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#600
by
RoboGoofers
on 16 Oct, 2013 21:36
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That O2 guess seems a good one. My intuition is this: Unless the avionics had shut the O2+kero valves of as soon as the engine starved, the unlit engine would have been spewing far more O2 than kero because gaseous O2 would still be at high pressure over its sump regardless of the liquid part having been centrifuged away.
The feed line for O2 is also much longer so it's likely that there was a large volume of liquid O2 that could have been expelled by the pressure of the gaseous O2.
I imagine it would be the same effect as a CO2 fire extinguisher.
I doubt the programers spent any time writing code to shut down the engines 1 second before landing, no matter what problems arise, but maybe they did out of an overabundance of caution.
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#601
by
meekGee
on 16 Oct, 2013 21:39
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I highly doubt the the final burn could have taken place even briefly if the stage was tumbling end over end. Just what is being implied by the term tumbling? Maybe you mean wobbling?
Not end over end, but a precession around the z axis. I used "tumbling" to mean a non-pure-Z rotation.
A rotating body (In a lossy situation, which this very much was) will tend to shift its axis of rotation from axial to end-over-end (max moment of inertia). But this object was under active RCS control, which is all but the axial direction, I'm assuming was effective. (Though "overwhelmed" RCS could have been referring to that as well)
Since the stage is shown centered on the ring-of-smoke, and the ring-of-smoke is long-lived, it shows no spin other than axial spin.
I agree that if the stage was precessing/wobbling/tumbling then it would have been torn up.
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#602
by
Maciej Olesinski
on 16 Oct, 2013 21:42
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GrassHopper was leveling its flight moments before landing.
Fuel problems there could result in angle landing.
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#603
by
Lars_J
on 16 Oct, 2013 21:52
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Comparing the sizes from a high-resolution image to the landing one, I get a ~18-20% difference in height - probably the stage was at a angle.
I agree 3m is an underestimation however, 3m would be ~5 pixels, and the distance between the bottom of the stage and the upper edge of the cloud is, at best, more than 10 pixels.
Same length, what you are seeing as the "end" of the stage is really just the end of the somewhat-clean part. Below that is more of the stage, covered in soot and not very visible.
.
I believe the 3m reference is quite correct, and the bottom of the stage is right in there with the "cloud", about to hit the surface.
Don't think so. I roughly agree to the pictorial representation by sittingduck: there's a clear hint of orange/red between the stage bottom and the cloud, which would be the plume.
I disagree. It makes sense that the area surrounding the engines would be darkest.... Here is what I believe is the true outline, in red. (your pic, modified)
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#604
by
eeergo
on 16 Oct, 2013 22:21
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I disagree. It makes sense that the area surrounding the engines would be darkest.... Here is what I believe is the true outline, in red. (your pic, modified)
Maybe, I just get that impression from the (very faintly redder) color difference that I think I see - with the haze washout and resolution, I agree it's a very subjective matter.
The engine area being darkest makes sense, but the majority of the soot coating would have happened during the braking burn, and that can be explained if the soot shot up in two rougly linear streams alongside the stage, as we hypothesized a bit ago in the discussion - of course, this is also speculative.
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#605
by
NovaSilisko
on 16 Oct, 2013 22:46
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Dunno about anyone else but I'm having a lot of fun watching everybody scramble over this blurry picture, makes me think of discussions about bigfoot
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#606
by
kevin-rf
on 16 Oct, 2013 22:57
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Dunno about anyone else but I'm having a lot of fun watching everybody scramble over this blurry picture, makes me think of discussions about bigfoot
Then you would really enjoy Thiery Legault's classic, "How Reliable is an Image"
http://legault.perso.sfr.fr/bad_astrophotography.htmlI would argue that the blurring in the image is causing both Eeergo and Lars_J to draw the stage wider than it is.
I was interesting to watch the grasshopper video again. The plume looked quite rectangular close to the ground.
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#607
by
billh
on 16 Oct, 2013 23:31
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I would argue that first photo is of the second burn. First burn was 40miles up or so and without specialized equipment they could not have take such picture/video.
Has it been mentioned yet that the first picture has the letters "usaf" in the file name? That immediately made me think "tracking camera". I believe the first photo was of the first burn, at high altitude, with three engines firing.
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#608
by
mlindner
on 17 Oct, 2013 00:12
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I would argue that first photo is of the second burn. First burn was 40miles up or so and without specialized equipment they could not have take such picture/video.
Has it been mentioned yet that the first picture has the letters "usaf" in the file name? That immediately made me think "tracking camera". I believe the first photo was of the first burn, at high altitude, with three engines firing.
It's got copyright "Scott Andrews" in the IPTC and TIFF info.
http://photoblog.nbcnews.com/_news/2011/07/08/7026327-spectacular-liftoff-how-he-gets-those-shuttle-imagesIs this maybe taken from Vandenberg on a telephoto and they are not from the same video?
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#609
by
Comga
on 17 Oct, 2013 05:14
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Both shots ARE from the video, which I've seen. And yes the second one is literally a sec before it hits the water.
I take it as read that if Padrat says "the video" there is one video. No ground tracking cameras, mlinder.
But many of us agree on a basic flow:
The second stage separates, demonstrating a new attach and release mechanism.
The first stage remains under control, and seconds later flips end over end and points back against its velocity.
It relights three engines, possibly the first time that has been attempted.
Its attitude remains controlled throughout the burn which kills off enough velocity to make atmospheric reentry survivable.
It falls from tens of kilometers through the atmosphere still under control.
It starts to roll from aerodynamic forces.
It relights the center engine, still somewhat under control.
It kills off most of the velocity, and remains burning to the point where its exhaust impinges on the surface of the ocean.
It flames out some seconds before reaching the surface and crashes, leaving some debris sufficiently intact to float.
Utterly astounding. There may be a fool who criticizes this as a failure, but it is such an amazing success. That SpaceX would get so far along on the first attempt is remarkable. It seems believable that they will land a first stage by the end of 2014. My hat is off to them.
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#610
by
meekGee
on 17 Oct, 2013 05:23
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We're not in agreement on the start of roll.
From the picture, it appears the engine was on "a plume's distance" away from the water. Maybe 100 m.
If the stage decelerates at 2g, and starts at 200 m/s, the engine needs to be at full thrust at 1000 m altitude.
The entire burn takes (under these assumptions) 10 seconds.
If that was the case, then they got through most of the burn, and it is possible that spin started after the engine re-lit.
(I have a bit of a problem with an uncontrolled spin which started high in the atmosphere. I think it would have taxed the RCS, exhausted its propellant, and the stage would have been tumbling by the time it had to relight. This is all very timing-sensitive of course)
But - yes -"Utterly astounding" is exactly right.
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#611
by
aero
on 17 Oct, 2013 05:47
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Here's a thought. Maybe Elon misspoke and it was fluid dynamic, not aerodynamic forces that caused the fuel to centrifuge. Fuel being sucked out of the tank like a toilet flush, swirling so gas gets into the pump intake. I have assumed that there was enough fuel in the tank for soft splash down, but maybe it was swirling around and riding up the tank wall a little. I wonder how many tons of RP-1 could still be in the tank under those conditions while still causing a flameout.
Of course that shouldn't cause the roll controls to be railed, I believe was the term. Or maybe it could, depends on the baffles inside the tank and a bunch of other factors.
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#612
by
llanitedave
on 17 Oct, 2013 06:11
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If that was the case, then they got through most of the burn, and it is possible that spin started after the engine re-lit.
No, this has been discussed ad nauseum. There's simply not enough time or rotational torque to start the spin after second ignition. It would have had to have started sometime during free fall, in atmospheric deceleration. This would have to be prior to second ignition.
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#613
by
Jason1701
on 17 Oct, 2013 06:15
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Do you think the stage might have been programmed to allow some roll so as to conserve cold gas propellant for pitch and yaw control? Roll is not as bad as the stage tumbling.
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#614
by
guckyfan
on 17 Oct, 2013 06:27
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Do you think the stage might have been programmed to allow some roll so as to conserve cold gas propellant for pitch and yaw control? Roll is not as bad as the stage tumbling.
That would require that they did expect the roll and did the programming to accomodate it. Roll is "not as bad" only as long as in the air. On landing with legs there must be no roll.
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#615
by
Lars_J
on 17 Oct, 2013 06:34
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A certain amount of roll could actually be a good thing, since it would give the stage added attitude stability during free-fall.
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#616
by
meekGee
on 17 Oct, 2013 07:01
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If that was the case, then they got through most of the burn, and it is possible that spin started after the engine re-lit.
No, this has been discussed ad nauseum. There's simply not enough time or rotational torque to start the spin after second ignition. It would have had to have started sometime during free fall, in atmospheric deceleration. This would have to be prior to second ignition.
I have been arguing against "rotational torques" (I assume you mean something to do with the turbo pump) - I'm very confident it was aerodynamic forces - but those can be show up only after the engines are thrusting.
But - if the burn nominally takes 10 seconds, and the effect kicked in as a result of ignition, and they did burn for 7-8 seconds (as the 3m picture seems to suggest), then that's plenty enough time to build up spin on a mostly-empty stage.
It's still an open race in my book.
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#617
by
kenny008
on 17 Oct, 2013 07:27
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I'm a little confused as to why some think that the stage roll had anything to do with engine ignition. It seems SO much more likely that the stage developed the roll durning the 300km/hr freefall, centrifuged the propellant away from the piping connection to the tank, and then started the engine on residual fuel in the piping. Why would we think that the stage remained stable all the way until the last few seconds, and then try to come up with some unusual mechanism of spinning up the stage?
Musk said it was aero, and they ran out of roll control authority. Doesn't it make more sense that slight aerodynamic proturbances at 300km/hr spun it up after running out of cold gas for the thrusters?
This discussion about torques on the stage in the final approach are confusing to me. My bet is the video will show a rapidly-spinning stage dropping into view, a short burn of the single engine in the last seconds, and the stage hitting the water at a pretty good velocity.
Still an amazing feat.
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#618
by
ChrisWilson68
on 17 Oct, 2013 07:43
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Was the stage tumbling as suggested before?
There was never any reason to believe the stage was tumbling, and every reason to believe it was not.
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#619
by
beancounter
on 17 Oct, 2013 08:13
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I'm a little confused as to why some think that the stage roll had anything to do with engine ignition. It seems SO much more likely that the stage developed the roll durning the 300km/hr freefall, centrifuged the propellant away from the piping connection to the tank, and then started the engine on residual fuel in the piping. Why would we think that the stage remained stable all the way until the last few seconds, and then try to come up with some unusual mechanism of spinning up the stage?
Musk said it was aero, and they ran out of roll control authority. Doesn't it make more sense that slight aerodynamic proturbances at 300km/hr spun it up after running out of cold gas for the thrusters?
This discussion about torques on the stage in the final approach are confusing to me. My bet is the video will show a rapidly-spinning stage dropping into view, a short burn of the single engine in the last seconds, and the stage hitting the water at a pretty good velocity.
Still an amazing feat.
In addition, Musk said that the legs would mitigate the spin. This lends more evidence toward the aero spin theory than engine startup. I also like to keep in mind the KISS principle.
