Author Topic: SpaceX Falcon 9 - NROL-76 - May 1, 2017 - DISCUSSION  (Read 218462 times)

Offline just-nick

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Re: SpaceX Falcon 9 - NROL-76 - May 1, 2017 - DISCUSSION
« Reply #480 on: 05/01/2017 03:52 PM »
Both yesterday and again today, the presenter of the webcast stated that the Falcon 9 is 12m in diameter. I am right that he is confusing the F9 - which I thought is 3.6m in diameter - with the Falcon Heavy, correct?

The presenter said 12ft (at about 15:50 into the webcast replay) which is close enough with rounding.

Since he said it two days in a row, it was an error in his script (meters instead of feet). He got it right later, or should I say the script was correct.
To be fair, the commentary was using a mix of units...imperial for the vehicle dimensions and metric for the velocity and irritating analogy units for vehicle performance (thrust of a 747? What model? What engines? What thrust rating?!?!). Has to get confusing when trying to keep up that cheerful patter.

Cheers!

Offline Kabloona

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Re: SpaceX Falcon 9 - NROL-76 - May 1, 2017 - DISCUSSION
« Reply #481 on: 05/01/2017 03:58 PM »
Another thing we can see from the excellent video footage is the ignition sequence at the start of the entry burn. The center engine starts first, making the "ring of fire," then about 3 seconds later the other two engines light, changing the shape of the plume:

https://www.instagram.com/p/BTjVdLVB1bO/
« Last Edit: 05/01/2017 04:00 PM by Kabloona »

Online LouScheffer

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Re: SpaceX Falcon 9 - NROL-76 - May 1, 2017 - DISCUSSION
« Reply #482 on: 05/01/2017 04:03 PM »
I was wondering why we can see the nitrogen jets.  After all, nitrogen is pretty transparent (we look through kilometers of it every day).  And it can't be that it's cold enough to condense water out of the air, since there is almost no atmosphere until it returns to much lower altitudes.

The only thing I can think of is that the free expansion cools the jet so much that droplets of liquid nitrogen condense, and scattering from these drops is what we see.  This seems plausible based on The Effects of Condensation on Gas Velocity in a Free Jet Expansion, where table 1 indicates that up to 30% of the mass of the jet might condense.

Offline Nomadd

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Re: SpaceX Falcon 9 - NROL-76 - May 1, 2017 - DISCUSSION
« Reply #483 on: 05/01/2017 04:04 PM »
Both yesterday and again today, the presenter of the webcast stated that the Falcon 9 is 12m in diameter. I am right that he is confusing the F9 - which I thought is 3.6m in diameter - with the Falcon Heavy, correct?

The presenter said 12ft (at about 15:50 into the webcast replay) which is close enough with rounding.

Since he said it two days in a row, it was an error in his script (meters instead of feet). He got it right later, or should I say the script was correct.
To be fair, the commentary was using a mix of units...imperial for the vehicle dimensions and metric for the velocity and irritating analogy units for vehicle performance (thrust of a 747? What model? What engines? What thrust rating?!?!). Has to get confusing when trying to keep up that cheerful patter.

Cheers!
I also heard him say 12 meters. I thought they'd built the ITS in secret and was using it to launch the satellite.

Offline hans_ober

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Re: SpaceX Falcon 9 - NROL-76 - May 1, 2017 - DISCUSSION
« Reply #484 on: 05/01/2017 04:05 PM »
Another thing we can see from the excellent video footage is the ignition sequence at the start of the entry burn. The center engine starts first, making the "ring of fire," then about 3 seconds later the other two engines light, changing the shape of the plume:

https://www.instagram.com/p/BTjVdLVB1bO/

I think they do this for the 3 engine landing burn too.

Might be doing it since it's very likely that both the side engines might not ramp up at the same time, so having the center engine active is good because it can be used to stabilize the stage. Another reason could be that they ignite the center first to minimize the 'jerk' on the stage.

They do the same at MECO. Center engine lights first (probably to prevent propellant from sloshing around), gimbals to control the rate of rotation and the outer 2 light towards the end.

Offline hans_ober

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Re: SpaceX Falcon 9 - NROL-76 - May 1, 2017 - DISCUSSION
« Reply #485 on: 05/01/2017 04:07 PM »
With this launch, LC 39 Pad A has handled more launches this year than any other launch center, let alone launch pad.   Likely temporary, but shows how hard SpaceX is pushing to recover from AMOS 6.

This was the tenth successful first stage landing and the fourth at LZ-1.  Nine first stages have now flown to recovery (one twice).  Something like four of those have apparently been retired.

Among v1.2 variants, this had the shortest first stage burn (137 seconds) and the second longest boost-back burn (40 seconds).

F9-34 performed the 98th LC 39A liftoff.

 - Ed Kyle

Did the longest bootback belong to one of the CRS flights?

My instinct tells me the higher the apogee, the shorter the boostback needs to be.

With a high enough apogee, they don't need to reverse their velocity, just cancel enough of it so by the time they coast up and back down, the Earth has rotated. OG2 is a good example.

Online abaddon

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Re: SpaceX Falcon 9 - NROL-76 - May 1, 2017 - DISCUSSION
« Reply #486 on: 05/01/2017 04:11 PM »
It's been noted elsewhere that this mission like USA-193 features a brown raptor bird in its patch.

https://m.facebook.com/photo.php?fbid=116648047531&l=169616826b
Let's hope this one isn't used for target practice...

Offline Star One

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Re: SpaceX Falcon 9 - NROL-76 - May 1, 2017 - DISCUSSION
« Reply #487 on: 05/01/2017 04:12 PM »

Offline Jim

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Re: SpaceX Falcon 9 - NROL-76 - May 1, 2017 - DISCUSSION
« Reply #488 on: 05/01/2017 04:15 PM »
I was wondering why we can see the nitrogen jets.  After all, nitrogen is pretty transparent (we look through kilometers of it every day).  And it can't be that it's cold enough to condense water out of the air, since there is almost no atmosphere until it returns to much lower altitudes.

The only thing I can think of is that the free expansion cools the jet so much that droplets of liquid nitrogen condense, and scattering from these drops is what we see. 

No it doesn't turn to liquid , we see like you can see any gas venting into a vacuum.  Just like the other engines.

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Re: SpaceX Falcon 9 - NROL-76 - May 1, 2017 - DISCUSSION
« Reply #489 on: 05/01/2017 04:20 PM »
I may have missed it but I don't recall the grid fins heating up as much as the last time. That could be due to other factors than upgraded fins of course... (a different reentry burn for example)
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Offline Norm38

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Re: SpaceX Falcon 9 - NROL-76 - May 1, 2017 - DISCUSSION
« Reply #490 on: 05/01/2017 04:21 PM »
Thinking about the rapid flip at stage sep, on past flights it appears that the interstage takes quite a blast from the second stage engine ignition.  Is the rapid flip an attempt to shield the interstage interior from the plume and reduce damage/wear?  The plume may also assist with the flip by pushing on one side, but I doubt that's the primary reason.

Offline Kaputnik

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Re: SpaceX Falcon 9 - NROL-76 - May 1, 2017 - DISCUSSION
« Reply #491 on: 05/01/2017 04:27 PM »
With this launch, LC 39 Pad A has handled more launches this year than any other launch center, let alone launch pad.   Likely temporary, but shows how hard SpaceX is pushing to recover from AMOS 6.

This was the tenth successful first stage landing and the fourth at LZ-1.  Nine first stages have now flown to recovery (one twice).  Something like four of those have apparently been retired.

Among v1.2 variants, this had the shortest first stage burn (137 seconds) and the second longest boost-back burn (40 seconds).

F9-34 performed the 98th LC 39A liftoff.

 - Ed Kyle

Did the longest bootback belong to one of the CRS flights?

My instinct tells me the higher the apogee, the shorter the boostback needs to be.

With a high enough apogee, they don't need to reverse their velocity, just cancel enough of it so by the time they coast up and back down, the Earth has rotated. OG2 is a good example.

Cancel relative to what, though? If you cancel it relative to the launch site, then the Earth will never rotate under the rocket, no matter how long it stays up for.
Waiting for joy and raptor

Offline Hauerg

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Re: SpaceX Falcon 9 - NROL-76 - May 1, 2017 - DISCUSSION
« Reply #492 on: 05/01/2017 04:28 PM »
They only glow on fast returns. This was an easy one.

Offline sevenperforce

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Re: SpaceX Falcon 9 - NROL-76 - May 1, 2017 - DISCUSSION
« Reply #493 on: 05/01/2017 04:28 PM »
I may have missed it but I don't recall the grid fins heating up as much as the last time. That could be due to other factors than upgraded fins of course... (a different reentry burn for example)
Way, way less aggressive entry. SES-10 had much higher horizontal velocity.

Thinking about the rapid flip at stage sep, on past flights it appears that the interstage takes quite a blast from the second stage engine ignition.  Is the rapid flip an attempt to shield the interstage interior from the plume and reduce damage/wear?  The plume may also assist with the flip by pushing on one side, but I doubt that's the primary reason.

In a near vacuum, the exhaust is going to spread out fast enough that first stage impingement shouldn't be problematic.

Online Welsh Dragon

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Re: SpaceX Falcon 9 - NROL-76 - May 1, 2017 - DISCUSSION
« Reply #494 on: 05/01/2017 04:32 PM »
As promised, to follow on my analysis of the whole flight, here are the detailed looks at the burns, in 1 second bins. Of course, this makes the data a lot rattier, but it's still interesting.

Boostback burn
Good data. We start the graph with MECO, we see the trust tailing off. Then we have 15 second or so of the stage being ballistic (stage is turning). Then we get the start of the boostback, reaching a peak of -26 m/s/s. There seems to be a bit of plateau in the engine start-up sequence, in the acceleration at seconds 162-166, however, this is quite likely just noise in the data. Following shut-down we can nicely see the stage going ballistic again. Acceleration is still negative, as it is still climbing to apogee. Altitude change during the entire burn is essentially constant, indeed suggesting the burn is done horizontally.

Reentry burn
Very nice data here. We start with the stage in a ballistic descent, still essentially in vacuum (82 km). We see a swift Merlin start-up. We can see a nice increase in acceleration during the burn, as fuel load decreases. (no throttling during the burn, I would think), reaching a maximum of -40 m/s/s around 455 seconds. Following completion of the burn the stage is ballistic again, although now considerably lower in the atmosphere (28 km)

Landing burn
This is a lot more problematic, data is nowhere near as solid. We start as the stage is hitting the dense lower atmosphere hard, and is decelerating fast. We can see the stage never quite reaches terminal velocity, the speed reached before engine start is 315 m/s at 511 seconds. The landing burn is very gentle compared to the three engine suicide burn GTO profile we've gotten used to. A full 30 seconds of burn. The acceleration data is very ratty, as the speed is low here. Highest derived value is -14 m/s/s at 521 seconds. This seems low to me, and I wouldn't trust this data too much. I suspect either I'm not sampling fast enough, or the update rate of the speed in the feed is too slow. The altitude data is way off here, it reaches 0 when speed is 80 m/s. Use these data at your own peril!

Again, data is available to anyone.
« Last Edit: 05/01/2017 04:38 PM by Welsh Dragon »

Offline Kabloona

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Re: SpaceX Falcon 9 - NROL-76 - May 1, 2017 - DISCUSSION
« Reply #495 on: 05/01/2017 04:34 PM »
Thinking about the rapid flip at stage sep, on past flights it appears that the interstage takes quite a blast from the second stage engine ignition.  Is the rapid flip an attempt to shield the interstage interior from the plume and reduce damage/wear?  The plume may also assist with the flip by pushing on one side, but I doubt that's the primary reason.

Probably propellant conservation is the main reason. The sooner the flip, the sooner boostback burn starts, and the less time the stage spends coasting downrange away from the landing pad, so less propellant is needed for boostback.
« Last Edit: 05/01/2017 04:44 PM by Kabloona »

Offline AncientU

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Re: SpaceX Falcon 9 - NROL-76 - May 1, 2017 - DISCUSSION
« Reply #496 on: 05/01/2017 04:46 PM »
Thinking about the rapid flip at stage sep, on past flights it appears that the interstage takes quite a blast from the second stage engine ignition.  Is the rapid flip an attempt to shield the interstage interior from the plume and reduce damage/wear?  The plume may also assist with the flip by pushing on one side, but I doubt that's the primary reason.

Probably propellant conservation is the main reason. The sooner the flip, the sooner boostback burn starts, and the less time the stage spends coasting downrange away from the landing pad, so less propellant is needed for boostback.

Could the thermal shock on the engine be lessened if it is rapidly restarted after separation?  Less thermal transient should mean less severe cracking in the turbine blades it would seem.
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Online edkyle99

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Re: SpaceX Falcon 9 - NROL-76 - May 1, 2017 - DISCUSSION
« Reply #497 on: 05/01/2017 04:47 PM »
With this launch, LC 39 Pad A has handled more launches this year than any other launch center, let alone launch pad.   Likely temporary, but shows how hard SpaceX is pushing to recover from AMOS 6.

This was the tenth successful first stage landing and the fourth at LZ-1.  Nine first stages have now flown to recovery (one twice).  Something like four of those have apparently been retired.

Among v1.2 variants, this had the shortest first stage burn (137 seconds) and the second longest boost-back burn (40 seconds).

F9-34 performed the 98th LC 39A liftoff.

 - Ed Kyle

Did the longest bootback belong to one of the CRS flights?
I have the longest boostback, at 45 seconds or longer, taking place during the CRS-9 flight, during which the first stage returned to LZ 1.  For some reason, a shorter 33 second burn was used for CRS-10, which also returned to LZ 1.  Both of these flights had a 141 second first stage MECO.

It does give me the sense that SpaceX is experimenting with fly-back trajectory shaping to minimize damage to the stage.  Note that none of the LZ 1 stages has re-flown to date.

 - Ed Kyle
« Last Edit: 05/01/2017 04:48 PM by edkyle99 »

Offline Davp99

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Re: SpaceX Falcon 9 - NROL-76 - May 1, 2017 - DISCUSSION
« Reply #498 on: 05/01/2017 04:48 PM »
Great to watch the Telemetry on S1, Speed & Altitude .....So Cool 8)
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Re: SpaceX Falcon 9 - NROL-76 - May 1, 2017 - DISCUSSION
« Reply #499 on: 05/01/2017 04:53 PM »
Note that none of the LZ 1 stages has re-flown to date.

That's hardly a note of significance since there have been a grand total of one reflight so far and it was statistically more likely to have been an ASDS stage, anyway.

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