re: Shuttle boosters
Yep - thanks TB. As von Braun put it at the time solids "lack abort modes".
Not only that, but the use of the pressure fed boosters as stand alone launchers mode would have given the US the lowest launch costs in the world. No one, not Arianespace, not Great Wall, not Energia, not Yuzhnoi, would have been able to compete.
As a matter of fact, it's a good question why NASA did not go to pressure fed liquid boosters after the Challenger disaster. My guess is that you wouldn't have to look much further than the Thiokol lobbyists of the time to find the answer to that one. But I was naive in those days...