Oppy Vs Spirit Power Consumption Options

[djellison]

The MER battery's have a better thermal profile that your average laptop battery, but the point is fairly true all the same

Doug

[tty]

As for freezing a battery to death, oddly enough it works the other way around too. At least for many battery types heating a flat battery will revive it temporarily. A number of wrecked sailors and airmen owe their lives to this old trick.

tty

[djellison]

I used to freeze tamagochi's for fun

Doug

[Bob Shaw]

This is easy to reproduce at home:

1. Take your girl friend's fancy new Apple notebook computer, pull out the plug and run it until it indicates a 'low on battery' status.
2. Put the thing in your freezer (at -12C or something) and leave it there for a few hours.
3. Take it out and try to recharge the battery.

See what happens? Nothing. Will not recharge! Now, if you bring it back to room temperature it might just work again, but if you repeat his experiment 2 or 3 times you will kill the battery permanently. (Go ahead, try it... don't believe me for it)

Now, remember that the MERs have exactly the same type of Li-ion batteries, so my guess is that when you can't maintain the temperature of the WEB above freezing the battery will be dead within 2 or 3 sols.

There is nothing that beats a bit of science @HOME! 

Er... ...no girlfriend, and a freezer full of Apples?

I won't mock, my wife used to go nuts at film in the fridge!

Bopb Shaw

[Bob Shaw]

As for freezing a battery to death, oddly enough it works the other way around too. At least for many battery types heating a flat battery will revive it temporarily. A number of wrecked sailors and airmen owe their lives to this old trick.

tty

Perhaps you're talking about *literally* freezing the battery after a certain point, with bits inside cracking and electrodes etc being damaged at Martian night-time temperatures? I presume there's a gel in there somewhere, any battery specialists out there?

Bob Shaw

[Cugel]

Actually not so difficult to grasp: a battery is nothing else than a chemical reaction in a casing. Chemical reactions are very sensitive to ambient temperature. When you freeze a battery it will grow crystals in its electrolyte that will increase its internal resitance.
Heating a Li-ion battery (>70C or so) will kill it instantly, it might even explode.
(Although this might work with other types)

Don't try that @HOME!

[ljk4-1]

February 9, 2006 | If NASA's Mars Exploration Rover Opportunity were human, it would be more than 600 years old.

The rover has lasted almost eight times its life expectancy, logging more than 700 Martian days on the Red Planet's surface. There is little wonder that the robotic dune buggy has developed joint problems.

As the rover entered its third Earth year on the Martian surface (two Martian years), mission engineers developed their plan to cope with Opportunity's arthritis: the steering motor on one of the rover’s front steering wheels has died and a broken wire in the instrument arm’s shoulder joint motor has hampered the mission. The arm contains many of the mission's science instruments, including the Rock Abrasion Tool used for grinding soil and rock samples, and the Microscopic Imager. However, the rover can still drive with its other steerable wheels, and with extra current the arm is still usable. By developing a new way to stow the arm, engineers have the rover driving again after more than an Earth month of analysis-related delays.

http://skyandtelescope.com/news/article_1666_1.asp

[helvick]

I've overhauled my solar power estimate charts with the most recent power generation information. I've also properly reworked the calculated data in these charts to include an accurate model of the available insolation for each rover and also included an estimated value that factors in the effects of Tau using the values published in the MER Analyst Notebook.


Spirit Key Dates:
If the current dust deposition rate holds (~0.08% power loss per sol) Spirit's normal power generating capability will drop below 300whr per sol on Sol 865 (June 9th) and stay below that until Sol 997 (October 22). Those power levels effectively eliminate any significant mobility unless she has a very favourable location.
The worst period is from sol 919 to Sol 943 when power will be no more than 275whr per Sol unless she is oriented towards the Sun.


My reworked figures actually put Opportunity in a much more favourable power situation. Aggregate dust loss over the past 100 Sols has been very close to 0.04% per Sol. That means that Opportunity should be able to generate more than 457whr throughout the winter period and while that is a restricted power budget it is more than Spirit currently has available.

[jamescanvin]

I've overhauled my solar power estimate charts with the most recent power generation information. I've also properly reworked the calculated data in these charts to include an accurate model of the available insolation for each rover and also included an estimated value that factors in the effects of Tau using the values published in the MER Analyst Notebook.

Thanks for the long range forcast Helvick - fantastic work.

Here's hoping for a few windy days on the slopes of McCool hill.

James

[Tom Tamlyn]

Very impressive work!

What is responsible for the occasional severe drops in power output? For example, on the Spirit chart there's one at around Sol 440 and another at around 580. Did she have a bad orientation the sun on those occasions?

I apologize if this has been discussed previously; it's hard to keep up with all the threads here.

TTT

[RNeuhaus]

I've overhauled my solar power estimate charts with the most recent power generation information. I've also properly reworked the calculated data in these charts to include an accurate model of the available insolation for each rover and also included an estimated value that factors in the effects of Tau using the values published in the MER Analyst Notebook.

Thanks Helvick! The report is probably one of the most wanted article to know among UMSF members. Now, we are more concerned about the power level survival for Spirit. So, we start to look for any signal from Spirit rover driver to leave "YA" from HP. The good probably ones is that Oppy might be traveling as forever all round year toward Victoria.

Rodolfo

[helvick]

Very impressive work!

What is responsible for the occasional severe drops in power output? For example, on the Spirit chart there's one at around Sol 440 and another at around 580. Did she have a bad orientation the sun on those occasions?

I apologize if this has been discussed previously; it's hard to keep up with all the threads here.

The x-axis labeling on my charts is a bit whacky, I need to fix that. The labels actually refer to the data point where a line underneath the text would intersect the x-axis. The points you refer to actually happen at Sol 418 and sol 557 respectively.

The biggest problem though is that I only have around 30 reliable data points for Spirit and 40 for Opportunity and they tend to be somewhat clustered so there are some odd artificacts in the chart as a result. The Spirit Sol 440 dip actually happens at around Sol 418 and was caused by an increase in atmospheric Opacity to 1.5 on sol 418. However that was immediately followed by a cleaning event so there was a fairly significant bounce back. I don't have detailed data for the period leading up to Sol 418 but I suspect it would have varied between 550 and 600 whr. By Sol 422 power was up to 800whr because of the cleaning event.
The Sol 557 event is, I think, more of a general pointer to the fact that power generation wasn't a huge priority in the drive up the side of Husband hill. By the time the summit was in view the orientation had improved significantly leading to the 956 whr number at the summit on Sol 581.

There are two main threats to the rovers Sol to Sol solar power generating capability:

• Orientation
  If the rover spends any significant amount of time in an unfavourable position at the wrong time then the amount of power generated on that Sol can be affected quite dramatically. The rover planners and drivers are very very good so this happens very rarely but there is one well documented example for Spirit on Sol 203 as she was approaching Clovis on the West Spur. The local slopes were up to 25deg and during the Clovis approach the rover ended up in a south facing hollow after a short autonomous driving sequence ended up ~1m further than planned. Power dropped from 370whr on Sol 202 to 288whr on Sol 203 as a result.
  
• Atmospheric Opacity
  The average atmospheric opacity (Tau) in spring\summer tends to be around the 0.8-0.9 range and that tends to drop down to 0.3-0.5 as they move through winter. However local\global dust storms cause that value to rise significantly. A large dust storm in the Margaritifer region affected Opportunity around Sol 627 when Tau rose from the summer average of around 0.9 to more than 1.6. The numbers in the Daily Reports for Sol 627 to 630 show the effect:
  Sol 625 675 whr Tau =~0.9
  Sol 627 593 whr Tau =1.6
  Sol 628 479 whr Tau = ~1.9
  Sol 629 470 whr Tau = ~2.0
  Sol 630 496 whr Tau = ~1.8
  The relationship between power generated and Tau is quite complex. Direct (ie beam) sunlight drops off according to the formula Beam=G0*exp(-tau). Where G0 is the amount of solar radiation that would be seen if there was no atmosphere. Indirect (diffuse) radiation increases with Tau but there isn't a nice simple formula for it. For the most part Tau changes between 0.5 and 0.9 don't have a severe overall effect. When Tau increases to 1.5 the power drops by about 25% and 2.0 causes a drop of about 42%. The Opportunity Sol 627-629 period seems to have been the one of the worst. There was a similarly significant storm around Sol 489-~520 which led to power dropping from around 600whr to 410whr.

To further complicate matters these two things interact. When Tau is very low then orientation effects are much more significant because direct sunlight predominates. In the equatorial and southern hemisphere regions of Mars Tau tends to be higher in Summer and lower in winter due to the overall weather patterns of Mars which tends to loft dust into the atmosphere during the SH Spring and Summer. The net effect for the rovers is generally beneficial but it means that orientation becomes critical in wintertime as mistakes can be punished by a power loss of 30% or more.
[Edited to Add]
There are one or two points where power generated exceeds the theoretical "Zero dust Loss" case on my charts. The most significant of those are at Sols 310-312 just inside the rim of Endurance where she was generating 10-17% more than she would have if she had been on a perfectly flat plain because she was optimally tilted towards the sun while still being high enough not to have early morning\late evening sunlight blocked by the crater rim. It happened again on Sol 339 when she was beside the heat shield but I don't have any explanation for the power surplus at that point.

[alan]

There are one or two points where power generated exceeds the theoretical "Zero dust Loss" case on my charts. The most significant of those are at Sols 310-312 just inside the rim of Endurance where she was generating 10-17% more than she would have if she had been on a perfectly flat plain because she was optimally tilted towards the sun while still being high enough not to have early morning\late evening sunlight blocked by the crater rim. It happened again on Sol 339 when she was beside the heat shield but I don't have any explanation for the power surplus at that point.

No explanation for the excess power when Oppy was at the heatshield?. I bet being parked next to a mirror had an effect.

[helvick]

No explanation for the excess power when Oppy was at the heatshield?. I bet being parked next to a mirror had an effect.

I'd considered that and although it must be a factor it doesn't add up to the whole story. The reported power was 836 whr. Tau on that day was 0.9. My zero dust loss calculation puts power for the horizontal case at 713whr. The difference is 17% and the actual improvement is probably closer to 30% since there was a non trivial amount of dust on the panels at the time.
From the Navcam pictures on Sol 339 the heatshield occupied around 1/4 of a Navcam frame or about 1/48th of the the total sphere around Opportunity. If you look at the pictures you'll see that most of the mirror sides are pointed away from Oppy at the time. My gut feeling for this is that it couldn't add more than 1% to the available power at the time.
Local albedo affects power especially when albedo is high and Tau is near or above 1. This is a non trivial effect but even though the area around Heat Shield seems quite bright in the raw images the reported data indicates that albedo was fairly constant on the plains (0.14 +_0.1). I used 0.15 for my Opportunity calculations which would probably be erring marginally on the high side in any case.

I suspect it might be a reporting error. ~600-650whr on that Sol would tie in much more closely with the nearest datapoints before (Sol 333) and after (Sol 401) that point in time.

Clearly I've spent far too much time thinking about this.

[helvick]

A slight update. Browsing back through this I realised I'd forgotton about the charts mars_loon had found that showed insolation charts for both sites and more importantly Spirit's actual solar panel relative efficiency between Sols 1 and 700. I extracted the data from the image and used it to calculate what should be a much more accurate estimate of solar panel power output per sol for Spirit. I applied that value to the Tau adjusted power value to see how it matches up with my estimated power curve which is based mostly on the occassional value in the Daily Reports. The new line is labelled "Efficiency Adjusted".

It makes the whole thing quite a bit messier but I think it highlights times where power has been gained or lost due to orientation.