Is there a wind speed at which rocket landing is impossible or extremely risky?
[quote author=El Commediante For SpaceX at present, just demonstrating reusability is a coup; It seems it's possible to land a stage without this control authority, in the winds present on good-weather days. It's only when they're using landing regularly for frequent launches, and especially when Falcon Heavy is landing stages in two locations at once, that edge conditions like wind become bottlenecks on fleet function.
I believe the landing wind limited for the orbcomm flight was 50mph.Unlike launch, there's no nearby launch tower or lightning protection to be blown into.
Quote from: Burninate on 12/28/2015 12:13 am[quote author=El Commediante For SpaceX at present, just demonstrating reusability is a coup; It seems it's possible to land a stage without this control authority, in the winds present on good-weather days. It's only when they're using landing regularly for frequent launches, and especially when Falcon Heavy is landing stages in two locations at once, that edge conditions like wind become bottlenecks on fleet function.I would assume that landing weather limits are not that different from launch weather limits, so In the case of RTLS the bottleneck won't be as significant as you think.
Quote from: cscott on 12/29/2015 04:43 pmI believe the landing wind limited for the orbcomm flight was 50mph.Unlike launch, there's no nearby launch tower or lightning protection to be blown into.What's the launch wind criteria ?50mph (44 knots / 81 km/h) is a LOT of wind. Ultra rare having even 40 mph winds.Even for ASDS 44 knot wind is a substantial margin.
Quote from: macpacheco on 12/30/2015 07:22 amWhat's the launch wind criteria ?50mph (44 knots / 81 km/h) is a LOT of wind. Ultra rare having even 40 mph winds.Even for ASDS 44 knot wind is a substantial margin.Launch: <20Landing: <50I've asked Chris if he could verify the landing number (we need gust limit as well).That 50mph seems huge in terms of landing controlabity... If true, very impressive... IMO.
What's the launch wind criteria ?50mph (44 knots / 81 km/h) is a LOT of wind. Ultra rare having even 40 mph winds.Even for ASDS 44 knot wind is a substantial margin.
The stage isn't an airfoil. In fact its the opposite. A round shape does the best job of allowing the wind to flow around the object while generating as little reaction force to wind flow as possible.
Quote from: macpacheco on 12/30/2015 11:19 pmThe stage isn't an airfoil. In fact its the opposite. A round shape does the best job of allowing the wind to flow around the object while generating as little reaction force to wind flow as possible.Not much could be further from the truth.A cylinder like that has a drag coefficient of around 1.2!
Quote from: Lee Jay on 12/30/2015 11:21 pmQuote from: macpacheco on 12/30/2015 11:19 pmThe stage isn't an airfoil. In fact its the opposite. A round shape does the best job of allowing the wind to flow around the object while generating as little reaction force to wind flow as possible.Not much could be further from the truth.A cylinder like that has a drag coefficient of around 1.2!What better shape could be used ?
Quote from: macpacheco on 12/30/2015 11:19 pmThe stage isn't an airfoil. In fact its the opposite. A round shape does the best job of allowing the wind to flow around the object while generating as little reaction force to wind flow as possible.Not much could be further from the truth.A cylinder like that has a drag coefficient of around 1.2!Ever seen this drawing? This is to scale and both have about the same drag.
Quote from: Lee Jay on 12/30/2015 11:21 pmQuote from: macpacheco on 12/30/2015 11:19 pmThe stage isn't an airfoil. In fact its the opposite. A round shape does the best job of allowing the wind to flow around the object while generating as little reaction force to wind flow as possible.Not much could be further from the truth.A cylinder like that has a drag coefficient of around 1.2!Ever seen this drawing? This is to scale and both have about the same drag.That's only true for the wind direction indicated. If the wind was 90 degrees to that, the results would be different! Real winds come from all directions and a cylinder is symmetric.
Quote from: CuddlyRocket on 12/31/2015 02:43 amQuote from: Lee Jay on 12/30/2015 11:21 pmQuote from: macpacheco on 12/30/2015 11:19 pmThe stage isn't an airfoil. In fact its the opposite. A round shape does the best job of allowing the wind to flow around the object while generating as little reaction force to wind flow as possible.Not much could be further from the truth.A cylinder like that has a drag coefficient of around 1.2!Ever seen this drawing? This is to scale and both have about the same drag.That's only true for the wind direction indicated. If the wind was 90 degrees to that, the results would be different! Real winds come from all directions and a cylinder is symmetric.Symmetric and very, very draggy.
Quote from: Rocket Science on 12/30/2015 10:13 pmQuote from: macpacheco on 12/30/2015 07:22 amWhat's the launch wind criteria ?50mph (44 knots / 81 km/h) is a LOT of wind. Ultra rare having even 40 mph winds.Even for ASDS 44 knot wind is a substantial margin.Launch: <20Landing: <50I've asked Chris if he could verify the landing number (we need gust limit as well).That 50mph seems huge in terms of landing controlabity... If true, very impressive... IMO.The stage isn't an airfoil. In fact its the opposite. A round shape does the best job of allowing the wind to flow around the object while generating as little reaction force to wind flow as possible.And the real concern isn't controllability while in flight, but the period between the last second or two before touchdown until a few seconds after, aka the landing flare, and toppling risk (which can't happen while in flight). (Not a rocket engineer, but as a hobbie cat sailor, skydiver and private pilot I know a thing or two about the wind).
Hi!I signed up to learn what is the influence of side wind on a rocket in the last seconds of landing. I thought it might be a problem, considering relatively big side surface of the rocket, high air pressure at the sea level or sudden changes in wind speed. Is there a wind speed at which rocket landing is impossible or extremely risky?
So what the heck is the point of your argument? That a stage should have a wing cross section, and rotate quickly to face whatever gusts happen?? C'mon.
Landing Weather Looks good
Up thread it clearly says that the LANDING criteria is <20 knots (this information from SpaceX). Then someone suggests without anything to back them up that it is 50mph and all of a sudden everybody takes this as gospel.>
The complicating issue to that is the desire to have as close to zero horizontal velocity as possible. So there has to be some tradeoff, either accept a bit of horizontal drift due to wind when it gets vertical for touchdown, or keep it tilted a little bit into the wind for zero horizontal drift, so the structure can handle the fact that one or two legs will touch down before the other legs and there will be some acceptable degree of settling-in rocking.
Quote from: mrhuggy on 12/21/2015 10:00 pmLanding Weather Looks goodSee the post above for the SpaceX's "Landing Commit Criteria" in ORBCOMM-2 update thread. It clearly shows that that wind speed limit for landing is 50mph.It is funny but another screen capture here:http://forum.nasaspaceflight.com/index.php?topic=37739.msg1395299#msg1395299entitled: "Launch Commit Criteria" shows item no. 3 - first stage landing (<10ft waves <20 kt winds).It seems to me that the <10ft waves <20 kt winds criteria refer to landing on an ASDS (barging) and <50 mph winds to RTLS type landing. Any better explanation for this discrepancy? It looks like they can tolerate higher winds due to a much bigger landing pad in LZ-1.
It looks like they can tolerate higher winds due to a much bigger landing pad in LZ-1.
Quote from: tleski on 01/04/2016 02:51 amIt looks like they can tolerate higher winds due to a much bigger landing pad in LZ-1.Or because the landing pad itself isn't moving around, no matter how fast the wind.The tighter limits on ASDS landing may be due to the limits of ASDS itself being able to stay stationary and level.
I think it's fairly certain that the latest success is largely due to the landing pad not moving around.
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...
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 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 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..
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.
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!
Quote from: meekGee on 04/19/2016 05:41 pmThe 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! They're probably not a little concerned about the effect a 3-engine hover-slam might have on the deck... paint.
I'd estimate surface winds at 25mph, based on the cloud of dust blowing away after touchdown in this video:
In the final few seconds, as the first stage approaches the drone ship and the landing burn begins, I can use either three engines or one engine for the landing burn. Three engines uses less fuel, but it's a bit like slamming on the brakes at the last second. That's harder on the rocket, and you don't very much time to correct, because the engines are only burning for a short period of time. A one engine burn uses more fuel, but it's a softer landing and you get more control. And that allows you to land in higher winds.
Interesting comment in the JCSAT-16 hosted webcast, when the SpaceX host indicated that the single-engine landing burn employed in this landing was more tolerant of wind, not less. The timestamp where it is discussed is starting at 13:00 in the below video. The quote:QuoteIn the final few seconds, as the first stage approaches the drone ship and the landing burn begins, I can use either three engines or one engine for the landing burn. Three engines uses less fuel, but it's a bit like slamming on the brakes at the last second. That's harder on the rocket, and you don't very much time to correct, because the engines are only burning for a short period of time. A one engine burn uses more fuel, but it's a softer landing and you get more control. And that allows you to land in higher winds.
Quote from: abaddon on 08/15/2016 09:02 pmInteresting comment in the JCSAT-16 hosted webcast, when the SpaceX host indicated that the single-engine landing burn employed in this landing was more tolerant of wind, not less. The timestamp where it is discussed is starting at 13:00 in the below video. The quote:QuoteIn the final few seconds, as the first stage approaches the drone ship and the landing burn begins, I can use either three engines or one engine for the landing burn. Three engines uses less fuel, but it's a bit like slamming on the brakes at the last second. That's harder on the rocket, and you don't very much time to correct, because the engines are only burning for a short period of time. A one engine burn uses more fuel, but it's a softer landing and you get more control. And that allows you to land in higher winds.Wind provides translation (horizontal) speed and torque. Both are very undesirable (platform landing requires also precise landing trajectory, i.e. rocket should land not only vertically but vertically in the specific point).So you want to compensate wind forces. By using significantly longer one engine burn they have better control i.e. more time to do exactly that.
Which is the opposite of what many people on here have been saying - that the faster the hoverslam, the less likely it is to be affected by wind and therefor its better/more accurate. It's one of the main arguments people have been saying against hovering...
Quote from: JamesH65 on 03/17/2022 11:43 amWhich is the opposite of what many people on here have been saying - that the faster the hoverslam, the less likely it is to be affected by wind and therefor its better/more accurate. It's one of the main arguments people have been saying against hovering...Exactly. It was some time ago (quite the necro-reply!) but it did run counter to the general thinking around here that was prevalent at the time, including my own.
Quote from: dondar on 03/14/2022 09:13 pmQuote from: abaddon on 08/15/2016 09:02 pmInteresting comment in the JCSAT-16 hosted webcast, when the SpaceX host indicated that the single-engine landing burn employed in this landing was more tolerant of wind, not less. The timestamp where it is discussed is starting at 13:00 in the below video. The quote:QuoteIn the final few seconds, as the first stage approaches the drone ship and the landing burn begins, I can use either three engines or one engine for the landing burn. Three engines uses less fuel, but it's a bit like slamming on the brakes at the last second. That's harder on the rocket, and you don't very much time to correct, because the engines are only burning for a short period of time. A one engine burn uses more fuel, but it's a softer landing and you get more control. And that allows you to land in higher winds.Wind provides translation (horizontal) speed and torque. Both are very undesirable (platform landing requires also precise landing trajectory, i.e. rocket should land not only vertically but vertically in the specific point).So you want to compensate wind forces. By using significantly longer one engine burn they have better control i.e. more time to do exactly that.Which is the opposite of what many people on here have been saying - that the faster the hoverslam, the less likely it is to be affected by wind and therefor its better/more accurate. It's one of the main arguments people have been saying against hovering...So now I am conflicted.