With SpaceX creating their launch facility in Boca Chica Texas, right next to the water, why would they bother building a huge launch pad to withstand the blast, as opposed to building it on top of the water? Why not let 20 feet of water handle the heat and blast? It would need a very good support structure below, but an unlimited water supply might make the job worth it.

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    $\begingroup$ Look at the answers to space.stackexchange.com/questions/1551/… $\endgroup$ – Eugene Styer Aug 19 '20 at 1:45
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    $\begingroup$ SpaceX is planning to launch from floating ocean platforms: businessinsider.com/… $\endgroup$ – D_Bester Aug 19 '20 at 12:54
  • $\begingroup$ No indication of whether the exhaust would blast into the ocean. Notice that Elon said it would be a floating platform. So any exhaust directly down into the ocean would create waves. Would the waves be big enough to make the floating platform unstable? $\endgroup$ – D_Bester Aug 19 '20 at 12:54
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    $\begingroup$ The marine fauna might have objections to toxic gasses being blasted into the water. $\endgroup$ – Florian F Aug 19 '20 at 22:09
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    $\begingroup$ @FlorianF the first stage engines are either using RP1 or liquid methane. RP1 is a highly refined kerosene specifically formulated for extremely low sulfur, and the carbon chain molecules are fractioned to optimize thermal stability (think about how diesel can turn to jelly when it's cold) and low polymerization (to reduce soot on the injector referred to as coking). It is far far cleaner than the basically raw crude oil that large cargo ships burn. Methane starts out cleaner, and produces even less carbon emission. $\endgroup$ – Aaron Aug 20 '20 at 16:54

Salt does all sorts of unpleasant things to just about every building material humans use. Hot salt spray, such as you'd get from a rocket launch, is even worse: spraying something with hot saltwater is one of the techniques used for corrosion testing. Build a launch pad over the ocean, and you'll need to clean it off after each launch to try to keep the corrosion down to reasonable levels.

Even without the rocket launches, building something to survive the ocean is difficult. Concrete, for example, is vulnerable to haloclastic weathering, where salt crystals deposited by evaporating water split it into fine sand. Steel, even stainless steel, will rust when exposed to saltwater.

Building a launch pad to withstand the blast of a rocket launch is simple compared to building it to withstand the ocean.

  • $\begingroup$ There are different qualities of stainless steel, a cheaper version not saltwater resistant and a more expensive version that resists saltwater parts. Not all stainless steel rusts when exposed to saltwater. $\endgroup$ – Uwe Aug 19 '20 at 6:47
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    $\begingroup$ @Uwe Yes, but most "saltwater resistant" steel alloys only resist saltwater under ambient conditions. Building a steel structure to resist all of salt water + the temperatures of a rocket launch + being shaken about by a departing rocket (stress corrosion cracking, anyone?) is going to be expensive. We're talking the kind of steels used for reactor pressure vessels. $\endgroup$ – TooTea Aug 19 '20 at 7:40
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    $\begingroup$ While the point about salt issues is absolutely true, I doubt it's really that relevant here since all they're doing so far is essentially one-off prototyping. No point building a platform that can withstand years of exposure to nature, if you destroy it anyway a month later with a static-fire explosion. More important, I'd guess, is that a chunk of concrete is just the simplest / cheapest / most reliable option to have a stable foundation that won't start sinking when you fuel up the rocket. $\endgroup$ – leftaroundabout Aug 19 '20 at 10:25

Environmental impact may be a major consideration

The effect on the local water environment from that sort of blast into it could be severe and far reaching in the neighborhood around a water based launch pad.

Also if there are any abort or need to dump fuel or uncombusted fuels from explosions could also have massive impacts that are immediately spread due to being in open water.

Where as on land you can tightly contain such effects to a much smaller area that can be cleaned up and/or doesn't spread easily and immediately through water (long term is different of course due to groundwater contamination, etc.).

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    $\begingroup$ a) Rocket's don't dump fuel like aircraft do sometimes. It's either just technically impossible (in solids) or would just exacerbate the problems. Oxygen may be vented, anything else at most pumped out back into the storage tanks, but that's independent of water vs. ground. b) Starship uses Methalox. Both components quickly evaporate and/or burn off, regardless of whether they're dumped on ground or water. I suppose it does also carry some amount of nastier stuff like hypergolics, but I'm not at all convinced that those can be “easily” cleaned up in case of a ground crash either. $\endgroup$ – leftaroundabout Aug 19 '20 at 14:08
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    $\begingroup$ Thanks, I did not know any of that, so I appreciate it all. I guess my focus was also more on explosions and potentially partially combusted stuff more than day to day contamination. However I see that's not what I said, I just write about dumping fuel. Rather then edit my answer right now I'll leave this in the comments. Maybe move up later - it will make your comment confusing though if we leave it then. $\endgroup$ – Michael Durrant Aug 19 '20 at 19:41
  • $\begingroup$ It's generally fine to fix problems in an answer and then reply to a comment pointing them out. Future readers need to understand that the most important purpose of comments is for improving posts; sometimes to the point where the comment becomes obsolete and can be removed, sometimes it's still interesting to have it. $\endgroup$ – Peter Cordes Aug 21 '20 at 20:20

Per the answers here the water around rocket launches is in spray or mist form to absorb energy through evaporation, with cooling being largely a happy side effect. A flat sea surface will instead tend to reflect energy back towards the launch structure and rocket. The rocket blast will tend to displace it downwards but not actually absorb the much energy So the over water platform is probably worse than a conventional 45 degree blast deflector that sends the exhaust sideways to dissipate.

Being at sea does have some other potential uses, in terms of fewer neighbors to get upset when launches fail, and if you make your full launch structure mobile there are potential commercial advantages but as per Marks answer, generally adding the ocean to any plan involves dealing with the fact it breaks everything

  • $\begingroup$ Absorbing energy through evaporation seems to mean absorbing thermal energy through evaporation, which means cooling. Do you mean it's absorbing kinetic energy from the blast, then thermalising it leading to evaporation? Or what did you mean? $\endgroup$ – Chris H Aug 19 '20 at 18:11
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    $\begingroup$ Due you have some references on flat sea “reflecting energy back”? This sounds mightily dubious to me. And the Leidenfrost effect is specifically when a solid gets in contact with a liquid of significantly lower boiling point, so that the vapour causes a thin isolating gas layer. But the rocket exhaust is already gas itself, and very effective at blowing any steam or whatever else “insulating” gas away. $\endgroup$ – leftaroundabout Aug 19 '20 at 22:53

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