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#560
by
eeergo
on 16 Oct, 2013 17:09
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Braking at 2g, it would take slightly more than 4 seconds (~1km height) to slow down from a terminal velocity of 300km/h. Considering engine startup transients, make that 5, maybe 6 (making it 1500-1700m height). Centrifugation won't have starved the engine right away, but rather thinned out the propellant supply: if the engine managed to start up to some degree, some acceleration will have built up, up to the nominal, notional 2g.
It should be also considered roll, if it was developed due to aerodynamic instabilities and was too strong to counteract with the gas thrusters, would probably involve some degree of tumbling (precession). Which would also make the "rectangular-ish" shape of the spray make more sense, if the stage was pointing roughly towards the observer, making an asymmetrical spray cloud towards the camera.
Plus the Merlins cough up a quite a bit of smoke while shutting down, as well as some flaming - which would be more prolongued in time due to the centrifugation effect and the irregular propellant supply.
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#561
by
Lar
on 16 Oct, 2013 17:17
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So I wonder if we[1] can work out how fast the stage was spinning
[2]
We have the estimated fuel load remaining or can get it, and we should be able to work out how long the engine had to burn to get to 3M above sea level, so that maybe can tell us how much fuel was left when it ran out? THAT should tell us the shape of the parabola and THAT should tell us the rotation
Easy peasy.
Ok maybe not.
1 - by "we" I mean people smarter than me
2 - yes I know, arrant uninformed speculation...
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#562
by
MP99
on 16 Oct, 2013 17:21
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Braking at 2g, it would take slightly more than 4 seconds (~1km height) to slow down from a terminal velocity of 300km/h. Considering engine startup transients, make that 5, maybe 6 (making it 1500-1700m height). Centrifugation won't have starved the engine right away, but rather thinned out the propellant supply: if the engine managed to start up to some degree, some acceleration will have built up, up to the nominal, notional 2g.
Could the engine have started using just prop already in the feed lines, which wasn't replaced because the sump was uncovered?
Depends whether the roll was already there before engine start. Would be implied, I think, if the roll was caused by aero effects during re-entry or descent.
cheers, Martin
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#563
by
MP99
on 16 Oct, 2013 17:24
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http://www.spacex.com/sites/spacex/files/11_c439042b-ee14-45c7-aa50-6f0f6396b0db.png
I'm confused about what we're seeing in this shot. Is the engine firing or not? It looks like it, but then some things don't add up.
This post has been ignored, but could it be the answer? IE a fast-falling stage will have a pressure wave in front of it and that may be what we're seeing.
The sea surface gets a prenotice when something large is falling fast but subsonic towards it.
cheers, Martin
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#564
by
eeergo
on 16 Oct, 2013 17:30
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So I wonder if we[1] can work out how fast the stage was spinning [2]
We have the estimated fuel load remaining or can get it, and we should be able to work out how long the engine had to burn to get to 3M above sea level, so that maybe can tell us how much fuel was left when it ran out? THAT should tell us the shape of the parabola and THAT should tell us the rotation
Easy peasy.
Ok maybe not.
1 - by "we" I mean people smarter than me
2 - yes I know, arrant uninformed speculation...
There are several unknowns yet to clear up before going into those calculations with any kind of "order of magnitude" precision.
- Where are the openings on the (oxidizer/fuel) tanks located? Just on the bottom, or somewhat to the sides?
- Was the stage tumbling as suggested before? If so, that would impart an important extra vertical pull on the lower tank (kerosene) that would require a higher rate of rotation to achieve the same level of centrifugation, than with a pure rotation around the vertical axis. The other tank (LOX), whose bottom is around the mid-height of the stage, wouldn't feel it, or would feel an upwards acceleration: this would starve the engines of oxygen before the fuel.
- Related to the above: was the engine cutoff due to software finding an inadequate mix ratio (or ECO sensors becoming locally dry) or to actual oxidizer/fuel deficit?
- We don't know what final reserve SpaceX was expecting for a nominal burn: I assume they don't expect to achieve zero velocity when both the fuel and the LOX are just about to run out.
Without at least this, the numbers can turn out to be anything.
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#565
by
cambrianera
on 16 Oct, 2013 17:31
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http://www.spacex.com/sites/spacex/files/11_c439042b-ee14-45c7-aa50-6f0f6396b0db.png
I'm confused about what we're seeing in this shot. Is the engine firing or not? It looks like it, but then some things don't add up.
This post has been ignored, but could it be the answer? IE a fast-falling stage will have a pressure wave in front of it and that may be what we're seeing.
The sea surface gets a prenotice when something large is falling fast but subsonic towards it.
cheers, Martin
I am with R7 (and you) on this. Definitely a pressure wave travelling in front of the stage.
Btw: three meters is less than the diameter of the stage.
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#566
by
Antares
on 16 Oct, 2013 17:35
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Why do people keep asking questions that are proprietary to SpaceX?
Since the dawn of this forum? 
Why get annoyed over it now? Asking is one thing... getting an answer is another.
It's different now. When the bulk of the content was Shuttle, all of that was public. Now that the government has more limited data rights on what it can do with the information it receives to do its risk analyses, there is not going to be as much public. Moreover, the veracity of what becomes public is more questionable.
In the incident question, how does Grasshopper hover if it's consuming propellant as it's hovering? Answer: the engine is throttlable. Ergo, a public question of what the thrust of the center engine is just before splashdown is essentially unknowable. Even T/W is a complete conjecture without seeing the video to see how fast the stage is slowing down.
NSF is just not as much fun when it's all swaggy guessing.
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#567
by
kevin-rf
on 16 Oct, 2013 17:37
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Which would also make the "rectangular-ish" shape of the spray make more sense, if the stage was pointing roughly towards the observer, making an asymmetrical spray cloud towards the camera.
Considering this was taken with a large zoom (meta data said 400mm) from a plane safely outside the drop zone, you are most likely looking at the image at a very oblique angle. Meaning, at it from the side. You are unlikely able to determine if you are looking at a circle, ellipse, or even trapezoid.
I suspect it is a nice symmetrical circular spray pattern observed from the side.
Edit: Eeergo I hand edit'd your processed image and added an ellipse to the spray pattern where my eye saw boundary of the plume.
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#568
by
AnjaZoe
on 16 Oct, 2013 17:44
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Ergo, a public question of what the thrust of the center engine is just before splashdown is essentially unknowable.
The question is perfectly knowable
but I know what you mean. However, if I recall correctly, the thrust of the engine at sea level is given by Space-X (at least in rough numbers), I also recall seeing a statement that is it throttleable to 70% or therebouts of max. thrust. That should give you an indication of the minimum thrust of the engine.
Now finding the answer to the question T/W ratio at time of landing, that is more tricky and the core of my original question, shouldn't an impossible challenge to people with relevant knowledge (not me).
Deriving a range of mass the nearly empty stage is most likely to have, one could estimate the resulting T/W ratio, considering that could lead to a very rough estimation of required burn time of the engine to have the stage reach about zero speed at zero height.
This isn't exactly complex rocket science.
To me that looks like an excellent basis for a technical discussion, but as usual, I might be mistaken.
Zoe
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#569
by
cambrianera
on 16 Oct, 2013 17:47
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One more curious thing from the video.
The final frames of the video are focused on second stage (I think to show it didn't blew up).
If you compare these two frames (T+10 min and T+36 min) you can see (circled in red) a buildup of material.
If this is ice from the small amount of humidity in LOX, then the engine has vented LOX all the time between SECO and the second frame.
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#570
by
eeergo
on 16 Oct, 2013 17:47
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Which would also make the "rectangular-ish" shape of the spray make more sense, if the stage was pointing roughly towards the observer, making an asymmetrical spray cloud towards the camera.
Considering this was taken with a large zoom (meta data said 400mm) from a plane safely outside the drop zone, you are most likely looking at the image at a very oblique angle. Meaning, at it from the side. You are unlikely able to determine if you are looking at a circle, ellipse, or even trapezoid.
I suspect it is a nice symmetrical circular spray pattern observed from the side.
I don't think so from the
image enhancement analysis from before, which was later verified: the background is all sea surface. Plus if the stage was in an uncontrolled "roll" at this point, it's very unlikely it has a pure z-rotation alone: some degree of tumbling will be present.
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#571
by
VatTas
on 16 Oct, 2013 17:57
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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.
It is hard to judge angle, but it looks like stage was still pretty high up above water when first photo/video frame was taken. If it burned for several seconds and then shut down, it had enough time to get some speed. If second frame was showing second or so after shutdown, stage would probably stayed intact.
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#572
by
ugordan
on 16 Oct, 2013 17:57
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I don't think so from the image enhancement analysis from before, which was later verified: the background is all sea surface.
So? A
very oblique angle does not imply seeing sky in a highly zoomed image. That is an image of a very distant object as the atmospheric haze and jitter distorting and blurring the stage image indicates.
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#573
by
LegendCJS
on 16 Oct, 2013 18:00
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Ergo, a public question of what the thrust of the center engine is just before splashdown is essentially unknowable.
The question is perfectly knowable but I know what you mean. However, if I recall correctly, the thrust of the engine at sea level is given by Space-X (at least in rough numbers), I also recall seeing a statement that is it throttleable to 70% or therebouts of max. thrust. That should give you an indication of the minimum thrust of the engine.
Just one detail folks are missing. I've noticed a few people assuming that 70% throttle is the plan, but to my memory it isn't the plan at all.. The most fuel efficient landing is at full throttle, not at 70%, so use the full value of merlin 1D thrust in all calcs, not the throttled value.
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#574
by
mr. mark
on 16 Oct, 2013 18:01
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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?
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#575
by
Lars_J
on 16 Oct, 2013 18:05
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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.
Doubtful. They couldn't start the 2nd burn that far up... This is a picture with sky background. And this isn't a very clear picture to begin with. I don't see why a good telephoto lens (with an unknown number of telephoto extensions) couldn't have captured it. And we don't know it it was attached to some tracking hardware or stabilizing platform. Nor do we know the length of the burn, and how far it descended during the burn.
It is hard to judge angle, but it looks like stage was still pretty high up above water when first photo/video frame was taken. If it burned for several seconds and then shut down, it had enough time to get some speed. If second frame was showing second or so after shutdown, stage would probably stayed intact.
How can you judge speed from that single blurry picture?
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#576
by
Nydoc
on 16 Oct, 2013 18:08
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I'm confused about what we're seeing in this shot. Is the engine firing or not? It looks like it, but then some things don't add up.
The second photo does not show the engine lit. If it were lit then it would be a lot brighter. This is not a cloud ring we are looking at. It is ocean spray from the impact of the stage like a whale hitting the water.
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#577
by
LegendCJS
on 16 Oct, 2013 18:09
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One more curious thing from the video.
The final frames of the video are focused on second stage (I think to show it didn't blew up).
That is an interesting theory about why they included the footage downlinked from the Antarctic station, but the footage included says "re-acquisition of signal" implying it is the first footage received from this station. When you combine that implication with the interview statements given by Elon that SpaceX waited until it had good telemetry/data links with the second stage over the Antarctic receiving window to attempt the restart, one would conclude that the restart happened after acquisition of signal from the Antarctic station, and therefore after the footage shown in the video. This would imply that reason for showing that last segment of video couldn't have been to prove the stage survived the restart attempt. (Or if it is then their left hand isn't talking to their right hand so to speak)
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#578
by
MP99
on 16 Oct, 2013 18:10
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that's only 10 feet off the water. Fully intact
Why is that surprising? If the stage had been torn up by aero, it would have happened at higher q (or q-alpha) when it was traveling much faster. Impact would have been much more destructive than low q aero.
I'd suggest that the delight is simply that the stage did indeed survive intact through re-entry and those higher q phases of descent, presumably reaching terminal velocity at sea level. People love the implication of major progress towards recovery (and I'm not implying anything negative about your statement **.)
Of course, the picture doesn't really convey much info over and above the previous statements that it
had survived, but people do love another chance to respond to things they're invested in.
cheers, Martin
** so hard to get correct tone across in a text-only medium.
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#579
by
Lars_J
on 16 Oct, 2013 18:14
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I'm confused about what we're seeing in this shot. Is the engine firing or not? It looks like it, but then some things don't add up.
The second photo does not show the engine lit. If it were lit then it would be a lot brighter. This is not a cloud ring we are looking at. It is ocean spray from the impact of the stage like a whale hitting the water.
No, according to Shotwell the picture shows the stage ~3m before impact:
https://twitter.com/jeff_foust/status/390503333561401344My speculation: the "spray" might be residual smoke from the engine that just shot down a second or two before the picture was taken/captured.
[2]
Why get annoyed over it now? Asking is one thing... getting an answer is another.