Quote from: mvpel on 01/11/2016 02:25 amIt's hard to grasp how massive seagoing platforms of this scale are unless you've stood on one. They're massive. And in any case I think SpaceX has undoubtedly run enough simulations to get to the point that it does not remain to be seen within the constraints of the landing commit criteria.Perhaps "massive" is a relative term... The platform in question is only 300' long and 100' wide and, lightly ballasted as it is and with no active stabilisation, will bob around like a cork out on the open ocean. Compared to, say, an aircraft carrier, it's a peanut.Yes, undoubtedly SpaceX have run enough simulations, and with the last landing have proven they can land a stage on a surface of equivalent size, so apparently all the other bugs are ironed out - but whether or not their commit criteria for a barge landing are correct, only time will tell.
It's hard to grasp how massive seagoing platforms of this scale are unless you've stood on one. They're massive. And in any case I think SpaceX has undoubtedly run enough simulations to get to the point that it does not remain to be seen within the constraints of the landing commit criteria.
The load lines tell a different story.When a ship carries more water ballast mass than it's light ship weight. (Mass of Steel) One can not use the term "lightly ballasted."
What is the definition of "bob"We calculate ship motions in terms of roll, pitch and heave. Roll and pitch are centered around the center if the waterplane area. Heave requires a waves longer than the perpendicular side to have any significant impact.
Quote from: Doesitfloat on 01/11/2016 01:24 pmThe load lines tell a different story.When a ship carries more water ballast mass than it's light ship weight. (Mass of Steel) One can not use the term "lightly ballasted." You have evidence of that?!? What's the "water ballast mass" and the "light ship weight" of the ASDS?? Quote from: Doesitfloat on 01/11/2016 01:24 pmWhat is the definition of "bob"We calculate ship motions in terms of roll, pitch and heave. Roll and pitch are centered around the center if the waterplane area. Heave requires a waves longer than the perpendicular side to have any significant impact.My apologies for using a nautical term I thought most here would understand.If you'd prefer to substitute "roll, pitch and heave" where I wrote "bob", I'm happy for you to do that.
While this may seem a bit off topic, I think using a specialized SWATH Hull designed structure would greatly reduce the pitch and roll issues that they would have landing the stage. (Ex Navy. Had some experience with this subject).
Congratulations to SpaceX on the successful launch of Jason-3!Barge needs much better stabilization in any sea state to keep landing area from moving. Barge moving up and down messes up with S1's sensors causing a hard landing. Landing would most likely have been successful if the barge was as stable as land.
Quote from: DJPledger on 01/17/2016 06:58 pmCongratulations to SpaceX on the successful launch of Jason-3!Barge needs much better stabilization in any sea state to keep landing area from moving. Barge moving up and down messes up with S1's sensors causing a hard landing. Landing would most likely have been successful if the barge was as stable as land.Looks like I was right after all...
Definitely harder to land on a ship. Similar to an aircraft carrier vs land: much smaller target area, that's also translating & rotating.
However, that was not what prevented it being good. Touchdown speed was ok, but a leg lockout didn't latch, so it tipped over after landing.
@elonmusk Does that mean the same thing would have happened on land?
@Uncle_Gus probably
However, that was not what prevented it being good. Touchdown speed was ok, but a leg lockout didn't latch, so it tipped over after landing.They'll fix the latch and try again and then we'll get real data on how sea state affects landing attempts.
Later tweets indicate the landing was soft enough, with the problem being that the collet locking one of the legs failed to (fully?) activate, resulting in the rocket toppling over. Elon indicated the problem would likely have occurred in a landing on land attemp..Will be a fun video to watch, when they release it.
made several software improvements, including a noteworthy one. Rather than the vehicle translating to land at the exact center of the pad, it now initially targets the center, but then sets down at a position of convenience on the pad, prioritizing vehicle attitude ahead of precise lateral positioning. It’s like a pilot lining up a plane with the centerline of the runway. If the plane is a few feet off center as you get close, you don’t swerve at the last minute to ensure hitting the exact mid-point. You just land a few feet left or right of the centerline. Our Monte Carlo sims of New Shepard landings show this new strategy increases margins, improving the vehicle’s ability to reject disturbances created by low-altitude winds.
Quote from: CameronD on 01/11/2016 02:58 amQuote from: mvpel on 01/11/2016 02:25 amIt's hard to grasp how massive seagoing platforms of this scale are unless you've stood on one. They're massive. And in any case I think SpaceX has undoubtedly run enough simulations to get to the point that it does not remain to be seen within the constraints of the landing commit criteria.Perhaps "massive" is a relative term... The platform in question is only 300' long and 100' wide and, lightly ballasted as it is and with no active stabilisation, will bob around like a cork out on the open ocean. Compared to, say, an aircraft carrier, it's a peanut.Yes, undoubtedly SpaceX have run enough simulations, and with the last landing have proven they can land a stage on a surface of equivalent size, so apparently all the other bugs are ironed out - but whether or not their commit criteria for a barge landing are correct, only time will tell. The load lines tell a different story.When a ship carries more water ballast mass than it's light ship weight. (Mass of Steel) One can not use the term "lightly ballasted." What is the definition of "bob"We calculate ship motions in terms of roll, pitch and heave. Roll and pitch are centered around the center if the waterplane area. Heave requires a waves longer than the perpendicular side to have any significant impact.
I know that's what's been said, but as an engineer with a few years experience working with floating platforms (amongst other things) I'm not sure that's the full picture. ISTM they didn't deploy the legs early enough to account for the swell ..and IMHO they probably would have landed on land quite okay - but without detailed analysis it's safer to agree that they wouldn't have.If what I think happened, had the latch not failed the entire stage may (worst case) have toppled into the sea.
Bump.I think it's fairly clear at this point that wind did have some effect on the CRS-8 landing.
Not sure about that figure. I know it's what Musk said but as a life long ocean sailor that landing didn't look anything near 50 mph.That's over 40 knots and at that wind you would expect a lot more blown wave tops and spray. Looked more like 25/30 kts max to me.
Quote from: kevinof on 04/18/2016 08:49 pmNot sure about that figure. I know it's what Musk said but as a life long ocean sailor that landing didn't look anything near 50 mph.That's over 40 knots and at that wind you would expect a lot more blown wave tops and spray. Looked more like 25/30 kts max to me.Agreed, but winds at surface are much lower than gusts at altitude.
As a rule of thumb, use the power law with exponent of 0.11 for vertical wind shear near the Earth's surface over water.So, if the wind is 9m/s at 10 meters, at 100m it would be 9*(100/10)^0.11 = 11.6m/s.
The best way to reduce wind impact is to land with three engines...These one-engine almost-hover landings are painfully slow to look at. Like watching paint dry.Someone give me some excitement!
