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#120 by CorvusCorax on 03 Sep, 2020 17:57
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I was looking at the fantastic data plots provided by OneSpeed and had a question for the rocket scientists in the forum. This may have been discussed at another forum. If so, I apologize in advance. My question is: As I recall, during the manned Dragon II launch Bob and Doug mentioned that the second stage ride seemed much rougher than first stage. The acceleration data in OneSpeed's chart seem to show a gradual increase in "noise" starting about halfway through the second stage burn. The noise, or vibration, gets progressively worse toward the end of the burn and the data from the latest launch (L11) even shows signs of periodic acceleration spikes. Does this relate to the "rough ride" noted by the astronauts?
That's quite possible. Small fluctuations in thrust will product small fluctuations in acceleration. As the fuel is consumed, the mass of the vehicle decreases and the amplitude of the acceleration fluctuations increases.
That being said, I would not read too much into the noise in these plots. We do not have the raw telemetry, only the video stream, which displays altitude above ground and velocity in the rotating earth reference frame in km/h at the frame rate of the video, which is being processed by on-screen OCR to read out these values.
At this point this data is already preprocessed with some filtering by SpaceX, then rounded in the velocity domain to km/h and in the time domain to 1/30s Sometimes there's gaps in the data or small jitter due to transmission delays and buffering effects on the SpaceX media side.
However, the acceleration is the derivative of velocity so the change in speed per time. Due to the rounding to full km/h this is inherently noisy, and the jitter adds additional noise, which also increases the faster the rocket accelerates.
So - although you would expect the actual noise in the acceleration to also increase as the rocket gets lighter and as such small thrust fluctuations have larger impact, I would expect this noise to be at higher frequencies than the 1/30 s resolution we can see in the stream. The noise in these plots is most likely dominated by the noise introduced by the way this is read out.
A hint at the actual noise is given by the Falcon9 payload user guide (current version: https://www.spacex.com/media/falcon_users_guide_042020.pdf ) which lists the maximum axial acceleration noise under 30 Hz at 0.8 g and the maximum axial acceleration noise at frequencies above 100Hz at 0.9g - with a noise maximum at around 160Hz
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#121 by Dean47 on 03 Sep, 2020 19:29
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Thanks for the explanation!!
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#122 by Jakusb on 03 Sep, 2020 19:32
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No news of how the fairings faired?
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#123 by kendalla59 on 03 Sep, 2020 21:13
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No news of how the fairings faired?
I think it's "fair" to say no news is bad news. -
#124 by abaddon on 03 Sep, 2020 21:49
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We've had no news before and then found out they scooped them out of the ocean intact. Not sure I'd call that "bad" news.No news of how the fairings faired?
I think it's "fair" to say no news is bad news. -
#125 by equiserre on 03 Sep, 2020 22:07
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this time it looked to me that the two bottom starlinks got tangled with each other and may have even bumped with each other. Does anybody see the same thing _ the left sat got almost at right angles with the stack rotating around the lower corner
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#126 by whitelancer64 on 03 Sep, 2020 22:33
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this time it looked to me that the two bottom starlinks got tangled with each other and may have even bumped with each other. Does anybody see the same thing _ the left sat got almost at right angles with the stack rotating around the lower corner
Not a problem. They will correct position after the individual satellites have more separation. -
#127 by SMS on 03 Sep, 2020 22:41
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For compare there are the lasts 4 starlinks deployment:
https://twitter.com/SpaceX/status/1268357720974823425
https://twitter.com/SpaceX/status/1295742448254115840
https://twitter.com/SpaceX/status/1291626962411126784
https://twitter.com/SpaceX/status/1301505913044439042 -
#128 by FutureSpaceTourist on 03 Sep, 2020 23:01
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https://twitter.com/spacexfleet/status/1301648543610961925Quote
Tracking shows that Falcon 9 and Of Course I Still Love You droneship are now underway from the LZ and heading towards Port Canaveral!
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#129 by FutureSpaceTourist on 03 Sep, 2020 23:35
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https://twitter.com/julia_bergeron/status/1301663454202388481Quote
Oh, hey! The Sisters are on their way back with scooped fairings. It appears they left around 3:30 ET and are happily speeding along.
#SpaceXFleet #Twins -
#130 by OneSpeed on 04 Sep, 2020 00:56
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The noise in these plots is most likely dominated by the noise introduced by the way this is read out.
Actually, from the raw SpaceX data, the acceleration noise looks more like this
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#131 by Prettz on 04 Sep, 2020 01:06
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Not caught = bad news
We've had no news before and then found out they scooped them out of the ocean intact. Not sure I'd call that "bad" news.No news of how the fairings faired?
I think it's "fair" to say no news is bad news. -
#132 by wannamoonbase on 04 Sep, 2020 02:09
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Not caught = bad news
We've had no news before and then found out they scooped them out of the ocean intact. Not sure I'd call that "bad" news.No news of how the fairings faired?
I think it's "fair" to say no news is bad news.
Yeah no panic yet. Last mission I don’t think we knew until they pulled into port.
That last mission was Sunday, I love this flight cadence. -
#133 by CorvusCorax on 04 Sep, 2020 09:04
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The noise in these plots is most likely dominated by the noise introduced by the way this is read out.
Actually, from the raw SpaceX data, the acceleration noise looks more like this
How exactly are you filtering that? Box filter? Gaussian? Infinite impulse response lowpass? FFT and then masking in frequency domain? -
#134 by Vettedrmr on 04 Sep, 2020 10:32
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Not caught = bad news
I disagree. Broken fairings = bad news. Fishing them out of water and reusing them isn't as cool as two autonomous vehicles coordinating a net landing, but isn't to the point of "bad news".
Have a good one,
Mike -
#135 by OneSpeed on 04 Sep, 2020 10:38
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How exactly are you filtering that? Box filter? Gaussian? Infinite impulse response lowpass? FFT and then masking in frequency domain?
Simple down sampling. Tried a few methods, most introduced artifacts and distortions not consistent with physics. -
#136 by FutureSpaceTourist on 04 Sep, 2020 15:50
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https://twitter.com/spacexfleet/status/1301909987560693762Quote
Ms. Tree and Ms. Chief are about two hours away from arriving at Port Canaveral
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#137 by FutureSpaceTourist on 04 Sep, 2020 16:34
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https://twitter.com/johnkrausphotos/status/1301921267508011009Quote
Love these scope views during flight!... Switched to manual control at the very last moment, shaky (🔭🤚) , but I like the blade runner feels. Watch till the end for some serious engine flames at liftoff🤩 #SpaceX #Starlink
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#138 by FutureSpaceTourist on 04 Sep, 2020 17:40
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https://twitter.com/kyle_m_photo/status/1301937827777785858Quote
One of the fairing catchers can just barely be seen on the horizon now. It looks like the fairing is still being held in the scoop net, this usually means it is damaged. #SpaceXFleet #SpaceX
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#139 by FutureSpaceTourist on 04 Sep, 2020 18:14
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https://twitter.com/kyle_m_photo/status/1301946581357395969Quote
Ms. Tree's fairing half is definitely broken. #SpaceXFleet #SpaceX
Edit to add:
https://twitter.com/trevormahlmann/status/1301947450094223363QuoteGO Ms. Tree's fairing half from the 12th Starlink mission is in a several pieces. #SpaceXFleet
https://twitter.com/trevormahlmann/status/1301950478394232833QuoteA few more close ups of GO Ms. Tree's fairing half from the 12th Starlink mission. Looks like Ms. Chief recovered much flatter pieces or they are below the deck line. Not much to see on its deck from here. #SpaceXFleet
