Falcon 9 wasn't designed for immediate reuse. Immediate reuse is just a theory right now, but SpaceX has an excellent track record of turning their theories into reality.
SpaceX did purchase some oil rigs with the intention to turn them into launch platforms, but later abandoned the idea. It's probably something they will return to later once Starship is flying regularly. You're right that avoiding the boostback burn is a big advantage. But maybe they don't need to bring the booster back after it lands on a platform, it can just launch again from there. Maybe they could have a bucket brigade of launch platforms ringing the Earth!
The issue is logistics, and in this case isnt an easy solve.
You have to get fuel, and the rocket, out to a pad in the ocean, and have to deal with a rocket lift on varying conditions.
If you dont want to do most of that, then the only option is putting your manufacturing on the rig too, which negates lifting a rocket, but instead makes the rig huge, and requires having a train of ships in and out 24/7 to keep it supplied.
There was even a company in the late 90s that tried oil rig launch platforms and ultimately abandoned it.
Immediate reuse means significantly less than 24 hours. Falcon 9's current cadence is fast enough to meet their current launch demand and doesn't need to improve, especially with Starship on the horizon. On the other hand, Starship will need to launch repeatedly in a short amount of time for orbital refueling to work.
So it was declared as a target, was possible but wasn't done because no demand? Knowing SpaceX they'd do it just for bragging.
The fact that it's required for Artemis to work, and the amount (nobody knows exactly but lower bound is like 15) of Starships required to launch in quick succession just highlights how risky, to the point of unsoundness, the project is.
They don’t need rapid reuse for Artemis. It would certainly help but Starship can just hang out in orbit for a few weeks while they do what they need to do to launch all their rockets.
It really depends - you can do direct injection to GEO now with cryogenic stages. And IIRC Soviets did some tests with kerolox stage (that should eventually launch on the ill fated N1) around the Moon, meaning multi-day flight times with liquid oxygen on board.
If you do the thermal design right, possibly use a sun shade (space is a large thermos bottle after all) or even use active cooling to remove the little heat that gets through, then it should work just fine. :)
The heat shield probably provides some degree of insulation from the solar radiation as well, and it's only needed on one side if you're far from Earth.
It would be good because there are many people who, roughly, refuse to believe their own eyes and keep moving goalposts, misrepresenting what they said or meant earlier, inventing additional conditions, changing their mind etc. If you would honestly believe that Starship can fly frequently when you see it flying frequently, you're already ahead of some.
I remember when people doubted that the full-flow staged combustion methalox engines could work... until SpaceX showed over a hundred of them working now.
Also catching a booster, also landing boosters at all, also achieving cost reduction via landings, also cheap enough phased arrays to make Starlink viable, also also also... SpaceX has a long history of proving doubters wrong.
I'd consider that cheating on the Gwynne side. It could be argued if chamber tilting mechanism is part of the engine - after all, it was added separately to NK-33 - but surely for Raptors it's considered an integral part, the fire test was then a test of chamber, not the engine :) . Though Gwynne had to answer positively, having little good choice.
The outer ring of engines on Superheavy do not tilt. They're rigidly mounted and exclude even the engine start hardware which is contained in the launch mount, further reducing weight. And they're only used during liftoff, not for boostback or landing.
Raptor is non-functional without tilting. That is, it's possible to build rocket where all Raptors are fixed, but that defeats some ideas of Raptors. And I haven't seen tilt mechanisms considered parts of the rocket.
So, while engines without tilting have been used - e.g. NK-15 - Raptors aren't from that category.
Raptor is used in a non-tilting configuration on both the booster and ship. You could certainly design a rocket to get to space with only non-tilting raptors, if you wanted. It would be silly to consider it incomplete without tilt, even if Starship does have some tilting ones too. Consider it two variants if you like, both complete on their own.
Differential thrust has been used to steer rockets for decades, and raptor has the sort of deep throttling capability required for it. Tilting is absolutely not required for anything but landing.
Yeah - you would still need roll-control, but given there are huge potato smashers already bolted on, it should not be an issue while in enough atmosphere. ;-)
So far this hasn't been shown even on much simpler Falcons. The barge gives quite some energy advantage for not having to boost back.
Ideally they'd build a capesize kinda barge with chopsticks to catch it in the ocean, then perhaps service what they have to while it's steaming back.