Author Topic: Landing rockets and the wind  (Read 50404 times)

Offline meekGee

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Re: Landing rockets and the wind
« Reply #40 on: 01/07/2016 01:32 am »

I think it's fairly certain that the latest success is largely due to the landing pad not moving around.


?

A corollary would be that the failures occurred because the barge was moving...

And since the corollary is false, then I think the original statement is not very certain...

The previous failures occurred for many reasons (perhaps even some we aren't privy to), only one of which being that, during the last attempt, the platform was moving (rocking)..  but I guess we have only to wait for the Jason-3 launch to know if SpX can overcome the effect of this in practice.

I know from many years of personal experience working with and around floating platforms of all kinds that it's a tough ask, but looking at what they've been able to achieve thus far, if it can be done at all, SpX will do it. :)
I think that's a stretch...  Like an aircraft coming in to land without hydraulic fluid and crashing because of a wind gust.

(Some may recognize the aviation scenario)

So yeah, the wind gust was a contributing factor, but to claim that it is "certain that it was the lack of wind gusts that made a subsequent landing possible"...  Meh.  It was the presence of hydraulic fluid....

Sure.  That they nailed this landing was an impressive feat and I've said as much elsewhere.  At each landing attempt we've seen them fix (change) more than one parameter - but ruling out an obvious one (eg. the lack of hydraulic fluid) sure helps for the next time around.

Equally, I think it's a stretch to expect the ability to successfully land at LZ-1 means they could successfully land on an ASDS - just like a successful landing at LAX does not mean you could land on an aircraft carrier..
Yes, but you went a lot further than saying that it is uncertain they'll land on the barge...

You said it was certain that this landing worked out because the land was not moving, and since I have nothing more productive to do tonight, I thought I'd show you the error of your ways....

And the sad part is that I actually do have other stuff I need to do tonight...
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Offline CameronD

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Re: Landing rockets and the wind
« Reply #41 on: 01/07/2016 01:48 am »
Yes, but you went a lot further than saying that it is uncertain they'll land on the barge...

You said it was certain that this landing worked out because the land was not moving, and since I have nothing more productive to do tonight, I thought I'd show you the error of your ways....

I guess my being "fairly certain the latest success is largely due.." wasn't uncertain enough.  For that you have my sincerest apologies and I shall try to be more vague next time.  8)

For the record, I meant to imply that, IMHO, the land not moving played a large part in the success of this latest landing.. the largest part being the work of the guys/gals at SpaceX.

And the sad part is that I actually do have other stuff I need to do tonight...

As do I this day.  Enjoy!
 
« Last Edit: 01/07/2016 02:39 am by CameronD »
With sufficient thrust, pigs fly just fine - however, this is not necessarily a good idea. It is hard to be sure where they are
going to land, and it could be dangerous sitting under them as they fly overhead.

Offline Lars-J

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Re: Landing rockets and the wind
« Reply #42 on: 01/07/2016 01:52 am »
Yes, but you went a lot further than saying that it is uncertain they'll land on the barge...

You said it was certain that this landing worked out because the land was not moving, and since I have nothing more productive to do tonight, I thought I'd show you the error of your ways....

I guess my being "fairly certain the latest success is largely due.." wasn't uncertain enough for you.  For that you have my sincerest apologies: I shall try to be more vague next time.  8)

Are you completely oblivious to the "largely" in your sentence, and what it means? Largely, meaning: to a great extent; on the whole; mostly. Do you not understand the reaction that gets, when you come up with a brand new explanation that according to you, you are fairly certain is the primary factor in the failure?

Offline CameronD

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Re: Landing rockets and the wind
« Reply #43 on: 01/07/2016 02:29 am »
Yes, but you went a lot further than saying that it is uncertain they'll land on the barge...

You said it was certain that this landing worked out because the land was not moving, and since I have nothing more productive to do tonight, I thought I'd show you the error of your ways....

I guess my being "fairly certain the latest success is largely due.." wasn't uncertain enough for you.  For that you have my sincerest apologies: I shall try to be more vague next time.  8)

Are you completely oblivious to the "largely" in your sentence, and what it means? Largely, meaning: to a great extent; on the whole; mostly. Do you not understand the reaction that gets, when you come up with a brand new explanation that according to you, you are fairly certain is the primary factor in the failure?

No, not oblivious.  It is my personal opinion (the "I think.." in my sentence) based upon videos posted here and personal experience of the difficulties working with floating platforms.

A brand new explanation?  No, not on the ASDS Thread it isn't.  Whether or not movement of the landing pad was the primary factor in the failure of the previous landing attempt I would not know - but in the absence of other information (can you point me to any?) I do believe it to be a factor at least worthy of consideration.  Whether or not it is indeed a problem for SpX will, I suppose, be known to us all following the Jason-3 launch.

(As a small aside, it isn't only floating platforms that move around: Problems at one customers' site were only resolved once we discovered the ground the place was built on was moving up and down with the tide!)
« Last Edit: 01/07/2016 02:33 am by CameronD »
With sufficient thrust, pigs fly just fine - however, this is not necessarily a good idea. It is hard to be sure where they are
going to land, and it could be dangerous sitting under them as they fly overhead.

Offline meekGee

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Re: Landing rockets and the wind
« Reply #44 on: 01/07/2016 03:14 am »
Let's let this one go...  We're down to semantics, and can keep it up till the 17th if we don't stop.



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Offline CameronD

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Re: Landing rockets and the wind
« Reply #45 on: 01/07/2016 03:43 am »
Let's let this one go...  We're down to semantics, and can keep it up till the 17th if we don't stop.

Fair enough..  I do have one question though that maybe someone here knows the answer to:

What maximum angle-of-tilt of the landing platform is the F9 landing system/landing legs designed to tolerate??
With sufficient thrust, pigs fly just fine - however, this is not necessarily a good idea. It is hard to be sure where they are
going to land, and it could be dangerous sitting under them as they fly overhead.

Offline meekGee

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Re: Landing rockets and the wind
« Reply #46 on: 01/07/2016 04:22 am »
Let's let this one go...  We're down to semantics, and can keep it up till the 17th if we don't stop.

Fair enough..  I do have one question though that maybe someone here knows the answer to:

What maximum angle-of-tilt of the landing platform is the F9 landing system/landing legs designed to tolerate??
Who knows, but my guess is that the engine throttle-down timing is designed to transfer weight from the engine bells to the legs gradually, sort of a hand-over.

This means that when one leg hits first, it's not the end of the world.

(This is assuming the rocket is not actively matching tilt...  They can have an altimeter on each leg, and the period of barge rocking is longer than the touchdown time)
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Offline abaddon

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Re: Landing rockets and the wind
« Reply #47 on: 01/07/2016 03:37 pm »
Whether or not movement of the landing pad was the primary factor in the failure of the previous landing attempt I would not know - but in the absence of other information (can you point me to any?) I do believe it to be a factor at least worthy of consideration.
Er, "other information", like SpaceX stating that the most recent barge landing failed due to valve stiction, and that this has been fixed for subsequent attempts?

Not trying to say a barge landing is a slam dunk but this is about as obvious a factor as it gets.

Offline JamesH

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Re: Landing rockets and the wind
« Reply #48 on: 01/07/2016 04:14 pm »
Whether or not movement of the landing pad was the primary factor in the failure of the previous landing attempt I would not know - but in the absence of other information (can you point me to any?) I do believe it to be a factor at least worthy of consideration.
Er, "other information", like SpaceX stating that the most recent barge landing failed due to valve stiction, and that this has been fixed for subsequent attempts?

Not trying to say a barge landing is a slam dunk but this is about as obvious a factor as it gets.

Indeed. The video shows the rocket coming in at the wrong angle and/or off centre. That's has nothing to do with the rocking of the barge, which is a sensible sea state is negligible.


Offline bstrong

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Re: Landing rockets and the wind
« Reply #49 on: 01/07/2016 04:41 pm »
The video shows the rocket coming in at the wrong angle and/or off centre. That's has nothing to do with the rocking of the barge, which is a sensible sea state is negligible.

Shotwell does not share this opinion:

Quote
"Just purely the boat moving, even in a low sea state, it's hard to imagine that vehicle is going to stay vertical," Shotwell said. "That vehicle is big and tall, compared to the itty-bity-greater-than-a-football-field-size ship."

http://www.defensenews.com/story/defense/air-space/space/2015/04/15/spacex-ground-attempt-reusable-landing-sea/25827625/

Edit to clarify: I'm disagreeing that the rocking is negligible. Not saying it was the cause of previous failures.
« Last Edit: 01/07/2016 04:43 pm by bstrong »

Offline mvpel

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Re: Landing rockets and the wind
« Reply #50 on: 01/07/2016 06:22 pm »
With respect to wind influence, I have a feeling that folks may be unaware of just how fast embedded processors are these days. Remember that cute little Falcon 9 landing game someone threw together? Imagine playing that at, let's say, one frame per second, and you can get an idea of how the software sees its universe.

"Oh, look, the sensors are indicating that some outside influence - maybe a gust of wind - is pushing me off-nominal by four centimeters so far. I guess we'll need to plan for a thruster firing and a bit of engine gimbal if this keeps up."

Some people expressed surprise that it landed dead center on the pad - I was like "well, it's a robot, where else would it land?"
"Ugly programs are like ugly suspension bridges: they're much more liable to collapse than pretty ones, because the way humans (especially engineer-humans) perceive beauty is intimately related to our ability to process and understand complexity. A language that makes it hard to write elegant code makes it hard to write good code." - Eric S. Raymond

Offline CameronD

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Re: Landing rockets and the wind
« Reply #51 on: 01/07/2016 10:19 pm »
With respect to wind influence, I have a feeling that folks may be unaware of just how fast embedded processors are these days. Remember that cute little Falcon 9 landing game someone threw together? Imagine playing that at, let's say, one frame per second, and you can get an idea of how the software sees its universe.

"Oh, look, the sensors are indicating that some outside influence - maybe a gust of wind - is pushing me off-nominal by four centimeters so far. I guess we'll need to plan for a thruster firing and a bit of engine gimbal if this keeps up."

Maybe processors are fast, but the real world is still analog - and that fast processor and software is totally reliant on sensors to know what is (and isn't) going on out there.  Garbage In, Garbage Out. ;)

To explain:  In my mind, it is a matter of prediction - but you can't predict something happening and make allowance for it if you can't sense it sufficiently within bounds of accuracy and time.  Of course the usual way to work out what you do and don't need to sense is by simulation and testing and often what you thought might be a problem (eg. wind gusts) can be overcome by other means than sensing (eg. "A gust? We're going fast enough to not worry about it") within the same bounds thus sacrificing some amount of accuracy for reduced complexity.

What is less straight-forward to me is how they plan to predict the angle of the surface they're landing on.  MeekGee suggested maybe an altimeter/range-finder on each leg?  That sounds feasible to me - although that would presumably only come into play in the final meters before touchdown.  Their commit criteria do specify the max sea-state they'll allow a landing, but that wouldn't cater for a rogue wave at the wrong time (certainly possible on the open ocean) tilting the platform outside acceptable limits at the last second.. and what do you do then?  They can't just hover (or can they?), so presumably they have safety margins in place to cover that scenario.

They could also measure instantaneous 3-axis platform tilt and use that to issue an earlier abort-to-water-landing if they find they're getting close to the edge of their safety margins in the minutes before landing... but to know whether or not that's feasible (or even necessary) you'd need to know the limits of the F9 landing leg design. Perhaps they are doing both??

Anyways, I'd be interested to know what others here think - and will be very interested in the landing video for Jason-3. 


EDIT:  I suspect I'm overthinking this.  It's little different to land the Apollo LEM on the Moon - just fly until one leg is in contact and throttle down.  Everything else should then sort itself out...
 
« Last Edit: 01/07/2016 11:01 pm by CameronD »
With sufficient thrust, pigs fly just fine - however, this is not necessarily a good idea. It is hard to be sure where they are
going to land, and it could be dangerous sitting under them as they fly overhead.

Offline CJ

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Re: Landing rockets and the wind
« Reply #52 on: 01/07/2016 11:02 pm »
With respect to wind influence, I have a feeling that folks may be unaware of just how fast embedded processors are these days. Remember that cute little Falcon 9 landing game someone threw together? Imagine playing that at, let's say, one frame per second, and you can get an idea of how the software sees its universe.

"Oh, look, the sensors are indicating that some outside influence - maybe a gust of wind - is pushing me off-nominal by four centimeters so far. I guess we'll need to plan for a thruster firing and a bit of engine gimbal if this keeps up."

Maybe processors are fast, but the real world is still analog - and that fast processor and software is totally reliant on sensors to know what is (and isn't) going on out there.  Garbage In, Garbage Out. ;)

To explain:  In my mind, it is a matter of prediction - but you can't predict something happening and make allowance for it if you can't sense it sufficiently within bounds of accuracy and time.  Of course the usual way to work out what you do and don't need to sense is by simulation and testing and often what you thought might be a problem (eg. wind gusts) can be overcome by other means than sensing (eg. "A gust? We're going fast enough to not worry about it") within the same bounds thus sacrificing some amount of accuracy for reduced complexity.

What is less straight-forward to me is how they plan to predict the angle of the surface they're landing on.  MeekGee suggested maybe an altimeter/range-finder on each leg?  That sounds feasible to me - although that would presumably only come into play in the final meters before touchdown.  Their commit criteria do specify the max sea-state they'll allow a landing, but that wouldn't cater for a rogue wave at the wrong time (certainly possible on the open ocean) tilting the platform outside acceptable limits at the last second.. and what do you do then?  They can't just hover (or can they?), so presumably they have safety margins in place to cover that scenario.

They could also measure instantaneous 3-axis platform tilt and use that to issue an earlier abort-to-water-landing if they find they're getting close to the edge of their safety margins in the minutes before landing... but to know whether or not that's feasible (or even necessary) you'd need to know the limits of the F9 landing leg design. Perhaps they are doing both??

Anyways, I'd be interested to know what others here think - and will be very interested in the landing video for Jason-3. 
 

They definitely can't hover, so that's out as an option.

Ships at sea almost always roll by varying amounts (in both magnitude and period) even in a steady sea state, because the base swells themselves vary a bit, as does their period. So, I think you're right, a greater than expected roll can occur at any time. It would also be a greater factor if the downhill side at landing was also the downwind side of the F9. They can't time the landing, but IMHO it's plausible that they might partially compensate for wind by changing the paramiters of the GN2 thrusters on the interstage, to have one firing during touchdown to get ahead of any tilt. My guess is that, at most, this is a future possibility; right now  they are focused on getting a baseline landing right. If an anemometer and wind direction gauge is added to the ASDS at some point, this would be my guess as to the reason (but it would require an uplink). 

My guess; they aren't going to build in an abort mode for this issue; too great a chance of losing a recoverable F9 when weighed against the $cost of damage to the ASDS.

Offline CameronD

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Re: Landing rockets and the wind
« Reply #53 on: 01/08/2016 03:20 am »
They definitely can't hover, so that's out as an option.

Ships at sea almost always roll by varying amounts (in both magnitude and period) even in a steady sea state, because the base swells themselves vary a bit, as does their period. So, I think you're right, a greater than expected roll can occur at any time. It would also be a greater factor if the downhill side at landing was also the downwind side of the F9. They can't time the landing, but IMHO it's plausible that they might partially compensate for wind by changing the paramiters of the GN2 thrusters on the interstage, to have one firing during touchdown to get ahead of any tilt. My guess is that, at most, this is a future possibility; right now  they are focused on getting a baseline landing right. If an anemometer and wind direction gauge is added to the ASDS at some point, this would be my guess as to the reason (but it would require an uplink). 

My guess; they aren't going to build in an abort mode for this issue; too great a chance of losing a recoverable F9 when weighed against the $cost of damage to the ASDS.

Good point.

1. We've seen the GN2 thrusters firing madly on a couple of landing attempts, but I had thought that was only to counter for wind - would they be large/powerful enough to (worst case) hold the stage on one leg for the second or three it took for the ASDS to come off of the back of a wave??

2. There was an anemometer/wind direction gauge fitted to JRtI - and presumably M'303 and M'304 also -  but that would be for current wind conditions only and would tell you nothing about sea-state or predicted sea-state.

I do agree with you though...  They'd be unlikely to abort a landing even if the sensors all indicated a high probability that the stage will end up in the ocean as soon as the engines shut down. Since there's no payload on board (valuable or otherwise) they've lost nothing trying.

« Last Edit: 01/08/2016 05:00 am by CameronD »
With sufficient thrust, pigs fly just fine - however, this is not necessarily a good idea. It is hard to be sure where they are
going to land, and it could be dangerous sitting under them as they fly overhead.

Offline joek

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Re: Landing rockets and the wind
« Reply #54 on: 01/08/2016 04:49 am »
1. We've seen the GN2 thrusters firing madly on a couple of landing attempts, but I had thought that was only to counter for wind - would they be large/powerful enough to (worst case) hold the stage on one leg for the second or three it took for the ASDS to come off of the back of a wave??

Those GN2 thrusters were dealing with other more serious problems: ran out of hydraulic fluid on one try; stuck valve on the other try.  We have never seen an attempted ASDS landing where everything performed nominally.

Offline CameronD

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Re: Landing rockets and the wind
« Reply #55 on: 01/08/2016 05:02 am »
1. We've seen the GN2 thrusters firing madly on a couple of landing attempts, but I had thought that was only to counter for wind - would they be large/powerful enough to (worst case) hold the stage on one leg for the second or three it took for the ASDS to come off of the back of a wave??

Those GN2 thrusters were dealing with other more serious problems: ran out of hydraulic fluid on one try; stuck valve on the other try.  We have never seen an attempted ASDS landing where everything performed nominally.

True.  I've edited my post above accordingly. :)

With sufficient thrust, pigs fly just fine - however, this is not necessarily a good idea. It is hard to be sure where they are
going to land, and it could be dangerous sitting under them as they fly overhead.

Offline JamesH

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Re: Landing rockets and the wind
« Reply #56 on: 01/08/2016 09:20 am »
The video shows the rocket coming in at the wrong angle and/or off centre. That's has nothing to do with the rocking of the barge, which is a sensible sea state is negligible.

Shotwell does not share this opinion:

Quote
"Just purely the boat moving, even in a low sea state, it's hard to imagine that vehicle is going to stay vertical," Shotwell said. "That vehicle is big and tall, compared to the itty-bity-greater-than-a-football-field-size ship."

http://www.defensenews.com/story/defense/air-space/space/2015/04/15/spacex-ground-attempt-reusable-landing-sea/25827625/

Edit to clarify: I'm disagreeing that the rocking is negligible. Not saying it was the cause of previous failures.

Negligible compared with what would actually cause a problem to the landing. So relative not absolute. But whether that is the case remains to be seen.

Offline mvpel

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Re: Landing rockets and the wind
« Reply #57 on: 01/11/2016 02:25 am »
It's hard to grasp how massive seagoing platforms of this scale are unless you've stood on one. They're massive. And in any case I think SpaceX has undoubtedly run enough simulations to get to the point that it does not remain to be seen within the constraints of the landing commit criteria.
"Ugly programs are like ugly suspension bridges: they're much more liable to collapse than pretty ones, because the way humans (especially engineer-humans) perceive beauty is intimately related to our ability to process and understand complexity. A language that makes it hard to write elegant code makes it hard to write good code." - Eric S. Raymond

Offline CameronD

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Re: Landing rockets and the wind
« Reply #58 on: 01/11/2016 02:58 am »
It's hard to grasp how massive seagoing platforms of this scale are unless you've stood on one. They're massive. And in any case I think SpaceX has undoubtedly run enough simulations to get to the point that it does not remain to be seen within the constraints of the landing commit criteria.

Perhaps "massive" is a relative term...  The platform in question is only 300' long and 100' wide and, lightly ballasted as it is and with no active stabilisation, will bob around like a cork out on the open ocean.  Compared to, say, an aircraft carrier, it's a peanut.

Yes, undoubtedly SpaceX have run enough simulations, and with the last landing have proven they can land a stage on a surface of equivalent size, so apparently all the other bugs are ironed out - but whether or not their commit criteria for a barge landing are correct, only time will tell.
 
With sufficient thrust, pigs fly just fine - however, this is not necessarily a good idea. It is hard to be sure where they are
going to land, and it could be dangerous sitting under them as they fly overhead.

Offline JamesH

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Re: Landing rockets and the wind
« Reply #59 on: 01/11/2016 10:57 am »
It's hard to grasp how massive seagoing platforms of this scale are unless you've stood on one. They're massive. And in any case I think SpaceX has undoubtedly run enough simulations to get to the point that it does not remain to be seen within the constraints of the landing commit criteria.

Perhaps "massive" is a relative term...  The platform in question is only 300' long and 100' wide and, lightly ballasted as it is and with no active stabilisation, will bob around like a cork out on the open ocean.  Compared to, say, an aircraft carrier, it's a peanut.

Yes, undoubtedly SpaceX have run enough simulations, and with the last landing have proven they can land a stage on a surface of equivalent size, so apparently all the other bugs are ironed out - but whether or not their commit criteria for a barge landing are correct, only time will tell.
 

It won't 'bob'. It has too much mass. It will roll with long wavelength waves, but short wavelengths should be OK. But of course, even an aircraft carrier will bounce around if the waves are big enough.

Tags: SpaceX rockets wind 
 

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