[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. Also, efforts to thrust vector solids have not been very successful. The liquid motors provide the throttling and thrust vectoring for stability and course corrections.The shuttle design restricted the acceleration to 3 g's. The solids provide the bulk of the thrust at low altitudes while the liquid is throttled back. As the solids burn out the liquid is throttled up allowing the accel to stay under 3 g's. There is no ideal geometry for a classic nozzle from sea level to a vacuum. The solid motor nozzle is optimized for low atmosphere and the liquid is optimized for low atmospheric pressure to orbit. Note that Rutan used a hybrid motor for thier effort. Hybrids are beginning to catch up. Hybrids are solid fuel and liquid oxidizer.