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

[meekGee]

It was the leg deployment that did most of the work.  And the same change in rotational inertial would have made it harder for the control system.

This assertion is contradicted by the onboard footage. The roll is mostly nulled by the time the legs begin to deploy.

I don't really agree with that, the instant the legs pop out the roll basically stops.

Both assertions are true.  Using, we can use the time the shadow passes the tip of the left fin to measure the rotation rate.  Times are measured as youtube time + frames.
1:47 + 15/30
1:49 + 14/30
1:51 + 13/30
1:53 + 17/30
1:55 + 24/30
1:58 + 3/30
2:00 + 19/30
2:03 + 13/30
2:66 + 25/30
----- Legs come out
then about 1/4 more turn in 4 seconds.

So the roll rate was dropped from 1+29/30 (1.966) seconds per turn to 3+12/30 (3.40) seconds per turn before the legs came out, and was decreasing quite quickly. (By extrapolation, the next turn would have taken about 4 seconds.) But the legs also had a big effect, reducing the remaining roll rate by a factor 2-4.

The difference between engine roll cancellation and leg-deploy is that the legs are a one-time thing, and can only take out a fixed fraction of the roll (the ratio of the moments of inertia) - not zero it.

The engine, as long as it continues to fire, takes out spin at a fixed rate, and doesn't slow down towards the end.

The legs helped at the end, for sure, but that's the extent of it.  It might also be that by enlarging the perpendicular moments of inertia, the on-axis engine was able to get more "grip" for canceling axial rotation, something that some poster above found baffling (and rightly so)

[freda]

In a rare case, at this moment, Port Canaveral Webcam is pointed at OCISLY at dock. I see no nearby activity; so I assume the booster remains slightly offshore, secured by the tug?  Waiting...

[flyright]

Go Quest is drifting south of the Channel according to an update they just gave the Coast Guard Sounds like they are meeting another ship, possibly "Pacific Talent".

Edit. "Pacific Talent" . appears to be a Bulk Carrier and Go Quest was being advised they were anchoring nearby. Probably unrelated to the recovery.

[Hauerg]

Frankly, I'm surprised the AFTS didn't terminate and it still executed a soft landing off shore.

AFTS is saved before the entry burn begins.

It is safed *after* the entry burn.

I looked it up and you are correct. But, that still leaves the stage without AFTS after the initial aiming point has been established (via the entry burn and assumed ballistic entry from that point forward).

In short: by the time the the flight computer senses that the grid fins are not responding there is no active AFTS present to trigger a self-destruct.

Not needed since the booster cannot reach the pad, it needs to be working ok to GET OVER THERE.

[OxCartMark]

In a rare case, at this moment, Port Canaveral Webcam is pointed at OCISLY at dock. I see no nearby activity; so I assume the booster remains slightly offshore, secured by the tug?  Waiting...

Position as reported by Go Quest 10 minutes ago;

edit: Where they were last night the water depth was 39-42 feet.  Now if they are south of the channel(?) it may be ~3 feet more (give or take tide).  Can someone refresh me on leg length please?

[flyright]

There's a lot of marine radio traffic involving our ships but I don't have time to listen and transcribe as I did last night.  Anyone?

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

I'm listening, but not making much sense of the transmissions. The Coast Guard is advising fishing vessels to remain more than 1/2 mile from the booster recovery operations which are apparently south of the channel along with Go Quest.

[ugordan]

Frankly, I'm surprised the AFTS didn't terminate and it still executed a soft landing off shore.

AFTS is saved before the entry burn begins.

It is safed *after* the entry burn.

I looked it up and you are correct. But, that still leaves the stage without AFTS after the initial aiming point has been established (via the entry burn and assumed ballistic entry from that point forward).

In short: by the time the the flight computer senses that the grid fins are not responding there is no active AFTS present to trigger a self-destruct.

True. I guess I was mostly surprised that the landing burn was actually initiated given the high roll rate and no way of making it to the desired landing point any more, but I guess there is good logic in burning off as much remaining RP-1 as possible before ocean impact and all that.

[mlindner]

I think the anti slosh baffles had a lot to do with stopping the spin.

Anti slosh baffles only inhibit accelerations, they don't inhibit velocities, because there is no sloshing going on. In fact anti-slosh baffles would help keep a spin steady and resist efforts to slow it down.

[sghill]

Go Quest is drifting south of the Channel according to an update they just gave the Coast Guard Sounds like they are meeting another ship, possibly "Pacific Talon".

"Pacific Talent"

It's a bulk carrier. It is headed to Port Canaveral from Brazil, where it departed on Nov. 21. The captain is on the radio talking about meeting the pilot boat to go into the port.

https://www.marinetraffic.com/en/ais/home/shipid:3935544/zoom:10

[kevinof]

It has nothing to do with recovery - just working it's way to the anchorage point to wait for clearance to enter port.

Go Quest is drifting south of the Channel according to an update they just gave the Coast Guard Sounds like they are meeting another ship, possibly "Pacific Talon".

"Pacific Talent"

It's a bulk carrier. It is headed to Port Canaveral from Brazil, where it departed on Nov. 21. I doubt it is going over to the party.

https://www.marinetraffic.com/en/ais/home/shipid:3935544/zoom:10

On the otherhand, it does have several VERY large cranes on its deck...

[Lee Jay]

Musk's tweet was;

Engines stabilized rocket spin just in time, enabling an intact landing in water! Ships en route to rescue Falcon.

My question is, how can a single engine landing burn (center engine) null a spin? I'm not claiming it didn't (the video makes it clear it did) I'm just trying to understand how a center engine could generate the needed torque. 

This was answered farther back in the thread
Musk is referring to the cold nitrogen thrusters as "engines".
There is protracted debate about when a single, on-axis engine can induce torque to increase or decrease roll rates, but the basic answer is it can't do much, and certainly didn't for this fist stage.

This seems correct to me, save for the fact that the center engine does contribute substantially by slowing the decent and therefore reduces the aerodynamic torque being applied by the wayward grid fins, thus allowing the relatively weak cold gas thrusters to do their jobs.

[flyright]

Per marine radio, a diving team is joining Tugboat Eagle and Go Quest.

[Pete]

Here's my question...If the stage knows it's aborting a pad landing to land on the ocean, why then lower the legs at all?  Would it then make lifting onto a barge eaiser with the legs connected to the stage?

It might allow the legs to take some of the impact instead of the engines taking it all.

It would also make the tip=over speed a good bit slower.

But I'm sure the stage knows nothing about that, it would have a VERY hardcoded and foolproof routine that says "ground detected at distance X, deploy legs" regardless of the nature of the ground, lateral velocity, etc. Point in case, the very failed landing of the FalconHeavy core started to try to deploy legs, despite approaching the sea at several hundred miles per hour.

[mlindner]

Musk's tweet was;

Engines stabilized rocket spin just in time, enabling an intact landing in water! Ships en route to rescue Falcon.

My question is, how can a single engine landing burn (center engine) null a spin? I'm not claiming it didn't (the video makes it clear it did) I'm just trying to understand how a center engine could generate the needed torque. 

This was answered farther back in the thread
Musk is referring to the cold nitrogen thrusters as "engines".
There is protracted debate about when a single, on-axis engine can induce torque to increase or decrease roll rates, but the basic answer is it can't do much, and certainly didn't for this fist stage.

This seems correct to me, save for the fact that the center engine does contribute substantially by slowing the decent and therefore reduces the aerodynamic torque being applied by the wayward grid fins, thus allowing the relatively weak cold gas thrusters to do their jobs.

It also slows the spin by removing any spin energy that is transferred into spins about other axes. The rocket does not want to spin along the axis that runs the length of the booster so it will start to turn into a flat spin over time. These other spins the rocket can counteract and remove.

[Dappa]

Musk's tweet was;

Engines stabilized rocket spin just in time, enabling an intact landing in water! Ships en route to rescue Falcon.

My question is, how can a single engine landing burn (center engine) null a spin? I'm not claiming it didn't (the video makes it clear it did) I'm just trying to understand how a center engine could generate the needed torque. 

This was answered farther back in the thread
Musk is referring to the cold nitrogen thrusters as "engines".
There is protracted debate about when a single, on-axis engine can induce torque to increase or decrease roll rates, but the basic answer is it can't do much, and certainly didn't for this fist stage.

This seems correct to me, save for the fact that the center engine does contribute substantially by slowing the decent and therefore reduces the aerodynamic torque being applied by the wayward grid fins, thus allowing the relatively weak cold gas thrusters to do their jobs.

Not only that, the grid fins will also have slowed down the rotation. Like a corkscrew, a given vertical speed will have a matching angular velocity where the grid fins slice through the air perfectly. As the vertical speed drops, the rotation will drop too.

[RoboGoofers]

I think the anti slosh baffles had a lot to do with stopping the spin.

Anti slosh baffles only inhibit accelerations, they don't inhibit velocities, because there is no sloshing going on. In fact anti-slosh baffles would help keep a spin steady and resist efforts to slow it down.

I suppose i should have said "the complex interactions of the friction from the remaining fuel and the tank walls and the internal obstructions such as baffles whilst the engine is firing" play a part in slowing the spin rate.

[OxCartMark]

Diving seems to be going on now.?.  Go Quest is on scene, Go Navigator is in port, Go Searcher is...hmm, I don't know where.

[AJW]

From the 'landing' video, it sure looks like the RCS thrusters are fighting the gridfin induced spin from the moment it starts until the booster hits the water.  It isn't always visible near the base of the booster, especially while the booster is at high speed, but a short plume is visible when in the right light.  Without the RCS system fighting it, the rate of the spin would have continued to increase.  As the landing burn starts, the speed of the booster slows, so the gridfins become less and less effective, allowing the RCS system to counteract the spin until it is nullified just before landing.  I think that releasing the legs may have had some impact but it was negligible compared to the RCS.

[Jim]

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

No, it is the grid fins and RCS that provide roll control.  A single engine can not provide roll control.

[Jim]

I am curious to see how exactly they'll approach bringing a rocket back into port that didn't land on an ASDS. Why not just fish it out, put it on OCISLY, and then take it back to port that way?

And how exactly would you 'just' fish it out?

Put the crane on OCISLY


That is partially serious, but understand stability issues associated with sea state and that OCISLY's deck might not be able to handle the load

There are such things as crane barges...my son worked on one when he was in the Army Reserves (he was a Watercraft Engineer) ... they trained to lift some hairy stuff in non-ideal sea states

not in Port Canaveral