Why does the Shuttle have Solid Rocket Boosters?

[DonRotolo]

Originally Posted by Gdeaver

Solid fuel motors can not be turned off. Once they are lit there is no turning back. The thrust curve from a solid can be preprogrammed by the grain geometry and chemical composition but they can not be modulated dynamically.

Exactly. At the max-Q point (where the aerodynamic stresses on the airframe peak) the SSMEs are throttled way down, and once the Max-Q has passed (the atmosphere is thinning fast at that altitude) they can throttle back up. Wide throttle control is necessary, but not easy with SRBs, and the timing (of thrust variations) has to be set during SRB manufacture, limiting flexibility with launch weight (for example).