Postby Shyster » Wed Feb 07, 2018 11:06 pm
There are multiple reasons. First, salt water is highly corrosive and would do bad things to the Merlin engines and other parts of the booster. Being solid rockets, the shuttle SRBs were mostly big steel tubes, and they didn't have the complicated plumbing and exotic materials involved in liquid engines like the Merlin 1D. And while the shuttle SRBs were reused, each of them had to be completely taken apart, carefully and thoroughly cleaned, and put back together. Dunking a Merlin engine in ocean water would at best require a complete disassembly for cleaning, and at worst it might outright ruin the engine. Either way, the amount of maintenance would run counter to SpaceX's goal of rapid reuse with virtually no significant refurbishment.
Second, the Falcon 9 itself partly depends upon pressurization for its structural integrity. As mentioned by Viva la Ben, If forces are applied in ways that the rocket isn't designed to support, it could buckle or rupture the booster, and a rupture would cause a loss of structural integrity. A rocket descending under a parachute could hit the water at all sorts of unexpected angles, or even get dragged along the surface. Wind would be an even bigger problem for an attempted land landing; you couldn't guarantee that the vehicle would come straight down onto its landing legs. When SpaceX was doing the early landing tests, they soft-landed multiple Falcon 9 boosters in the ocean. Even if the boosters managed to splash down softly, they often still broke up from tipping over or from the forces imparted by waves. They're designed to be strong up and down, but they are vulnerable to forces like bending or twisting.
Third, parachutes of sufficient size to show a Falcon 9 booster for a soft landing would be big and heavy. The ones on the shuttle SRBs were huge. Those would significantly impact the vehicle's payload. I believe the fuel/oxidizer needed for the boostback, entry, and landing burns might actually weigh less than a parachute system would.
SpaceX is eventually hoping to get the landing precision to the point where the landing legs are no longer necessary; the booster will land directly back on its launch mounts. At that point you just crane up another second stage, fuel up, and launch again.
Arianespace is evaluating a system called Adeline where after staging the back of the booster with the engine and electronics would detach and fly back to land horizontally on a runway using a pair of wings and maybe even some deployable propellers. So you'd need new fuel tanks for each launch, but you'd be able to reuse the engine(s), avionics, and engine mounts/gimbals—basically all of the expensive stuff. ULA has proposed a somewhat similar system where the engine structure for the upcoming Vulcan rocket would detach and deploy a big parafoil. A heavy duty helicopter with a hook system would "catch" the parafoil on the way down. Both companies have said that those systems wouldn't be ready until 2025 at the earliest.