SpaceX Falcon 9 : CRS-16 (Dragon SpX-16) : December 5, 2018 - DISCUSSION

[mainmind]

Re-upping this question so it doesn't get lost among the booster recovery talk. What is the constraint that they had to do a Dragon back-away to the 30m hold when they weren't going to have TDRSS at the planned capture time? Abundance of caution? Is the command to send Dragon to free drift right before grapple coming in over TDRSS and not a direct command from ISS? It sounded  like the back-away command was sent from ISS.

Thank you

info probably over here SpaceX CRS-16 Dragon - RNDZ, ISS Ops, EOM - UPDATES

Sadly no. It just notes the ground issue with TDRSS causing a retreat to the 30 meter hold from the capture box and later the resumption of approach to capture point. No explanation of why ISS to ground link is required to perform the capture with SSRMS. Good thought, but the mods seem to keep the "update" threads clean of extra info.

[meekGee]

It's SpX internal investigation, or with NASA?

As booster recovery isn't a NASA requirement, I assume it is a purely SpaceX investigation

I'm pretty sure there will be some sort of landing mishap investigation.  It will likely be SpaceX only with possible involvement/positions for USAF Range Safety and the FAA as those are the two organizations responsible for approving RTLS landings operations.  And maybe an observer from NASA just to keep their hand in.  The technical focus of exactly what failed and why won't be of much interest to any one but SpaceX.  However a review of the performance of in-place safety rules/precautions for RTLS and an examination of potential areas of improvement or needed changes will be of interest to the other groups.  If Range Safety and FAA aren't actually represented on the board, for sure they will be CC'd on the report as well as be doing their own analysis as to safety impacts.

I agree with this, it would be a missed opportunity for the FAA and USAF ranges not to review what happened on this flight.  They definitely have an interest in the SpaceX landing processes.

Absolutely.  And it's 100% in SpaceX's benefit for this investigations to happen, since SpaceX came out of this looking really good - there was an anomaly, and existing plans and designs worked, plus later the vehicle even saved itself, for extra credit.

When the time comes for P2P talks, this is as good a demonstrator SpaceX can have for their approach.

[RedLineTrain]

Really liked the rendezvous and berthing webcast.  Great dry-run for DM-1 & 2.  As a suggestion to SpaceX, it would be nice if there were some information on the screen:

*the speaker identified (as in many videoconference programs); and
*some sort of timeline or on-screen notations to indicate what is happening or about to happen, as on the launch webcasts.

Another suggestion:

*auto fade-in and fade-out on the music when speakers are talking.  Love to listen to the radio comms.

Overall, I thought the talking heads did a great job, but they narrated things a bit more heavily than is my taste over such a long webcast.

[Rocket Science]

[wannamoonbase]

After a nice salty soak and the initial quenching it’s hard to see how this bird flys again.

But I want to see them try.  Just recovering it is impressive.

[Mixi55]

Great video - Falcon 9 booster landing in ocean with music from the "Interstellar".
The descent looks even more suspenseful and exciting this way 

[webdan]

Interesting video analysis, his voice reminds me of Craig Ferguson.

I'm agreeing with most except where the fins actually do operate as he mentions.

In other words, I would have thought that each is individually controlled, because they do have separate actuators.

If not, does anybody have any reference so I can update my mental knowledge base? Thanks!

[JBF]

Interesting video analysis, his voice reminds me of Craig Ferguson.

I'm agreeing with most except where the fins actually do operate as he mentions.

In other words, I would have thought that each is individually controlled, because they do have separate actuators.

If not, does anybody have any reference so I can update my mental knowledge base? Thanks!

They are individually controlled, but there is only 1 pump to pressurize the system.

[Joffan]

Interesting video analysis, his voice reminds me of Craig Ferguson.

I'm agreeing with most except where the fins actually do operate as he mentions.

In other words, I would have thought that each is individually controlled, because they do have separate actuators.

If not, does anybody have any reference so I can update my mental knowledge base? Thanks!

As far as I can tell, grid fins have individual actuation (and they wouldn't be very useful without that). For the divert maneuver, they would likely operate roughly in the way that Scott Manley described,  with one pair driving the pitch-over to retarget onto the landing pad, although that wouldn't be the only way they could operate. Based on Elon's remarks since the failure, all the fins are supplied with pressure fluid from a common hydraulic reservoir which is pressurized with just the one pump.

[AndyH]

Re-upping this question so it doesn't get lost among the booster recovery talk. What is the constraint that they had to do a Dragon back-away to the 30m hold when they weren't going to have TDRSS at the planned capture time? Abundance of caution? Is the command to send Dragon to free drift right before grapple coming in over TDRSS and not a direct command from ISS? It sounded  like the back-away command was sent from ISS.

Thank you

info probably over here SpaceX CRS-16 Dragon - RNDZ, ISS Ops, EOM - UPDATES

Sadly no. It just notes the ground issue with TDRSS causing a retreat to the 30 meter hold from the capture box and later the resumption of approach to capture point. No explanation of why ISS to ground link is required to perform the capture with SSRMS. Good thought, but the mods seem to keep the "update" threads clean of extra info.

Apparently the arm is at least initially controlled from the ground, and there was a ground failure that prevented the TDRS link required.  They moved Dragon back until the ground crew had a good satellite link, then they resumed the process.

[MattBaker]

Well, they didn't let them do an RTLS at Vandenberg earlier this week in case something happens, I wonder how the Air Force looks at that decision after this failure.

On the one hand if the exact same failure happens, seems like there's no danger to Vandenberg at all, the stage just lands in the Pacific (that recovery would be worse though, seeing as the West Coast SpaceX fleet is based further away than "just pop down the road"-Cape Canaveral).

On the other hand, if something happens a couple of minutes later during the landing burn or something else just doesn't work in general...sure shows there'll always be some danger to this whole thing.

Actually now that I think about it, having your landing site be directly next to your launch pad is probably convenient but if something goes wrong...especially at your only West Coast pad compared to the two Florida pads you have...well, I hope all their failures end as smoothly and entertainingly as this one.

[meekGee]

Well, they didn't let them do an RTLS at Vandenberg earlier this week in case something happens, I wonder how the Air Force looks at that decision after this failure.

On the one hand if the exact same failure happens, seems like there's no danger to Vandenberg at all, the stage just lands in the Pacific (that recovery would be worse though, seeing as the West Coast SpaceX fleet is based further away than "just pop down the road"-Cape Canaveral).

On the other hand, if something happens a couple of minutes later during the landing burn or something else just doesn't work in general...sure shows there'll always be some danger to this whole thing.

Actually now that I think about it, having your landing site be directly next to your launch pad is probably convenient but if something goes wrong...especially at your only West Coast pad compared to the two Florida pads you have...well, I hope all their failures end as smoothly and entertainingly as this one.

We know the answer to your first question.

The stage doesn't shift the IIP onto shore until the main engine relight happens and the engine is confirmed good.

We can also assume that any other health checks that fail also result in an offshore landing.

They can also walk the IIP along a safe corridor as it approaches the landing pad.  There's no reason that the post-relight path must be collinear with the pre-relight path.

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ABCD: Always Be Counting Down

[MattBaker]

What if the hydraulic pump stalls during the final main engine relight? Especially during the transition of the landing point so it can't stabilise to stop lateral movement?

Which of course is an unfair question because we don't know what caused it or what the stage's reaction to it would be so I can use the hydraulics as "magical failure turning this into dangerous hypothetical that probably never happens".

But when their secret billion dollar projects are on the line the Air Force can probably be pretty unfair, too.

[CorvusCorax]

What if the hydraulic pump stalls during the final main engine relight? Especially during the transition of the landing point so it can't stabilise to stop lateral movement?

Which of course is an unfair question because we don't know what caused it or what the stage's reaction to it would be so I can use the hydraulics as "magical failure turning this into dangerous hypothetical that probably never happens".

But when their secret billion dollar projects are on the line the Air Force can probably be pretty unfair, too.

Actually, we had that before. Remember a certain drone ship landing that grid fins went into hardover due to lack of hydraulic fluid?

The stage still managed to hit the deck of the ship, but wasn't able to land. If it can hit the deck then it can certainly hit its landing pad.

The drone ship mishaps provide a useful case study for minimum safe distance and debris impacts. I think the COPV reach the furthest since they are mini-helium-rockets in their own right

[MattBaker]

Oh man, that was, what, 30 launches ago? Forgot about that.

Hope we never have to find out what's the worst case on the West Coast then because it's pretty close from the landing pad to everything else, the Cape is sorta more forgiving there.

[Jcc]

Interesting video analysis, his voice reminds me of Craig Ferguson.

I'm agreeing with most except where the fins actually do operate as he mentions.

In other words, I would have thought that each is individually controlled, because they do have separate actuators.

If not, does anybody have any reference so I can update my mental knowledge base? Thanks!

As far as I can tell, grid fins have individual actuation (and they wouldn't be very useful without that). For the divert maneuver, they would likely operate roughly in the way that Scott Manley described,  with one pair driving the pitch-over to retarget onto the landing pad, although that wouldn't be the only way they could operate. Based on Elon's remarks since the failure, all the fins are supplied with pressure fluid from a common hydraulic reservoir which is pressurized with just the one pump.

Soon to be two pumps.
You wonder why they didn't consider this previously, but they were a part way through grasshopper landing test when they realized that they needed grid fins.

Makes me wonder how well the " fins" on the New Glenn booster will work out for BO, or if they will have to iterate the design.

[meekGee]

What if the hydraulic pump stalls during the final main engine relight? Especially during the transition of the landing point so it can't stabilise to stop lateral movement?

Which of course is an unfair question because we don't know what caused it or what the stage's reaction to it would be so I can use the hydraulics as "magical failure turning this into dangerous hypothetical that probably never happens".

But when their secret billion dollar projects are on the line the Air Force can probably be pretty unfair, too.

We in the current generation, the engine was able to recover a wildly spinning ship and still land.

So the answer to your question is that if the fins failed after relight, the stage will land at the pad.  The fins are relatively ineffective at that point.

But you may as well alao ask what if the engine failed after relight...  To which the old aviation joke applies: "well sir, in that case, I guarantee you that we won't stay in the air forever".



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ABCD: Always Be Counting Down

[kdhilliard]

I only see 3 grid fins.

I think the fourth one is still there, just hidden behind the blue cradle that the booster is resting on.

Thanks to guckyfan for pointing me to some Ken Kremer tweets (1, 2) with photos accounting for all four grid fins.

Edited to add: Not that we all haven't seen photos of the top three fins (that second photo is for completeness), but all the other photos of the stage on shore that I've seen only hinted at the possibility of a grid fin hiding behind that blue support structure.  This is the first one that makes it clear.

[OxCartMark]

Some component didn't work as it should have, and it wasn't noticed until that component was needed in flight.

Obviously adding redundancy might help. But if an investigation comes to the conclusion that overlooking something in assembly or during test procedures contributed to a faulty part ending up on an operational flight, this might end up with suggestions for operational, test or QC procedure changes that have implications for other flight hardware, relevant to commercial and possibly crewed launches too.

Suggestion: Do a test flight before using a booster for launches that count.  That's the way I think they do it in the airplane world.

Now shifting a bit... There's currently only one hydraulic pump because recovery isn't mission critical.  And the leading candidate change from this is to add a redundant pump.  But you also lack redundancy in the center engine during the recovery burns.  Just staying with pumps there's only one turbopump, one thrust vectoring hydraulic pump (or is it electric motor (doesn't matter)) and one of just about everything else on the engine.  Are you going to double those up now that recovery is a proven real part of flying F9?

Which brings me to... We've seen 3 engine landings, we've seen in this flight that roll control authority can be a challenge in some off script scenarios, we know that its not only the center engine but two additional ones (the ones which are used for re-entry burn) that have an enhanced TEA/TEB capacity and re-startability... I think this leads them to want to write some more software to allow F9 to make a choice to go to a 2 engine (the 2 outboard engines from the re-entry burn) landing burn in some off nominal situations such as the landing we had here.  Perhaps 2 engines could have controlled the roll early and allowed an early decision to go for a normal landing.?.

...Or not as a 2 engine landing burn would have been started at a much lower altitude thus cutting off the option of diverting to the LZ.  Hmm, I can see that this would work beneficially for an ASDS landing where the water impact point is probably much closer to the intended LZ (as evidenced by FH core's impact close to the ASDS) but maybe not so useful in an RTLS scenario where more horizontal divert is needed.  Corollary thought - intentional 3 engine burns are unlikely to be planned for RTLS not only because ASDS is the way to go in propellant challenged recoveries but also because 2 or 3 engine landings significantly reduce the distance between default impact point and intended LZ.

[jak Kennedy]

 2 hydraulic pumps seems the simplest option but perhaps an accumulator or electric servos would be useful even with 1 or 2 pumps to give enough control to return the grid fins to a neutral position instead of them locking in a hard over position.

Edit, spelling