MSL Cruise Phase Options

[brellis]

hi Emily

algorimancer pinpoints my concern -- we're landing six wheels on Mars! Hanging from a Skycrane! As you wisely mentioned on the current Opportunity thread, after all the complexity of these fantastic machines why do we have problems with the WHEEL?

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.

I'm glad that my favorite informed reporter is nervous about EDL! What did cause the launch delay? My 'outsider' internet search only speaks of 'inadequate test time'.

Thanks to you and Doug for answering my questions. I really do poke around the webz before daring to ask!

[elakdawalla]

I hope you will not be surprised to learn that that was discussed in depth on this forum. There is, in fact, a thread titled "MSL Launch Delay".

[brellis]

Emily, I am happy to tell you that I am not surprised to learn of this, but I couldn't find it on my searches. The Space Review article cited in that thread answers my question. I'll try to keep quiet now!

[siravan]

I really want to understand why people think MSL is worse than MER.

I guess it is just simple psychology. We are not comparing MER before landing to MSL before landing; we are actually comparing our feeling about MER after couple of successful landings to our risk assessment about MSL EDL.

[centsworth_II]

algorimancer pinpoints my concern -- we're landing six wheels on Mars!

Thus eliminating potential problems that could occur driving off a platform down a ramp.

[centsworth_II]

My only real concern with MSL's EDL is the deployment on that cable....

MER was deployed on a cable. See beginning at 2:30.
http://www.youtube.com/watch?v=tZRXwRybb1I

[djellison]

Curiosity is just out there hanging by what looks like a thread,

Just like Pathfinder, Spirit and Opportunity. Actually - they were hanging by a single thread...Curiosity will be hanging by three.

For those wondering - yes - there were tests of that deployment for MSL - here's a video of just one of them : http://www.youtube.com/watch?v=YasCQRAWRwU

A bit more video of other tests here - http://www.youtube.com/watch?feature=playe...Z-6snF0Q#t=147s

[elakdawalla]

I'll admit to having a brief "oh my god what were they thinking" moment a few weeks ago. Someone from JPL's outreach office did a presentation about MSL at my daughter's school. She had brought a box of goodies that she had convinced engineers to part with -- bits of spent test hardware. One of these was a sample of the cable on which MSL is descending. In my mind, I'd imagined steel cable. It is not steel. It is braided nylon. It looks a hell of a lot like clothesline. I was, briefly, aghast. Then she handed me another piece of hardware: a spent pyro cutter device, one of the things that will cut that nylon cable when it's time for the descent stage to break away. I almost dropped it, it was so unexpectedly heavy: solid steel, or maybe even something heavier. This is a spacecraft; mass is important. Nylon is clearly strong enough (and reliable enough) for the purpose, while being very very light. That guillotine of a cable cutter is made of what it needs to be made of. You can be quite sure that every material in this rover has been carefully thought through and optimized for mass, strength, and whatever other properties it needs.

brellis: Google has made us all very lazy. I don't always find what I'm looking for on UMSF with Google. But we admins work very hard to keep this forum well-organized. I found that thread by clicking on the MSL forum link at the top of the page and browsing backward in time.

[MahFL]

I love these insights to MSL. Sometimes we followers of MSL fret that the thing won't work. In reality though many talented people designed and tested MSL to work, not fail .

[climber]

I think Doug wrote it, we know MER worked and we have fear what is "unknown".
Anyway, I'm also very confident in the system for several reasons.
This technique has been developped for landing larger masses than previously so it's a stepping stone for the future.
When I "accidently" met the EDL team (see here: http://www.unmannedspaceflight.com/index.php?showtopic=3765) it was in early 2007 and they were already working on it for several years. I've seen with my eyes how much those guys are dedicated to their job.
I certainly will be bitting my nails at EDL but well, these tense moments are what keep our passion for Space alive.

[Guest_Oersted_*]

My fear with regard to the landing would stem from the fact that we have a dynamic two-body system, where the lower body has to arrive at a certain position with quite a low tolerance for accelerations on arrival. The MER airbag clusters had to arrive at an altitude whose precision was measured in meters, and with rather forgiving tolerances for arrival accelerations, compared to MSL.
There was an ancient computer game twenty-odd years ago in which you had to control a two-body system where the jet pack was on one of the two bodies only. It was fiendishly difficult. Two bodies connected by a three-stringed non-rigid bridle arrangement sounds even more challenging. However, I have great confidence in the engineers at JPL and their expertise easily cancels out my gut instincts and fears.

[nprev]

Yes, I believe that it's quite safe indeed to assume that the MSL landing system is not based on 20-yo gaming technology.

[elakdawalla]

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.

[djellison]

The sky crane phase is also very brief, just a few seconds. We intentionally made it longer in the project animation to give editors etc plenty of B-roll to use at a later date.

[Stu]

Bottom line: the guys behind this are almost supernaturally clever and have almost magical levels of technology. If they weren't sure this would work they wouldn't be *doing* it. Let's all trust them, ok?