Author Topic: Falcon 9 1st stage landing accuracy and progress towards ITS landing cradle?  (Read 7005 times)

Offline hkultala

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Is there any compilation of F9 landing precision statistics?

The latest was 0.7 meters off-center from the platform.

This is quite good but might not be enought for the cradle landing.

What about earlier ones?

Has the accuracy been about the same with some random variation, or been improving?

(and has it always been off-center to same direction or to random direction?)


I wonder if thy are constantly tweaking their landing control algorithms(so that they can in the future get it to the precision required by the cradle), or have they decided that the current algorithm is accurate enough for F9 and don't want to risk introducing some bug by changing it?
« Last Edit: 08/25/2017 08:29 AM by hkultala »

Offline meberbs

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The latest was 0.7 meters off-center from the platform.

This is quite good but might not be enought for the cradle landing.
I assume this post was prompted by Musk's tweet of landing accuracy.

He also had a follow up where someone asked what accuracy is needed for ITS landing mount.
Quote from: Elon Musk
Probably 2m or so
The rest of your questions bring up some good points though, for a landing cradle, you would want to make sure you can be within the accuracy even in a worst case situation.

The "probably" in Musk's response is almost certainly due to it still being way to early in the design for the accuracy needed to be fixed to an exact number.

Offline IRobot

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The cradle itseft might move to match the stage. I've seen a 400mT crane in an well intervention ship (oil & gas) moving sideways and up/down by 2.5 meters to each side, to account for sea motion.
« Last Edit: 08/25/2017 08:33 AM by IRobot »

Offline cppetrie

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Is there any compilation of F9 landing precision statistics?

The latest was 0.7 meters off-center from the platform.

This is quite good but might not be enought for the cradle landing.

What about earlier ones?

Has the accuracy been about the same with some random variation, or been improving?

(and has it always been off-center to same direction or to random direction?)


I wonder if thy are constantly tweaking their landing control algorithms(so that they can in the future get it to the precision required by the cradle), or have they decided that the current algorithm is accurate enough for F9 and don't want to risk introducing some bug by changing it?
There was a plot recently from the landing bingo game that showed the winning location for each landing. (BTW did I miss landing bingo for these last two launches?) it showed there was a consistent drift direction that we suspected was wind related. It is probably possible to calculate the distance from center for each one and plot over time to determine how landing accuracy has changed over time. Can anyone find that plot to post into this thread? I'm on my phone only and searching abilities are limited.

Continuing the trend of landing on the aft half.


Edit: found the post
« Last Edit: 08/25/2017 02:34 PM by cppetrie »

Offline wannamoonbase

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0.7 meters on an ASDS is seriously impressive.

They are getting better and more consistent.  Very cool to be watching this evolution.

This is an exciting time to be a spacenut.
Excited to be finally into the first Falcon Heavy flow, we are getting so close!

Offline Lar

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Landing bingo may be getting kinda pointless...

I don't know that anyone did this analysis for land landings.
"I think it would be great to be born on Earth and to die on Mars. Just hopefully not at the point of impact." -Elon Musk
"We're a little bit like the dog who caught the bus" - Musk after CRS-8 S1 successfully landed on ASDS OCISLY

Offline Jim

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There was a plot recently from the landing bingo game that showed the winning location for each landing. (BTW did I miss landing bingo for these last two launches?) it showed there was a consistent drift direction that we suspected was wind related. It is probably possible to calculate the distance from center for each one and plot over time to determine how landing accuracy has changed over time. Can anyone find that plot to post into this thread? I'm on my phone only and searching abilities are limited.


Data for landings on the barge shouldn't be included since there are errors in the location of the barge.  Only data on ground based landings should be looked at.

Offline Comga

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There was a plot recently from the landing bingo game that showed the winning location for each landing. (BTW did I miss landing bingo for these last two launches?) it showed there was a consistent drift direction that we suspected was wind related. It is probably possible to calculate the distance from center for each one and plot over time to determine how landing accuracy has changed over time. Can anyone find that plot to post into this thread? I'm on my phone only and searching abilities are limited.


Data for landings on the barge shouldn't be included since there are errors in the location of the barge.  Only data on ground based landings should be looked at.

I am not sure I agree, Jim.

Of course these should be considered. They bound the accuracy and we have been told the ASDS finds the target position to a meter or so.
Plus we have this data. We don't get as good data for the pad landings.
Mediocre data with a fixed target or very accurate data with a compounding error.  Both have value especially with such a small number of data points.

edit/Lar: PoliteJim/3000 works for inbound as well as outbound messages.
« Last Edit: 08/25/2017 06:11 PM by Lar »
What kind of wastrels would dump a perfectly good booster in the ocean after just one use?

Online Lars-J

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The cradle itseft might move to match the stage. I've seen a 400mT crane in an well intervention ship (oil & gas) moving sideways and up/down by 2.5 meters to each side, to account for sea motion.

It doesn't even have to move... If the cradle has some sort of vanes/glove that can shift the landing vehicle to its final resting position. Building something like this that can handle a 2m horizontal error is certainly possible. And more likely than a dynamic moving system, IMO.

Think of the landing cradle as the equivalent of the passive cone in a probe & drogue docking system. (see image)
« Last Edit: 08/25/2017 06:00 PM by Lars-J »

Offline matthewkantar

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The cradle itseft might move to match the stage. I've seen a 400mT crane in an well intervention ship (oil & gas) moving sideways and up/down by 2.5 meters to each side, to account for sea motion.

It doesn't even have to move... If the cradle has some sort of vanes/glove that can shift the landing vehicle to its final resting position. Building something like this that can handle a 2m horizontal error is certainly possible. And more likely than a dynamic moving system, IMO.

Think of the landing cradle as the equivalent of the passive cone in a probe & drogue docking system. (see image)

Seems to me that tis would require some large, robust (heavy) surfaces on the returning booster. The equipment on the ground can be arbitrarily robust, the modifications to the booster have to be significantly lighter than landing gear for the effort to pay off.

Matthew

Online Lars-J

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The cradle itseft might move to match the stage. I've seen a 400mT crane in an well intervention ship (oil & gas) moving sideways and up/down by 2.5 meters to each side, to account for sea motion.

It doesn't even have to move... If the cradle has some sort of vanes/glove that can shift the landing vehicle to its final resting position. Building something like this that can handle a 2m horizontal error is certainly possible. And more likely than a dynamic moving system, IMO.

Think of the landing cradle as the equivalent of the passive cone in a probe & drogue docking system. (see image)

Seems to me that tis would require some large, robust (heavy) surfaces on the returning booster. The equipment on the ground can be arbitrarily robust, the modifications to the booster have to be significantly lighter than landing gear for the effort to pay off.

Matthew

Not necessarily. The catching mechanism could have dampening and very low friction (even as low tech as heavy duty rollers on the surface)

And the base is already *incredibly* robust on rockets... The octaweb equivalent will need to support the holding the entire fully loaded rocket before takeoff (launch hold downs attach to it), and with the the purpose of the three fins in the ITS concept is to make sure that only they need to be the strongest, they would guide the booster into the proper position. So for the base to handle the an empty booster being pushed slightly sideways is nothing compared to the loads before and during liftoff.

Offline intrepidpursuit

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The landing fins on the BFR are exactly the hard point vanes you are talking about. We already know they are there and can reverse engineer what the other side would look like.

Elon said they need about 2m of accuracy. I'm sure he's put some thought into it.

Offline spacenut

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I agree with Jim.  The ship is moving or trying to stay in one position in a moving sea.  It is usually more windy at sea also.  The landing pad(s) are fixed and never move.  It is all up to the rocket to hit the center on land.  I think they need a lot more return to launch site and see how accurate they can get before using a landing cradle. 

Maybe the could land on a movable landing cradle, that can land it away from the launch pad.  The cradle could then carry the rocket over the launch pad for refueling and launching.  Then having a spare movable cradle in case of crash so as not to keep other rockets from launching and landing.  Something like the mobile platform for Saturn V, shuttle, and SLS, but either on tracks for a traditional locomotive to move or wheels. 

Offline FutureSpaceTourist

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I think they need a lot more return to launch site and see how accurate they can get before using a landing cradle.


Edit to add: itís not clear to me whether youíre referring to F9 RTLS or BFR.

I suspect just this yearís F9 RTLS has given SpaceX a pretty good handle on F9 accuracy, coupled with any planned further improvements for block 5. Given increased frequency of RTLS with block 5, it wonít take long in 2018 to get significant more RTLS experience.

So I think the question here is the extent to which F9 landing accuracy does, or doesnít carry over to BFR. My guess is that SpaceX have a pretty good handle now on what drives/limits F9 accuracy.
« Last Edit: 08/26/2017 02:59 AM by FutureSpaceTourist »

Offline darkenfast

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The wind out at sea is fairly steady (most of the time).  It think some of what we see is the drift that happens right at the end of flight, when the engine starts shutting down.  You can often see a little drift, almost like a skid.  Take a look at where the rocket is when it's 15 feet up and we may get a better idea of what will happen when the vanes enter the guides on the pad.  Pure speculation, etc.

Offline spacenut

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I was referring to RTLS landings of Falcon 9 to within the 2m of accuracy that Elon is shooting for.  I didn't think they had 9 return to launch site, most have been on the landing ships.  I know they have had 9 landings this year, but most have been on the landing ships. 

Online LouScheffer

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Is there any compilation of F9 landing precision statistics?

The latest was 0.7 meters off-center from the platform.

This is quite good but might not be enought for the cradle landing.
If they can routinely get 0.7 meters, or even close, it's plenty good enough for landing two boosters on one ship. 

That would result in a big boost in FH payload, at the cost of two ships in one ocean (one to  catch the outside boosters, and one further downrange to catch the central core).

Online meekGee

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I agree with Jim.  The ship is moving or trying to stay in one position in a moving sea.  It is usually more windy at sea also.  The landing pad(s) are fixed and never move.  It is all up to the rocket to hit the center on land.  I think they need a lot more return to launch site and see how accurate they can get before using a landing cradle. 

Maybe the could land on a movable landing cradle, that can land it away from the launch pad.  The cradle could then carry the rocket over the launch pad for refueling and launching.  Then having a spare movable cradle in case of crash so as not to keep other rockets from launching and landing.  Something like the mobile platform for Saturn V, shuttle, and SLS, but either on tracks for a traditional locomotive to move or wheels.
Don't forget that unlike the audience, SpaceX knows the internal state of the control systems, so that if the barge was not where it was supposed to be, they can subtract that out.

So they know, and certainly can use the data.

For us, all we know is that barge landing accuracy is degraded by the additional motion of the barge, and by conditions that are typically more adverse, so land landing accuracy will be better.
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Offline wannamoonbase

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Is there any compilation of F9 landing precision statistics?

The latest was 0.7 meters off-center from the platform.

This is quite good but might not be enought for the cradle landing.
If they can routinely get 0.7 meters, or even close, it's plenty good enough for landing two boosters on one ship. 

That would result in a big boost in FH payload, at the cost of two ships in one ocean (one to  catch the outside boosters, and one further downrange to catch the central core).

Might want to build the second ship to be larger.

What would happen to boosters that close together?  If one was landed briefly before the other.  What would the affects of the second landing (engine exhaust) be on the first?

It's an interesting idea though.  The performance increase would be pretty impressive but currently unwarranted for anything on the manifest I believe.
Excited to be finally into the first Falcon Heavy flow, we are getting so close!

Online LouScheffer

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If they can routinely get 0.7 meters, or even close, it's plenty good enough for landing two boosters on one ship. 

That would result in a big boost in FH payload, at the cost of two ships in one ocean (one to  catch the outside boosters, and one further downrange to catch the central core).

Might want to build the second ship to be larger.

What would happen to boosters that close together?  If one was landed briefly before the other.  What would the affects of the second landing (engine exhaust) be on the first?

A back of the envelope calculation indicates no problem with the second booster blowing over the first.  So it should work if the accuracy is enough.

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