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MSL Cruise Phase
brellis
post Jun 12 2012, 05:32 AM
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What are the numbers on "chance of failure" during the procedures of the 7 minute plunge from outer space to Gale crater? I'm really scared about this.

Is the scariest part getting into the atmosphere at the correct angle?

Does the sky crane have a failure potential based on the fact that it's never been tried on Mars?

Narrowing the landing ellipse eases my fears about the early parts of this process. The sky crane part has me all twisted up in concern!
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djellison
post Jun 12 2012, 06:52 AM
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QUOTE (brellis @ Jun 11 2012, 09:32 PM) *
Is the scariest part getting into the atmosphere at the correct angle?


That's something we've done many times

QUOTE
Does the sky crane have a failure potential based on the fact that it's never been tried on Mars?


Yup. As has every landing system sent to Mars. When Viking went, it hadn't been tried on Mars. Nor had Pathfinder. Nor had MER ( which was similar to, but far from identical to Pathfinder ) And Phoenix's only previous test was a failure. There are many many ways in which MSL could fail during EDL.

QUOTE
Narrowing the landing ellipse eases my fears about the early parts of this process. The sky crane part has me all twisted up in concern!


Many many people seem to point at the Sky Crane and express concern, but no one's ever pointed out to me what it is that has them all concerned. Why does it make you any more or less concerned than Viking, Pathfinder, MER or Phoenix?

It's worth noting that the very same team that put Spirit and Opportunity on the surface of Mars - is the team responsible for MSL EDL.



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Paolo
post Jun 12 2012, 08:17 AM
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QUOTE (nprev @ Jun 12 2012, 03:08 AM) *
Do you have a link for this teflon problem, or was this a comment from a press conference?


see also the Science blog
http://news.sciencemag.org/scienceinsider/...minate-its.html
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brellis
post Jun 12 2012, 01:41 PM
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"Why does it make you any more or less concerned than Viking, Pathfinder, MER or Phoenix?"

Lack of redundancy makes this trip scarier to me. There was some comfort on the Viking and MER missions because we had two shots at everything. It took ten years for the Polar Lander mission to rise again as Phoenix.

Regarding feeling 'scared' about the Sky Crane: it just seems that there are so many more phases to the procedure that have to be executed perfectly as compared to MER. I do take comfort from the success of the Phoenix landing. In the case of the Sky Crane, a thousand things can go perfectly but if the 'umbilical cord' stage of the landing fails, this giant SUV might clunk down to the Martian surface, or get dragged away if the cord doesn't detach, or...or...or smile.gif
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djellison
post Jun 12 2012, 02:51 PM
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QUOTE (brellis @ Jun 12 2012, 06:41 AM) *
it just seems that there are so many more phases to the procedure that have to be executed perfectly as compared to MER.


Really?

Put it this way....think of MER.

Now take off the solid rocket RAD motors over which there is no control authority once ignited. Replace them with throttlable liquid engines. How different are they, really?

There was an umbilical cord between an the MER airbags and backshell. That had to be cut at the exact right moment or lander would have been dragged away by residual RAD motor thrust or have the backshell collapse ontop of the airbags.

Take the EDL timelines and put it side by side with previous ones...honestly - there are not any more phases to MSL.

Cruise stage sep. (same)
MSL has a phase here - ballast jettison.
Entry. (difference here - MSL's is guided using a lifting body design, but Vikings was 3-axis RCS as well, just without the nav algo)
MSL has another here - more ballast jettison.
Chute Deployment (same)
Heat shield sep (same)
Radar lock on the ground (same)
Backshell Sep (difference here - backshell sep on MSL is like PHX/Viking. MER's was more like the skycrane phase of MSL)
Powered decent (same as PHX and Viking. MER was very susceptible to cross winds at this phase. PHX, VIking, MSL are not. MSL, PHX and Viking can control this phase. MER's RAD motors, fired later in EDL, are solids with no adjustments once fired)
Constant velocity phase ( same as PHX and Viking)
At this moment - MER had to do DIMES imaging and airbag inflation. MSL doesn't.
Spacecraft separation ( like the MER rappelling down it's bridle from the back shell)
Mobility deploy (like deploying Viking of PHX's landing legs - at this moment in the MER timeline, it's firing the RAD motors and TIRS motors. Once fired, zero control authority on the spacecraft attitude etc)
Touchdown detection (far easier on MSL. Viking and PHX had touchdown sensors on the legs - potentially the cause of the failure of MPL. MSL just waits for the throttle to decrease on its engines to maintain constant velocity as the rover is offloaded onto the ground)
Bridle cut ( MER had this as well. It had to be timed perfectly. There is several seconds of margin for MSL here)

From this point on - VIking and PHX are safely on the ground and finished.

MER and MSL have more work to do.

MSL's Descent stage turns up the descent engines to full throttle for 4 seconds and it flys away at 45 degrees to get away from the rover.
MSL is now safely on the ground on its wheels, on the dirt. The only critical deployment left is the camera mast, probably done 2 days later which all these vehicles ( apart from Viking ) had to do.

MER however - has to conduct all of its bounces safely. Once the bounces have stopped...
Deflate the airbags
Retract the airbags
Identify which petal is face down, and begin to open the lander, righting itself if necessary.
Deploy the solar arrays.
Standup the rover using the lift mechanism.
Unfold the front wheels and lock in place. Drive the rear wheels out and lock in place.
Cut bridle to the lander
Drive off the lander (Spirit of course, had to turn around on the lander to do this)
NOW you've got the rover safely on the ground on its wheels in the dirt.

MSL has none of that to do.

Honestly - I look at the two side by side and I see MORE steps of complexity for MER. MER was far less robust, those spacecraft took massive impact G's on those bounces. A big nasty rock on any of them and it could have been game over. MSL will be the most gentle touchdown on any planetary surface in history - including Apollo. Once that touchdown is over, we're done. With MER, that just began another final phase to landing... impact to egress.

Moreover - the MSL landing ellipse has been entirely mapped, at 25cm/pixel, with 1m/pixel DTMs. MER was not. HiRISE and CRISM were not available when MER's landing sites were picked. Gale is the safest landing site we've sent a vehicle to.

I know that there's a gut reaction to the skycrane phase of MSL that make it look scary.

So was MER. We just forget how scary it was because it worked twice.

As for redundency....it's worth noting there is almost zero redundancy within each MER itself. The redundancy was two vehicles. MSL has two flight computers etc etc. There is system redundancy within it.

I worked on this : http://www.youtube.com/watch?v=P4boyXQuUIw...;feature=relmfu : as technical director for about a year. Lots of meetings with the MSL EDL team. The breadth and depth to the testing, the fidelity of simulations, the options and trade spaces they've looks at, the way they've gone about designing building and testing this system....it gives me far more confidence than I ever had for MER.

I really want to understand why people think MSL is worse than MER. So - after me saying all that - what is it that still stands out? Sorry if this comes across as bit agressively - but I really want to understand why people seem to be more scared by this than MER. It doesn't make sense to me at this point.
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brellis
post Jun 12 2012, 03:15 PM
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Doug, thanks for the detailed reply. I'll downgrade my current emotion from "scared" to "encouraged, but still a bit worried". smile.gif

Great vid -- you helped design/produce that? Very impressive!

QUOTE
I really want to understand why people think MSL is worse than MER. So - after me saying all that - what is it that still stands out?


I don't think MSL is worse, it's a wonderful upgrade. It's also bigger, which had me worried for a while, but now I feel better, thanks to your response. I'm still gonna save some munchies for the nervous nellie moments when the Curiosity SUV arrives!
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elakdawalla
post Jun 12 2012, 03:41 PM
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During my research for my Sky & Tel article on MSL, I was surprised to discover that of all the problems facing MSL that caused its launch delay, the EDL system was not one of them. EDL was ready two years before they launched. In the unexpected extra two years, they've had time to simulate all kinds of bad situations during landing -- unexpectedly high winds, low/high air density, landing on steep slopes, on big rocks, and all of these at once; and they've found that the EDL system not only meets its engineering requirements, it copes with really terrible situations. They told the landing site selection committee that they could land this rover practically anywhere. This is not to say that nothing bad can happen -- it surely can -- but there's nothing specific for us to fear; the engineers really have built a system that should be robust to even serious problems. I'm going to be having an anxiety attack on landing day, but not about any specific issue, and frankly, I'm very glad to be moving away from a landing system that intentionally bounces a half-billion-dollar spacecraft several times.


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Tesheiner
post Jun 12 2012, 03:44 PM
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QUOTE
I really want to understand why people think MSL is worse than MER.

My impression is that this ("MSL is more dangerous") is a subjective conclusion because MSL EDL ***looks*** much more dynamic than MER. Someone really have to go into the details of each EDL sequence to make an objective assessment.
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algorimancer
post Jun 12 2012, 04:43 PM
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My only real concern with MSL's EDL is the deployment on that cable. I've seen too many cases of variants of snagging occur during that sort of scenario -- case in point being the Shuttle-deployed tethered-satellite-system. I'm hoping no spool is involved in MSL, I'm envisioning something more like the stowed loops of tether as used in parachutes, designed to smoothly deploy without any intrinsic mechanism for snagging. Probably this mechanism has been well-tested, and won't be a problem.
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elakdawalla
post Jun 12 2012, 05:06 PM
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QUOTE (algorimancer @ Jun 12 2012, 09:43 AM) *
Probably this mechanism has been well-tested, and won't be a problem.
That is what I would call a safe assumption.
QUOTE (Tesheiner @ Jun 12 2012, 08:44 AM) *
My impression is that this ("MSL is more dangerous") is a subjective conclusion because MSL EDL ***looks*** much more dynamic than MER. Someone really have to go into the details of each EDL sequence to make an objective assessment.
I think it's also a basic fear of change.


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ElkGroveDan
post Jun 12 2012, 05:20 PM
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I'm going to guess that because the MERs have the appearance of a ballistic delivery (dropping a beach ball from high up) they intuitively seem simpler to someone not taking into account the complexities and timing of RAD firings, bag inflation, ground lock radar, bridle cut, airbag deflation, etc.


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Explorer1
post Jun 12 2012, 05:26 PM
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We could easily extrapolate and say any moving part is a point of failure, right? Galileo's HGA, Mars Express' radar (which was solved eventually), Odyssey (and Hayabusa's) reaction wheels, Genesis, whatever happened to Mars 3, Phoenix's oven doors, etc, etc.
But of course no one launches a solid block of metal that can't do anything. The entire point is to have scientific instruments, spacecraft control, and yes, complex but well-test EDL methods. They are a necessity, so cheer up doubters!
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elakdawalla
post Jun 12 2012, 05:34 PM
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Amen, Explorer1!

Let me add a general comment on responding to MSL EDL fear. For those of us who have actually been paying attention, repeated questions like "are they crazy?" and "how can that possibly work?" and "have they tested that?" drive us batty. I urge people (and sometimes need to remind myself) to respond to questions like these with firmness but also patience both at UMSF and elsewhere. Probably every person just waking up to MSL will ask these questions. We will do a greater service to Mars exploration by not treating EDL worries as being ridiculous but instead by calmly responding to it with answers rooted in facts. It's a tightrope to walk; the message is that landing on Mars is hard and failure can happen, but also that the engineers really have thought this through and the landing mechanism is a robust and well-tested one.

XKCD had a relevant cartoon about this recently: http://xkcd.com/1053/

If you're not familiar with XKCD, note that mousing over the cartoon will produce more text. In this case it's:
QUOTE
Saying "what kind of an idiot doesn't know about the Yellowstone supervolcano?" is so much more boring than telling someone about the Yellowstone supervolcano for the first time.


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djellison
post Jun 12 2012, 08:48 PM
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QUOTE (brellis @ Jun 12 2012, 08:15 AM) *
Great vid -- you helped design/produce that? Very impressive!


Yup - I was the technical director, the interface between JPL and the animation team. We didn't quite nail everything, but given the schedule and budget, we got pretty damn close.
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Gsnorgathon
post Jun 12 2012, 09:58 PM
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I think part of the unease with MSL EDL - especially the "L" part of EDL - is that the rover is *naked*. Everything else that's landed on Mars had legs or airbags. It's not that either of those are any guarantee of safe landing (e.g. all the Soviet landers, Beagle 2, MPL), it's just that they *look* safer. Curiosity is just out there hanging by what looks like a thread, as part of a landing that looks right out of a Gerry and Sylvia Anderson production. It just seems so improbable.
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