Author Topic: SpaceX Falcon 9 - NROL-76 - May 1, 2017 - DISCUSSION  (Read 217894 times)

Offline hans_ober

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Re: SpaceX Falcon 9 - NROL-76 - May 1, 2017 - DISCUSSION
« Reply #440 on: 05/01/2017 12:15 PM »
Staged 10 m/s faster than CRS-10, and 4 seconds earlier. Seems very likely to be an upgraded booster.

Lighter payload or are you implying Block 4? My bet will be lighter payload ... the Apogee was looking a bit high on the S1, like they were expecting 130-ish km when they were at 160-ish at the peak.

Not necessarily. Could be that the trajectory was somewhere between the very steep OG2 (vert velocity of ~1500m/s , horizontal ~600m/s = 1670m/s at MECO.) and the flatter CRS-9/10.

MECO was ~1670m/s, but there could have been more of a horizontal component than OG2, which makes sense since OG2 tried burning directly to ~600km.

I think the presenter said 1.7million lb at liftoff, which makes this Block 4 since this is the uprated thrust figure; unless he was just reading data off their website. Changes to S2 make it probable that this was a B4.

Best way to figure out is to skim over velocity+altitude data from the webcast. Figure out vertical v from altitude, and plot horizontal & acceleration from that data.

Was it just me, or did anyone else notice that the altitude data was not consistent, but was jumping to rounded up values of kilometers. Fudged data?

Offline Antares

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Re: SpaceX Falcon 9 - NROL-76 - May 1, 2017 - DISCUSSION
« Reply #441 on: 05/01/2017 12:18 PM »
Was it me or was the re-entry burn vectored sharply off to one side during the middle phase? I wonder why they would have done  that; to give asymmetric braking?
It always does that, the elongated plume is visible from the onboard camera in many previous launches. It does look incredible from the outside though. I guess the supersonic airflow does strange things.

Does the IIP remain offshore through both entry and landing burn or just landing?  It may be adjusting the track at that point.
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Online CorvusCorax

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Re: SpaceX Falcon 9 - NROL-76 - May 1, 2017 - DISCUSSION
« Reply #442 on: 05/01/2017 12:21 PM »
Did you notice that the telemetry altitude is heavily filtered?

From 99km down to 0, it displayed the alt with seemingly 0.1km accuracy, but the actual altitude went down in jumps, as if the telemetry feed was first reduced to 1km resolution and later filtered to give a "smooth" appearance during ascent - but whatever interpolation they have for that smoothing seems to be coded for ascent under asceleration and shows weird stairstep -behaviour when being run on descent during deceleration.

This has obvious implications for any attempts to recreate accurate trajectory and acceleration information from the telemetry via OCR as has been done in the past.  Obviously the minimum trustworthy altitude resolution is 1km, but the question is about timing.

This is very well visible in the time between entry burn shutdown and actual landing

The nonlinearity on velocity is less obvious, somone would need to run OCR on the webcast and analyse it.

I wonder if they would go as far and deliberately distort also the absolute values (such as MECO velocity and max altitude) to obfuscate the classified NRO trajectory. Since the data is obviously postprocessed that might as well be the case - with the additional benefit of misleading SpaceX competitors.

But that's speculation. The only thing that's certain is that the accurate altitude resolution (and timing) is less than it seems.

Offline IanO

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Re: SpaceX Falcon 9 - NROL-76 - May 1, 2017 - DISCUSSION
« Reply #443 on: 05/01/2017 12:26 PM »
Did you notice that the telemetry altitude is heavily filtered?

From 99km down to 0, it displayed the alt with seemingly 0.1km accuracy, but the actual altitude went down in jumps, as if the telemetry feed was first reduced to 1km resolution and later filtered to give a "smooth" appearance during ascent - but whatever interpolation they have for that smoothing seems to be coded for ascent under asceleration and shows weird stairstep -behaviour when being run on descent during deceleration.
No, it had the same problem during ascent.  Altitude resolution appeared to be integral km only, with some hokey algorithm to jump between km when <100 km.
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Offline dawei

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Re: SpaceX Falcon 9 - NROL-76 - May 1, 2017 - DISCUSSION
« Reply #444 on: 05/01/2017 12:34 PM »
When the strongback retracted a bit before launch I experienced an optical illusion that the strongback was standing still and the rocket was tipping over.  My brain told me it was an illusion but my heart skipped more than a few beats!  So glad my brain was right.

Online AC in NC

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Re: SpaceX Falcon 9 - NROL-76 - May 1, 2017 - DISCUSSION
« Reply #445 on: 05/01/2017 12:41 PM »
Was it me or was the re-entry burn vectored sharply off to one side during the middle phase? I wonder why they would have done  that; to give asymmetric braking?
It always does that, the elongated plume is visible from the onboard camera in many previous launches. It does look incredible from the outside though. I guess the supersonic airflow does strange things.

Does the IIP remain offshore through both entry and landing burn or just landing?  It may be adjusting the track at that point.

As I understand, IIP is remains stable until the final transition from off-target IIP to Landing.  They don't put the IIP on the LZ until a good attempt at landing is assured.  Almost any burn would move the IIP and therefore vectoring in required to maintain it. 
« Last Edit: 05/01/2017 12:45 PM by AC in NC »

Offline Kabloona

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Re: SpaceX Falcon 9 - NROL-76 - May 1, 2017 - DISCUSSION
« Reply #446 on: 05/01/2017 12:46 PM »
Looks like aerodynamic braking is pretty strong, as expected.  After the entry burn, it accelerated for a while in the thin air, then decelerated at about 2G.   Terminal velocity looked like about 300 m/s.  This is about as expected from something of that size, shape, and mass.

The entry burn seems to be timed so that, at cutoff, the aerodynamic drag has increased almost to the point of beginning to be able to decelerate the stage by itself. The timing wasn't perfect, as cutoff occurred around 740 m/sec and the stage accelerated under gravity to about 840 m/sec before aero drag was able to begin decelerating. That period of acceleration lasted about 17 seconds.

So the "matching" of the end of propulsive deceleration with the start of aerodynamic drag deceleration wasn't perfect, but it was close, and it's reasonable to think that's how they tried to time the entry burn sequence, ie with a given quanitity of propellant, and thus a given burn duration, time the start of the burn so that, at the end of the burn, aero drag will be just enough to continue deceleration.

(See Welsh Dragon's excellent speed plot below for the graphic illustration of the transition from entry burn deceleration to aero drag deceleration.)
« Last Edit: 05/01/2017 01:03 PM by Kabloona »

Offline ugordan

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Re: SpaceX Falcon 9 - NROL-76 - May 1, 2017 - DISCUSSION
« Reply #447 on: 05/01/2017 12:49 PM »


Looks like it leaned into the wind to counteract the drift during the lower portion of the descent (notice the cloud movement direction)

Offline Welsh Dragon

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Re: SpaceX Falcon 9 - NROL-76 - May 1, 2017 - DISCUSSION
« Reply #448 on: 05/01/2017 12:53 PM »
Given the fantastic footage of first stage, and the continual telemetry data, I decided to plot out the data for this stage, thought it would be interesting. Relevant information is in the figure, but took data in 5 second bins, and derived acceleration in 5 second bins too. Interesting the see the significant deceleration due to air resistance between entry and landing burns. Happy to share raw data with anyone.

EDIT: Updated attachment to correct labelling error. Data has not changed.
« Last Edit: 05/01/2017 01:22 PM by Welsh Dragon »

Online saliva_sweet

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Re: SpaceX Falcon 9 - NROL-76 - May 1, 2017 - DISCUSSION
« Reply #449 on: 05/01/2017 12:53 PM »
Interesting periodic "breathing" from the main engines was noticed by reddit user Brusion. From around T+4.30 to reentry burn.
« Last Edit: 05/01/2017 12:54 PM by saliva_sweet »

Offline rsdavis9

Re: SpaceX Falcon 9 - NROL-76 - May 1, 2017 - DISCUSSION
« Reply #450 on: 05/01/2017 01:10 PM »
Interesting periodic "breathing" from the main engines was noticed by reddit user Brusion. From around T+4.30 to reentry burn.

I think this periodic venting is something the head of mission assurance(Koenigsmann) said was going to be done to fix bubbles in lines that were preventing even starting of engines for landing burn. All he said was that there was a problem of heating on the intake lines to the engines. So to fix it they can insulate better and didn't say what the other was.
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Offline LouScheffer

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Re: SpaceX Falcon 9 - NROL-76 - May 1, 2017 - DISCUSSION
« Reply #451 on: 05/01/2017 01:12 PM »
Interesting periodic "breathing" from the main engines was noticed by reddit user Brusion. From around T+4.30 to reentry burn.
Venting from the side would upset the attitude, as well.  So maybe they vent from the engines to avoid this:

A few observations from this wonderful footage:

There is a "heartbeat" like glow (not the nitrogen thrusters) at the base of the rocket every few seconds.  My guess is that they are releasing pressure through the engines so (as opposed to the vents) so they don't upset the attitude.

The jets are navigating in 3d,not just two.  You can see them thrusting fore and aft as well as sideways.


Offline sevenperforce

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Re: SpaceX Falcon 9 - NROL-76 - May 1, 2017 - DISCUSSION
« Reply #452 on: 05/01/2017 01:24 PM »
Because we know that the boostback burn is always going to be just about perfectly horizontal (as anything off-horizontal is going to waste propellant), we can now determine for the first time with reasonably good accuracy exactly what the trajectory was at MECO, based on separation altitude vs apogee.

Separation: 1.68 km/s at 69.6 km altitude, T+2:23
Apogee: 0.293 km/s at 166 km altitude, T+4:46

Offline Kabloona

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Re: SpaceX Falcon 9 - NROL-76 - May 1, 2017 - DISCUSSION
« Reply #453 on: 05/01/2017 01:26 PM »
  Terminal velocity looked like about 300 m/s.  This is about as expected from something of that size, shape, and mass.

Interesting. That terminal velocity is 2x the 150 m/sec terminal velocity observed on the first F9 landing at the Cape. F9 has bulked up since then...

http://forum.nasaspaceflight.com/index.php?topic=39100.msg1465116#msg1465116

Offline Elmar Moelzer

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Re: SpaceX Falcon 9 - NROL-76 - May 1, 2017 - DISCUSSION
« Reply #454 on: 05/01/2017 01:26 PM »
It looks like we can attribute most of the "toastiness" of the first stage after landing to merely to soot deposited from the boost- back-, re- entry-, and landing- burns. With the re- entry burn being the one that deposits most of the soot. The button of the stage seemed slightly darker before boost back already, but that may have been an optical illusion.

Offline ugordan

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Re: SpaceX Falcon 9 - NROL-76 - May 1, 2017 - DISCUSSION
« Reply #455 on: 05/01/2017 01:31 PM »
Interesting periodic "breathing" from the main engines was noticed by reddit user Brusion. From around T+4.30 to reentry burn.

Engine(s) need to be chilled down prior to ignition to get them to the engine start box so it might also be that.

Online saliva_sweet

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Re: SpaceX Falcon 9 - NROL-76 - May 1, 2017 - DISCUSSION
« Reply #456 on: 05/01/2017 01:37 PM »
Interesting periodic "breathing" from the main engines was noticed by reddit user Brusion. From around T+4.30 to reentry burn.

I think this periodic venting is something the head of mission assurance(Koenigsmann) said was going to be done to fix bubbles in lines that were preventing even starting of engines for landing burn. All he said was that there was a problem of heating on the intake lines to the engines. So to fix it they can insulate better and didn't say what the other was.

Could be, they appear to be dumping quite a lot though.

There is a "heartbeat" like glow (not the nitrogen thrusters) at the base of the rocket every few seconds.  My guess is that they are releasing pressure through the engines so (as opposed to the vents) so they don't upset the attitude.

I'm not sure how that could be done. Dumping fuel from the bottom of the tank to relieve pressure dosn't seem like a good idea. They are perfectly capable of symmetrical venting and asymmetrical venting could be easily used to correct attitude instead of disturbing it, sparing nitrogen.
« Last Edit: 05/01/2017 01:38 PM by saliva_sweet »

Offline Welsh Dragon

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Re: SpaceX Falcon 9 - NROL-76 - May 1, 2017 - DISCUSSION
« Reply #457 on: 05/01/2017 01:40 PM »
  Terminal velocity looked like about 300 m/s.  This is about as expected from something of that size, shape, and mass.

Interesting. That terminal velocity is 2x the 150 m/sec terminal velocity observed on the first F9 landing at the Cape. F9 has bulked up since then...

http://forum.nasaspaceflight.com/index.php?topic=39100.msg1465116#msg1465116
Velocity at start of landing burn was about 320 m/s.

Offline old_sellsword

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Re: SpaceX Falcon 9 - NROL-76 - May 1, 2017 - DISCUSSION
« Reply #458 on: 05/01/2017 01:44 PM »
Interesting periodic "breathing" from the main engines was noticed by reddit user Brusion. From around T+4.30 to reentry burn.

Engine(s) need to be chilled down prior to ignition to get them to the engine start box so it might also be that.

It's most likely this, we can see the LOX bleeding from the octaweb in the Eutelsat/ABS M2 webcast here (17:40): https://www.youtube.com/watch?v=ckjP8stlzxI?t=17m40s
« Last Edit: 05/01/2017 01:45 PM by old_sellsword »

Offline sevenperforce

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Re: SpaceX Falcon 9 - NROL-76 - May 1, 2017 - DISCUSSION
« Reply #459 on: 05/01/2017 02:00 PM »
"...the execution of 100s of people at SpaceX "

I had heard that this approach was used in North Korea to encourage hard work among the survivors, but I didn't expect it to be used in the USA as well.  Still, these days, who knows?

Haha, I lol'ed...  :-)

Amazing footage indeed. Would have loved a closer ground-to-booster view on landing but that's my only quibble.

Congrats to the NRO on being able to snoop even more on all of us
Elon just posted a nice view on Instagram.

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