mars 2020 Rover
« Reply #63 on: 05/15/2016 08:23 am »
There's a Mars 2020 update from March here:

http://mepag.nasa.gov/meeting/2016-03/21_MEPAG_160303_FINAL%20v2.pdf

They are scheduled to go into Phase C (Design and Development) in April/May, so presumably they are about to do that any time now. Considering that they already have a lot of hardware in hand, they should move through this faster than MSL/Curiosity did.

Hmm...so not as much detail on how the samples will be processed although it says adaptive cache is baselined.  It's also interesting to see they're still considering the helicopter idea, more imaging in general EDL, and microphones.  Inversely, cubsats and a ring parachute are off the list; both make sense if they're trying to duplicate Curiosity as much as possible.

It appears that they've divided the landing sites into fluvial versus hydrothermal sites.  I'm tempted to lean more towards the hydrothermal group because it includes Columbia Hills i.e. Gusev Crater, which has the advantage of better study, along with prominently considered Nili Fossae.  They have a good variety to chose from, although I hope consideration for MSR will be considered.

Like the helicopter I hope the microphones do fly as well. Vision is all very well but sound is also an important part of how we interpret things.

Offline redliox

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Re: NASA - Mars 2020 Rover
« Reply #64 on: 05/15/2016 09:04 am »
Like the helicopter I hope the microphones do fly as well. Vision is all very well but sound is also an important part of how we interpret things.

I think they're allowing the helicopter (as a possibility) because it's small and pseudo-independent of the rover (probably still needs it for communication), not to mention having a mini-scout able to check ahead on a sample-critical mission has merit.

The paper said the microphones are already baselined; I'm pretty sure the Planetary Society supported it - their blog here on it: http://www.planetary.org/blogs/bruce-betts/2016/0215-mars-2020-microphone.html

Again I long to hear more about the cache plans; if they're going adaptable I hope there's some way to bundle the cores together to ease the future burden on a retrieval rover or likewise an astronaut.
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Offline Don2

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Re: NASA - Mars 2020 Rover
« Reply #65 on: 05/20/2016 12:46 am »
I hope they will find a way to collect enough samples for 2 sample return flights. Sample return is complex, and there is a reasonable chance that the first attempt will fail. If there is enough sample for a second attempt, it would avoid the time and cost needed to fly a second sample collecting rover.

Another reason for doing this is that while the rover has a design life of 2 years the expected life is more like 10-20 years. They will want to collect enough samples for a return flight fairly quickly, before they have had a chance to really explore and understand the landing site. If the rover lasts a long time they will discover a lot of interesting things in the extended mission and there will be a strong desire to return those rather than the original samples. That will only be possible if they have plenty of sample return containers.

I understand this would add weight but I don't see why it should add a lot of additional cost. All that is required is more copies of hardware that the project already plans to design and build.

Offline Blackstar

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Re: NASA - Mars 2020 Rover
« Reply #66 on: 05/20/2016 12:13 pm »
Another reason for doing this is that while the rover has a design life of 2 years the expected life is more like 10-20 years.

Where do you get that latter figure?

What is the available Pu-238 energy level at 10 years?

Offline Blackstar

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Re: NASA - Mars 2020 Rover
« Reply #67 on: 05/20/2016 12:14 pm »
They have a good variety to chose from, although I hope consideration for MSR will be considered.

Sample return is going to drive landing site selection. It is the primary reason they are doing this mission.

Would you say the launch needs of a future MAV be part of that?  If so I am assuming some leeway is being considered beyond a strictly equatorial site.

Yeah, I'm sure that the MAV requirements are factored in there somehow.

Offline Don2

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Re: NASA - Mars 2020 Rover
« Reply #68 on: 05/20/2016 08:49 pm »
Another reason for doing this is that while the rover has a design life of 2 years the expected life is more like 10-20 years.

Where do you get that latter figure?

What is the available Pu-238 energy level at 10 years?

There's a couple of ways of looking at that. Half-life of Pu-238 is 88 years, so heat production drops off very slowly. Power conversion components inside the RTG also degrade. Several sources state a minimum lifetime of 14 years for a MMRTG, but many spacecraft have exceeded that. If the RTG is manufactured 3 years before launch and the journey to Mars takes 1 year, then the RTG should last at least 10 years on the surface. Wikipedia claims 125W when new, and 100W after 14 years.

 Another source states 122W when new, and 54W after 17 years. The same source shows MSL discretionary energy falling to zero after 13 years on the surface. That source admits they are modelling decay as a linear rather than an exponential process for reasons of conservatism. That will give a much faster decay in power output than would be expected in practice.

http://www.lpi.usra.edu/opag/july2014/posters/15-eMMRTG_PosterV5.pdf

What about other things besides the power supply? The average life of Opportunity and Spirit works out to 9 years. Hopefully they have learned how to avoid getting stuck in the sand as Spirit did. Opportunity shows that motors, mechanisms and circuit boards can all last at least 12 years on the surface.

Orbiter experience is similar. Mars Global Surveyor failed at 10 yr, but MRO (11 yr), Mars Express (12 yr) and Odyssey (15yr) are still going strong.

Cassini is still scientifically productive 18yr after launch, while Ulysses was switched off 19yr after launch due to transmitter problems. I think 10-20 years is a very reasonable guess for the lifetime of the 2020 rover.

Offline vjkane

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Re: NASA - Mars 2020 Rover
« Reply #69 on: 05/20/2016 09:24 pm »
There's a couple of ways of looking at that. Half-life of Pu-238 is 88 years, so heat production drops off very slowly. Power conversion components inside the RTG also degrade. Several sources state a minimum lifetime of 14 years for a MMRTG, but many spacecraft have exceeded that. If the RTG is manufactured 3 years before launch and the journey to Mars takes 1 year, then the RTG should last at least 10 years on the surface. Wikipedia claims 125W when new, and 100W after 14 years.
Another issue is battery life.  The RTG actually powers the battery, which is used for the bulk of Curiosity's operations.  (RTG also provides ample heat, eliminating or minimizing the need for electric heaters).  Curiosity's long life suggests that the Energizer bunny is alive and well on Mars.

Offline Jim

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Re: NASA - Mars 2020 Rover
« Reply #70 on: 05/21/2016 01:09 pm »

Orbiter experience is similar. Mars Global Surveyor failed at 10 yr, but MRO (11 yr), Mars Express (12 yr) and Odyssey (15yr) are still going strong.


Spacecraft are not relevant comparisons to rovers.  Different systems and different environments.

Offline ccdengr

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Re: NASA - Mars 2020 Rover
« Reply #71 on: 05/21/2016 03:31 pm »
RTG also provides ample heat, eliminating or minimizing the need for electric heaters. 
The RTG on MSL can only heat the internal electronics via a fluid loop; all of the external actuators and instruments still have to be electrically heated to use them and this requires a fair amount of battery power.  See, e.g., https://ttu-ir.tdl.org/ttu-ir/bitstream/handle/2346/59520/ICES-2014-295.pdf?sequence=1

Offline ccdengr

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Re: NASA - Mars 2020 Rover
« Reply #72 on: 05/21/2016 03:36 pm »
Mars Global Surveyor failed at 10 yr
The root cause of the MGS failure was errors in ground commanding, not a hardware failure.
See http://llis.nasa.gov/lesson/1805
Quote
The mission loss was attributed to a High Gain Antenna (HGA) positioning command sent by the spacecraft operations team five months earlier that, in the process of updating several parameters, created a bad memory load...

Offline vjkane

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Re: NASA - Mars 2020 Rover
« Reply #73 on: 05/21/2016 03:55 pm »
RTG also provides ample heat, eliminating or minimizing the need for electric heaters. 
The RTG on MSL can only heat the internal electronics via a fluid loop; all of the external actuators and instruments still have to be electrically heated to use them and this requires a fair amount of battery power.  See, e.g., https://ttu-ir.tdl.org/ttu-ir/bitstream/handle/2346/59520/ICES-2014-295.pdf?sequence=1
Thanks ccdengr.  For outer planet missions, RTGs also provide this double blessing (heat + electricity).  Almost half of Juno's power goes to heaters.  At Saturn, powering the heaters makes solar power iffy.  Low data rate missions like the Enceladus Life Finder can just manage while missions with any serious data requirements can't (per a JPL engineer I talked with).

Offline Don2

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Re: NASA - Mars 2020 Rover
« Reply #74 on: 05/21/2016 08:23 pm »

Another issue is battery life.  The RTG actually powers the battery, which is used for the bulk of Curiosity's operations.  (RTG also provides ample heat, eliminating or minimizing the need for electric heaters).  Curiosity's long life suggests that the Energizer bunny is alive and well on Mars.

I've been digging into the battery issue a little. With lithium-ion, if you discharge them all the way, that is called a deep cycle, and it wears out the batteries quickly. A consumer battery might be killed by 400 deep cycles. If you only discharge them 10%, you can extend the life of the same battery by a factor of 10 to 4000 cycles or so.

What do they mean by 'dead'? This seems to be defined as a 30% loss of capacity rather than a complete failure. So it might still be usable up to a point.

NASA did a test of the batteries used on the Mars rover and found they could cope with 12000 cycles at 40% depth of discharge.  20 years on Mars would be 7105 cycles, so the batteries might not be life limiting if they are not deeply discharged on a regular basis.

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20080008855.pdf

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. The power output goes down more than the heat because there is some deterioration in the heat to electricity conversion components. 20 years after manufacture is probably equivalent to 16 years after landing.

The bottom line is that the battery is more life limiting than the RTG, but even that looks like it could last 20 years or more.

Offline Don2

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Re: NASA - Mars 2020 Rover
« Reply #75 on: 05/21/2016 08:29 pm »
Mars Global Surveyor failed at 10 yr
The root cause of the MGS failure was errors in ground commanding, not a hardware failure.
See http://llis.nasa.gov/lesson/1805


Thanks, I didn't realize that it wasn't a hardware failure. I think it is still a relevant data point though. A bad command load could  kill a rover if it causes it to drive off a cliff.

Offline savuporo

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Re: NASA - Mars 2020 Rover
« Reply #76 on: 05/21/2016 09:11 pm »
I've been digging into the battery issue a little. With lithium-ion, if you discharge them all the way, that is called a deep cycle, and it wears out the batteries quickly. A consumer battery might be killed by 400 deep cycles. If you only discharge them 10%, you can extend the life of the same battery by a factor of 10 to 4000 cycles or so.

What do they mean by 'dead'? This seems to be defined as a 30% loss of capacity rather than a complete failure. So it might still be usable up to a point.

NASA did a test of the batteries used on the Mars rover and found they could cope with 12000 cycles at 40% depth of discharge.  20 years on Mars would be 7105 cycles, so the batteries might not be life limiting if they are not deeply discharged on a regular basis.

Yes, but lithium ion also has calendar life based capacity loss. You can excercise them at 10K cycles rapidly and estimate the capacity loss, but they do degrade over time too. Slow cycling over a few years over the same number of cycles will hence result in bigger capacity loss - which also somewhat depends on average state of charge and temperature over the period.
So cycle life isn't the full story, but fortunately for established battery chemistry and construction types there are fairly good models for estimating calendar life too.
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Offline Blackstar

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Re: NASA - Mars 2020 Rover
« Reply #77 on: 05/23/2016 01:09 am »
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.


Offline Don2

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Re: NASA - Mars 2020 Rover
« Reply #78 on: 05/23/2016 09:12 pm »
I don't know anything specific about the MMRTG being built for Mars 2020. If they were no longer able to meet the specifications for initial power output then I think that would have been well publicized.

In one sense it doesn't matter how old the Pu-238 is. Whatever the current heat output is, it will take 88 years to fall to half of current levels. That is the basic physics of how radioactive decay works.  If the Mars 2020 RTG delivers as much power when new as Curiosity's did, then power production will decline in exactly the same way. If it delivers less when new, it will still take the same amount of time to fall by 10% or 20% or 50% as Curiosity's did.

I'm sure the Pu-238 stockpile has all sorts of different batches with different heat outputs depending on their age. I don't know exactly how they manage all that to hit a certain specification for RTG power. As the stockpile ages, at some point it will become impossible to make RTGs with the specified initial output.

There are some other things that might be life limiting. They use a coolant loop to circulate heat from the RTG to the rover interior. The plumbing, pumps and cooling fluid in that won't last forever.

There is also the flash memory in the computer. That wears out after a certain number of read/write cycles. Opportunity is having all sorts of troubles with its flash memory, so maybe there is something in the Martian environment that doesn't agree with flash.  It should be good for 10 years, but maybe not 20.

Offline a_langwich

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Re: NASA - Mars 2020 Rover
« Reply #79 on: 05/23/2016 09:35 pm »
RTG also provides ample heat, eliminating or minimizing the need for electric heaters. 
The RTG on MSL can only heat the internal electronics via a fluid loop; all of the external actuators and instruments still have to be electrically heated to use them and this requires a fair amount of battery power.  See, e.g., https://ttu-ir.tdl.org/ttu-ir/bitstream/handle/2346/59520/ICES-2014-295.pdf?sequence=1

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.

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