SpaceX is spending considerable effort to catch the Falcon 9 first stage onto a giant drone ship. I am aware that salt water ruins almost everything in the long run, but the first stage is a composite material and seems to be painted. If it can survive a 8 km/s reentry, it should be able to survive a little time in the water. Empty of all fuel, the Falcon 9 first stage way just 75,000 pounds. Considering how much surface area it has, it should be able to float. There are carbon fiber (and fiberglass) boats, so why can't the Falcon 9 stages just touch the water (even for brief time)?
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$\begingroup$ Some misconceptions in the question that might not be covered by that answer: the Falcon 9 first stage is not composite, it's mostly aluminum. Some of it's painted, some of it's covered with a sort of hydrophobic ceramic felt. And boats aren't dropped vertically into the water and allowed to fall over, and even if more gently capsized or submerged, are likely to require major repair work, and may not be salvageable at all. Even a car that is immersed in saltwater is likely to be considered totaled, and rockets are considerably more complex and less forgiving. $\endgroup$– Christopher James HuffApr 10, 2022 at 21:52
$\begingroup$ @ChristopherJamesHuff more to the point; even if the structural pieces are OK taking a bit of a bath, there's a lot of complex innards which would have to be gutted. As mentioned in the linked question, where the SRB of the shuttle needed a lot of teardown of the internals to re-fly. $\endgroup$– fyrepenguinApr 11, 2022 at 8:28
$\begingroup$ @fyrepenguin that's why I used the example of a capsized or sunken boat. Boats are supposed to be in the water, but only certain parts are supposed to be in extended contact with it. The F9 booster's tanks might float, but the engines and grid fin/stage separation machinery don't have a hull keeping the seawater away. $\endgroup$– Christopher James HuffApr 11, 2022 at 17:54
$\begingroup$ There could also be issues with Very Hot materials coming into contact with relatively Cold seawater. $\endgroup$– FlaStorm32Apr 14, 2022 at 22:28
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