Why don't they let the Falcon 9 splash down IN water, and then recover it? Wouldn't this reduce both fuel use & potential damage?
The first several attempts to recover the Falcon 9 did include water landings:
However there are many problems with water landings.
There's no way for the Falcon 9 to remain upright on water. To get an idea of the magnitude of the problem, try standing by a swimming pool and falling forward face first into the water. It's going to hurt. Now do it while being fifty meters tall and weighing 28 tons.
According to Elon Musk this can cause kaboom (technical term).
All attempts to recover the rocket intact after water landings failed. Although one core survived the 90° fall it quickly broke up from wave action.
Moreover, even if the stage survived tipping into the sea there's the problem of salt water corrosion. Salt water is really nasty stuff if you're a rocket. After a dip in the sea actuators are going to need striping down and rebuilding, combustion chambers are going to need careful cleaning, the whole rocket will probably need rebuilding, and if even a drop of water gets into the computers you can kiss them goodbye for good.
The days of drone platform landings are probably numbered (even before they fully begin). SpaceX has stated that they want to start returning the cores to land in the near future. They've even begun planning and construction of the first landing facilities.
Finally (as pointed out by Luggage in the comments) the fuel required to land on a floating platform is almost identical to that required to perform a splash landing. There would be some extremely minor increase in fuel consumption (probably measurable in 10s of litres) for the platform landing. The Falcon would have to modify it's velocity slightly to aim for a small target, rather than just landing anywhere in the ocean with the most efficient "go where the wind takes it" trajectory.
1$\begingroup$ Might be helpful to compare with the Space Shuttle SRBs which were successfully reused after splashdowns. They contained a lot of electronics and moving parts (the rocket nozzles were steered with hydraulics which had their own power systems, etc.). Obviously expensive, damage-prone etc., but apparently still more economic than abandoning and ordering up fresh sets for every launch. So the question may be more about how expensive it was to recover and refurbish the SRBs (and what could be predicted for Falcon 9) after a splashdown vs. soft landing on a dry surface. $\endgroup$ Apr 18, 2015 at 20:59
3$\begingroup$ The SRBs were 8 mm thick steel casings, so they traded lots of payload weight for being able to splash them. F9 stages are much thinner and prone to damage. $\endgroup$– HobbesApr 18, 2015 at 21:29
2$\begingroup$ Note that after splashdown the SRBs had to be completely overhauled. For example, the actuators that controlled the engine gimbals (3 per SRB, it would be many more for the 9-engined Falcon 9) were sent to Moog in Buffalo, NY to be completely torn apart, refinished, and re-assembled before they were used again. $\endgroup$– regdougApr 18, 2015 at 22:17
8$\begingroup$ There was nothing "economic" about SRBs refurbishment. $\endgroup$– ErikApr 18, 2015 at 22:28
1$\begingroup$ I'd like to add that it would not reduce fuel use, as asked, since you'd still want to hit the water as close to 0 velocity as possible. $\endgroup$– LuggageApr 19, 2015 at 23:10
Well, they seem to have managed it, albeit by accident, today (31 January 2018).
SpaceX were testing a 3-engine suicide burn with a booster they didn't plan to reuse, so they were landing on the sea rather than the droneship.
And, well, it wasn't destroyed on contact with the ocean:
1$\begingroup$ Where's the kaboom? There's supposed to be an Earth-Shattering kaboom! $\endgroup$– SaibooguFeb 1, 2018 at 2:48
The SRB and Falcon 9 first stage examples are very different.
The SRB is made of fairly heavy steel, considering that a burning SRB generates pressure throughout the body of the SRB. In fact to stop thrust, you pop the top off so that the thrust is equal in both directions.
The Falcon 9 first stage is an empty tin can, that is strong enough to hold itself up empty, but not much more. It gets pressurized somewhat during flight, to add strength to survive launch.
A 140 foot tall, 12 foot wide pop can, falling over in the ocean is not survivable.
The SRB also seems to enter heavy end first (engine) fairly quickly, submerges and bobs back up. The F9 first stage is not strong enough to do that.