Comment by kirrent | Hacker News Reader

Comment by kirrent | Hacker News Reader

For those wondering, this isn't a simple repeat of CRS8 which landed on a drone ship about a month ago. CRS8 was a mission to low earth orbit which left the first stage with plenty of fuel to effect a landing. The landing was made as slow as possible and limited only by how low a single engine could be throttled. The re-entry burn which slows the rocket down before the landing burn was also more agressive.

This mission was to launch JCSAT-14 to geosynchronous orbit which requires the falcon 9 to move the already heavy satellite into a geosynchronous transfer orbit. That meant the first stage had to do a lot more work and was left with a lot less fuel as a result. Therefore, the re-entry burn was less aggressive, the first stage came in with twice the speed, and SpaceX needed to do a far harder landing with three engines lit in the quicker and riskier suicide burn. Somehow, despite playing down expectations, they managed an even more precise landing than last time.

To expand on this slightly the CRS8 (and OG2, the land landing) landing was done using only 1 engine, which even then was presumably throttled. This landing burn was done using 3 engines so they spend less time below terminal velocity, and thus use less fuel. The total deceleration should be ~40 m/s during the burn.

So only about 4Gs? I somehow though it would be more and unsurvivable for humans.

Check out the flight profile on Flightclub.io -- I'm not sure how accurate their instantaneous data is but they're showing ~12g acceleration upon landing the booster. Seems more like what I'd expect.

Impatient to know if the 1st stage is reusable after a 12g phase. The entire concept hangs on this and it looks like we don't have a firm confirmation yet.

12g but empty. A structure meant to survive being filled with 10+ times (fuel+upper stage) it's own weight will survive 10+Gs without effort. In other words, the stress of a 12g acceleration isn't much different than the stress of the stage sitting on the pad a few minutes previously.

The overall structure actually goes through all sorts of stresses, not all of which are measured in G. The combination of push from the rocket and air resistance compresses the rocket from both ends, something that isn't reflected in G numbers.

I am not a rocket scientist, but:

I think this is only sort of true. Yes, the fueled rocket is much, much heavier. But not all parts of the rocket are load bearing as it sits on the pad waiting for launch. All parts of the rocket experience the 12g (or whatever it was) deceleration.

Also, rocket skins are really, really thin (relative to their size, the forces they endure, etc.) I've read that without the fuel inside to reinforce the rocket, it is quite fragile - think of trying to crush an empty soda can vs a full one. This is part of why rockets which deviate too far from prograde on launch are ripped apart mid-air.

Regarding your last part, some rockets have balloon tanks, which means that the tank requires pressurization to keep its shape.

The Falcon 9 has a sort of partial balloon tank. It's strong enough for ground handling without pressurization, but it requires pressurization to handle flight loads.

Note however that it's pressure, not fuel, that provides the extra strength. These tanks are still pressurized even when almost empty.

TheVehicleDestroyer (who makes FlightClub) actually commented [1] to say that the 12G is an error due to how he models the deployment of the landing legs.

The actual force would have been closer to 5.1G

[1] https://www.reddit.com/r/spacex/comments/4htenu/rspacex_jcsa...

Ugh sorry, there's a bug of some sort in Flight Club there. There's no way it was that large. A constant deceleration down from terminal velocity over 6 seconds is

a = (300 - 0)/6 = 50m/s2 = 5.1g

I'll need to look into that

https://www.reddit.com/r/spacex/comments/4htenu/rspacex_jcsa...

So it looks like that peak is due to the sudden increase in drag during my landing legs deployment. I deploy them at 500m altitude which is a time when the stage is probably still moving a bit too fast. It's a really large increase in cross-sectional area, but I'm probably not treating it correctly. I'll revise before Thaicom-8

Wow, thanks for the update -- the internet can be damn cool sometimes.

40 m/s is a velocity, not an acceleration (deceleration). I guess that you mean 40 m/s^2

He did say total deceleration, so I believe he meant the landing burn starts at a velocity of 40 m/s and consequently the total ΔV of the landing burn is 40 m/s.

That said, I have no idea where he got that number, whether 40 m/s or 40 m/s^2; both are incorrect.

The best estimate available for the flight dynamics can be found at flightclub.io, specifically for JCSAT-14: https://flightclub.io/results/?id=eeaf889e-f3d8-4f9b-be98-b7...

This shows the landing burn starting at a velocity of 269 m/s and experiencing a peak deceleration of 12.3g = 121 m/s^2

I don't know how accurate these numbers are, but some fans put quite a bit of effort into getting the flightclub.io profiles as close to reality as possible, so they tend to be pretty good.

Oops, you are right, I did miss a ^2.

The approximation I had is almost certainly less reliable than flightclubs, not sure why it was so low, will have to look at the math I did and figure out what I screwed up.

From the video it looks like it lands on a ship in rough seas and they do nothing to secure it. Won't it just fall over if the seas are rough enough, and if so, why don't they have some sort of robotic system that locks it down tight the moment it lands?

All of the centre of gravity is low down with the engines. The rest is basically just a thin empty tube.

Plus the platform has four computer controlled engines on basically universal joints to stabilize it. A crew then boards the barge and puts 'shoes' over the landing legs then welds them to the deck.

To add to above: In the previous barge landing they did weld the landing legs to the barge using some kind of bracket. But they discovered that the centre of gravity is low enough to not need any welding. During the previous attempt the sea was heavier than this time.

Maybe they still weld it this time, but that is done by having someone go to the barge and do it.

Actually no they did not weld anything to the legs. Elon said that they realize it wasn't needed. They did use octojack though to support the weight off the legs.

What's octojack? Can't seem to find any other references to that.

Sorry meant Octoweb jacks

https://imgur.com/5GTW1CO

To add to the other comment.

http://www.spacex.com/news/2006/01/01/octaweb

Everyone seems worried about waves and tilting, but you are saying the rocket is pretty much a metal sail. Can wind blow it over?

If the sea is still enough to land, it's still enough to take time securing. The previous stage was secured by tying down the thrust structure ("octaweb") to tack-welded fixings on the deck with hoofing big chains. There may also have been shoes, we couldn't see clearly.

Tack-welding fixtures to a steel deck is apparently SOP in big shipping, so it's not as radical as it sounds.

Securing is purely a safety precaution, it's not actually required.

https://twitter.com/elonmusk/status/726218218109444096

With respect to Musk, that is total bunk. Any structure like this will have to be secured to the deck. The fact that it is a rocket doesn't matter. He may think that it is safe from falling over, but the insurance company and/or the health and safety people will tell him that cargo that tall needs to be secured. Ropes, chains, welding ... it cannot just sit there. A Volvo station wagon, on that deck at sea, would be tied. A box of onions would at least be secured so that it doesn't shift onto someone's foot. Boats move, cargo cannot.

Somehow I suspect they have run the numbers.

What numbers? Unsecured cargo is unsecured cargo. A metal object sitting on a flat metal deck gets tied/braced/chained.

I think you're trying to apply rules designed to govern shipping in general to a very specialised and controlled situation.

A lot of rules around shipping actually have to do with physics. See my other comment. Shifting cargo can be very bad news. It has caused a lot of deaths. It's probably not a big deal in this case, except that booster is worth about (I'm guessing) 20 million dollars, SpaceX's cost? It's tall, but the center of gravity is low and it's on a barge.

Engineering is conservative "just in case." So are a lot of practical activities that happen on the sea. In neither case, is this irrational or a waste of time.

They mentioned multiple times that because the stage is empty, its center of gravity is much lower than what you'd expect.

By whom? The drone ship has no on-board crew.

... which means there's nobody around to be injured or killed if the rocket falls post-landing. Maybe it's a precaution worth taking when they intend these stages for commercial re-use; for now, the whole thing is one big experiment.

There is a ship a few miles away with engineers on it. Once the stage has vented all excess liquid oxygen and is safe to approach, they can return to the ASDS and secure it.

They will of course have modelled the cargo scenario, and I suspect that Musk means that it's nice and stable when they do that -- but then shipping norms come into play because, like you say, unsecured cargo is unsecured cargo.

It needs to be both stable and secured, because only one is inviting woe.

By the time the rocket lands, there's very few fuel left, basically you see a near empty Pepsi can. It's as stable as it can get.

Ever see an empty pepsi can in a stiff breeze?

More like a Pepsi can with a bloody big brick in the bottom 9*470 kg merlins). Main threat to stability would seem to be wind, but doesn't seem to be the case in practice

Just to elaborate: due to wave action and leverage on the underside of the hull, parts of the deck of a ship can accelerate downward faster than the acceleration from Earth's gravity, and while the deck underneath the rocket has fallen faster than the rocket and its heavy engines it can also lurch to a side. Even if the center of gravity makes it equivalent to a sheet of rolled steel, so that it wouldn't tip over, it could become displaced, then go overboard or cause unwanted damage.

While I suppose that the rocket could be heavy enough to lower the center of gravity of the entire vessel below the troughs of any waves enough to make it incredibly unlikely, it isn't just the prospect of tipping over that makes it important to secure cargo.

Of course if there is any chance of the rocket moving, it isn't even safe to pull up alongside, let alone send people onto the deck to secure it.

There are protocols for securing unsecured cargo. Whether delivered by rocket or crane, things land on decks every day and are tied down by hand.

In conditions where it's going to be sliding around on the deck?

Yes, see this example : https://www.youtube.com/watch?v=q3idQKi5EqM

how much money and time would a robotic lashing system cost? hint: a shitload.

how much money would a few ditched rockets cost, in comparison? hint: less than a shitload

would a recovery crew approach this platform in rough seas, even if a robotic lash were in place? hint: nope

then what exactly is the point of securing this 'cargo'? hint: there isn't one.

problem solved.

What port will let them in with giant explosive unsecured cargo? Hint no port.

and yet, they seem to be doing just fine.

Everything I've seen says they secure their load. Is there something I'm missing?

i'm referring to the landing platform.

The point of securing the cargo is to prevent it being lost to toppling, sliding off the side, or being catapulted off the side of a rolling deck.

And yes, it will be secured. The previous recovered rocket was secured, this one will be too.

i'm talking about the landing pad.

I suspect he is in a unique situation to know the numbers for this much better than you do, because he runs the company that has successfully done this.

Bonus points that the overwhelming majority of ships lack the giant oil rig thrusters that all of the ASDS armada contain to keep them as stable as feasible and prevent listing too much in normal ocean condiditons.

Hence the phrase "purely a safety precaution"

Cargo shifting is a big deal. The yearly casualty rate for Bering Sea crab fishing used to be over 1% per year, because the shifting weight of crab pots on overloaded vessels in heavy seas. I saw a training film when I was aboard a tender boat in Alaska. Those boats could look fine one minute then capsize and be as good as sunk in the next 2. Shifting weight on vessels has and does cause rapid capsizing and deaths at sea.

This is pretty light cargo relative to the weight of the ship. It's a few tons of rocket and thousands of tons of ship. I don't think it's going to matter to the ship if it shifts, the only risk is losing the rocket over the side.

What you say is True. My comment explains why the shipping industry and regulatory bodies seem paranoid about shifting weights at sea. There are good reasons to be conservative about certain things and brook no exceptions.

It is amusing that you insist your speculation about something you've never done is more valuable than the experience of someone that has pioneered it.

Ya, because SpaceX is the only company to have ever transported something on the deck of the ship. It was a rocket until the moment it landed. Now it's cargo.

This isn't my area of expertise and what you've said certainly makes sense, however from my understanding ship-to-ship, or in this case ship-to-barge, boardings at sea aren't without their own inherent risks.

It could be that having the first stage untethered is deemed less risky to personnel than having a team of welders board the barge and tie it down.

If they don't need it, they don't need it, but I wonder if there would be any other way do the secure the rocket without requiring welders to board the barge. Coat the deck with solder and put heaters in the feet to attach? A robotic Roomba-style welder? Giant arms that rise up to clasp the rocket regardless of the exact landing position? Fun to think about...

Maybe something with magnets?

The key is movement. Doing something like moving objects from one ship to another is pretty standard, but only in good conditions and normally not when the ship is moving. The rocket may well be safe to stand on the ship at the moment it lands. The conditions at that moment are good enough to make the choice to do the move. Once it is on the ship, that choice is over. Conditions change, most dramatically in that the stationary ship is going to drive away to port. Not bringing the object aboard is no longer an option, as it was prior to the rocket landing. Under those conditions the cargo has to be secured not against current conditions, but against all possible conditions.

Thanks for the clear explanation.

Before it landed, the most likely scenario was that the rocket was going to crash into the deck and be destroyed. (That's happened several times before.) If they can deal with that, they can deal with the rocket landing safely and then falling over.

They can until it gets to a busy port with other ships and people around. Then it becomes a huge safety issue.

Copying my other comment:

Apparently they do secure to the deck after landing.

Discussed here: http://space.stackexchange.com/q/6403/574

From the last landing, Elon said (on twitter), that the ASDS can list up to 8% and the rocket can still land or stay upright.

Apparently they do secure to the deck after landing.

Discussed here: http://space.stackexchange.com/q/6403/574

And they did this in the dark! ;) (yes I know it does not matter)

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