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

[speedevil]

I was under the impression that the center engine had essentially zero roll control.   Am I wrong?

Zero, not essentially zero as I understand it.
The legs popping out considerably slowed the stage roll.

[Lars-J]

I was under the impression that the center engine had essentially zero roll control.   Am I wrong?

You are correct. If you are parsing Elon's tweet, note that he said "engineS". The RCS is not the main engines, but they are tiny rocket engines responsible for roll control in this scenario.

The grid fins were stuck in a position that increased roll, and the RCS was fighting them all the way down.

[whitelancer64]

In the video just posted by Musk on twitter it looks like the engines touch water, presumably shut down, stage starts tipping over and then there's a small fireball, what's up with that?

I presume the booster started venting its fuel and it somehow ignited? But first where would it ignite, looks like the engines are shut down at this point, can LOX or RP-1 just ignite of the hot smoke around it like that?

And second, is the stage really venting like two seconds after what would have been a landing if it'd be on the ground? Or is this another "well, that wasn't supposed to happen like that"? Or does it have an automatic "emergency vent" mode when it thinks it landed but starts tipping over to prevent a large explosion.

Yep, RP-1 fireball right after splashing down, but before tipping completely over, that would be the venting fuel. Presumably something was hot enough to ignite it.

[martin_nv]

Does anyone know how far ahead/behind the Dragon is in relation to the station? There is a station sighting opportunity here tonight and clear skies so I'm hoping to see the Dragon as well.

[Lars-J]

https://twitter.com/elonmusk/status/1070446975642812416

I think the blast of fire you see after it sets down in the water is from venting the fuel tank which must have had some RP-1 vapor and/or droplets which were ignited by something down by the engines. It seems too controlled to have been the tank springing a leak.

It is, look at it frame by frame and you'll see how there is a release from the lower third of the stage, and this is the vapor or propellant that ignites.

[MattBaker]

So the stage vents the RP-1 in a way where it can ignite, which as far as I know RP-1 is like diesel where it has to be atomised to ignite? I guess a vent is like an oversized spray bottle but...well, seems funny to me that a safety feature is expelling your fuel in a way where it can blow up.

[matthewkantar]

So the stage vents the RP-1 in a way where it can ignite, which as far as I know RP-1 is like diesel where it has to be atomised to ignite? I guess a vent is like an oversized spray bottle but...well, seems funny to me that a safety feature is expelling your fuel in a way where it can blow up.

Not blow up. Burn up.

[eriblo]

I was under the impression that the center engine had essentially zero roll control.   Am I wrong?

You are correct to first order, but if you are not spinning exactly around the long axis you can get a roll-canceling tangential component (preferably you should have TVC actuation faster than the rate of rotation). A really advanced control system would first initiate/increase the axis offset and then gimbal slightly sideways, all while compensating for control coupling and the failed grid fins as well as steering the stage to a soft landing using a minimum thrust larger than the stage weight - that would be impressive indeed

[acsawdey]

So the stage vents the RP-1 in a way where it can ignite, which as far as I know RP-1 is like diesel where it has to be atomised to ignite? I guess a vent is like an oversized spray bottle but...well, seems funny to me that a safety feature is expelling your fuel in a way where it can blow up.

Not blow up. Burn up.

To be fair, normally the stage is shaken by the landing, not stirred by an out-of-controlled spin.

You can see a white cloud jetting from the vent location near the top of the fuel tank. It gets partway down before igniting.

I think there might have been more RP-1 vapor and droplets in the vent than usual. But they do start venting the stage with seconds of touchdown; on normal RTLS landings it's obvious because 2 opposing gas jet nozzles vent all the remaining nitrogen.

[andrewsdanj]

I was under the impression that the center engine had essentially zero roll control.   Am I wrong?

You are correct to first order, but if you are not spinning exactly around the long axis you can get a roll-canceling tangential component (preferably you should have TVC actuation faster than the rate of rotation). A really advanced control system would first initiate/increase the axis offset and then gimbal slightly sideways, all while compensating for control coupling and the failed grid fins as well as steering the stage to a soft landing using a minimum thrust larger than the stage weight - that would be impressive indeed

Also don't forget the grid fins are aero devices, control authority roughly goes with dynamic pressure, so density and the square of velocity. The roll moment from the grid fins would be dropping right through the landing burn, until such a point that the RCS wins. At touchdown the grid fins are purely decorative ;-)

[speedevil]

So the stage vents the RP-1 in a way where it can ignite, which as far as I know RP-1 is like diesel where it has to be atomised to ignite? I guess a vent is like an oversized spray bottle but...well, seems funny to me that a safety feature is expelling your fuel in a way where it can blow up.

It is moderately hard to make a vent in a liquid/gas tank that will always output either liquid or gas, when the tank is tipped.
It's likely that it would exhaust a mixture of gas/liquid in a flammable mist if the liquid is only partially or intermittently covering the intake.
Adding mass to cope with landing in unusual attitudes where the stage is assumed to be destroyed isn't worth it.

[acsawdey]

I was under the impression that the center engine had essentially zero roll control.   Am I wrong?

You are correct to first order, but if you are not spinning exactly around the long axis you can get a roll-canceling tangential component (preferably you should have TVC actuation faster than the rate of rotation). A really advanced control system would first initiate/increase the axis offset and then gimbal slightly sideways, all while compensating for control coupling and the failed grid fins as well as steering the stage to a soft landing using a minimum thrust larger than the stage weight - that would be impressive indeed

Not out of the question that it could have in fact done this -- in one of the videos you can see that there was a large yaw motion just before the legs deployed and some large TVC deflection along with it.

[meekGee]

How does a single engine counter roll?  This was a single-engine landing, not a three-engine landing, unless I'm mistaken.

Single-engine landing burns allow for more control authority

A single M1d engine provides no roll control authority.

If the engine control system is taking the grid fins into account, then I think even a single engine can do it.

-----
ABCD: Always Be Counting Down

[OxCartMark]

Earlier marine radio would have me believing that they're 2 miles north of the port but that's not what I see when I plot the lat/lon.  And that seems to be confirmed by the buoy numbers they're near.

More audio (semi-quotes)-

"We have a line on it N. of 5 &6 and I'm going to spin Eagle around and hold it north of the channel"

"...Hang out overnight, operations manager is coming from Ft. Lauderdale"

"Going to stay here where its 3-4'.  When I got out by its original position it was 6-8'  "

"We'll let you know the plan when we've talked it over with the operations manager ~ maybe, we'll see"  (my interpretation, sounds like that's hours away, maybe tomorrow)

Planning for operations manager: "not a good plan to transfer him in this sea state.  ~~platform not good~~ RIB to stern probably not a good idea.  ...Ladder, scurry up the side"

Brevard county boat was on station with blue lights and CG small boat came in and relieved them.  Seems to be only there for observation.

Coast Guard small boat to Eagle once CG came in and saw the layout: "What is the plan, are you going to stay on scene until first light?"  Eagle: Let's go to Ch. 22 (I don't have access to hear what was said there)

[eriblo]

I was under the impression that the center engine had essentially zero roll control.   Am I wrong?

You are correct to first order, but if you are not spinning exactly around the long axis you can get a roll-canceling tangential component (preferably you should have TVC actuation faster than the rate of rotation). A really advanced control system would first initiate/increase the axis offset and then gimbal slightly sideways, all while compensating for control coupling and the failed grid fins as well as steering the stage to a soft landing using a minimum thrust larger than the stage weight - that would be impressive indeed

Not out of the question that it could have in fact done this -- in one of the videos you can see that there was a large yaw motion just before the legs deployed and some large TVC deflection along with it.

Yeah, quite possible. We know that they have by rad-hard rocket standards obscene processing power and are updating the state-space and running control and trajectory optimization in real time. I don't think they just got lucky

[OxCartMark]

So the stage vents the RP-1 in a way where it can ignite, which as far as I know RP-1 is like diesel where it has to be atomised to ignite? I guess a vent is like an oversized spray bottle but...well, seems funny to me that a safety feature is expelling your fuel in a way where it can blow up.

It is moderately hard to make a vent in a liquid/gas tank that will always output either liquid or gas, when the tank is tipped.
It's likely that it would exhaust a mixture of gas/liquid in a flammable mist if the liquid is only partially or intermittently covering the intake.
Adding mass to cope with landing in unusual attitudes where the stage is assumed to be destroyed isn't worth it.

Slosh, Centerfugeness could raise the fuel level quite a bit beyond normal.

Does anyone remember what the sea state was when the previous non-kaboom F9 water landing was?  I'm hypothesizing that in calm seas when the whole cylinder slaps the water at once we get kaboom but in high seas (6-8' where it flopped) it has a more gradual entry to the water thus nonkaboom.

Now best guess is that it'll be ~8pm ET when the operations manager gets there.

Correction to earlier, its sheriff that's there and CG that went home.

[eeergo]

It was ice and it came off the booster at the lower end, not where the fins are.

No, go to the launch video and watch starting around T+5:50. The perspective is messing with you but you can clearly see that it is a small ring up close, not a bigger ring farther away from the camera. There's frames where you can see it in front of the gridfin, i.e. between the camera and the gridfin.


It may have been ice, no comment on that.  Just that it was from the area around the gridfins and actually bumped into the one on the left before it started floating up and away.

That looks like it's coming from a feedthrough (also supported by its round shape) on the interstage, probably the one highlighted in the attached image.


On the other hand, why the conditions in this flight allowed for it to be detached, as opposed to in other (daylight) missions? It also seemed the gridfins deployed "hiccuping" rather than slowly but smoothly like in previous flights...

[mlindner]

Earlier marine radio would have me believing that they're 2 miles north of the port but that's not what I see when I plot the lat/lon.  And that seems to be confirmed by the buoy numbers they're near.

More audio (semi-quotes)-

"We have a line on it N. of 5 &6 and I'm going to spin Eagle around and hold it north of the channel"

"...Hang out overnight, operations manager is coming from Ft. Lauderdale"

"Going to stay here where its 3-4'.  When I got out by its original position it was 6-8'  "

"We'll let you know the plan when we've talked it over with the operations manager ~ maybe, we'll see"  (my interpretation, sounds like that's hours away, maybe tomorrow)

Planning for operations manager: "not a good plan to transfer him in this sea state.  ~~platform not good~~ RIB to stern probably not a good idea.  ...Ladder, scurry up the side"

Brevard county boat was on station with blue lights and CG small boat came in and relieved them.  Seems to be only there for observation.

Coast Guard small boat to Eagle once CG came in and saw the layout: "What is the plan, are you going to stay on scene until first light?"  Eagle: Let's go to Ch. 22 (I don't have access to hear what was said there)

Can anyone give a link to where this can be listened to live on the web?

[martin_nv]

Can anyone give a link to where this can be listened to live on the web?

https://www.broadcastify.com/listen/feed/21054

Seems kind of quiet now though.

[CraigLieb]

I miss the old landing game 

Water squares would have to be added to the LZ1 grid