Re:Not really a surprise (Score 1, Informative)

Actually, you can, and NASA does, throttle the SRB's. Actually, throttling isn't really the correct word, but by changing the shape of the solid fuel, SRB thrust is reduced by about 33% 50 seconds into the burn. After MAX Q, it is returned to maximum thrust.

See the section on SRB's here http://www.daviddarling.info/encyclopedia/S/Space_ Shuttle.html