SpaceX F9 : Starlink 5 (v1.0 L4) : Feb. 17, 2020 : Master Thread

[gongora]

The unified thread idea was under the impression of expected rapid launch cadence and boring routine.

Unfortunately (or fortunately for those who like stuff to stay "interesting") booster landings on barges are not as boring routine yet as we'd all assumed after 50 49 + 2 splashdowns of them.

Since this is likely a rare occurence, we could make a new thread about the booster-splashdown+recovery(maybe)+causes (in the hope that we will ever get to know these), move all the post splashdown posts there and keep this one clean for the next launch.

This might be a good precedent on what to do if stuff on a routine launch doesn't go as planned.


To add to the speculation - a dead easy cause for either barge or rocket not being where it's supposed to be would be a GPS malfunction.

GPS can be prone to outages and drift, and in this case you rely on two autonomous vessels to hold respectively reach the correct position in space independently from each other. If something was wrong with GPS sats, signal paths, receivers or modules on either vehicle could lead to this with no hardware issue whatsoever.

Not saying that is it, but without knowing any other suspect, this is among the most obvious plausibilities.

My understanding is that terminal guidance is done with radar during the landing burn. Since the splashdown was very near the ASDS, if GPS location was the issue for either the booster or the ASDS, the radar should still have been able to pick up the ASDS and the booster should have been able to correct to the ASDS's true position.

Unless, of course, my understanding is not correct.

I've never heard such a thing.  All I've heard is that radar is used for the altitude, and both vehicles target a common location.

[kevinof]

far as I understand that is not how it works. There is a GPS location that it has to hit and the drone ship positions on that spot. After that it's up the radar altimeter to choose the right moment to fire up and shut down.

The unified thread idea was under the impression of expected rapid launch cadence and boring routine.

Unfortunately (or fortunately for those who like stuff to stay "interesting") booster landings on barges are not as boring routine yet as we'd all assumed after 50 49 + 2 splashdowns of them.

Since this is likely a rare occurence, we could make a new thread about the booster-splashdown+recovery(maybe)+causes (in the hope that we will ever get to know these), move all the post splashdown posts there and keep this one clean for the next launch.

This might be a good precedent on what to do if stuff on a routine launch doesn't go as planned.


To add to the speculation - a dead easy cause for either barge or rocket not being where it's supposed to be would be a GPS malfunction.

GPS can be prone to outages and drift, and in this case you rely on two autonomous vessels to hold respectively reach the correct position in space independently from each other. If something was wrong with GPS sats, signal paths, receivers or modules on either vehicle could lead to this with no hardware issue whatsoever.

Not saying that is it, but without knowing any other suspect, this is among the most obvious plausibilities.

My understanding is that terminal guidance is done with radar during the landing burn. Since the splashdown was very near the ASDS, if GPS location was the issue for either the booster or the ASDS, the radar should still have been able to pick up the ASDS and the booster should have been able to correct to the ASDS's true position.

Unless, of course, my understanding is not correct.

[DigitalMan]

far as I understand that is not how it works. There is a GPS location that it has to hit and the drone ship positions on that spot. After that it's up the radar altimeter to choose the right moment to fire up and shut down.

The unified thread idea was under the impression of expected rapid launch cadence and boring routine.

Unfortunately (or fortunately for those who like stuff to stay "interesting") booster landings on barges are not as boring routine yet as we'd all assumed after 50 49 + 2 splashdowns of them.

Since this is likely a rare occurence, we could make a new thread about the booster-splashdown+recovery(maybe)+causes (in the hope that we will ever get to know these), move all the post splashdown posts there and keep this one clean for the next launch.

This might be a good precedent on what to do if stuff on a routine launch doesn't go as planned.


To add to the speculation - a dead easy cause for either barge or rocket not being where it's supposed to be would be a GPS malfunction.

GPS can be prone to outages and drift, and in this case you rely on two autonomous vessels to hold respectively reach the correct position in space independently from each other. If something was wrong with GPS sats, signal paths, receivers or modules on either vehicle could lead to this with no hardware issue whatsoever.

Not saying that is it, but without knowing any other suspect, this is among the most obvious plausibilities.

My understanding is that terminal guidance is done with radar during the landing burn. Since the splashdown was very near the ASDS, if GPS location was the issue for either the booster or the ASDS, the radar should still have been able to pick up the ASDS and the booster should have been able to correct to the ASDS's true position.

Unless, of course, my understanding is not correct.

But it's also apparent from prior observations that the IIP is in the water just past the barge.

[Captain Crutch]

My understanding is that terminal guidance is done with radar during the landing burn. Since the splashdown was very near the ASDS, if GPS location was the issue for either the booster or the ASDS, the radar should still have been able to pick up the ASDS and the booster should have been able to correct to the ASDS's true position.

Unless, of course, my understanding is not correct.

My understanding is that the barge and landing pads have a (insert invisible to the eye light wave here) reflective coating on them so the flight computer can do final positioning of the rocket. Could be wrong though...

[kevinof]

Yes it appears that it aims slightly off target (we're talking maybe 100m I'd say) and then use the gimbal to kick over to the correct GPS location at late as it can but this is just pure GPS and altitude.

[whitelancer64]

In other words, the booster is dumb as far as terminal guidance goes, relying 100% on GPS for its position. Booster does not use any other input from radar other than to gauge altitude.

Correct?

[rpapo]

In other words, the booster is dumb as far as terminal guidance goes, relying 100% on GPS for its position. Booster does not use any other input from radar other than to gauge altitude.

Correct?

That is how it has been described over the years.  For it to work, both the rocket and the barge require GPS position control, and the GPS receivers had better be accurate.  The altimeter is used because GPS is not very precise in the Z direction.

[thirtyone]

When has SpaceX released details of *exactly* what the booster uses to guide the rocket to the droneship (citations please  )? I doubt any serious aerospace company *only* uses a radar altimeter or even just *only* GPS for guiding anything anywhere. There are quite a few other things you can use to get much finer and robust position/relative position measurements.

Also, I don't know why there's so much focus on just recovering parts for value. It's almost certainly because they want to investigate and figure out what actually caused the booster to miss landing. The awkward thing is I recall hearing LOS before landing, which makes me think there's a possibility they might have not even gotten all the telemetry of the failed landing (not sure if there is booster -> droneship telemetry link).

And keep in mind there's the outright possibility, apart from booster energy, that this was just a fluke - perhaps even an error during refurbishment. While SpaceX seems to have been doing relatively well in that respect (QA), it's probably good for them to recover as much as possible and hopefully identify what failed.

[Mandella]

Although this is true, if I am remembering correctly that is not how they are mechanically linked. Apparently both opposite pairs are slaved together but one pair reversed, effectively making it so that one pair is for pitch and the other for roll. To yaw the Falcon has to roll to the correct orientation.

Again, working from memory, but apparently I saw the same explanation as Captain Crutch...

I have never heard of such a thing, and it would make very little sense. Pitch and yaw axes as defined here are basically symmetric, and you would generally want equal control in both. I have seen no evidence of shared axles in the interstage, which while it would eliminate 2 actuators, the remaining actuators would need double the power, and would otherwise add significant complexity, and serve no purpose other than to reduce functionality by removing an axis of control authority.

4 actuators, no linking and it would not make sense.

You're probably correct and it may have just been some authoritative sounding forum post I'm remembering and took as gospel. When I'm home later I'll see if I can search out where I read it.

[mme]

When has SpaceX released details of *exactly* what the booster uses to guide the rocket to the droneship (citations please  )? I doubt any serious aerospace company *only* uses a radar altimeter or even just *only* GPS for guiding anything anywhere. There are quite a few other things you can use to get much finer and robust position/relative position measurements.

...

This really has been discussed ad-nauseam. Every time someone "in the know" describes how the F9 performs the landing it is ALWAYS just GPS and landing radar. When they've decided to call off a landing attempt, they explicitly stated they just moved the barge before launch.

Here's one example: https://www.youtube.com/watch?v=Nz60GcmKOvc#t=16m20s

[thirtyone]

When has SpaceX released details of *exactly* what the booster uses to guide the rocket to the droneship (citations please  )? I doubt any serious aerospace company *only* uses a radar altimeter or even just *only* GPS for guiding anything anywhere. There are quite a few other things you can use to get much finer and robust position/relative position measurements.

...

This really has been discussed ad-nauseam. Every time someone "in the know" describes how the F9 performs the landing it is ALWAYS just GPS and landing radar. When they've decided to call off a landing attempt, they explicitly stated they just moved the barge before launch.

Here's one example: https://www.youtube.com/watch?v=Nz60GcmKOvc#t=16m20s

Yeah, I've heard that before. SpaceX has, on multiple occasions, explicitly said the droneship is largely GPS guided, which I totally accept and makes tons of sense as it's a slow, heavy vessel.

I don't remember anyone saying something about the booster itself. I'm mostly emphasizing that there's almost certainly a lot more sensor fusion for the booster - I'm pretty sure most high speed / high performance guidance systems will use a combination of inertial guidance and GPS, and most likely GPS is only used to as an occasional absolute position input to "calibrate" inertial guidance. I don't know how you could possibly land a booster without at least all the sensors in an IMU in addition to GPS.

[starsilk]

GPS signal lost? GPS jamming is becoming a major issue for shipping in some areas. that would be the tinfoil hat version.. might explain Elon's silence 

alternatively, perhaps the signal loss (that we've seen before during the reentry burn) which is presumably due to flying through its own exhaust also killed the GPS lock and it couldn't re-establish in time. GPS is a notoriously weak signal.

[kevinof]

Any chance we can take this discussion to the other thread? As
Mme said it's been discussed over and over for the last 3 years.  Doesn't belong in the starlink thread.

When has SpaceX released details of *exactly* what the booster uses to guide the rocket to the droneship (citations please  )? I doubt any serious aerospace company *only* uses a radar altimeter or even just *only* GPS for guiding anything anywhere. There are quite a few other things you can use to get much finer and robust position/relative position measurements.

...

This really has been discussed ad-nauseam. Every time someone "in the know" describes how the F9 performs the landing it is ALWAYS just GPS and landing radar. When they've decided to call off a landing attempt, they explicitly stated they just moved the barge before launch.

Here's one example: https://www.youtube.com/watch?v=Nz60GcmKOvc#t=16m20s

Yeah, I've heard that before. SpaceX has, on multiple occasions, explicitly said the droneship is largely GPS guided, which I totally accept and makes tons of sense as it's a slow, heavy vessel.

I don't remember anyone saying something about the booster itself. I'm mostly emphasizing that there's almost certainly a lot more sensor fusion for the booster - I'm pretty sure most high speed / high performance guidance systems will use a combination of inertial guidance and GPS, and most likely GPS is only used to as an occasional absolute position input to "calibrate" inertial guidance. I don't know how you could possibly land a booster without at least all the sensors in an IMU in addition to GPS.

[Alastor]

Just after the entry burn, around T+00:07:28, someone says "Stage 1 LOS expected". What does LOS mean in this context?
<snip>
It could also mean "Loss Of Signal" referring to the downlink though, or lots of other things.

Your final guess is correct.  LOS in this context = Loss Of Signal

Welcome aboard!

Thanks!
So the loss of the downlink signal at that point wasn't unexpected, which could be for a number of different reasons.

Usually, LoS during reentry is due to the reentry plasma preventing communications.
It's true that we don't always have LoS during F9's reentry as evidenced by the fact that they sometimes stream video all the way through (and honestly, I'm not sure how), but we also have video feed cut out on a regular basis at that time, which indicates at the very least a downlink transfer rate drop.
It might all be linked with reentry profile. This one was slower that usual, but might have been steeper (this is pure speculation, I didn't check if it's true, but it might make sense), leading to less slow down in the upper atmosphere and therefore worse conditions for communications and for some hardware, or something like that.

The LoS itself is not particularly worrying, as they had expected it, but since it's not usual, it might point to something indeed.

[eriblo]

Just after the entry burn, around T+00:07:28, someone says "Stage 1 LOS expected". What does LOS mean in this context?
<snip>
It could also mean "Loss Of Signal" referring to the downlink though, or lots of other things.

Your final guess is correct.  LOS in this context = Loss Of Signal

Welcome aboard!

Thanks!
So the loss of the downlink signal at that point wasn't unexpected, which could be for a number of different reasons.

Usually, LoS during reentry is due to the reentry plasma preventing communications.
It's true that we don't always have LoS during F9's reentry as evidenced by the fact that they sometimes stream video all the way through (and honestly, I'm not sure how), but we also have video feed cut out on a regular basis at that time, which indicates at the very least a downlink transfer rate drop.
It might all be linked with reentry profile. This one was slower that usual, but might have been steeper (this is pure speculation, I didn't check if it's true, but it might make sense), leading to less slow down in the upper atmosphere and therefore worse conditions for communications and for some hardware, or something like that.

The LoS itself is not particularly worrying, as they had expected it, but since it's not usual, it might point to something indeed.

I do not think that the conditions during first stage reentry are even close to energetic enough to block the signal. When the stage goes below ~30 km it drops below the horizon as seen from tracking antennas at the cape. Any video beyond that point likely has to be relayed by the recovery fleet (via satellite).

[NosFi]

In other words, the booster is dumb as far as terminal guidance goes, relying 100% on GPS for its position. Booster does not use any other input from radar other than to gauge altitude.

Correct?

That is how it has been described over the years.  For it to work, both the rocket and the barge require GPS position control, and the GPS receivers had better be accurate.  The altimeter is used because GPS is not very precise in the Z direction.

There is a landing radar involved. At least there used to be:
https://twitter.com/elonmusk/status/871228411494014976

Yeah, pretty much dead center. We painted the target area with radio reflective paint, which helps the radar be more precise.

[Jcc]

F9 also has inertial guidance that works together with GPS. If the GPS drifted off by 100 m, or even 10m, then INS would see the error. From where it landed, it appears that navigation was close to on target, but F9 was diverted away (or not diverted to) the drone ship due to some other off nominal conditions, which we are all anxious to hear about.

[JamesH65]

F9 also has inertial guidance that works together with GPS. If the GPS drifted off by 100 m, or even 10m, then INS would see the error. From where it landed, it appears that navigation was close to on target, but F9 was diverted away (or not diverted to) the drone ship due to some other off nominal conditions, which we are all anxious to hear about.

Which implies that navigation and grid fins worked all the way down. Perhaps a landing engine gimbal problem? It landing softly so not fuel. Anyway, speculation seems fairly pointless so stopping now!

[leetdan]

Which implies that navigation and grid fins worked all the way down. Perhaps a landing engine gimbal problem? It landing softly so not fuel. Anyway, speculation seems fairly pointless so stopping now!

An engine / TVC problem would almost certainly prevent a soft, and therefore upright, landing.  The only system that comes in to play that late in the sequence is the legs -- maybe one didn't fully deploy or lock a la Jason 3.

[mark_m]

An engine / TVC problem would almost certainly prevent a soft, and therefore upright, landing.  The only system that comes in to play that late in the sequence is the legs -- maybe one didn't fully deploy or lock a la Jason 3.

That's an interesting theory. Do the landing legs deploy/lock early enough to allow for this? It always seems like the booster is already over the landing area before the legs deploy. Also, that seems to me like the kind of failure that SpaceX/Elon would have discussed publicly by now. But it does explain the nearby soft water landing.