SpaceX Falcon 9/Dragon CRS-2 SpX-2 MISSION GENERAL DISCUSSION

[Kabloona]

My one observation you said "this motion is later reversed." I assume you base this on the appearance, at the end of the tumbling portion of the video, of the view forward to the clouds, when the clouds appear to freeze motion for a second or two before the camera view cuts away.

Sorry to not have made this clear enough, I meant that the (almost imperceptible) motion of s1 caused by s2 engine ignition is reversed by the tumble that happens later for unknown reasons.

Thanks, I see now What you meant.

[meekGee]

Thanks a lot Sitting Duck - this was a long and painful argument...

Can you, in the simulation, indicate the moment that S2 ignites?  Can you estimate the range between S1 and S2 at that time?

[sittingduck]

Thanks a lot Sitting Duck - this was a long and painful argument...

Can you, in the simulation, indicate the moment that S2 ignites?  Can you estimate the range between S1 and S2 at that time?

Yes I can and will.

[mlindner]

Thanks a lot Sitting Duck - this was a long and painful argument...

Can you, in the simulation, indicate the moment that S2 ignites?  Can you estimate the range between S1 and S2 at that time?

Don't think accurate range is possible because of unknown camera field of view.

[sittingduck]

Thanks a lot Sitting Duck - this was a long and painful argument...

Can you, in the simulation, indicate the moment that S2 ignites?  Can you estimate the range between S1 and S2 at that time?

Don't think accurate range is possible because of unknown camera field of view.

I've actually got the FOV relatively close right now (did not in 1st/2nd video) and also have a pretty good idea where the camera is nested in the first stage.  I believe this can get us a distance +/- 10%.

[sittingduck]

Been fooling around today with the models and don't yet have a satisfactory video showing the moment that the second stage ignites.  However I can show my best estimate for the distance between the two stages when this occurs, with error bars.  I do not know the height of the first stage alone so cannot calculate this distance.

[Antares]

If the second stage went to closed loop guidance and vectored the engine a tiny bit, it could easily have imparted the noted moment.

[sittingduck]

If the second stage went to closed loop guidance and vectored the engine a tiny bit, it could easily have imparted the noted moment.

This is roughly the separation between the two stages when it begins to be visible.

[meekGee]

"it" being the rotation, yes?

wow.

[Kabloona]

If the second stage went to closed loop guidance and vectored the engine a tiny bit, it could easily have imparted the noted moment.

Good point. If you look closely at the video, the plume halo is centered around S2, but at the moment S1 starts to move it does look like the halo goes off-center for a moment, in the same direction as S1 motion.

[meekGee]

Actually, I don't think so.

Even if S2 is vectoring, it just means the plume center is point somewhere else, but S1 is still seeing particles moving in a straight line from S2, and since it sampling the plume over a cross section of only 3.6 meters, there would hardly be any difference between forces on either side of it.  Even if there were, the moment arm is very very small. 

It's not like S2 can, by vectoring, push on the side of S1.

That rotation looks like it was induced by something on S1.

[sittingduck]

Alright this is the final video I will be making in the attempt to illuminate exactly what happened during these seconds.

This is the most accurate version of the simulation.  I placed cameras with the correct FOV on both S1 and S2 within centimeters of their actual locations.  Every 1/3 of a second I updated the position of each stage relative to each other.  Both stages stay in view of each other until the moment that S1 begins its most significant downwards rotation.  It is at this exact instant that the view of S1 from S2 is lost!

You will notice S2 makes a very odd looking wobble around engine ignition.  I doubt that the movement is entirely accurate and that it actually represents the jolt that S1 receives during S2 ignition, obscuring the view from both stages and making accurate placement difficult.  More likely is that S1 should have rotated "downwards" (direction of Earth which is down in this video) by less than 5° during this time instead of that "wobble" that you see from S2, would barely have been perceptible.

The small exhaust effect indicates the moment of stage ignition.

S2 is almost 610m downrange (+/- 100m) by the time S1 reverses its direction of rotation.

Finally it becomes clear that S2 does seem to 'kick' S1 moments after engine ignition.  This kick starts around 0:12 and ends at 0:19 when S1 begins to rotate in the opposite direction.  There is a very slight clockwise roll component to the latter movement as well.

Extrapolating from the movement at the end of the video it would have taken S1 ~104s to make a full rotation.



Finally once more I must emphasize that I do not know around which point S1 is rotating, around the engine block, around the CG, around the exact center, I have no idea and cannot know from the available data.

[meekGee]

I must say I never noticed the first small rotation.  IIRC by the time the large rotation started, S1 looked practically head on from S2, but you'd be able to deduce this motion much more accurately from the S1->S2 view.

Time to go look at the videos again.

This is lovely.

[mlindner]

What kind of torque would be imparted by the engines gimbaling to some "park" orientation?

I still don't believe that this is cold gas thruster testing as there's no reason for the test and any testing they might want to conduct, could be done on the ground.

The rotation is indeed very interesting though, especially after seeing these videos, thank you sittingduck.

[douglas100]

...I still don't believe that this is cold gas thruster testing as there's no reason for the test and any testing they might want to conduct, could be done on the ground.

The rotation is indeed very interesting though, especially after seeing these videos, thank you sittingduck.

I agree. The testing they plan on the upcoming Cassiope flight will cover the use of the cold gas thrusters and will be on the stage that the system was actually designed for.

[meekGee]

What kind of torque would be imparted by the engines gimbaling to some "park" orientation?

I still don't believe that this is cold gas thruster testing as there's no reason for the test and any testing they might want to conduct, could be done on the ground.

The rotation is indeed very interesting though, especially after seeing these videos, thank you sittingduck.

You mean S1 engines?   The answer is zero.

You might get a temporary angular displacement when the engines are in motion, but when they reach the parked position and stop, by conservation of angular momentum, so will any S1 angular motion that they caused.

A possible motivation for inducing rotation had to do with trying something during the stage's initial reentry interface.  Maybe.  I actually don't have a good explanation to the "why" - just that when compared with the first flight where S1 dropped out of view so cleanly, this motion seemed very distinct and abrupt.

[Jim]

You might get a temporary angular displacement when the engines are in motion, but when they reach the parked position and stop, by conservation of angular momentum, so will any S1 angular motion that they caused.

And then the gases from second stage come into effect.

[sittingduck]

And then the gases from second stage come into effect.

Do you think this is what causes the small rotation 13 seconds after stage separation or the larger rotation 19 seconds after stage separation ?

[meekGee]

You might get a temporary angular displacement when the engines are in motion, but when they reach the parked position and stop, by conservation of angular momentum, so will any S1 angular motion that they caused.

And then the gases from second stage come into effect.

So your theory is that:
a) S1 engines gimballed after separation to some "park position"
b) During this engine motion, for a fraction of a second, S1 deviated from its nominal orientation
c) The exhaust from S2 had enough of an effect to start rotating the stage at (Sitting Duck:  how fast?  We can calculate the angular momentum from that, and since we know the moment arm, can calculate the force supposedly imparted by the gasses...
d) The motion started at +19 seconds, so somehow the stage stored this angular momentum for about 10 seconds (fuel slosh?  maybe it did't burn to depletion?)...
e) ... and reversed it, since the +19 rotation is opposite the initial rotation.

Well, it's a theory.  But you'll have to fill out some serious holes before it can hold water.

[meekGee]

SittingDuck - if you can dedicate any more time to this, can you plot out the deduced rotation rate on a time axis?

... and note the ignition time point, and maybe also your data source (since we lose one camera just after rotation starts, right?)

Thanks