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#620
by
ChrisWilson68
on 17 Oct, 2013 08:16
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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.
Yes, I was watching through handheld binoculars from near the launch site and I was able to continue following the first stage after separation. I could even see the thrusters activating. It was only a dot in my binoculars, but I'm sure a DSLR with a serious lens would have had no trouble taking that picture.
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#621
by
R7
on 17 Oct, 2013 09:22
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stage developed the roll durning the 300km/hr freefall
Where did the 300 figure come from?
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#622
by
cambrianera
on 17 Oct, 2013 10:10
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I've decided to put some numbers in my integration spreadsheet to see some possible results for the final burn of F9 v1.1.
Mass of incoming stage assumed 20000 kg (18000 kg structure, 2000 kg propellant)
Terminal velocity (with front area 10m2 and Cd=1) is about 180 m/s
Assuming an incoming velocity of 200 m/s I get that the final burn is:
Throttle % Time Braking height Propellant used
100 less than 7 s 660 m 1600 kg
85 less than 9 s 810 m 1700 kg
70 about 11 s 1000 m 1800 kg
Usual disclaimer: not for reference.
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#623
by
VatTas
on 17 Oct, 2013 10:38
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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?
I can't. All I was saying that if engine shut down just 1-2s before second picture, then stage would have
probably stayed intact. Or at least not in pieces. Burn must have killed most of velocity already.
I still think that first image is of the second burn (unless USAF in file name really means USAF tracking cameras), as Padrat stated that it is from the same video. It was also mentioned that video was taken from small plane near recovery zone. That probably means - no stabilized platform for photo equipment, so no matter how good your lens is, it would be very hard to take video/pictures of something that is tens of kilometers away. And cambrianera's calculations show that stage should have been pretty high when second burn started - so no wander that background is darker.
Anyway, it doesn't really matter. I'm still hoping for video
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#624
by
kenny008
on 17 Oct, 2013 10:48
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stage developed the roll durning the 300km/hr freefall
Where did the 300 figure come from?
Don't take that number to mean anything more than "very fast." No calculations or inside knowledge
stage developed the roll durning the 300km/hr freefall
Where did the 300 figure come from?
. I just know skydiver freefall is somewhat less than that. I see that Cambrianera has much more precise calculations (showing even higher speeds).
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#625
by
douglas100
on 17 Oct, 2013 10:55
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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.
I doubt that scenario. I think the engine would be shut down by the engine controller as soon as the propellant depletion sensor(s) had been uncovered. I think the valves would have closed.
I didn't get a chance to post yesterday. I'd just like to add my congratulations to SpaceX on pulling off what is probably a genuine "first."
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#626
by
ChefPat
on 17 Oct, 2013 13:06
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Is there any reason the "roll" had to be an end over end tumble & not a wobbly barrel roll with the business end oriented correctly?
ISTM that would better explain the plume we're seeing in the picture.
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#627
by
Mader Levap
on 17 Oct, 2013 13:11
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I still think that first image is of the second burn
It would mean stage falling almost exactly on head of photographer. Forget about it.
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#628
by
Norm38
on 17 Oct, 2013 13:47
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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.
That's why I posted in the Grasshopper thread on the subject. The latest GH1 flight was over 70 seconds, and at no point did roll forces overcome RCS authority. So how could it be possible for v1.1 to spin up in 7-8 seconds as a result of ignition if GH1 shows no sign of that?
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#629
by
kevin-rf
on 17 Oct, 2013 14:54
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It has legs ...
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#630
by
eeergo
on 17 Oct, 2013 15:45
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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.
Is there any reason the "roll" had to be an end over end tumble & not a wobbly barrel roll with the business end oriented correctly?
ISTM that would better explain the plume we're seeing in the picture.
This^
As explained before, I referred to *some degree* of tumbling, which you may call wobbling, precession, non-purely-Z axis rotation, or the likes.
If the roll control authority by the cold gas thrusters was exceeded, as Elon mentioned (let's remember he only mentioned "
the stage was spinning to a degree that was greater than we could control with the cold gas thrusters and it centrifuged the propellant") it's more likely it was present than not. It could also help explain some features of the "water-spray" ocean picture.
Of course all this theory is difficult to sustain with a single blurry picture (see the conversation Lars_J, me and others had during the last few pages), but I think it's an interesting discussion.
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#631
by
blazotron
on 17 Oct, 2013 16:19
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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 
I would argue that the blurring in the image is causing both Eeergo and Lars_J to draw the stage wider than it is.
Completely agree. If you have one pixel of blur around the object's actual perimeter, it will make a much higher percentage error in measured width than length just due to one pixel being a larger fraction of the width pixels than the length pixels. So for a body with a high aspect ratio, extrapolating length from measured width has high uncertainty and will tend to overestimate length.
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#632
by
mrmandias
on 17 Oct, 2013 16:21
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This is amazing. A whole bunch of new stuff had to go right to get that close landing successfully.
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#633
by
cambrianera
on 17 Oct, 2013 16:24
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Someone with a better explanation than a mistake for those two frames showing exactly the same speed?
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#634
by
Space OurSoul
on 17 Oct, 2013 17:12
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My explanation: Those values were added to the video by an editor after the fact. They represent a snapshot of the value taken approximately (with error arbitrarily large depending on how conscientious the editor was) at the time the values first appear on the video.
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#635
by
ugordan
on 17 Oct, 2013 17:17
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Apart from the 1st stage restart video (which was blown up unnecessarily, cropping the 4:3 aspect to fit 16:9 in the process), the mission overview video was pretty disappointing.
Seriously, no tracking camera shots?
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#636
by
aero
on 17 Oct, 2013 17:31
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I'm a little surprised that we haven't seen photos of some major rocket parts recovered. I guess they broke up and sank.
Its been commented that SpaceX tanks are "semi-balloon" tanks. They require some level of internal pressure for maximum strength. Could fuel depletion on landing the stage have caused internal pressure loss in the tanks, hence weakening the structure allowing more serious break-up on touch down than we might have expected?
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#637
by
Lars_J
on 17 Oct, 2013 17:35
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I'm a little surprised that we haven't seen photos of some major rocket parts recovered. I guess they broke up and sank.
Most of the trunk and some engine components were apparently recovered. I don't think we'll see any photos of them for a while, for two reasons:
1) No good PR from displaying heavily damaged hardware
2) To not give competitors more information than they need bout how far along SpaceX is (or isn't) towards reusability
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#638
by
meekGee
on 17 Oct, 2013 18:32
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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.
I agree 100% it was aerodynamic forces.
However, since the aerodynamic environment around the leading face of the rockets gets upended completely after the engine ignites, and since it seems (from the 3m picture) that the engine might have been on for the majority of the planned descent (not certain by any means, it could have been LOX) then there was definitely enough time to acquire the spin post-ignition.
There's also the argument that if the stage was spinning during the entire descent, it would have tried to wobble/precess/tumble, and that would tend to exhaust the lateral RCS, so it would not have been stable for that final burn.
I'm also not clear that the fuel in the center tubes was enough for a 7-8 second burn.
So as before - both options are possible, and none is a slam dunk.
I hope the video is clear enough to tell the amount of spin pre- and post- ignition.
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#639
by
mlindner
on 17 Oct, 2013 22:10
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Apart from the 1st stage restart video (which was blown up unnecessarily, cropping the 4:3 aspect to fit 16:9 in the process), the mission overview video was pretty disappointing.
Seriously, no tracking camera shots?
There weren't any working cameras. They all were disabled by launch overpressure if I remember correctly. Forget where this was reported.
