But the question is... is there an advantage of being able to hover the stage (F9 and BFR) to give more time to land accurately? More relevant to BFR since the Rapture is going to be a powerful bit of kit with I suspect a higher T2W ratio than the Merlin.* I personally think the 'hoverslam' will work. I'm just wondering about hovering advantages.
Quote from: llanitedave on 01/13/2015 03:49 pmGeez, people, you're driving me crazy here! Is everyone Rube Goldberg in disguise?They have a system, it's going to work. Let them tweak it.They have a system, it's *probably* going to work. Fixed that for you. Unless you have a crystal ball and already know.But the question is... is there an advantage of being able to hover the stage (F9 and BFR) to give more time to land accurately? More relevant to BFR since the Rapture is going to be a powerful bit of kit with I suspect a higher T2W ratio than the Merlin.* I personally think the 'hoverslam' will work. I'm just wondering about hovering advantages.
Geez, people, you're driving me crazy here! Is everyone Rube Goldberg in disguise?They have a system, it's going to work. Let them tweak it.
However, a straight in, fast but accurately controlled landing is much easier for a computer than hovering and stabilizing above the landing surface.
Quote from: nadreck on 01/13/2015 06:28 pm However, a straight in, fast but accurately controlled landing is much easier for a computer than hovering and stabilizing above the landing surface.Depending on if you have adequate control authority and target vector flexibility to ignore the gusts.In principle though even for extreme gusts - 'brute force'' solutions - a thousand tiny quadcopters in rings 50m apart giving 5s warning of winds to allow some feed-forward would help _enormously_.
But, what I am trying to point out, is that under automated control the slower you go, the harder it is to ignore the gusts, the less control authority your aerodynamic surfaces have, the more ping ponging you will get with your gimballed engine, etc. with manual control you have no option but to go slow because a human can't do the fast/accurate decelerate to zero at zero in real time. So for a human to control the landing you need far more control authority to make a successful 'soft' landing of any type, for a machine, if only the last 2 seconds of the landing has effectively zero aerodynamic control authority as opposed to a human controlled one where there is maybe 10 or more seconds of that, then the automated landing needs 1/5th or less the control authority because there is 4/5th less possible deviation from when you had aerodynamic control authority.
Quote from: nadreck on 01/13/2015 07:52 pmBut, what I am trying to point out, is that under automated control the slower you go, the harder it is to ignore the gusts, the less control authority your aerodynamic surfaces have, the more ping ponging you will get with your gimballed engine, etc. with manual control you have no option but to go slow because a human can't do the fast/accurate decelerate to zero at zero in real time. So for a human to control the landing you need far more control authority to make a successful 'soft' landing of any type, for a machine, if only the last 2 seconds of the landing has effectively zero aerodynamic control authority as opposed to a human controlled one where there is maybe 10 or more seconds of that, then the automated landing needs 1/5th or less the control authority because there is 4/5th less possible deviation from when you had aerodynamic control authority.I quite agree.If, and only if the vehicle can't hover for a long time, and does not have the control authority to reliably fly through gusts at maximum safe deceleration speed.If it can hover roughly around the pad waiting for a calm moment - that's quite another thing.Also unlikely for near-term rockets for both of these to be true.
VTVL hovering rockets (F9R isn't going to be hovering, but whatever) can withstand pretty good side winds. Here's "rocket tug of war" from Armadillo Aerospace (or Masten?):Jon Goff probably has some more war stories.
Quote from: Robotbeat on 01/14/2015 02:00 amVTVL hovering rockets (F9R isn't going to be hovering, but whatever) can withstand pretty good side winds. Here's "rocket tug of war" from Armadillo Aerospace (or Masten?):Jon Goff probably has some more war stories.Yes, but the main point of the argument (I think) is that hovering doesn't benefit you at all. Just set down directly. The longer you hover, the worse your throttling response will be (craft getting lighter and lighter), and you increase the time that something can go wrong. Waiting for calmer winds is not a good idea, it is just as likely that a stronger gust could come. It is safest to land as quickly as possible, don't waste time.
https://en.wikipedia.org/wiki/Pendulum_rocket_fallacy
"Look, if we built this
Another reason not to mount engines, or parachutes or arresting hooks or whatever, to the top of the stage might be that it's built to withstand compressive forces (e.g. max-Q) but not necessarily tensile forces. I'm no engineer but I'd imagine that subjecting the stage to cycles of compressive, then tensile forces each launch would not be great for its lifespan unless significant re-engineering was done.
Yes, but the main point of the argument (I think) is that hovering doesn't benefit you at all. Just set down directly. The longer you hover, the worse your throttling response will be (craft getting lighter and lighter), and you increase the time that something can go wrong. Waiting for calmer winds is not a good idea, it is just as likely that a stronger gust could come. It is safest to land as quickly as possible, don't waste time.
without the ability to hover you have precisely one chance to get it right
Quote from: Lars-JYes, but the main point of the argument (I think) is that hovering doesn't benefit you at all. Just set down directly. The longer you hover, the worse your throttling response will be (craft getting lighter and lighter), and you increase the time that something can go wrong. Waiting for calmer winds is not a good idea, it is just as likely that a stronger gust could come. It is safest to land as quickly as possible, don't waste time.I think I agree, but without the ability to hover you have precisely one chance to get it right - the accuracy of descent needs to be perfect, the engine needs to ignite at exactly the right time, the throttle response need to be perfect, the wind prediction need to be almost perfect (this should be feasible with the barge telling the rocket what the current deck wind conditions are). Hopefully it works every time. But without a low thrust engine, there are no second chances.Point being, is it worth sacrificing a bit of performance (by having a lower thrust centre engine - no other changes) to enable a second chance? I suspect not.
Musk said 50% more hydraulic fluid is all they need to nail the landing. Why should we doubt that?
Musk said 50% more hydraulic fluid is all they need to nail the landing.
Quote from: ChrisWilson68 on 01/14/2015 10:28 amMusk said 50% more hydraulic fluid is all they need to nail the landing. He never said that, certainly not as confidently as you make it sound. "With the next flight we have 50% more hydraulic fluid margin. Something else could go wrong, certainly, but at least with respect to that, it should cover. So there's a real decent chance, within three weeks, of landing it."