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MSL Approach Phase
nprev
post Jun 23 2012, 05:32 PM
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We're now 45 days from landing, so as of 23 Jun please post all comments related to the end of the transit to Mars here.

Go Curiosity!!!!


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Stu
post Jun 23 2012, 06:54 PM
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Approach Phase.

Blimey.

blink.gif

Buckle up, people.



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imipak
post Jun 23 2012, 10:23 PM
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...and extinguish all cigarettes? Doesn't time fly in retrospect, but crawl when looking forwards...


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Guest_Oersted_*
post Jun 25 2012, 12:54 PM
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Good write-up by elakdawalla here:
http://www.planetary.org/blogs/emily-lakda...and-part-1.html
-Looking very much forward to the upcoming parts!

Just have a question, though. It says the "nose will tip upwards by about 20 degrees" when the first set of mass balances are thrown. I thought the nose would tip downwards, to enable a bit of lift by the airshell and heat shield? I.e. that the "plate" of the spacecraft would "lie more flat", in stead of "standing more on its edge", with respect to the Mars gravity field.
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mcaplinger
post Jun 27 2012, 03:41 AM
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QUOTE (Oersted @ Jun 25 2012, 04:54 AM) *
Just have a question, though. It says the "nose will tip upwards by about 20 degrees" when the first set of mass balances are thrown. I thought the nose would tip downwards, to enable a bit of lift by the airshell and heat shield?

The lift vector/bank angle is steered during entry in whatever direction is needed to hit the aim point. See http://trs-new.jpl.nasa.gov/dspace/bitstre...8/1/08-0255.pdf -- section 4.


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Lightning
post Jun 27 2012, 09:06 AM
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A third (and short) TCM occured yesterday. The spacecraft gained 50 mm/s, shifting as planned the landing site to about 7km toward Sharp Moutain.
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Guest_Oersted_*
post Jun 27 2012, 10:19 PM
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Ok, I found the answer to which way the entry configuration is influenced by the shedding of the cruise balance masses. As I thought, the "nose" of the spacecraft does indeed tip downwards by twenty degrees, to enable a guided lifting entry:

Described on p. 2 of
"Mars Science Laboratory. Entry, Descent, and Landing System Overview"
http://trs-new.jpl.nasa.gov/dspace/bitstre...9/1/10-1775.pdf

Attached Image


mcaplinger, yes, the spacecraft doers indeed maneuver around during entry, but the jettison of the CBMs gives it a new stable configuration, with the nose pointing downwards so the lift vector points slighty forwards.

- Another interesting fact, new to me, is that the thrusters may actually fire after parachute deploy, to dampen any unwanted rotation of the capsule under the chute. That's never been done before. "Wrist mode damping is active throughout parachute descent and ensures a safe heatshield separation, good TDS surface acquisition, and a safe backshell separation" (p. 8).

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brellis
post Jun 28 2012, 01:25 AM
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A question I had for Emily's *very entertaining and fear-reducing* Google-hang, still can't find an answer: how much more than .75m/sec vertical drop can the rover take, for example if the crane lets go too soon? Can it take the drop the Vikings withstood?

Edit: pospa's post in the Cruise thread provides the basis for my question.
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PDP8E
post Jun 28 2012, 02:28 AM
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I assume the cruise stages 'burn-up' (no heat shield).
Is there any evidence that there are pieces laying about in 'new craters' beyond or before the landing ellipses?
(i.e. Pathfinder, MER-A/B, Phoenix, others, and now MSL)


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djellison
post Jun 28 2012, 05:21 AM
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I've had a look downrange for Opportunity in whatever MOC and HiRISE images I could - but found nothing.

Most would burn up - a few small components might make it thru - but I doubt a thorough analysis has been done.

As for touchdown rates - this is interesting reading
http://trs-new.jpl.nasa.gov/dspace/bitstre...7/1/06-1785.pdf
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brellis
post Jun 28 2012, 12:25 PM
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Thanks Doug. So, on flat terrain, MSL can handle as much as 1.25m/sec vertical velocity whereas the Vikings landed at 2.5m/sec. I'm not nervous -- I'm perfectly calm! unsure.gif
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djellison
post Jun 28 2012, 02:11 PM
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And we have 30+ years of improvements in radar, IMU's, software etc etc.

Moreover - it's not like 1.26m/sec will result in complete and utter devastation - a dinged wheel, a slightly bent suspension strut etc etc - the rover would still be able to carry on.

I'l wager the actual touchdown vertical velocity will be < 0.8m/sec
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ElkGroveDan
post Jun 28 2012, 04:51 PM
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I'd also wager that those are spec tolerances. I'd be willing to bet it could take a bigger thump than that with no appreciable damage.


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MarsEngineer
post Jun 29 2012, 01:11 AM
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Hi again,

Doug is correct (he seems to have the knack, me on the other hand...). There is margin well above 0.8 m/s (I forget the number but it depends a lot on surface characteristics if my memory serves). With these sorts of multi-DOF control systems, either it is coming down well within the spec or ... it won't and something is wildly wrong.

Ironically the Phoenix and Viking landers could not afford to land slower due to dynamics issues with slower velocity (need for leg stroke for touchdown detection, surface-plume interaction, fuel cost, etc). However the velocity knowledge and control accuracy for Viking and Phoenix was fantastic and about the same as MSL's (in fact MSL and Phoenix both use the same inertial measurement unit (IMU) design). Because of the skycrane architecture, MSL is simply able to capitalize on the IMU and on the fact that the decent engines are a long way from the surface and wheels, that the engines are dynamically uncoupled with the rover touchdown event to allow a much slower terminal descent velocity. If we were willing to use more fuel we could probably have reduced the velocity even more, but we did not need to. The big benefit of a slower touchdown is that the rover's wheels (aka "legs") can be used as landing gear plus that slow velocity really broadens the spectrum of Mars surfaces that are considered "safe". (Of course I am wildly biased - opinions expressed are those of the co-co-co-co-inventer and do not reflect NASA/JPL/Caltech).

With "somewhat controlled velocity" landing systems like MER (6 solid rockets) or "nearly controlled velocity" landing systems like Pathfinder (3 solid rockets) the landing system obviously has to be robust to a much wider range of impact velocities (let alone surface characteristics).

You know I can't tell you how much fun it is to come by here (I wish I had more time!!!). I get a huge kick from the thoughtful discussions of risk, and the fun walks through memory lane (like the link that SFJCody left on Post#169 to a 1997 bulletin board about the weirdness of Pathfinder's landing system and even a discussion about my old web page on EDL I created back then - what a hoot!)

In my opinion, all of these missions (especially the ones that have to land safely on Mars) are experimental vehicles and have a rather substantial element of risk. For all of the Mars lander missions I have worked on (MPF, MER, PHX and MSL) like everyone else, I am initially daunted by the vast array of all the "things that must go right". The mountain ahead seems insurmountable. But then I (we) look down at my (our) feet and move myself one step at a time, one minute detail at a time, oftentimes with insufferable pauses as we ruminate, test and argue over the safety of each tiny step, sometimes having to go backwards and find another path or add new paths that we thought would never be there. Slowly, ever so slowly and with infinite patience we gain altitude, only vaguely aware of the progress we have made. The really hard part is knowing when the mountain has been scaled. Too often it appears that the summit is ahead and we can relax, only to discover that the top is further ahead than it appears, and yet another push must be mustered. Of course the top really can NOT be seen and no one knows for certain, until it is over.

I do find though that there is a feeling I get that tells me the top is there in front of me (if only I could see it). It is really a feeling that we have run out of places to put our feet. No more tests left to ponder, no more problem reports to close, no more reviews to hold, only a far away machine waiting for Mars to arrive. I think we have nearly run out of places to put our feet. Could it be that we are there? Almost, I see a couple of more places to step. Next couple of week perhaps?

-Rob Manning
MSL Chief Engineer and faux climber

Opinions expressed are indeed those of the author and do not reflect the opinion of NASA/JPL/Caltech

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nprev
post Jun 29 2012, 01:18 AM
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Rob, and we are absolutely delighted when you do have a chance to visit, believe me! smile.gif

Great post.


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