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Nasa announces new rover mission to Mars in 2020
fredk
post Dec 11 2012, 08:40 PM
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He says "a 2020 launch would be favorable", but he doesn't say with respect to what. That's the crucial question. The 2018/20 opportunities should be compared with the 2010 2011 flight.
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centsworth_II
post Dec 11 2012, 08:58 PM
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QUOTE (fredk @ Dec 11 2012, 03:40 PM) *
...The 2018/20 opportunities should be compared with the 2010 flight.
That would be an interesting comparison, but the payload of a future rover is dependent only on the 2018 vs 2020 dates.

From the same article I linked in my previous post:
John Grunsfeld, NASA's science chief... however, cautioned that "2020 is ambitious, and a lot of it has to do with the science instrument development. ... It might be possible to do it in 2018, but it would be a push. What it might do is exclude certain science investigations that might be possible if we had the extra two years. That's something downstream."

I don't know what use extra payload would be at the expense of "certain science investigations", especially if a geochronometer is a possibility in 2020 but not in 2018.
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mcaplinger
post Dec 11 2012, 09:02 PM
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QUOTE (djellison @ Dec 11 2012, 01:04 PM) *
You could add approx 100kg to the mass of MSL within the current architecture - but it's not coming from the ballast.

http://trs-new.jpl.nasa.gov/dspace/bitstre...2011_216988.pdf is an interesting study of potential improvements to MSL; one of the options discusssed is replacing the entry balance masses with an actively-controlled trim tab. Of course it's not clear how many changes to the MSL architecture are going to be possible for cost and schedule reasons.


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JRehling
post Dec 11 2012, 09:06 PM
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The distance between Earth and Mars will reach a local minimum in the 2018 opposition, and will grow in each successive opposition through 2027 then decrease again before reaching the next local minimum in 2035.

2020, however, is only slightly farther than 2018. Then there's a steep climb with each successive opposition.

The Earth-Mars opposition distance isn't quite the same thing as trajectory energy for the launch opportunity, but I think they correlate very well.
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mcaplinger
post Dec 11 2012, 09:17 PM
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QUOTE (JRehling @ Dec 11 2012, 02:06 PM) *
2020, however, is only slightly farther than 2018.

Here's the C3 in km2/sec2 for the opportunities from 2009 to 2022. You can clearly see that 2020 is much higher (worse) than 2018.
Source: table 2 in Interplanetary Mission Design Handbook: Earth-to-Mars Mission Opportunities and Mars-to-Earth Return Opportunities 2009–2024, NASA/TM—1998–208533.

2009: 10.27
2011: 8.95
2013: 8.78
2016: 7.99
2018: 7.74
2020: 13.17
2022: 13.79


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centsworth_II
post Dec 11 2012, 09:24 PM
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At least it's good to see that a mission planned for 2020 won't be much affected by a launch in 2022. On the other hand, a mission design based on a 2018 launch could run into big trouble with a slip of launch date.
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JRehling
post Dec 11 2012, 09:32 PM
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I certainly stand corrected. It looks like there's a phase shift in the relationship between opposition distance and energy with the shift being about one launch opportunity.
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djellison
post Dec 11 2012, 09:33 PM
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QUOTE (mcaplinger @ Dec 11 2012, 01:02 PM) *
one of the options discusssed is replacing the entry balance masses with an actively-controlled trim tab.


Yeah - that (and others) were even looked at earlier in MSL development (my favorite was using tanks of mercury that could be pumped around the backshell) - but they were dumped just to keep the architecture simple. Mass wasn't a problem - complexity, reliability and schedule were - so they went with the simplest option.

To be honest, I'd expect them to do the same this time around.
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fredk
post Dec 11 2012, 10:09 PM
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QUOTE (mcaplinger @ Dec 11 2012, 10:17 PM) *
2009: 10.27
2011: 8.95
2013: 8.78
2016: 7.99
2018: 7.74
2020: 13.17
2022: 13.79

So getting back to my question, this shows that 2020 is worse than 2011 in terms of delta v^2, so all else being the same a 2020 MSL2 could carry less payload than the current MSL. How easily can this delta v^2 difference be translated into a payload mass difference?

2018, on the other hand, is a bit better than 2011.
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mcaplinger
post Dec 11 2012, 10:26 PM
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QUOTE (fredk @ Dec 11 2012, 03:09 PM) *
So getting back to my question, this shows that 2020 is worse than 2011 in terms of delta v^2, so all else being the same a 2020 MSL2 could carry less payload than the current MSL.

Not a foregone conclusion, since MSL probably wasn't using all of the C3 available.

In general I think C3 scales as the square of injected mass, but I haven't seen a detailed analysis of the 2020 opportunity. For 2018 there is a detailed breakdown in http://www.nap.edu/reports/13117/App%20G%2...gy-Explorer.pdf


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brianc
post Dec 11 2012, 10:54 PM
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QUOTE (mcaplinger @ Dec 11 2012, 09:02 PM) *
http://trs-new.jpl.nasa.gov/dspace/bitstre...2011_216988.pdf is an interesting study of potential improvements to MSL; one of the options discusssed is replacing the entry balance masses with an actively-controlled trim tab. Of course it's not clear how many changes to the MSL architecture are going to be possible for cost and schedule reasons.


Why not replace the entry balance masses with something useful such as Penetrators or Micro-probes

http://www.planetaryprobe.eu/IPPW7/proceed...ion7B/pr401.pdf


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mcaplinger
post Dec 11 2012, 11:07 PM
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QUOTE (brianc @ Dec 11 2012, 03:54 PM) *
Why not replace the entry balance masses with something useful such as Penetrators or Micro-probes

The usual reasons: cost, complexity, increased mission risk.

Volumetrically it's impossible to get a microprobe to weigh as much as a piece of tungsten. At some point it just wouldn't fit in the available space.


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Explorer1
post Dec 11 2012, 11:27 PM
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And they wouldn't be balanced very well either, which is the whole point of a ballast mass. Penetrators and others would need a dedicated mission of their own.
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stevesliva
post Dec 12 2012, 12:45 AM
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It would be neat happenstance if there happened to be a seismometer relatively near the impacts of those ballast masses.
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MahFL
post Dec 12 2012, 12:47 AM
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QUOTE (stevesliva @ Dec 12 2012, 01:45 AM) *
It would be neat happenstance if there happened to be a seismometer relatively near the impacts of those ballast masses.


The ones they have now don't need to be anywhere near the impact.
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