Author Topic: SpaceX 'Octagrabber' (Rocket Grabbing Robot) - Updates and Discussion  (Read 325898 times)

Offline Comga

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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.

Why?  Because they want to optimize the legs for landing, not for standing during transit.
They are extended by gas pressure, then locked in place.
They don't keep enough gas for the trip back to port, and they can't or don't want to rely on the locking mechanisms, so they have to remove the weight from the legs. 
Now it is a manual operation at sea, with significant risks. 
So they build a machine to remove the weight from the legs and hold the rocket to the deck, either brute force with mass or with some way to attach to the deck.
What kind of wastrels would dump a perfectly good booster in the ocean after just one use?

Online 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.

Why?  Because they want to optimize the legs for landing, not for standing during transit.
They are extended by gas pressure, then locked in place.
They don't keep enough gas for the trip back to port, and they can't or don't want to rely on the locking mechanisms, so they have to remove the weight from the legs. 
Now it is a manual operation at sea, with significant risks. 
So they build a machine to remove the weight from the legs and hold the rocket to the deck, either brute force with mass or with some way to attach to the deck.
The legs are built to carry the landing impact, so are strong enough.

If a leg didn't lock, those tiny jacks and stands would never hold the overturning moment during wave action.  They'd buckle out of the way in a second.

If a leg isn't locked, they need to pump it up actively to make up for any gas lost.

Holding it down from inside a 3.5 m base won't do it.
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Offline Jim

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Holding it down from inside a 3.5 m base won't do it.

Then how does it stand on the launch pad?  Especially during a static firing.

BTW, those jack stands aren't "tiny"
« Last Edit: 03/17/2017 03:17 pm by Jim »

Online meekGee

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Holding it down from inside a 3.5 m base won't do it.

Then how does it stand on the launch pad?  Especially during a static firing.
Because the launch pad is anchored to th ground.

The robot, from what we hear, is mobile.

If we hear about stud welding, and lots of it (more than would have been required at the feet) then maybe.

But holding a rocket by the thrust structure, in waves, would required a very strong torque-carrying connection to the deck.
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Offline docmordrid

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Or, big electromagnets powered by an onboard generator & cables to match.
DM

Offline AAPSkylab

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Holding it down from inside a 3.5 m base won't do it.

Then how does it stand on the launch pad?  Especially during a static firing.
Because the launch pad is anchored to th ground.

The robot, from what we hear, is mobile.

If we hear about stud welding, and lots of it (more than would have been required at the feet) then maybe.

But holding a rocket by the thrust structure, in waves, would required a very strong torque-carrying connection to the deck.

What if the Roomba had extendable outrigger feet?  Many types of cranes, lifts, construction equipment, and even firetrucks have these.  It is very much off the shelf equipment.

Yes, I know this is added complexity, but it does mean that the main body and its attachment points do not need to have a large footprint.

Online meekGee

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Or, big electromagnets powered by an onboard generator & cables to match.
Right, and if we hear about any of that, we can rely on it.  Right now, it's made up stuff.

All we know is that (most likely) there is a single large robot that drives under and connects to the thrust frame.

Since it doesn't have a large footprint, it needs to pull down.

If it had a way to secure to the deck, reliably, with enough strength to mimic a ground mount (more, really, because of waves and winds) then yes, it can lift up.
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Online meekGee

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Holding it down from inside a 3.5 m base won't do it.

Then how does it stand on the launch pad?  Especially during a static firing.
Because the launch pad is anchored to th ground.

The robot, from what we hear, is mobile.

If we hear about stud welding, and lots of it (more than would have been required at the feet) then maybe.

But holding a rocket by the thrust structure, in waves, would required a very strong torque-carrying connection to the deck.

What if the Roomba had extendable outrigger feet?  Many types of cranes, lifts, construction equipment, and even firetrucks have these.  It is very much off the shelf equipment.

Yes, I know this is added complexity, but it does mean that the main body and its attachment points do not need to have a large footprint.
How wide?  Regular outriggers add a bit of width, but mostly take out the suspension and tires.  (And conform to the ground)

 
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Online philw1776

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Why does the roomba need to pull down?  It needs to attach securely to the thrust frame and use its mass to increase the normal force static friction so the stage doesn't walk.  No need to add many tons of force stress to the legs.
FULL SEND!!!!

Online meekGee

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Why does the roomba need to pull down?  It needs to attach securely to the thrust frame and use its mass to increase the normal force static friction so the stage doesn't walk.  No need to add many tons of force stress to the legs.
It might, but then the up arms will be working in bending.

The main problem IMO was stage walking, not sliding.

During to dynamic tilting, the normal force on one or two legs would decrease, and they would move a bit. Then the tilt would go the other way, and they'd move.

Cinching it down solves everything, and I don't think the legs are anywhere near their load limits.

If the legs are both unlocked and losing pressure, the only remedy is to pump them up.  Might as well suppose they installed the piping to allow the robot to do that.
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Offline AAPSkylab

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Holding it down from inside a 3.5 m base won't do it.

Then how does it stand on the launch pad?  Especially during a static firing.
Because the launch pad is anchored to th ground.

The robot, from what we hear, is mobile.

If we hear about stud welding, and lots of it (more than would have been required at the feet) then maybe.

But holding a rocket by the thrust structure, in waves, would required a very strong torque-carrying connection to the deck.

What if the Roomba had extendable outrigger feet?  Many types of cranes, lifts, construction equipment, and even firetrucks have these.  It is very much off the shelf equipment.

Yes, I know this is added complexity, but it does mean that the main body and its attachment points do not need to have a large footprint.
How wide?  Regular outriggers add a bit of width, but mostly take out the suspension and tires.  (And conform to the ground)

Is it always to take out suspension and tires?  Sometimes outriggers are much larger than base otherwise can be.

Can Stud welders be on ends of outriggers?


Online envy887

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Why does the roomba need to pull down?  It needs to attach securely to the thrust frame and use its mass to increase the normal force static friction so the stage doesn't walk.  No need to add many tons of force stress to the legs.
It might, but then the up arms will be working in bending.

The main problem IMO was stage walking, not sliding.

During to dynamic tilting, the normal force on one or two legs would decrease, and they would move a bit. Then the tilt would go the other way, and they'd move.

Cinching it down solves everything, and I don't think the legs are anywhere near their load limits.

If the legs are both unlocked and losing pressure, the only remedy is to pump them up.  Might as well suppose they installed the piping to allow the robot to do that.

Wasn't the walking only an issue on the stage that had one short leg? If the 4 feet are in plane, that shouldn't be a problem. If they aren't in plane, then adding mass under the current CG isn't going to help - the mass needs to be offset to keep the stage from rocking from one 3-point stance to the other.

Offline matthewkantar

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The legs are built to carry the landing impact, so are strong enough.

If a leg didn't lock, those tiny jacks and stands would never hold the overturning moment during wave action.  They'd buckle out of the way in a second.

If a leg isn't locked, they need to pump it up actively to make up for any gas lost.

Holding it down from inside a 3.5 m base won't do it.

The legs are supplied with a crush core to protect the stage from damage upon landing. Once the cushion is used in a hard landing, leg loads go directly into the side of the stage/tank. In waves, with even a little asymmetry in the landing gear, this will hammer the stage for a few days before unloading.

The legs, with the crush core intact, are designed to take one impact, after that any loads on the air frame where it is not strongest are undesirable. SpaceX would not employ jacks if they deemed the stage tough enough to just ratchet it down to the deck.

Dragging those aircraft jacks around on a rolling deck is no picnic, why would they be doing that if they don't need to?

Matthew

Offline chrisking0997

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envy kinda beat me to it...not sure how this pull down thought is affected if one or more legs had a hard landing and used some of the crush core.  Also not sure if pulling down on the stage would actually cause some of the crush core to be used?
Tried to tell you, we did.  Listen, you did not.  Now, screwed we all are.

Online meekGee

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The legs are built to carry the landing impact, so are strong enough.

If a leg didn't lock, those tiny jacks and stands would never hold the overturning moment during wave action.  They'd buckle out of the way in a second.

If a leg isn't locked, they need to pump it up actively to make up for any gas lost.

Holding it down from inside a 3.5 m base won't do it.

The legs are supplied with a crush core to protect the stage from damage upon landing. Once the cushion is used in a hard landing, leg loads go directly into the side of the stage/tank. In waves, with even a little asymmetry in the landing gear, this will hammer the stage for a few days before unloading.

The legs, with the crush core intact, are designed to take one impact, after that any loads on the air frame where it is not strongest are undesirable. SpaceX would not employ jacks if they deemed the stage tough enough to just ratchet it down to the deck.

Dragging those aircraft jacks around on a rolling deck is no picnic, why would they be doing that if they don't need to?

Matthew
Forget the crush core for a second.

On regular landings, there is some impact, and one leg always touches first.  So the legs, with suspension, can take more than the weight of the rocket.

What you're not explaining is how an "up lifting" robot is able to hold the combined rocket-robot structure from tipping over.

If we find out about a serious weld-down arrangement, then ok.  But we haven't yet, even though some details are out.

That's all there is to it really.

Unless you somehow anchor to the deck, lifting up makes the stage less stable.

We also absolutely know that stage walking is a real issue, but have never heard of leg unlocking and slow leak being an issue... except for the one that tipped over, and the robot would not have helped there.
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Online 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.
Why?

Because the legs need to be light-weight since they fly, and designing them to take enough compression load for friction to hold it in place under all conditions (think along the lines of quadrupling the stage's weight) is going to make them heavier.

Offline TripD

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Why does this device need to provide either up or down force?  Seems to me that if it merely snugs up the slack, it then serves to impede any up or down motion.  By avoiding said motion, the walking and any subsequent weakening of the legs would be prevented.

Online Lee Jay

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Unless you somehow anchor to the deck, lifting up makes the stage less stable.

No, the base device could be heavier than the stage, and it could have a higher frictional coefficient with the barge deck (i.e. rubber).  If properly attached to the stage, it could therefore add an enormous amount of lateral resistance to movement that the stage needs to keep from moving around.  It doesn't need over-turning stability, but if lifted the device would have to provide that as well.  An easy way to do that would be the way the stage does it - with outriggers (called legs on the stage).

Crushing the legs down against the deck sounds like the worst of all worlds to me - even worse than putting rams in bending (which is a total no-no).

Online Lee Jay

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Why does this device need to provide either up or down force?  Seems to me that if it merely snugs up the slack, it then serves to impede any up or down motion.  By avoiding said motion, the walking and any subsequent weakening of the legs would be prevented.

It doesn't need up or down force, but it does need to provide resistance to shear (lateral force) and that requires more than just avoiding up and down motion.

Offline JamesH65

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Unless you somehow anchor to the deck, lifting up makes the stage less stable.

No, the base device could be heavier than the stage, and it could have a higher frictional coefficient with the barge deck (i.e. rubber).  If properly attached to the stage, it could therefore add an enormous amount of lateral resistance to movement that the stage needs to keep from moving around.  It doesn't need over-turning stability, but if lifted the device would have to provide that as well.  An easy way to do that would be the way the stage does it - with outriggers (called legs on the stage).

Crushing the legs down against the deck sounds like the worst of all worlds to me - even worse than putting rams in bending (which is a total no-no).

But isn't pulling it down to the deck exactly what they do now?

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