[bantunes]

Could someone explain to me why they attempt this? Wouldn't inflating a set of floating devices when it goes down the ocean an easier way of retrieving it? Or something other than this REALLY HARD TO PULL OFF maneuver? Genuinely interested.

[benjaminl]

The key thing to remember about rockets is that they are incredibly fragile. The rocket is about 15 stories tall and is designed for force to only be applied at the end either via the rocket engines or the landing gear.

Any type of lateral forces will irreversibly damage the rocket. As an illustration, extreme care is taken when the rocket needs to be transported on its side. Some rockets, not the Falcon 9, can't even be transported on their side without being pressurized.

So pretty much anything other than a perfect soft touch landing would leave the rocket unable to fly again. Anything touching the 3/16" thick skin of the Falcon 9, would cause the very least extensive refurbishment at the least, but more likely irreversibly damage the frame of the 15 stories tall 1st stage. So this rules out every alternative that has been suggested in this thread.

[Dylan16807]

While that's true, the metal tube is not the part that's expensive to build.

[sbierwagen]

You'd be surprised: https://en.wikipedia.org/wiki/Isogrid

[tavert]

SpaceX mostly uses friction stir welding to avoid the costs of isogrid

[ruffmuff]

Two main reasons I can think of: 1) Sea water is extremely corrosive. If your booster takes a dunk in the drink, it's going to be very hard to refurbish for another launch. Lots of components (electronics, valves, etc) would need to be replaced and the structural integrity of the tanks themselves may be compromised by rust.

2) Flotation devices attached to the booster = extra payload. It would be the canvas/dinghys AND compressed gas canisters for inflating them. This is arguably (a lot?) more weight than just landing struts + fins. Any extra payload on a rocket represents a massive cost and means less stuff you can haul for paying customers.

[martindevans]

In addition to that the vehicle is extremely weak when lying on its side. I should think there's a good chance the tanks could be damaged by waves bending the vehicle.

[Gravityloss]

Correct. Also the terminal velocity even with a parachute can be high, and if you hit a rising wave, even more.

Actually parachuting and fishing the stage from the sea was their first idea, but they never recovered a single stage that way, and moved to active controlled recovery, which is much much much better in my opinion.

[refurb]

Flotation devices attached to the booster = extra payload

Yes, but how much extra fuel does the rocket have to carry to land? That's a ton of extra payload as well.

[ramidarigaz]

Estimates I've heard say 15% loss of payload mass for a barge landing, 30% for land.

[refurb]

What do you mean by "loss of payload mass"? If the rocket lands on a barge it has to reduce it's payload by 15%?

Any ideas why it's higher on land? Seems like it wouldn't differ that much.

[tavert]

Ascent trajectory is much different. Orbit is mostly about horizontal velocity, the altitude is just to get above the atmosphere and is the easy part. For the first stage to return to its launch site for landing, it has to cancel out all of the horizontal velocity it builds up while lifting the second stage and payload, then turn around and come back. A barge landing can use a more optimal ascent trajectory and have the first stage land several hundred kilometers away from the launch site.

[chad_oliver]

You have to accelerate the rocket back to the launchpad if you want to land on solid ground -- there's no convenient landmasses at the right distance. That takes fuel.

[refurb]

I assume the rocket is in geosynchronous orbit? Otherwise it would just be a timing issue (i.e. fire the rockets when you're over land).

[dchesterton]

They want to use this tech to eventually land rockets on other planets so the rocket needs to be able to land upright. Short term it might be easier to land on inflatables but they're looking at the bigger picture.

[el_zorro]

The utility here is not really landing on other planets, but instead the order of magnitude decrease in launch costs if they don't have to build a new first stage for every launch.

[dchesterton]

This could theoretically be achieved by either means. The question was specifically why a vertical landing is favoured over a cushioned inflatable landing.

Musk has stated in the past that they're looking to eventually use this tech to create rockets which can be used on Mars, hence why parachutes and inflatables are out of the question.

[ansible]

Parachutes also have the problem that you can't as easily control the landing point.

[jsumrall]

No, its just not possible to maintain the integrity of the rocket if you're going to somehow lay it down on an inflatable. How could an inflatable withstand the temperature of the firing engines?

[gbhn]

For weight, the sides of a rocket are very thin. Think tin can. Vertical is OK but horizontal can't work.

[delecti]

Which makes sense. It's designed specifically to withstand a huge amount of vertical stress.

[ohyes]

They want to be able to relatively quickly send it back into space. If they have to pull it out of the ocean and dry it out/fix it, I assume this makes their turnaround time less than ideal. Think what you do with a 747, it lands, a technician checks it out, it gets refueled, it takes off again. We'd be able to build things in space so much faster with that sort of round-trip lifting capability. I assume there are also implications for exploring other planets. This is my guess as someone who has not studied the issue closely.

[JoeAltmaier]

You don't want to carry heavy landing-maneuver gear to space and back. Thus the skeletal landing struts that were not-quite-strong-enough for rough seas.

Further, the ocean platform it was landing on was built to sustain heavy seas and stay relatively flat. Recovering a rocket bobbing in the waves could be much harder than this. Also a challenge: quenching a very hot engine in cold ocean water.

No, this platform-landing is probably one of the simplest, safest ways to recover/reuse a stage.

[alkonaut]

Rockets have always been able to land in the ocean more or less intact (i.e. without disintegrating). However, if the rocket is dipped in salt water, the restoration would be enormously expensive, so it needs to stay dry to be re-used.

Of course, you could just try to land it vertically on the water on some cushion and have more cushions deploy to make it tip over horizontally and float (all the time staying above the surface of the ocean). Such a solution isn't as compatible with recovery on dry land. If you want to do e.g. space tourism missions and land on dry land, the vertical landing seems better.

[zardo]

Rockets have always been able to land in the ocean more or less intact (i.e. without disintegrating).

No they haven't. SpaceX started doing that in 2013 before the barge landings. I don't think anyone had ever even tried to land(sea?) a liquid fuel booster before that.

You may be thinking of the STS solid boosters, but solid boosters are really a different game. They can be built sturdy enough to hit the ocean at 60mph, but they couldn't possibly be simply refueled and reflown.

[jsumrall]

How do you expect it to be able to land on an inflatable raft? The point of landing the rocket is to reuse it, which is not possible if it falls over or gets wet.

Landing in the ocean is only done for certain types of missions, where its not possible to carry enough fuel to turn the first stand around a fly it all the way back to the launch site. Instead, they can carry just enough fuel to land wherever they are finished boosting the 2nd stage, to put it simply.

[Already__Taken]

Keeping it out of the sea water is a key part of reusability.

[agumonkey]

Or even a different kind of barge. A floating 'jar' rather than a flat surface, the thing could tip and not fall all the way.

[startupfounder]

Salt water isn't good for rockets. Once you solve this really hard problem of landing a rocket on a barge on a dynamic ocean you have a real competitive advantage.

[valhalla]

Is that the reason they've been using the barges for landing? I'd always assumed they were just being extra cautious since it could endanger civilians or crew if the reusable stage was supposed to land on solid earth.

I'd imagine that the barge landings introduce a lot of uncertainty and complicating factors that are impossible to completely nullify. High waves and (comparatively) strong open-ocean winds seem (to me at least) to make consistently successful lands much harder.

[estel]

There was definitely a safety issue that lead to earlier landings taking place on a barge, but it also requires a lot less fuel to land somewhere downrange (which is always over the ocean) rather than boost all the way back to the launch site.

[AaronM]

Some flights wouldn't have enough fuel to make it back to the launch site. That's the reason I've seen. There is also a large amount of water, so that increases the places you can land.

[el_zorro]

The corrosion from the salt water would completely ruin the engines, obviating any benefits from returning the first stage.

[Grue3]

Yeah, I don't understand why they won't keep landing them on the ground like everyone else does. It's not like there's not enough space on Earth.

[typicalbender]

Depending on the mission it may not be possible to get back to the landing spot and sometimes the most efficient place to land is in the middle of the ocean. Musk answers that question here[1] in a tweet regarding the launch. You would have to have launch sites all around the earth and get approval to land or carry a lot of extra fuel to get back to where you started from which would be prohibitively expensive and heavy.

[1] https://twitter.com/elonmusk/status/688844190826942465

[dkbrk]

> like everyone else does

Nobody else does this. I have absolutely no idea what you could possibly be referring to. SpaceX lands on land preferably when it is an option.

> I don't understand why they won't keep landing them on the ground ... It's not like there's not enough space on Earth

As a general rule, you not understanding the reason for something does not mean there is not a good reason. For reference, see the Dunning-Kruger effect. Furthermore, I honestly do not understand how you could even contemplate "SpaceX does not think there is enough space on land" as a serious argument and treat its dismissal as legitimate support for your beliefs. This is some of the most egregiously lazy reasoning I have ever seen on the internet.

SpaceX launches rockets in the direction of water so that an accident won't threaten property or people on land. To return to land after staging the first stage of the Falcon 9 must perform a burn—the "boost back"—to point its lateral velocity back to land; this is the first of three burns required for landing and the most expensive in fuel by a considerable margin. If the first stage is travelling too quickly (greater than approximately 1.6 km/s lateral velocity) it does not physically have enough fuel to return to the launch site and perform the landing. In this case, a far more mild burn can put the booster on a re-entry trajectory and it can land on an autonomous barge in the ocean. As the rocket trajectory is chosen to put the ground path well away from land, the choice is between either returning to the launch site or landing in the ocean. If a return to the launch site is prohibited by the rocket's capabilites and physics then an ocean landing is the only option.

In the case of Jason-3 a barge landing was the only option as SpaceX does not yet have permission to use their landing facility at Vandenberg Air Force Base. In the future, the central core of the Falcon Heavy will almost certainly have to land in the ocean due to its velocity at separation.

[Grue3]

>Nobody else does this. I have absolutely no idea what you could possibly be referring to.

Believe it or not, we've been landing spacecraft for decades. Does "space shuttle" ring a bell?

[benjaminl]

The Space Shuttle was a space craft, not a rocket. The SpaceX equivalent to the Space Shuttle is the Dragon spacecraft. Dragon V2 does land exclusively on the ground.

[anarazel]

In addition to all that (spacecraft, srbrs, atmosphere), a space shuttle launch literally cost more than a billion.

[Dylan16807]

The shuttle throws its lower stages into the ocean.

[joegaudet]

Which were recovered and refurbished to keep cost down.

[mikeash]

OK, that's one orbital rocket that's recovered on land. You said "everybody else" which implies at least several, so what other examples are there? (Hint: none.)

[Grue3]

Buran. It only flew once, but it did work!

Apollo Lunar Module, if Moon counts.

[mikeash]

Buran, fair.

Lunar module doesn't count, the reduced gravity makes everything a hundred times simpler.

[rkallos]

The space shuttle couldn't land and take off on a planet whose atmosphere doesn't support it, hence why SpaceX is trying to do the same via rocket propulsion.

[ceejayoz]

> Yeah, I don't understand why they won't keep landing them on the ground like everyone else does.

No one else lands first stages. SpaceX is the first to do it.

[el_zorro]

It has to do with the payload mass. If it's too massive, it won't have enough fuel to return to the launch site, so they do a landing at sea.

[brianwawok]

So if you launch out of Cali going west and separate stage 2 300 miles out.... What is under you to land on?

[mikeash]

Not to change your overall point, but you almost never launch west. When possible, you want to launch east, to gain the benefit of the Earth's rotational speed. Sometimes you launch north or south so you can go into a polar orbit which allows viewing more of the Earth. But there's pretty much no reason to launch west. One exception to this is if your launch site is located in a place where that's the only option, which is why Israel does it like this.

For American launch sites, you have KSC for launching east, and Vandenberg for launching south. (North and south are equivalent in terms of which orbits they can reach, so you don't need both.) Vandenberg can be used for launching west, but pretty much never is, unless you count ICBM tests and the like.

[brianwawok]

Well the launch Space-X launch was South-West, clearly not over any land.

[mikeash]

Southeast, not southwest. But yes, as I said in the original comment, it doesn't change your overall point.

[alkonaut]

I assume the parent poster meant that the launch should take place from somewhere on earth where there is earth 2300 miles out. It's likely not convenient to do so, at least not for all launch directions.

[lmm]

It would be convenient enough but the FAA isn't too happy with the idea of a 12-story missile full of explosive fuel and oxidiser flying over populated areas.

[alkonaut]

Yes, part of the inconvenience of course is that it's unlikely to be anywhere near the continental US.

[lmm]

If you just needed to find two bits of ground with room for rocket pads and one 2300 miles east of the other it really wouldn't be that hard to have both in the continental US. It's the fact that the whole flight path has to be safe that makes it virtually impossible.

[Grue3]

The entire world = California.

Hacker News has officially become a parody of itself.

[brianwawok]

I don't even live in Cali.

But some rocket launches come form the west coast, not all are Florida.