Author Topic: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3  (Read 602363 times)

Online meekGee

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #480 on: 04/09/2016 04:04 PM »
Quote
Also, I'm pretty sure we saw the shoes before, somewhere. Buried in one of the old ASDS threads, no doubt.


Or ASDS is maintaining predetermined position as well as orientation? What factor decides what orientation ASDS should maintain? Is it something related to waves like aligning to have them along length than width.
That's a good question. And a complicated one. Boats sitting still act nothing like boats moving. Bow into the waves on a still barge could make the thing pitch more than side to the waves. And, do they worry more about pitching and rolling or sliding sideways? Those Thrustmasters aren't instantaneous.
How would a boat know if it is "standing still", GPS wise...

I think the difference is between "free drifting boats" and "thrusting boats" (whether stationary or not", and also "moored boats".
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Online meekGee

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #481 on: 04/09/2016 04:06 PM »
Word is it may come back to JAX, not to the cape

I can see they might have wanted to get close to shore for overnight and/or ASAP, to get out of the higher winds offshore, and JAXPORT would have been closest to their position at the landing site, but why go to the trouble of offloading and trucking the stage from there when it's already loaded for transport on the drone ship. I think when when the stage comes in close to it's final resting place, it will be on the drone ship, not a truck.

They have experience of trucking the stage for thousand of km, they never experienced vertical barge transport.
That said, I don't put a dime on a JAX stop.
Deleted...   New "triumphant barge return" thread!
« Last Edit: 04/09/2016 04:19 PM by meekGee »
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Offline rpapo

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #482 on: 04/09/2016 04:07 PM »
Word is it may come back to JAX, not to the cape

I can see they might have wanted to get close to shore for overnight and/or ASAP, to get out of the higher winds offshore, and JAXPORT would have been closest to their position at the landing site, but why go to the trouble of offloading and trucking the stage from there when it's already loaded for transport on the drone ship. I think when when the stage comes in close to it's final resting place, it will be on the drone ship, not a truck.

They have experience of trucking the stage for thousand of km, they never experienced vertical barge transport.
That said, I don't put a dime on a JAX stop.
They have to go to JaX first.  There is an outstanding $20 bet between Elon and someone about whether the stage will fit under that bridge.
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Offline apirie98

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #483 on: 04/09/2016 04:23 PM »
The barge sways A LOT, definitely more than 10 degrees, but the stage seems pretty stable, no signs of movement, which is impressive. Looked close to 20 degrees.

I noticed that the RCS thrusters are still firing after the stage is landed. I'm wondering if that is actually helping to keep the stage upright, or if it's not doing anything much to help but it's just the control system firing as the stage tilts, because that's what the control system does.

If it were keeping the stage upright and stable, how long would the RCS system be able to sustain this (i.e. in strong winds/waves)?
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Offline gospacex

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #484 on: 04/09/2016 04:42 PM »
A non-trivially off-center landing. I think we will see somewhat bigger barges in the future.

Offline OxCartMark

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #485 on: 04/09/2016 04:44 PM »
I noticed that the RCS thrusters are still firing after the stage is landed
They are firing N2 in all four directions simultaneously with a net thrust of zero just to get rid of the potential energy in the pressure vessels as part of safing the stage.  This happened on their momentarily successful previous landing as well.  The time that they landed with way too much sideways motion it was different, there was just one thruster at the top fighting to keep it upright.

If it were keeping the stage upright and stable, how long would the RCS system be able to sustain this (i.e. in strong winds/waves)?
The thrust isn't that significant.  You'd be better off having a pelican land on one of the legs.  The duration would be 4x the duration you see in this video.
« Last Edit: 04/09/2016 04:47 PM by OxCartMark »

Offline cambrianera

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #486 on: 04/09/2016 05:12 PM »
A non-trivially off-center landing. I think we will see somewhat bigger barges in the future.
Disagree...
I think we will see bullseye landing in the future!
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Offline Ronsmytheiii

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #487 on: 04/09/2016 05:14 PM »
Latest Terraserver update of Port Canaveral Shows "Of Course I still Love you" in port:

http://bit.ly/1N1dzeb
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Offline 411rocket

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #488 on: 04/09/2016 05:22 PM »
Latest Terraserver update of Port Canaveral Shows "Of Course I still Love you" in port:

http://bit.ly/1N1dzeb

It also looks like, there is a hole in the deck too. We know that hole, has since been repaired.

Offline georgegassaway

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #489 on: 04/09/2016 05:32 PM »
FWIW, here is an image I modified to show an estimated CG location of the F9 core after landing. I’m not claiming this is the real CG, it may well be lower than this, just that from old discussions on the forum that it would be pretty low down with nearly all of the fuel gone (And thruster gases vented shortly after landing).  So, it is easier to visualize how hard it would be for it to fall over, once you see how low the CG is relative to the legspan.



The off’center landing, I do not think is random, I think it is directly related to the high wind and the fact that it had to stop tilting to fight the wind to be level at touchdown, so it drifted off-center in that brief time.  Here is an edited version of a post I made elsewhere:

The tilt seen early as it was coming down, I thought at first it had been off course horizontally and was trying to maneuver a bit laterally during the descent to make up for an error. So I was relieved (and super excited) to see it landed safely.

Turned out that it was coming down pretty much vertically.... but it was VERY windy. Musk said 50 mph, not sure of that though SpaceX had said it could land in winds as high as 50 mph. Anyway, the tilt was needed to fly it "into the wind", so the net descent path seemed pretty vertical (If the wind was say 40 mph, then it would need to tilt to fly laterally 40 mph into the wind so as not to drift downwind).

But for landing it needs to be level, the legs are not designed for one to land first and for one leg to support the landing loads of the whole booster (never mind rocking moments that would be generated as a result of that).

So, it had to straighten up at the last moment to land level. And in that time, the wind started to move it downwind. The off-center location seems to be pretty much aligned with the wind direction, so it may have bene dead center until it leveled off for touchdown. Also, the video shows how, as it is landing, the steam (water sprayed on deck) from the exhaust moves quickly from right to left, showing how fast the wind was and the direction that matches the tilt direction the core was doing to fight that wind during descent.

IIRC, the Orbcomm-2 landing in December (RTLS back to the Cape) also was "downwind" of the center of the circle. Was not as windy, was not off by much, but notably the location was downwind (Later there was a great fixed near-pad camera that showed the landing closeup, where it was visibly obvious it was descending vertically, then as it straightened up 2-3 seconds before touchdown it drifted horizontally a bit).

Maybe sometime they will tweak the landing software to "lead" the targeted landing spot to be a few feet upwind of center, so the descent will be "over" that upwind spot and try to land a few feet upwind, but when it levels out to point vertically, the wind will push it closer to the center. Of course if it really was 50 mph wind.... then it works without the need for doing that. But inevitably there'd be some day with even higher wind, where such a landing software tweak might allow it to land safely without drifting too far and put a leg over the side, leading to it falling overboard.

An update I’ll add to the last paragraph, is that on rematching a HD view of the landing, it looks like the core might have been a little bit upwind of center as it came down. So possibly the landing software DID “lead” some of the landing to be somewhat upwind of center. Of course with winds that high, there would likely be gusts, including gusts at different altitudes.  As well, the issue of vertical Wind Gradient, wind velocity is lower closer to the surface.
https://en.wikipedia.org/wiki/Wind_gradient

- George Gassaway
« Last Edit: 04/09/2016 05:46 PM by georgegassaway »

Offline alang

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #490 on: 04/09/2016 05:42 PM »
Could a mechanical engineer here say if that skid might be a good thing, to reduce the bending stress on the stage?

Offline OxCartMark

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #491 on: 04/09/2016 05:54 PM »
...

Yes!  Exactly as you said from beginning to end.

Offline cambrianera

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #492 on: 04/09/2016 05:59 PM »
That CG is too low.
Right position is about 12-13 m high.
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Offline StuffOfInterest

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #493 on: 04/09/2016 06:01 PM »
Maybe sometime they will tweak the landing software to "lead" the targeted landing spot to be a few feet upwind of center, so the descent will be "over" that upwind spot and try to land a few feet upwind, but when it levels out to point vertically, the wind will push it closer to the center. Of course if it really was 50 mph wind.... then it works without the need for doing that. But inevitably there'd be some day with even higher wind, where such a landing software tweak might allow it to land safely without drifting too far and put a leg over the side, leading to it falling overboard.

Something I put in the mission discussion thread, rather than changing the rocket landing target why not just shift the barge a couple meters downwind a few minutes before landing?   The rocket isn't going to change in real time but the barge can do a readjustment and the wind measurement tools are right there.

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #494 on: 04/09/2016 06:02 PM »
A non-trivially off-center landing. I think we will see somewhat bigger barges in the future.
Maybe they will make bigger barges, but I don't think that is the "next step."

It touched down pretty darn close to center in a stiff wind with 50 mph gusts (it looks like a couple of meters off to me, max).  Then it "bounces" a couple times and scoots downwind. There may be ways to mitigate the "bounce."  There are definitely ways for the rocket to programmatically adjust it's target point to adjust for "fighting" the wind.  It can estimate how far it will drift in the last moments when it has to "get vertical" for the landing.

I'm not saying it "easy", but refining landing software and dampening the bounce is where to start, IMO.
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Offline mme

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #495 on: 04/09/2016 06:17 PM »
Maybe sometime they will tweak the landing software to "lead" the targeted landing spot to be a few feet upwind of center, so the descent will be "over" that upwind spot and try to land a few feet upwind, but when it levels out to point vertically, the wind will push it closer to the center. Of course if it really was 50 mph wind.... then it works without the need for doing that. But inevitably there'd be some day with even higher wind, where such a landing software tweak might allow it to land safely without drifting too far and put a leg over the side, leading to it falling overboard.

Something I put in the mission discussion thread, rather than changing the rocket landing target why not just shift the barge a couple meters downwind a few minutes before landing?   The rocket isn't going to change in real time but the barge can do a readjustment and the wind measurement tools are right there.
My only concern is that the amount of drift for the touch down point is related to when the rocket "goes vertical" for the landing.  So changes to the flight software/timing would require changes to the barge offset calculation.  Possible, but my gut is to let the rocket figure out how to hit the target.  It doesn't know the airspeed directly, but it has a good idea what the wind speed is based on how much it has to gimbal and "lean in" to the wind.
Space is not Highlander.  There can, and will, be more than one.

Offline CJ

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #496 on: 04/10/2016 04:41 AM »
From the Return of OCISLY thread;

We saw the support ship approaching the ASDS soon after the landing.
You are the second person that I've heard say that.  Perhaps I need to consider that its true.  What did you see to suggest that (what did I miss?)?

I know saw a smudge/water drop on the on-board view that I briefly thought looked like a ship. Can someone provide a picture of anything more definitive?

Do you mean this shot?

It started with a photo Ohsin attached at 4:58pm yesterday and then I embedded it, again below. There's a hazy outline or two and a light in the background to the right, but clearer in this shot than in Bargemanos'.



Cindy, I have utterly no clue what that light is (though if anyone has any ideas, please enlighten) but those things on what looks like the horizon that look like ships, aren.t. IMHO, the forward blast wall top is in line with the horizon, so in that area you're looking over the top of the blast wall, and the things we see there are IMHO parts of OCISLY; the generator exhaust stacks, and, I think, the satcom dome.




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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #497 on: 04/10/2016 04:53 AM »
We saw the support ship approaching the ASDS soon after the landing.
Cindy, I have utterly no clue what that light is (though if anyone has any ideas, please enlighten) but those things on what looks like the horizon that look like ships, aren.t. IMHO, the forward blast wall top is in line with the horizon, so in that area you're looking over the top of the blast wall, and the things we see there are IMHO parts of OCISLY; the generator exhaust stacks, and, I think, the satcom dome.

Gosh I'm sorry.  CJ is obviously correct.  My statement was wrong.
But it was a minor point.  The ship is somewhere maybe an hour or two away. 
The basic conclusion remains.  The first stage was probably secured within hours of landing.
What kind of wastrels would dump a perfectly good booster in the ocean after just one use?

Offline CJ

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #498 on: 04/10/2016 05:32 AM »

Cindy, I have utterly no clue what that light is (though if anyone has any ideas, please enlighten) but those things on what looks like the horizon that look like ships, aren.t. IMHO, the forward blast wall top is in line with the horizon, so in that area you're looking over the top of the blast wall, and the things we see there are IMHO parts of OCISLY; the generator exhaust stacks, and, I think, the satcom dome.

Gosh I'm sorry.  CJ is obviously correct.  My statement was wrong.
But it was a minor point.  The ship is somewhere maybe an hour or two away. 
The basic conclusion remains.  The first stage was probably secured within hours of landing.

IMHO, you are quite correct that the ships are somewhere fairly close (an hour or less IMHO). Five miles (about half an hour at ten knots) should be enough - it's enough for RTLS (It's about 6 miles from LC-13 to the restaurants and cruise terminals at Port Canaveral).

 

Offline cscott

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Re: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 3
« Reply #499 on: 04/10/2016 07:49 AM »
A non-trivially off-center landing. I think we will see somewhat bigger barges in the future.
I think the barge will stay the same size and the landings will improve.

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.
« Last Edit: 04/10/2016 07:52 AM by cscott »

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