SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 2

[PhilW]

That crane has been there for a while now.

And all those Thrustmaster thrusters do is maintain position no roll, pitch, yaw or heave stabilization. That is some serious stuff and I don't think such capability can be just added to barge.

Nice computer graphic. Now try that in the real ocean.

[speedevil]

I think I'm misinterpreting the thrustmasters; I've been assuming that they also operate in a vertical axis, to actively damp wave motion. After seeing some of the comments on this thread, I checked thrustmaster's site, and my read now is that they don't. This renders my prior speculation as to what the upgrade might be (swing-out beams with the thrustmasters mounted at the tips) as utterly absurd.

Unfortunately - yes.
Keeping the barge on station  in 10 knot seas takes a few tons of thrust per thruster.

Keeping it stationary in the vertical axis  takes a few thousand tons of thrust - varying rapidly with the swell.

Just upgrading the thrustmasters so that the platform can stay stationary plus or minus a few meters would do a lot of good things.
Even if you get a crunch again due to excessive Z motion, or it washes off the deck - you've still demonstrated it landing in the middle of the deck.

Random comment.
A 100mm*20mm diameter magnet spaced .5mm from a steel deck will exert a pull of about 200kg.

A 60cm diameter foot could contain enough to hold the stage if it was completely upside-down.

A 40cm diameter foot would add 50% clamping force - 10 tons - to the feet, in addition to the 20 ton weight, and add maybe 80kg to the stage weight.

[meekGee]

That crane has been there for a while now.

And all those Thrustmaster thrusters do is maintain position no roll, pitch, yaw or heave stabilization. That is some serious stuff and I don't think such capability can be just added to barge.

Nice computer graphic. Now try that in the real ocean.

.... and that's why I don't think 3 m "under worst conditions" can be interpreted as some people have done upthread.

[Ohsin]

Well in CRS 5 pre launch presser Hans said 14 ft isn't a problem, ~30 ft is extreme and that carnage video falls in crazy extreme.

[CJ]

I think I'm misinterpreting the thrustmasters; I've been assuming that they also operate in a vertical axis, to actively damp wave motion. After seeing some of the comments on this thread, I checked thrustmaster's site, and my read now is that they don't. This renders my prior speculation as to what the upgrade might be (swing-out beams with the thrustmasters mounted at the tips) as utterly absurd.

Unfortunately - yes.
Keeping the barge on station  in 10 knot seas takes a few tons of thrust per thruster.

Keeping it stationary in the vertical axis  takes a few thousand tons of thrust - varying rapidly with the swell.

Just upgrading the thrustmasters so that the platform can stay stationary plus or minus a few meters would do a lot of good things.
Even if you get a crunch again due to excessive Z motion, or it washes off the deck - you've still demonstrated it landing in the middle of the deck.

Random comment.
A 100mm*20mm diameter magnet spaced .5mm from a steel deck will exert a pull of about 200kg.

A 60cm diameter foot could contain enough to hold the stage if it was completely upside-down.

A 40cm diameter foot would add 50% clamping force - 10 tons - to the feet, in addition to the 20 ton weight, and add maybe 80kg to the stage weight.

It wouldn't take hundreds of tons of thrust in the vertical axis to significantly damp rolling and pitching, but it would take dozens to have much effect. If I'd bothered to look up the thrust capacity of those thrustmasters, I'd have known at once that I had it all wrong.

Interesting on the magnets... that'd help a lot with the moist likely issue, sliding. My own guess for what thay've done is high-friction paint on the deck. I live in snow country, and it's common to see hillside homes with steep driveways use paint mixed with sand as a driveway treatment to improve traction, so that made me wonder if that's what they've done on ASDS. 


 

That crane has been there for a while now.

And all those Thrustmaster thrusters do is maintain position no roll, pitch, yaw or heave stabilization. That is some serious stuff and I don't think such capability can be just added to barge.

True, but, a barge hull form already has an ideal hullform for a form of passive stabilization, which I've long assumed (perhaps wrongly) that ASDS uses. Basically, a barge hullform is a thick rectangle, hollow inside. Thus, if it was about 1/4 flooded, and had anti-slosh baffling to lower the rate of water movement, it'd be a passive stabilization system for both pitch and roll (Sort of akin to the tuned weights placed atop skyscrapers to minimize wind sway). It wouldn't be fully effective, of course, but it'd help. (the detriment would be reduced freeboard).


 

[MP99]

The "success" of the water landing seems to prove that bad weather is no problem for the returning stage.  They just need to nail the ASDS side.

But do we know what the lateral velocity of the stage was when it landed?

I'd think it would have a tilt if it had any lateral velocity, but apparently it didn't :-

Elon Musk (elonmusk):
Rocket soft landed in the ocean within 10m of target & nicely vertical! High probability of good droneship landing in non-stormy weather.

http://twitter.com/elonmusk/status/565659578915115011

Cheers, Martin

[jzjzjzj]

[..]

A 60cm diameter foot could contain enough to hold the stage if it was completely upside-down.

A 40cm diameter foot would add 50% clamping force - 10 tons - to the feet, in addition to the 20 ton weight, and add maybe 80kg to the stage weight.

Wouldn't the magnetic field of four 40cm magnets rip the legs/stage apart upon meeting the deck?

[speedevil]

[..]

A 60cm diameter foot could contain enough to hold the stage if it was completely upside-down.

A 40cm diameter foot would add 50% clamping force - 10 tons - to the feet, in addition to the 20 ton weight, and add maybe 80kg to the stage weight.

Wouldn't the magnetic field of four 40cm magnets rip the legs/stage apart upon meeting the deck?

16 10cm magnets - there would be approximately no attraction until the last 5cm of travel.

[Hankelow8]

I have always thought the welding of the legs to the deck after landing will be fraught with danger.

I guess it will require boarding of the drone ship by space x staff to carry out the job. In anything but a dead calm sea  there will always be a danger of the stage moving, coupled with unused fuel still on board it makes for a very dangerous job.

Elon mentioned a big upgrade to the drone ship. I hope it includes some form of  temporary securing of the rocket without the need for Space x staff to carry this out.  Some form of a drone clamp to secure the rocket before a more secure clamp can be fitted.

[harveyb]

.... and that's why I don't think 3 m "under worst conditions" can be interpreted as some people have done upthread.

Don't be obtuse. You know that any engineering specification of this type (as opposed to simplified PR-speak) will include the probability range and environmental conditions. The real specification is something like "3m CEP (xx probability) in xx wave height sea conditions" with larger and smaller CEP depending on the wave height.

If forecast wave conditions push the CEP beyond margins, then they either scrub the launch or decide not to land, just like they did Wednesday. For all we know, other issues associated with sea conditions (such as risk of toppling a landed stage) may be the limiting factor.

[kevinof]

I think SpaceX made a call on safety grounds - They won't ever (could be wrong) postpone a launch just because they can't land. Certainly not right now - launch is first, landing is second.

Assuming the conditions out at the landing site were bad, the biggest factor would be getting people back on board a deck that was shipping big seas. If conditions were that bad then you can also assume that the stability of the landed stage would also be a problem - I realize it's a low center of gravity but there is still only so much tilting it can take.

Maybe SpaceX made a call that it's hard to keep station, it's probably going to fall over even if it does land and how the hell are we going to get people on board (ship to ship transfer) in bad conditions to help secure it.

[jzjzjzj]

I think SpaceX made a call on safety grounds - They won't ever (could be wrong) postpone a launch just because they can't land. Certainly not right now - launch is first, landing is second.

Assuming the conditions out at the landing site were bad, the biggest factor would be getting people back on board a deck that was shipping big seas. If conditions were that bad then you can also assume that the stability of the landed stage would also be a problem - I realize it's a low center of gravity but there is still only so much tilting it can take.

Maybe SpaceX made a call that it's hard to keep station, it's probably going to fall over even if it does land and how the hell are we going to get people on board (ship to ship transfer) in bad conditions to help secure it.

Judging by the commitment to blow the stage up against ASDS on CRS-5, I'd say even video footage of first stage "rolling" aboard JRTI or over the board would have been acceptable by SpaceX standards.

[JamesH]

I have always thought the welding of the legs to the deck after landing will be fraught with danger.

I guess it will require boarding of the drone ship by space x staff to carry out the job. In anything but a dead calm sea  there will always be a danger of the stage moving, coupled with unused fuel still on board it makes for a very dangerous job.

Elon mentioned a big upgrade to the drone ship. I hope it includes some form of  temporary securing of the rocket without the need for Space x staff to carry this out.  Some form of a drone clamp to secure the rocket before a more secure clamp can be fitted.

Any sort of work done at sea has the same level of danger - moving deck, equipment etc. But the barge is pretty damn big, so even in a half decent swell it would be pretty stable. No need for dead calm.

[Kabloona]

On the subject of when EIII/ASDS will return to port...GO Quest made the trip in ~18 hrs, arriving around noon Wednesday. I had figured EIII/ASDS would take twice as long (5 kts vs. 10 kts for GO Quest), putting ETA sometime this morning.

But now I'm wondering if, due to the heavy seas, perhaps EIII postponed tying onto ASDS until, say, Wednesday morning, when at least they would have had the benefit of full daylight for safer operations, and perhaps calmer seas.

In which case they wouldn't make port until tonight at earliest.

[MP99]

I think SpaceX made a call on safety grounds - They won't ever (could be wrong) postpone a launch just because they can't land. Certainly not right now - launch is first, landing is second.

Today, SpaceX are pricing and selling launches for F9 expendable. I agree with your comment for these jobs.

If/when they sell a cheaper contract for a reused / reusable launch, I would expect it to include clauses allowing them not to schedule when they can't land. It's a reasonable trade-off for the cheaper price (and won't be an issue for RTLS, anyway).

Cheers, Martin

[meekGee]

.... and that's why I don't think 3 m "under worst conditions" can be interpreted as some people have done upthread.

Don't be obtuse. You know that any engineering specification of this type (as opposed to simplified PR-speak) will include the probability range and environmental conditions. The real specification is something like "3m CEP (xx probability) in xx wave height sea conditions" with larger and smaller CEP depending on the wave height.

If forecast wave conditions push the CEP beyond margins, then they either scrub the launch or decide not to land, just like they did Wednesday. For all we know, other issues associated with sea conditions (such as risk of toppling a landed stage) may be the limiting factor.

That's exactly what I said.   Some folks interpreted it to be "barge will always be under 3 m from target".  I'm saying it is an expected value (or similar as you suggest) up to some weather level. 

And is marginal if you need to add to it a second independent variable in the form of the rocket error.

[Rocket Science]

I have always thought the welding of the legs to the deck after landing will be fraught with danger.

I guess it will require boarding of the drone ship by space x staff to carry out the job. In anything but a dead calm sea  there will always be a danger of the stage moving, coupled with unused fuel still on board it makes for a very dangerous job.

Elon mentioned a big upgrade to the drone ship. I hope it includes some form of  temporary securing of the rocket without the need for Space x staff to carry this out.  Some form of a drone clamp to secure the rocket before a more secure clamp can be fitted.

Cable retraction reels that slide on tracks that would allow the 4 cables to move across the deck and side up the legs to the ram mounting points. It basically would work like the end effector on the Canadarm to snare the Falcon. This system would allow the Falcon to be secured no matter how off center it landed. The crew can then move in to weld the shoes on....

[harveyb]

.... and that's why I don't think 3 m "under worst conditions" can be interpreted as some people have done upthread.

Don't be obtuse. You know that any engineering specification of this type (as opposed to simplified PR-speak) will include the probability range and environmental conditions. The real specification is something like "3m CEP (xx probability) in xx wave height sea conditions" with larger and smaller CEP depending on the wave height.

If forecast wave conditions push the CEP beyond margins, then they either scrub the launch or decide not to land, just like they did Wednesday. For all we know, other issues associated with sea conditions (such as risk of toppling a landed stage) may be the limiting factor.

That's exactly what I said.   Some folks interpreted it to be "barge will always be under 3 m from target".  I'm saying it is an expected value (or similar as you suggest) up to some weather level. 

And is marginal if you need to add to it a second independent variable in the form of the rocket error.

So, it's fair to say that IF conditions are within weather limits, barge error will be less than 3m, to some acceptable probability. If not, they won't attempt landing. Which is in line with what "some folks" have been saying.

As far as whether <3m is suitable, that depends on a lot of factors that we don't know anything about. I'm going to make one assumption: when they designed the barge, they considered whether the specified CEP in expected conditions would be suitable and decided that it would be.

[Ohsin]

I have always thought the welding of the legs to the deck after landing will be fraught with danger.

I guess it will require boarding of the drone ship by space x staff to carry out the job. In anything but a dead calm sea  there will always be a danger of the stage moving, coupled with unused fuel still on board it makes for a very dangerous job.

Elon mentioned a big upgrade to the drone ship. I hope it includes some form of  temporary securing of the rocket without the need for Space x staff to carry this out.  Some form of a drone clamp to secure the rocket before a more secure clamp can be fitted.

Cable retraction reels that slide on tracks that would allow the 4 cables to move across the deck and side up the legs to the ram mounting points. It basically would work like the end effector on the Canadarm to snare the Falcon. This system would allow the Falcon to be secured no matter how off center it landed. The crew can then move in to weld the shoes on....

mmmm cables sliding on deck touching loose heavy load.. Have you seen the movie Ghost Ship?

[meekGee]

.... and that's why I don't think 3 m "under worst conditions" can be interpreted as some people have done upthread.

Don't be obtuse. You know that any engineering specification of this type (as opposed to simplified PR-speak) will include the probability range and environmental conditions. The real specification is something like "3m CEP (xx probability) in xx wave height sea conditions" with larger and smaller CEP depending on the wave height.

If forecast wave conditions push the CEP beyond margins, then they either scrub the launch or decide not to land, just like they did Wednesday. For all we know, other issues associated with sea conditions (such as risk of toppling a landed stage) may be the limiting factor.

That's exactly what I said.   Some folks interpreted it to be "barge will always be under 3 m from target".  I'm saying it is an expected value (or similar as you suggest) up to some weather level. 

And is marginal if you need to add to it a second independent variable in the form of the rocket error.

So, it's fair to say that IF conditions are within weather limits, barge error will be less than 3m, to some acceptable probability. If not, they won't attempt landing. Which is in line with what "some folks" have been saying.

As far as whether <3m is suitable, that depends on a lot of factors that we don't know anything about. I'm going to make one assumption: when they designed the barge, they considered whether the specified CEP in expected conditions would be suitable and decided that it would be.

Not my interpretation.

I think under some conditions, the barge has a 3m expected error.  If both barge and rocket are independently aimed at a common reference frame (GPS), you also need to add a second expected error (of the rocket), and then any mismatch in how the two platform perceive the common reference frame.

Those three random variables add up, let's say by RMS.

I argued earlier that since the absolute maximum landing error allowed is 16 m, and I proposed that by design you want to be 3-sigma safe (if you're going to attempts 10s or 100s of landings) then your average error should be more like 5 m.

I will relax this, since the error can occur in any direction, and 16 m was the limit only in the width direction.  Let's make it 16*1.4, or ~24 m.  So still, you want an expected error of 8 m.

So are we there?  I said it was marginal.

There's the barge error (3 m), the rocket error (which we don't know, but was 10 m under windy conditions), and the GPS-toGPS error (1-2 m?).  It seems marginal.

OTOH,  if the rocket is homing on the barge, you can remove both the first and last terms.  This will make it easier.

That's the extent of what I'm saying.

I don't know how SpaceX implemented it. Maybe they went for simple and accept "marginal" is ok, at least at this stage.  Maybe not.

I'll add that I'm almost 100% sure they have vertical radar, for altitude and for Doppler measurement of rate of descent - just because GPS sucks at that, the errors are larger, and the allowed errors are much smaller.

And then, since you've already got some measurements going on, it's really tempting to put a few retroreflectors / transponders on the barge, and have the stage know exactly where the it is relative to the true reference frame of the barge.