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MSL Cruise Phase
pospa
post Jun 14 2012, 07:19 AM
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For those who still doubt about skycrane system robustness this presentation might help a bit. smile.gif

Some terminal descent challenges and strategy is described here.
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djellison
post Jun 14 2012, 10:41 PM
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Someone was asking about the actual mechanism that lowers the rover under the descent stage...

http://photojournal.jpl.nasa.gov/catalog/PIA11428
http://photojournal.jpl.nasa.gov/catalog/PIA11426

On the second image the S/N001

I love the optimism that there might be cause to make as many as 999 of them smile.gif

Not as optimistic as MSSS who use S/N00001 smile.gif
http://www.msss.com/images/science/MAHLI_PP0121_wb_cb.jpg
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elakdawalla
post Jun 15 2012, 12:04 AM
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Oy. I was on the Google+ Space Hangout this morning and another space blogger was giving an account of the story of the landing ellipse being moved. They said that the reduction in ellipse size meant that the rover was more likely to crash into the mountain on descent. The amount of wrongness in that summary was hard to bear. I corrected the statement, but clearly it's an uphill battle to explain how landing on Mars actually works.

Thanks, Doug, for that photo; I hadn't seen it before. And pospa, those links are very useful.



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SFJCody
post Jun 15 2012, 02:01 AM
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People were skeptical of airbags before Pathfinder landed. Novelty is often treated with suspicion.

http://groups.google.com/group/sci.space.t...finder+airbags#
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Guest_Oersted_*
post Jun 16 2012, 10:40 AM
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QUOTE (elakdawalla @ Jun 14 2012, 05:56 AM) *
John Grotzinger told me that at the outset the engineers had some concern about the three-bodies-connected-by-strings problem but after poking at the situation in all kinds of ways, both simulated and empirical, they found it to be remarkably (to them) stable. I'm afraid I didn't follow his explanation at any level of detail deeper than that, but it's not a concern.


Glad that my posting elicited this interesting tidbit of information. It doesn't surprise me that the JPL engineers "had some concern" at the outset. But, if the right attenuation and active dampening mechanisms are built into the system, then of course the "dynamic-bodies-connected-by-string" problem can be solved. A lot of systems - at least since the F-16 - have been inherently unstable, but work just fine with high-speed computers in the drivers' seat (F-16 "negative stability" described here: http://www.fas.org/programs/ssp/man/uswpns...ghter/f16.html).

Didn't know that the skycrane phase is much shorter than indicated in the movie, thanks for explaining that Doug! - So, how short would it nominally be then?
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Guest_Oersted_*
post Jun 16 2012, 10:43 AM
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QUOTE (elakdawalla @ Jun 15 2012, 02:04 AM) *
Oy. I was on the Google+ Space Hangout this morning and another space blogger was giving an account of the story of the landing ellipse being moved. They said that the reduction in ellipse size meant that the rover was more likely to crash into the mountain on descent. The amount of wrongness in that summary was hard to bear. I corrected the statement, but clearly it's an uphill battle to explain how landing on Mars actually works.


Just like Phoenix that almost crashed into Heimdall crater... - wasn't that what the MRO/HiRISE shot showed? rolleyes.gif

http://dalsgaard.eu/Pics/2008-06.Phoenix-o...n-Dalsgaard.jpg
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pospa
post Jun 16 2012, 12:52 PM
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QUOTE (Oersted @ Jun 16 2012, 12:40 PM) *
Didn't know that the skycrane phase is much shorter than indicated in the movie - So, how short would it nominally be then?

See my previous post with link to presentation, page 10.
The Sky Crane maneuver / rover deployment starts about 19 m above the ground and 14-15 sec before touchdown.
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Guest_Oersted_*
post Jun 16 2012, 06:01 PM
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Thanks!
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DavidVicari
post Jun 18 2012, 07:47 AM
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Does the decent stage have its own computer? I figure it must at least have something simple to perform the fly-away maneuver. Do the rovers redundant computers control most of the decent? If not, does the decent stage have redundant computers like the rover?

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pospa
post Jun 18 2012, 10:06 AM
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The spacecraft’s main computer inside the rover controls all activities during EDL.

Regarding DS fly-away the MSL EDL System Overview can explain all details of this maneuver:

"Once touchdown is declared, the DS halts vertical motion and the triple bridles are cut. The BUD has built-in retraction springs to retract the now free bridles away from the Rover top deck. At this point, control is transferred to the Flyaway Controller on the DS and the command to cut the umbilical is issued. Once the flyaway controller on the DS assumes control, it first holds the current altitude for 187 msec to allow sufficient time for the umbilical to be cut. After the
requisite hold time, the MLEs throttle up and the DS ascends vertically for a predetermined amount of time. Then, the DS begins to execute a turn to approximately 45 pitch. The DS holds this attitude with the MLEs at 100% until the fuel depletes. The hold, ascent, and turn take place within 2 seconds, and the remaining time is variable depending on the amount of fuel remaining. The DS will then ballistically fall to the surface at a distance of at least 150 m from the Rover."
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djellison
post Jun 18 2012, 01:38 PM
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Only change to that I know of is that the flyaway has been changed from until-depletion, to a 4 second burn. There is sufficient fuel margin to allow that to happen, and it's preferable to get that burn in, than a potential explosion in a depletion event after a burn that might be even longer.
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pospa
post Jun 18 2012, 02:07 PM
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Thanks for the update, good to know. smile.gif
So, if flyaway burn will be fixed time now then the distance of DS wreck from just landed Curiosity should be clear as well (not only "at least 150 m from the rover").
Doug, do you know that more accurate value?
Thx
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djellison
post Jun 18 2012, 03:29 PM
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Several hundred meters. It depends exactly how much fuel is left at that moment, how it tumbles at burnout etc etc. There's a fairly large dispersion to it I'd expect.

With no rover to carry, and almost all its fuel exhausted - even just using 4 of its 8 engines, that descent stage is going to haul out of there.

VERY crude approximation....4 x 3060N of thrust on about 800kg of descent stage with maybe 100kg of fuel remaining.... 13.6m/s/s - so after 4 seconds it'll be at about 54.4 m/sec.

http://www.calctool.org/CALC/phys/newtonian/projectile

45 degrees, 54m/sec, gravity of 3.711. Max height is 343m, distance is 785m and it'll take 20.6 seconds ( that's assuming no drag etc)

I'd put error bars of 50% on that..but it'll be something like that.



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ElkGroveDan
post Jun 18 2012, 04:01 PM
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Is there any return of telemetry from the descent stage after the bridle is cut? I would imagine that data on low-altitude flight performance would certainly be valuable -- even chaotic flight.



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djellison
post Jun 18 2012, 04:18 PM
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Nope - that would invoke a bunch of engineering requirements on the vehicle it just doesn't need. All we actually care about is the rover. The redundent UHF radios are in the rover itself, and the three UHF antennae used during EDL ( Parachute cone, Descent Stage and Rover ) are all driven by the rover UHF radios. The descent stage doesn't have UHF transmitters of its own

This is the MSL telecom bible - it's amazing! http://descanso.jpl.nasa.gov/DPSummary/Des...MSL_Telecom.pdf

Block diagram on Pg 44. Timeline on Pg 25

The descent stage does have X-Band transmitters - but that's simply to transmit the tones we're used to seing a-la MPF/MER. It's transmitter is 100 watts to try and hammer thru the plasma and it does so via two LGAs on the backshell (one at 17.5 degrees to account for the pitch of the capsule during entry) and the LGA on the descent stage. The rover can pick up and transmit as well, but its X-Band is only 15 watts

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