I recall that the booster motor only burns for about 10 seconds which is a long time for a small booster motor burn. The MCM (mission control module) event sequencer lowers the engine air scoop, starts to spool the engine, the fins erect, the wing plug doors blow off, the wing doors open, the wings sweep forward, the wing doors close. The booster motor is jetissoned when the pyrotechnic charge breaks the attachment ring off as the engine arrives at about half thrust. Tghe wing plug doors are airfoil shaped, and when blown off allow for the wing to fill up the hole they originally occupied. All of this is accomplished by a high pressure (6000 or 7000 PSI... I can't remember which) helium bottle located above the engine inlet cavity.

More later...
Harley Condra
BVM REP
JetCat REP