In article ,
says...

Booster engines fully shut before pneumatic pistons detach the boosters.

Would it be correct to state that such a manoeuver would only appen in
thin enough atmpsphere so that the boosters do not become a big drag on
stack between the time their engines shut and booster separation ?

They're a big drag because the center stage is still firing placing the
stack under continuous acceleration. Air drag wouldn't change that
much.

The problem with air drag is more the worry that it could cause lateral
forces which cause the boosters to recontact the core. This happened
with the D-21.

http://www.wvi.com/~sr71webmaster/M21_Crash.htm

Jeff
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