Quote from: fake_name on 06/30/2017 01:27 pmI think it's quite easy to rule out friction alone, \mu = F_r/F_nIf the barge is moving up and down in the swell, F_n is changing, it will increase as the barge rises (accelerates up) and then decrease as the barge falls (accelerates down).Wouldn't take much more to find the coefficient of steel on steel or steel on painted steel, work out what the swell size is, figure out the reduction in the normal force then find the force that would overcome friction in that situation.My gut feel, friction is no where near enough, not with the surface area of the roomba, the contact patch is only the underside of the outside steel beams? If there are any fasteners or such sticking out on the bottom, would make it worse.The other options off hand are:- some type of fastener (drill and tap a hole in the deck and drop a bolt in? or a rivet?)- weld some lugs to the deck then chain block the roomba to the lugs (what happened previously, I saw some welded lugs in early barge photos)- ?Maybe the roomba is full of lead ballast?It won't get 'lighter' as the barge drops, because they are both falling under the same force of gravity. It will get 'heavier' when starting upwards as the barge will be pushing against it as it accelerates up due to wave action. There will also be a side to side force from rocking, the torque on the base would be considerable, but that's not a grip problem, that's a hold down problem.To me, simple rubber on the underside would increase fraction dramatically, but TBH, that thing must weigh at least 10000kg and won't be any easy thing to move even when holding the stage, even with just steel on steel.
I think it's quite easy to rule out friction alone, \mu = F_r/F_nIf the barge is moving up and down in the swell, F_n is changing, it will increase as the barge rises (accelerates up) and then decrease as the barge falls (accelerates down).Wouldn't take much more to find the coefficient of steel on steel or steel on painted steel, work out what the swell size is, figure out the reduction in the normal force then find the force that would overcome friction in that situation.My gut feel, friction is no where near enough, not with the surface area of the roomba, the contact patch is only the underside of the outside steel beams? If there are any fasteners or such sticking out on the bottom, would make it worse.The other options off hand are:- some type of fastener (drill and tap a hole in the deck and drop a bolt in? or a rivet?)- weld some lugs to the deck then chain block the roomba to the lugs (what happened previously, I saw some welded lugs in early barge photos)- ?Maybe the roomba is full of lead ballast?
To me, simple rubber on the underside would increase fraction dramatically, but TBH, that thing must weigh at least 10000kg and won't be any easy thing to move even when holding the stage, even with just steel on steel.
Quote from: JamesH65 on 06/30/2017 02:03 pmTo me, simple rubber on the underside would increase fraction dramatically, but TBH, that thing must weigh at least 10000kg and won't be any easy thing to move even when holding the stage, even with just steel on steel.Steel on steel, particularly when wet is a bit slippy. The simplest answer is to leave the tracks in contact with the deck - that would probably be enough to prevent it moving. The alternative would be dense rubber pads on the underside of the octagrabber, but all they would do is act as a substitute for the tracks if the tracks were lifted, so why bother lifting the tracks?The slight issue about rubber pads on the underside of the octagrabber would be that if the tracks were partially retracted and it tries to move, the pads would be ripped off.Similarly, the fitting of pads would increase the maximum height of the octagrabber - for example, 1" thick pads would increase the height accordingly, which could create an issue trying to get under a stage with fully-collapsed crush cans. Simplest answer is to rely on the tracks for friction, which has the additional benefit of keeping the overall height lower.
Quote from: vanoord on 07/04/2017 11:36 amQuote from: JamesH65 on 06/30/2017 02:03 pmTo me, simple rubber on the underside would increase fraction dramatically, but TBH, that thing must weigh at least 10000kg and won't be any easy thing to move even when holding the stage, even with just steel on steel.Steel on steel, particularly when wet is a bit slippy. The simplest answer is to leave the tracks in contact with the deck - that would probably be enough to prevent it moving. The alternative would be dense rubber pads on the underside of the octagrabber, but all they would do is act as a substitute for the tracks if the tracks were lifted, so why bother lifting the tracks?The slight issue about rubber pads on the underside of the octagrabber would be that if the tracks were partially retracted and it tries to move, the pads would be ripped off.Similarly, the fitting of pads would increase the maximum height of the octagrabber - for example, 1" thick pads would increase the height accordingly, which could create an issue trying to get under a stage with fully-collapsed crush cans. Simplest answer is to rely on the tracks for friction, which has the additional benefit of keeping the overall height lower.My thinking is they rely on gravity and friction, but they do need to bring the frame into contact with the deck, because the tracks alone are not stable enough, but in so doing some of the weight is removed from the tracks, so friction becomes less effective, unless there is high friction material on the bottom of the frame.Alternatively, they could attach a robotic welding arm to each side and have steel plates which are bolted on to the frame be welded to the deck. Probably unnecessary.
I went to @PortCanaveral to visit @SpaceX #OCISLY since it's been a while. I was greeted with autonomous open arms. Yes, we miss you too!
I do believe that is Rhoomba sitting on the dock. I guess it is still in repair status.
Late edit - Pics from https://twitter.com/Mike_Seeley/status/880420558386733058
Over the last couple of launches, I've been trying to remember to check when the boosters return on OCISLY. And I may have missed one, but so far I don't remember ever seeing the grabber bot using all 4 arms to hold a booster. Each time I've been able to see it in use, it's always been with just 3 of the 4 arms engaged. Have others noticed this as well? Or have I just not seen the best pictures?
Quote from: deruch on 09/13/2018 11:07 amOver the last couple of launches, I've been trying to remember to check when the boosters return on OCISLY. And I may have missed one, but so far I don't remember ever seeing the grabber bot using all 4 arms to hold a booster. Each time I've been able to see it in use, it's always been with just 3 of the 4 arms engaged. Have others noticed this as well? Or have I just not seen the best pictures? It almost looked like only 2 this time? This got some discussion in the Facebook SpaceX group but I didn't see any firm theories why. Are they testing how secure it is with less than 4 arms? That's a good thing to know because there will be mechanical failures from time to time
Quote from: Lar on 09/13/2018 05:48 pmQuote from: deruch on 09/13/2018 11:07 amOver the last couple of launches, I've been trying to remember to check when the boosters return on OCISLY. And I may have missed one, but so far I don't remember ever seeing the grabber bot using all 4 arms to hold a booster. Each time I've been able to see it in use, it's always been with just 3 of the 4 arms engaged. Have others noticed this as well? Or have I just not seen the best pictures? It almost looked like only 2 this time? This got some discussion in the Facebook SpaceX group but I didn't see any firm theories why. Are they testing how secure it is with less than 4 arms? That's a good thing to know because there will be mechanical failures from time to timeLooks like 3 to me: https://twitter.com/ken_kremer/status/1040025868544946176