@John Smith 19
I'll try and address most of your counter points in order. You say the modifications for interstage pipework would be minor. Have you considered the fact that the pipework would have to run on the outside of the vehicle as the engine gets in the way of having the pipes run inboard? Have you considered what effect this would have on the vehicles centre of mass or aerodynamic profile? How would you sort out pumping fuel from one tank to the other once the pressure between the first and second stage tanks equalizes? How do make sure all tanks are fully purged of fuel before impact? (you'd better hope they are or you'll have a very large explosion) What effect are these changes going to have on the structural integrity of the tank and the vehicle as a whole?
Still think these are minor changes?
You say the stack would be going at mach 12 rather than mach 6. I've tried looking this up without much success so I'll assume you're correct on this one. Either way it's still going chuffing fast, horizontally. Any significant manouveres like this 3 point turn you talk about are going to subject the vehicle to lateral stresses it was not designed to stand up to. Rockets are built to be as light as possible remember.
Even if it could do this, it's still moving in the same direction at the same speed, just now pointing the wrong way. How do you control a rocket pointing arse first? Your aerodynamic profile is all different, the main engines and any manouvering thrusters are all pointing the wrong way, aerodynamic heating becomes are real problem as it decends back through the atmosphere (even at mach 6, let alone mach 12) and any shielding on the vehicle is at the front and now pointed the wrong way. These are not simple problems to solve.
I know I refered to this arrangement as a guided landing rather than an abort but the important word there was "guided". You rocket is over the sea remember? How do you propose to get it back over land? Even if you manage that, how do you get it to land upright and concertina neatly as you describe without some very precise control of speed and direction. All with empty fuel tanks?
My point is not to suggest that these problems are insurmountable, only to highlight issues that going by the content of your previous posts, you appear not to have considered and also to point out that the design changes required to address all these problems would not be "minor" but would actually result in a radically different vehicle from the one that launched to other day.
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