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 In a later tweet that was subsequently withdrawn, Musk then indicated that "the issue was stiction in the biprop throttle valve, resulting in control system phase lag."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.

Comments

Video from the barge

[Bruce Perens]

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.

Re:Video from the barge

[TWX]

Damn that was close. Kind of makes me wonder if using the barge as such a small target is contributing to the hard landings, simply because it's such a tiny target relative to the area that the rocket has to come down on. It's about the length of a football field; makes me wonder if they could set-down on an area ten times that large if most of these control problems simply wouldn't matter.

Re:Video from the barge

[bledri]

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.

Re:Video from the barge

[DerekLyons]

Kind of makes me wonder if using the barge as such a small target is contributing to the hard landings, simply because it's such a tiny target relative to the area that the rocket has to come down on

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

[Bruce Perens]

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.

Re: Video from the barge

[Anonymous Coward]

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.

Re:Video from the barge

[cbhacking]

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.

Re:Video from the barge

[Firethorn]

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.

Re:Video from the barge

[catchblue22]

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 times. If you notice, those landings come down at pretty much the exact location they began at.

Re:Video from the barge

[SternisheFan]

Elon Musk Verified account 12:49 PM - 13 Apr 2015

@elonmusk

Odds of rocket landing successfully today are still less than 50%. The 80% figure by end of year is only bcs many launches ahead.

https://twitter.com/elonmusk/s...

You Can See

[The Raven]

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.

Re:You Can See

[Anonymous Coward]

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.

Doesn't look close

[tomhath]

FTFA:

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.

Re:I wonder why he bothers...

[bledri]

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?

Re:Alighting on land

[Bruce Perens]

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.

He's actually testing bombproof barges

[ankhank]

which, when perfected, will be stationed in his volcano lair ....

Hysteresis

[Whiteox]

"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.