Is it worth my mentioning again that the mechanics of bringing a hovering F9 first stage down onto the deck of a barge (or to an eventual land based location) are far more demanding than the direct in, reduce to zero Vx + zero Vy + zero Vz @ zero altitude and zero x, y and z offset from the centre of the X on the barge. Once you are hovering, you now are being exposed continuously to the local winds, when you were coming in you could adjust your engine angle by a much smaller amount to compensate for wind (10s before landing your velocity is 100m/s a 5m/s gust/shear requires 1/10th the correction that it would at 10m/s 1s before landing and 1/20th the correction that you would need hovering 5 meters above the deck (presuming that you descend from hover at .5 g to gain speed and then decelerate with 1.5g thrust the remainder of the time). As well, your grid fins had some authority down to a handfull of seconds before touch down and the longer you hover the longer you have to put up with buffeting at the level where it is most gusty/sheary.Either the controls are accurate enough to allow the computer to manage engine thrust, reduce to zero altitude zero V at the target at 1G deceleration straight on to the deck, or it is not good enough to manage to go from hovering anywhere and get to the deck and you need a human remotely piloting it to land it. If the engine responds variably to gymballing and thrust level control inputs the piloting program must respond immediately to that adjusting the control signals, but it already had to vary all of these things continuously as the weight changed and air density changed to match its course profile.
I salute your optimism. In fact I agree with you.Everything is solved. We just need to wait until the solution can be implemented. Rocket science is easy, rocket engineering is hard. I'm not arguing against the software or the science behind the technique. That is clearly sound. I'm less optimistic that the engineering can ensure the landing are reliable. A lot needs to work just right, guidance, hydraulics, engine, environmental conditions and there are no second chances with the hoverslam approach. WIth regard to one point above, the F9R-dev cannot have done a test flight with an engine TW >1 at all times. That basically the definition of going up, not down. It carried a fuel load to ensure the engine could be throttled back enough to give TW<1.The returning stage does not have that much fuel, so at no point has a TW<1 except when it's engine off, so that is an invalid argument (and the F9R-dev can hover, as we all know, so really, it's flights are not entirely relevant wrt this discussion)
Quote from: JamesH on 01/21/2015 06:51 pmI salute your optimism. In fact I agree with you.Everything is solved. We just need to wait until the solution can be implemented. Rocket science is easy, rocket engineering is hard. I'm not arguing against the software or the science behind the technique. That is clearly sound. I'm less optimistic that the engineering can ensure the landing are reliable. A lot needs to work just right, guidance, hydraulics, engine, environmental conditions and there are no second chances with the hoverslam approach. WIth regard to one point above, the F9R-dev cannot have done a test flight with an engine TW >1 at all times. That basically the definition of going up, not down. It carried a fuel load to ensure the engine could be throttled back enough to give TW<1.The returning stage does not have that much fuel, so at no point has a TW<1 except when it's engine off, so that is an invalid argument (and the F9R-dev can hover, as we all know, so really, it's flights are not entirely relevant wrt this discussion)I explicitly listed the problems that were solved, what the remaining problems were and why I believe they are solvable. I don't appreciate the cherry picking nor the explicit misrepresentation of what I wrote.Grasshopper and F9R-Dev land with the engines running TW > 1. What happens before then is irrelevant.Clearly you are committed to "hovering is better." But guess what? If guidance, hydraulics, or engines fail or if environmental conditions are outside the operational limits of a rocket that can hover - it still crashes.
My point was that the F9R-dev, for it entire flight, has the ability to have a TW < 1 and clearly uses it.
I would like to see the SuperDracos available in this capacity specifically to aid in solving all ~12 degrees of freedom that need to be correct for a soft touchdown, amidst a moving reference frame. 3D position, 3D orientation, 3D velocity, 3D rotational velocity all need to be near-zero relative to the landing zone at impact, and long after the grid fins cease to be effective. Fighting any significant wind from terminal velocity to ship velocity with the main engines essentially requires that some of these variables diverge from zero. More wind, more problems. SuperDracos provide extra thrust in the right directions at very high frequency. This is *far* more useful than merely slowing down the hoverslam descent to lower G-ratings, because it expands the operational envelope of first stage reuse, something that exponentially increases the lifespan of a first stage in SpaceX's fleet given a fraction of missions targetting specific orbital windows.Two problems with this, though:1) While CoM does not have a huge effect during the flight of a pure rocket, contrary to intuition... it does have a large effect on stability for the duration that the landing legs touch the deck.2) Nitrogen tetroxide and monomethyl hydrazine. If the landing requires these in quantity, the landing zone is a HAZMAT zone, and approval for such landings is even harder. Green hypergolic propellants would be very strongly preferred.
Quote from: speedevil on 01/13/2015 09:06 pmQuote 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.I don't see it that way. The decision to hover or not would require foresight of gusts, and the sea is just inherently not very gusty; Wind is steady and relatively undisturbed by turbulence. The problem is it doesn't have the control authority to land in *steady state winds* above some level. Hovering doesn't help. More fuel doesn't help. The grid fins don't help. The barge will be positioned several hundred to several thousand (FH center core) kilometers downrange, far enough that launchsite weather conditions are no guarantee of landing site weather conditions; And we're probably talking about not very many knots of wind in the first place.I don't *know* the maximum wind criteria - maybe it won't affect this barge at all (as wind-driven swells might make the barge unusable before windspeed does), but once this is proven the next landing pad can be more seaworthy.The issue is that the demands of stable flight in a moving airmass with fixed position *using only thrust from the business end of the rocket*, conflict with the demands of touching all four feet to the ground at nearly the same time without significant rotation rate or horizontal velocity. You can squash any of the variables to zero, but doing so raises at least one of the other variables.
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.
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: 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, rockets can be made very effective at holding position *in the air*, but as soon as they touch down, the inherent structural stability and any dynamic momentum in the structure come into play, as well as any pressure from the wind; The F9R first stage is a giant 18 ton hollow metal tube, with the top ~45 meters off the ground, standing on legs only ~18 meters wide. A tiny little rocket like the one pictured, with wide, strong legs and a compact, fuel-filled tank, and a short, squat mass distribution, isn't directly comparable.Maybe if the F9R's legs were actuated to be a dynamic structure which cushions impacts rather than a rigid fixed shape?
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: mme on 01/17/2015 07:49 pmQuote from: JamesH on 01/17/2015 12:46 pmQuote from: mme on 01/17/2015 06:50 amQuote from: Hotblack Desiato on 01/17/2015 01:41 amis wind really such a problem?...No, it's not. This entire thread is an attempt to solve a problem that doesn't exist. The F9 landing is basically a smart bomb targeting a specific set of GPS coordinates. It's just a smart bomb that starts out higher and faster than normal and that hoverslams instead of exploding (assuming it's on target.)For the very first attempt they discovered they need more hydraulic fluid to have precise control to the target. The next attempt will hit the barge more directly. Maybe even "land." Autopilots deal with wind all the time without any external information beyond how the aircraft is actually flying.Odd how you can say its a problem that doesn't exist, when no-one has EVER landed and recovered a first stage successfully. Close, but no cigar, to quote a phrase.Once a stage has been landed multiple times successfully, then you say its a solved problem, but not yet.Hoverslam != explode on impact.The stage exploded because it hit the side of the barge while doing a hard divert. It was doing a hard divert because it could not maintain an on target trajectory. It could not maintain an on target trajectory because it did not control over it's primary aerodynamic control surfaces for the last minute of "flight." It didn't have control of the grid fins because it ran out of hydraulic fluid.Wind was not the problem. Complex systems to anticipate the wind are not the solution. The solution is to have control authority for the entire landing process.The hoverslam itself is not some intractable problem and the term hoverslam is overly dramatic compared to the reality. All the stage needs is good information regarding the distance to the ground and it's current deceleration.I do not mean to imply that the targeting and landing control systems are trivial, just that with enough simulation, testing and experimentation I am confident they can be done and that they can be done reliably.I feel like I've repeated myself too much so I'll do my best to refrain unless I have something new to add the the conversation.I feel like my point has been lost in the noise, and maybe I didn't do a great job explaining myself, and people elaborated on misconceptions.I am positing that a wind limit exists (not that I know what it is, just that it exists) beyond which the stage will not be able to land, because of the mismatch between the constant velocity of the air, and the fixed position and zero velocity of the landing pad. This limit exists for a specific reason: The only way the rocket has of holding position against a wind, is by tilting itself.1) Vertical descent, fighting wind, tilted profile: If the rocket is substantially tilted when the first landing leg touches the ground during a vertical descent, the force of the dropping rocket will cause it to torque around and maybe topple over.2) Diagonal descent, static with wind, straight profile: If the rocket tries to track with the wind, coming in diagonally (requiring foreknowledge of wind), but with the tube itself perfectly vertical - that's a neat trick that introduces another problem: Now your rocket is travelling sideways, which when the all four landing legs touch the ground at the same time, induces a torque and maybe topples it.3) Vertical descent, fighting wind up until last moment, dynamic profile: If the rocket tries to land as with 1) and then rapidly pitches to get all four legs on the ground at the same time for just the moment of impact - Then your rocket is now a rotating body, with angular momentum, and it might topple of its own volition.4) Diagonal descent, static with wind, until last moment, dynamic profile: If the rocket tries to land as with 2) and then rapidly accelerates sideways to null horizontal velocity - well, the only way to accelerate sideways (induce a horizontal acceleration of center of mass) simultaneously induces a rotation... which might topple it.Complicating all this is the CoM vs CoP issue in flight, and the fact that large shuttlecocking effects would be observed in wind.Above some limit of wind, the fact that the only actuator you have simultaneously induces a horizontal acceleration, and induces a rotational acceleration, at the same time, prevents you from landing. I don't know what that limit is, but it exists in principle. It's a problem with bringing a high profile rigid object into rendezvous with a surface that has a different velocity than the wind, while still fighting gravity & vertical momentum, on only a rear actuated thruster.A set of SuperDracos, while a bit overkill for the task, gives very resilient control of orientation if positioned at the top of the rocket, until all four legs are on the ground and velocity & momentum have been zeroed out. There's still the issue of whether a gale might blow a *standing* landed stage over, but I imagine this limit is well above the limit at which you encounter problems at the landing interface event.Again: All of this assumes a constant, predictable, prevailing wind, and has nothing to do with turbulent gusts; It has no bearing on hovering, and hovering has no bearing on it.
Quote from: JamesH on 01/17/2015 12:46 pmQuote from: mme on 01/17/2015 06:50 amQuote from: Hotblack Desiato on 01/17/2015 01:41 amis wind really such a problem?...No, it's not. This entire thread is an attempt to solve a problem that doesn't exist. The F9 landing is basically a smart bomb targeting a specific set of GPS coordinates. It's just a smart bomb that starts out higher and faster than normal and that hoverslams instead of exploding (assuming it's on target.)For the very first attempt they discovered they need more hydraulic fluid to have precise control to the target. The next attempt will hit the barge more directly. Maybe even "land." Autopilots deal with wind all the time without any external information beyond how the aircraft is actually flying.Odd how you can say its a problem that doesn't exist, when no-one has EVER landed and recovered a first stage successfully. Close, but no cigar, to quote a phrase.Once a stage has been landed multiple times successfully, then you say its a solved problem, but not yet.Hoverslam != explode on impact.The stage exploded because it hit the side of the barge while doing a hard divert. It was doing a hard divert because it could not maintain an on target trajectory. It could not maintain an on target trajectory because it did not control over it's primary aerodynamic control surfaces for the last minute of "flight." It didn't have control of the grid fins because it ran out of hydraulic fluid.Wind was not the problem. Complex systems to anticipate the wind are not the solution. The solution is to have control authority for the entire landing process.The hoverslam itself is not some intractable problem and the term hoverslam is overly dramatic compared to the reality. All the stage needs is good information regarding the distance to the ground and it's current deceleration.I do not mean to imply that the targeting and landing control systems are trivial, just that with enough simulation, testing and experimentation I am confident they can be done and that they can be done reliably.I feel like I've repeated myself too much so I'll do my best to refrain unless I have something new to add the the conversation.
Quote from: mme on 01/17/2015 06:50 amQuote from: Hotblack Desiato on 01/17/2015 01:41 amis wind really such a problem?...No, it's not. This entire thread is an attempt to solve a problem that doesn't exist. The F9 landing is basically a smart bomb targeting a specific set of GPS coordinates. It's just a smart bomb that starts out higher and faster than normal and that hoverslams instead of exploding (assuming it's on target.)For the very first attempt they discovered they need more hydraulic fluid to have precise control to the target. The next attempt will hit the barge more directly. Maybe even "land." Autopilots deal with wind all the time without any external information beyond how the aircraft is actually flying.Odd how you can say its a problem that doesn't exist, when no-one has EVER landed and recovered a first stage successfully. Close, but no cigar, to quote a phrase.Once a stage has been landed multiple times successfully, then you say its a solved problem, but not yet.Hoverslam != explode on impact.
Quote from: Hotblack Desiato on 01/17/2015 01:41 amis wind really such a problem?...No, it's not. This entire thread is an attempt to solve a problem that doesn't exist. The F9 landing is basically a smart bomb targeting a specific set of GPS coordinates. It's just a smart bomb that starts out higher and faster than normal and that hoverslams instead of exploding (assuming it's on target.)For the very first attempt they discovered they need more hydraulic fluid to have precise control to the target. The next attempt will hit the barge more directly. Maybe even "land." Autopilots deal with wind all the time without any external information beyond how the aircraft is actually flying.
is wind really such a problem?...
There are likely dynamics-based solutions here, but they require extremely fine-grained (temporally and spatially) control of the throttle, and several seconds building up substantial horizontal velocity into the wind, at low vertical velocity, so that the horizontal velocity zeroes out at just the right second without shuttlecocking... but that doesn't factor in...It's complicated. More degrees of freedom of control, and specifically much more control authority in pitch and yaw, makes things much, much less fragile in those last few seconds.
Quote from: nadreck on 01/21/2015 09:13 pmIs it worth my mentioning again that the mechanics of bringing a hovering F9 first stage down onto the deck of a barge (or to an eventual land based location) are far more demanding than the direct in, reduce to zero Vx + zero Vy + zero Vz @ zero altitude and zero x, y and z offset from the centre of the X on the barge. Once you are hovering, you now are being exposed continuously to the local winds, when you were coming in you could adjust your engine angle by a much smaller amount to compensate for wind (10s before landing your velocity is 100m/s a 5m/s gust/shear requires 1/10th the correction that it would at 10m/s 1s before landing and 1/20th the correction that you would need hovering 5 meters above the deck (presuming that you descend from hover at .5 g to gain speed and then decelerate with 1.5g thrust the remainder of the time). As well, your grid fins had some authority down to a handfull of seconds before touch down and the longer you hover the longer you have to put up with buffeting at the level where it is most gusty/sheary.Either the controls are accurate enough to allow the computer to manage engine thrust, reduce to zero altitude zero V at the target at 1G deceleration straight on to the deck, or it is not good enough to manage to go from hovering anywhere and get to the deck and you need a human remotely piloting it to land it. If the engine responds variably to gymballing and thrust level control inputs the piloting program must respond immediately to that adjusting the control signals, but it already had to vary all of these things continuously as the weight changed and air density changed to match its course profile.Three separate problems. The "Accurate Hypersonic reentry problem" is something that they seem to have *very quietly* solved. This was a Big Deal behind the scenes, and may even prove the little-researched concept of supersonic retropopulsion.If you solve that, you get to -The "Hoverslam Problem" is landing at all, transitioning from terminal velocity to pad safely in still air, with a TWR minimum of much more than 1. I don't think there's anything standing in the way of that, at this point. It doesn't need SuperDracos. To my mind this had a first tentative solution with the arrangement Grasshopper was running, and it will be refined as time goes on. Hovering is unnecessary if guidance is good enough to hit a ~150ft target (which, again, was the Accurate Hypersonic Reentry Problem), and in the first barge test, guidance was good enough to hit the edge of the barge even without the supposedly critical grid fins, though not good enough to land.If you solve that, you get to -The "Windy Landing Problem" is something that is not a problem as long as the landing pad is not windy. It has to do with what happens as, and just after, the first leg hits the ground. This isn't a priority early on, solving the first two problems is. It only becomes a mild problem as Falcon 9 starts to operate reusably all the time - a few hundred kilometers is enough for a slightly different weather system. and not all launch windows can be pushed back easily. It then becomes a moderate problem for Falcon Heavy centercore reuse, which has to happen much farther downrange.There are indications from Grasshopper that the Windy Landing Problem isn't so severe, but we do not have the data. The Hoverslam Problem and the Windy Landing problem are very nearly unrelated - and solving the one does not solve the other. This thread originally proposed SuperDracos for the Hoverslam Problem, and I think that it's been shown by Grasshopper that this is unnecessary. The Windy Landing Problem... maybe - they could certainly improve capabilities in that respect, at some significant cost. We'll have to see.Enough said.
Elon Musk @elonmusk · 5h 5 hours agoLooks like Falcon landed fine, but excess lateral velocity caused it to tip over post landing Elon Musk @elonmusk · 4h 4 hours ago@kwrzesien It is a lot like Lunar Lander, except with 6X higher gravity and a tiny landing areaElon Musk @elonmusk · 3h 3 hours ago@teknotus There are nitrogen thrusters at top of rocket. Either not enough thrust to stabilize or a leg was damaged. Data review needed.
Do you believe me now?
Quote from: OxCartMark on 04/15/2015 01:38 am3) a) Discussion of wind here in this chat room of the internets is useless. Wind on that cylinder whether its a significant force or not is easy to model and has been thought through vastly more than our words and the wind from our mouths will accomplish.I have an aerospace engineering degree - the wind-induced drag on a large cylinder is significant but not as significant as the vertically-assymmetrical drag on the entire stage once those legs deploy.Quoteb) Observations of a flag which is seen to be flying briskly in a direction radially away from an active rocket engine are not reliable indications of a meterological wind. A review of the Apollo 11 lunar surface flag during liftoff would under that logic lead one to believe that it happened on a windy day on the moon. I watched that moon walk and I can attest that there was no appreciable wind on the surface until they lit that thing.The reported wind conditions at the landing site were in the range of 14 kts. That's not gale-force but it IS significant to a dynamic control system. The moon has nothing to do with this discussion.
3) a) Discussion of wind here in this chat room of the internets is useless. Wind on that cylinder whether its a significant force or not is easy to model and has been thought through vastly more than our words and the wind from our mouths will accomplish.
b) Observations of a flag which is seen to be flying briskly in a direction radially away from an active rocket engine are not reliable indications of a meterological wind. A review of the Apollo 11 lunar surface flag during liftoff would under that logic lead one to believe that it happened on a windy day on the moon. I watched that moon walk and I can attest that there was no appreciable wind on the surface until they lit that thing.
@elonmusk Congratulations! How many engines are lit for landing? Can you differentially throttle for more degrees of control?
@ID_AA_Carmack Thanks! 3 of 9 engines are lit initially, dropping to 1 near ground. Even w 1 lit, it can't hover, so always land at high g@ID_AA_Carmack Looks like the issue was stiction in the biprop throttle valve, resulting in control system phase lag. Should be easy to fix.
Quote from: Burninate on 04/15/2015 02:37 amDo you believe me now?No, still too early to pat yourself on the back
