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#80
by
apace
on 28 Jun, 2011 13:05
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Don't understand the question. Also, it is a descent stage and not a skycrane
My question was, why to develop an additional method (sky crane) of lowering something to the ground and not use a more traditional method like a normal lander with long legs as we can see in the animations for future human landings (where the payload is between the descent stage rocket motors). Or can this sky crane also be used for more heavy payloads?
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#81
by
Nomadd
on 28 Jun, 2011 13:10
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I found no answer on the MSL web page, so my question here to the MSL Sky Crane and I hope someone knows it.
Why using a Sky Crane, where a lot can go wrong instead of a normal landing. If I watch the animations, there should be no problem to land the rover attached to the Sky Crane part and after landing releasing it. Of course, there need extended legs, but such a landing would be easier in my opinion and the way to go for future landings.
Greetings,
Daniel
JPL calls the manuever Skycrane. Not the actual descent stage hardware.
This a a pretty good article on the reasons for going this route.
http://www.space.com/3163-heavy-lift-helicopter-inspires-nasas-mars-lander.html
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#82
by
apace
on 28 Jun, 2011 14:40
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I found no answer on the MSL web page, so my question here to the MSL Sky Crane and I hope someone knows it.
Why using a Sky Crane, where a lot can go wrong instead of a normal landing. If I watch the animations, there should be no problem to land the rover attached to the Sky Crane part and after landing releasing it. Of course, there need extended legs, but such a landing would be easier in my opinion and the way to go for future landings.
Greetings,
Daniel
JPL calls the manuever Skycrane. Not the actual descent stage hardware.
This a a pretty good article on the reasons for going this route.
http://www.space.com/3163-heavy-lift-helicopter-inspires-nasas-mars-lander.html
thanx!
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#83
by
Proponent
on 29 Jun, 2011 03:03
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I like the article, but I think it's a little bit wrong about some of the reasons for using a Skycrane approach.
Pointing to a hazard of traditional landings on legs, the article states "Failure of any of the thrusters to cease firing at just the right time could send the lander hopping across the surface, as happened with NASA's Surveyor robotic lunar lander in 1967." To my knowledge, one Surveyor hopped --
Surveyor 6 -- but the hop was intentional. Well after landing, the engines were briefly re-ignited. After re-landing, the probe
photographed its previous footprint.
The next paragraph reads "Getting a rover off a legged lander after touchdown poses additional challenges, Steltzner said. Ramps are customarily used, but there is no guarantee that the martian terrain and an imprecise landing will not conspire to deny the rover a safe path to the surface. On the 1996 Mars Pathfinder mission, for example, only one of the landing platform's two ramps opened onto a clear path for the tiny Sojourner rover. NASA could have just as well found both paths blocked."
In the case of Pathfinder, wasn't the ramp blocked by one of the airbags rather than by martian terrain (maybe that should be "areain"?
, as the article suggests?
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#84
by
STS-200
on 29 Jun, 2011 08:14
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I still don't understand the advantage of lowering the payload (rover) on a set of cables and making the descent bus hover then fly away.
What is the reason for separating the two prior to landing? It seems like a very time-critical event which could lead to all sorts of instability (i.e. the rover swinging on its cables or getting hung up) immediately prior to landing. Why not land together, then separate once you are stable on the surface.
It would mean the bus having to have landing legs or some sort of "slide off" mechanism to allow the rover to get out from underneath, but would seem to save on winches, cables and flyaway fuel.
(I understand why the rover is at the bottom of the lander - stability & the risks of driving down a ramp).
I have little doubt there is a good reason - but what?
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#85
by
mrbliss
on 29 Jun, 2011 15:31
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STS-200, I feel the same way. Time-critical "must works" make my skin crawl.
However, I'm guessing (as an armchair engineer) a factor in favor of skycrane is the 'clean sheet' landing. That is, once the rover is on the surface, a fairly standard pyro cuts the cables, and the rover is ready to go -- there's no "exit the lander" transition. From my limited reading, this exit maneuver has been a challenging part of past rover mission planning, it doesn't contribute any return science or data, and adds significant uncertainty/risk.
The skycrane sounds risky at first, but I think it actually adds little risk -- the descent rockets have to work right anyway, or you don't even get to the skycrane phase. The rover oscillating on the cable is possible, but the descent stage should be able to damp that out -- at least, oscillation is something that can be planned for, modeled, and tested. If the cable unreels partially (or not at all?), I'm guessing the descent stage will just keep descending until the rover is on the ground. The cable-cutter has to work, but NASA has a huge history with that operation.
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#86
by
Proponent
on 30 Jun, 2011 03:43
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Good relevant points, but I think one of STS-200's observations is that you could have a clean-sheet landing without the crane: with the the rocket-and-landing-craft assembly rigidly attached to the top of the rover, touch down, sever the connection between the two, and have the top part fly away.
I'm wondering if maybe it's a control issue. Presumably, especially since the rover will be touching down on wheels designed for traversing rather than on landing gear designed for shock absorption, the touch down needs to be very gentle. Maybe it's easier to get a lander to hover at constant altitude while the rover is winched down than to get a lander to descend very slowly. Just a guess.
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#87
by
MKremer
on 30 Jun, 2011 05:02
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Let's not forget that the rover needs to stay as clean as possible at landing - free from flying dust (+ rocks) and rocket exhaust contamination. Having the descent/landing module hovering well above the rover when it touches down is probably the best compromise solution, I think.
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#88
by
Proponent
on 30 Jun, 2011 05:15
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Limiting the amount of surface debris kicked up by the rocket engines makes sense to me. It's not clear to me, though, that winching the rover down while the rocket engines are burning isn't going to result in a lot of plume contamination.
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#89
by
MKremer
on 30 Jun, 2011 05:31
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Limiting the amount of surface debris kicked up by the rocket engines makes sense to me. It's not clear to me, though, that winching the rover down while the rocket engines are burning isn't going to result in a lot of plume contamination.
Better a very small amount than a whole heck of a lot if the descent module lands with the rover. And additional shields or coverings, etc. required to protect against the rocket plumes/backblast/landing debris are additional mass penalties against the rover itself.
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#90
by
STS-200
on 30 Jun, 2011 09:47
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I hadn't thought of the dust issue. Thanks
As you all say - no doubt it is a compromise - or a series of compromises. I wondered whether it might have anything to do with the packaging of the complete spacecraft (i.e. there isn't room for some critical piece/mass distribution is wrong if you do it another way).
Does anyone know if there is any intention to control the final descent of the bus, or is it just set up to "fly away until you run out of fuel".
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#91
by
MKremer
on 30 Jun, 2011 11:16
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Does anyone know if there is any intention to control the final descent of the bus, or is it just set up to "fly away until you run out of fuel".
Fly away. As soon as the harness is released it's going to start ascending rather quickly anyway... then within 2 seconds it's supposed to roll/pitch to 45 degrees at full thrust and maintain that attitude until the fuel is gone.
The latest landing simulation video includes that flyaway command.
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#92
by
Jim
on 30 Jun, 2011 11:55
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I hadn't thought of the dust issue. Thanks
As you all say - no doubt it is a compromise - or a series of compromises. I wondered whether it might have anything to do with the packaging of the complete spacecraft (i.e. there isn't room for some critical piece/mass distribution is wrong if you do it another way).
Does anyone know if there is any intention to control the final descent of the bus, or is it just set up to "fly away until you run out of fuel".
There is no real control of the descent stage after the cables are cut. The guidance system is in the rover
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#93
by
Jim
on 30 Jun, 2011 11:57
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As you all say - no doubt it is a compromise - or a series of compromises. I wondered whether it might have anything to do with the packaging of the complete spacecraft (i.e. there isn't room for some critical piece/mass distribution is wrong if you do it another way).
It really boils down to two things
a. Rolling off a lander has many risks
b. the rover already has "landing gear"
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#94
by
STS-200
on 30 Jun, 2011 13:32
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It really boils down to two things
a. Rolling off a lander has many risks
b. the rover already has "landing gear"
OK - that makes sense:
Mass of landing legs needed for the complete unit > Mass of cable & winch.
Many thanks.
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#95
by
grakenverb
on 02 Aug, 2011 19:46
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Does MSL use the downward facing camera to avoid placing the rover on top of a large boulder, or will it be used solely for mapping the immediate locale after landing?
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#96
by
Norm38
on 03 Aug, 2012 16:50
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If I'm reading the elevation data correctly, Curiosity's landing site in Gale crater is 2-3km lower in elevation than either of the MER sites.
Is the sky expected to be any bluer due to the greater atmospheric density?
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#97
by
knotnic
on 03 Aug, 2012 21:48
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Does MSL use the downward facing camera to avoid placing the rover on top of a large boulder, or will it be used solely for mapping the immediate locale after landing?
No, there is no hazard avoidance. Per one of the rover drivers on UMSF.
Camera is for documentation and radar is for altimetry.
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#98
by
Pheogh
on 03 Aug, 2012 21:52
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Does anyone here know about what kind of twisting motion they might expect from the skycrane bridles. In other words I am dying to find any information about what kind of tolerance there is to the difference in angle between the descent stage and the rover might be expected?
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#99
by
JimO
on 04 Aug, 2012 03:18
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What are the degraded landing conditions, in the event of impact beyond nominal survival levels? Even if the vehicle's wheels/chassis is damaged/destroyed, even if the vehicle does not come down right-side-up, what communications [omni?] and power [nuc] capabilities might remain to allow SOME command and control? And what indications might be received that suggest such a degraded landing has occurred?
, as the article suggests?