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An Engineering Analysis of the Falcon 9 First Stage Landing Failure
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 In a later tweet that was subsequently withdrawn, Musk then indicated that "the issue was stiction in the biprop throttle valve, resulting in control system phase lag."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.
A video from the barge itself is here. Everything goes wrong in the last second of landing, with over-correction putting it down on one leg, and then the leg crumples.
Bruce Perens.
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.
"I will trust Google to 'do no evil' until the founders no longer run it." Hello Alphabet.
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.
As I noted in a ./ post a few months ago, there would be a huge penalty in trying to bring down the booster in Florida. Much of Palm Beach County is indeed east of the longitude of Cape Canaveral, but the orbit would have to be close to polar for that landing to work. If the plan were to have the booster change direction and go back maybe a hundred miles would mean having to budget *lots* of fuel for the purpose--and have much less payload go to orbit.
No, I can't see putting down the booster on land except in the Bahamas.
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.
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.
Live today, because you never know what tomorrow brings
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.
Kind of reminds me of when Neil Armstrong crashed the Lunar Lander Simulator
Looks like they just need to add some no-skid pieces to the platform. That looks like it was really close alright. If that leg had planted just a tad more for half a second I think it would have made it ;) Almost feel sorry for it...it is trying sooo hard to stand up straight.
Really, that did look very good considering the difficulty. Get that lateral squared away a touch sooner, make the last second adjustment at the next to last second and they should be golden ;)
I wonder how much the barge affects that. How much variation in height from waves? Another foot or two in drop might have made a difference.
which, when perfected, will be stationed in his volcano lair ....
Why don't they just use a splashdown? It seems that corrosion resistance is relativly easier to solve than landing a rocket on a platform thats swaying in the ocean. You could even have it splashdown in a freshwater lake or a barge half filled with fresh water. You would also save the weight penalty of landing gear.
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.
In the end, that makes far less difference than you think because while you can reduce the amount of horizontal velocity that needs to be nulled you cannot eliminate it. (Not without launch criteria that include "near zero wind at the recovery site", which is beyond impractical.) The result is, with the current vehicle, you still have to null horizontal velocity at the last second before touch down. The basic problem is that the vehicle is badly designed for what it's being asked to do.
Both times they've hit the barge almost dead center - I fail to see how that's an arguement for a larger landing area since neither failure was caused by the landing area being too small. Both vehicles would have crashed regardless of the size of the landing area due to control system failures. (Attitude control on the first, throttle control on the second.) That's what neither you nor the OP seem to grasp.
"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.
Don't be apathetic. Procrastinate!
"Rocket landed on droneship, but too hard for survival."
https://twitter.com/elonmusk/s...
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.
Elon, please read this! You need ADAPTIVE CONTROL on your systems. Don't settle for less! Automatic loop stabilization compensation in the algorithms. The underdamped control loop is obvious from the video. Adaptive control would have seen the control delay and would have saved you lots of big $$.
Why not land the rocket on a large floating net instead of trying to land it on a small barge upright?
Wow. That really comes off like astroturfing. Also, really nice timing on the press release from ULA.
That said, the ULA scheme looks decent, but Musk is clearly after more than just recovering the engine. His plan clearly serves the dual purpose of perfecting automatic powered landings of large spacecraft, not just for recovery on Earth, but for landing on other large bodies as well.
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 ratio is so far out of the optimal range and a larger target area won't make it all that much easier.
That's "good at targeting" a couple of orders of magnitude better than what they've demonstrated to date (which is, pardon my french, already pretty fucking amazing). You're talking about some kind of articulated arm (which can survive being essentially inside rocket exhaust)... Which is, quite frankly, makes things much harder and more complicated and introduces a metric buttload of additional possible points of failure. Much easier to simply re-engineer the throttle valve.
Why not just let it fall on the ocean and have it inflate couple balloons so boats can pick it up again?
Seems like the whole landing it on a barge is just stupid.
It looks like the video shows the lag of the rocket thrusts reaction. First the thrust is vectored right and it stays right even after the tipping point. And after being several seconds late it vectors to left, causing the excess force.
From the video (https://vid.me/i6o5 - mentioned earlier) one wonders if the leg didn't collapse, but the landing momentum rotated the falling into the zone between the two legs, and being top-heavy it fell over. Just before the end of the video you can see the landed leg facing the camera - hope that wasn't mechanical failure rotating it.
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.
just have some electromagnets engage the legs upon touching and boom instant grab, horizontal velocity or not, you can accommodate horizontal velocities this way, second fix have some of the thrust rerouted with some tubing exiting at the top, this would help with the stabilization at the top as well with only minor modifications and nearly insignificant penalty in weight, too easy.. they need problem solvers these space-xers