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Comment: Re:Wait a minute (Score 1) 248

by robbak (#48872663) Attached to: SpaceX Landing Attempt Video Released

Because any electric hydraulic pump is going to be slow at pumping fluid, you still need the pressurized accumulator, so you can move your control surfaces quickly when you need to. So the reservoir, pump and the batteries are an additional weight, that you would want to omit if you could.

The reason why you have never seen a total loss pressurized system is that the conditions that call for it are ones that you rarely see - a strong mass constraint (which has to include the power source), and a short time period when it is required. Your plane's system needs to operate over a period of many hours, there is normally a power source on hand (the engine's alternator), and the mass is not really that constrained. Really, a rocket is the only place where a pressurized total loss hydraulic system makes sense.

(Note that this crash could probably have been avoided with more complex programming. The programming could have kept count of how much hydraulic fluid it was using, and driven the fins to neutral before it ran out. This sort of capability - or even just a fluid level sensor - will doubtless be added before the landing system leaves the testing stage. If dealing with early exhaustion of fluid does turn out to be this easy, then the need for the extra mass completely disappears.)

Comment: Re:The problem was the control fins. (Score 1) 248

by robbak (#48842777) Attached to: SpaceX Landing Attempt Video Released

The rocket decelerates quickly during quite a short landing burn, so they would have had a strong effect until the last few seconds. Indeed, the loss of that force as the rocket comes to a stop would have been an important part of the crash - the rocket would have been countering the influence of the grid fins pushing the top of the rocket away from the camera, while tilting the rocket toward the camera to get it back to the platform. Then the rocket slows and that force dies away. Now the rocket has to go from working hard forcing the rocket to tilt toward us against that force, to trying to push it back upright with that force suddenly gone. You can see that it was trying, because the rocket flame is directed away from us, illuminating the far side of the rocket, leaving the near side in darkness.

Nope. Grid fins explain what we see very well.

Comment: The problem was the control fins. (Score 1) 248

by robbak (#48838347) Attached to: SpaceX Landing Attempt Video Released

The fault that caused this failure was the control fins running out of pressurized hydraulic fluid. When this happened, they were driven fully to one side, pushing the rocket over. The engine tried it's best to counter that, but it didn't have a hope.

A fellow fan tried something similar in the Kerbal Space Simulator. I imagine the real flight was very much like this:

Comment: Solid boosters vs. liquid rockets. (Score 1) 248

by robbak (#48838329) Attached to: SpaceX Landing Attempt Video Released

Solid booster casings are a very different beast. A solid booster rocket needs to be very strong, because the combustion chamber of a SRB is literally the entire rocket. The whole thing needs to withstand combustion chamber pressure. So it is strong, tough (and heavy), so you can do what you like with it.

A liquid fuel rocket is a much more fragile beast. If allowed to tumble through the atmosphere, or hit the water at parachute speeds, it would be totally destroyed.

Comment: Do stores take pictures of rocket engines in fog? (Score 1) 248

by robbak (#48838295) Attached to: SpaceX Landing Attempt Video Released

The initial reason for not releasing video was that it was dark and foggy, and the video was not fit to release. While this may have been more about controlling the news cycle by forcing the media to use pictures of the successful launch, it is clear that this video required a lot of levels adjustment to make it acceptable, and that has created noise in the image. However, apart from the drops of water on the lens, which is unavoidable, the quality is quite good.

Comment: Investigation was over ~20 seconds before landing. (Score 1) 248

by robbak (#48838273) Attached to: SpaceX Landing Attempt Video Released

The fact that the next launch was already going to carry 50% more fluid indicates that they had an idea that there might not have been enough. That decision about how much fluid was needed would have been made early on, and they could not have fixed it later, as this secondary experiment could not be allowed to interfere with the primary mission.

The engineers monitoring the landing would have seen the fins be driven to hardover and known instantly that they'd run out of fluid (if they didn't have a sensor for that). Elon tweeted that they'd run out of hydraulic fluid within hours of impact.

As others have stated, this was testing anyway.

Comment: Re:Wait a minute (Score 1) 248

by robbak (#48838227) Attached to: SpaceX Landing Attempt Video Released

The pressurized fuel used to gimbal the engines is way down at the bottom of the rocket, and the grid fins are at the top. The engines providing that pressure are not running for most of the descent. For these reasons, you need a separate system at the top for these fins, and a simple pressure-activated total loss system would provide everything that they need (or, at least, would have if provided with a few pints more fluid!)

Comment: Fins went hard-over when the system ran dry. (Score 3, Interesting) 248

by robbak (#48838201) Attached to: SpaceX Landing Attempt Video Released

Elon stated while being questioned last week that the steering fins went hard-over (which means they were driven to their maximum angle) when the fluid ran out. With the fins pushing the rocket over, it didn't have much hope of landing. And, yes, a pressurized accumulator is the most likely design of this system.

/u/DixieAlpha over at reddit programmed a Kerbal Space Program model to try to land with grid fins fixed at 30 degrees. The results were scarily similar to this landing.

Comment: Re:No video? (Score 1) 213

by robbak (#48783365) Attached to: SpaceX Rocket Launch Succeeds, But Landing Test Doesn't

In the reddit AMA, Musk stated that the 50% wasn't calculated from anything, but was just a guess. And we didn't expect to get video of a failure, because of persons using such video to create bad publicity.

All of us over at r/spacex are ecstatic about this. The mission was a 100% success (so far), and the landing, 90%.

Comment: With a RTG, it couldn't have got to the comet. (Score 2, Informative) 523

by robbak (#48423289) Attached to: What Would Have Happened If Philae Were Nuclear Powered?

It's a question of weight. No matter how you build them, nuclear Radioisotope Thermal Generators are heavy. This mission was heavily mass-constrained. What they wanted it to do was at the limit of what the rockets were capable of.

Add a several-hundred-kilogram RTG to to mix, and the 'rocket equation' kills you. You just cannot get the probe to the comet. Solar panels were the only option.

"There is no statute of limitations on stupidity." -- Randomly produced by a computer program called Markov3.