SpaceX F9 : Starlink v1.0 L19 : CCSFS SLC-40 : 15/16 Feb 2021 (0359 UTC)

[Semmel]

If end of life issues were at play here and they strike at the reentry burn, then they were incredibly lucky. Who is to say that the end of life has to happen during the relatively short reentry burn? The ascend burn is much longer and would have a much higher probability to run into any end of life situation. I hope this is not the case and there is some other explanation.

[Prettz]

Are vacuum starts of M1D more stressful than ground starts? To the point an engine might fail during firing, after the ignition?

[edzieba]

A relatively minor point, but we normally hear the Stage 1 FTS safe callout after the entry burn: makes sense, entry burn has the capability to impart a lot of delta-V and grow the potential IIP area (extreme failure case: booster is sideways for some reason and burns perpendicular to trajectory, then slams into atmosphere sideways, and subsequently showers some poor boat outside the keep-out zone with hot Merlin turbomachinery), aerodynamic descent can only slow and contract that area (starts out with the greatest extent aerodynamic divert could achieve and contracts as altitude decreases).
For L19, that callout was before the entry burn.
I don't think it has any relation to the issue, and the fact that the impact point was close enough to the ASDS to be visible on the webcast means that trajectory deviation was within the local horizon shows that FTS would not have been needed (in this case) to prevent debris exiting the keep-out area, so the entry burn has probably been judged as sufficiently well controlled that FTS would not be required afterwards. But an interesting procedural change nonetheless.

[Vettedrmr]

If end of life issues were at play here and they strike at the reentry burn, then they were incredibly lucky. Who is to say that the end of life has to happen during the relatively short reentry burn? The ascend burn is much longer and would have a much higher probability to run into any end of life situation. I hope this is not the case and there is some other explanation.

Statistically mechanical devices are at their happiest when in a steady state condition.  Once F9 is satisfied that the engines are performing nominally it commits to launch.  After that point probably your highest risk of engine failure on ascent is at throttle transients.  Obviously it can happen, but even then F9 has demonstrated the ability to put a payload in orbit AFTER an engine failure.

I expect the entry burn is probably the highest risk startup.  Local heating due to super/hypersonic heating, chaotic flow around the nozzles, etc.

[quagmire]

A relatively minor point, but we normally hear the Stage 1 FTS safe callout after the entry burn: makes sense, entry burn has the capability to impart a lot of delta-V and grow the potential IIP area (extreme failure case: booster is sideways for some reason and burns perpendicular to trajectory, then slams into atmosphere sideways, and subsequently showers some poor boat outside the keep-out zone with hot Merlin turbomachinery), aerodynamic descent can only slow and contract that area (starts out with the greatest extent aerodynamic divert could achieve and contracts as altitude decreases).
For L19, that callout was before the entry burn.
I don't think it has any relation to the issue, and the fact that the impact point was close enough to the ASDS to be visible on the webcast means that trajectory deviation was within the local horizon shows that FTS would not have been needed (in this case) to prevent debris exiting the keep-out area, so the entry burn has probably been judged as sufficiently well controlled that FTS would not be required afterwards. But an interesting procedural change nonetheless.

L18 also had FTS safe before entry burn.

FTS may safe later for RTLS landings and safe's earlier for droneship landings.

EDIT:It does appear to be a difference between landing sites.

Transporter-1 also had FTS Safe before entry burn( droneship)

SXM-7 FTS safe before entry burn( droneship)

SAOCOM 1B FTS safe after entry burn( LZ-1).

NROL-108 FTS safe after entry burn( LZ-1).

[abaddon]

If end of life issues were at play here and they strike at the reentry burn, then they were incredibly lucky. Who is to say that the end of life has to happen during the relatively short reentry burn? The ascend burn is much longer and would have a much higher probability to run into any end of life situation. I hope this is not the case and there is some other explanation.

Luck favors the prepared.  F9 has had engine out on ascent before and has completed its primary mission.  Chances are, it will do so again.

[Herb Schaltegger]

I expect the entry burn is probably the highest risk startup.  Local heating due to super/hypersonic heating, chaotic flow around the nozzles, etc.

I agree generally, but that said, though the flow is hypersonic at entry burn startup, the atmosphere is really, really thin. Heat flux from the atmosphere at the START of the burn is almost certainly a lot less than heat flux from the engines and hot engine exhaust gas recirculation. In any case, it's certainly a more dynamic start environment than that at launch though I don't know if it's overall more challenging or dynamic than at the start of the landing burn. Yes, the vehicle is moving a lot more slowly, but also the engine is starting up at near-sea level pressure rather than in near-vacuum, that creating a much more chaotic and energetic pressure and flow field around the base of the landing rocket.

But in any case, the whole process has worked well for 70+ landings so all of these environmental variables are probably pretty well understood by SpaceX's Falcon 9 EDL team. But it's the unknowns that get you, or the minor stuff you just discount or miss entirely (such as the tiny bit of isopropyl cleaning fluid that was missed in an M1D cleaning/refurb that caused the in-flight Merlin failure last year).

[meekGee]

...
But in any case, the whole process has worked well for 70+ landings so all of these environmental variables are probably pretty well understood by SpaceX's Falcon 9 EDL team. But it's the unknowns that get you, or the minor stuff you just discount or miss entirely (such as the tiny bit of isopropyl cleaning fluid that was missed in an M1D cleaning/refurb that caused the in-flight Merlin failure last year).

I think the bottom line is that if the failed engine showed signs of failure on reentry startup in such a way that had it been during the launch it would have aborted, then the main mission was never at risk.

That's on top of engine-out capability for S1 on ascent (at the likely expense of recovery)

So you have two layers of why an engine failure at reentry is very weakly indicative of launch failure.

[scr00chy]

Hans Koenigsmann, SpaceX: The cause of the Falcon 9 landing failure on the most recent launch was “related to heat damage.” He adds it’s an ongoing investigation and declines to say more.

https://twitter.com/StephenClark1/status/1364247180463263746

[abaddon]

SpaceNews write-up: https://spacenews.com/spacex-blames-failed-falcon-9-booster-landing-on-heat-damage/ adds a little bit more info:

“This has to do with heat damage, but it’s a running investigation,” he said, adding that the company was “close to nailing it down” and correcting the problem. “That’s all I can say at this point in time.”

There are some further forward-looking statements about reuse limits.

[FutureSpaceTourist]

https://twitter.com/stephenclark1/status/1366454776259629061

SpaceX’s Benji Reed: The Falcon 9 landing failure last month was caused by a hole in a “boot” on one of the Merlin engines that allowed in hot gas, triggering an engine shutdown. The boot on this particular engine had flown more times than any other F9 engine boot.

https://twitter.com/bluemoondance74/status/1366454874850869255

The more you fly the more you learn and it has been an opportunity to learn about Falcon on the Starlink missions. On the failed Booster return, we have a cover called "boots" this boot had a hole, got some hot gas where it should not have been...

Engine shut down as it should have, we've been using the boosters with the most flights on them for Starlink, so we're learning much about Falcon for the long term.

[FutureSpaceTourist]

https://twitter.com/nasaspaceflight/status/1366455049036136450

Falcon 9 B1059.6 landing failure update. A Merlin engine boot (a life leader) developed a hole and sent hot gas to "where it wasn't supposed to be" and shut down during first stage flight. Not enough thrust for landing.

[Lars-J]

So this did happen during ascent, and it affected one of the 3 essential engines for landing. Similar to an earlier landing failure, but a different cause it seems. (the boot gas leak)

[leetdan]

So this did happen during ascent, and it affected one of the 3 essential engines for landing. Similar to an earlier landing failure, but a different cause it seems. (the boot gas leak)

How do you know it was during ascent?  "First stage flight" could mean during retropropulsion, which certainly tracks with hot gas entering a compromised engine boot.

Also note identical telemetry: https://forum.nasaspaceflight.com/index.php?topic=52895.msg2192524#msg2192524

[OnWithTheShow]

https://twitter.com/stephenclark1/status/1366454776259629061

SpaceX’s Benji Reed: The Falcon 9 landing failure last month was caused by a hole in a “boot” on one of the Merlin engines that allowed in hot gas, triggering an engine shutdown. The boot on this particular engine had flown more times than any other F9 engine boot.

https://twitter.com/bluemoondance74/status/1366454874850869255

Interesting that they chose to fly this boot with a known hole that presumably they suspected could cause thermal sensors in the engine bay to shut down the engine. Or they have been flying boots with damage and just never had that as a criteria for swapping the part. Or its the first boot to develop this time of damage and it was missed during inspection. Wonder if they caught it just by reviewing close out photos and had an ah-hah moment.

[Vettedrmr]

How do you know it was during ascent?  "First stage flight" could mean during retropropulsion, which certainly tracks with hot gas entering a compromised engine boot.

Also note identical telemetry: https://forum.nasaspaceflight.com/index.php?topic=52895.msg2192524#msg2192524

His description implied it shut down on ascent, but IIRC there was no visible plume change (unless it was the center engine), and I don't remember the launch data showing any kind of degraded performance on ascent either.  So I'm guessing it was a failure on descent (although the burnthrough might have occurred on ascent, as seen on TM later). 

And no, I seriously doubt the hole was there prior to flight.

[LouScheffer]

Falcon 9 B1059.6 landing failure update. A Merlin engine boot (a life leader) developed a hole and sent hot gas to "where it wasn't supposed to be" and shut down during first stage flight. Not enough thrust for landing.

So how do you get a life-leader boot on a non-life-leader booster?

The only boosters with more flights are still around, I believe.  So it would be odd for a boot to be swapped off of one of them.

Maybe all others boosters and engines had their boots replaced before flight 6?

Maybe boots are in one big stockpile (they might have to remove them for refurbishing).  So after refurbishing, you go grab 9 boots from the stockpile, and they happened to get a well-used one?

Curious minds want to know....

[OnWithTheShow]

And no, I seriously doubt the hole was there prior to flight.

Then how would they be able to determine the cause? If they cant see boot damage on the closeout photo how can they definitively say that boot damage and not something else caused the thermal spike in the engine bay?

[Vettedrmr]

And no, I seriously doubt the hole was there prior to flight.

Then how would they be able to determine the cause? If they cant see boot damage on the closeout photo how can they definitively say that boot damage and not something else caused the thermal spike in the engine bay?

Telemetry is the primary method, although post-processing video can sometimes reveal clues as well.

[Comga]

And no, I seriously doubt the hole was there prior to flight.

Then how would they be able to determine the cause? If they cant see boot damage on the closeout photo how can they definitively say that boot damage and not something else caused the thermal spike in the engine bay?

Because they are "rocket scientists "?
They can draw conclusions from telemetry.
Perhaps they don't know the exact size or shape of the "hole" but can sense that the "hot gas" was consistent with a modest opening, rather than something failing entirely.