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Nasa announces new rover mission to Mars in 2020
PaulH51
post Sep 13 2019, 03:07 PM
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I'm not quite sure how you can accurately measure the C of G without the MMRTG being installed or at least a dummy unit of the correct size / mass being used...
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djellison
post Sep 13 2019, 07:38 PM
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As I understand it - these balance masses are entirely unrelated to EDL. The two cruise balance masses on one side of the backshell, and entry balance masses on the opposite side are designed so the vehicle is balanced during the spin stabilized cruise, then unbalanced before entry to give it the angle of attack required for guided entry, and then rebalanced before parachute deployment. I believe these total something around 150kg ejected both before entry, then before 'chute deployment.

The rover's CoG also needs to be characterized as part of the whole stack of the vehicle. It needs to match a conservatively predicted mass and CoG so as not to throw the rest of the vehicle off. The mass properties of the RTG are well known - you can 'balance' the vehicle without it, knowing how it will change the properties. You are not literally 'balancing' the vehicle on a spin table like that - you're simply characterizing the CoG. This is where the analogy of balancing a tyre kind of falls apart..... on one of those machines you actually need to balance it right then and there.
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mcaplinger
post Sep 13 2019, 08:59 PM
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QUOTE (djellison @ Sep 13 2019, 11:38 AM) *
and then rebalanced before parachute deployment...

As part of the amusingly-named "straighten up and fly right" (SUFR) maneuver. See https://trs.jpl.nasa.gov/bitstream/handle/2...629/10-1775.pdf


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Floyd
post Sep 13 2019, 10:13 PM
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Thanks to all who responded for adding insight into mass balance issues. A very interesting topic as most space craft issues are.


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djellison
post Sep 13 2019, 10:17 PM
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QUOTE (mcaplinger @ Sep 13 2019, 01:59 PM) *
As part of the amusingly-named "straighten up and fly right" (SUFR) maneuver.


Yeah - at one point they considered try to do it by pumping mercury around a fluid loop or something laugh.gif
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nprev
post Sep 14 2019, 12:52 AM
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...what could possibly go wrong there...? laugh.gif


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John Whitehead
post Sep 16 2019, 11:40 PM
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QUOTE (Floyd @ Sep 13 2019, 11:51 AM) *
Article on getting Mars 2020 center of gravity balanced Link
I'm a bit surprised...44 pounds of tungsten weights...44 pounds that scientist would love to add.
Maybe someone can explain why the position of components can't be re positioned slightly.

The article refers to 9 separate weights, so they average about 5 pounds, not the same thing as being able to accommodate a 44-pound instrument. The large number of weights and the use of a spin table seems consistent with "dynamic balancing," which is more than simply adjusting the c.g. position. Dynamic balancing orients the principal axes of inertia to avoid wobble when rotating. The engineer quoted in the article likened it to tire balancing at a gas station, perfectly appropriate except that the procedure is now done at automotive tire shops, rarely at "gas stations" anymore.
Repositioning components would be complicated, considering many hard constraints on component locations, and presumably it would be taboo to drill new mounting holes in the frame at this late stage of assembly in the clean room. Adjustable mounting brackets for components could be an engineering nightmare, adding extra weight and the possibility of things shaking loose and changing position.
Thanks Floyd for posting the September 12 article, I was wondering why the rover has been absent from the regular clean room webcam view this past week. <https://mars.nasa.gov/mars2020/mission/where-is-the-rover/>
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atomoid
post Oct 7 2019, 09:30 PM
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interesting quick read on the landing vision system on arsTechnica
i'd almost forgotten about this addition, more info
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mcaplinger
post Oct 8 2019, 01:01 AM
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QUOTE (atomoid @ Oct 7 2019, 01:30 PM) *
the landing vision system...

Huh. I wonder who built this camera? I haven't seen anything public about it anywhere. rolleyes.gif


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atomoid
post Oct 12 2019, 12:18 AM
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I havent seen that either, although some interesting (if brief) details on p.7,8 of this pdf here
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mrpotatomoto
post Oct 31 2019, 08:12 PM
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I'm curious: are there reasons to suspect that Jezero crater is more likely to preserve signs of ancient life (had it existed) compared to Gale crater?

I know that they're both ancient lake systems, with Jezero being an open lake system and Gale being a closed one. Does that or other differences influence biosignature preservation?
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elakdawalla
post Oct 31 2019, 08:44 PM
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It's because of the delta within Jezero. More here: https://www.planetary.org/blogs/emily-lakda...2020-rover.html


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mrpotatomoto
post Oct 31 2019, 08:57 PM
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QUOTE (elakdawalla @ Oct 31 2019, 09:44 PM) *
It's because of the delta within Jezero. More here: https://www.planetary.org/blogs/emily-lakda...2020-rover.html


Thanks! Great article.
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