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

[Lars-J]

Excuse me while I put a momentary break into the "I know physics better than you **** measuring" with this...

From the update thread:

heard:  There is nowhere to put his feet to pull it up on the other side.

I wish I could suggest them to throw a rope over the top or something.

We're not there. Let's assume that they know what they are doing, and are being instructed by SpaceX who certainly know the booster and how to handle it.

[DigitalMan]

Excuse me while I put a momentary break into the "I know physics better than you **** measuring" with this...

From the update thread:

heard:  There is nowhere to put his feet to pull it up on the other side.

I wish I could suggest them to throw a rope over the top or something.

We're not there. Let's assume that they know what they are doing, and are being instructed by SpaceX who certainly know the booster and how to handle it.

relax, it was a joke.  clearly I need a new one.

[Chris Bergin]

Ok, calm it down, everyone. This is not the party thread, nor is it the party pooper thread.

[HVM]

Landing burn slows down the airflow over (through) the grid fins, and fin's sides start to works as speed brakes, for the roll induced airflow. The landing leg deployment is not only change moment of inertia*, legs work as speed brakes too and reduce roll rate.

*moment of inertia and rotational speed changes, angular momentum remains constant. -elementary school physics.

[CorvusCorax]

Its fun to see you argue over nothing. You are both right, yet fail to see that the other is right too.
1. The stage was rotating with high roll rate but also significant precession, especially early in the descent as the fins induced both a rolling and a pitching force.
2. The stage was fighting both roll and pitch/yaw with cold gas thrusters.
3. As the air density increased the roll rate did too, reducing prevession. In thos phase the booater is effectively spin stabilized.
4. Once the stage was subsonic the roll rate decreased as airspeed decreased.
5. The precession increased too, leading to quite an extreme lean during powered flight.
6. At the beginning of powered flight, main engine was almost in line with rotation axis, no authority over roll rate, effect of cold gas thrusters only.
7. With the "lean" seconds before touchdown, rotation axis was not inline and main engine could decrease rotation rate using yaw gimbal.
8. the booster uprights itself, but the rotation rate is now greatly reduced. Engine is now again in line with rot axis but booster is too slow for grid fins to do much harm. Gas thrusters still figgt rotation.
9. Throwing the legs out has a very visinle "ballerina" effect and instantly reduces rotation rate to a fraction of the already slowed rate.
10. Legs hit the water, arresting the remaining rotation. Booster seems to undergo some twisting oscillation where the interstage with grid fins overshoots by a few degrees, then twists back.
11. Booster goes horizontal and splashed down.

[edzieba]

Theory:

The hydraulic pump failure happened extremely early in descent (not long after - if not during - grid-fin deployment).

Evidence:

- The grid-fins do not move at all after deployment until after the roll has started, at which point one of the two visible grid fins 'flops' over to the end of travel and remains there.

- RCS thruster pulses are visible after atmospheric entry, something I can't recall seeing before. Likely the stage attempting to use whatever control authority it has available to it.

- Stage does not perform normal 'side on' descent to gain free aerobraking using the grid-fins (fins 'kick out; the top of the stage to the side against airflow, giving it a larger cross-section so more drag), so would be descending faster than normal

- Flight Club's CRS-16 sim has a nominal descent going transonic at T+460 (7 m 40s). The grid fin 'flops' over at 7m25s, which could match up with a faster than normal descent.

- The known 'transonic control inversion' issue with grid-fins would explain why at that time the stage rotation changes direction dramatically: while supersonic the free-floating grid fins were oriented slightly off and imparting a small clockwise roll moment. As the stage goes transonic, the grid-fin briefly acts as a big flap tangential to the airflow rather than a fin parallel to it, imparting a large momentary force to the fin that flops it over in the opposite direction. As the stage slows further, the grid fin then returns to acting as a fin again, and is now applying a grater roll moment in the opposite (anticlockwise) direction. We see this in the on-board video: clockwise roll slowly increasing in rate, grid fin flops over, roll stops, roll direction reverses with dramatic increase in roll rate.

[haywoodfloyd]

Logan reports to Eagle: Divers are clear, but there are some "guys around the leg on the surface"

Mental image of Slim Pickings riding that bomb...

I think you meant Slim Pickens.

[Okie_Steve]

I apparently misunderstood Chris B's request to keep the spin discussion L2, because my question got deleted, so I'll ask it here instead.

The last time they had a floating stage recovery was discussed but eventually it was scuttled because IIRC there was no way to vent and it was too dangerous. So, is there any information about what changes were made to make it more feasible to try this time?

[meekGee]

Logan reports to Eagle: Divers are clear, but there are some "guys around the leg on the surface"

Mental image of Slim Pickings riding that bomb...

I think you meant Slim Pickens.

Heh yes ...

-----
ABCD: Always Be Counting Down

[pb2000]

I apparently misunderstood Chris B's request to keep the spin discussion L2, because my question got deleted, so I'll ask it here instead.

The last time they had a floating stage recovery was discussed but eventually it was scuttled because IIRC there was no way to vent and it was too dangerous. So, is there any information about what changes were made to make it more feasible to try this time?

The flight computer on the booster continued transmitting after touchdown+tip and reported that the safeing procedure was completed. Also, they want this one back, but 1032.2 was always meant to be expended.

[whitelancer64]

I apparently misunderstood Chris B's request to keep the spin discussion L2, because my question got deleted, so I'll ask it here instead.

The last time they had a floating stage recovery was discussed but eventually it was scuttled because IIRC there was no way to vent and it was too dangerous. So, is there any information about what changes were made to make it more feasible to try this time?

The stage was still transmitting data after splashdown, so they were able to confirm it had gone through its safing procedures, including venting fuel and LOX.

*edit* NINJA'D

[edkyle99]

I apparently misunderstood Chris B's request to keep the spin discussion L2, because my question got deleted, so I'll ask it here instead.

The last time they had a floating stage recovery was discussed but eventually it was scuttled because IIRC there was no way to vent and it was too dangerous. So, is there any information about what changes were made to make it more feasible to try this time?

In my view, SpaceX has not made any appreciable changes to account for a water "landing".  This is simply not the intended mode of recovery.  The last one sank due to wave action.  This one is likely also being stressed by wave action, albeit smaller waves, and won't last forever on the surface, not to mention the lengthy salt water exposure.  I don't see how this stage could ever be reused.  If the company wants to make adjustments for this scenario, it should design an easier way to sink the floating stage.

 - Ed Kyle 

[rakaydos]

I apparently misunderstood Chris B's request to keep the spin discussion L2, because my question got deleted, so I'll ask it here instead.

The last time they had a floating stage recovery was discussed but eventually it was scuttled because IIRC there was no way to vent and it was too dangerous. So, is there any information about what changes were made to make it more feasible to try this time?

it was probably a failure of imagination last time around. The stage was supposed to come to 0 velocity just above the water, then shut down, crash and blow up. They missed the target in the EXACT way needed to splash down without blowing up, something the designers never considered.

Now, of course, they know it's possible, and probably even aim for a safe water landing at least for RTLS. Auto-safing is just common sense now that we know it's possible.

[StuffOfInterest]

Just saw on the update thread that GO Quest has recovered a leg.  I guess they are detaching the legs in the water.  Has to be some tricky work, but I guess if they do it will at least make it possible to tow the stage in and/or lift it out of the water.

https://twitter.com/spacexfleet/status/1070777385946103809

GO Quest has confirmed to the coast guard over marine radio that they recovered a rocket leg earlier today.

[meekGee]

Just saw on the update thread that GO Quest has recovered a leg.  I guess they are detaching the legs in the water.  Has to be some tricky work, but I guess if they do it will at least make it possible to tow the stage in and/or lift it out of the water.

https://twitter.com/spacexfleet/status/1070777385946103809

GO Quest has confirmed to the coast guard over marine radio that they recovered a rocket leg earlier today.

It's the new procedure..  detach the one that's underwater, then the stage auto-rotates to submerge the next one, repeat x4...

Either that, or one leg was lost during the topple maneuver...

[dnavas]

Just saw on the update thread that GO Quest has recovered "a leg".  I guess they are detaching the legs in the water.  Has to be some tricky work, but I guess if they do it will at least make it possible to tow the stage in and/or lift it out of the water.

If the stage was in trouble, I would have thought that the titanium fins would be higher priority, so I'm going to assume the plan continues to be full recovery of the stage (though, let's face it, *reuse* seems thoroughly unlikely).

[joncz]

Just saw on the update thread that GO Quest has recovered a leg.  I guess they are detaching the legs in the water.  Has to be some tricky work, but I guess if they do it will at least make it possible to tow the stage in and/or lift it out of the water.

I don't believe that's the case.  Coast Guard asked them if the piece they recovered was from the interstage.  They said it was from the leg, and CG asked them to confirm that was the only piece recovered.

[ellindsey]

I think it's a bit optimistic to assume the leg was removed intentionally.  I suspect it was broken off either by wave action or accidentally by hitting a tugboat or tow rope.

[Lars-J]

https://twitter.com/KillianPhoto/status/1070781952398118912

Note - some (on the UPDATE thread) are interpreting the first image as showing a gash/hole in the interstage. But what looks like  a hole is actually an gray inflated bag attached to the grid fin. The bag has weird shadows thrown on it from the grid fin, making it look like water.

[saliva_sweet]

The fin is there. There is a gash, the wall is bent. There is no bag attached to the fin.