Why is SpaceX landing rockets on ships in the ocean?

Just because it looks hard to prove how advanced they are? Or is it a safety idea? I know some have landed back on land.

It takes too much fuel to make it back to land.

It depends on the desired orbit and the payload. For many launches, there isn’t enough fuel left to get all the way back to land.

Yep. These rockets are launched eastward from Florida over the Atlantic. If the payload is light enough, the booster may have enough fuel to turn around and make it back to the landing zones at Cape Canaveral. If it’s heavy, they will not be able to make it back, so they land on the drone ship.

A CT I heard is it is because they want to show off the accuracy of their navigation systems and drone AI to sell to the highest bidder that wants to improve its arsenal.

Reportedly there are two such drone ships. One is named “Just Read The Instructions,” and the other is named “Of Course I Still Love You.” The current JRI is a reincarnation of an earlier vessel with the same name. Another ship under construction is to be named “A Shortfall Of Gravitas.”

Just Read the Instructions (I call it the SS RTFM) is used in the Pacific for launches from Vandenburg AFB.

Recovering the rockets is what makes the costs of SpaceX launches much more affordable than traditional rocket launches, where the rockets are not recovered.

The rockets launch upward and curve across toward horizontal. When more fuel is needed (heavy load or higher orbit) it is simpler and cheaper to land downstream from the launch site rather than build bigger rockets or settle for lesser payload. After first stage separation, the first stage will turn and fire to slow its horizontal motion; this also slows it so it doesn’t overheat coming back down through the atmosphere at high speed. Then it has enough fuel left to land softly on the ship. IIRC there was one landing where the engine appeared to cut out a few feet above the deck - possibly fuel exhaustion. (They also lost one in rough seas because the clamp-down robot that latches rockets to the deck was not ready yet) Conveniently though, an almost empty tin can (or stainless steel) is lighter for its wind resistance and does not fall too fast. However, it falls bottom first so does not take full advantage of wind resistance the way a sideways fall would. If you recall the news footage of twin rockets landing back at the cape, those were the side boosters for a high-capacity launch, so they could drop off and come all the way back from a closer position than regular first stages.

Launching of the ocean was a decision made when locating NASA’s launch facilities back in the days when letting a rocket fall into the ocean was the expected launch procedure. It’s more convenient than Russia’s letting then plunk down off in the Siberian wilderness.

The first stage is going at 5000 to 6000 mph at MECO (main engine cut-off, or the stage separation point). To land back at the launch site, the booster needs to cancel that velocity, and then add some more, and then do all the other operations needed for landing–a reentry and landing burn.

For very light payloads, the Falcon 9 can fly a more vertical trajectory so that there is a less horizontal component. Only the horizontal component needs to be cancelled. The vertical component can be partially handled by the atmosphere (it keeps going up, and then falls and experiences atmospheric drag). This is called the boostback burn.

A boostback burn isn’t needed for barge landings, since they just put the barge where the arc of its trajectory would have taken it anyway. Less delta-V (change in velocity) needed, so more left over for the second stage (and a flatter, more efficient trajectory), and therefore more payload.

Check out the trajectories in the upper half of this image. The ones that loop back are landing at the launch pad; they take a steeper trajectory than the ones that land on the barge.

Incidentally, while saying things like “not enough fuel” isn’t exactly wrong, it might give the wrong impression of what’s going on (it’s not about the cost of fuel or anything like that). A rocket can fit only so much fuel and from this extract a certain amount of delta-V. Getting to orbit takes delta-V. Getting to a higher orbit, like geostationary, takes more delta-V. Adding payload mass takes away how much delta-V your fuel gets you. And using delta-V for other purposes, like landing, also means it’s unavailable for other purposes.

These factors conspire to mean that if you have a light payload going to LEO (low Earth orbit), you have enough delta-V left over for the first stage to make it back to the launch pad. For heavier payloads or ones going to geostationary, you need to save delta-V and the barge landing is the only option. For very heavy payloads, there isn’t enough delta-V even for that–and so the booster is expended (at this point, SpaceX would rather launch a Falcon Heavy for those payloads).

Bijou, are you possibly also asking why are they landing boosters…at all? Just in case you are, it’s to save a ton of money. The first stage and its 9 Merlin engines cost a huge amount of money, as do pretty much any other rocket’s first stage. Historically, and even continuing into the future (see the big SLS they’re building), these have been used just once and discarded, making spaceflight extra expensive.

But if the first stage really can be cheaply reused for dozens, or hopefully even hundreds of flights, that may well cut the per-flight cost down so much that spaceflight becomes almost affordable. SpaceX certainly beats the other space launch providers so badly it seems unfair. They spent the time and effort to develop the technology to make spaceflight cheaper, now they get to enjoy being the lowest cost provider…provided that Elon doesn’t lose business by pissing off everyone else in the space community with his strange antics.

Launching over the ocean is also a safety measure. You don’t want rockets crashing at all, but sometimes they do, and when it happens, you don’t want them crashing into people’s homes.

Launching over the ocean is also a safety measure. You don’t want rockets crashing at all, but sometimes they do, and when it happens, you don’t want them crashing into people’s homes.

I know they are doing it to save money.

Landing payloads also increases reliability.

Any flight has the potential for marginal events that don’t quite cause a failure. Wire insulation partially rubbed through due to vibration. Partial burn-through of an engine component. Minor buckling of some structural part. Small leaks of propellant or hydraulic fluid. Etc.

When you get the booster back, you can see all these sub-critical anomalies and correct them so that they don’t turn into critical ones. Otherwise, you have only the telemetry to go with, and it just isn’t going to tell you everything.

Of course, you have act on these observations as well. Prior to the Challenger explosion, NASA noted unexpected burn-through of the o-rings–something they only found due to recovery. But rather than correct the problems, they deemed it acceptable risk and kept flying.

You can say that again.

USSR/Russia has never launched from a coastal location have they? And they also don’t splashdown now, maybe they did in the past. They end the missions on land. ESA launches their Ariane rockets from the east coast of South America in French Guiana.

Just curious, how much extra fuel does this recovery require? This added fuel obviously reduces the orbital payload capacity.

The rocket gets the full amount of fuel each time. To enable landing, the first stage has to cut out earlier (possibly also flying a more lofted trajectory), which means the second stage has to burn for longer, which means less payload.

Going to a geostationary transfer orbit, the latest version of the F9 can deliver 8300 kg expendable, 5500 kg while landing at the drone ship, and 3500 kg when landing at the launch site (RTLS, return to launch site).

To low-Earth orbit (LEO), they can get 22800 kg expendable and 15600 kg at the drone ship. I don’t know about RTLS, but LEO should be a bit less sensitive to booster performance than GTO. I suspect it can do around 10-12000 kg in RTLS mode (i.e., more like a 50% hit compared to 60%).

There’s another factor, which is stress on the vehicle and risk during burns. It’s possible to reduce the performance loss by literally coming in hot–reducing the reentry burn (and thus depending more on atmospheric drag), and by doing the landing burn with three engines instead of one. These increase the risk that the landing will fail just due to random variation.

Landing on the drone ship vs. RTLS means that they can reserve more performance against this variation (back to using a longer reentry burn, and a 1-engine landing burn). So that’s likely to be the preferred option even when there is technically enough performance for RTLS. Only when there is a very large performance margin will they use RTLS (i.e., some of the lighter Cargo Dragon missions).

IIRC the Soyuz can theoretically land in water in an emergency.