An Engineering Analysis of the Falcon 9 First Stage Landing Failure 113
schwit1 writes: AviationWeek has posted an analysis of SpaceX's latest attempt to land its Falcon 9 rocket on an ocean barge. Quoting: "SpaceX founder and chief technology officer Elon Musk tweeted that "excess lateral velocity caused it [the booster] to tip over post landing." In a later tweet that was subsequently withdrawn, Musk then indicated that "the issue was stiction in the biprop throttle valve, resulting in control system phase lag." In this statement, Musk was referring to "stiction" — or static friction — in the valve controlling the throttling of the engine. The friction appears to have momentarily slowed the response of the engine, causing the control system to command more of an extreme reaction from the propulsion system than was required. As a result, the control system entered a form of hysteresis, a condition in which the control response lags behind changes in the effect causing it.
Despite the failure of the latest attempt, SpaceX will be encouraged by the landing accuracy of the Falcon 9 and the bigger-picture success of its guidance, navigation and control (GNC) system in bringing the booster back to the drone ship. The GNC also worked as designed during the prior landing attempt in January, which ended in the destruction of the vehicle following a hard touchdown on the edge of the platform." In related news, SpaceX is hoping to attempt its next landing on solid ground.
Despite the failure of the latest attempt, SpaceX will be encouraged by the landing accuracy of the Falcon 9 and the bigger-picture success of its guidance, navigation and control (GNC) system in bringing the booster back to the drone ship. The GNC also worked as designed during the prior landing attempt in January, which ended in the destruction of the vehicle following a hard touchdown on the edge of the platform." In related news, SpaceX is hoping to attempt its next landing on solid ground.
Video from the barge (Score:5, Informative)
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Re:Video from the barge (Score:5, Informative)
Since the first attempt hard landed because it ran out of attitude control gas, and the second hard landed because of a control valve problem... how would a larger target have helped? In case of the first attempt, you've still got to control your attitude regardless of the size of the field. In the second, the size of the field is irrelevant if you can't properly control the vehicle in the first place.
Seriously, don't be misled by the frantic activity in the final seconds of the most recent attempt. That burst of activity was the vehicle attempting to null it's horizontal velocity and then trim it's attitude before landing - something it has to do regardless of the size of the field.
The basic flaw in the landing sequence isn't the size of the target, it's the design of the vehicle. Its minimum T/W ratio is well over unity at landing, meaning it can't hover, can't ease itself down, and you have to take great care to not end up with positive vertical velocity. The only way it can land (with any reasonable sized target) is to approach at high speed, then at the last second try to null horizontal velocity without excessively reducing vertical velocity (I.E. bouncing), followed by a return to vertical and touchdown.
You could avoid this by having a circle of paved ground a quarter to half a mile in diameter - but that's not cheap to build or maintain given the need to resist a rocket's exhaust. Long term, given that the tests are essentially free*, it's cheaper and easier to figure out how to land precisely on a smaller target.
* The first stage is bought and paid for by the launch customer - and so long as the added equipment for landing poses no undue risk during ascent, they don't care what happens to it after separation.
Re:Video from the barge (Score:5, Interesting)
Nitpick: The first attempt ran out of hydraulic fluid (for the guidance fins), not out of propellant for the RCS thrusters.
The rest of what you say is generally true, although a larger target *would* help. The advantage of a larger target is that, while you still have to zero your horizontal velocity, you don't have to zero it anywhere terribly precise. You can pick an optimal set of thrusts that results in the correct orientation and velocities (horizontal and vertical) without worrying overmuch *where* that series of thrusts has you touching down. Both attempts so far clearly demonstrate the ability to do an excellent good job of targeting a (relatively) tiny barge, but currently, if the rocket would come down even 100' (30m) to one side of its target spot, it needs to induce a horizontal momentum (which requires leaving a vertical attitude as well, it can't just translate sideways) and then null that momentum at the right moment (and fix its attitude). That's hard.
To clarify for the person who keeps misunderstanding my posts: they should, of course, plan for the barge-level of landing precision. They should aim for a precision of inches, and within a year, they may get it... 90% of the time. Stuff goes wrong, though, and (especially early in the testing of such a system) it behooves them to use a larger landing area so that there's some margin for error. I'd say their land attempt (possibly next CRS launch, in a couple months) has a very good chance of being their first success.
Neither failure was due to target size (Score:2)
Yeah, you do. Given the narrow footprint and the low CG of the vehicle, if the horizontal velocity wasn't as close to zero as you can get at touchdown - it's very likely to tip over. (Even if you don't damage the landing legs in the process.) The upper part of the vehicle isn
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Yeah, you do. Given the narrow footprint and the low CG of the vehicle, if the horizontal velocity wasn't as close to zero as you can get at touchdown - it's very likely to tip over. (Even if you don't damage the landing legs in the process.) The upper part of the vehicle isn't heavy, but it has a very long lever arm.
You're not getting what he was saying. To put it in car terms, he's saying that 'stop the car in front of that building' is a much easier task than 'stop the car on that postage stamp', which is what they're trying to do with the barge.
In the former scenario, if you under or overshoot the location of your stop, you're still fine. You'd miss the barge, but with a large piece of flat land, you're still landing on flat terrain. You can tune your control jets to always use 'small' corrections, and wait for t
It doesn't work that way. (Score:2)
I do get what he's saying. What neither of you seem to grasp is that the size of the target isn't as relevant as you think, because you have to null your horizontal velocity regardless of the size of the target. It doesn't matter whether you're stopping on a postage stamp or anywhere in a given block - either way you still have to stop. It's the stopping that's problem, not the deciding where to stop. Stopping is very difficult for the Falcon 9 because it's T/W ra
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What neither of you seem to grasp is that the size of the target isn't as relevant as you think, because you have to null your horizontal velocity regardless of the size of the target. It doesn't matter whether you're stopping on a postage stamp or anywhere in a given block - either way you still have to stop.
The problem of having to null both velocity and position at the same time is harder than just having to null velocity. The latter gives you a bigger solution space.
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What neither of you seem to grasp is that the size of the target isn't as relevant as you think, because you have to null your horizontal velocity regardless of the size of the target.
As itzly, says, you can say 'null your horizontal velocity' all you like, but we agree with you there - what we're saying is that arranging for velocity AND position to be 'null' at the same time is harder than simply arranging for velocity to be null and position to be +/- 100m(or so).
You're talking about some kind of articulated arm (which can survive being essentially inside rocket exhaust)
I think you're picturing something different. I'm picturing something pretty big that comes in from the sides, staying well away from the exhaust.
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*Sigh*
I understand what you're saying - but as with my previously reply, you don't grasp the problem.
The appearance of the vehicle "working hard at the last second" during the first attempt was a consequence of running out of hydraulic fluid - and would have occurred regardless of the size of the target. The a
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The engine can throttle between 50% and 100%. It is true that even 50% of just one of the nine engines is enough to shoot the rocket back towards space. Which is why it can not hover.
But hover is not relevant. Hitting zero velocity at height zero is.
Let the rocket drop freely while continuously calculating needed thrust to hit the target velocity zero at zero height. Keep dropping until your algorithm says you need 75% thrust. At this moment you relight your engine at 75%.
Now you can do a control loop with
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I thought that was him estimating an 80% chance of at least one success by year's end, which (considering that they have a lot of launches this year) is a very different thing. Do you have a source for that?
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I'm not actually sure which of use that tweet backs up; I thought he meant "I estimate we have an 80% chance of at least one landing by year's end, because there are lots of launches between now and then and there's a pretty good chance at least one will succeed." It would be pretty awesome if what he meant was "By the end of the year, we'll have so much more launch practice that, even though each launch has less than 50% chance of recovery right now, by year's end it should be about 80%." I hope the latter
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At the cost of increasing the difficulty of targeting the landing, and still having to null any horizontal velocity due to the wind. TAANSTAFL.
Re: Video from the barge (Score:2, Informative)
Their ultimate goal is to land this on another planet, then take back off. That is why they are working so hard on unassisted landing.
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Watch the vid posted in the parent, you will see the flexing of the middle once it starts tipping. Nets wont keep it from destroying itself, it would just crumple, then explode (probably).
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Is there a reason the rocket has to land upright unassisted? What about some sort of bungee-mesh around the edges to catch it if it topples?
Parachutes are too heavy for a nine engine rocket, and would either land in corrosive seawater or on land with a hard damaging landing. To be truthful, they have already accomplished the most difficult part, namely bringing a rocket from 140km altitude to a tiny location in the middle of the ocean. They have also already demonstrated the landing part several [youtube.com] times [youtube.com]. If you notice, those landings come down at pretty much the exact location they began at.
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So, I asked this in the last thread but the discussion there was already mostly dead: what would it cost (presumably mostly a matter of weird) to upgrade the nose thrusters? These are cold-gas (nitrogen) thrusters, and I can't imagine they have a lot of power.
The Dragon uses hydrazine-based "Draco" thrusters for its RCS system; might it be worth adding a hydrazine thruster with a few seconds of fuel in place of the cold-gas thrusters, enabling the rocket to correct its orientation in the moment of touchdown
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Agh, bloody autocorrect and stupid failure to proofread. First paragraph:
What would it cost (presumably a matter of weight) to upgrade the nose thrusters?
Re:Video from the barge (Score:5, Insightful)
So, I asked this in the last thread but the discussion there was already mostly dead: what would it cost (presumably mostly a matter of weird) to upgrade the nose thrusters? These are cold-gas (nitrogen) thrusters, and I can't imagine they have a lot of power.
The Dragon uses hydrazine-based "Draco" thrusters for its RCS system; ...
Thoughts?
You, like many people, are trying to solve the wrong problem. Fix the over-correction and there is no need for rocket powered thrusters in place of the cold gas thrusters. Fix the root cause, don't mask it with a heavy/expensive kludge that will come with a host of it's own failure modes.
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I'm not trying to fix the wrong problem, I'm trying to add a backup for the fix. Shit happens. Parts will fail, valves will stick, unexpected winds or waves will occur.
I thought the fact that the primary goal was to correct the problem that caused the excessive lateral velocity was so bloody obvious that it didn't need saying, but I guess I forgot I'm on the Internet. The purpose of my idea was not "fuck it, fixing a little problem is hard, let's do something much more complicated", it's "shit happens. What
Re:Video from the barge (Score:4, Informative)
If there's one thing they should work on, it's not thrusters but having the capability to throttle to hover. That would potentially change the entire low approach. It is complicated by the fact that engine performance goes nonlinear in the low range.
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I'm not trying to fix the wrong problem, I'm trying to add a backup for the fix. Shit happens. Parts will fail, valves will stick, unexpected winds or waves will occur.
And when they happen, you loose the vehicle...I think you are trying to address the wrong problem. IMHO the smart money is on making what they have work though some software adjustments or if they really can't do it that way the minimum adjustments in the hardware to smooth out the valve responses. They are really close here, and from what I just was reading they are a software change away from making this work. Just remove the oscillation problem in software by tweaking the delay calculations in the fee
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And when they happen, you loose the vehicle..
It stinks if you're going to lose the vehicle when 1 thing fails. Though programming wise, they might 'fix' the thruster issue the same way they did some LCD panels - change the request curve so that when ordering the valve to change a small amount - IE from shut to 10%, it very briefly orders a larger change to overcome the static friction, such as 50%, then countermands that with the 10% order a millisecond or so later.
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Is "The Vehicle" a euphemism for something? ;) What exactly are you talking about loosing [loseloose.com]?
(And I hardly think that suggesting a more powerful RCS as a backup (backups being critically fundamental rocket design) is "redesigning the whole thing"). I don't know why they went with cold gas thrusters, but hydrazine RCS thrusters are mature tech, one that even SpaceX themselves uses - they're reliable and have a good power to weight ratio for their size. I presume there's a reason they went with nitrogen instead
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So the grammar police are still out in force.... OK, what ever floats your boat.
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Not necessarily. A stronger RCS system would probably have for example saved this last rocket - even though that wasn't the fundamental problem. Again, that's the purpose of backup systems. They're not the primary - they're there for if the primary fails, which it will sooner or later even if you've ironed out the major bugs.
I'd wager that they chose nitrogen not because of the best thrust / mass combination for their needs, but to try to make regulatory approval for landing on land easier.
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I'm not trying to fix the wrong problem, I'm trying to add a backup for the fix. Shit happens. Parts will fail, valves will stick, unexpected winds or waves will occur.
I thought the fact that the primary goal was to correct the problem that caused the excessive lateral velocity was so bloody obvious that it didn't need saying, but I guess I forgot I'm on the Internet. The purpose of my idea was not "fuck it, fixing a little problem is hard, let's do something much more complicated", it's "shit happens. What can we do to survive the likely error modes?"
And I stand by my response, it's better to reduce the likelihood of shit happening. There are ways to reduce sticton and there are ways to handle it better when it happens. Trying to fight an oscillation induced by a lag/overcorrection of a main engine with a thruster is a losing battle. If the thruster is up top it will just rotate the stage around the center of mass (which is really what a thrusters job is, to orient the stage. Not move it through space laterally.) If you look at the thruster firing
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I'm not trying to fix the wrong problem, I'm trying to add a backup for the fix. Shit happens. Parts will fail, valves will stick, unexpected winds or waves will occur.
A backup is nice but from a control perspective when you have an unstable control scheme additional power (gain) is about the worst possible situation.
The problem occurred because of an overshoot of a control reaction. The only thing achieved by better thrusters would be more overshoot.
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You, like many people, are trying to solve the wrong problem. Fix the over-correction and there is no need for rocket powered thrusters in place of the cold gas thrusters. Fix the root cause, don't mask it with a heavy/expensive kludge that will come with a host of it's own failure modes.
I partially agree with this. Correcting the root of the problem seem to be the best solution, but one thing my years in precise robot precision though me is that usually, "asservissement" (french word because I don't know the exact English translation, but I would guess it's control with feedback) is more efficient while correcting the root isn't always possible.
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You want to add a tank of highly toxic fuel to rocket that tends to explode when it lands?
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Explode when it crashes, there's a difference. SpaceX has demonstrated powered vertical landings on their test vehicles, there were no explosions.
Hydrazine is nasty stuff, but you don't need very much of it (a few second's worth, maybe) and the *entire point* is to avoid the crash, so what happens in the event of a crash is much less important.
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Did you not see the thrusters it's already got were more than capable of manoeuvring it? In fact, they provided too much thrust and over-corrected it, apparently due to a problem with the valve or control software.
Video shows the thruster control lag ? (Score:1)
You Can See (Score:3)
The 'stiction' is evident when the rocket is initially coming down and swinging to the left of the video frame, before you see it (over-)correct and swing back to (and past) vertical. I watched that section wondering why the rocket went excessively to the left in the first place, and a stuck valve makes a lot of sense.
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That would require strengthening the body to withstand forces from other directions. More weight. This is a kludge and the wrong way to solve the problem.
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Although I'm willing to take it on faith that it might be 'stiction' on the actuator side, I'm pretty sure extreme variations which can cause unanticipated phase lag from a measurement system (say a gyro or a flow meter) is equally disruptive to a control system and can generically explain control system oscillation events as well.
Sometimes you never really know until you set up the conditions and simulate the crap out of it in an controlled testbed as it is much harder to distinguish the difference in a "l
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Although I'm willing to take it on faith that it might be 'stiction' on the actuator side, I'm pretty sure extreme variations which can cause unanticipated phase lag from a measurement system (say a gyro or a flow meter) is equally disruptive to a control system and can generically explain control system oscillation events as well.
Sometimes you never really know until you set up the conditions and simulate the crap out of it in an controlled testbed as it is much harder to distinguish the difference in a "live" situation.
True, but if you have good enough instrumentation and data collection built into your system it's not *that* hard to figure out what exactly happened. I'm sure they have this system well instrumented so it won't take them long to narrow down on a solution to the problems they are seeing. My guess is that they just need to tune their control systems a bit to account for the valve movement delays and all will be better. They might need to work on the actuators for the valves some if they cannot smooth thin
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Microminiature accelerometers are really cheap and very very light, and you don't have to wait for them to spin up or deal with their mechanical issues. I doubt you will see a gyro used as a sensor any longer.
Similarly, computers make good active stabilization possible and steering your engine to stabilize is a lot lighter than having to add a big rotating mass.
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Microminiature accelerometers are really cheap and very very light, and you don't have to wait for them to spin up or deal with their mechanical issues. I doubt you will see a gyro used as a sensor any longer.
Gyros are still required to detect changes in rotation. However, modern MEMS gyroscopes are tiny, and they don't rotate but use a vibrating element. http://en.wikipedia.org/wiki/V... [wikipedia.org]
You can get combinations of accelerometers and gyroscopes (and sometimes even magnetometers) all integrated in a single tiny device that's within reach of school kid projects: http://www.geekmomprojects.com... [geekmomprojects.com]
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sure you will, they have different uses.
thats why your modern normal smartphone has both a gyro and an acceleration detector chip. older phones tend to just have the acceleration detector. you can use the accel chip for detecting orientation, if you are not slowing or speeding up into some direction...
sure.. the gyrochips might not have an actual old school spinning device inside but they function the same.
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I've run into this kind of thing a few times in my career. I'm curious how they fix it. In my experience it is never an "eas
Doesn't look close (Score:5, Funny)
Falcon 9 was seconds away from what would have been the first successful landing of a used booster stage on SpaceX’s Autonomous Spaceport Drone Ship
I suppose that's one way to look at it. But actually it was seconds away from exploding in a huge ball of fire.
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That's why they used the words "WOULD HAVE BEEN".
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You saw the part where the reaction control thruster at the top (little white plume) is trying to keep the rocket upright, didn't you? Lasts about four seconds. The rocket had already touched down (on at least one leg, with engines shut off) at that point. If the stage had been even a *little* closer to upright, or the thruster a *bit* more powerful, the rocket would have settled onto all four legs and that would be it.
Also, the stage swings through vertical in the moment before touchdown. It's that half-se
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It's that half-second of overcorrection that doomed it.
Maybe. Or maybe it was coming in at the wrong angle. Or maybe the thrusters had no chance. Or maybe the legs wouldn't have stood up to it even if the thrusters had worked as planned. All we know *for sure* is that it crashed. I'm sure they'll keep working on until the kinks are ironed out though.
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The thrusters aren't even supposed to be needed there, actually. They're only supposed to fire in very short bursts, not a continuous stream like in the video. As for the legs holding up, just one of them supported the whole rocket for a few seconds; all four should have had no trouble. We know a hell of a lot more than just that it crashed. To claim otherwise is to embrace ignorance.
There is literally no point at all to living in a world where you are only concerned with the things that you absolutely know
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While watching the last landing attempt I had to wonder if the rocket experiences different amounts of lift when it is over water, as opposed to being over the barge...
Will this be a factor that is alleviated by landing on terra firma?
If they do make their next attempt on land, what are the potential take off and landing points?
Many people noted advantages to landing down range of the launch point, and not being able to launch over the continental US
Does that leave the options of
1. Launching from Brownsvill
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The booster can indeed make it back uprange to Kennedy Space Center, and they've leased a landing pad for it there. Besides the turn-around burn, they tilt the booster against the airstream and let aerodynamics push it back uprange during that 78 mile descent.
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An empty stage with no payload gets about 1 1/2 orders of magnitude more delta-V for its last kilogram of fuel than it got for its first kilogram of fuel when the countdown hit zero at the pad. And on top of that it's already got altitude, and can use the atmosphere to shed unwanted lateral momentum or aerodynamically redirect it to change direction, with little consumption of fuel. Its these things that make flyback a lot easier than it sounds at first glance.
Still not "easy", but a lot easier.
Input from a UI designer instead? (Score:1)
Engineers can be useful in the beginning of a project but I think I'd like to hear from a UI designer instead, in the last couple years these guys have really been showing their stuff around the web.
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Thanks. I really needed that laugh.
I wonder why he bothers... (Score:2)
In a later tweet that was subsequently withdrawn, Musk then indicated that "the issue was stiction in the biprop throttle valve, resulting in control system phase lag."
Anything he leaves for more than 0.5 seconds is going to be reported, retweeted, screenshotted and several articles posted. Just google "musk stiction biprop" and you get plenty hits, no real "undo" button for such a public figure.
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I'm pretty sure Elon Musk doesn't want to risk falling off a cliff
Re:I wonder why he bothers... (Score:5, Insightful)
In a later tweet that was subsequently withdrawn, Musk then indicated that "the issue was stiction in the biprop throttle valve, resulting in control system phase lag."
Anything he leaves for more than 0.5 seconds is going to be reported, retweeted, screenshotted and several articles posted. Just google "musk stiction biprop" and you get plenty hits, no real "undo" button for such a public figure.
He's tech savvy enough to know it's not scrubbed from the internets nor the collective consciousness. This just seems to be how he uses twitter. He regularly tweets things he leaves up, but he also uses twitter to have conversations with people and then he deletes those tweets after the conversation is over. One theory is that he likes a "clean" twitter history. But who the hell knows?
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Holy Stiction, Batman! WTF is hysteresis? (Score:2)
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I don't think I've ever seen such big words in a summary before. I may still have a dictionary on the bookshelf somewhere.
Um...you have the Internet right in front of you.
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A) You've mastered "dictionary", "bookshelf", and "somewhere", but an eight-letter word is too big for you?
B) Welcome to the Internet. You probably got here using "the blue e", right? Tip: highlight (whoops, big word) the scary word with your mouse, right-click on it, and click the option that will search or define the word for you! No need to go over to your bookshelf at all, and you get to avoid looking like a lazy ignoramus (sorry, is that one too long?) at the same time!
Seriously, I get that you're prob
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It's ok, I have a car analogy for hysteresis:
So, my first car was a 92 Plymouth Laser RS Turbo. It had a (rudimentary) cruise control system that wasn't quite capable enough for a turbocharged engine. The problem was that cruise control behaved as if throttle input was linear, but a 90's era turbocharged car is the very definition of non-linear throttle input. Thus, any time cruise was on and you hit a decent slope, the system would begin to oscillate. When the car slowed from climbing a hill, cruise contro
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The car ended up stuck in this long loop of "too slow, too slow, more gas more gas more more mooooOOOOH CRAP TOO MUCH TOO MUCH OFF OFF OFF WAY Ooooh that's better, much better this is nice, I'm right on, wait, no no no wait too slow, too slow more gas more more MOOOOAAAAHHHH TOO FAST TOO FAST..." which could sometimes last for miles, unchecked.
I had that happened to me when the fuel regulator for my car decided to go out. Line pressure leveled out once I got up to 65MPH. Slower than that it was all over the place. Drove straight to the mechanic to get it fixed.
Stuff happens (Score:2)
He's actually testing bombproof barges (Score:5, Funny)
which, when perfected, will be stationed in his volcano lair ....
splashdown (Score:1)
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When you last flew a jet somewhere, why wasn't it a seaplane? Surely such things would be an easier problem to solve than building airports.
Short of giving you the starter course in rocket engineering, I can only say no, it's not easier.
Hysteresis (Score:2)
"entered a form of hysteresis, a condition in which the control response lags behind changes in the effect causing it."
I had a girlfriend with that condition.
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"entered a form of hysteresis, a condition in which the control response lags behind changes in the effect causing it."
I had a girlfriend with that condition.
haha
Understatement of the year... (Score:2)
"Rocket landed on droneship, but too hard for survival."
https://twitter.com/elonmusk/s... [twitter.com]
Too hard for survival? By which you really mean 'it went SPLOOIE in an impressive fireball'? ;-)
In other news, a small amount of smoke was reported aboard the Hindenburg.
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Not clear whether or not you realize this, so to give you the benefit of a doubt:
A) In orbital launch booster rocketry terms, that's a pretty damn small fireball.
B) At the time he sent that tweet, nobody (Musk included) had seen the video from the barge. I doubt he even had the video from the airplane, probably just the telemetry data that showed touchdown followed by a loss of telemetry.
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No, I agree on both points. But when I watched the video from the airplane a few hours after seeing his tweet, with it still ringing in my head, I literally LOLed.
Almost there! (Score:2)
That video is really impressive. It's damn hard to hard a rocket on Earth and it looks like SpaceX has almost done it in a just a few iterations of their design. My *guess* is that the next attempt will succeed.
Once they do, the cost to put stuff in orbit will drop by an order of magnitude.
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You've got to slow the top half down too. Inertia is still a thing. If you suddenly stop the bottom but the top keeps going, you get half a stage. I'm not sure the body is built to take a load from that direction.