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#580
by
AnjaZoe
on 16 Oct, 2013 18:16
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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.
Fair and interesting remark. But using the engine at full throttle to slow down the stage, wouldn't that actually reduce the tolerance on the firing time? Too short and it crashes down, too long and it takes off again? (assuming that the thrust is higher than the mass of the stage of course, which I would guess it (a) is and (b) needs to be anyway, but please correct me if I'm wrong). And would fuel efficiency be of great importance at that time of flight?
Zoe
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#581
by
Nydoc
on 16 Oct, 2013 18:18
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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/390503333561401344
My speculation: the "spray" might be residual smoke from the engine that just shot down a second or two before the picture was taken/captured.
That's just what Jeff Foust wrote when he saw the image the first time. Did Shotwell actually say 3m or is Jeff interpreting that?
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#582
by
Lars_J
on 16 Oct, 2013 18:19
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You can ask him, but I don't think Jeff would waste part of 140 chars of a tweet to invent a detail like that.
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#583
by
sittingduck
on 16 Oct, 2013 18:22
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Is the first stage + interstage roughly ~48 meters in length? I don't see how the stage can be 3 meters above the water surface in that photo then, given the apparent gap between tail-end and surface. What's wrong with this picture? Am I totally misinterpreting the various fuzzy objects here?
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#584
by
RoboGoofers
on 16 Oct, 2013 18:23
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One possibility is that after the fuel ran dry the engine was still expelling O2. I would think the cold O2 would create a lot of condensation at the humid surface of the open ocean.
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#585
by
Lars_J
on 16 Oct, 2013 18:25
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Is the first stage + interstage roughly ~48 meters in length? I don't see how the stage can be 3 meters above the water surface in that photo then, given the apparent gap between tail-end and surface. What's wrong with this picture? Am I totally misinterpreting the various fuzzy objects here?
I think the bottom part of the stage is darker - the area you mark as 'plume?' appears to be part of the stage. It is just charred/discoloured a bit.
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#586
by
cambrianera
on 16 Oct, 2013 18:31
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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)
Fine, you've cut away the really curious thing to discuss a useless detail......
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#587
by
Space OurSoul
on 16 Oct, 2013 18:34
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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 kero has almost zero gaseous pressure by comparison, so would still be mostly stuck in the tank.
But I'll put my money on hot exhaust from the brief burn hitting that cold ocean and creating good old fashioned steam. I doubt the stage was traveling fast enough, even if it were still close to terminal velocity, to have produced a shockwave of sufficient energy to kick up spray.
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#588
by
Lars_J
on 16 Oct, 2013 18:34
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That's just what Jeff Foust wrote when he saw the image the first time. Did Shotwell actually say 3m or is Jeff interpreting that?
Here is another blog's partial transcript of her talk:
http://www.transterrestrial.com/?p=51818 Three meters was mentioned again, so this is unlikely to be Jeff's invention.
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#589
by
ugordan
on 16 Oct, 2013 18:39
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If you compare these two frames (T+10 min and T+36 min) you can see (circled in red) a buildup of material.
Also, flashes of light can be seen on the left side, also could be seen after SECO-1 but less prominently. I wonder if those are pulses from an ACS thruster. Settling propellant?
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#590
by
Nydoc
on 16 Oct, 2013 18:44
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That's just what Jeff Foust wrote when he saw the image the first time. Did Shotwell actually say 3m or is Jeff interpreting that?
Here is another blog's partial transcript of her talk: http://www.transterrestrial.com/?p=51818 Three meters was mentioned again, so this is unlikely to be Jeff's invention.
Fair enough. One thing I think we can be certain of however: the engine is not lit in the second photo.
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#591
by
MP99
on 16 Oct, 2013 19:08
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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.
But, 100% doesn't seem to be the lowest risk? (Agreeing with AnjaZoe's post.)
If the stage starts at below max throttle, then it can throttle up if acceleration starts late (slow engine startup), or is below expectation (perhaps because residuals are greater than expected). That assumes the engine can start at anything other than 100%, or can quickly throttle down if that looks to be required.
Question is whether lowest risk is to assume 70% then throttle up if required, or somewhere intermediate followed by throttle up or down as required.
If acceleration is higher than expected, that seems to imply that residuals are lower than expected. If the stage has to light up and verify before it can redirect itself from an ocean disposal to land landing, then being light on residuals seems to imply that the stage should abort the landing attempt. (ISTM the burn would continue to depletion to avoid contaminating the ocean with unburnt RP-1, but not re-direct towards land.)
I wonder, also, whether this flight carried a lot more residuals post-MECO (and maybe did a more substantial pre-reentry burn) than they'd hope to need in a "routine" flight that needs most of F9R's available performance. Not forgetting, as well, that there will also need to be prop retained for a boost-back burn to target RTLS (but this is the least difficult addition if they've already nailed a re-entry burn, and proved that M1Dvac can light in µg).
[NB question re whether the reentry burn was conservatively large may be interesting next time someone has a Q&A with Elon or Gwynne.]
cheers, Martin
PS to mods: suggest this is on-topic for F9 #006 in attempt to understand the throttle settings that SpaceX were targeting for this landing attempt.
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#592
by
Okie_Steve
on 16 Oct, 2013 19:44
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I would think that 85% which allows +/- 15% would be a good place to aim for in planning.
Since the still frame is from a video, why was this particular frame chosen? Would the next frame show damage or maybe just be obscured by surface mist. Depending on the frame rate and velocity it obviously "moves" in discreet steps from frame to frame. If this is not the "3 meter" image then probably the next frame was.
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#593
by
eeergo
on 16 Oct, 2013 19:58
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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.
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.
Maybe we're just saying the same thing with different words: I was replying the above quote from kevin, which seemed to imply the picture was taken from the side of the "spray cloud".
If said cloud is parallel to the sea surface, and the background is all sea (that is, the photo was taken from a height), then the cloud cannot be symmetric - which was what I was arguing in the response.
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#594
by
eeergo
on 16 Oct, 2013 20:02
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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.
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#595
by
matthewkantar
on 16 Oct, 2013 20:15
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Weighing in for the first time after much lurking here. Doing a simple width to height ratio of the image of the first stage as it is coming down, some of the first stage is in the white cloud/splash/plume object.
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#596
by
eeergo
on 16 Oct, 2013 21:07
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Is the first stage + interstage roughly ~48 meters in length? I don't see how the stage can be 3 meters above the water surface in that photo then, given the apparent gap between tail-end and surface. What's wrong with this picture? Am I totally misinterpreting the various fuzzy objects here?
Weighing in for the first time after much lurking here. Doing a simple width to height ratio of the image of the first stage as it is coming down, some of the first stage is in the white cloud/splash/plume object.
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 in the attached image, and the distance between the bottom of the stage and the upper edge of the cloud is, at best, more than 10 pixels.
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#597
by
Noborry
on 16 Oct, 2013 21:25
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Ok, so the so the deceleration and descent went fine, then the final burn died due to fuel starvation, caused by excessive spin.
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Just how much disruption of the fuel supply would be needed, to kill the burn? That baby is burning a lot of fuel per second, surely even a tenth of a second disruption by an "air bubble" in the fuel feed would cause a severe hiccup, very likely flame-out, likely turbopump shredding?
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The roll was more than the control system could compensate, yes, but it need not have been very great in absolute terms to cause problems for the engine. A good solid "slosh" at the wrong time could have caused the outage. No need to envisage it spinning like a top, centrifuging all the fuel to the walls.
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#598
by
Noborry
on 16 Oct, 2013 21:30
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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.
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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.
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#599
by
eeergo
on 16 Oct, 2013 21:36
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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.
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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.
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.