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Sliding into 'Home Plate North', Heading for Spirit's 2008 Winter Retreat
fredk
post Nov 14 2007, 10:10 PM
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QUOTE (djellison @ Nov 14 2007, 08:54 PM) *
2 : We're VERY good at VERY low power survival following the worst of the dust storm ( i.e. the traditional 240 Whrs minimum was cut almost in half for a while)

Remembering that the dust storm was summer, and this is winter we're talking about now, so colder temperatures. This will be new territory.
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climber
post Nov 14 2007, 10:32 PM
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QUOTE (djellison @ Nov 14 2007, 09:54 PM) *
2 : We're VERY good at VERY low power survival following the worst of the dust storm ( i.e. the traditional 240 Whrs minimum was cut almost in half for a while)
Doug

Can we hold such low level for several months AND do some science or at least communications ?


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Aussie
post Nov 14 2007, 11:09 PM
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There is a limit to power conservation. The RHU provide 8 W heating but during the dust storm they had to keep the electronics operating to generate sufficient heating to prevent the heaters cutting in. So they will almost certainly do some science and data transmission as a pragmatic way of keeping the WEB above -40C. Since the heaters cut in last winter there is a high probability that they will do so again. Anyone have any knowledge as to the power draw for these heaters?
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djellison
post Nov 14 2007, 11:31 PM
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QUOTE (climber @ Nov 14 2007, 10:32 PM) *
Can we hold such low level for several months AND do some science or at least communications ?


That's the way they held such low levels - by doing no science save for a single Tau observation one day in three, and communicating one-day-in-three



Doug
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Del Palmer
post Nov 14 2007, 11:51 PM
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QUOTE (Aussie @ Nov 14 2007, 11:09 PM) *
Since the heaters cut in last winter there is a high probability that they will do so again. Anyone have any knowledge as to the power draw for these heaters?


According to the MER thermal design document at JPL's reference archive, the MiniTES survival heater would consume 55 W on a cold night. Of course, Spirit could opt to use Deep Sleep to avoid survival heater usage at night. During the day, the WEB survival heaters use 60 W, so you'd want to avoid those from kicking in by running the computer for longer periods, as was done on Oppy.


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djellison
post Nov 15 2007, 12:02 AM
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During the dust storm - they didn't do that. They didn't have the power to run the thing in the afternoon just to pump heat into the system. They just left the basic level circuitry for charging and clock running and the survival heaters kicked in when necessary. The most efficient way to keep the vehicle above a critical temperature is to use a heater when, and only when, it is about to hit that temperature. Anything other than that and you're wasting Whrs keeping the vehicle warmer than it essentially needs to be.

Doug
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helvick
post Nov 15 2007, 12:45 AM
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QUOTE (Del Palmer @ Nov 14 2007, 11:51 PM) *
.. the MiniTES survival heater would consume 55 W on a cold night. ..

Lets get our units straight folks. The resource that we are worried about is Watt-hours [per sol]. The individual systems draw Watts [per second\minute\hour\sol\whatever ] so you have to know the power draw _and_ the duration of the event in order to figure out how much of an issue it might be. My understanding is that the temperature triggered survival heaters draw 15 Watts and the total number of Watt-hours they draw in a given sol depends on when the switches get turned on [ in the evening as temp drops < -40 ] and turned off [ in the early morning as temp rises above something slightly higher than -40].

I can't throw any light on the critical numbers here apart from pointing out that they survived substantially lower power levels during the dust storm than any of us had believed possible before the storm so I am now firmly of the opinion that [a] The rover management team knows best and [b] they will only stop working when they are good and ready.
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Del Palmer
post Nov 15 2007, 01:08 AM
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Yes, sorry I wasn't clearer. W = Watt-hours per sol.
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Aussie
post Nov 15 2007, 05:49 AM
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QUOTE (fredk @ Nov 14 2007, 10:10 PM) *
Remembering that the dust storm was summer, and this is winter we're talking about now, so colder temperatures. This will be new territory.


Squires statement from the Planetary Society Update 31 Jan 07: 'What the rover needs to survive in winter is something between 200 and 250 watt-hours'.

Given current power levels I think we can appreciate why an extra 10 Whrs is so critical and why they want to be in place as early as possible.
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centsworth_II
post Nov 15 2007, 09:25 PM
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QUOTE (Aussie @ Nov 15 2007, 12:49 AM) *
'What the rover needs to survive in winter is something between 200 and 250 watt-hours'. -- Steve Squires

Without another cleaning event, does this even look possible?
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Aussie
post Nov 16 2007, 02:04 AM
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QUOTE (helvick @ Nov 15 2007, 12:45 AM) *
My understanding is that the temperature triggered survival heaters draw 15 Watts and the total number of Watt-hours they draw in a given sol depends on when the switches get turned on [ in the evening as temp drops < -40 ] and turned off [ in the early morning as temp rises above something slightly higher than -40].


Probably the most cold vulnerable module is the battery pack and I think the -40 C operating limit is a function of battery electrolyte freezing temperature, so this temperature is a critical limit, not a heater activation level. As I understand it heater switching is autonomous and the battery heaters activate at - 20 C. I also believe that 6 of the 8 RHU are dedicated to the battery pack . So rather than the WEB being maintained at a common temperature are there more vulnerable sections within the WEB that have additional insulation and heating, with 'last ditch' WEB survival heaters that activate at -40 C to reduce the temperature gradient from vulnerable areas should electronic activity not provide sufficinet buffering. An incremental defence makes for good energy efficiency. This would certainly maximise the effect of the RHU and explain why the heaters did not kick in last winter until the external temperature dropped to around a - 95 C minimum, but when the electronics were shut down, in the comparatively warm dust storm environment WEB temperatures reached -37 C . Activating some electronic activity was probably a more energy effective way of minimising the temperature gradient and associated risk than survival heaters.
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fredk
post Nov 16 2007, 05:26 AM
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Tracks from 616 sols ago (that's pushing two years!) visible in the left half, a bit below centre, of this new pancam frame.

There was a tiny bump on sol 1375. Open up this sol 1373 navcam and this sol 1375 navcam in identically positioned windows and flip from one to the other for a cute flicker animation between the two sites.
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climber
post Nov 16 2007, 05:48 PM
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May be a wild thought, but anyway.
Instead of hoping for a cleaning event, do you think that the (actual) dust, coat Spirit enough to preserve temperature exchanges with the atmosphere i.e. keep more heat inside ?


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djellison
post Nov 16 2007, 06:19 PM
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I doubt it makes much of a difference be honest. It's a very thin, very fine layer. Indeed - because it's a lighter colour than the dark solar arrays themselves, it might even make the rover less able to absorb what ever solar heating there is.

Doug
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Stu
post Nov 17 2007, 10:27 AM
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Time for a few new views, I think...


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