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

[freddo411]

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

I thought there might be some second order effects.   Watching the stage it looked like it did retard the rotation a bit

[OxCartMark]

"The man" is being brought out to Eagle on a fast boat now, to approach from the side in a few minutes.  Hopefully they have a spot for him to sleep because i don't see much opportunity to make progress until the sun comes up. There seems to be more of the conversation going on on alternate channels recently but maybe perking up on the main channel that we can hear through the internets.

edit: Onboard now.

[Okie_Steve]

Watching the stage it looked like it did retard the rotation a bit

Some one definitely deserves a gold star for the control system robustness.

Edit  - typo

[rickl]

Watching the stage it looked like it did retard the rotation a bit

Some one definitely deserves a gold star for the control system robustness.

I have to say, that was a hell of a good landing for a "failure".

[Scylla]

Changed my link to this video on the update thread to the actual producer, Kerbal Space Academy.

Question: Does anybody know why they have Nasaspacflight.com in the corner of the screen as well as their own?

[rocketguy101]

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

[deruch]

Changed my link to this video on the update thread to the actual producer, Kerbal Space Academy.

Question: Does anybody know why they have Nasaspacflight.com in the corner of the screen as well as their own?

He and ChrisG were live streaming the launch/landing together, ChrisG "on behalf" of NSF.  One of the voices you hear in that video is Chris'. 

https://twitter.com/ChrisG_NSF/status/1070358919833374721

[spacenut]

Is the Dragon ok on its way to the ISS?  I know the solar panels deployed.  It was the primary purpose of the flight. 

[RotoSequence]

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.

https://twitter.com/elonmusk/status/1070399755526656000

[CJ]

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. 

[meekGee]

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.

If the stage was in vacuum, then it'd be impossible.

If there are aerodynamic forces, especially way out on the far end, then absolutely they can, but I am floored that they did in practice.

Not only is it difficult to control roll, the stage was at such an extreme state that all assumptions that hold "near nominal" such as small angle approximations and such - all those were out the window.

This is one of the most amazing space videos ever!

-----
ABCD: Always Be Counting Down

[mlindner]

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.

https://twitter.com/elonmusk/status/1070399755526656000

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

[zubenelgenubi]

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!

Dare I say, the avionics aboard B1050.1 has "The Right Stuff"?!

Maybe this "black box" could be Time magazine's Computer of the Year?  (Is Time still relevant?)

[RotoSequence]

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

The tracking shots all show that the vehicle's roll rate visibly decreases well before the legs pop out. The leg deployment helped, but the engine clearly did much, or even most, of the work first.

[DigitalMan]

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

The tracking shots all show that the vehicle's roll rate visibly decreases well before the legs pop out. The leg deployment helped, but the engine clearly did much, or even most, of the work first.

One of the things I was pondering was this:

High pressure helium is used to extend the legs, no?  Is it possible the release of the high pressure helium during leg deploy could have an effect on the MOI before it noticeably moves the legs?

[Comga]

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.

[catdlr]

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

The tracking shots all show that the vehicle's roll rate visibly decreases well before the legs pop out. The leg deployment helped, but the engine clearly did much, or even most, of the work first.

One of the things I was pondering was this:

High pressure helium is used to extend the legs, no?  Is it possible the release of the high pressure helium during leg deploy could have an effect on the MOI before it noticeably moves the legs?

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?

[RotoSequence]

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.

Its always possible I'm simply overrating my interpretation of the Mark 1 eyeball, so you could be right. It could also be that the vehicle's quick rotation is obliterating the visibility of the cold gas plumes, but it certainly doesn't look very much like previous large "firings" of their nitrogen RCS thrusters.

[CorvusCorax]

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

The tracking shots all show that the vehicle's roll rate visibly decreases well before the legs pop out. The leg deployment helped, but the engine clearly did much, or even most, of the work first.

One of the things I was pondering was this:

High pressure helium is used to extend the legs, no?  Is it possible the release of the high pressure helium during leg deploy could have an effect on the MOI before it noticeably moves the legs?

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?

Those legs create quite a bit of drag when.pressed under water as the stage is tilting and falling over, possibly enough to slow the falling stage down enough to prevent loss of structural integrity on splashdown. Only 2 stages survived their water landing up 2 date.  Both had legs deployed.

[Nomadd]

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.