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Comment by snovv_crash | Hacker News Reader

Did anyone else notice that the engines shut off while the rocket was still a small ways up, after which it fell quite heavily onto the landing legs?

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Yes. Probably due to pitching drone ship deck due to heavy seas, the stage reached zero vertical velocity above the deck and had to shut down. Even at the lowest throttle setting the first stage still has a TWR > 1 and cannot hover without accelerating upwards again, so calculating the "suicide burn" timing is really crucial.

edit Corrected "TWR > 0" to "TWR > 1".

Is that TWR (Thrust to Weight Ratio) for when all 9 Merlin engines are lit? Landing burn today had 3 lit if I recall correctly.

No, that's for a single engine. At landing, the first stage is nearly empty, and thus very light. Even the single center engine alone, throttled as low as it will go (40% or something like that, I don't think we have the exact number for the Merlin-D), produces too much thrust to be able to hover. The Grasshopper and F9R testing vehicles carried extra ballast to be able to hover and descend under thrust without cutting their engines.

That's pretty cool. Makes sense, especially with stage 1 being so light on re-entry that a single engine would have a TWR > 1.

The landing burn actually used one engine initially, then three, then one again for the last bit. It doesn't burn three all the way through the landing burn.

Might be an artifact of the weather conditions, which was said to be less than ideal.

https://twitter.com/elonmusk/status/878828849722671104

Probable wind gust right before landing.

He's talking about friday's landing that came down very hard and used up pretty much all the emergency crush core in the legs, because it came down on the side of the ship, then overcorrected to the other side, then corrected again and touched down (speculation, but pretty obvious when you look at the scorch marks on the ship)

Yesterday's "jump" for the last few feet was probably due to the ship going up and down with the waves and the rocket not expecting it

they call it a "hover-slam" landing

The idea of the hoverslam is to arrive at zero vertical velocity at exactly zero altitude. This is complicated, but can be done fairly reliably when "zero altitude" is stationary - on land landings or drone ship in calm seas. In heavy seas as today, the drone ship deck is pitching up and down, and it's really hard to predict the altitude it will be at the exact moment of touchdown. If I had to guess, I'd say the calculations are done against the highest possible or average position of the deck, and during today's landing it was at its lowest point at the moment of arrival, causing the stage to drop the last meter or so. This is what the crush cores in the landing legs are for :)

I wonder if they will build more stable landing platforms. If you look at oil drilling rigs, they stay calm even in relatively heavy waters.

I believe oil rigs are heavily anchored. I believe the barge either has no anchors, or less (or less massive ones.)

It's not about having anchors, it's about not being affected by the top layer of water which moves up and down due to waves. If you submerge (your center of gravity) just a few meters under the surface, you'll suddenly be much more stable and much less affected by the waves. Oil rigs, just like ice bergs, have a large part that is submerged and invisible on the surface. Look at this for example: https://s-media-cache-ak0.pinimg.com/736x/73/64/d6/7364d6da3...

An even better example of this: https://en.wikipedia.org/wiki/RP_FLIP

The problem with a rig is all that vertical structure. It limits where the vessel can be towed to for offloading. To get this rocket off, they need a regular commercial dock.

I'm pretty sure they already flood the ballast tanks for stability.

That's presumably part of the reason that Blue is planning on using a much heavier ship.

But oil rigs aren't easily movable, and they will want to get the rocket back to land. I suppose they could crane it across onto a drone ship barge, but landing directly on the barge in the first place seems to be working out ok so far and saves a huge amount of extra cost, time and trouble.

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