What's left untested, therefore, is just the deviation of real-life grid fin behavior from the modeled one. This, in the grand scheme of things, is a pretty low risk.
Quote from: meekGee on 01/02/2015 09:30 pmWhat's left untested, therefore, is just the deviation of real-life grid fin behavior from the modeled one. This, in the grand scheme of things, is a pretty low risk.I think you're overly glossing the fact that they haven't tested grid fins at transsonic speeds. I recall Musk and John Carmack having a Twitter chat about that tending to go very badly; Musk's responses weren't terribly confident.
Quote from: dcporter on 01/02/2015 10:02 pmQuote from: meekGee on 01/02/2015 09:30 pmWhat's left untested, therefore, is just the deviation of real-life grid fin behavior from the modeled one. This, in the grand scheme of things, is a pretty low risk.I think you're overly glossing the fact that they haven't tested grid fins at transsonic speeds. I recall Musk and John Carmack having a Twitter chat about that tending to go very badly; Musk's responses weren't terribly confident.I think John Carmack talked about steering aero surfaces, not gridfins. Gridfins are for trassonic speeds and their behaviour is generally well known, though not at objects this size but that can be modeled. Still it is the reason why the chance of success is lower on the first try. But I have no doubt at all that they will learn what they need for success at second try if the first one fails.
.....What's left untested, therefore, is just the deviation of real-life grid fin behavior from the modeled one. This, in the grand scheme of things, is a pretty low risk....
Looks like SpX has built a mini F9 pad next to the ASDS dock and SpX trailer. The 4 grey structures seem to be similar to the hold downs / supports used for the F9 VTVL tests.
I find it interesting to note, in passing, that the CRS-5 NASA Press Kit makes absolutely NO mention of the first stage recovery attempt. In fact, there is only a single reference to first stage recovery, referencing the April 2014 soft landing water recovery.What the heck!? One would think there would be SOME official notice of this potentially huge milestone event advancing booster technology, even if (from NASA's point of view) it was Not Invented Here. Granted, it is not "NASA business" but rather "SpaceX business", but still. Perhaps concern that a SpaceX successful, or even partially successful first stage recovery will overshadow CRS and/or ISS operations in the public view? I think this is likely, especially of SpaceX releases "news candy" , i.e., photogenic, video of the landing.
Quote from: meekGee on 01/02/2015 09:30 pm.....What's left untested, therefore, is just the deviation of real-life grid fin behavior from the modeled one. This, in the grand scheme of things, is a pretty low risk....I agree they've retired the vast majority of the overall risks, but I would NOT say that "deviation of real-life grid fin behavior from the modeled one" is low-risk. I'm thinking of specific instances where rockets were lost because real-life fin behavior deviated from the modeled one. Usually this happens when the vehicle goes transonic, a regime which the grid fins have yet to see in flight.Just because they've done most of the hard parts doesn't mean their probably of success on this particular flight should thus be greater than 50%. It really does only take one thing to screw up your multi-million-dollar vehicle. So I think Musk is making an accurate assessment of the risk.For instance, see this exchange on Twitter:
Quote from: Robotbeat on 01/02/2015 11:39 pmQuote from: meekGee on 01/02/2015 09:30 pm.....What's left untested, therefore, is just the deviation of real-life grid fin behavior from the modeled one. This, in the grand scheme of things, is a pretty low risk....I agree they've retired the vast majority of the overall risks, but I would NOT say that "deviation of real-life grid fin behavior from the modeled one" is low-risk. I'm thinking of specific instances where rockets were lost because real-life fin behavior deviated from the modeled one. Usually this happens when the vehicle goes transonic, a regime which the grid fins have yet to see in flight.Just because they've done most of the hard parts doesn't mean their probably of success on this particular flight should thus be greater than 50%. It really does only take one thing to screw up your multi-million-dollar vehicle. So I think Musk is making an accurate assessment of the risk.For instance, see this exchange on Twitter:Look. John Carmack said "inversion" and now it's the internet risk de jour, but this is not some exotic unexpected phenomena that nobody understands. The mere fact that Musk is going with grid fins means he's not clueless about it, and a very wild guess says that they have an aerodynamicist (or two) on staff.If I was to place a bet on why the landings might fail, this won't be a leading cause.And besides, I never said "low risk", mainly because I can't quantify " low". I said that the risks during the first (ever!) propulsive reentry were higher, and that this time almost all stages of the process have been validated already, or that "in the grand scheme of things, it's a low brisk" - so please quote my entire statement, not just half sentences.
Fascination is due in port shortly. Wonder if #BargeX / Falcon's Nest will still be there when she arrives?http://www.cruisin.me/ship_tracker/carnival_cruise_lines/carnival_fascination.php
Quote from: meekGee on 01/03/2015 01:39 amQuote from: Robotbeat on 01/02/2015 11:39 pmQuote from: meekGee on 01/02/2015 09:30 pm.....What's left untested, therefore, is just the deviation of real-life grid fin behavior from the modeled one. This, in the grand scheme of things, is a pretty low risk....I agree they've retired the vast majority of the overall risks, but I would NOT say that "deviation of real-life grid fin behavior from the modeled one" is low-risk. I'm thinking of specific instances where rockets were lost because real-life fin behavior deviated from the modeled one. Usually this happens when the vehicle goes transonic, a regime which the grid fins have yet to see in flight.Just because they've done most of the hard parts doesn't mean their probably of success on this particular flight should thus be greater than 50%. It really does only take one thing to screw up your multi-million-dollar vehicle. So I think Musk is making an accurate assessment of the risk.For instance, see this exchange on Twitter:Look. John Carmack said "inversion" and now it's the internet risk de jour, but this is not some exotic unexpected phenomena that nobody understands. The mere fact that Musk is going with grid fins means he's not clueless about it, and a very wild guess says that they have an aerodynamicist (or two) on staff.If I was to place a bet on why the landings might fail, this won't be a leading cause.And besides, I never said "low risk", mainly because I can't quantify " low". I said that the risks during the first (ever!) propulsive reentry were higher, and that this time almost all stages of the process have been validated already, or that "in the grand scheme of things, it's a low brisk" - so please quote my entire statement, not just half sentences.Because SpX engineers know about the Inversion issue, their grid fin flight control software should be able to compensate for it and still maintain the desired F9 attitude control capability.
Quote from: PhilW on 01/03/2015 01:41 amFascination is due in port shortly. Wonder if #BargeX / Falcon's Nest will still be there when she arrives?http://www.cruisin.me/ship_tracker/carnival_cruise_lines/carnival_fascination.phpGo Quest is underway at 0.1 knots with destination Blake Plateau at this time.No - nevermind - its track looks like it's just jostling around. Keep an eye on it, though ...
... And now, back to the Fascination vid feed, tonight's main event.