SpaceX Falcon 9 FT - ORBCOMM-2 - Dec. 21, 2015 (Return To Flight) DISCUSSION

[Steven Pietrobon]

I wonder if adaptive optics could be used to get an improved image. Here's a possible source.

http://www.stellarproducts.com/

[OnWithTheShow]

Wouldnt the thrust fluctuations have caused an abort anyway? Maybe thats why its so short?

[terryy]

Looks like it only lit for a second or so to me... ~1:50 mark in the video

It sounded like a pretty long fire to me.  It takes a few second for the sound of the rocket motor firing to reach the camera's location.  But at around the 2:00 mark you can start to hear the distinct rumble of the engine firing and it pretty much lasts all the way till the end of the video.

[Tonioroffo]

From this cool angle viewing with black line inserted as a centerline, the 1st stage seems to have shifted as much as 1.5 meter to the right at the last 2 meter without any issues!

...if the cam was stable.  It simply might have shifted sideways under the forces of the landing?

[somepitch]

Looks like it only lit for a second or so to me... ~1:50 mark in the video

It sounded like a pretty long fire to me.  It takes a few second for the sound of the rocket motor firing to reach the camera's location.  But at around the 2:00 mark you can start to hear the distinct rumble of the engine firing and it pretty much lasts all the way till the end of the video.

And you can see the plume of smoke billowing up on the far right side of the video out of the flame duct. It clearly lit for an extended period of time.

Yeah my mistake... serves me right for watching on on a phone in a loud place and making a comment.  Carry on...

[NovaSilisko]

Could someone please interpret 'debris ingestion' for the layman. It sounds like something that could have happened during ascent. It's concerning that he jumped to that explanation. One would expect the rocket is designed to be free of debris-generating parts, fuel fully filtered, etc. Then again Merlin was actually qualification tested with a loose nut in the feed line, or so it was said.

"Debris" can mean "anything that's not propellant", really. My suspicion is that it's some soot that got blown back up into the tank or engine piping somehow, possibly from airflow during EDL, from having a valve that got stuck open or something similar. Not really macroscopic chunks of debris, but more like impurities that mess with the combustion. Given nothing seems to have blown up, we're not talking about an N-1 style "ate a bolt and died horribly" sort of scenario.

[laika_fr]

Could someone please interpret 'debris ingestion' for the layman.

Some speculations about the "debris",

A while ago SpaceX came up with his own turbo pump model ( formerly it was bought from Barber Nichols ).
We did not had much informations about this new one, whether is it was vastly improved or not. Where it stands exactly, in fact we don't know.
It's one of the big question marks about this experiment (at least for the public).

On the other hand, Max Vozoff stated at the Space Show that the M1D could withstand some debris for instance coming from the turbo pump fan without exploding, and actually even keep running.
So you can go this way and imagine that something didn't  went well during the relight.

Or on a brighter side you could also consider that SpaceX, is standing much more further than that, thanks to extensive Merlin test runs. Therefore the so called "debris" may be relatively benign.

By the way, i have no answers, but Europe is eager to know of course.

[MP99]

Could someone please interpret 'debris ingestion' for the layman. It sounds like something that could have happened during ascent. It's concerning that he jumped to that explanation. One would expect the rocket is designed to be free of debris-generating parts, fuel fully filtered, etc. Then again Merlin was actually qualification tested with a loose nut in the feed line, or so it was said.

"Debris" can mean "anything that's not propellant", really. My suspicion is that it's some soot that got blown back up into the tank or engine piping somehow, possibly from airflow during EDL, from having a valve that got stuck open or something similar. Not really macroscopic chunks of debris, but more like impurities that mess with the combustion. Given nothing seems to have blown up, we're not talking about an N-1 style "ate a bolt and died horribly" sort of scenario.

Perhaps even more likely is dust, etc blown up during the landing. Doesn't seem plausible this would affect the active (centre) engine, but the others have no protection of an active exhaust plume.

I'd speculate that this was a concern identified as part of the design of the landing process, which might explain why Elon jumped to that as a possible explanation in his tweet.

Cheers, Martin

[Jakusb]

I hate it when they're all secretive like that.

You mean, when they share details of partial failures so openly, what no other company ever would dare to?
It keeps amazing me Elon keeps doing that. As if he has his head inside the engine while tweeting it as it happens, almost live.
A man with a mission.

[LouScheffer]

Could someone please interpret 'debris ingestion' for the layman. It sounds like something that could have happened during ascent. It's concerning that he jumped to that explanation. One would expect the rocket is designed to be free of debris-generating parts, fuel fully filtered, etc. Then again Merlin was actually qualification tested with a loose nut in the feed line, or so it was said.

This seems weird to me, too.  Presumably it worked normally on the way up. And it's hard for me to imagine that the fluctuations were caused by the turbo machinery.  It is well instrumented, presumably under tight closed loop control, and since it's the main RUD risk, I'd think they'd stop the static fire if there were fluctuations here.

The landing might kick up debris (chunks of concrete, small rocks, etc) and these might strike the outer engines.  But they inspected the engine bell, and must have found no big dents.  it's hard to imagine debris getting much further than the injector.  Can damage to these surfaces cause thrust fluctuations?

Maybe, from past experience, when an engine basically works, but experiences thrust fluctuations, it's problems with the injector?  And since it worked on the way up, debris is the suspected cause?  That would be my best guess....

[CorvusCorax]

I think the term "thrust fluctuations" is interesting word choice. There's lots of terms to describe non-nominal engine burn from a thrust point of view -
- loss of thrust
- reduced thrust
- thrust fluctuations
- thrust excursion
...
all of which have slightly different semantics. Now if that tweet was from a politician or public relations spokesman with limited technical expertise, then there would be no point in dwelling on the subtle subtext of every single word.
But the Tweet is by Elon Musk, and Elon Musk is way too focused on these things to use any technical term inappropriately.

If the engine had had loss of thrust from some debris ingestion leading to emergency shutdown/ static fire abort, I don't think "thrust fluctuations" would have been the choice of words. Instead I think we can take them very literally. They did the burn, possibly for its full duration, then looked at the data and found outliers on engine #9 engine thrust.

Fluctuations likely suggest one or more temporary drops in engine thrust.(Although temporary increases would also qualify as fluctuations, I don't see how that would suggest debris ingestion - unless the 'debris' were drops of unbunt TEA/TEB or similar volatiles)

More importantly a temporary drop in thrust (literal fluctuation) does not suggest damage to turbopump/turbine, but of course better safe than sorry and borescope it

[LouScheffer]

I think the term "thrust fluctuations" is interesting word choice. There's lots of terms to describe non-nominal engine burn from a thrust point of view -
- loss of thrust
- reduced thrust
- thrust fluctuations
- thrust excursion
...
all of which have slightly different semantics.

Also, thrust is a vector quantity.  So it could it be side-to-side variations, perhaps caused by the flow sticking, or not sticking, to the surface, which might be rougher after being dinged.  Or maybe some mild form of combustion instability.  Of course lessons like this, whatever the cause, are exactly what these tests were intended to discover.  So in some sense the test has already succeeded.

[OxCartMark]

Anyone have any idea / can educate me how a water deluge system that's designed to spew its contents in a few seconds during launch can be made to support a full duration firing?

My *guess* on "thrust fluctuations" and "debris" - something has gummed up the throttle valve and its (once again) not responsive, over or undershooting.

I find it odd that a) the ignition fluid flash was so pronounced (don't recall ever seeing it at all) and b) that there was not just one flash but two separate blue / green flash events.

It appears to me that those folks that are still hanging onto extravagant refurbishment conspiracy theories have a much decreased perch upon which to hang their hopes.

[Jim]

My suspicion is that it's some soot that got blown back up into the tank or engine piping somehow, possibly from airflow during EDL, from having a valve that got stuck open or something similar. Not really macroscopic chunks of debris, but more like impurities that mess with the combustion. Given nothing seems to have blown up, we're not talking about an N-1 style "ate a bolt and died horribly" sort of scenario.

Not really.  Anything with carbon in it would have ignited when exposed to LOX.

[NovaSilisko]

My suspicion is that it's some soot that got blown back up into the tank or engine piping somehow, possibly from airflow during EDL, from having a valve that got stuck open or something similar. Not really macroscopic chunks of debris, but more like impurities that mess with the combustion. Given nothing seems to have blown up, we're not talking about an N-1 style "ate a bolt and died horribly" sort of scenario.

Not really.  Anything with carbon in it would have ignited when exposed to LOX.

Hmmm. What about the feasibility of something (anything, like dust) going back up a non-burning engine in general? Is the path for that too complex for it to be a reasonable possibility, even assuming valves got stuck open? I'm trying to think of things that are specific to the reentry/landing procedure, rather than just extended numbers of firings, which have been done on individual engines/stages before.

[John Alan]

I will speculate that the fluctuation is a result of a sensor malfunction issue...   
Sensors can and sometimes will lie to you... lesson learned in my line of work...   

That said... it fired up... 
Proved it still is in 'basic' working order... 

Take S1 back to 39 and use it as a checkout mule for that pad... as already planned...
Swap engine 9 and take it to Texas for a engine stand run to find true root issue with it...

[Lars-J]

The noise to signal ratio in these speculations are off the chart. If you don't know much about it, writing paragraphs is not going to make you know more.

[ugordan]

Wouldnt the thrust fluctuations have caused an abort anyway? Maybe thats why its so short?

Watching that grainy video I do get the impression that the burn was cut short of the intended duration, for a couple of reasons. Usually, when there's an abort at ignition, you can see the LOX and RP-1 tanks being immediately vented, this is what is seen here. Secondly, one would not expect the burn to end with another TEA/TEB flash. Also, chamber ignition and thrust ramp-up on M1Ds seems to start 2 seconds before T-0, the actual burn here seems to have been shorter.

[RonM]

So far, this is a great success, even with one engine acting up during the test.

If this was a stage being prepared for another launch, all they would have to do is swap out the engine in question. Then do another test with the replacement engine. If it passed, send the stage to the next processing step. The swapped out engine would then go back to the shop to be fixed.

Looks like SpaceX will soon be able to do a rapid turnaround. Of course, they need to take it slow at first to confirm the condition of the returned stages, but so far this is a good sign that the design is on the right track.

[deadman719]

While a lot of people are focused on the engine fluctuations engine 9 experienced in the static fire, the importance of this test may be overlooked. On the pad was a first stage which endured the rigors of launch, entry, and landing.  With little maintenance, based on public knowledge, after those events, it was once again erected on the launch pad and fired up. While it remains to be seen as to the root cause of the engine fluctuations, eight of nine engines operated with no reported issues. This is huge!