Quote from: meekGee on 01/07/2016 01:14 amQuote from: CameronD on 01/07/2016 01:10 amQuote from: meekGee on 01/07/2016 12:52 amQuote from: CameronD on 01/07/2016 12:37 amI think it's fairly certain that the latest success is largely due to the landing pad not moving around.?A corollary would be that the failures occurred because the barge was moving...And since the corollary is false, then I think the original statement is not very certain...The previous failures occurred for many reasons (perhaps even some we aren't privy to), only one of which being that, during the last attempt, the platform was moving (rocking).. but I guess we have only to wait for the Jason-3 launch to know if SpX can overcome the effect of this in practice.I know from many years of personal experience working with and around floating platforms of all kinds that it's a tough ask, but looking at what they've been able to achieve thus far, if it can be done at all, SpX will do it. I think that's a stretch... Like an aircraft coming in to land without hydraulic fluid and crashing because of a wind gust.(Some may recognize the aviation scenario)So yeah, the wind gust was a contributing factor, but to claim that it is "certain that it was the lack of wind gusts that made a subsequent landing possible"... Meh. It was the presence of hydraulic fluid....Sure. That they nailed this landing was an impressive feat and I've said as much elsewhere. At each landing attempt we've seen them fix (change) more than one parameter - but ruling out an obvious one (eg. the lack of hydraulic fluid) sure helps for the next time around.Equally, I think it's a stretch to expect the ability to successfully land at LZ-1 means they could successfully land on an ASDS - just like a successful landing at LAX does not mean you could land on an aircraft carrier..
Quote from: CameronD on 01/07/2016 01:10 amQuote from: meekGee on 01/07/2016 12:52 amQuote from: CameronD on 01/07/2016 12:37 amI think it's fairly certain that the latest success is largely due to the landing pad not moving around.?A corollary would be that the failures occurred because the barge was moving...And since the corollary is false, then I think the original statement is not very certain...The previous failures occurred for many reasons (perhaps even some we aren't privy to), only one of which being that, during the last attempt, the platform was moving (rocking).. but I guess we have only to wait for the Jason-3 launch to know if SpX can overcome the effect of this in practice.I know from many years of personal experience working with and around floating platforms of all kinds that it's a tough ask, but looking at what they've been able to achieve thus far, if it can be done at all, SpX will do it. I think that's a stretch... Like an aircraft coming in to land without hydraulic fluid and crashing because of a wind gust.(Some may recognize the aviation scenario)So yeah, the wind gust was a contributing factor, but to claim that it is "certain that it was the lack of wind gusts that made a subsequent landing possible"... Meh. It was the presence of hydraulic fluid....
Quote from: meekGee on 01/07/2016 12:52 amQuote from: CameronD on 01/07/2016 12:37 amI think it's fairly certain that the latest success is largely due to the landing pad not moving around.?A corollary would be that the failures occurred because the barge was moving...And since the corollary is false, then I think the original statement is not very certain...The previous failures occurred for many reasons (perhaps even some we aren't privy to), only one of which being that, during the last attempt, the platform was moving (rocking).. but I guess we have only to wait for the Jason-3 launch to know if SpX can overcome the effect of this in practice.I know from many years of personal experience working with and around floating platforms of all kinds that it's a tough ask, but looking at what they've been able to achieve thus far, if it can be done at all, SpX will do it.
Quote from: CameronD on 01/07/2016 12:37 amI think it's fairly certain that the latest success is largely due to the landing pad not moving around.?A corollary would be that the failures occurred because the barge was moving...And since the corollary is false, then I think the original statement is not very certain...
I think it's fairly certain that the latest success is largely due to the landing pad not moving around.
Yes, but you went a lot further than saying that it is uncertain they'll land on the barge...You said it was certain that this landing worked out because the land was not moving, and since I have nothing more productive to do tonight, I thought I'd show you the error of your ways....
And the sad part is that I actually do have other stuff I need to do tonight...
Quote from: meekGee on 01/07/2016 01:32 amYes, but you went a lot further than saying that it is uncertain they'll land on the barge...You said it was certain that this landing worked out because the land was not moving, and since I have nothing more productive to do tonight, I thought I'd show you the error of your ways....I guess my being "fairly certain the latest success is largely due.." wasn't uncertain enough for you. For that you have my sincerest apologies: I shall try to be more vague next time.
Quote from: CameronD on 01/07/2016 01:48 amQuote from: meekGee on 01/07/2016 01:32 amYes, but you went a lot further than saying that it is uncertain they'll land on the barge...You said it was certain that this landing worked out because the land was not moving, and since I have nothing more productive to do tonight, I thought I'd show you the error of your ways....I guess my being "fairly certain the latest success is largely due.." wasn't uncertain enough for you. For that you have my sincerest apologies: I shall try to be more vague next time. Are you completely oblivious to the "largely" in your sentence, and what it means? Largely, meaning: to a great extent; on the whole; mostly. Do you not understand the reaction that gets, when you come up with a brand new explanation that according to you, you are fairly certain is the primary factor in the failure?
Let's let this one go... We're down to semantics, and can keep it up till the 17th if we don't stop.
Quote from: meekGee on 01/07/2016 03:14 amLet's let this one go... We're down to semantics, and can keep it up till the 17th if we don't stop.Fair enough.. I do have one question though that maybe someone here knows the answer to:What maximum angle-of-tilt of the landing platform is the F9 landing system/landing legs designed to tolerate??
Whether or not movement of the landing pad was the primary factor in the failure of the previous landing attempt I would not know - but in the absence of other information (can you point me to any?) I do believe it to be a factor at least worthy of consideration.
Quote from: CameronD on 01/07/2016 02:29 amWhether or not movement of the landing pad was the primary factor in the failure of the previous landing attempt I would not know - but in the absence of other information (can you point me to any?) I do believe it to be a factor at least worthy of consideration.Er, "other information", like SpaceX stating that the most recent barge landing failed due to valve stiction, and that this has been fixed for subsequent attempts?Not trying to say a barge landing is a slam dunk but this is about as obvious a factor as it gets.
The video shows the rocket coming in at the wrong angle and/or off centre. That's has nothing to do with the rocking of the barge, which is a sensible sea state is negligible.
"Just purely the boat moving, even in a low sea state, it's hard to imagine that vehicle is going to stay vertical," Shotwell said. "That vehicle is big and tall, compared to the itty-bity-greater-than-a-football-field-size ship."
With respect to wind influence, I have a feeling that folks may be unaware of just how fast embedded processors are these days. Remember that cute little Falcon 9 landing game someone threw together? Imagine playing that at, let's say, one frame per second, and you can get an idea of how the software sees its universe."Oh, look, the sensors are indicating that some outside influence - maybe a gust of wind - is pushing me off-nominal by four centimeters so far. I guess we'll need to plan for a thruster firing and a bit of engine gimbal if this keeps up."
Quote from: mvpel on 01/07/2016 06:22 pmWith respect to wind influence, I have a feeling that folks may be unaware of just how fast embedded processors are these days. Remember that cute little Falcon 9 landing game someone threw together? Imagine playing that at, let's say, one frame per second, and you can get an idea of how the software sees its universe."Oh, look, the sensors are indicating that some outside influence - maybe a gust of wind - is pushing me off-nominal by four centimeters so far. I guess we'll need to plan for a thruster firing and a bit of engine gimbal if this keeps up."Maybe processors are fast, but the real world is still analog - and that fast processor and software is totally reliant on sensors to know what is (and isn't) going on out there. Garbage In, Garbage Out. To explain: In my mind, it is a matter of prediction - but you can't predict something happening and make allowance for it if you can't sense it sufficiently within bounds of accuracy and time. Of course the usual way to work out what you do and don't need to sense is by simulation and testing and often what you thought might be a problem (eg. wind gusts) can be overcome by other means than sensing (eg. "A gust? We're going fast enough to not worry about it") within the same bounds thus sacrificing some amount of accuracy for reduced complexity.What is less straight-forward to me is how they plan to predict the angle of the surface they're landing on. MeekGee suggested maybe an altimeter/range-finder on each leg? That sounds feasible to me - although that would presumably only come into play in the final meters before touchdown. Their commit criteria do specify the max sea-state they'll allow a landing, but that wouldn't cater for a rogue wave at the wrong time (certainly possible on the open ocean) tilting the platform outside acceptable limits at the last second.. and what do you do then? They can't just hover (or can they?), so presumably they have safety margins in place to cover that scenario.They could also measure instantaneous 3-axis platform tilt and use that to issue an earlier abort-to-water-landing if they find they're getting close to the edge of their safety margins in the minutes before landing... but to know whether or not that's feasible (or even necessary) you'd need to know the limits of the F9 landing leg design. Perhaps they are doing both??Anyways, I'd be interested to know what others here think - and will be very interested in the landing video for Jason-3.
They definitely can't hover, so that's out as an option. Ships at sea almost always roll by varying amounts (in both magnitude and period) even in a steady sea state, because the base swells themselves vary a bit, as does their period. So, I think you're right, a greater than expected roll can occur at any time. It would also be a greater factor if the downhill side at landing was also the downwind side of the F9. They can't time the landing, but IMHO it's plausible that they might partially compensate for wind by changing the paramiters of the GN2 thrusters on the interstage, to have one firing during touchdown to get ahead of any tilt. My guess is that, at most, this is a future possibility; right now they are focused on getting a baseline landing right. If an anemometer and wind direction gauge is added to the ASDS at some point, this would be my guess as to the reason (but it would require an uplink). My guess; they aren't going to build in an abort mode for this issue; too great a chance of losing a recoverable F9 when weighed against the $cost of damage to the ASDS.
1. We've seen the GN2 thrusters firing madly on a couple of landing attempts, but I had thought that was only to counter for wind - would they be large/powerful enough to (worst case) hold the stage on one leg for the second or three it took for the ASDS to come off of the back of a wave??
Quote from: CameronD on 01/08/2016 03:20 am1. We've seen the GN2 thrusters firing madly on a couple of landing attempts, but I had thought that was only to counter for wind - would they be large/powerful enough to (worst case) hold the stage on one leg for the second or three it took for the ASDS to come off of the back of a wave??Those GN2 thrusters were dealing with other more serious problems: ran out of hydraulic fluid on one try; stuck valve on the other try. We have never seen an attempted ASDS landing where everything performed nominally.
Quote from: JamesH on 01/07/2016 04:14 pm The video shows the rocket coming in at the wrong angle and/or off centre. That's has nothing to do with the rocking of the barge, which is a sensible sea state is negligible.Shotwell does not share this opinion:Quote"Just purely the boat moving, even in a low sea state, it's hard to imagine that vehicle is going to stay vertical," Shotwell said. "That vehicle is big and tall, compared to the itty-bity-greater-than-a-football-field-size ship."http://www.defensenews.com/story/defense/air-space/space/2015/04/15/spacex-ground-attempt-reusable-landing-sea/25827625/Edit to clarify: I'm disagreeing that the rocking is negligible. Not saying it was the cause of previous failures.
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.
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.