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

[Jim]

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

Nah, the video confirms my point.   It is still spinning at the beginning of leg deployment. The leg deployment stops the roll.

[Jim]

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.

No, the engine has no role in stopping the roll.

[Jim]

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.

Great answers:  Thanks CorvusCorax and Nomadd.

Legs have the radar altimeter for landing too.

[groundbound]

No, the engine has no role in stopping the roll.

I often miss your humor but I caught that one.

[Jim]

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

No, the engine can not provide any torque on the longitudinal axis.  It is on the centerline, it can only provide pitch and yaw.

[Jim]

I think that releasing the legs may have had some impact but it was negligible compared to the RCS.

It isn't "negligible".  The change in the moments of inertia was large and did remove most of the roll.  Just like a skater.  A skater's arms are a small fraction of the mass of the skater but it can greatly slow the spin.

[Orbiter]

Conservation of angular momentum, guys. Jim's right, the legs stopped the roll. Think about the axis of rotation here in relation to the force of the engine, how could the engine possible stop the roll?

[Lars-J]

A neat combination of views of the landing:



Sadly the camera guy for the lower left feed "had one job", and managed to miss the most interesting part of the early wobble. Grumble. 

[sfxtd]

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.

I think this analysis makes the most sense.

[meekGee]

Conservation of angular momentum, guys. Jim's right, the legs stopped the roll. Think about the axis of rotation here in relation to the force of the engine, how could the engine possible stop the roll?

Conservation of angular momentum means that the legs can only diminish the rate of rotation by the ratio of the moments of inertia (before and after).  "Ratio" means that by definition they can't stop the rotation, only reduce it.

The ratio can't be large either. The legs are not that heavy, their initial radius is about 2 m, and when they open up, most of their mass moves only a small distance.  Even with the r² term, I'd be surprised if the ratio was 2x.

The engine, OTOH, can reduce the rate of rotation to zero, but coupling an axial engine to axial rotation is tricky and only works when there are things like grid fins to help cause precession.  I think the main contribution of the legs was to make the engine much more effective, plus give the obvious one time reduction from their moment of inertia.

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ABCD: Always Be Counting Down

[meberbs]

Nah, the video confirms my point.   It is still spinning at the beginning of leg deployment. The leg deployment stops the roll.

Wrong.
Conservation of angular momentum, the stage has just as much angular momentum before and after the leg deploy. It may be spinning slower, but there is just as much momentum to cancel. The only way in which the legs actually helped bring the spinning to a true stop is the additional rotational air drag on them.

No, the engine has no role in stopping the roll.

Wrong.
As has been said in other posts on here, the roll bled into other axes, which seems to be true if you look carefully at the video. Once the spin axis is no longer lined up with the main axis of the booster, the engine has some useful control authority.
Also, of the effects that allowed the booster to stop the roll, one of the most important would be the change in torque due to the grid fins. There are 2 effects, one is that as the booster slows down, the aerodynamic force on the fins decreases, which enables the RCS thrusters to start winning the fight. Second is that for a given downward speed, there should be a preferred spin rate at a stable equilibrium where the grid fins apply no torque. Decreases in the downward air speed reduce that spin rate, causing the grid fins to actually apply some torque in a helpful direction. Your pithy statements which ignore all of this are not helpful.

[Jim]

Wrong.
As has been said in other posts on here, the roll bled into other axes, which seems to be true if you look carefully at the video. Once the spin axis is no longer lined up with the main axis of the booster, the engine has some useful control authority.

Wrong.  We are not talking spin but roll.  The other axis are not part of the discussion, of course the engine can affect those.

Your pithy statements which ignore all of this are not helpful.

Those are not part of the discussion.  The vehicle was no longer in a flat spin at the point of the discussion.  There only was roll remaining.

[Jim]

"Ratio" means that by definition they can't stop the rotation, only reduce it.

That is why the term "remove most of the" was used.

The engine, OTOH, can reduce the rate of rotation to zero,

No, it has no affect on rates on the roll axis

[meekGee]

The engine, OTOH, can reduce the rate of rotation to zero,

No, it has no affect on rates on the roll axis

That's categorically false.  A) See Musk's statement.  B) This is not a classical rigid body in vacuum problem.  The axes of rotation are coupled, as has been pointed above.  C) You can see the rate of rotation (roll) decreasing once the engine turns on, and before leg deployment.

EDIT: clarified this is about roll as well.

[Jim]

No where in the discussion have I said "spin".  I am only talking about roll and that was only happening near the end when vehicle was vertical.    Of course, the engine damps out pitch and yaw.

[ugordan]

Legs have the radar altimeter for landing too.

Are you sure the radar is not actually mounted on the base of the rocket, I think a white rectangular addition to the side of the base was speculated in (now older) images?

Landing legs deploy mere meters before touchdown, I'd think it would be to late to incorporate radar altimetry into the thrust profile at that point.

[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...

[ugordan]

As has been said in other posts on here, the roll bled into other axes

Because the grid fins were locked into such a position that they induced a lift vector in addition to a roll torque?

[meberbs]

Wrong.
As has been said in other posts on here, the roll bled into other axes, which seems to be true if you look carefully at the video. Once the spin axis is no longer lined up with the main axis of the booster, the engine has some useful control authority.

Wrong.  We are not talking spin but roll.  The other axis are not part of the discussion, of course the engine can affect those.

They aren't perfectly separable concepts in the real world, spins are not simply tied to a single body axis except in certain cases of perfect symmetry.

Your pithy statements which ignore all of this are not helpful.

Those are not part of the discussion.  The vehicle was no longer in a flat spin at the point of the discussion.  There only was roll remaining.

The things I described just before the quoted statement apply to the ideal pure roll case. You cannot redefine yourself out of the fact that your statements were incorrect.

"Ratio" means that by definition they can't stop the rotation, only reduce it.

That is why the term "remove most of the" was used.

Again, the moment of inertia change from leg deploy removes none of the angular momentum. The angular velocity may reduce, but the remaining work other mechanisms need to do to stop the spinning is unchanged.

No where in the discussion have I said "spin".  I am only talking about roll and that was only happening near the end when vehicle was vertical.    Of course, the engine damps out pitch and yaw.

If you think there is a difference, then you don't know what you are talking about. Unless there is perfect symmetry, a pure roll will turn into a generic spin which can then be removed.

[vanoord]

As has been said in other posts on here, the roll bled into other axes

Because the grid fins were locked into such a position that they induced a lift vector in addition to a roll torque?

Presumably, if the grid fins were locked into different orientations that would induce sufficient instability to turn the roll into a wobble.

Going back to the point about the legs reducing the roll - yes, they had a marked effect but the roll was very much reduced by that time compared to the peak.

If anyone doesn't believe that, go the the onboard view uploaded by Elon and count each revolution, preferably out loud (the reflection of the sun on the sea is an obvious marker).

My guess would be that as the speed of the stage decreased, the effect of the stuck grid fins reduced and allowed the gas thrusters to start to get control.