Spirit EDL Options

[Guest_Analyst_*]

It has been mentioned here several times that Spirits EDL has been on the edge of failure, that is was a close call. The vehicle was only one or two seconds away from failure.

Is there any evidence (links, papers) available in the public regarding this? Everything I have suggests EDL was within the typical expected uncertainties (3sigma) and the deployment timeline left enough time for an even later parachute deployment and all the subsequent events. Even without DIMES the impact velocity would not be above of the airbag design limits. There exactly has been this close call?

Analyst

PS: Sorry Doug, wrong forum. Please move to Spirit. Thanks.

[MarkL]

That's an interesting, if academic question. You can bet that with hundreds of sensors, pyros, ic's, connectors, hinges, gyros, and accelerometers having to interact perfectly for a successful delivery, EDL scares the pudding out of the mission scientists so there are probably all kinds of apocryphal tales out there. It wouldn't be much of a surprise to hear about plenty of things that almost went wrong. Hopefully, any near disasters have showed up in telemetry and been analyzed carefully. It may be, however, that NASA does not want any record of near failure in the public domain. Perhaps this is something Steve Squyres would be willing to discuss sometime.

JPL posted extensive animations of Opportunity's EDL but nothing similar for Spirit which I've often thought a bit strange.

[Pertinax]

Would not the animations posted on Sol0020 (http://marsrovers.jpl.nasa.gov/gallery/video/spirit01.html) qualify?

Cheers.


-- Pertinax

[AndyG]

Hopefully, any near disasters have showed up in telemetry and been analyzed carefully. It may be, however, that NASA does not want any record of near failure in the public domain. Perhaps this is something Steve Squyres would be willing to discuss sometime.

I'd like to think that all the data - if it exists - of a near-failure would be made public. Who knows what value it might have on other multi-million dollar missions in the future, by whatever nation?

Science - very much with a capital S - is based on admitting mistakes and learning from them. I don't see what JPL/NASA would achieve by suppressing such information.

Andy

[djellison]

JPL posted extensive animations of Opportunity's EDL but nothing similar for Spirit which I've often thought a bit strange.

Wrong way around - we have two anims of the Spirit EDL Sequence, but not of Opportunities.

http://marsrovers.jpl.nasa.gov/gallery/vid...s_Animation.mpg
http://marsrovers.jpl.nasa.gov/gallery/vid...S_Animation.avi

Doug

[nprev]

I thought Oppy was the one that had a close call due to the lower-than-expected density of the atmosphere over Meridiani on landing day...

[Eluchil]

My recollection was that the "close call" was not so much a matter of the design limits but rather simple luck. If the DIMES system had not engaged, Spirit would likely have impacted into the wall of Bonneville crater with unknown consequences according to Roving Mars. It was not a designed as a crater avoidance system but it work as one in this case.

Eluchil

[mcaplinger]

It has been mentioned here several times that Spirits EDL has been on the edge of failure, that is was a close call. The vehicle was only one or two seconds away from failure.

I might argue that even in the best of circumstances the MER EDL system is only "seconds away from failure" -- the margins are pretty tight.

Many MER close calls were described in http://pweb.ae.gatech.edu/people/rbraun/cl...-ugly-truth.pdf
but the one that seems like it fits the bill is "dust storm 10 days before Spirit landing reduces atmospheric density" -- http://sirius.bu.edu/aeronomy/withersmericarus2006.pdf has a lot of technical detail about the atmosphere's state. There was a timing parameter that could be adjusted by ground command to factor in the atmospheric density, and this was commanded shortly before EDL (see http://www.nasa.gov/offices/oce/llis/1480.html and http://www.nasa.gov/pdf/160654main_Mars%20...onsultation.pdf ) That alone would make anyone nervous, although I don't know what the sensitivity of this parameter really was. If Rob Manning drops by this forum again, obviously he would have some insight.

The story in ROVING MARS about DIMES saving the day is certainly based on fact, though the DIMES team understandably would like to believe that their work was critical. I don't think we know for sure what would have happened without DIMES.

[Guest_Analyst_*]

Thank you for the links!

All I have so far is that the MER EDL system performed within its design envelope (timeline, velocity, etc.), twice. Avoiding ground obstacles has not been the purpose of this system. If it has been "a close call" because of a crater, this is nothing the EDL system was planned to avoid and therefore can't be praised or blamed for.

Analyst

[mcaplinger]

Thank you for the links!

All I have so far is that the MER EDL system performed within its design envelope (timeline, velocity, etc.), twice.

If you read the NESC position paper (which is an odd mixtuire of human factors stuff and technical details of EDL) you find that

"Apparently, as a consequence of the initial
low-density encounter, parachute deployment time, triggered at a specified
dynamic pressure of 725 Pa, was later in time and at a lower altitude
(approximately 2-sigma) than expected. Although this reduced the time margins
to complete descent and landing to a low level, as measured by the parachute
deployment altitude, margin was regained because the parachute descended more
slowly than expected. The cause of this fortuitous “over-performance” of the
parachute was not understood."

Since MER didn't include temperature and pressure sensors, doing the EDL reconstruction is problematic. There are many unknown aspects of the system performance, and it's hard to tune the adjustable parameters given limited knowledge of the atmosphere. The Spirit EDL was 2-sigma off in one parameter and made up for that with unexpectedly good chute performance.

That said, I don't disagree with your assessment, but how close it really was is pretty hard to tell.
I think there would be some hard thinking were the MER system to be flown again.

[Guest_Analyst_*]

This paper (sorry, I don't have a link):

Desai, P.N./Knocke, P.C.: "Mars Exploration Rovers Entry, Descent, and Landing Trajectory Analysis"

gives the minimal time needed from parachute deploy to RAD firing with 57 seconds. The actual times are: 88.4 seconds for Spirit, 86.0 second for Opportunity.

Parachute deployment altitude 3sigma: 6.1 - 11.1 km for Spirit and 6.4 - 11.0 km for Opportunity. Actual heigths: 7.54 km for Spirit and 7.52 km for Opportunity.

Looks like pretty solid margins.

Analyst

[Mark Adler]

Wrong way around - we have two anims of the Spirit EDL Sequence, but not of Opportunities.

It turns out that we know a fair bit more about the Spirit landing than Opportunity's. The reason is simple. We had in place the entire EDL team, plus a review board in residence, whose job for three weeks was nothing other than to analyze the Spirit landing in detail to see if there was something we could learn to improve the chances of a successful Opportunity landing. Once Opportunity landed, the motivation to analyze its EDL data was purely academic.

Since then the Opportunity data has been analyzed, but not with the intensity that Spirit's was for Opportunity's sake.

Mark Adler

[Mark Adler]

There was a timing parameter that could be adjusted by ground command to factor in the atmospheric density, and this was commanded shortly before EDL. That alone would make anyone nervous, although I don't know what the sensitivity of this parameter really was.

Those were the parameters for the parachute deployment backup timer. In case the accelerometer failed, or somehow the algorithm for deploying the parachute based on accelerometer data was fooled, there was a timer to force deployment of the parachute within certain bounds. We could in fact get a very high probability of deploying the parachute within its qualification mach and dynamic pressure just with the timer, if we had the atmosphere density profile close to correct. So when our assessment of the atmosphere changed, so did the timer values.

However the accelerometer and associated algorithm did deploy the parachute, so the adjusted parameters never came into play.

Mark Adler

[Mark Adler]

I don't think we know for sure what would have happened without DIMES.

Well, for one thing, Spirit would not have landed in Gusev. Without DIMES to counter the higher modeled winds at Gusev, we would not have been able to certify Gusev as a "safe enough" landing site. We would have instead landed at the "wind-safe" but potentially boring Elysium landing site.

Mark Adler