Author Topic: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3  (Read 791030 times)

Offline Req

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #520 on: 04/11/2016 10:53 AM »
Why are people trying to solve a problem that doesn't appear to exist?

It landed, in fairly extreme conditions.

What, exactly do they need to change? Accuracy? Why?

"Extreme conditions" is highly debatable.  Estimates here on this very forum bound the winds to under 25kn(look at how fast the exhaust cloud moves).  Nothing terribly out of the ordinary.  And in any case, if it's a barge landing, they have to work with what the conditions are or delay the launch, it's not like there's much wiggle room.

That said, it wouldn't surprise me if they have done all of the modelling and everything will work great no matter what, even if a stage ends up bouncing off of the railing.  It also wouldn't surprise me if they haven't.

Put simply, the problem DOES exist.  This stage came too close for comfort to the edge for such a more or less nominal mission(let's not forget that they DID actually do a boost-back burn, just not a full one - this mission was as "controlled" as any other mission will be);  If they expect to successfully launch ~66% of their missions on-time and then recover them with barges going forward, then something will need to change.  I have full confidence in them to make the appropriate adjustments, though.  And my personal WAG is that they won't need a larger barge, or any type of uplink system between the barge and the stage, or to move the barge 10 minutes before landing - it'll just be updates to the software on the stage itself. (I understand where the "move the barge 10 minutes before landing" idea comes from, but if you're going to invest money in that accuracy, obviously you want to invest it in the stage.  It's just better to have the stage land on target from a system level, that's obviously where you want to put your effort - do you plan on having mobile landing platforms on Mars that can move 10 minutes before you land an MCT?  Full accuracy is pretty much on the critical path)
« Last Edit: 04/11/2016 11:47 AM by Req »

Online Jet Black

Why are people trying to solve a problem that doesn't appear to exist?

It landed, in fairly extreme conditions.

What, exactly do they need to change? Accuracy? Why?

indeed, and also how would they do it?  Those 50mph winds are fairly close to the surface, you can't see them and there are changes in wind speed all the way up. What they've achieved is tremendous, but it also shows the massive challenges they face.
For a successful technology, reality must take precedence over public relations, for Nature cannot be fooled. -- Richard Feynman

Offline JamesH65

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #522 on: 04/11/2016 11:39 AM »
Why are people trying to solve a problem that doesn't appear to exist?

It landed, in fairly extreme conditions.

What, exactly do they need to change? Accuracy? Why?

"Extreme conditions" is highly debatable.  Estimates here on this very forum bound the winds to under 25kn(look at how fast the exhaust cloud moves).  Nothing terribly out of the ordinary.  And in any case, if it's a barge landing, they have to work with what the conditions are or delay the launch, it's not like there's much wiggle room.

That said, it wouldn't surprise me if they have done all of the modelling and everything will work great no matter what, even if a stage ends up bouncing off of the railing.  It also wouldn't surprise me if they haven't.

Put simply, the problem DOES exist.  This stage came too close for comfort to the edge for such a more or less nominal mission(let's not forget that they DID actually do a boost-back burn, just not a full one - this mission was as "controlled" as any other mission will be);  If they expect to successfully launch ~66% of their missions on-time and then recover them with barges going forward, then something will need to change.  I have full confidence in them to make the appropriate adjustments, though.  And my personal WAG is that they won't need a larger barge, or any type of uplink system between the barge and the stage, or to move the barge 10 minutes before landing - it'll just be updates to the software on the stage itself. (I understand where the "move the barge 10 minutes before landing" idea comes from, but if you're going to invest money in that accuracy, obviously you want to invest it in the stage.  It's just better to have the stage land on target from a system level, that's obviously where you want to put your effort)

Er, I not sure anyone outside of SpaceX is in a position to say there is even a problem here. My opinion, no there isn't one, and nothing written above seems to say otherwise.

25knts of wind is actually pretty windy. It's at the high end of a Force 6, which is bordering on gale force. That classes as pretty damn high. And that, I suspect is a conservative guess at speed, at sea level, speeds further up will be much higher.

The fact this was a mission with plenty of fuel is completely irrelevant. It makes no difference to the end result whether this was an easy boostback or a hard one, the accuracy has been shown to be there in all the attempts so far.

So, since this came in within 5m accuracy wise, how do you propose a 'software fix' is going to make it more accurate? The barge is unable to tell the stage the wind conditions, and since they vary rapidly anyway, this would appear to be irrelevant.

I'm still not seeing any sort of problem here.

EDIT: taken another look at the video. Wave state is pretty indicative of force 6, with some streaks possibly indicating close to a 7. Looking closely at the way the stage moves' as it touches down and the angle of the barge, indicated it landing first on the right leg (as seen from video) on a slightly sloping deck, This caused it to hop slightly to the left, which is pretty much what you would expect when coming down on a sloping deck. And I'm pretty damn sure there's nothing you can do about a sloping deck, except avoid flying when the sea state means it's likely to be too angled.
« Last Edit: 04/11/2016 11:50 AM by JamesH65 »

Offline Req

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #523 on: 04/11/2016 11:49 AM »
Why are people trying to solve a problem that doesn't appear to exist?

It landed, in fairly extreme conditions.

What, exactly do they need to change? Accuracy? Why?

"Extreme conditions" is highly debatable.  Estimates here on this very forum bound the winds to under 25kn(look at how fast the exhaust cloud moves).  Nothing terribly out of the ordinary.  And in any case, if it's a barge landing, they have to work with what the conditions are or delay the launch, it's not like there's much wiggle room.

That said, it wouldn't surprise me if they have done all of the modelling and everything will work great no matter what, even if a stage ends up bouncing off of the railing.  It also wouldn't surprise me if they haven't.

Put simply, the problem DOES exist.  This stage came too close for comfort to the edge for such a more or less nominal mission(let's not forget that they DID actually do a boost-back burn, just not a full one - this mission was as "controlled" as any other mission will be);  If they expect to successfully launch ~66% of their missions on-time and then recover them with barges going forward, then something will need to change.  I have full confidence in them to make the appropriate adjustments, though.  And my personal WAG is that they won't need a larger barge, or any type of uplink system between the barge and the stage, or to move the barge 10 minutes before landing - it'll just be updates to the software on the stage itself. (I understand where the "move the barge 10 minutes before landing" idea comes from, but if you're going to invest money in that accuracy, obviously you want to invest it in the stage.  It's just better to have the stage land on target from a system level, that's obviously where you want to put your effort)

Er, I not sure anyone outside of SpaceX is in a position to say there is even a problem here. My opinion, no there isn't one, and nothing written above seems to say otherwise.

25knts of wind is actually pretty windy. It's at the high end of a Force 6, which is bordering on gale force. That classes as pretty damn high. And that, I suspect is a conservative guess at speed, at sea level, speeds further up will be much higher.

The fact this was a mission with plenty of fuel is completely irrelevant. It makes no difference to the end result whether this was an easy boostback or a hard one, the accuracy has been shown to be there in all the attempts so far.

So, since this came in within 5m accuracy wise, how do you propose a 'software fix' is going to make it more accurate? The barge is unable to tell the stage the wind conditions, and since they vary rapidly anyway, this would appear to be irrelevant.

I'm still not seeing any sort of problem here.

It was obviously aware of the wind conditions because it came in at a higher angle than usual.  Simple refinements to the model given the high quality telemetry from this landing and others in the future.
« Last Edit: 04/11/2016 11:50 AM by Req »

Offline JamesH65

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #524 on: 04/11/2016 11:54 AM »
Why are people trying to solve a problem that doesn't appear to exist?

It landed, in fairly extreme conditions.

What, exactly do they need to change? Accuracy? Why?

"Extreme conditions" is highly debatable.  Estimates here on this very forum bound the winds to under 25kn(look at how fast the exhaust cloud moves).  Nothing terribly out of the ordinary.  And in any case, if it's a barge landing, they have to work with what the conditions are or delay the launch, it's not like there's much wiggle room.

That said, it wouldn't surprise me if they have done all of the modelling and everything will work great no matter what, even if a stage ends up bouncing off of the railing.  It also wouldn't surprise me if they haven't.

Put simply, the problem DOES exist.  This stage came too close for comfort to the edge for such a more or less nominal mission(let's not forget that they DID actually do a boost-back burn, just not a full one - this mission was as "controlled" as any other mission will be);  If they expect to successfully launch ~66% of their missions on-time and then recover them with barges going forward, then something will need to change.  I have full confidence in them to make the appropriate adjustments, though.  And my personal WAG is that they won't need a larger barge, or any type of uplink system between the barge and the stage, or to move the barge 10 minutes before landing - it'll just be updates to the software on the stage itself. (I understand where the "move the barge 10 minutes before landing" idea comes from, but if you're going to invest money in that accuracy, obviously you want to invest it in the stage.  It's just better to have the stage land on target from a system level, that's obviously where you want to put your effort)

Er, I not sure anyone outside of SpaceX is in a position to say there is even a problem here. My opinion, no there isn't one, and nothing written above seems to say otherwise.

25knts of wind is actually pretty windy. It's at the high end of a Force 6, which is bordering on gale force. That classes as pretty damn high. And that, I suspect is a conservative guess at speed, at sea level, speeds further up will be much higher.

The fact this was a mission with plenty of fuel is completely irrelevant. It makes no difference to the end result whether this was an easy boostback or a hard one, the accuracy has been shown to be there in all the attempts so far.

So, since this came in within 5m accuracy wise, how do you propose a 'software fix' is going to make it more accurate? The barge is unable to tell the stage the wind conditions, and since they vary rapidly anyway, this would appear to be irrelevant.

I'm still not seeing any sort of problem here.

It was obviously aware of the wind conditions because it came in at a higher angle than usual.  Simple refinements to the model given the high quality telemetry from this landing and others in the future.

It knows the wind conditions from it's change in GPS positioning, but that doesn't indicate what the conditions are like close to the surface. And as we have been told multiple times, there is apparently NO communications from the barge to the stage, so there's nothing being sent to tell it of the conditions on the surface.

They MIGHT preprogram approximate conditions in to the stage before flight, to give it a hint on how to deal with them, but on  the whole it seems like a closed loop system that knows where it is and where it has to go, and compensates for conditions on the way down.

Offline Req

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #525 on: 04/11/2016 12:03 PM »
I understand that it only has it's own GPS and/or intertial reference to work with.

I've written quite a few closed-loop systems in my time, and I refuse to believe that they can't just make it land on target rather than hopping and skidding, now that they know that can happen and have high quality telemetry of exactly that happening.  At least for this case - They may also need high quality telemetry from an actual ~50MPH winds landing to refine the closed-loop code to nail that landing as well.

Edit - I can't find the exact abort criteria for this mission, but would just like to point out that going from memory, 25kn is either below or close to typical green launch criteria for the stage itself.  Characterizing that as "extreme conditions" is a bit of a stretch if you want to be realistic about launching 66% of your missions on-time, because off-shore landing conditions seem to be characteristically worse than launch conditions, at least so far.
« Last Edit: 04/11/2016 12:28 PM by Req »

Offline Citabria

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #526 on: 04/11/2016 01:01 PM »
As pilot of a light plane, I am the closed-loop system. A smooth landing requires bringing 6 parameters to zero all at the same instant: height; vertical speed; distance from centerline; sideways speed; yaw; and yaw rate. Doing that in variable, gusty winds is quite a challenge. If any of those has to be sacrificed for the sake of the others, it's probably distance from centerline, within limits of runway width.

Falcon has 12 to zero simultaneously:  height; vertical speed; distance from center in two axes; sideways speed in two axes; and pitch, yaw, and roll, plus their rates. That's why it's so impressive when it works, even if a few meters off center.

Offline Req

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #527 on: 04/11/2016 01:05 PM »
As pilot of a light plane, I am the closed-loop system. A smooth landing requires bringing 6 parameters to zero all at the same instant: height; vertical speed; distance from centerline; sideways speed; yaw; and yaw rate. Doing that in variable, gusty winds is quite a challenge. If any of those has to be sacrificed for the sake of the others, it's probably distance from centerline, within limits of runway width.

Falcon has 12 to zero simultaneously:  height; vertical speed; distance from center in two axes; sideways speed in two axes; and pitch, yaw, and roll, plus their rates. That's why it's so impressive when it works, even if a few meters off center.

There does, however, exist closed-loop software that can do things like land an X-47B(which is an aerodynamically unstable airframe in it's own right) on an aircraft carrier in a comms contingency situation.  I have some insight into that particular software.
« Last Edit: 04/11/2016 01:15 PM by Req »

Offline Kabloona

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #528 on: 04/11/2016 01:16 PM »
Edit - I can't find the exact abort criteria for this mission, but would just like to point out that going from memory, 25kn is either below or close to typical green launch criteria for the stage itself.  Characterizing that as "extreme conditions" is a bit of a stretch if you want to be realistic about launching 66% of your missions on-time, because off-shore landing conditions seem to be characteristically worse than launch conditions, at least so far.

For past ASDS landings, the criterion was 20 kts wind max. For the OG2 landing at LZ-1, it was 50 mph wind max.

Judging from the well-damped pitch motion of the stage during the landing, staying pitched into the wind then righting itself pefectly with no overshoot, it looks like they've got the attitude control loop dialed in perfectly for wind compensation, and I wouldn't be surprised if a limit higher than 20 kts is now the "new normal" for the stage landing limit.

But winds a bit above that are going to make ASDS grappling/towing dangerous, with high waves as we saw during the "storm" launch that banged up the ASDS.

So we may have reached the point where the limiting factor for landing is no longer the effect of winds on the stage autopilot, but the sea conditions caused by high winds and the resulting operational difficulties.
« Last Edit: 04/11/2016 01:20 PM by Kabloona »

Offline Req

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #529 on: 04/11/2016 01:21 PM »
Edit - I can't find the exact abort criteria for this mission, but would just like to point out that going from memory, 25kn is either below or close to typical green launch criteria for the stage itself.  Characterizing that as "extreme conditions" is a bit of a stretch if you want to be realistic about launching 66% of your missions on-time, because off-shore landing conditions seem to be characteristically worse than launch conditions, at least so far.

For past ASDS landings, the criterion was 20 kts wind max. For the OG2 landing at LZ-1, it was 50 mph wind max.

Judging from the well-damped pitch motion of the stage during the landing, staying pitched into the wind then righting itself pefectly with no overshoot, it looks like they've got the attitude control loop dialed in perfectly for wind compensation, and I wouldn't be surprised if a limit higher than 20 kts is now the "new normal" for the stage landing limit.

But winds a bit above that are going to make ASDS grappling/towing dangerous, with high waves as we saw during the "storm" launch that banged up the ASDS.

So we may have reached the point where the limiting factor for landing is no longer the effect of winds on the stage autopilot, but the sea conditions caused by high winds and the resulting operational difficulties.

Out of curiosity, what makes you think it wouldn't skid twice as far in 50MPH winds?  Not trying to be confrontational at all.  To me it looks like the system didn't take any translation after MECO(?) into account at all despite doing admirably well up to that point.
« Last Edit: 04/11/2016 01:24 PM by Req »

Offline Kabloona

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #530 on: 04/11/2016 01:43 PM »
Quote
Out of curiosity, what makes you think it wouldn't skid twice as far in 50MPH winds?  Not trying to be confrontational at all.  To me it looks like the system didn't take any translation after MECO(?) into account at all despite doing admirably well up to that point.

The stage landed successfully despite the skidding, and I guess we could quibble about whether that makes the wind compensation "perfect," which I assume is your point. Could they refine the control loop even further to eliminate skidding? Maybe.

But my point was that barge landings in 50 mph winds (or therabouts) are not even going to be possible because of the operational difficulties. So why bother worrying about more skidding at 50 mph? And 50 mph winds at LZ-1 at the Cape will violate LCC's and cause a scrub anyway.

So if the autpilot's wind compensation is not yet "perfect," it's quite good enough, and now the limiting factors are probably ASDS ops in high winds, and launch wind constraints.
« Last Edit: 04/11/2016 02:24 PM by Kabloona »

Offline JamesH65

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #531 on: 04/11/2016 02:17 PM »
I think the skid was more to do with the 'right' leg landing before the others, making it momentarily susceptible to wind pressure, with only one leg down. As soon as all legs were on deck it stopped moving. *

There's not a lot you can do about that since the barge orientation is unpredictable/unfollowable, ie you cannot reorient the stage angle to match the barge.


* This is presumably why there is an upper limit of deck angle - the legs can only absorb so much, so need to get all legs down before the absorption capacity of any one leg is exceeded.

Offline cscott

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #532 on: 04/11/2016 02:37 PM »
There's a control inversion issue once one leg is touching.  That's actually probably the cause of the bump.  Solution is either to explicitly model the control inversion in software, or else just come down a little "harder" to minimize it.  The legs can probably take a bit harder landing.  My opinion is still that we are seeing a little bounce due to the shutdown transient, and that can be modelled away.

Offline OxCartMark

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #533 on: 04/11/2016 03:03 PM »

Offline Kaputnik

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #534 on: 04/11/2016 03:05 PM »
The fact this was a mission with plenty of fuel is completely irrelevant. It makes no difference to the end result whether this was an easy boostback or a hard one, the accuracy has been shown to be there in all the attempts so far.


It is highly relevant.
The stage would have been, in effect, ballasted by the additional remaining propellants. This would ease up the constraints on the final burn by lowering the thrust to weight ratio and giving the engine and guidance system more leeway to set down safely.
Waiting for joy and raptor

Offline JamesH65

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #535 on: 04/11/2016 03:42 PM »
The fact this was a mission with plenty of fuel is completely irrelevant. It makes no difference to the end result whether this was an easy boostback or a hard one, the accuracy has been shown to be there in all the attempts so far.


It is highly relevant.
The stage would have been, in effect, ballasted by the additional remaining propellants. This would ease up the constraints on the final burn by lowering the thrust to weight ratio and giving the engine and guidance system more leeway to set down safely.

Make no difference. Stage knows how much fuel it has, compensates. There is no leeway. There is simply stage weight vs engine thrust.

Offline Comga

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #536 on: 04/11/2016 04:17 PM »
Why are people trying to solve a problem that doesn't appear to exist?

It landed, in fairly extreme conditions.

What, exactly do they need to change? Accuracy? Why?

I agree, but let's add some numbers.
Here is an image from the CRS-8 Barge Bingo thread.
Scaling from a 53 meter width of the ASDS, the legs spread across ~16.5 m.
If it comes down at the center, and square to the deck, there are 18 m left on each side.
The CRS-8 first stage came to rest with about 9.5 m from the rail.
That means it used up about half the margin in a 23 knot wind. 
We can (probably can't resist the urge to) debate whether or not that qualifies as "extreme" but twice that would definitely be extreme.
They will continue to refine the control system.
It remains hard to make the case that a more complex system is required.

edit: OxCartMark:  "I feel your pain" but it's hopeless.  I will not make the excuse that people discussing adding windage to the ASDS location makes this relevant to the thread title.
In which thread would you have us discuss landing dynamics?
« Last Edit: 04/11/2016 04:20 PM by Comga »
What kind of wastrels would dump a perfectly good booster in the ocean after just one use?

Offline Req

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #537 on: 04/11/2016 04:23 PM »
The fact this was a mission with plenty of fuel is completely irrelevant. It makes no difference to the end result whether this was an easy boostback or a hard one, the accuracy has been shown to be there in all the attempts so far.


It is highly relevant.
The stage would have been, in effect, ballasted by the additional remaining propellants. This would ease up the constraints on the final burn by lowering the thrust to weight ratio and giving the engine and guidance system more leeway to set down safely.

Make no difference. Stage knows how much fuel it has, compensates. There is no leeway. There is simply stage weight vs engine thrust.

On SES-9, it wasn't even known whether the stage would structurally survive re-entry, much less with all of it's critical components intact.  It's critical components WEREN'T intact, and it under-thrusted.  Could this be because of physical deformation(or full-on detachment) of an expansion nozzle?  SpaceX will probably never tell us.

Anyway, I mentioned that it was a normal profile for that reason.

Offline Kaputnik

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #538 on: 04/11/2016 04:36 PM »
The fact this was a mission with plenty of fuel is completely irrelevant. It makes no difference to the end result whether this was an easy boostback or a hard one, the accuracy has been shown to be there in all the attempts so far.


It is highly relevant.
The stage would have been, in effect, ballasted by the additional remaining propellants. This would ease up the constraints on the final burn by lowering the thrust to weight ratio and giving the engine and guidance system more leeway to set down safely.

Make no difference. Stage knows how much fuel it has, compensates. There is no leeway. There is simply stage weight vs engine thrust.

In an ideal world, yes. But in that ideal world there are no unknown thrust transients at startup and shutdown, there are no variable crosswinds, or slight inaccuracies in positioning data.
Fact is, a ballasted stage is going to be easier to land than an empty one. Look at the early GH flights- a much heavier vehicle that could actually hover. Of course that is going to be easier to fly, you have a wider range of control authority.
Waiting for joy and raptor

Offline Ohsin

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #539 on: 04/12/2016 06:14 AM »
Shoes.  :)
"Well, three cheers to Sharma, but our real baby is INSAT."

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