A few months ago, TRF/YORF member luke strawwalker was posting summaries of old NASA reports...and I was summarizing his summaries. A lot of those old studies involved adding SRBs to Saturn-derived rockets. If interested, follow the 'historic' tag below and cruise back a ways.
Aside: As many have pointed out, the images released today are 'rocket p0rn'. That is, they were made to look pretty and may not accurately reflect the final product.