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

[DanielW]

I would wonder if the engineers might talk Musk out of adding a redundant pump if they can just make the one more robust. I seems like understanding and fixing the issue is better than redundancy for a non-mission critical system.

[RDMM2081]

Observation: During the initial grid fin deployment, they deployed to their "flight configuration position" very slowly.  I assume this is because of the hydraulic pump issue, however the announcer clearly said something to the effect:  "The grid fins are deploying nice and slow so (some reason)".

Which is true or more likely given that we now know there was a serious issue with the grid fin hydraulic pump system?

The TI fins have always deployed much more slowly than the aluminum ones. That didn't look unusual to me.

Not sure how I never caught onto that fact but thank you for pointing it out.

[Lars-J]

It's amazing watching the entire structure flex and bend from the torques being put on the grid finds and then the whole structure twist when it impacts the water but holds together! That's some incredible engineering!

I think most of the bending is actually the camera not being firmly enough attached to the stage. (due to G-forces)

No, the camera doesn't move with respect to the rocket body, only the fins twist.

Look again. Or point out where you see bending. The camera is certainly shaken loose in the water impact.

[ugordan]

I would wonder if the engineers might talk Musk out of adding a redundant pump if they can just make the one more robust. I seems like understanding and fixing the issue is better than redundancy for a non-mission critical system.

Anyone count up how many flights it took for a pump to fail, counting all the other failed landings like running out of hydraulic fluid, running out of prop, etc.?

[CorvusCorax]

...

Spin stabilized?

Heh. Back to the roots. Has been the tool of choice since the first rockets ever.

Its what I always do in Kerbal Space Program when I have asymmetric thrust failures and the rocket wants to pitch over during launch. Full roll, spin it up, and hope that the forces cancel out

What's remarkable is that the rocket was able to ignite and perform its landing burn while spinning. Didn't SpaceX first water landing attempt fail because a spin centrifuged the propellant to the tank side walls and the pump inlets were running dry?

Obviously that got fixed

[StuffOfInterest]

I wonder if the control software could be modified so that when a fin freezes the fin on the opposite side can be placed in a position to dampen the action of the first one and then let the two other grids fins do all the work?  There wouldn't be as much control but if it can avoid a roll then there may be enough to get the booster down.

How do you propose to do that if all the fins are powered by the single pump?

Ouch.  I thought originally it was independent pumps but I guess it is one pump driving all four actuators.  Definitely nothing that can be done there without a redundant string.

[Doesitfloat]

Well no one has shown this yet.
During stage separation the 2nd Stage M-Vac engine ignited and blew bunch of fire'n stuff into stage 1.
Here are a couple screen grabs from the SpaceX feed. 
If there is only one pump for all the gridfins a break in any hydrolic line will eventualy  drain the entire system.
So system was probably fine till it got blasted by stage 2. 

[Oersted]

However, to your point, I'm not sure what you said really matters that much at this point, while of course there will be more robust/redundant systems, this landing attempt undeniably illustrates how much further we need things to be for propulsive landing of people (especially for the purposes of leisure travel) to become something. We can't claim BFR will be more reliable considering it hasn't even done a hop test.

Propulsive landing wasn't the issue here. Actually propulsive landing probably saved the stage and it made a soft landing. Had the stage been recovered using parachutes such spin I'd assume would tangle them and kill the chute system. So the propulsive landing IS the right choice if one were just looking at this footage Loss of a major control surface that actually worked against the system is what caused the issue here and gives though on how this will impact the BFS landings if one of the control surfaces seriously locks up.

Perhaps they will have a jettison system with explosive bolts / pyrotechnics for grid fins or control surfaces that get stuck in an unhelpful position. With partially redundant control surfaces that could "take over" for each other that would be helpful.

[ugordan]

Block 5.1

By the time they got to their ten reflights of a single booster goal, I fully expected it to get to something like 5.3 regardless of this...

[kevinof]

There she blows..

https://twitter.com/ChrisG_NSF/status/1070407175611277314

[ugordan]

Well no one has shown this yet.
During stage separation the 2nd Stage M-Vac engine ignited and blew bunch of fire'n stuff into stage 1.
Here are a couple screen grabs from the SpaceX feed. 
If there is only one pump for all the gridfins a break in any hydrolic line will eventualy  drain the entire system.
So system was probably fine till it got blasted by stage 2. 

That happens on literally every flight. You'd think that if the effect was so severe, they'd figure out by now that hardware was getting seriously damaged by those "fireworks".

[mme]

Well no one has shown this yet.
During stage separation the 2nd Stage M-Vac engine ignited and blew bunch of fire'n stuff into stage 1.
Here are a couple screen grabs from the SpaceX feed. 
If there is only one pump for all the gridfins a break in any hydrolic line will eventualy  drain the entire system.
So system was probably fine till it got blasted by stage 2.

This is pretty normal for RTLS quick flips. Maybe it's related, but I would not assume. I also think Musk is assuming in a way. Unless telemetry told them something specific, all they know is the grid fins locked up. I think they'll figure it out but I would wait for a real analysis of the first stage that will hence forth be named "Bob."

[Oersted]

Everyday Astronaut played a video of it a few minutes ago. Looked wild and then much calmer as landing burn started.

It looked like the leg deploy reduced the roll quite swiftly, much like an ice skater but aerodynamic drag may have slowed it more than the shifting weight.

As it slowed down, the stuck grid fins would have less authority (because there is not as much air flowing through them) and the RCS would have more authority. Makes sense that it looked more stable right as it neared a dead stop.

That makes sense, and the legs are inline with the fins. As soon as the legs start to deploy, what little airflow there is over the fins will be turbulent. I couldn't see it in the video, but likely to be RCS that killed the roll.

Transfer of momentum. When the legs deploy they transfer momentum slowing the roll. When that occurred the control system had enough authority to finish the roll dampening just before surface contact. It might have even survived a land landing.

If a stuck grid fin caused the roll I would say that the slowdown in speed had more to do with stopping the roll than any transfer of momentum from the leg deployment. At slow speed the reaction control thrusters could counteract the aerodynamic forces.

[vanoord]

Well no one has shown this yet.
During stage separation the 2nd Stage M-Vac engine ignited and blew bunch of fire'n stuff into stage 1.
Here are a couple screen grabs from the SpaceX feed. 
If there is only one pump for all the gridfins a break in any hydrolic line will eventualy  drain the entire system.
So system was probably fine till it got blasted by stage 2.

It does that most of the time. Check Monday's footage and the same thing happened.

[DigitalMan]

Block 5.1

By the time they got to their ten reflights of a single booster goal, I fully expected it to get to something like 5.3 regardless of this...

I seem to recall a tweet about S2 reliability improvements during the tweet-fest about mini-BFS for F9 when it was determined SpaceX would not be doing mini-BFS.

[Lars-J]

Everyday Astronaut played a video of it a few minutes ago. Looked wild and then much calmer as landing burn started.

It looked like the leg deploy reduced the roll quite swiftly, much like an ice skater but aerodynamic drag may have slowed it more than the shifting weight.

As it slowed down, the stuck grid fins would have less authority (because there is not as much air flowing through them) and the RCS would have more authority. Makes sense that it looked more stable right as it neared a dead stop.

That makes sense, and the legs are inline with the fins. As soon as the legs start to deploy, what little airflow there is over the fins will be turbulent. I couldn't see it in the video, but likely to be RCS that killed the roll.

Transfer of momentum. When the legs deploy they transfer momentum slowing the roll. When that occurred the control system had enough authority to finish the roll dampening just before surface contact. It might have even survived a land landing.

If a stuck grid fin caused the roll I would say that the slowdown in speed had more to do with stopping the roll than any transfer of momentum from the leg deployment. At slow speed the reaction control thrusters could counteract the aerodynamic forces.

Nope. With the legs extended the "wider momentum" makes it much harder for the RCS fins to affect roll. This is another reason why the legs are deployed at the last second. Sure, lower speed helps in that it makes the stuck grid fins less effective, but note the timing of the leg deploy and the slowdown of the roll. It's all from the legs.

[ugordan]

I would wait for a real analysis of the first stage that will hence forth be named "Bob."

I like that name.

Personally, I don't see this stage flying again regardless of what Musk says, at least not as a whole unit. Maybe the tankage and interstage could be reused, but the rest will probably be scrapped and tested to see the effects of salt-water corrosion on flown hardware.

[Hauerg]

I wonder if the control software could be modified so that when a fin freezes the fin on the opposite side can be placed in a position to dampen the action of the first one and then let the two other grids fins do all the work?  There wouldn't be as much control but if it can avoid a roll then there may be enough to get the booster down.

How do you propose to do that if all the fins are powered by the single pump?

Ouch.  I thought originally it was independent pumps but I guess it is one pump driving all four actuators.  Definitely nothing that can be done there without a redundant string.

Except for ejecting/blasting off all the grid fins. Which, of course introduce new failure modes nobody needs.

[sghill]

Based on the full landing video that Elon posted, looks like the leg deployment momentum shift had the most impact in reducing the roll. (like a spinning figure skater)

And it looks like the grid fins were locked in a position to increase the roll... So the RCS fought that roll all the way down.

If this were the 1950-60s, we'd be talking about the unbelievable skills of the pilot who wrestled the craft all the way to the ground and regained control at literally the last moment to soft land his aircraft intact....

Good job Major Tom!

[leetdan]

GO craft are on the move, per redacted local surveillance resources.

Edit: GO Quest is underway heading offshore, after shuffling with GO Navigator.