Main Question or Discussion Point
When a space shuttle launches, how is simultaneous SRB cutoff assured so as not to create an asymmetrical thrust situation? (assuming that SRB cutoff occurs prior to separation).
http://en.wikipedia.org/wiki/Space_Shuttle_Solid_Rocket_Booster#PropellantSRB separation is initiated when the three solid rocket motor chamber pressure transducers are processed in the redundancy management middle value select and the head-end chamber pressure of both SRBs is less than or equal to 50 lbf/in² (345 kPa). A backup cue is the time elapsed from booster ignition.
The separation sequence is initiated, commanding the thrust vector control actuators to the null position and putting the main propulsion system into a second-stage configuration (0.8 second from sequence initialization), which ensures the thrust of each SRB is less than 100,000 lbf (445 kN). Orbiter yaw attitude is held for four seconds, and SRB thrust drops to less than 60,000 lbf (267 kN).
http://www.csar.uiuc.edu/F_info/space_shuttle_srb.htmThe SRBs are used as matched pairs and each is made up of four solid rocket motor segments. The pairs are matched by loading each of the four motor segments in pairs from the same batches of propellant ingredients to minimize any thrust imbalance.