Comment by nkoren | Hacker News Reader

Comment by nkoren | Hacker News Reader

One of the best things to happen during this launch is that an engine exploded. You can see engine 1 eat itself at 1:31 in this video:

http://www.youtube.com/watch?v=tRTYh71D9P0&feature=youtu...

[Edit: First the video was public, then it was made private, now it seems to be "unlisted" but viewable. So no guarantee that it'll still be there in another minute...]

Why is this a good thing? Because it proves a point: the rocket automatically adjusted its trajectory, and continued on its course as if nothing had happened.

The Falcon 9 is the first American rocket since the Saturn 5 with an engine-out capability. Its multiple symmetrically-arrayed engines allow for it to compensate for a sudden loss of thrust from one side: the other side reduces thrust as well to stay balanced, and everything else just burns a little bit longer. That was the theory, anyways. Today they put it into practice.

For any other rocket that you've seen launched in the last 35 years, that would have ended the mission catastrophically -- but the Falcon shrugged it off like nothing had happened. They always said they could do that, and now they've done it. Congratulations, SpaceX! Creating a fault-tolerant rocket is much better than creating a faultless one (since that's impossible).

[Edit]: In the absence of the video, the engine anomaly is being widely reported on Twitter: https://twitter.com/spaceteam/status/255128401927610368 https://twitter.com/visionik/status/255128010653593600

Here's a high quality 1080P slow-mo clip of the explosion. You can clearly see huge pieces of debris falling off for several seconds afterwards.

http://www.youtube.com/watch?v=y6zsZiVa998&feature=playe...

While I'm glad that everything worked out well for SpaceX, I would not be so quick to consider it a smashing success. Correcting for an ineffective engine is a fantastic feat of engineering, but consider that the engine obliterated itself due to some other unknown flaw. The bigger issue with blowouts like tonights is that the violent nature of the event can damage surrounding engines, or the failure could be caused from a common system. SpaceX was lucky that it was a corner engine that exploded, as it was least-likely to damage any of the other engines. Excellent redundancy engineering allowed them to continue the climb to orbit, but only after they got lucky.

Then again, it is unfair of me to make relatively uninformed declaratives about tonight's launch. The engine blowout could have been cause by an incredibly unlucky set of circumstances, and the engineers at SpaceX could have built a vehicle that survived a catastrophic failure that would have doomed any other craft to date.

Given what I've seen from SpaceX, I am positive that they will get to the bottom of the incident and have it corrected before their next launch.

Each engine is surrounded by a kevlar blast shield, in anticipation of exactly this kind of scenario. While there have been previous rockets with an engine-out capability (the Saturn 5 first stage demonstrated this, for example), I believe this is the first rocket explicitly designed to survive an explosive engine failure.

No doubt they should do an investigation to see if the cause of failure can be determined and corrected. It's possible that the fault may no longer be an issue; this was flying the soon-to-be-retired Merlin C engines, and the design or manufacturing flaw may not have carried over to the Merlin D engines. But what they shouldn't do is spend billions trying to make the rocket perfect, which has been the traditional approach. A few million + redundancy is much better.

A very important distinction is that liquid rocket engines should be shut down when problems are detected, and an explosion should not occur.

This has been done many times in history, in Saturn V and Space Shuttle for example.

But in this case, since the engines are so small, it seems the explosion tactic paid off very well.

Hopefully they can test Merlin-1D adequately. If you aim for reusability, your reliability must go up considerably. If you plan to fly something for 100 times, you probably can not tolerate 98% per-flight reliability. A lot depends on assumptions of course.

All very true. Although as I note in a thread below, I'm starting to wonder whether this was truly an explosion (ie of the combustion chamber and everything), or simply a nozzle collapse following a commanded shutdown of the engine. The latter would be relatively benign, aside from some aerodynamic effects which the flight dynamics software obviously handled without trouble.

And you're right -- for reusability, they do need to get their per-engine reliability above 98%. That would produce a mission reliability of 97.1% (assuming that 2 engines out causes a loss of vehicle, which I understand is only the case during the earlier phases of flight). At 99% engine reliability, you get 99.3% per-flight reliability -- which strikes me as the point where reusability starts to become very sensible. I have considerable confidence that SpaceX will be able to do this.

Well spotted! I missed that during the live webcast, there was absolutely no mention of it. Some people at SpaceX should be pretty hard to play poker against, there was 0 indication that they were aware of any issues.

Bringing a RAID approach to engine configurations seems to have paid off quite handsomely.

There was a point where they said something along the lines of "the rocket is on an anomalous trajectory", and I was wondering what they hell they meant by that, since they didn't follow up on it.

Way to go SpaceX! I'm just as thrilled by the fault tolerant system working as designed as I am for the launch itself. SpaceX is proving again and again just how well thought out and robust their system is.

Wow, that's a great catch - I did notice the turbulence from the explosion in the webcast and thought "whoah, that didn't look like it was planned", but everything went so smoothly afterwards that I just wrote it off as the supersonic shockwave or something like that.

Well, to be fair, I may have jumped to conclusions by calling it an "explosion". Something disassembled itself for sure, but it may have just been the bell nozzle rather than the whole engine (combustion chamber, turbopump, etc.).

SpaceX is calling it an "unplanned shutdown", but it's unclear at what point the shutdown was commanded. It's possible that some off-nominal parameter triggered an engine shutdown, at which point the sudden vacuum inside the engine bell would have caused the nozzle to collapse. (Note that this happened at Max-Q, the moment of maximum aerodynamic pressure, so the outside force on the nozzle would have been significant). This would have had some impact on the aerodynamics of the vehicle (with the flight dynamics software compensating perfectly), but it wouldn't really have been a threat to the other engines, and is not too dissimilar from some unplanned shutdowns which the Saturn V experienced. It wouldn't really count as an "explosion". It might just be a (relatively benign) software bug which triggered the shutdown.

On the other hand, the engine might have disassembled itself first, followed by the shutdown order to keep it from dumping uncombusted propellant. This would be an actual explosion of the engine -- contained by the kevlar shielding to keep it from damaging the surrounding engines. If this is the case, then it's a huge validation of SpaceX's RAID-like design philosophy, since I don't believe that any launch vehicle has ever survived such an explosion, much less completed its mission to perfection.

New video (not private) with engine blast around minute 5:21

http://www.youtube.com/watch?v=6kGaKsSFS6E

I checked out the video you linked. It's now all of a sudden marked Private for some reason... Is there any corroboration from SpaceX that there was indeed an engine failure? It looked to me like it could have been ice breaking off near max Q or some kind of shockwave interacting with the exhaust plume. I'm not questioning your observation but it didn't look definitive to me. I agree that having redundancy is good though.

Yes, the loss of engine 1 was apparently confirmed in a press conference.

https://twitter.com/i/#!/search/?q=falcon+engine&src=typ...

Nice. Thanks for pointing this out.

The video is now private, does anyone have a mirror?

'This video is private'

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