Gubbo: I feel the spaceships are designed for acceleration around very specific conditions. Mostly with the flame-y bit pointed towards the ground.

Exactly.  They're built to take a lot of force in specific situations.  They are not built to resist forces in other directions.  We have two videos of large rockets that turned too quickly in the sky (and they aren't built with much ability to turn because they're not able to withstand it) and both end up with the rocket coming apart and turning into a huge fireball.  (Admittedly, one of them the boom is augmented as the rocket sensed it's doom and suicided.)

DoBeDoBeDo: While true I have to imagine that at 4,000MPH and 5.75 Million pounds a side shear of 74MPH isn't out of the question for normal operation.

Actually, I would think those conditions would never occur.  By the time it's up to 4,000 mph there's going to be very little atmosphere around and thus very little shear.  Rockets start out basically straight up because you have to get above most of the atmosphere before you pile on too much velocity.  Maximum efficiency is actually attained by going straight sideways (but the rocket wouldn't be pointed straight sideways) from the launch pad--but you can't do that in atmosphere and even on an airless body you won't have the turn authority to actually do it.  (Now, if you're playing KSP with it's ridiculously overpowered turning ability that's another matter.  I've lifted from Minmus with a 1-second burn, rotate to horizontal and then burn for orbit--more efficient than anything MechJeb can do.)