Author Topic: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3  (Read 711311 times)

Online CJ

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #500 on: 04/10/2016 10:02 PM »
Over in the CRS-8 discussion thread, there's quite a bit of concern regarding the off-center location where the stage ended up after some downwind sideways motion on landing.

My suggestion in that thread was that if SpaceX is concerned about this, they might address it by, for future windy landings, positioning the ASDS so it's centered a yard or two upwind of the coordinates for touchdown.

So my question here is, is there anything known about the ASDS control system that would preclude making such a simple adjustment in case of need? My guess as to how they could do it is simply change the ASDS's target coordinates by a yard ot two in the appropriate direction.

Doing the above just seems, to me, vastly simpler and cheaper than some of the proposed (in that thread and others) changes to the ASDS and the F9, to fix a problem that we don't, yet, know is actually a problem or not.


Offline Mongo62

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #501 on: 04/10/2016 10:54 PM »
My suggestion in that thread was that if SpaceX is concerned about this, they might address it by, for future windy landings, positioning the ASDS so it's centered a yard or two upwind of the coordinates for touchdown.

I assume you mean downwind? Being upwind of the GPS location would make the stage's final stopping point even further from the center of the deck.

Online CJ

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #502 on: 04/10/2016 11:17 PM »
My suggestion in that thread was that if SpaceX is concerned about this, they might address it by, for future windy landings, positioning the ASDS so it's centered a yard or two upwind of the coordinates for touchdown.

I assume you mean downwind? Being upwind of the GPS location would make the stage's final stopping point even further from the center of the deck.

And this is why SpaceX would never be crazy enough to hire me; a goof like I made there would turn a successful landing into a kaboom.

Thanks for the correction.

Online CameronD

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #503 on: 04/10/2016 11:55 PM »
Over in the CRS-8 discussion thread, there's quite a bit of concern regarding the off-center location where the stage ended up after some downwind sideways motion on landing.

My suggestion in that thread was that if SpaceX is concerned about this, they might address it by, for future windy landings, positioning the ASDS so it's centered a yard or two updownwind of the coordinates for touchdown.

So my question here is, is there anything known about the ASDS control system that would preclude making such a simple adjustment in case of need? My guess as to how they could do it is simply change the ASDS's target coordinates by a yard ot two in the appropriate direction.

Doing the above just seems, to me, vastly simpler and cheaper than some of the proposed (in that thread and others) changes to the ASDS and the F9, to fix a problem that we don't, yet, know is actually a problem or not.

It would be really very easy to tweak the location reference in the Thustmaster's position system to do this, but I, for one, don't think they would do it for the following reasons:
1. A barge moving in any direction has significant momentum - it can't stop instantly - and presumably they want it stationary at the moment of impact (jumping onto a moving platform is more hazardous than jumping onto a stationary one - try it at home!).
2. If something went screwy with the comms link at the last second, the barge could start driving off underneath the landing stage, which would be a bad day all around.
3. The landing deck is big enough - it's simply not worth the risk.

« Last Edit: 04/11/2016 12:05 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.

Online CameronD

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #504 on: 04/11/2016 12:02 AM »
Also: some of the bounce might be velocity overshoot---getting to positive a few fractions of a meter per second up, instead of precisely to zero.  Engine shutdown transient might do that, it's notoriously difficult to predict the amount of thrust from the shutdown transient.

I can't see them eliminating the bounce entirely in anything other than calm seas - because it's impossible for anyone to predict the exact vertical position of the barge deck at the exact moment of touchdown whenever you have waves striking the hull.  It's a far easier engineering ask to put design constraints on the amount of bounce..
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 sewebster

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #505 on: 04/11/2016 12:49 AM »
To me, it seems like if this "skidding off the deck" thing were a real problem, then it would be easiest to get a bigger boat. Unless they are currently at some particular width limit etc...

Online CameronD

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #506 on: 04/11/2016 01:15 AM »
To me, it seems like if this "skidding off the deck" thing were a real problem, then it would be easiest to get a bigger boat. Unless they are currently at some particular width limit etc...

Bigger deck cargo barges do exist (Marmac 400, for example) but 300' is the largest most folks need for the sort of transportation tasks these barges are used for and increasing size does increase handling and berthing costs.  This means there aren't so many to choose from, berthing options become limited and costs increase... so it's not an option they'll want to entertain unless they have to.
« Last Edit: 04/11/2016 01:18 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 robertross

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #507 on: 04/11/2016 01:49 AM »
Also: some of the bounce might be velocity overshoot---getting to positive a few fractions of a meter per second up, instead of precisely to zero.  Engine shutdown transient might do that, it's notoriously difficult to predict the amount of thrust from the shutdown transient.

I can't see them eliminating the bounce entirely in anything other than calm seas - because it's impossible for anyone to predict the exact vertical position of the barge deck at the exact moment of touchdown whenever you have waves striking the hull.  It's a far easier engineering ask to put design constraints on the amount of bounce..

Maybe they can build and float a moving wall around the barge. It would be buoyant on the trip out, filled with water to partially submerge it, and that would become a circular wall around the barge to reduce wave impact during landings.

(They could also erect it on station & then fill it to submerge it, taking it off the barge via the support ship). There are a few ways to do this.
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Online CJ

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #508 on: 04/11/2016 01:50 AM »
Over in the CRS-8 discussion thread, there's quite a bit of concern regarding the off-center location where the stage ended up after some downwind sideways motion on landing.

My suggestion in that thread was that if SpaceX is concerned about this, they might address it by, for future windy landings, positioning the ASDS so it's centered a yard or two updownwind of the coordinates for touchdown.

So my question here is, is there anything known about the ASDS control system that would preclude making such a simple adjustment in case of need? My guess as to how they could do it is simply change the ASDS's target coordinates by a yard ot two in the appropriate direction.

Doing the above just seems, to me, vastly simpler and cheaper than some of the proposed (in that thread and others) changes to the ASDS and the F9, to fix a problem that we don't, yet, know is actually a problem or not.

It would be really very easy to tweak the location reference in the Thustmaster's position system to do this, but I, for one, don't think they would do it for the following reasons:
1. A barge moving in any direction has significant momentum - it can't stop instantly - and presumably they want it stationary at the moment of impact (jumping onto a moving platform is more hazardous than jumping onto a stationary one - try it at home!).
2. If something went screwy with the comms link at the last second, the barge could start driving off underneath the landing stage, which would be a bad day all around.
3. The landing deck is big enough - it's simply not worth the risk.

Regarding point #1, I didn't mean to imply any momentum. I was thinking something like this; ten minutes or so before launch, check wind speed an direction (I'm assuming the ASDS has an anemometer). If needed, command the ASDS to move a yard or two downwind. That (a movement of 6 feet or less) wouldn't take long, and it should be at a relative standstill again by launch.

As for trying it at home; I steadfastly refuse to try jumping on a moving ASDS at home - I live 7000 feet up a mountain, so getting an ASDS here would be a tad problematic.  :)

Points 2 and 3 are very valid IMHO. Of course, the only reason to try something like this is if there's a problem that needs fixing. My guess is that for the conditions we saw, there isn't one, but stronger winds might change the equation on that and thus might make trying something along these lines worthwhile. At least, that's my guess as to what they might have in their high winds contingency folder.   

 

Online CameronD

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #509 on: 04/11/2016 02:00 AM »
Over in the CRS-8 discussion thread, there's quite a bit of concern regarding the off-center location where the stage ended up after some downwind sideways motion on landing.

My suggestion in that thread was that if SpaceX is concerned about this, they might address it by, for future windy landings, positioning the ASDS so it's centered a yard or two updownwind of the coordinates for touchdown.

So my question here is, is there anything known about the ASDS control system that would preclude making such a simple adjustment in case of need? My guess as to how they could do it is simply change the ASDS's target coordinates by a yard ot two in the appropriate direction.

Doing the above just seems, to me, vastly simpler and cheaper than some of the proposed (in that thread and others) changes to the ASDS and the F9, to fix a problem that we don't, yet, know is actually a problem or not.

It would be really very easy to tweak the location reference in the Thustmaster's position system to do this, but I, for one, don't think they would do it for the following reasons:
1. A barge moving in any direction has significant momentum - it can't stop instantly - and presumably they want it stationary at the moment of impact (jumping onto a moving platform is more hazardous than jumping onto a stationary one - try it at home!).
2. If something went screwy with the comms link at the last second, the barge could start driving off underneath the landing stage, which would be a bad day all around.
3. The landing deck is big enough - it's simply not worth the risk.

Regarding point #1, I didn't mean to imply any momentum. I was thinking something like this; ten minutes or so before launch, check wind speed an direction (I'm assuming the ASDS has an anemometer). If needed, command the ASDS to move a yard or two downwind. That (a movement of 6 feet or less) wouldn't take long, and it should be at a relative standstill again by launch.

They may well be doing this already.. but I guess we will never know unless someone for SpX pipes up here.

Points 2 and 3 are very valid IMHO. Of course, the only reason to try something like this is if there's a problem that needs fixing. My guess is that for the conditions we saw, there isn't one, but stronger winds might change the equation on that and thus might make trying something along these lines worthwhile. At least, that's my guess as to what they might have in their high winds contingency folder.

The entire barge-landing scenario is a series of engineering compromises.  Stronger winds result in larger seas which would quickly reach the point where a repeat of previous attempts is likely.  Now that they've had a success, my guess is they'll compare the environmental (wind/waves) conditions and telemetry present this time with previous attempts and determine what conditions are "go" and which aren't, so repositioning the barge due to wind conditions alone rapidly becomes unnecessary.

« Last Edit: 04/11/2016 02:02 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.

Online launchwatcher

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #510 on: 04/11/2016 03:59 AM »
Regarding point #1, I didn't mean to imply any momentum. I was thinking something like this; ten minutes or so before launch, check wind speed an direction (I'm assuming the ASDS has an anemometer). If needed, command the ASDS to move a yard or two downwind. That (a movement of 6 feet or less) wouldn't take long, and it should be at a relative standstill again by launch.
seems simpler to send the same wind data to the landing stage and have it adjust its aim to maximize the likelihood that during both the initial touch and any subsequent dancing all four feet are on or above the barge deck.

of course that requires a model (with things like sea state and wind velocity as input) for how the stage dances after landing. 

Offline Req

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #511 on: 04/11/2016 04:02 AM »
Upthread(or maybe on the live thread) somebody brought up the concern of the sheer number of cycles that the structure within the stage will have to endure during this return trip - which raises the obvious question - is any structure that is able to handle the bending structural loads of flight and return more or less impervious to this type, magnitude, frequency and duration of repetitive cycling, full stop?  Or is this something worth digging into?
« Last Edit: 04/11/2016 04:06 AM by Req »

Online Elmar Moelzer

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #512 on: 04/11/2016 05:28 AM »
Hmmmm, maybe they could just make some sort of retractable or unfold- able deck extension to give the stage just a few more yards to land on. Maybe with an inflatable floatation device of sorts to give it a tiny bit of extra stability underneath. Not sure if that would make sense.

Offline Req

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #513 on: 04/11/2016 05:48 AM »
Hmmmm, maybe they could just make some sort of retractable or unfold- able deck extension to give the stage just a few more yards to land on. Maybe with an inflatable floatation device of sorts to give it a tiny bit of extra stability underneath. Not sure if that would make sense.

Let's not forget that any extendable/modular extensions to the deck size will also require something on the barge that is able to transport the stage back to center so that they can then be detached(re-sunk, whatever)/retracted before the trip home.

Edit - On to a totally different topic from earlier in the thread, but since I looked it up - As far as rocking induced by an off-center landing, a falcon 9 S1 dry mass is estimated to be less, but in the same ballpark as the standard maximum load for a single full-size cargo container.  Just for visualization/reference.
« Last Edit: 04/11/2016 06:41 AM by Req »

Offline Hankelow8

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #514 on: 04/11/2016 08:26 AM »
At Elon's pass conference after the CRS-8 launch,he commented that they are expecting to be able to launch every 2/3 weeks.

I think they will have to build an extra drone ship to cover the possibility of a hard landing, especially when attempting the more extreme conditions. The repair times and turnaround to be ready for the next launch would be very difficult to meet, if the next launch was just a couple of weeks way.

The cost of building an extra drone ship to be on standby for such a situation has to be cost effective. One saved Falcon first stage  alone could be worth the cost of a standby drone ship.

Offline JamesH65

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #515 on: 04/11/2016 08:45 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?

Offline Jarnis

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #516 on: 04/11/2016 09:44 AM »

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

Yes, gotta get it closer to center so I win the landing bingo next time. This one was way to the side and totally fouled my chances :(

 :P

Offline miki

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #517 on: 04/11/2016 10:08 AM »
It's interesting to note that on every ASDS landing, landing legs are parallel to the sides of the deck. If they weren't one leg could end up over the edge and the stage would tip over. They thought of everything. Great job...

Offline FutureSpaceTourist

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #518 on: 04/11/2016 10:22 AM »
seems simpler to send the same wind data to the landing stage

But I thought the stage has no mechanism to receive such data. Isn't the range safety termination signal (which I believe is completely separate from the rest of the avionics) the only thing the stage can receive, as the stage is fully autonomous?

Ok so something could be added to the stage but I'm really not sure that's warranted. If any adjustment is needed I think it's simpler to get the ASDS to do it.

Offline The Amazing Catstronaut

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #519 on: 04/11/2016 10:30 AM »
I feel the more we complicate the guidance system the greater the danger of feedback loops. Accuracy has never really been the primary problem with ASDS, it's always been a propulsion related issue, a control surfaces issue or an issue with the landing legs which have caused stages to avoid ending up where we want them. Each time those issues get iterated out (or, at least, we haven't seen them again).

The bounce this time was due to windsheer, not guidance - and hey, they recovered it.
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