[X] My Assistant

[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.

[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.