Author Topic: SpaceX 'Octagrabber' (Rocket Grabbing Robot) - Updates and Discussion  (Read 326166 times)

Offline AncientU

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It seems that personnel risk is the driver here.  Even approaching (let alone boarding) the barge with a wandering stage is quite risky.  We do extraordinary things to keep astros safe... deck crew are people, too.  Also, cannot come into port with an unsecured stage -- what do you do if it is unsafe to board, but also unsafe to enter port because stage isn't secure?

Speed is nice, too, but secondary.
« Last Edit: 03/14/2017 05:16 pm by AncientU »
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Offline meekGee

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It seems that personnel risk is the driver here.  Even approaching (let alone boarding) the barge with a wandering stage is quite risky.  We do extraordinary things to keep astros safe... deck crew are people, too.  Also, cannot come into port with an unsecured stage -- what do you do if it is unsafe to board, but also unsafe to enter port because stage isn't secure?

Speed is nice, too, but secondary.


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Offline FutureSpaceTourist

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I can imagine and see the benefit of automated anchoring after landing. I'm not so sure about using this to move the stage. Unless there's a lot of ballast in the 'roomba' isn't the centre of gravity going to be higher than it? Also the stage would have to be lifted a bit to give leg clearance. Must be a risk of toppling?

Offline Lar

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I can imagine and see the benefit of automated anchoring after landing. I'm not so sure about using this to move the stage. Unless there's a lot of ballast in the 'roomba' isn't the centre of gravity going to be higher than it? Also the stage would have to be lifted a bit to give leg clearance. Must be a risk of toppling?

My thinking (which I think is shared by some) is that the ASDS is so massive that it doesn't matter if the stage is exactly in the very center. As long as it doesn't slide off, it can be anywhere... the barge that became ASDS has many ballast compartments and if it's a bit off dead level, that can be fixed by taking on or releasing ballast, which can be done without humans aboard. So the device just has to hold the stage from not moving. That's it. It doesn't, at least at sea, have to move the stage. Safing the stage is another matter but even if the device doesn't do that, once the stage is stationary, it's a lot safer for humans to board and do their thing.

"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 JamesH65

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I can imagine and see the benefit of automated anchoring after landing. I'm not so sure about using this to move the stage. Unless there's a lot of ballast in the 'roomba' isn't the centre of gravity going to be higher than it? Also the stage would have to be lifted a bit to give leg clearance. Must be a risk of toppling?

My thinking (which I think is shared by some) is that the ASDS is so massive that it doesn't matter if the stage is exactly in the very center. As long as it doesn't slide off, it can be anywhere... the barge that became ASDS has many ballast compartments and if it's a bit off dead level, that can be fixed by taking on or releasing ballast, which can be done without humans aboard. So the device just has to hold the stage from not moving. That's it. It doesn't, at least at sea, have to move the stage. Safing the stage is another matter but even if the device doesn't do that, once the stage is stationary, it's a lot safer for humans to board and do their thing.

The stage is so light in comparison to a barge's normal cargo, I doubt they even need to shift water ballast.

Offline CameronD

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I can imagine and see the benefit of automated anchoring after landing. I'm not so sure about using this to move the stage. Unless there's a lot of ballast in the 'roomba' isn't the centre of gravity going to be higher than it? Also the stage would have to be lifted a bit to give leg clearance. Must be a risk of toppling?

My thinking (which I think is shared by some) is that the ASDS is so massive that it doesn't matter if the stage is exactly in the very center. As long as it doesn't slide off, it can be anywhere... the barge that became ASDS has many ballast compartments and if it's a bit off dead level, that can be fixed by taking on or releasing ballast, which can be done without humans aboard. So the device just has to hold the stage from not moving. That's it. It doesn't, at least at sea, have to move the stage. Safing the stage is another matter but even if the device doesn't do that, once the stage is stationary, it's a lot safer for humans to board and do their thing.

The stage is so light in comparison to a barge's normal cargo, I doubt they even need to shift water ballast.

If the stage is slightly off-center, they still might.  Trim is just as important for ships & barges as it is for aeroplanes, since excess drag means increased fuel consumption during the tow.

..but it seems recent landings have been close enough to the bulls-eye to not warrant any obvious changes in trim, so it's a moot point.
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 Norm38

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Interesting.  So this is essentially a big hunk of steel that bolts itself onto the bottom of the stage and weighs it down?
Alternatively they could drive those jacks with a feedback loop and try to actively counter the rocking of the ship, to keep the stage perfectly upright as the deck tilts.  But that might be a lot more complexity than they need, given that the stages have made it back to port without this system so far.

Offline Lar

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The stage is so light in comparison to a barge's normal cargo, I doubt they even need to shift water ballast.
My expectation is that the Roomba is 10x the weight of the empty stage.[1]  So that's enough that maybe a bit of trim adjustment is warranted. Just pump water from one tank to another and Bob's your uncle.

Interesting.  So this is essentially a big hunk of steel that bolts itself onto the bottom of the stage and weighs it down?
Alternatively they could drive those jacks with a feedback loop and try to actively counter the rocking of the ship, to keep the stage perfectly upright as the deck tilts.  But that might be a lot more complexity than they need, given that the stages have made it back to port without this system so far.

The stage has to survive maxQ and some serious buffeting on the way up, so if it can't take a bit of tilting back and forth I'd be surprised. It doesn't have to be held perfectly upright, just needs to not slide around and walk right off the edge of the barge in heavy seas...

If the Roomba stays "live" in the unlikely event the stage is still moving towards the edge a bit in really heavy chop, it could actively move it back to the center, even....
1 - That's just a WAG, not based on anything but if I was doing it, that's what I'd do. I don't think I'd do magnets or welders or etc, that's too complicated, I'd just give the Roomba rubber treads for grip and a LOT of ballast. Yes, it might do the bolt welding thing, that's true, but I bet against it.
« Last Edit: 03/16/2017 06:35 pm by Lar »
"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

Online matthewkantar

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If they have another stage with unequal length legs due to an asymmetric landing, I will be impressed if the robot can go out and grab it while it is tilting back and forth. I would think a wedge-bot would have to go out first and stop the rocking. Sugar packs and napkins won't do it.

Matthew

Offline leetdan

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The simple solution would be to add hardpoints on the deck.  The robot would first secure and lift the stage, then move to the hardpoints and latch itself down.  It could then be left safely inert for the return trip.

Offline CameronD

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The simple solution would be to add hardpoints on the deck.  The robot would first secure and lift the stage, then move to the hardpoints and latch itself down.  It could then be left safely inert for the return trip.

Hard-points might have some merit, but nothing and no-one would ever try to "lift the stage" at sea - at least not with anything that couldn't provide at least the same amount of lateral support/stability as the splayed legs currently do.  The next wave would topple both the stage and the robot into the ocean.

 
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 Lee Jay

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Based on the sketches, the thing is going to prevent lateral movement by putting hydraulic or pneumatic rams in bending while at full extension?

That would be, ummm..., shocking.

Offline CameronD

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The stage is so light in comparison to a barge's normal cargo, I doubt they even need to shift water ballast.
My expectation is that the Roomba is 10x the weight of the empty stage.[1]  So that's enough that maybe a bit of trim adjustment is warranted. Just pump water from one tank to another and Bob's your uncle.

Just keep in mind that trimming the barge (by pumping water from one tank to the other) is a manual procedure that would be carried out by the crew aboard.  They physically have to:
1. Open the access ports on each of the two tanks (there''' be a hole in the deck for the purpose)
2. Connect a hose to the cam-lock fittings (or lower it into the well of one tank, and through the hole of the other if there isn't one)
3. Connect the hoses to their ballast (diaphragm) pump - which we've seen then using before
4. Turn it on an monitor progress, stopping when they think it's about right.

It is not normally automated - nor is it likely to be.  Pumping seawater around a barge's ballast compartments in the open ocean is a pretty big deal.  Think rust, corrosion, clogged filters x 24!
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 meekGee

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Based on the sketches, the thing is going to prevent lateral movement by putting hydraulic or pneumatic rams in bending while at full extension?

That would be, ummm..., shocking.

I don't think so.  I think the Roomba is strictly a "pull down" device.  It increases the force on the four legs, compressing the suspension, and thus also eliminates rocking.

It also lowers the c.g. even lower, further reducing the dynamic effects cause by rocking ("walking")

It can be made to weigh as much as the entire stage if they so desire (each cm of thickness, in steel, will weigh about a ton) though I don't think you need THAT much extra mass.

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Online matthewkantar

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I think the main purpose of the prospective gizmo will be to take load off of the legs, not put more onto them.

Matthew

Offline CameronD

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I think the main purpose of the prospective gizmo will be to take load off of the legs, not put more onto them.

No, the main purpose is to stop the stage supposedly/potentially sliding on the deck and the subsequent danger to the barge crew.

"Repel boarders!.."
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 speedevil

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I was idly doodling a device sort-of-like the above, but with wheels on really quite large electric motors, steering very fast indeed, and magnets to hold the whole thing down, so it could acellerate at ~1g, with a peak of perhaps 20m/s.


Online matthewkantar

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I think the main purpose of the prospective gizmo will be to take load off of the legs, not put more onto them.

No, the main purpose is to stop the stage supposedly/potentially sliding on the deck and the subsequent danger to the barge crew.

"Repel boarders!.."

 The actual stage securing SpaceX has done it the past involves putting jacks under the stage and then pulling the stage down onto the jacks from the deck.  Task of securing the stage would be much easier if they just snugged it down to the deck, they don't seem to want that sort of load on the legs/tank walls for very long.

Matthew

Offline Lee Jay

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Based on the sketches, the thing is going to prevent lateral movement by putting hydraulic or pneumatic rams in bending while at full extension?

That would be, ummm..., shocking.

I don't think so.  I think the Roomba is strictly a "pull down" device.  It increases the force on the four legs, compressing the suspension, ...

That would be even worse.

Offline meekGee

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Based on the sketches, the thing is going to prevent lateral movement by putting hydraulic or pneumatic rams in bending while at full extension?

That would be, ummm..., shocking.

I don't think so.  I think the Roomba is strictly a "pull down" device.  It increases the force on the four legs, compressing the suspension, ...

That would be even worse.
Why?

Stage walking and stage sliding is a result of too little reaction force on the legs.

If a stage is doing that, you run under it and pull it down to stop it.  Not push up, since that will reduce the force in the legs.

The pulldown chains did the same trick. The jacks the stage was pulled into didn't lift the stage, they just limited how far down the stage was winched down.

The robot does both things with a single set of jacks.
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