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Comment by crispyambulance | Hacker News Reader

Here's one thing I don't understand... The rocket lands upon a floating platform in the ocean. Presumably the platform rocks along with the waves. How do they keep the rocket from tipping over after the landing? If it were as huge as an aircraft carrier I can imagine that it might be possible but the platform is relatively small.

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A rocket is basically a giant fuel tank with some very heavy engines at the bottom. By the time it lands, the tanks are almost empty, so the center of mass is _extremely_ low.

There are two main tanks in the stage, the top ~half is liquid oxygen, and the bottom ~half is kerosene. The liquid oxygen is lighter, and boils off pretty quickly once they open up the vents. So what little fuel is left (the kerosene in the bottom tank) also settles to the bottom, which helps push the CoM even lower.

Actually, liquid oxygen is almost 40% denser than kerosene (1.14 vs 0.82 g/cm^2). And this is the reason most rockets [1] put oxygen on top/forward: they want the center of gravity to be as far forward as possible, ahead of the aerodynamic pressure center, so that the rocket will be as stable as possible when ascending through the atmosphere. Just as we would put a weight on the front of the arrow, and feathers in the back, to make it more stable.

For the vertical landing, this would seem to work against them, but I guess the mostly empty tanks don't dominate the considerations. Falcon has oxidizer on top in the conventional manner, as far as I know (they mention LOX feed lines going through the fuel tank).

[1] The N1 rocket had oxidizer below the fuel in every stage, apparently because they wished to use spherical tanks, and this allowed packing them in a conical shape - the oxidizer tanks are much larger.

Yeah, you're absolutely right. Not only is it denser, but they carry more of it (due to oxidizer:fuel ratios and to account for boil off in-flight). No idea what I meant by "lighter"... I'll blame it on a pre-caffeinated state.

It is however true that they can vent it pretty quickly though, leaving the only remaining propellant in the bottom tank.

For the vertical landing, this would seem to work against them

Actually, an inverted pendulum -- where most of the weight is on the top -- is easiest to balance on top of the rocket thrust. Coming down at high speed, this would be aerodynamically unstable, but this is why they have the grid fins, to move the center of aerodynamic pressure behind the center of gravity.

The center of gravity for the rocket is actually very low. The engines are on the bottom, and the rest is just a light aluminum tube (with other smaller tubing and tanks inside but still.)

Also, after the landing they vent the remaining oxygen, and afterwards people come aboard to weld the rocket legs to the surface so it wouldn't slide off.

Maybe very strong winds would be an issue, or very rough seas immediately after landing but they wouldn't launch in rough weather anyway.

Elon mentioned at the news conference that the pitch/roll of the ship was 2-3 degrees for this landing, and that it should be easy to handle twice that, with a maximum of about 8-9 degrees.

https://youtu.be/VNygOavo2mY?t=934

He further mentions that they weld the rocket to the ship's deck before heading to shore.

That's insane. In offshore heavy lift we stick to barge motion limits around 2 degrees. Maybe we're just far more risk averse, but I cannot fathom how this would work with 8-9 degrees rotation. Even being on a ship with 2-3 degrees rotation you really feel like you're walking up and down a hill.

I agree it's insane but looking at the few previous attempts it seems its a regular expected sea state so I suppose there is no option but to work around it.

Some fan calculations suggest it could handle even more, but SpaceX knows more about their coefficients of friction and center of gravity than we do.

The barges are autonomous. The lack of people on board presumably significantly increases their tolerance for rough seas, both in terms of risk of human life and tolerance for human factors like seasickness and ability to perform labor in a bumpy environment.

That's true but there still have to be people coming aboard to seafasten the rocket. To my knowledge it's not welding itself to the deck.

If not already, this seems like a job that could be done by specially designed robots.

To clarify I'm talking about a double significant amplitude limit, which for a given typical three hour sea state is the mean of the highest third of double amplitudes. Does anyone know the reasonably precise location of the landing? I can then find what the typical waves are in the region.

After watching the video I don't know what Musk is talking about. Maybe he's talking about a maximum double amplitude.

When landings are routine, SpaceX could apply for an on land license. Blue Origin already has that.

SpaceX already landed one on land, December 22, 2015. And received a license to do so.

The weight of the engines puts the center of gravity in a mostly empty rocket fairly low.

they also weld the feet to the deck

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