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Here's a much better quality video of the previous attempt: https://www.youtube.com/watch?v=rAzwuEmZcmE

⬐ benihana

That thrust vectoring!

Landing attempt video: https://youtu.be/rAzwuEmZcmE

⬐ atroyn

I can't help but feel bad for that one poor little RCS thruster doing its level best to keep the first stage upright, but ultimately running out of fuel.

⬐ hyperbovine

Judging from the ensuing explosion there was still fuel left over.

⬐ darkmighty

Making it a little puzzling ask to why the engines didn't try harder to lower the vertical velocity on approach. Perhaps there are improvements to be made in the control systems.

⬐ fleitz

I have a feeling their engineers may have considered this. This is after all rocket science.

⬐ TeMPOraL

You'd be surprised what Bill lets Jeb fly at times... ( ;) ).

⬐ darkmighty

I also have this feeling, but from my limited experience with control systems you might want to try the simplest first.

There is a tremendous amount of variables to consider, and the analysis of how to take each into account is non trivial -- if you don't measure some variable precisely enough it might make the whole system less stable, the solution might get worse due to increased computational load, the conditioning might degrade, etc.

So with the number of controls they have and the predictive models available, I'm sure they're actually scrambling to refine their systems right now.

⬐ will_hughes

The thrust to weight ratio of a single engine firing is so high that they have no option but to fire at the last moment. Firing for longer would result in the rocket going back up, it can't hover.

⬐ TeMPOraL

Vertical was fine, it's the residual lateral velocity that tipped over and killed the rocket. Also, I can't find the tweet now (I think it was deleted) but initially Musk said they determined that 'static friction in bipropellant valves introduced a lag into system response that the control system wasn't prepared to handle'.

⬐ toomuchtodo

When near fuel exhaustion, the Merlin engine has a thrust to weight ratio above 1. Its firing is timed in order to land at a distance above the landing surface that will allow for a reusable vehicle (close enough to not snap off the landing legs). It carries only enough fuel to do this while also using that same fuel for hydraulically actuating grid fins for aerodynamic control while in a controlled fall to the ground.

Stopping a first stage rocket traveling back to an ocean going barge from supersonic speeds with substantial lateral velocity is hard, no matter what engine is performing its suicide burn while coordinating with RCS thrusters for attitude keeping.

⬐ justin66

> When near fuel exhaustion, the Merlin engine has a thrust to weight ratio above 1.

Wouldn't it have a thrust to weight ratio well above 1 all the time?

The ratio will be highest just as it's about to run out of fuel. About 160 to 1?

⬐ will_hughes

For the final stage of landing only a single Merlin engine is lit.

They're also limited in how much they can control the throttle on it.

Even at minimum throttle, it's not possible for it to hover.

⬐ justin66

Odd responses. I guess I shouldn't have phrased my response in the form of a question.

Here's another try:

> When near fuel exhaustion, the Merlin engine has a thrust to weight ratio above 1.

The rocket will always have a thrust to weight ratio well above 1.

⬐ manicdee

While your statement is technically correct in isolation, you are ignoring the context: "the rocket will have a thrust to weight ratio of greater than what is needed to hover and land safely as the Grasshopper demonstrated. Instead the F9R is limited to using the overpowered Merlin as a braking engine with the intent of reaching 0 soeed at 0 altitude with 0 lateral or rotational motion."

⬐ justin66

Yeah, it's easy to be a little too conversational sometimes in these comments, which leads to misunderstanding. In retrospect I get the feeling everyone understands the limitations of the hardware here.

⬐ None

None

⬐ peeters

I still don't understand the point of your comment then. Yes, the rocket's (potential) TWR will be above 1 at all points of the flight.

What does that have to do with the comment you're replying to? The Falcon 9 has 9 engines. The combined thrust of all of them has to be greater than the weight of the fully loaded rocket. That doesn't mean a single engine will always have more thrust than the weight of the entire rocket.

⬐ jmgao

That's a thrust to weight ratio above 1 when only one of its 9 engines are firing, and running at minimum throttle to boot.

⬐ sfeng

The RCS thrusters are powered by compressed nitrogen, not the rocket fuel that powers the main engines.

⬐ acadien

Musk commented himself on twitter something to the effect that the explosion is from the pressure difference of the fuel canister when it is breached. Also the fuel mostly burned at high heat and didn't explode per say.

⬐ olex

Looks like the landing leg on that side collapsed during touchdown. The thruster did all it could, but alas, it couldn't keep firing forever... nor did it have enough thrust to actually hold the rocket vertical all by itself, for that matter. An A for effort though.

⬐ dperfect

As if this weren't impressive enough already, according to a comment made by Scott Manley[1] it sounds like the final maneuver (where it appears the rocket is making a fairly strong lateral correction) is entirely intentional. Not sure if anyone here can confirm or deny the claim, but apparently the trajectory prior to landing is not supposed to be exactly above the barge as one would expect - I understand it may be an attempt to limit exposure/damage to the barge in case of failure.

[1] https://www.youtube.com/watch?v=C9I55o8hQgs

⬐ Shivetya

just the fact they can hit a barge after falling back from space impresses me. I wonder if there are any islands they could use instead, surely they had to have looked for one. Would a stationary oil platform work better?

⬐ vidarh

A volcano base if one is to judge by Elon's tweets.

⬐ noselasd

They do launches with different trajectories, so you'll run out of fuel if the landing site is fixed and you're launching the rocket in another direction. Besides, you'll anyway need something to transport it back to mainland.

If all this goes well though, the intention is to eventually fly it back and land it on the mainland.

⬐ zaroth

I was wondering about that too! I guess it does make sense if the final burn fails to not have a fairly large bomb dropping at terminal velocity directly onto the barge! On land I guess less of a difference though...

⬐ andygates

Not terminal velocity, just fairly fast: there's a deceleration burn earlier in the descent plan. If that doesn't work, the big red abort button can be used to blow it up at high altitude.

⬐ spectrum1234

This is pretty fascinating and makes a lot of sense. You obv want to land very slowly, but going slow means less control to keep it vertical. So the clever work around is use thrust at the last second which will allow it to also be vertical and stable. And that means coming in from a non vertical angle beforehand which is exactly what they do.

⬐ grecy

Elon says it's in slow motion too

"High resolution, color corrected, slow motion rocket landing video https://youtu.be/BhMSzC1crr0"

https://twitter.com/elonmusk/status/588463193070022656

⬐ cryptoz

If it's slow motion, I suggest changing the YouTube setting to 1.25x and watching it at that speed. It seems quite natural actually.

Edit: Or maybe 1.5 or 2x! I don't know..

⬐ Throwaway90283

I can't figure it out either. It's really difficult to determine the speed because of the scale involved here.

⬐ Daneel_

2x looks the most correct to me, based on the speed of debris falling at the end.

⬐ grecy

I replied to Elon's tweet asking for full-speed video.

Fingers crossed :)

⬐ None

None

⬐ None

None

⬐ monk_e_boy

How high did it get? Does it burn the whole way down (I assume it uses those little X-wings to 'fly down?) When does it start the descent burn? How fast is it going at this point? How much does it weigh, what sort of forces are we talking about? How hard is landing this puppy in terms of human achievement? Top 10% of tricky things humans have so far achieved?

[edit] wouldn't it be easier to hover it a few meters above land and grab it with some big robot arms?

⬐ jccooper

Stage separation is somewhere around 100km and Mach 6 or more. The exact figures will depend on the mission profile.

There's a "boostback burn" at altitude to slow the horizontal velocity right after separation, an entry burn to put it into the atmosphere, and a landing burn just before it hits the deck. The guidance in between those last two burns is aerodynamic from the grid fins.

The first stage mass is ~18,000kg empty (which it nearly is on landing), and it's coming in at its terminal velocity, which is, as a WAG, maybe 70m/s.

What's more, it cannot hover, as even one engine has a greater thrust at minimum throttle than the rocket weighs. So it must do what SpaceX calls a "hover-slam", and reduce its velocity vectors to 0 at the same moment its altitude gets to 0 in a single burn. Burn too short, you crash, burn too long, you go back up (and then crash.) And the horizontal vectors also need to be very small, and you can't end up too tilted either, or you fall over. And explode, because you're a thin-shelled tube with plenty of fuel and oxidizer still left. Oh, and you have to hit a very small target within a couple meters.

So it's a pretty tricky task. Plenty of rockets can hover these days, but landing a full-sized launch stage coming in from the edge of space is pretty tricky. And they almost got it. Looks like it overreacted just a hair to a gust of wind or something.

Your solution has two problems: (1) it cannot hover and (2) you'd have to design and build and test big robot arms. And put them everywhere you want to land. It's probably easier to make your big robot spacecraft, which you already have, do the landing itself on a plain concrete pad.

⬐ will_hughes

According to a (now deleted?) tweet from Elon, it was stiction in a control valve that caused a lag in response time, causing repeated over corrections.

⬐ lutorm

In general, burning while you descend is a terrible waste of fuel, because you have to generate 1G of acceleration just to keep your speed constant. (See http://en.wikipedia.org/wiki/Gravity_drag.) That's fine if you're not fuel limited (like the grasshopper tests) but if you want to land with minimal use of propellants, you want to use as high acceleration as possible. So the idea is to let the rocket drop freely and then light the engines just early enough that you can stop before hitting the ground.

⬐ None

None

⬐ teraflop

According to http://www.spaceflight101.com/falcon-9-v11.html, the first stage has an empty weight of about 20 metric tons.

⬐ sfeng

It burns three times after the second stage detaches. The first is to get the landing zone closer to the takeoff, the second slows it down from 1300 m/s to about 250 m/s, the third slows it to (hopefully) 0 m/s on the landing pad. Here's a diagram: http://media2.s-nbcnews.com/i/newscms/2014_51/812491/141217-...

If the 'x-wings' are what I think you're referring to, they're the landing legs. They are shaped like that to have as minimal impact on the air resistance of the ascending rocket as is possible. They open just before landing.

It can't hover. Even just using a single one of the nine engines, the engine can't be throttled down enough for the rocket to hover. The landing method relies on what they call a 'hover-slam', where they fire the engine during landing to slow it down just before it touches.

Landing this is extremely impressive. It is going four times faster than the speed of sound, and they are landing it on a pad smaller than a football field. Vertical landing of a returning rocket has never been done. It's also the key step in the process of dramatically lowering the cost of spaceflight which could usher in a new era of technology around earth, and on other planets.

⬐ swamp40

Maybe it needs to hover? There must be something that can be squeezed to throttle it down some more. Call the NASCAR guys with their restrictor plate designs.

⬐ TeMPOraL

IMO that landing clearly proved suicide burning is working perfectly fine - they just need to fix something to get rid of the remaining lateral velocity.

⬐ monk_e_boy

Aah, grid fins were the things I was thinking of. That first burn must have some complex maths behind it. Do we know anything of their simulation software?

⬐ None

None

⬐ 69_years_and

What's the difference between the tests on earth (as seen on youtube etc) where the engine runs the whole time and seems to display good throttle control and this type of landing?

⬐ Two9A

The Grasshopper tests had a full tank of fuel and (if I recall) additional ballast, to counteract the thrust of the engines. That's what allowed them to hover and perform the various low-velocity translation tests.

Of course, these barge landings are the first high-velocity tests, so there'll be kinks to work out.

⬐ 69_years_and

Thanks, that would explain that.

I was impressed by how small the barge looked compared with the F9 it's clever that the F9 can autonomously target such a small area. If they keep targeting 'Just Read the Instructions' by the time land based landings come to be it will be nailed. Awesome stuff!

⬐ zaroth

It was jaw dropping as-is, but when they finally do stick a landing I imagine the engineers will be running around like little kids screaming with the pure joy and exhilaration of it all. I hope they share the video at mission control as well when the day does come!

The sheer drama of how fast it comes down, the awesome power of the Merlin engine just blasting away delta-v... I wonder how many g's it takes on that final burn?

Would a drougue have simply added too much weight or too much complexity to deploy? I guess slowing down the freefall velocity before the burn just doesn't really help all that much?

Amazing that Falcon Heavy will have 27 Merlins on board.

⬐ justin66

> Would a drougue have simply added too much weight or too much complexity to deploy?

Never mind the weight: the problem sited with this landing was excessive lateral velocity. A drogue chute would have the potential to make that problem worse rather than better.

⬐ frakkingcylons

A drogue chute strong enough to avoid shredding at these speeds would be too heavy.

⬐ jccooper

A drogue would seem unnecessary; a Merlin can scrub off plenty of m/s real fast, and if it can't do it fast enough, they have more of them. In fact, it might be easier if the thing is coming in faster, because you get to run the engine a bit longer and have more time to adjust your aim with the engine's greater control authority.

Anyway, they don't seem to have any problem managing the vertical velocity. It's been the horizontal each time. And this time, only a tiny bit off.

⬐ staunch

The best part of their reusable rocket project is how incremental the progress has been. I'm not sure I would have appreciated the difficulty of what they're doing as much without these failures.

⬐ bane

Even cooler is how they're pulling off these tests on production flights.

⬐ trose

THE best part in my opinion is that they live stream every mission and are very transparent about failures. I think they realize how many geeks and engineers are watching and that we understand that failure !== defeat.

⬐ jozzas

Getting paid to develop product features is the best way to do it!

⬐ calinet6

A ton of lessons here.

If you deliver your primary objectives well and keep improving at everything else in the process, your customers will keep on loving you for it.

⬐ corywright

This is an "unpublished video" on a non-SpaceX YouTube channel. Who uploaded this?

⬐ corywright

SpaceX has now published the video on their channel: https://www.youtube.com/watch?v=BhMSzC1crr0

⬐ lutorm

I guess that's what "This video is unlisted. Be considerate before sharing." gets you in today's world...

⬐ stevesearer

Also somewhat interesting that it appears to be the same angle as the official video posted to Vine [1], only in drastically higher quality.

[1] https://vine.co/v/euEpIVegiIx

⬐ smackfu

And the vine stops before the explosion.

⬐ atroyn

The video appears to be a leak from inside Space X.

⬐ palebluedot

From reading the comments over on /r/spacex, this looks to originally have been an unlisted video in SpaceX's youtube channel, and shortly after appearing on reddit it was made private. Not before several people had made copies, however, and re-uploaded it (one of those people has the same reddit username as the youtube channel linked here).

That said, impressive video. This shows how close they are truly getting.

Unfortunately, it doesn't stop clickbait headlines that imply that this was a setback (for a while, drudgereport was running a story with the headline akin to "SpaceX rocket fails to land on landing pad"). Maybe that is why the video was made private, out of fear of appearing like a failure.

Of course, now that the cat is out of the bag they are better off embracing the video.

⬐ hydrogen18

One great thing about being a government contractor: Public opinion isn't something you need to worry about.

⬐ agumonkey

According to a comment from a collaborator of Musk, saying they dealt with larger public distrust before, clickbait headlines won't make a difference to Musk and his team. I'm sure they'll get through with this one.

⬐ filvdg

this footage give a real sense of how close they are to getting it right

⬐ waterlesscloud

Watching it made me realize I only had an abstract idea of how difficult it was to even attempt this. It's insanely hard, and they're so close!

⬐ 31reasons

Yes its a very hard problem but what makes it hard ? Is it engineering of the components, algorithms or expensive tests and economics of it ? Maybe people before Elon Musk were not interested in solving this problem not because its technically hard problem but because their existing business model is much more lucrative.

⬐ InclinedPlane

It's hard not because the engineering is fundamentally difficult (it's challenging but compared to a lot of engineering problems not terribly so) but rather because it's very unforgiving and it takes a great deal of foresight and risk management / business savvy to do what SpaceX has done efficiently and cost effectively.

Look at a couple of the issues they've faced and managed to deftly conquer. Take vehicle design, for example. If you design a purely expendable launcher you tend to want to minimize engine count and concentrate on 2nd stage performance, which leads to a design with an expensive 2nd stage and a cheaper 1st stage with very little throttle authority (due to using a small number of engines). That makes reuse very difficult and not very worthwhile because you'd need to either try to reuse the 2nd stage (a much more challenging task) or to heavily modify the 1st stage, which would then only save the cheapest component of the launcher. So if you want to concentrate on reuse you need a vehicle like the F9, with lots of 1st stage engines, and most of the cost in the 1st stage.

But even then you need to figure out how to run your business. It would be easier to envision the launcher with reuse an integral part of the design from day 1, which would lead you towards testing the reusability aspects of the design prior to the first launch. But that would be tremendously expensive and would make it more difficult to keep a company running while that was going on. SpaceX has been able to provide numerous orbital launches with their rocket and earn a tremendous amount of revenue before it is capable of actually being operated in a reusable fashion. A common engineering dictum is "you ain't gonna need it", to avoid introducing unnecessary design elements that aren't strictly immediately necessary. But without designing the Falcon 9 with reuse in mind from day 1 they would have had a much more difficult time in developing the reusable aspects. Note that SpaceX has made use of hundreds of millions of dollars of flight hardware in their various landing tests, all of which they not only didn't have to pay for out of pocket but actually earned a profit for. That's smart business. Meanwhile, for all of the launches where they haven't attempted landings they've added operational experience and real-world testing of their overall vehicle design and components.

Big, bureaucratic companies that are bringing in billions a year in revenues (like ULA or Arianespace) don't tend to engage in behavior like this, because they don't have the hunger, they don't have the boldness, they don't have the coherent foresight. What SpaceX has done is something that a lot of other companies are going to replicate, likely fairly successfully, once it's been proven. But getting to that point has required not just engineering acumen but enormous risk management skills and business savvy. They'll have a significant market advantage due to their head start for quite some time, and hopefully they will retain enough boldness and savvy to continue moving ahead even as others attempt to catch up.

⬐ imaginenore

You have to deal with real physics and the real physical world - turbulence, friction, lag, wind, waves, none of which are predictable.

⬐ jccooper

From a technical standpoint, it's tough. You've got big expensive machinery that operates at or past any reasonable limits in extreme conditions and moreover gets destroyed with every test if you make the slightest mistake. I know that when I end up with an iteration interval of an hour it seems like development takes forever. SpaceX is able to test, what, every couple months?

But the economics is probably more important. The incumbents have such sky-high development costs that reusability seemed like an impossible task to do economically. Last time anyone tried they came up with STS, which was highly capable but ultimately dangerous and ruinously expensive. A new reusable system seemed like it would take half a trillion dollars in development, and would probably never have the flight rate to make it worthwhile. Elon thought that he could do it better. NASA believes designing a reusable system would be too expensive for them. They're probably both right.

⬐ ansible

It's called a "suicide burn" for a reason. Even with just one engine, the thrust/weight ratio for the nearly-empty stage makes landing difficult. The engine can't throttle down enough.

So you can't just come to a near-hover, and then ease you way onto the landing pad. You have to cancel your velocity right at touchdown, with little margin for error.

⬐ mrep

I agree. I feel like if they had a larger area to land on they would be able to get it. That way, they could focus less on pinpointing the exact position, and more on keeping it upright and stable.

⬐ joeyspn

Yup, looks like they're about to crack it.. exciting times

⬐ NDizzle

Watching this footage made me immediately go to SpaceX's website and look at job openings.

⬐ Udik

Come on people, have a go at it yourselves:

http://moonlander.seb.ly/

⬐ will_hughes

Someone needs to make a version of this for a Falcon9.

⬐ Veratyr

Or even better:

https://kerbalspaceprogram.com/

⬐ Rapzid

I wonder what happened there. Looks at 4 seconds the guidance system figured it needed to pull an ultra manoeuvre to slow the the horizontal speed(was drifting left) but ended up with a solution putting the rocket upright again at nearly 0m. If it would have left some altitude for stabilizing I think they would have had it.

Arm chair rocket science is fun.

⬐ InclinedPlane

As mentioned, it's control system lag, which led to over-correction oscillations. This is very straightforward to deal with.

⬐ tsotha

I find this a bit surprising given the amount of testing they did with F9R.

⬐ TeMPOraL

According to Elon's tweet that seems to have disappeared by now, they had static friction in bipropellant valves introduce a lag that the control system couldn't handle.

⬐ shostack

Can you or someone else please explain what that means in slightly less technical terms without dumbing it down too much?

⬐ jffry

So presumably this is a valve controlling fuel flow to the engine. The valve is hooked up to some form of motor/actuator, and the rocket's control system needs to open/close the valve to control some aspect of the thrust.

Since valves cannot open instantaneously, the control system has to know how long the valve takes to open/close, and start that process a little bit early so the valve is open when it needs to be open. If something goes wrong with the valve, it doesn't open as quickly, and so the computer model thinks it should be open and isn't, which could easily go off into an unstable state.

⬐ TeMPOraL

A control system is a feedback loop that connects measurements (orientation, speed, acceleration) and steering (grid fins orientation, engine power and gimbaling, RCS/other means of attitude control) in order to steer the entire system into a desired state (in this case, upright orientation and 0m/s velocity exactly at the point of touchdown). The control system measures the state of the rocket and directly adjusts the control inputs - i.e. it's not a series of if-else instructions, but more like "engine_gimbal_x = P * yaw + I * integral(yaw) + D * dyaw/dt"[0] - the inputs and outputs are wired together through a set of mathematical transformations.

The "lag" is a delay between your steering signal and the state of the system changing in response. In this case, because of static friction, valves controlling fuel flow reacted with delay (I'm not sure if they're talking about Merlin's valves or attitude control engines though) - so the rocket did not react immediately after being told to do so.

There's a whole math field around that topic, known as "control theory". There are ways to determine the limits of inputs and outputs that will allow your control system to keep everything in the desired state. In this case however, the input lag wasn't accounted for and it pushed the system out of the space of stable states, beyond the ranges where the control unit could keep up with the changes, so it ended up oversteering. Fortunately, in this case it's mostly a software problem, i.e. they could tweak the controller to deal with the lag.

The best way to imagine it is to recall your first experiences with a bike or ice skates - trying to balance yourself on them is exactly the kind of thing the rocket was doing, probably with similar results.

My control theory is a bit rusty nowadays, but I hope this clears up some things.

[0] - if you ever hear about "PID controller", this is exactly where the name comes from - proportional, integral, derivative.

⬐ shostack

This was exactly what I was looking for. Thanks for taking the time to go into this level of detail.

So looks like if they can solve for modeling this latency (likely a software problem), there's a decent shot they'll nail it the next time around?

⬐ TeMPOraL

If this turns out to be the only problem then yes, they have a decent chance of doing a proper landing the next time around.

⬐ Rapzid

It made a solid effort.

⬐ TeMPOraL

That it did, and it almost succeeded.

⬐ gridspy

Based on this quote "static friction in bipropellant valves introduce a lag that the control system couldn't handle"

It sounds like the fuel valves which control the engine's throttle took longer to turn up or down the rocket than expected.

The control system that is constantly processing rocket position and turning that into throttle outputs couldn't handle how long it took for decisions to turn into actions. That tends to cause wobbling in most controllers (overshoot, undershoot, etc).

⬐ shiftpgdn

Why did it explode in such a dramatic fashion? I thought the booster was almost entirely empty when it returns to the barge.

⬐ revelation

It's hard to appreciate the energy density of the fuel they're using, but this makes for a good example.

⬐ jccooper

The rocket is, at the very least, a thin pressurized tube full of kerosene vapor and oxygen, and probably some decent amount of liquid kerosene and liquid oxygen. Crack it open and it's gonna make a pretty good flash. But as rocket explosions go, that wasn't very big. Get to YouTube if you want to see some real rocket explosions.

⬐ TeMPOraL

Check out, for instance, Proton-M going full Kerbal in 2013: https://www.youtube.com/watch?v=Zl12dXYcUTo.

⬐ jccooper

And here's what it looks like when a rocket of similar class, but fully-fueled, explodes: https://www.youtube.com/watch?v=bx1CeHFeea0

⬐ unspecified

Thank you so much for this. The anticipation while waiting for the sound of the explosion to arrive got a cackle out of me.

⬐ richeb

If you think that was dramatic, look up on YouTube some rocket explosions on the launchpad when the tanks are full.

⬐ agumonkey

It felt like the first time I saw the Death Star blow up in SW. I didn't expect so much energy, more like slow metal wreck.

⬐ benihana

Yep, it's almost empty. It shows how much stored energy rockets deal with and how hard it is to get out of this gravity well we call home.

⬐ nickff

There is always some extra fuel and oxidizer, as it is used to slow the vehicle, and is needed until touchdown. If the vehicle were to run out of propellant before landing, it would crash. In addition, the LOX makes any combustion spectacular.

⬐ trothamel

It probably would explode before crashing. The various machinery in the engine expects fuel to be there, providing resistance. From what I understand, if gas was to be sucked into the engine, the end result would be a rapid disassembly.

⬐ erobbins

In addition, the tanks are slightly pressurized which adds to the effect.. probably 30-50psi begging to be released :)

⬐ melling

Complete destruction is obviously expensive. Finding a way to prevent an explosion if it falls over after landing seems worthwhile.

If it hovered for 5 seconds at .5 meters, for example, could it burn off enough extra fuel?

⬐ CHY872

Their eventual goal is for every rocket to land and not have problems. Providing enough fuel for five seconds of hovering is a waste of valuable fuel (see how they're already so low that they run out in the process).

Also, falling over almost certainly also leads to a writeoff, as otherwise they'd just fly the rocket into a big pit of fire retardant foam blocks.

I'd imagine that the lateral stresses involved in a crash landing are more than enough to write off the rest of the rocket, and so the explosion would just make the barge slightly harder to clean up.

⬐ melling

Of course, their plan is to not have problems. That's everyone's plan. It's planning for when things don't go right that makes the difference.

⬐ TeMPOraL

I imagine making the rocket sturdy enough to withstand falling over would be prohibitive in terms of both costs and reduced capacity, so it makes sense to them to ignore this problem completely and work on perfecting the landing so that the rocket does not fall over.

⬐ mikeash

They are planning for when things go wrong, which is why the landing ship is unmanned and in the middle of a big exclusion zone in the ocean.

⬐ melling

I'm talking about the economics of failures. Insurance prices will go down, the cost to orbit will be cheaper. Could probably land at the Cape, decreasing turn around time. We'll have flights everyday!

⬐ mikeash

The explosion isn't going to cause significant damage to the landing ship or the landing pad. There isn't much fuel left onboard at that point anyway, so it's fairly superficial. The rocket is scrap if it falls over regardless of whether or not it explodes, and the explosion won't hurt much, so there's not much reason to put effort into reducing the risk of an explosion in a crash. The plan is to eventually land at the Cape once the technique is proven, but there's no need to make a rocket that can tip over without blowing up to achieve that.

⬐ melling

Ok, just thought the rocket could be salvaged if it didn't get blown to pieces. That's the goal. Reusable rockets are going to lower the cost.

⬐ mikeash

I think that once it falls over, it's scrap. There's so little margin that once you even suspect things are bent, you pretty much have to rebuild it anyway.

⬐ vonklaus

> Finding a way to prevent an explosion if it falls over after landing seems worthwhile.

They probably are spending more effort into not letting it fall over.

⬐ cryptoz

> If it hovered for 5 seconds at .5 meters, for example, could it burn off enough extra fuel?

The Falcon 9 cannot hover. The thrust of the single landing engine, even when throttled down to minimum, will provide significant lift to the empty rocket. That's part of what makes this maneuver so difficult and so awesome: it's called the hoverslam, where you achieve 0m altitude and 0m/s velocity at time = 0.

For reference, though, the Grasshopper/F9R-dev1, could hover due to it only having 3 engines, not 9, and because they could ballast it with additional fuel.

⬐ TeMPOraL

> it's called the hoverslam, where you achieve 0m altitude and 0m/s velocity at time = 0

Also known as making a "suicide burn", at least in the KSP world.

⬐ mladenkovacevic

It must be amazing working in a field where failure is almost as exciting as success.

What's the main obstacle to slowing down and stabilizing the rocket on its descent to the platform? Is it just a matter of speed or also the shape of the rocket and its center of gravity? Would a set of 3 parachutes might slow it down a little bit or at the very least straighten it before the rockets need to kick in saving some fuel?

⬐ jccooper

SpaceX seems to be able to stabilize fine with RCS and/or their grid fins. The rocket design must be reasonably stable--even tail-first.

I don't think incoming velocity is much of a problem. Terminal velocity is gonna be no more than 90 m/s. The engines on that stage are more than capable of slowing it down--and fast. In fact, they're too powerful--it can't hover.

⬐ mladenkovacevic

If it's a matter of control then all it needs is just a bit more fine tuning. My money is on success next time they try it.

⬐ ufmace

Anybody know anything about the tech they're using to guide the stage to the barge? Since the flight is so short, I suppose they could get a GPS position on the barge at liftoff and pre-program that as the initial target, but I'd think they need some kind of terminal guidance actually tracking the barge to hit it reliably. Maybe visual or radar, or a radio beacon or something?

⬐ mark-r

I've always imagined that the big bulls-eye wasn't painted on the barge just for decoration. I don't have any hard info though. Come to think of it, it would be hard to get video through the rocket exhaust.

⬐ grecy

On the live feed, just after +1:30 I heard "Recovery platform has acquisition signal"

⬐ grecy

We know it does three separate burns while coming back down (Boostback, entry burn, landing burn).

I wonder if each burn starts with the RCS thrusters to turn the stage "nose up".

i.e. when it's free falling, does it always roll around and fall nose down, requiring RCS to get it back to nose up?

It would be awesome to see footage from the entire return trip. Do they have cameras mounted on stage 1?

⬐ rdoherty

From what I've seen there are vanes on the top that guide it down, keeping it upright during reentry and descent.

http://images.gizmag.com/inline/crs-5-crash-5.jpg

⬐ cfreeman

They're called grid fins:

http://en.wikipedia.org/wiki/Grid_fin

I believe they are less to "keep it upright", but for better accuracy. The rocket doesn't really having a hard time staying upright because all of the weight is on the bottom where the engines are.

⬐ socialist_coder

Is there a reason you couldn't have things on the side of the landing pad that "grab" onto it and secure it?

It could be as simple as a huge ring of cable that just closes around the top of the rocket, then pulls it tight on all sides- like the guidelines on a tower to keep it stable.

Maybe they don't want to do that because they think they can do it without it?

⬐ polemic

It might also be that the structural changes required to handle a grabbing force would be too extreme. At that point it's basically a thin shell, so a grabber or cable might just slice it.

I like the 'but then it can't land on mars' answer too though.

⬐ iyn

I was thinking the same, seems that it would be the easiest way to stabilize the rocket. I'm curious what were the reasons for not using this idea - for sure they though about it, but concluded that the current way is better.

⬐ Lorento

If you want a more intuitive mechanical system, see the Boeing/somebody 1st stage engine recovery plan that uses a heat shield and parachute to land just the engine, not the whole stage body.

Don't forget the barge is also actively controlled to keep it stable. If you have sufficient control systems, you don't need mechanical aids. Just like humans can walk on two legs every day our whole lives using our brains, so we don't need four for "extra safety".

⬐ TeMPOraL

> Maybe they don't want to do that because they think they can do it without it?

If they master vertical landing without external support equipment, they can pull it off on Mars as well. I think that's their other goal here.

⬐ NamTaf

That's certainly the case. It has to be entirely self-contained to the rocket.

⬐ swamp40

What happens once it lands successfully?

Won't one stray wave tip it right over into the ocean?

Those landing struts are only 1/10 the length of the rocket.

If this wasn't so tall, it wouldn't be nearly the problem that it is.

Are they perhaps really trying to develop a Dragon landing system?

And using this extra tall rocket as a worst-case scenario?

⬐ Osmium

I'm not an expert, but here's my understanding:

> What happens once it lands successfully?

It becomes a lot easier to get permission to attempt a landing on dry land. But, as I understand it, they will always need to land some rockets at sea. In the long run, they hope that they can then re-use the rocket, which will drastically reduce the cost of launches.

> Are they perhaps really trying to develop a Dragon landing system?

They almost certainly are, but this is unrelated to that, I believe.

> And using this extra tall rocket as a worst-case scenario?

No. The length of the rocket is ultimately determined by the amount of fuel it needs to carry. The first stage will always be very long like this. Note that this rocket came from a successful mission which launched a Dragon capsule to the ISS on a re-supply/science mission.

⬐ smackfu

>But, as I understand it, they will always need to land some rockets at sea.

Yeah, this is something I didn't realize. Flying back to land requires fuel, which cuts into the payload capacity. If a payload is too close to capacity, a sea landing may be the only option.

⬐ smackfu

Since it's empty of fuel, and has big metal engines at the very bottom, the center of gravity is surprisingly low. So it should be fairly stable immediately after landing. And then they plan to secure it so it stays upright.

⬐ atroyn

Space X would like to return Falcon to dry land, but given the high chance of failure, they're first trying to do it on the drone ship "Just Read the Instructions" at sea. That way when things go wrong, it's not such a big deal.

⬐ swamp40

But if the sea is the problem...

⬐ atroyn

It's a carefully considered tradeoff. They've managed to get to the drone ship successfully twice now, and they will need this kind of accuracy in the future.

⬐ stevesearer

The goal of the barge landings is to prove they can land precisely in a targeted position repeatedly to prove they are able to touchdown on land without accidentally hitting populated places.

⬐ dandelany

From Elon's AMA: How would the Falcon 9 would be secured onto the deck of its autonomous spaceport drone ship?

"Mostly gravity. The center of gravity is pretty low for the booster, as all the engines and residual propellant is at the bottom. We are going to weld steel shoes over the landing feet as a precautionary measure."

⬐ trurl

"Just Read the Instructions"? Sounds very Culturey.

⬐ TeMPOraL

That's straight from Banks. As is the other barge, that is to be named "Of Course I Still Love You".

http://www.tor.com/blogs/2015/01/elon-musk-iain-m-banks-just...

⬐ peeters

Anyone know how fast the engine's gimbal is? The video makes it look like the gimbal lags significantly behind the angle of the rocket, so even when you're rotating one way, you end up burning more toward that angle as the engine slowly gimbals to the other side.

Granted, this is in slow motion. I just would've expected the engine to be gimballing to the other side even before there was a noticeable rotation.

⬐ jordanthoms

It's interesting to think that if you are building rockets, up til now you've never really gotten to take the rocket apart _after_ flying it - it's burnt up in the atmosphere or at the bottom of the ocean somewhere. I imagine there will be lots of interesting insights that come out of recovering, undamaged, a flown first stage and taking it apart to see what's experienced wear etc.

⬐ vinkelhake

NASA has been recovering their rockets from ocean landings for some time.

See: https://www.youtube.com/watch?v=Gbtulv0mnlU

⬐ TeMPOraL

But only after getting them totally messed up by salt water. Also, those were solid-fuel rockets, which are significantly simpler than liquid-fuel ones. There's probably a lot to learn from an undamaged liquid-fuel first stage that went to space.

⬐ hurin

Question: Why don't they land into the water instead of a hard surface? (Wouldn't this give them more leeway in terms of impulse?)

Edit: i.e. landing on a net stretched (under the lake/ocean-surface) by 3 or 4 ships - reel the net it in to raise stage out of the water, I'm pretty sure seawater is not significantly corrosive from 2-3 hours of exposure.

⬐ hydrogen18

This how NASA recovered the SRBs. They landed upright, much like SpaceX's rocket. They were bouyant and floated vertically. The tender would approach the SRB, hook onto it, pump more water into it so it would sink. It would then carry it horizontally back to land.

Seawater is incredibly corrosive and as it evaporates it leaves behind a fine layer of sea salt. Since these were SRBs, they had few moving parts. They were completely disassembled and the individual components were inspected and recertified prior to the next launch.

Also, just to give you some scale of how crazy the Space Shuttle program was: the SRBs are the largest lift platform that was ever in use. ICBMs are fireworks compared to them. To land in the water, the SRBs needed not one but 3 of the largest parachutes ever produced.

⬐ TeMPOraL

Correct me if I'm wrong, but AFAIR SRBs weren't really reused, it was cheaper to make new ones than refurbish those they fished out.

⬐ hydrogen18

https://en.wikipedia.org/wiki/Space_Shuttle_Solid_Rocket_Boo...

Some of the parts from the first missions were re used on the last. The SRBs weren't disposable, but they were essentially refurbished after each launch.

⬐ olex

Currently they land on an autonomous ship (huge flat barge with stationkeeping engines and autonomous control). Reason is, they haven't yet proven that their approach and landing are accurate enough to get permission to try a landing on land. If the rocket misses the ship, it goes for a swim and damages nothing except itself. It it goes out of control during approach over land, there's much more damage it can do to all kinds of things.

Also, there's no land downrange from the launch site, and returning the stage to land at the launch site costs a lot of extra fuel. It is possible and actually planned to do so later (SpaceX is constructing a landing pad for this purpose at the Cape at this time), but for now they are trying to get the ship landings down properly.

⬐ rdoherty

They have landed it in the water once as a proof of concept. Seawater is very corrosive and would require cleaning & inspection of nearly all the components.

Landing on a dry surface means they can eventually reuse rockets soon after landing (a few days I believe).

⬐ elif

same reason they don't land planes in water. it would damage the plane, and then you'd have to get it out of the water somehow.

⬐ ZoFreX

The eventual goal is to land it on land, they're only doing water landings right now due to the high chance of failure. In the future, the first stage will land back at the rocket pad, which massively reduces the logistics for re-using it.

There's a cool mock-up video of what that might look like: https://www.youtube.com/watch?v=4Ca6x4QbpoM

⬐ pbreit

Sorry, but I'm failing to see how sending a large barge out into the ocean constitutes "massive logistics".

⬐ hurin

So you're saying the real cost-savings don't kick in until you're landing it back on the pad, hence salvage from sea is only important as far as perfecting the technology?

⬐ benihana

Yep. They're doing it at sea because they're in the late alpha stage of this reusable program.

⬐ solve

Seems that the control system only tries to guide it on the X & Y axis, but if I was controlling it and saw it coming in at that steep angle, I'd fire hard on the Z axis to try to raise it up, fix the angle, and then lower it down again.

Is it correct to say that this landing control system never tries raising the craft vertically?

⬐ pjc50

They can't turn it off and on. They can restart it once and it has too much minimum thrust to hover.

⬐ grecy

It's tricky because it can't hover, one engine on minimum provides too much thrust.

So if you do raise it up, you'd have to stop the engine, then wait a little and re-light it again.. which I think is tricky to do, and the engines only have so many "re-light" cycles in them, so you don't want to use up too many , else you can't re-use the thing for another real flight.

⬐ ezegolub

If any of the engines has a TWR above one when landing, even at it's lowest setting, i can't help but wonder why don't they use a smaller engine which can be more effectively used to land.

I'm sure theres a perfectly good reason for this, there's a reason SpaceX has its reputation, but i'm curious as to why.

⬐ dlgeek

Because the primary job of the engine is to get as massive a payload as they can to orbit, not to land. That's just a bonus.

⬐ sfeng

For one, developing reliable rocket engines takes a long time, you can't just whip up a new one. The current Merlin has a very large throttling range already when compared to existing engines. The size of the engines they use now is defined by the need to get payload's to space, the landing has to be secondary.

⬐ simonh

Grasshopper was able to hover, or even perform a powered descent and was probably lighter than an F9 first stage, so I don't think this can't be it. They likely do have an engine configuration capable fo a lower minimum thrust, but they're choosing not to use it presumably because it's not optimal for the primary mission, and they believe it's not necessary for the landing.

⬐ jccooper

Mostly, I think, because they'd prefer to avoid having several types of engine involved. SpaceX is big on cost savings through commonality. The development and tooling and testing etc. for creating a smaller landing-only engine would probably cost darn near as much as for the main engines, so if you can get it done with them, it makes sense to do so.

If they can't get them on the ground reliably with just the Merlins after trying enough, then maybe they'll do something like that. But if it's a simple matter of software and a handful of tests, as it seems may be the case, then they've saved a lot of effort and cost.

The other answer is that the F9 architecture was designed (and flying) before this mode of recovery was "the one". And they'd prefer to not redesign the thing if it can work as-is. So they'll try it and see before spending gobs of money on a re-design. Rockets aren't software (except to the extent they are) and changing things about them is dangerous and expensive.

⬐ magic5227

I almost think it would be easier to let it do what this video shows, and build a giant metal net to just prevent it from sinking. Or is that harder than landing on a small platform?

⬐ mb_72

My thoughts too, but I guess any ingress of sea water makes it necessary to junk the engines. Also, once you've solved the 'landing problem' once, presumably the method can be reused on each and every subsequent landing; if you are looking at many such landings it becomes worth doing it 'once' and properly.

⬐ TeMPOraL

> if you are looking at many such landings it becomes worth doing it 'once' and properly.

Yup; the point is, SpaceX is not trying to just recover the first stage, they're trying to solve the vertical-landing of rockets for rapid reusability - which is vital not only for cost reduction, but for their plans to go to Mars. Because if they solve landing here, they can use the very same technique to land on Moon, Mars and other planets.

⬐ Throwaway90283

It looks small in the video, but that first stage is the height of a 20 story building. It can't tip over without destroying itself, and it's not something you want to get wet.

⬐ ansible

The goal is not to recover the stage per-se, but to land successfully.

If they get a the 1st stage down on the drone ship, with all four legs touching, the main engine shut off, and it stays upright for even a few seconds, there will be much celebration. They would, of course, want to then actually examine the rocket afterwards, but that is a bonus.

⬐ bane

Pretty inspiring after all this time that the problem is not so much getting stuff up, but landing it successfully. I kind of feel like this is where we should have been in the 70s.

⬐ jfoutz

Pretty sure the original plan was mars after the moon. The cold war shifted away from the space race, and we built skylab instead. Some of the mars plans were pretty crazy. send 3 rockets. 1 with crew and supplies, 2 for the return trip. 2 just in case something went wrong.

⬐ lotsofmangos

I wonder if you could put thrust deflectors on the legs, then vary the fold out to help stabilise it just before landing.

⬐ agumonkey

Someone on ArsTechnica said it was a sticky valve causing thrust lag and trajectory swing.. anyone thought the same ?

⬐ paulddraper

That's tricky stuff.

What was the intention behind not doing a water landing, like with the Space Shuttle boosters/tank?

⬐ pixie_

Mars doesn't have water.

⬐ scott_karana

I think the plan is to eventually land it somewhere that it can be checked, refueled, and sent straight back to space.

The barge, I suspect, is simply for safety/liability at this point ;)

(Even if I'm dead wrong, and they want to land it on a barge forever: seawater is some nasty, corrosive shit that I wouldn't want my rocket dunked into! :)

⬐ DannoHung

Such a bummer! So close. Keep it up guys, you'll get it soon!

⬐ atroyn

That's two for two for getting to the ship, definitely not a bummer!

⬐ ezegolub

Any info on how much damage the barge took from that explosion?

⬐ sheerun

"Droneship is fine. No hull breach and repairs are minor. Impact overpressure is closer to a fast fire than an explosion."

https://twitter.com/elonmusk/status/588490238936240129

⬐ rtpg

this might be a silly question but why can't they use parachutes? wouldn't a mostly empty rocket float? or is there an issue with the salt water and whatnot

⬐ MrSourz

They intend for it to land on land in the future, not out at sea. For that they cannot use parachutes.

⬐ obituary_latte

Was this filmed from a drone? I couldn't find any info.

⬐ sesutton

The only thing I could find describes it as a "chase plane". I know one time they used Musk's private jet but I think that was an emergency replacement.

⬐ tommoor

In the middle of the ocean It seems doubtful... I'm also intrigued by the angle - maybe a helicopter?

⬐ obituary_latte

Plausible it could be launched from the landing platform prior to the attempt. Although, you'd think a drone could have a closer viewpoint. Anyway, very cool to see.

⬐ mikeyouse

SpaceX has a chase plane that films the landings from afar;

https://twitter.com/elonmusk/status/588142879245238273

I can't seem to find what type of plane it is though.

⬐ dude3

It looks like the main engines didn't cut quick enough.

⬐ natarius4k

Makes me wonder if they just leave them on the ground of the ocean or recover them?

...I bet otherwise the Russians or Chinese would love to get their hands one of them :)

⬐ 31reasons

Chinese are just waiting until it succeeds. Their hackers are probably on stand-by to download the blueprints and the code. lol

⬐ transfire

Its tail is wagging way to much. Landing will always be a crap shoot if they don't get that stabilized.

⬐ TeMPOraL

You mean thrust vectoring? It's intentional, it's how the rocket steers itself.

⬐ transfire

You mean it's not pixie dust? ;-P

⬐ TeMPOraL

Naah, pixie dust is what the rocket is covered with before launch; you can see it falling off during lift-off. Check out the launch videos. ;).

⬐ rebootthesystem

Non-engineers see this as a failure. Engineers and scientists see this as an awe-inspiring success from which to learn, iterate and try again. I don't fault reporters for exagerated headlined focusing on failure. They don't know any better. That is not what their lives are about and most might be challenged to understand that nothing in that video constitutes failure in the context of evolving a design. In fact, failure is absolutely necessary in order to better understand the problem you are trying to solve.

⬐ tjradcliffe

This.

This video is one of the most beautiful things I have ever seen. This is not a failure. This is an incredibly rich mine of data on how to do better next time.

This is how we improve the world, and it's beautiful.

⬐ 31reasons

We are witnessing 0-1 technology of space exploration and its amazing. Its going to be an inflection point in the history of our species.

⬐ _Adam

I don't agree that failure is absolutely necessary. I think success is generally more informative, because it gives you data about something that works end to end.

If your rocket blows up on the launch pad, you just learned that you shouldn't overpressure your fuel tank (or w/e). But if your rocket lifts off, goes to space, and lands, you learn thousands of things that work that you can use again.

⬐ simonh

I've +1'd because while I disagree, yours is a reasonable opinion that's provoking a useful discussion, and I think the downvotes you're getting are not appropriate.

The problem with aiming for success from the start is that it promotes a highly conservative, over-engineered solution. If it does work, it also acts as a brake on iterative improvement because your system already works, why risk failure by trying to make it more optimal? Also if you have a proven system, people start making bigger and bigger bets on it's success (more expensive payloads, for example), so the cost of any failure goes up.

If you aim for optimal design form the start, you're almost bound to experience failures. That's because say there are 100 parameters you need to tone and you aim for the estimated optimum for all of them. Statistically, half of those estimates are goign to turn out to be sub-optimal. As you iterate on the design, you fix those untill you end up with a truly optimal design. At each iteration, you have a positive incentive to make the necessary changes.

Depending on the cost of failure, this can be more expensive than aiming for a conservative, over-engineered design to start with, but not necessarily. Over-engineered systems, for a high chance of initial success, are by definition more costly than optimised designs and there's still a non-zero chance of initial failures anyway.

⬐ reneherse

Consider this: If you're conducting tests and measuring your points of failure over numerous iterations, you eventually find the "threshold" of success. Exactly how much howsit and whatsit you need to produce a desirable outcome. Then you can pile on a margin of safety as needed. Continuous improvement of this sort eventually results in an efficient system or structure along with robust documentation of failure modes which you can then anticipate and accomodate.

Also, there are different ways things can fail, E.G. catastrophically vs gracefully. For example, when your steel bike fails it will generally start creaking and slowly bend, allowing you to come to a stop or even ride home. Your carbon bike bike on the other hand is more prone to snapping or shattering, a catastrophic failure mode with potentially grave results.

⬐ deeviant

I strongly disagree.

I have seen a company be destroyed because they got things almost right, from the beginning. This "success" informed their every decision and created an enormous resistance to trying anything new and in the end it turned out almost simply wasn't good enough. The initial success destroyed them.

⬐ robryan

I agree, this video makes it look like there was a strong chance that it could have been a success if the random variables on the way down aligned better. If it had of been they may not have learned and iterated based on this possibility and may have had a failure down the track.

⬐ hga

That destroyed two Space Shuttles and killed their crews.

Both the failing field joints ("O-rings") and detaching external tank insulation weren't quite bad enough to kill a Shuttle, were all but ignored, until of course they couldn't be ignored.

⬐ mark-r

For an example of how this works in practice, just go back to the original rocket programs of the 50's and early 60's. Those guys dealt with failures much worse than this, and ended up with some of the most awesome achievements in human history.